Ecological Engineering 148 (2020) 105791

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Ecological Engineering

journal homepage: www.elsevier.com/locate/ecoleng

Review Municipal with technology: Historical review T and future outlook ⁎ Long Ho , Peter L.M. Goethals

Department of Animal Sciences and Aquatic Ecology, Ghent University, Ghent, Belgium

ARTICLE INFO ABSTRACT

Keywords: Facing an unprecedented population growth, it is difficult to overstress the assets for wastewater treatment of waste stabilization (WSPs), i.e. high removal efficiency, simplicity, and low cost, which have been re- High algal rate pond cognized by numerous scientists and operators. However, stricter discharge standards, changes in wastewater Pond upgrade compounds, high emissions of greenhouse gases, and elevated land prices have led to their replacements in many Pond modeling places. This review aims at delivering a comprehensive overview of the historical development and current state Greenhouse gas of WSPs, and providing further insights to deal with their limitations in the future. The 21st century is witnessing Emerging contaminant Climate change changes in the way of approaching conventional problems in pond technology, in which WSPs should no longer be considered as a low treatment technology. Advanced models and technologies have been integrated for better design, control, and management. The roles of , which have been crucial as solar-powered aeration, will continue being a key solution. Yet, the separation of suspended algae to avoid deterioration of the effluent remains a major challenge in WSPs while in the case of high algal rate pond, further research is needed to maximize algal growth yield, select proper strains, and optimize harvesting methods to put algal biomass pro- duction in practice. Significant gaps need to be filled in understanding mechanisms of greenhouse gas emission, climate change mitigation, pond services, and the fate and toxicity of emerging contaminants. From these insights, adaptation strategies are developed to deal with new opportunities and future challenges.

1. Introduction and one-third of the WWTPs in Germany (Mara, 2009). WSPs are also the most popular application of wastewater purification in New Zealand Although wastewater treatment is considered a recent technology, with over 1000 anaerobic ponds (APs) (Heubeck and Craggs, 2010). its first application was implemented as early as 4000 B·C when ancient Despite the global presence of WSPs, this low-cost technology is Sanskrit and Greek applied water treatment methods such as filteration currently facing a junction of either being upgraded or being replaced through charcoal, light exposure, and water boiling and straining (US as a result of stricter regulation on the standard of effluent discharge. EPA, 2011a). Later on, pond-based technology was employed over Moreover, it becomes more challenging for pond treatment technology 3000 years ago (US EPA, 2011a). The first man-made , due to the fast development of other wastewater treatment technologies named Mitchell with 275 ha of surface area and 1.4 m of average for the last decades, including activated , membrane technology, depth, was accidentally built in San Antonio, Texas, the US in 1901 and technology (Fig. 1). Although the number of (Gloyna, 1971). Since then, waste stabilization ponds (WSPs) have been publications related to WSPs occupies 10% of the total number of sci- progressively applied for municipal wastewater treatment worldwide. entific publications studying on wastewater treatment technology, this WSPs can be found in many countries located from polar areas, e.g. number is only half of the publications related to (AS) North America or Europe, to the equator, e.g. Africa or South Asia, technology or one-fourth of the publication studying on advanced treating wastewater from large metropolitan to small municipal areas. membrane technology. In fact, in contrast to the slow pace of pond In fact, above half of wastewater treatment plants (WWTPs) are WSPs in technology development, AS-based systems continue to apply in- the US, accounting for 8000 facilities, which are also the first choice for novative design and controlling processes, such as Instrumentation, nearly all remote communities in Northern Canada due to their low Control, and Automation (ICA) technology, aerobic granular sludge costs and simplicity (Ragush et al., 2015; US EPA, 2011b). In Europe, system, nutrient recovery technology (NRT) to increase both removal WSPs contribute to 20% of the total number of urban WWTPs in France and energy efficiencies, and minimize space requirement. The stagnant

⁎ Corresponding author. E-mail address: [email protected] (L. Ho). https://doi.org/10.1016/j.ecoleng.2020.105791 Received 2 January 2020; Received in revised form 28 February 2020; Accepted 8 March 2020 0925-8574/ © 2020 Published by Elsevier B.V. L. Ho and P.L.M. Goethals Ecological Engineering 148 (2020) 105791

Fig. 1. The number of scientific published documents related to main wastewater treatment technologies on Scopus databases from 1990 to 2019. Details ofsearch queries can be found in Supplementary materials.

Fig. 2. Milestones in the chronological evolution of pond treatment technology. Related references can be found in this section. development reflects the negative impact of being labeled as a low-cost the case of As-Samra WSP in Jordan, one of the largest WSPs in the 90s' technology with basic operation and maintenance, leading to a highly serving near 2.6 million inhabitants (Pearson et al., 1996; Saqqar and conservative approach towards accepting and applying innovative Pescod, 1991), pond engineers need a novel and comprehensive ap- technologies. Most of the guidelines for pond operation and main- proach, which should be an integration of maximizing the merits of tenance were written around three decades ago, i.e. Pearson et al. pond technology while avoiding its negative drawbacks. Recent reviews (1987a), and Mara and Pearson (1998) while the newest guideline from have been conducted to deliver an overview on different subjects re- US EPA (2011b) overlooked the benefits of the integrated mechanistic lated to pond technology, such as design and operation (Ho et al., models or intelligent monitoring, communication and control in system 2017), removal (Dias et al., 2017b; Liu et al., 2018), hydro- management. dynamics (Li et al., 2018a; Passos et al., 2016), micropollutants To avoid being replaced due to the overload and odor nuisance as (Gruchlik et al., 2018), removal (Verbyla and Mihelcic, 2015).

2 L. Ho and P.L.M. Goethals Ecological Engineering 148 (2020) 105791

However, a complete picture of research status and current challenges period (Fitzgerald and Rohlich, 1958; Ludwig and Oswald, 1952; of pond technology remains unclear. To this end, this review aims at Ludwig et al., 1951; Malchow-Møller et al., 1955; Marx, 1956; Oswald delivering an overview of the historical development and current state and Gotaas, 1957; Oswald et al., 1957; Oswald et al., 1953a,b). As a of wastewater pond technology from which valuable lessons and in- result of taking into account the algal biomass in the BOD measurement novative ideas are withdrawn for their future. Specifically, the chron- of the system effluent, Malchow-Møller et al. (1955), Merz et al. (1957), ological history of pond development is systematically and critically and Silva and Papenfuss (1953) believed that the algae exerted con- inspected from early development to modern times. Subsequently, the siderably the amount of organic matter in the receiving . Con- potential future of pond technology in the 21st century is thoroughly versely, William J. Oswald had a different thought. Recognizing the discussed with the most urgent issues related to design and operation, symbiosis between and algae, he concluded that “the principal pond modeling, energy recovery, greenhouse gas emissions, and work of treatment is accomplished by aerobic bacteria, which oxidize emerging contaminants. These perspectives are finally summed up in the organic carbon contained in the to carbon dioxide. The the last section which serves as a guideline for the development of pond algae, through , convert much of this carbon dioxide to technology for pond engineers and operators. algal cell material … through photosynthesis, produce an excess of above their own respiratory requirement” (Ludwig and Oswald, 2. Chronological history 1952; Ludwig et al., 1951; Oswald and Gotaas, 1957; Oswald et al., 1957; Oswald et al., 1953a,b). Due to these findings, Oswald pioneered In this section, a description of the historical development of pond the use of algae in wastewater treatment, which was intensively applied treatment technology is written in detail with its milestones being il- in the US as as many countries in the world. Until 1963, a total of lustrated in Fig. 2. 1647 stabilization ponds were used for sewage water treatment from around four million inhabitants, and 827 industrial ponds treated 2.1. Early development wastewater from 31 industries in the US (Porges and Mackenthun, 1963). In Melbourne, Australia, pond technology was also applied for Pond technology was invented in Texas in 1901, which is the same purifying the sewage of a population of 1,250,000 as a replacement of year when Cleophas Monjeau received the US patent for his innovative the previous land disposal system in 1958 (Parker et al., 1959). design of a , another natural system for wastewater To optimize the performance of pond technology, many novel ideas treatment (Monjeau, 1901). A short time thereafter the application of and guidelines for pond design and operation were established from the pond technology started to be recognized, but more by accident than 1960s onwards. The first issue of conventional oxidation ponds was design. In 1924, the sewage purification capacity of oxidation ponds because of poor hydraulic performance as a result of their deep-water was accidentally revealed at Santa Rosa in California where the clog- body. To tackle this issue and increase removal efficiency, the use of ging of a prepared gravel seepage area formed these ponds (Caldwell, high rate algal pond (HRAP), a shallow (0.3 to 0.4 m in depth) raceway- 1946). It was observed that the amount of BOD and suspended solids in type pond with mechanical mixing, was proposed by Oswald (1963b). the sewage water were reduced after being stored in these settling and HRAP promoted algal growth and prevented biomass settling, allowing holding basins. After discovering this purification power, sewage oxi- to increase nutrient and organic matter removal efficiencies in waste- dation ponds were designed and constructed as a secondary or tertiary water treatment (Oswald et al., 1967). Moreover, to deal with over- treatment to treat the effluent of septic tanks in the US and Europe. The loading problems of existing oxidation ponds in industrial wastewater first intentional design of a pond system in Europe was in Munich, treatment (Lee, 1969), these ponds were modified with the addition of Germany in 1920 (Vuillot and Boutin, 1987). This pond combined two mechanical aeration, creating the very first systems of aerated ponds, tasks, i.e. growing valuable crops of fish and receiving part of the ef- increasing considerably their removal capacity with low land require- fluent of the biological treatment plant of the city(Monte and Marecos, ments (Gloyna et al., 1969). Also noteworthy is that during this early 1992). From the late 1920s onwards, favorable experiences with oxi- stage, basic foundations of modern pond design were developed with dation ponds promoted their presence. From 1929 to 1946, about 30 early experiences from the pond treatment systems in Dandora, Cali- oxidation ponds were installed for treated or untreated sewage in only fornia, and Melbourne, in which, pioneer engineers mathematically California (Caldwell, 1946). With these expanded observations, the first designed pond size with respect to their types, surface/volume organic insights into pond removal efficiency and behaviors were withdrawn. loads and retention time (Cooper, 1968; Fisher et al., 1968; Meiring Especially noteworthy is the study of Caldwell (1946), which was solely et al., 1968; Oswald, 1968; Oswald et al., 1967; Parker et al., 1959; based on observations and collected data, covered very well pond Parker et al., 1950). However, high disagreements among these rules of ability to remove organic matter, suspended solids, and coliform-type thumb were found, which was a result of limited understanding of pond bacteria as well as its advantages and disadvantages compared to other mechanisms. To optimize the pond design and operation, Marais and treatment systems of the time, such as and activated Shaw (1961) developed a sophisticated first-order kinetic model, the sludge. In this early stage of pond development, the role of algae as an first model taking into account the hydraulic performance of WSPs.By oxygen producer was gradually appreciated as researchers observed the applying a differential equation governing the mass balance of organic coincidence of their presence in the ponds with a higher removal effi- matter and fecal bacteria within boundary conditions representing the ciency of organic matter or pathogen (Abbott, 1948). However, the hydraulic regime of an oxidation pond, Marais and Shaw (1961) con- amount of the appreciation remained very little as, according to the cluded that the performance of maturation ponds in series is superior to Chief of Bureau of Sanitary Engineering, California State Department of a single pond with an equivalent total volume. This study initiated the Health, Mr. C. G. Gillespie, “oxidation ponds are not more used can be use of a series of MPs for final polishing of the wastewater and the explained by their crudity compared to neat engineering structures, the addition of hydraulic factors in pond design and operation. Subse- unattractive green coloration of the effluent and perhaps by the fact quently, several attempts were made to develop designs built on sci- that too few engineers are aware of the potentialities for oxidation of entific and mathematical principles regarding chemical manufacturing organic matter” (Caldwell, 1946). processes and unit concepts, in which ideal or non-ideal hydraulic re- gimes integrated with the first-order reaction rate to calculate the mass 2.2. From the 1950s to 1970s balance of within a pond (Oswald, 1963a; Thirumurthi, 1969; Thirumurthi and Nashashibi, 1967). These studies were a starting Until the early 1950s, the beneficial role of algae in pond treatment point for numerous studies on complex hydraulic problems in WSPs systems was not yet well recognized. The correlation between algae and afterward. organic matter removal was discussed in numerous studies during this

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2.3. From the 1970s to 1990s physical and biogeochemical variables. A novel instance was proposed by Fritz et al. (1979) by developing first biogeochemical model using Offering a significant economic benefit with high removal efficiency Monod equations to simulate the mass balance of 12 key variables, e.g. and simple operation, WSPs were prevalent regardless of different cli- organic matter, organic , and . This model was matic conditions during the period of the 1970s and 1990s. According referred to in the development of many later biokinetic modeling to the survey of WHO, WSPs were in use in at least 39 countries from (Colomer and Rico, 1993; Kayombo et al., 2000; Mayo and Abbas, Australia, Asia, Europe, and America until the early 1970s (Gloyna, 2014; MorenoGrau et al., 1996; Prats and Llavador, 1994; Senzia et al., 1971), in which the number of oxidation ponds increased to almost 2002; Wen et al., 1994). 7000 in the US and 1400 in France until 1983 (Boutin et al., 1987; US EPA, 1983). Apart from municipal wastewater treatment sector, ponds 2.4. From the 1990s to 2010s was also popular in industrial sectors, in which almost 1000 ponds were applied by 31 industrial groups in the US, especially in canning, meat During this period, the number of pond applications in the world and poultry industries (Gloyna, 1971). The prevalence led to the in- was saturated, as this number remained relatively the same until 2010s. ternational and national recognition of the important role of pond Pond researchers and engineers focused more on optimizing pond re- technology in wastewater treatment serving both populated urban areas moval and energy efficiency, advancing the understanding of its me- as well as small rural communities. This was illustrated via many chanisms, and modifying pond design. To overcome problems of the guidelines of WHO and EPA in many countries, such as the US, Aus- odor nuisances, sludge accumulation, and low discharge quality, many tralia, South Africa, and East Africa (Gloyna, 1971; Mara, 1975; Meiring solutions were introduced. Oswald (1991) presented a modified pond et al., 1968; SA EPA, 1997; US EPA, 1983; WHO, 1987). More im- system, the Advanced Integrated Wastewater Pond Systems (AIWPS), portantly, in June 1987 the IWA specialist group on WSPs was founded including Advanced Facultative Ponds (AFPs), High Rate Algal Ponds after the first conference in Lisbon, cementing international collabora- (HRAPs), Algae Settling Ponds (ASPs) and Maturation Ponds (MPs). tion on pond technology. This invention was a result of the prevalence of the HRAP installations During this period, pond engineers began to include more affecting in many countries as well as the widespread recognition of the benefit variables to have higher accurate designs and operation, e.g. bio- of algal biomass in recovering potential energy from wastewater, chemical, hydraulic, and meteorological variables, which created so- namely when biodiesel from microalgae was received increasing at- phisticated models. Three familiar equations were established for tention as a renewable fuel (Mata et al., 2010). AIWPS integrating computing pond loading rates, which are still very handy tools for pond ecological engineering principles with multiple treatment processes, engineers, i.e. (i) Arceivala equation, (ii) McGarry and Pescod equation, were able to achieve both high treatment efficiency and energy re- and (iii) Gloyna method. Specifically, the impacts of meteorological covery via high production of algal biomass (Craggs et al., 2014; Green conditions was highlighted on the removal of organic matter in et al., 1995; Oswald, 1991). As such, this advanced system has been Arceivala (1973) and McGarry and Pescod (1970), while Gloyna et al. applied to treat municipal wastewater in many cities in the US and (1976) accounted for several variables, i.e. sunlight, algal toxicity, and Australia (Craggs et al., 2003), which led to an increasing number of sulfate concentration, to calculate the removal of organic matter re- studies on optimizing the system operation and enhancing the pro- moval and odor problems. duction of algal biomass. Extra carbon sources are needed as the C:N Also noteworthy is that many hydraulic-related parameters were ratio of typical municipal wastewater, between 100:5 and 100:10, is added in models based on chemical reactor concepts, i.e. hydraulic higher than the optimal range for algal biomass production from 100:1 loading, depth, detention period, mixing, pond size and shape, re- to 100:2 (Craggs et al., 2011). To work around the problem, pH control circulation, and inlet and outlet systems (Shilton, 2001). However, and CO2 supplement were applied (Craggs et al., 2014; Mehrabadi choosing an optimal flow pattern representing pond flow behavior was et al., 2016; Park et al., 2011). For example, utilizing the flue gas from controversial. Unlike ideal flows, i.e. plug flow and complete mix, pond fossil fuel power plants for CO2 supplement can substantially minimize hydraulics has been often considered non-ideal, represented in tanks-in- the operational costs of a full-scale application (Campbell et al., 2011; series (TIS) or dispersion models. Thirumurthi (1974) emphasized that Craggs et al., 2011; Lardon et al., 2009). However, to be economically an optimal flow could be chosen among plug flow, dispersion, and viable, further study is needed on optimizing algal yields and improving modified-dispersion models. Similarly, Polprasert and Bhattarai (1985) harvesting techniques and biomass control (Munoz et al., 2004; le concluded that dispersion models could predict total and fecal coliform Williams and Laurens, 2010). die-off more precisely than completely-mixed models while Marais During the same period, the hydraulic performance of the pond (1970) and Uhlmann (1979) highly recommended the application of systems was found to affect tremendously pond removal performance. TIS. On the other hand, Ferrara and Harleman (1981) concluded that Moreover, as an open system, their flow patterns are under strong in- ideal flows were more relevant due to its simplicity and low datare- fluences from many design factors, e.g. baffling, inlet/outlet config- quirement for model development and validation. uration, wind forces, sludge accumulation, and thermal balances Until the early 1970s, pathogen removal mechanisms in the oxida- (Ouedraogo et al., 2016; Sweeney et al., 2005). From this point-of-view, tion ponds were little known. Berg (1973) concluded that virus removal Computational Fluid Dynamic (CFD) with a high capability of simu- of WSPs was considered erratic as a result of short-circuiting. The first lating fluid flow in a more precise manner was applied. The firstap- temperature-dependent model by Marais (1974) suggested that under- plication of CFD on a WSP was presented in the study of Wood et al. lying process for pathogen removal in WSPs was high temperature. (1995) for assessing pond hydraulic performance under several con- Herein, numerous studies tried to explain bacterial removal with sev- figurations with/without baffles in the inlet and outlet, andusing eral hypotheses of the underlying mechanisms (Cherry and Guthrie, aerators. CFD modeling in WSPs has substantially improved the un- 1975; Moeller and Calkins, 1980; Parhad and Rao, 1974; Polprasert derstanding of pond hydraulics and resulted in a widespread applica- et al., 1983). It was realized that the pathogen removal in WSPs in- tion of baffling and other modifications of pond inlet/outlet config- volves a complex coexistence of several living species which is very urations. Additionally, the roles of other factors, such as wind, thermal different from pure culture behavior (Polprasert et al., 1983). As such, stratification, and sludge accumulation on pond hydraulic performance the effect of pH, retention time, and pond dispersion on pathogen re- were applied with 3D CFD models. The chronological evolution of the moval were simulated in complex subsequent models (Calkins et al., main CFD applications on WSPs can be found in Fig. 3. 1976; Parhad and Rao, 1974; Polprasert et al., 1983; Saqqar and Together with the development of hydraulic modeling, the under- Pescod, 1991). Similarly, increasing numbers of more comprehensive standing of the complex biogeochemical interactions with diverse mi- models for predicting pond performance based on the variety of crobial niches, and external conditions was also significantly improved.

4 L. Ho and P.L.M. Goethals Ecological Engineering 148 (2020) 105791

Fig. 3. Chronological evolution of major studies on CFD models applied in WSP systems. References in the Figure: (Aldana et al., 2005; Alvarado et al., 2012a; Alvarado et al., 2012b; Coggins et al., 2018; Dias et al., 2017a; Karteris et al., 2005; Li et al., 2013a; Passos et al., 2019; Passos et al., 2014; Salter et al., 2000; Shilton, 2000; Shilton et al., 2008a; Sweeney et al., 2003; Sweeney et al., 2005; Wood et al., 1995; Wood et al., 1998). (Adapted from Ho et al. (2017))

Despite its quiet appearance, the reactions inside WSPs vary con- underlying mechanisms of these influencing factors and their interac- siderably in response to the seasonal and diurnal fluctuation of weather, tions. In the modern period of pond technology, many studies focused such as light intensity, wind speed, temperature, and precipitation. on clarifying these mechanisms, which was meticulously described in Many studies were conducted on the performance comparison of WSPs two critical reviews of Maynard et al. (1999) and Davies-Colley et al. in different climatic conditions, from the tropical zones to extreme (2000). It is noteworthy that sunlight exposure was recognized as the continental climates or from low to high altitude (Heaven et al., 2003; most important factor in pathogen removal in WSPs (Curtis et al., 1992; Juanico et al., 2000; Pearson et al., 1987b; Recio-Garrido et al., 2018). Davies-Colley et al., 1997; Davies-Colley et al., 1999; Nelson et al., Because of this widespread application, rules of thumb and basic cor- 2018). Despite its intricate correlations with other factors, including relation models, which were popular in many pond design guidelines, temperature, pH, dissolved oxygen and algal concentrations, the me- generated a very wide range of outputs, i.e. pond dimension, when chanisms of sunlight-mediated disinfections for mainly bacteria were applying them in different regions (Ho et al., 2017). To avoid this established via three main pathways, including direct inactivation, in- variation and obtain more reliable extrapolations, complex mechanistic direct endogenous inactivation, and indirect exogenous inactivation models were increasingly applied. In fact, after the 1990s, mechanistic (Davies-Colley et al., 2000; Nelson et al., 2018; Silverman and Nelson, models began to be dominant as a result of the fast growth of computer 2016). While the first mechanism involves solar UVB absorption technology and the increase in the understanding of pond underlying (300–320 nm) causing direct damage to DNA, the last two mechanisms mechanisms. The last decade of the twentieth century witnessed a rely on the presence of oxygen to form highly reactive photo-oxidizing higher number of their applications in pond literature and is expected agents, which are able to damage the internal and external structure of to continue in the future (Ho et al., 2017). This indicates the increasing the micro-organisms (Nelson, 2000). In contrast to the clear insights interest of the pond designer in seeking better cost-effective pond into sunlight inactivation process, other disinfection processes, in- configurations, operation optimization, and system upgrade which can cluding algal toxins and biological disinfection (predation, starvation, be accomplished by the application of integrated numerical models (Ho and competition), are still difficult to quantify. Additionally, tempera- et al., 2017). The first integrated model was developed by Sah et al. ture and retention time, which were previously considered as major (2011) which integrated a 3D semi-dynamic submodel with an ex- influencing factors for pathogen removal, were evaluated (Maynard tended biokinetic matrix to evaluate the impact of baffling and wind et al., 1999). As such, the temperature-dependent model of Marais direction on a hydraulic and removal performance of a FP. (1974) became inappropriate to simulate the pathogen removal in Regarding pathogen removal, it was acknowledged that the disin- WSPs. Two well-known statistical models were developed in the early fection processes in WSPs depend strongly on multiple pond char- 1990s by Curtis et al. (1992) and Mayo (1995) including other influ- acteristics, namely water temperature, pond depth, light intensity, and encing factors, such as light intensity, pH, and pond depth, or recently, hydraulic efficiency. However, there remained uncertainty about the a second-order kinetic model for virus inactivation was validated by

5 L. Ho and P.L.M. Goethals Ecological Engineering 148 (2020) 105791

Fig. 4. Plan view (top) and longitudinal cross-section (bottom) of the layout composed of a maturation pond and a free water surface constructed wetland. This combination has been installed as a tertiary treatment in conventional waste stabilization pond layout.

Mattle et al. (2015). Deterministic models of E. coli removal in con- 3.1.1. Ponds with rock filters ventional WSPs have been developed by Nguyen et al. (2015), The most known and broadly used WSP layout is composed of a Silverman and Nelson (2016). These models predicted the inactivation sequence of anaerobic (AP), facultative (FP), and (a series of) matura- of E. coli, and pigmented and non-pigmented enterococci that accounts tion ponds (MPs). Biochemical reactions and physical processes of this for endogenous and exogenous sunlight inactivation mechanisms and layout can be seen in Fig. 3. This conventional pond system can be dark processes (Nguyen et al., 2015; Silverman and Nelson, 2016). simply constructed and operated, resulting in lower costs compared to However, despite many mechanistic models combining pathogen re- other treatment technologies (Mara, 2004). Moreover, they are also moval with CFD (Banda et al., 2006; Sah et al., 2011; Shilton and very resilient towards both organic and hydraulic shock loads due to Harrison, 2003; Vega et al., 2003), there remains a shortage of me- their long HRT (Ho et al., 2018b). However, due to high concentration chanistic models of pathogen removal in conventional WSPs. The of algae in the effluent, the removal efficiency of nutrients andsus- combination of such models with CFD would be very useful for pond pended solids are relatively low and highly variable in the conventional designers to improve pathogen removal efficiency of WSPs. layout (Mara, 2004). To polish pond effluent, rock filters have been widely used as an effective and inexpensive solution (Mara and Johnson, 2007). In rock filters, algae and suspended solids can be set- 3. Pond technology of the future tled on or attached to rock surfaces where pollutants can be further biologically decomposed. Often rock filters can be installed at or near 3.1. Pond upgrades the effluent end of the final cell in a WSP. Rock systems cancontain submerged layers of rocks, which allow pond effluent to pass through Despite the popularity of WSPs, it appears that the number of pond horizontally and vertically. Well-designed systems can deliver high −1 installations has not changed much over the last 30-odd years, which is quality effluent with less than 30 mg·L of BOD5 and total suspended in contrast to the increasing application of other wastewater treatment solids (US EPA, 2002). However, despite its simplicity and low cost, the technologies, such as activated sludge or constructed wetlands (Matos ammonia concentration of the final effluent can be increased as aresult et al., 2019; Miranda et al., 2019; Racault and Boutin, 2005). This state of nitrogen release from ammonification and dead algal cells (Mara and of pond technology reflects their current challenges, such as overload, Johnson, 2006). Although some studies indicated that using aeration sludge accumulation, odor nuisances, and nutrient removal. Pond up- can alleviate this problem as aeration can provide high dissolved grades with innovative design and technology were developed, such as oxygen (DO) and warmer temperature supporting the nitrifying bac- the integration of ponds with different add-ons (e.g. rock filters (Mara terial communities (Mara and Johnson, 2007), numerous applications and Johnson, 2007), constructed wetlands (Andrade et al., 2017; in the US have been replaced by another add-on using constructed Gschlossl et al., 1998; Manjate et al., 2015), activated sludge (Shipin wetlands (US EPA, 2002). As such, rock filters can be applicable to et al., 1999)), and the modification of different configurations (Ad- small communities where no ammonia discharge limit is applied. In vanced Integrated Wastewater Pond System (AIWPS) (Oswald, 1991), fact, a treatment system consisting of UASB reactor, maturation ponds, wastewater-fed ponds (WFPs) (Polprasert and Koottatep, 2018), and and coarse rock filters have been in used in many places of Brazil whose aerated ponds (Houweling et al., 2005)). In general, these upgrades aim discharges meet local required criteria for reuse (Rodrigues et al., 2015; to maximize the benefits from algae while minimizing the potential von Sperling, 2015; von Sperling and de Andrada, 2006). harms of algal overgrowth. Particularly, suspended algae will be either trapped in rock filters, and constructed wetlands, and their predators inthe food chain (i.e. zooplankton and fish) after being discharged from 3.1.2. Ponds with constructed wetlands conventional WSPs, or grown with an optimal yield in HRAPs for bio- Another add-on of pond technology, which has been increasingly mass production. Also noteworthy is that baffling FPs and MPs is an- applied in both tropical and temperate regions, is constructed wetlands other significant improvement of pond design to increase pond hy- (CWs) (Fig. 4)(Gschlossl et al., 1998; Steinmann et al., 2003). Sharing draulic behavior, which has become a must for modern pond the same features of natural systems for wastewater treatment, i.e. high configuration (Dias et al., 2017a; Dias et al., 2018; Mara, 2009). robustness, low costs, simplicity, constructed wetlands can supplement

6 L. Ho and P.L.M. Goethals Ecological Engineering 148 (2020) 105791 conventional WSPs as well as HRAPs as a tertiary treatment. Specifi- continuously mixed by a paddlewheel and a HRT of 2–8 days (Craggs cally, algae concentration in the effluent can be constrained as a result et al., 2011). Continuous mixing and shallow depth promote the of shading by macrophytes and suspended solids can be trapped by homogeneity of pond conditions and algal suspension, which max- enhanced when passing through emergent vegetation andbed imizes photosynthetic efficiency and the symbiosis between microalgae gravel media which also provide a suitable habitat for grazing and and aerobic heterotrophic bacteria (Oswald, 1963b). As such, this pond filter-feeding invertebrates (Tanner et al., 2005). Furthermore, large enhances aerobic of the remaining dissolved organic wetland areas can also promote nutrient biological removal while fur- matter, and nutrient assimilation, volatilization, and precipitation. ther P removal is enhanced by specific P-sorptive media and plant Disinfection is also improved as a result of strong exposure to the UV in uptake (Tanner and Sukias, 2003). Thus, numerous installations of this solar radiation, and high DO and pH levels in the water (Montemezzani integrated system are currently in use in the US, New Zealand, Spain, et al., 2015). Details of the biogeochemical processes in the HRAP can Brazil, France and Thailand (Shilton, 2005). All three conventional be found in the Supplementary Materials. The third pond (ASP) aims to types of CWs, i.e. free water surface CWs (FWS), horizontal subsurface remove algae-bacterial suspended biomass in the effluent of HRAP flow CWs (HF), and vertical subsurface flow CWs (VF), canbein- through sedimentation. ASP's are normally designed with a length to tegrated with the pond systems (Vymazal, 2010). However, it is im- width ratio of 5:1, an inlet pipe located near to pond bottom, and weir portant to keep in mind the differences in terms of removal perfor- outlet at the surface (Green et al., 1996). Settled algae, which contain mance among the three types. Treatment efficiency is comparable normally 3–4% of solids, can be recovered from the bottom of a ASP at between FWS and HF CWs while VF CWs are reported to have higher intervals of 4–6 months (Green et al., 1995). It is reported that a HRT of removal efficiency due to higher inflow concentrations (Vymazal, 1–2 days is adequate to capture 50% to 80% of the algae biomass in a 2010). Comparisons between different treatment layouts, i.e. using ASP. Besides a main function of disinfection, the MP is designed to MPs, HF CWs, and VF CWs, for the tertiary treatment were conducted in remove the remaining unsettled algae biomass by zooplankton grazing Brazil showing that each system has its own applicability, advantages and pathogen by sunlight-mediated inactivation, sedimentation, and and limitations, and no generalization can be made on the best option predation (Nelson, 2003; Silverman and Nelson, 2016). Details of the (von Sperling, 2015; von Sperling et al., 2010). In fact, while pond biogeochemical processes in a MP can be found in the Supplementary technology still needs to deal with its odor issue and sludge accumu- Materials. In short, AIWPS was considered less expensive to construct, lation, clogging of the filtration substrate remains a major problem of operate, and maintain than other mechanical systems that produce CW systems (Matos et al., 2019). Recent studies indicated strong in- comparable removal efficiency (Green et al., 2003). However, its fluences of substrate type and position of plant species on clogging and widespread application is hinder due to higher energy costs for op- hydrodynamics of CW systems, suggesting the importance of analyzing eration due to the paddlewheel in HRAPS and skilled labor regarding the characteristics of the solids and the effect of pore space expansion design and maintenance are required compared to conventional WSPs. by the roots (Miranda et al., 2019). Besides, large land occupation around 6 m2 p.e.−1 of HRAPs compared Furthermore, several studies reported that this add-on can be ef- to 0.5 m2 p.e.−1 of conventional systems, makes this advanced pond fective in purifying toxic compounds and heavy metals from pond ef- suitable only with rural communities where land is available (Arashiro fluent, hence, is suitable to treat agricultural wastewater, such asswine et al., 2018). As such, pond engineers need to determine a suitable wastewater (Stone et al., 2002), dairy farm wastewater (Tanner and approach to exploit vast potential benefits from algae biomass recovery Sukias, 2003), and piggery wastewater (Sezerino et al., 2003). In these to make this system more financially attractive. studies, it was reported that N removal was improved due to enhanced N mineralization and nitrification, reaching from 49% to 85%, while P 3.1.4. Wastewater-fed ponds removal was ineffective which could be because of low areal loadings or Wastewater-fed ponds (WFPs) is another pond upgrade system that low P adsorption/uptake rate of the media and macrophytes. More maximizes the symbiotic relationship between decomposers, primary recently, Park et al. (2018) proposed a novel Enhanced Pond and producer, and consumers in an aquatic food web (Fig. 6). In these Wetland (EPW) system which consists of HRAPs followed by gravity ponds, primary producers, such as algae and plants, obtain nutrients Algal Harvesters, Surface Flow CWs, and a Woodchip Denitrification from input supply and decomposing processes, while decomposers, Filter. Occupying a small land area compared to conventional WSP and mostly bacteria, receive carbon and nutrients from pond influent, and normal HRAP systems, resulting in good-quality discharge removal, the excretion, respiration, and decay processes of the primary producers + − 3− 95% of NH4 , 77% of NO3 , 65% of PO4 and above 3-log E. coli and consumers in the food chain, such as zooplankton and fish. Fish can removal. Harvesting algae and nutrients as vegetation biomass, EPW graze on abundant algal cells and zooplanktons, which diminishes system can also give the mutual benefits from algal biomass production suspended solids and nutrients in final effluent while providing valu- and nutrient recovery for fertilizer use. able fish biomass which can be used as animal feed and human food. WFPs can be categorized into monoculture ponds where only one fish 3.1.3. Advanced integrated wastewater pond systems species can be raised and polyculture ponds where multiple fish species Among the upgrading approaches, Advanced Integrated Wastewater can be grown in a same pond which can allow higher total fish pro- Pond System (AIWPS) appears promising. It was reported that by up- duction. According to Polprasert and Koottatep (2018), fish to be raised grading to this system, most disadvantages of conventional WSPs, i.e. in WFPs should be herbivorous or omnivorous, which can be tolerant to odor nuisances, sludge accumulation, poor discharge quality regarding low DO levels, diseases, and adverse environmental conditions, such as nutrients and , can be tackled (Green et al., 1996; Oswald, Tilapia, Chinese carp, and Indian carp. Due to high economic interests, 2003). AIWPS is composed of an advanced facultative pond (AFP), a WFPs have been popular in many countries, including Germany, India, high rate algal pond (HRAP), two algae settling ponds (ASP) and a MP Mexico, China, Vietnam Tunisia, Bangladesh and the US (Mara et al., (Fig. 5). Designed as a modular system, these ponds can be installed for 1993). For example, a WFP in India can produce yearly from 5 to 7 tons upgrading existing conventional systems in different combinations. of silver carps and tilapia per hectare while its effluent can reach the AFPs (3–4 m of depth) are deeper than conventional FPs with an of conventional (Shilton, 2005). anaerobic fermentation pit surrounded by a wall. The main aim of this However, the application of WFPs is currently being constrained by design is to prevent wind-driven short-circuiting and oxygenated water concerns related to public health and safety from harvested products. intrusion, hence, improving the removal efficiency of both organic Three major public health problems raised in WHO (2006) are: (i) matter and nitrogen, and avoid generating malodorous gases, such as passive transference of pathogens via fish, (ii) transmission of specific

H2S(Craggs et al., 2015). On the other hand, the second pond in the helminths requiring fish as intermediate hosts, (iii) transmission of system (HRAP) is a shallow raceway (0.2–1.0 m deep) which is schistosomiasis helminths via intermediate hosts, e.g. freshwater snails.

7 L. Ho and P.L.M. Goethals Ecological Engineering 148 (2020) 105791

Fig. 5. Plan view (top) and longitudinal cross-section (bottom) of the layout composed of Advanced Integrated Wastewater Pond Systems composed of Advanced Facultative Ponds (AFPs), High Rate Algal Ponds (HRAPs), Algae Settling Ponds (ASPs) and (a series of) Maturation Ponds (MPs). (Adapted from (Shilton, 2005))

Besides, there is another public health risk related to the high con- bottom of a pond, and decomposed anaerobically. In general, faculta- centration of heavy metals, toxic substances, pharmaceutical residuals tive aerated ponds behave similarly to conventional FPs (Fig. 7). that can present in the fish tissue as a result of bioaccumulation and However, due to the presence of machines, the former is less simple but, biomagnification. Moreover, like other made-from-waste products, one on the other hand, occupies less land than the conventional FPs (Von of the most challenging issues is public acceptance, which depends Sperling, 2007). The second type is a completely mixed which heavily on sociocultural aspects (Mara, 1989). While some areas, such normally requires around 10 times more energy for additional mixing as Asian countries like China, Japan, Vietnam, India, and Indonesia, to enhance biodegradation processes. Better contact between biomass excreta or waste use has long been appreciated, the responses from and wastewater increases system efficiency, which allows reducing its Africa, American, Europe, and Islamic societies normally range from volume. Typical HRT of a completely mixed aerated pond is in the abhorrence through disaffection and indifference (WHO, 2006). As range from 2 to 4 days (Von Sperling, 2007). Similar to AS systems such, unless the public is assured that wastewater-grown fish are safe where biomass also stays suspended, a settling compartment is required and more economically beneficial, the application of WFPs is still lim- after the completely mixed aerated pond. Hence, a sedimentation pond ited. with short HRT, around 2 days, is normally recommended (Von Sperling, 2007). 3.1.5. Aerated ponds An aerated pond is a variant of treatment pond technology. Its main 3.2. Pond monitor and operation functions are to remove organic matter and nutrients, hence, it can be an alternative for APs or FPs in a pond series. In fact, over 450 aerated Another pathway to improve pond effluent quality could be ac- ponds are currently being used in Quebec for treating BOD and nu- complished by advancing their operation and maintenance, which are trients from municipal wastewater (Houweling et al., 2005). Different mostly following two guidelines written around three decades ago, i.e. from the natural source of oxygen in conventional FPs, microbial floral, Pearson et al. (1987a), and Mara and Pearson (1998). Note that the oxygenation is done by mechanical equipment, such as surface aerators newest guideline from US EPA (2011b) neglected the benefits of in- or propellers, in aerated ponds. Two types of aerated ponds are classi- tegrated mechanistic models or intelligent monitoring, in system con- fied, based on the required energy for aeration, i.e. partially mixedand trol. This suggests the negative impact of being considered decen- completely mixed aerated ponds. In the case of the former, the energy tralized treatment systems with low-cost technology and basic for aerators is only sufficient for oxygen supply but not for maintaining operation and maintenance as it is not normally expected that highly all solids in suspension. Therefore, sludge deposits still occur at the complex biotechnologies with serious supervision can be suitable to

Fig. 6. Simplified food web in a waste-fed pond.

8 L. Ho and P.L.M. Goethals Ecological Engineering 148 (2020) 105791

Fig. 7. Schematic overview of possible biochemical processes of an aerated pond. their applications (Wilderer and Schreff, 2000). This misconception has least three different locations (e.g. input, middle, and output), and led to a highly conservative approach towards accepting and adopting different depths of a pond. Pond operators should also take extra care innovative technology. Consequently, pond engineers are still comfor- on monitoring TSS/BOD ratio to control the algal biomass and sludge table with a conventional way of pond design and operation with a high accumulation as algal overgrowth and old sludge age can be indicated margin of uncalculated safety factors, generating over-sized systems by high TSS/BOD ratio. Moreover, as hydraulic performance can affect with the poor controlling system, exacerbates main disadvantages of directly and strongly pond removal efficiency, frequent monitor of pond WSPs, i.e. high land requirement and high nutrient effluent con- hydraulics is an essential step in pond operation and maintenance. centrations. On the other hand, mechanical treatment systems like ac- Hence, tracer study and bathymetry mapping are recommended to tivated sludge continue to develop and apply new technologies, such as identify potential problems related to short-circuiting and sludge ac- ICA technology, aerobic granular sludge system, and nutrient recovery cumulation in a pond (Coggins et al., 2018). technology (NRT) to increase both removal and energy efficiencies or minimize space requirement. Bringing both environmental and eco- 3.3. Pond modeling nomic benefits, these new technologies have been successfully applied not just in lab-scale experiments but also in full-scale WWTPs. For ex- 3.3.1. Data-driven and process-driven modeling ample, aerobic granular sludge system can reduce by at least 20% total Like any industrial sectors, modeling plays a crucial role in design, O&M cost and 75% space requirement (de Bruin et al., 2004) while operation, and optimization of WSPs. There have been two respective advanced control ICA has resulted in some 20–30% improvement of the schools of pond modeling, i.e. empirical approach and mechanistic treatment plant capacity for a 2–5% cost increase(Olsson et al., 2005). approach. The former approach normally applies statistical inference On the other hand, considered an effective decentralized wastewater and machine learning techniques to automatically construct a parsi- treatment system, the O&M of small-scale ponds is normally omitted monious model from training data obtained from a real system. The due to financial reasons while in large-scale WSPs acting as acen- second approach, on the other hand, is typically based on first en- tralized treatment system, new advanced technology is rarely applied. gineering principles, meaning that models are developed via principal This fact leads to the failures of many pond treatment systems, such as balances of mass, energy or other conserved quantities from physical, fourteen WSPs in Mexico with their under-performance due to lack of chemical or economic laws, resulting in a set of differential equations. proper control and poor operation (Lloyd et al., 2002). Contrary to the Each of the two approaches has its own pros and cons however it is traditional manner of problem-solving by enlarging pond area as being concluded that the mechanistically based models have greater potential suggested by Juanico et al. (2000), it would be more useful and cost- for application due to their better extrapolation outside model effective in the longer term by applying advanced technologies for boundary (Henze et al., 2008). In fact, many wastewater treatment system control and monitoring (Olsson et al., 2005). In fact, remote systems have developed their standard mechanistic models, such as sensing applications based on unmanned aerial vehicles and advanced Activated Sludge Models (ASMs) (Henze et al., 2000), Anaerobic Di- equipment such as autonomous surface vehicles are suitable for con- gestion Model (ADM1) (Batstone et al., 2002), Constructed Wetland tinuously monitoring and controlling water quality as well as green- Model (CWM1) (Langergraber et al., 2009), and Benchmark Simulation house gas (GHG) emission (Arzamendia et al., 2016; Khan et al., 2012; Models (BSMs) (Gernaey et al., 2014). From this point of view, WSPs Pajares, 2015). Particularly, compact, low power, and lightweight laser- are still struggling to develop theirs. In the most advanced pond model based sensors on these autonomous technologies allow for high spatial of Sah et al. (2011), some important processes remain missing, such as mapping of measurements at a large scale which can be suitable for removal by chemical precipitation and phosphorus-accu- pond monitoring and control. mulating organisms (PAO), sedimentation, ammonia volatilization, and Furthermore, as a result of large area and strong dependence on resuspension. Similarly, research on the influence of pond design, such climatic conditions, pond performance is normally subject to vast spa- as input and output configurations, flow and wind velocities, and pond tiotemporal fluctuations, which hence requires a more complex online geometry, on sludge accumulation should be further carried out monitoring approach than conventional grab sample technique. Recent (Coggins et al., 2017). Hence, a standard model developed by the IWA findings showed that, besides common measurements, such asCOD, specialist group on WSPs is needed. On the other hand, a lack of me- nutrients, and heavy metals, DO appeared to be very important to de- chanistic models of pond pathogen removal has been eased by the re- tect common problems of pond performance, i.e. overload of organic cent development of models on sunlight mediated inactivation me- and nutrients, sludge accumulation, and algal overgrowth (Ho et al., chanisms (Nguyen et al., 2015; Silverman and Nelson, 2016). 2018a; Ho et al., 2018c) In fact, DO, pH, and chlorophyll a, are highly More importantly, as an open natural system, mechanistic models of correlated and can be used as indicators for algal photosynthesis in WSPs often have to confront to overparameterization issue, leading to WSPs. Hence, it is recommended that, instead of monitoring only the poor identifiability problem, in which the number of available data for influent and the effluent of every pond in the systems, the determina- parameter estimation is of a significantly lower order than the com- tion of these parameters should be measured via online monitoring in at plexity of pond models (Omlin and Reichert, 1999). The problem of

9 L. Ho and P.L.M. Goethals Ecological Engineering 148 (2020) 105791 complex numerical models has well been investigated and conveyed in pond HRAPs (> 100 ha), high productivity of more than 100 tons of conventional WWTPs (Brun et al., 2002; Van Veldhuizen et al., 1999; algal biomass per hectare per year can be achieved (Benemann et al., Weijers and Vanrolleghem, 1997). In contrast, there remains an absence 2003). However, since due to high-quality requirements, public ac- of research dealing with this issue in pond modeling (Ho et al., 2019). ceptance, and the possibility of heavy metal contamination, algal bio- Similarly, even though ponds as complex open water bodies are in- mass from HRAPs is normally not suitable for producing food, high- herently subject to substantial variability leading to large uncertainty in value chemicals or fertilization, the only option remains biofuel pro- its models, uncertainty analysis remains missing in most of WSP de- duction (Munoz and Guieysse, 2006). In general, by digesting waste to signs. Until now, to authors' knowledge, four studies, i.e. von Sperling biogas and utilizing methane as a fuel source, this option offers two (1996), Lukman et al. (2009), Sweeney et al. (2012), and Ho et al. major benefits. Energy required for treating solid waste aerobically by (2019), included uncertainty analysis in their models. However, apart activated sludge can be avoided and the utilization of methane for from the most recent model, Monte Carlo simulations were simply ap- energy production can mitigate CO2 emission from using fossil fuel for plied for empirical equations or first-order models, which is insufficient the same purpose (Shilton et al., 2008b). to represent the important uncertainty sources, causing the under- Many bottlenecks remain for algae biomass production. A major estimation in model outputs. In fact, according to the precautionary challenge is to find a proper technique to separate suspended algae principle in the field of water policy imposed by the EU Water Fra- from wastewater, which is difficult due to a small size of algal cells mework Directive, implementing uncertainty analysis is required in (Campbell et al., 2011; Christenson and Sims, 2011). It is reported that integrated models (EC, 2000). Furthermore, due to the bottleneck of the most common approaches, including centrifugation, filtration, and high data requirement, the severe shortage of calibration and validation flotation, have been proven not economically feasible for industrial- discredits the applicability and validity of mechanistic models in WSPs scale production (Craggs et al., 2011; Lardon et al., 2009; Rastogi et al., (Ho et al., 2017). Therefore, we highly recommend future pond mod- 2018). On the other hand, bio-flocculation, which applies the me- eling to have quality assurance (QA) guidelines for pond virtual mod- chanisms of bridging or patching for cell-to-cell adhesion, is considered eling and simulation for decision support systems (Aumann, 2007; a potential solution as one of the most cost-effective harvesting methods Jakeman et al., 2006; Refsgaard et al., 2005). (Fig. 8)(Salim et al., 2011). This method uses microbial flocculants, i.e. sticky polymer exopolysaccharides (EPS), to attach algae together to 3.3.2. Hybrid modeling form flocs, which can then be coupled with sedimentation orcen- While empirical or mechanistic models both have to confront their trifugation to improve the harvesting productivity (Craggs et al., 2011). disadvantages, their advantages can be combined in a hybrid model or a This harvesting technique is expected to make microalgae-driven bio- grey-box model where the first principles that can be applied in amodel fuel production economically viable, however, these mechanisms are with particular functionalities that are already estimated with collected still poorly understood (Rastogi et al., 2018). Further research is also data. Each hybrid model can consist of a set of nonlinear differential needed for other issues related to maximizing the growth yield of algae, equations in combination with backpropagation (or feedforward) and oil extraction efficiency, select the algae strains with high tolerance neural network model (Cote et al., 1995). According to Thibault et al. and resistance against inhibitors or grazers (Lundquist et al., 2010; (2000), system phenomena can be described as much as possible by the Montemezzani et al., 2015, 2017). Before these issues are solved, mass set of differential equations while artificial neural network (ANN) algae production is, even though technically feasible, yet not econom- models predict key parameters, which are crucial elements of a phe- ically viable (Rastogi et al., 2018). Until then, although thousands of nomenological model. Boger (1992) concluded that main advantage of HRAPs are existing, it will remain that only few harvest algae (Salerno this hybrid approach is its ability to obtain the underlying mechanisms et al., 2009). of a system via their historical data without expert knowledge about its processes. This ability of a hybrid model can be useful and applicable in 3.5. Greenhouse gas emission and climate change a complex natural system like WSPs in which not all of the processes are fully understood. Furthermore, an ANN model can be used to optimize 3.5.1. Greenhouse gas emissions error measurements between simulation output of a mechanistic model Greenhouse gases (GHGs), such as carbon dioxide (CO2), nitrous and corresponding collected data (Gernaey et al., 2004). oxide (N2O), and methane (CH4), are released from WWTPs. It was concluded that WWTPs are subject to be potentially high contributors 3.4. Algal biomass control to GHG emission as a result of high and regular supply of organic matter and nutrients (Daelman et al., 2012). In fact, WWTPs are the sixth-

Throughout a century of pond development, the perception of the largest contributor to N2O emission according to the Environmental role of algae in pond treatment technology has been substantially Protection Agency of the United States in 2006. Note that N2O and CH4 changed from a green-yellowish suspended , which deterio- have high global warming potential (GWP), i.e. 265 and 28 times rates effluent quality (Caldwell, 1946), to a valuable energy resource as higher than CO2 for a 100-year timescale, respectively. As such, despite a fertilizers, agricultural food, animal feed or a biofuel, which can their small emissions from wastewater treatment plants (WWTPs), their compensate the energy use and GHG emissions of WWTPs (Rastogi contributions to greenhouse effect are high, around 91.9% of the total et al., 2018). In fact, pond engineers have been focusing on optimizing effect when converting 2to CO equivalent(Daelman et al., 2013). In the algal biomass production to maximize its profits while minimizing its case of WSPs, CO2 and CH4 are mainly released via the decay of organic potential detriment to pond removal efficiency. For example, the pond matter during bacterial decomposition processes while nitrifying and upgrade solutions mentioned above were proposed to either keep sus- denitrifying microorganisms are considered major generators of N2O pended algae in pond effluent into rock filters, constructed wetlands, (Glaz et al., 2016). Since the emissions are strongly dependent on pond and their predators, or harvest them in a HRAP with optimal growth configuration, size, and condition as well as climatic conditions, they rate and harvesting method. Hence, algal biomass separation is very varied significantly from one system to another. Glaz et al. (2016) crucial to enable a better removal performance in WSPs and higher concluded that the percentage contribution of CO2 was almost zero in energy recover efficiency in HRAPs. In the case of HRAPs, to increase WSPs in Western Austria which can be explained by photosynthetic the productivity of algae, CO2 from power plants can be utilized consumption which has previously been shown in (Brix et al., 2001; (Benemann et al., 2003). This possibility can diminish 90% costs of Teiter and Mander, 2005) while this number was 93.8% in another algal growth period, which is the most expensive operating cost and pond system in Quebec, Canada. Additionally according to Hernandez- investment of current algae cultivation systems (Collotta et al., 2018). It Paniagua et al. (2014), the range of GHG emissions from WSPs are −2 −1 was calculated that if applying this integrated system on large open- 1025–218,099 and 5457–102,505 mg·m ·d for CO2 and CH4,

10 L. Ho and P.L.M. Goethals Ecological Engineering 148 (2020) 105791

Fig. 8. Schematic illustration of two main bio-flocculation mechanisms: (a) bridging and (b) patching. respectively. intracellular accumulation and its reduction by Nitrate reductase in There is a shortage of studies on the GHG emissions from WSPs Chlorella vulgaris emit N2O, which may be affected by the concentration − compared to numerous studies on this subject in conventional WWTPs, of NOx and photosynthesis repression. Especially noteworthy is the leading to few insights into their emission pathways and main affecting findings of Alcantara et al. (2015) which indicated a vast change in N2O −1 −1 factors. In fact, the potential pathways of N2O formation and its main emission, from 2 to 5685 nmol h g TSS, by replacing the N-source drivers in WSPs are not yet discovered (Alcantara et al., 2015; Bauer from ammonia to nitrite. From this outcome, a potential solution to et al., 2016; Plouviez et al., 2018). Besides two pathways of nitrifier reduce N2O emission from pond treatment system can be found, how- denitrification and heterotrophic denitrification, which also occur in ever, further research is needed to understand this process (Plouviez conventional WWTPs, an extra pathway for N2O emission from the et al., 2017; Plouviez et al., 2018). metabolism of green algae was first discovered by Hahn and Junge Equally important, it was concluded that CH4 contributed the most (1977) (Fig. 9). The study of Guieysse et al. (2013) showed that nitrite to the global warming potential of WSPs, as it accounted for 99% of the

total CO2 equivalent emissions. 90% of this emission originates from uncovered APs from which odor nuisances mainly originate (Hernandez-Paniagua et al., 2014). Furthermore, release of biogas is also useful energy which can be a economically beneficial. Currently, there are more than 1000 uncovered APs used for wastewater treatment from dairy farms, piggeries, and industry in New Zealand. From the estimation of Park and Craggs (2007), these APs in New Zealand were releasing considerable amounts of CH4, contributing a significant point source of GHG emissions. By conversing the amount of CH4 to elec- tricity, around 1500 kWh/day can be produced and 5.6 tons of CO2- equivalent emission per day can be captured. This benefit of energy recovery and GHG abatement is a reason for an increasing shift to anaerobic technologies in the wastewater treatment framework (Shilton et al., 2008b). In this case, further research incorporated with cycle assessment is needed to evaluate the benefits of APs compared to other anaerobic treatment processes.

Fig. 9. Potential pathway of the N2O emissions in WSPs. (?): unclear pathway.

11 L. Ho and P.L.M. Goethals Ecological Engineering 148 (2020) 105791

3.5.2. Climate change impacts dioxide (CeO2), zinc oxide (ZnO), and siver nanoparticles (Ag NPs), There is a mutual interaction between WWTPs and climate change. were found in fishes (Johnston et al., 2010; Yang et al., 2020; Zeumer On the one hand, WWTPs are one of the GHG sources contributing to et al., 2020). On the other hand, it was found that microalgae-based climate change. On the other hand, the processes happening inside the systems can contribute to the removal process of micropollutants via plants are affected by the unprecedented change of climate in thenext four pathways, including photodegradation (direct and indirect), bio- decades (Zouboulis and Tolkou, 2015). To deal with extreme condi- degradation (anaerobic, anoxic, and aerobic), sorption (adsorption and tions, such as intensive rainfall, storms, and drought, which are pro- absorption), and evaporation (Garcia-Rodriguez et al., 2014; jected to happen more frequently as a consequence of climate change, Matamoros et al., 2016b). Among these pathways, microalgae adsorp- more resilient systems are needed (US EPA, 2013). In fact, 1.7 billion tion is proved to be effective with green algae as a low-cost adsorbent dollars was planned for reconstruction of the treatment system to be with a high fraction of proteins and cellulosic polysaccharides content more resilient after Hurricane Sandy in 2012 by the New Jersey De- to form glycoproteins (Abdel-Aty et al., 2015; Ali et al., 2018; Silva partment of Environmental Protection (Kenward et al., 2013). Having et al., 2009). As such, algae ponds for secondary polishing are re- an extensive HRT, WSPs are normally considered more robust with the commended as a promising treatment method for a wide range of fluctuation of the influent, however, scenario analysis is aneedfor municipal derived ECs in the study of Petrie et al. (2015). more comprehensive assessment. The temperature increase can be an- Another issue of EC management in wastewater treatment sector is other opportunity for WSP application. As a natural-based system, the to create a holistic approach for monitoring and sampling in order to biological treatment processes inside the ponds are considerably in- obtain representative data (Petrie et al., 2015). This approach should fluenced by temperature. It was recorded that the surface loading rate allow determining the fate and effect of ECs in both aquatic and ter- of pond increased almost 2.5 times when changing from temperate restrial environments. Indeed, apart from aquatic environment, the regions to the tropical and subtropical climate (Ho et al., 2017). Higher measurement of the occurrence of pollutants should be conducted in temperature means that lower land requirement and higher removal and sludge in the bottom layer of a pond, with the con- and energy efficiency, which can possibly put pond treatment systems sideration of their leakage and runoff to groundwater. The toxicity of in an advantageous position in competition with other treatment terrestrial products of WSPs, such as fertilizers and agricultural foods, technologies. needs to be assessed across their life cycle. Moreover, the sampling technique by grab sample, which is typically conducted once per day, is 3.6. Emerging contaminants considered inaccurate given the extensive HRTs and the performance fluctuation of WSPs where strong dependence on climatic conditions Another major challenge for pond engineers is an exponentially causing wide diurnal fluctuation of their performance. Therefore, on- increasing number of emerging contaminants (ECs) in wastewater line monitoring technology is needed to ensure the stability and re- (Mannina et al., 2017). Emerging pollutants are new compounds whose presentation of these measurements (Hillebrand et al., 2013; Ho et al., impacts on the environment and human health are still not fully clear, 2018c). leading to a lack of their regulation (Deblonde et al., 2011). ECs can be divided into several groups, i.e. pharmaceuticals and personal care 4. Summary of current challenges/knowledge gaps and future products (PPCPs), antibiotics, illicit drugs, to endocrine disrupting research directions compounds (EDCs) (Bolong et al., 2009). Until now, the number of studies on the removal of these pollutants in WSPs remains very low. In Table 1 summarizes main current challenges/knowledge gaps and fact, there have been only nine scientific publications studying ona potential future research directions of pond treatment technology in small number (40) out of hundreds of micropollutants (Gruchlik et al., this 21st century. Different review and research articles in multi- 2018). Most of these studies showed a comparable, or better perfor- disciplinary studies were meticulously compiled and assessed in order mance efficiency of WSPs compared to conventional biological waste- to deliver a comprehensive picture of future pond technology. water treatment systems (Camacho-Munoz et al., 2012; Coleman et al., 2010; Hijosa-Valsero et al., 2010; Hoque et al., 2014; Li et al., 2013b; 5. Conclusions Matamoros et al., 2016a; Pessoa et al., 2014; Qing et al., 2013; Ying et al., 2009). Having been developed over more than one century, WSPs are ap- Equally important, given that WSPs and HRAPs are major treatment plied in various corners of the world. However, since stricter regula- systems for livestock and farming wastewater management, it becomes tions of wastewater discharge are being enforced, this low-cost tech- more urgent that the fate and impacts of veterinary antibiotics and nology is now facing a crossroads of either being upgraded or being pesticides in these systems are determined (de Godos et al., 2012). In substituted. Considering the current and future challenges of water fact, as a result of very limited studies in WSPs, understanding the fate, shortage and pollution, it is difficult to overstress the importance of formulation, and toxicity of most of the micropollutants remains still existing assets of pond treatment technology, especially in the devel- unexplored (Gruchlik et al., 2018). Most of the published research has oping world where economic affordability is still a bottleneck for mainly focused on PPCPs and EDCs while missing a wide range of other wastewater treatment installation. Integration of this system into in- organic micropollutants, including pesticides, perfluorinated com- novative and cutting-edge technologies together with advanced mod- pounds, artificial sweeteners, and illicit drugs. More importantly, there eling techniques for better control and management emerges to be a is no study on microplastics in WSPs, which have been increasingly way forward as WSPs should no longer be considered as just a low concerned due to their ubiquitous presence and potentially harmful technology for wastewater treatment. Instead, advanced online monitor effects. Since many studies indicated the toxic effects of microplastics and control can help to increase the quality of pond effluent discharge on marine microalgae (Li et al., 2018b; Long et al., 2015), it is im- and resolve the issue of system overload, sludge accumulation, odor portant for pond treatment technology, especially HRAPs, to assess the nuisance, and nutrient removal/recovery. Besides, a standard con- impacts of these microplastic particles on freshwater microalgae. Si- ceptual model combining CFD and a standard biokinetic model is an- milarly, no studies have been conducted on the potential risk of ex- other advanced tool for pond analysis and management. However, posure to engineered nanomaterials (ENMs) in WSPs despite their systematic quality assurance including consistent terminology, proper growing production and widespread use (Scown et al., 2010). Despite model evaluation, and thorough uncertainty assessment is needed for their absences in a list of priority substances for water quality mon- pond modeling to avoid the mistakes in model development and vali- itoring in the Water Framework Directive (Auvinen et al., 2017), sig- dation. nificant uptakes of ENMs, such as titanium dioxide (TiO2), cerium Algal biomass control remains a crucial factor to enable a better

12 L. Ho and P.L.M. Goethals Ecological Engineering 148 (2020) 105791

Table 1 Overview of main current challenges/knowledge gaps and potential future research directions of pond treatment technology.

Topics Main current challenges/knowledge gaps Potential research directions

Pond upgrades HRAP/AIWPS − A higher requirement for energy and expert skills compared to − Integrated system between HRAP and biogas production conventional WSPs emerged to be more environmentally friendly than a − Large required land area compared to conventional WWTPs combination between HRAP and biofertilizer production − Understanding of influencing factors on algal production: (1) (Arashiro et al., 2018) environmental conditions such as light and temperature; (2) − Full evaluation via Life Cycle Assessment (LCA) from which

operational variables including DO, CO2, pH, depth and their impacts and costs can be optimized

nutrients; (3) biological factors, e.g. zooplankton, fungal and − Using inexpensive CO2 from biogas or flue gas to reduce O&M viral infection costs of HRAP system − Economic benefits compared to other bioreactors for algal − Natural aggregation/bioflocculation of algal biomass cultivation − Select resistant and tolerant algal species which can deal with inhibitors or grazers and facilitate biomass recovery Wastewater-fed agricultural ponds − Three major public health problems raised in WHO (2006) − Developing a national and local policy framework for safe regarding the transference and transmission of pathogens, waste-fed production with strict legislation and helminths, and schistosomiasis, creating public health problems regulation on food production and processing (WHO, 2006) − High risks of contamination of heavy metals or toxic substances − Detailed planning and implementation involving diverse in harvested products stakeholder groups in the development of public health − Public acceptance of made-from-waste products strategies (WHO, 2006) − Involvement of mass communication in promoting made-from- waste products − Developing food-web models to assess the bioaccumulation and biomagnifications of heavy metals or toxic substances throughout the food chain in WFPs Integrated pond and wetland systems − Large required land area compared to conventional WWTPs − LCA to assess the environmental impact of integrated systems − Algal biomass production − Role of different macrophytes in reducing algal concentration − Contamination risks of heavy metals or toxic substances in the and TSS in the effluent of pond treatment systems harvested products − Developing food-web models to assess the bioaccumulation and − Economic feasibility biomagnifications of heavy metals or toxic substances − Clogging issues in constructed wetlands throughout the food chain

Pond monitoring and operation Advanced monitoring and control − High requirement of expert skills and high investment costs for − Application of these technologies on small lab-scale to full- advanced equipment, such as online and intelligent monitoring scale WSPs and HRAPs − Limited research and application of advanced control such as ICA − LCA to evaluate the environmental impact and economic technology feasibility of these technologies in different scale application of − Conservative approach on accepting and adopting new pond treatment technology technologies on decentralized wastewater treatment systems − Close collaboration between the university and industrial which are normally considered a low-cost technology without sectors is necessary to keep research and development of novel serious supervision technology in line with the actual field requirement − Developing and updating insight of novel technology in the curricula in schools, universities, and training courses − Developing detailed planning and implementation plans involving decision makers and stakeholders to improve the awareness of the benefit of these advanced monitoring and control systems in the long term Pathogen removal − Remaining uncertainties about the underlying mechanisms of the − A comprehensive deterministic model of E. coli and virus influencing factors and their interactions on pathogen removal removal in conventional WSPs needs to be developed − Algal toxins and biological disinfection including predation, − Improving the understanding of dark die-off processes including starvation, and competition are not yet understood algal toxins and biological disinfection − The effect of hydraulic performance on the removal efficiency of − The accumulation of helminth eggs in the sludge layers must be virus and helminth eggs is not yet understood (Verbyla and investigated in order to promote resource recovery from pond Mihelcic, 2015) effluent in agriculture and aquaculture activities − Integrated CFD model with the deterministic model of pathogen removal is necessary to understand the impact of hydraulic performance on pathogen removal Ecosystem services − No research on economic benefits from the ecosystem services − A comprehensive evaluation of provisioning services provided by WSPs including the supply of food and raw materials, cultural − The potential added services as a natural system of WSPs, besides services including recreation and education, aesthetics, and regulating services including water quality, air quality, and biodiversity services, needs to be investigated in WSP systems greenhouse gas regulation, have not yet concerned − Exploit more of these significant, but often overlooked, values of WSPs to increase their competitiveness

Pond modeling Model development − Shortage of the application of, CFD models, integrated models or − Developing integrated standard models combining a compartment models hydraulic submodel and a biokinetic submodel (Ho et al., − No adequate model for describing pathogen removal processes in 2019) WSPs − Developing hybrid models which maximize the advantages of − Further studies for modeling the impact of pond configurations both black-box and white-box models, such as artificial neural and hydraulic performance sludge accumulation are necessary. network models − Applying an adequate modeling technique for analyzing temporal trends Model identification − Shortage of mechanistic models that are calibrated and validated − Implementation systematic and extensive sampling campaign − Issue of overparameterization, leading to poor identifiability to ascertain sufficient number of collected data for model problem in large mechanistic models for natural systems like calibration and validation (Ho et al., 2019) WSPs (continued on next page)

13 L. Ho and P.L.M. Goethals Ecological Engineering 148 (2020) 105791

Table 1 (continued)

Topics Main current challenges/knowledge gaps Potential research directions

− Lack of uncertainty-based models, leading to conservative design − Applying sensitivity analysis and identifiability analysis to outputs determine the most influencing parameters, hence avoiding overparameterization − Developing intensive quality assurance guidelines for pond virtual modeling and simulation, which involve multiple tools for uncertainty assessment

Algal biomass production Harvesting techniques − The mechanisms of bio-flocculation using EPS and its influencing − Understanding and controlling the mechanisms of factors autoflocculation and bioflocculation (Munoz and Guieysse, − The economic feasibility of these bio-flocculation techniques 2006) − Developing a new method for increasing harvesting efficiency − The potential combination of bio-flocculation with sedimentation or centrifugation to improve the harvesting productivity (Rastogi et al., 2018) − Applying pilot scale and full scale of a biofilm-based system for integration of algal production, harvesting, and dewatering operations (Christenson and Sims, 2011) − Developing a bypass method for harvesting and dewatering process by performing non-destructive oil extraction of live cells or using genetically modified algae (Christenson and Sims, 2011) Oil/biomass extraction − Consuming a large part of total energy, the process of − As the dewatering process is energetically expensive, it can be dewatering/drying of microalgae is one of the major obstacles in beneficial if lipid is directly extracted from wet feedstock

biomass extraction. It is not clear which technology would be − Supercritical CO2 technology has a great potential for large- more cost-effective (Halim et al., 2012; Rastogi et al., 2018) scale microalgal lipid extraction as a green solvent with high − No current extraction methods are being considered a standard extraction efficiency method for maximizing lipid yields for producing algal fuel − LCA should be applied to identify the drawbacks of technologies − Wet extraction appears promising, but the info used to calculate and increase their environmental performance and its impacts and mass flows is still questioning (Halim et al., 2012) cost-efficiency. − Further research on selecting microalgae strains is needed as most of the studies only focus on Chlorella vulgaris Algal strain selection − Selection of potent microalgal strains with higher growth and − Full-scale experiments for detailed studies of biomass lipid productivity composition of different potent microalgal strains − Genetic modification of microalgae for advanced lipid/TAG − Advancing the insight on lipid biochemistry and metabolism in production microalgal biosynthesis − Easily harvestable algae species which are also tolerant to specific − Role of different cellular machinery in lipid biosynthesis for climatic conditions, inhibitors, and zooplankton genetic modification is required for different algal species − Food web modeling can be useful to investigate the interaction between algae, zooplankton, and zooplankton predators to discover the most beneficial zooplankton controlling methods. Mass production − Impact of different wastewater composition on the algal biomass − Scale-up and applying models to provide a better production, such as the presence of heavy metals or toxic understanding of microalgae metabolism

substances can constraint the algal growth rate and reduce the − CO2 utilization from power plants and other industries quality of the harvested products − Genetic modification of algae can be promising but also can − Difficulty in controlling the productions due to complex create high environmental risks microbial biosynthesis, interaction with climatic conditions, − Microalgae biomass can be used for the production of various interconnected with other biotic factors, such as zooplankton, other products, such as animal feed, biogas, or fertilizers , fungi, and virus − High cost and restricted supply are main obstacles for biofuels from microalgae to be a competitor with petroleum fuels

Greenhouse gas emission and climate change

Greenhouse gas emission and − A severe shortage of studies on greenhouse gas emission and − Isotopic analysis of N2O to assess the microbial pathways

abatement climate change mitigation in WSPs producing N2O in WSPs

− Understanding of N2O formulation pathways by green algae in − LCA for methane recovery to reduce GHG emission and energy both conventional WSPs and HRAPs need − Assessing the methane emission from anaerobic ponds − Produce a complete data set of GHG emissions from wastewater − Spatiotemporal impacts of climatic conditions on GHG emission urban systems. of WSPs − Develop mathematical models using results from laboratory and − No studies have been conducted to quantify or reduce GHG field sampling campaign to quantify the impact of mitigation emissions wastewater urban systems integrating WSPs measures in WSPs

− The use of CO2 from flue gas of fossil fuel power plants in algal cultivation Climate change adaptation − The negative impacts and potential benefits of climate change on − SWOT analyses of the long term impact of climate change to pond treatment technology pond treatment technology − Adaptation strategies on different scenarios of climate change − Scenario analysis should be implemented via numerical modeling and time series modeling − Risk and cost analyses of potential solutions for climate change adaptation are highly recommended

Emerging contaminants Removal of organic micropollutants, − Only 40 organic micro-pollutants have been studied in pond − Advance the understanding of removal mechanism, especially antibiotics, and microplastics treatment systems. by microalgae adsorption, for better removal efficiency − Understanding about the removal of a wide range of organic − Advance the understanding of the fate, formulation, and toxicity micropollutants, including pesticides, perfluorinated compounds, of transformation products in pond systems artificial sweeteners, and illicit drugs − Environmental risk assessment of the EC and their transformation products in pond treatment technology (continued on next page)

14 L. Ho and P.L.M. Goethals Ecological Engineering 148 (2020) 105791

Table 1 (continued)

Topics Main current challenges/knowledge gaps Potential research directions

− No studies on the removal of microplastics and engineered − Pond modification and upgrades for better removal ofEC nanomaterials are conducted in WSPs Impact of emerging contaminants − No research on the impact of antibiotics, engineered − Assessment of the concentrations of EC, especially nanomaterials, and microplastics on the performance of WSPs microplastics, engineered materials, antibiotics, and EDCs, in − The fate and levels of ECs in WSPs, especially the transformation, sludge layers of WSPs as they can be ingested and degradation, and accumulations in the influent and effluent ofthe accumulated within microorganisms systems − Bioaccumulation and biomagnifications of EC in microorganisms, especially in algal biomass, which can affect the quality of harvested products

removal performance in conventional WSPs and higher energy recover References efficiency in HRAPs. To optimize algal separation and production, pond engineers have proposed many upgrade solutions of either integrating Abbott, W.E., 1948. Oxygen Production in Water by Photosynthesis. Sew. Work. J. 20 (3), ponds with add-ons (e.g. rock filters, constructed wetlands, activated 538–541. Abdel-Aty, A.M., Gad-Allah, T.A., Ali, M.E.M., Abdel-Ghafar, H.H., 2015. Parametric, sludge) or designing in different configurations (AIWPS, wastewater-fed equilibrium, and kinetic studies on biosorption of diuron by Anabaena sphaerica and ponds, and aerated ponds). However, many bottlenecks still remain, Scenedesmus obliquus. Environ. Prog. Sustain. Energy 34 (2), 504–511. such as the difficulties in maximizing yield growth, optimize harvesting Alcantara, C., Munoz, R., Norvill, Z., Plouviez, M., Guieysse, B., 2015. Nitrous oxide emissions from high rate algal ponds treating domestic wastewater. 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