water

Article Transformation of Waste Stabilization : Reengineering of an Obsolete Treatment System

Silvânia Lucas dos Santos 1 and Adrianus van Haandel 2,*

1 Department of Civil Engineering, Federal University of Rio Grande do Norte, 59.078-970 Natal, Brazil; [email protected] 2 Department of Civil Engineering, Federal University of Campina Grande, 59.429-350 Campina Grande, Brazil * Correspondence: [email protected]; Tel.: +55-83-99133-0196

Abstract: Waste Stabilization Ponds (WSPs) are commonly used for . These systems are composed of a series of ponds: (1) anaerobic ponds, (2) facultative ponds, and (3) maturation ponds. WSPs generally produce good-quality effluent in terms of organic matter and removal, but their application has disadvantages. The most serious disadvantages are a long retention time, the release of biogas, and the impossibility of removing nutrients. A promising alternative to the use of WSPs is replacing the anaerobic and facultative pond with an upflow anaerobic blanket (UASB) reactor, with the advantages of greatly reducing the retention time and the biogas capture. The post-treatment ponds of the UASB reactor effluent involve production and the biological consumption of carbon dioxide, which raises the pH. An experimental investigation showed that it is possible to use polishing ponds in a sequential batch regime instead of continuous flow. This modification accelerates the decay of and accelerates the increase in pH, which,  in turn, facilitates the removal of and . This produces a good-quality effluent  with low concentrations of biodegradable organic material, nutrients, and pathogens. This good- Citation: dos Santos, S.L.; quality effluent is obtained in a system without energy consumption or auxiliary materials and with van Haandel, A. Transformation of a much smaller area than conventional stabilization ponds. Waste Stabilization Ponds: Reengineering of an Obsolete Sewage Keywords: waste stabilization ponds; UASB reactor; polishing ponds; sequential batch regime; Treatment System. Water 2021, 13, investment costs 1193. https://doi.org/10.3390/w 13091193

Academic Editor: Long Ho 1. Introduction

Received: 3 February 2021 At the beginning of the last century, when primary sewage treatment proved to Accepted: 16 April 2021 be insufficient for the protection of water resources, options were Published: 25 April 2021 developed to provide additional biological treatment for the removal of organic material. One of these methods was the waste stabilization pond (WSP) system, which was later Publisher’s Note: MDPI stays neutral improved to increase its treatment capacity. Parker et al. (1950) [1] consolidated the with regard to jurisdictional claims in results of this experiment over the first half of the last century and developed the so- published maps and institutional affil- called Australian system, which showed that the ideal configuration was to subdivide iations. the stabilization pond system into three sequential parts: (1) the anaerobic pond (AP), which receives the influent and is in an anaerobic condition; (2) the facultative pond (FP) that receives the effluent from the AP, which is at least partially aerobic because oxygen is generated due to -mediated ; and (3) maturation ponds, which are

Copyright: © 2021 by the authors. predominantly aerobic, complement the removal of organic material, and improve the Licensee MDPI, Basel, Switzerland. hygienic quality of the final effluent. This article is an open access article A second important development of the stabilization pond project was the recognition distributed under the terms and that the pond system also resulted in an improvement in the hygienic quality of the sewage. conditions of the Creative Commons Marais and Shaw (1964) [2] showed that the decay of thermotolerant coliforms could be Attribution (CC BY) license (https:// described as a first-order process. Later, Marais (1974) [3] developed a model to optimize creativecommons.org/licenses/by/ stabilization ponds for the removal of thermotolerant coliforms. By joining the models 4.0/).

Water 2021, 13, 1193. https://doi.org/10.3390/w13091193 https://www.mdpi.com/journal/water Water 2021, 13, 1193 2 of 15

of Parker et al. (1950) and Marais (1974) [1,3], it is possible to design a stabilization pond system that efficiently removes both organic material and pathogenic organisms. In the 1960s, it became evident that in addition to the removal of organic material and pathogens, the removal of the nutrients, nitrogen and phosphorus, is of vital importance to protect recipient bodies. However, the research aimed at removing nutrients by Pano and Middlebrooks (1982) [4], Bastos et al. (2018) [5], Zimmo et al. (2003) [6], and Camargo Valero and Mara (2010) [7] showed that nitrogen removal is, at best, partial, and the phosphorus removal is poor (see Gomez et al. (2000)) [8]. The reason for this failure is that the pH in a WSP remains close to the neutral point. In this article, it is shown that for efficient nutrient removal, it is necessary for the pH to increase significantly. However, in conventional WSPs, there is an equilibrium between oxygen and carbon dioxide production and consumption. As a result, the pH remains essentially constant, and nutrient removal is incomplete. WSPs have some advantages, but they also have major drawbacks. The advantages of WSPs are associated with their operational simplicity and low cost of implementation (if there are favourable topographic conditions and the area is available at a low price). The disadvantages, however, are numerous and -established, such as the following: (1) Oxygen production is slow, so the WSP’s area must be large (a required area of 3 m2/IE and total retention time of approximately 1 month, Mara (1976) [9]), to maintain at least a partially aerobic environment in the facultative pond and a predominantly aerobic environment in maturation ponds, which is essential for the efficient removal of organic material. (2) The consequence of the large areas of WSPs is a considerable loss of water via evap- oration, resulting in an increase in of the effluent, thus impairing its applicability as water for purposes (Mara and Pearson (1998)) [10]. (3) WSPs are not suitable for the removal of nutrients (nitrogen and phosphorus), so effluent discharge can result in eutrophication of the receiving body. Moreover, the WSPs’ effluents with nutrients have little use as industrial water reuse. (4) The application of AP results in the biogas production that emanates from the liquid phase and generates bad odours around the WSP system due to the presence of hydrogen sulphide gas, which greatly contributes to the unpopularity of WSPs with the surrounding population. (5) Biogas from the AP represents an ecological problem: Methane in the biogas contributes significantly to the release of greenhouse gases and is 21 times more harmful than CO2. As a result, WSPs have the largest GHG footprint of all treatment systems (Van Haandel and Van der Lubbe (2019)) [11]. (6) There is a significant accumulation of non-biodegradable solids in the AP as a result of settling, which leads to the need to remove solids (every 3–5 years)—a complicated operation (Cavalcanti (2003)) [12]. (7) The need to remove the pond and its odours from the urban region entails an extra cost for the collection network with a long outfall, representing a major investment factor. (8) WSPs are almost invariably built as single treatment systems for cities, so segmen- tation of the sewage collection network is not possible, again leading to large investment costs for the network. The treatment of WSPs is basically reduced to the removal of organic material and pathogens; for that reason, the effluent does not comply with legal standards in most cases. Specifically, nutrient removal is insufficient. In view of these serious drawbacks of WSPs, a novel treatment system is proposed. This system comprises a combination of efficient anaerobic pre-treatment and post-treatment ponds. The most widely applied anaerobic sewage treatment unit is the upflow anaerobic sludge blanket (UASB) reactor, which on its own offers greater efficient BOD (Biologi- cal Oxygen Demand) removal than the AP and the FP combined [11] and produces a clear effluent. Water 2021, 13, 1193 3 of 15

2. Polishing Ponds as a Post-Treatment Alternative for Digested Sewage As a result, in a subsequent post-, photosynthesis is stimulated (with more transparency), and oxygen consumption is reduced (leading to less BOD). Therefore, photosynthesis tends to dominate over the oxidation of organic materials, leading to net oxygen production. This opens up the possibility of operating the post-treatment pond as a sequential batch unit, leading to a shorter retention time and decreased pond volume than treatment in flow-through units, thus reducing the pond area. Oxygen production is accompanied by carbon dioxide consumption, increasing the pH, which can lead to the removal of the nutrients, N and P. Thus, the substitution of WSPs with a combination of a UASB reactor and post- treatment sequential batch ponds potentially offers several important advantages: (1) ef- ficient BOD removal in the UASB reactor reduces the retention time in the subsequent pond; (2) capturing the biogas production avoids methane emissions and odour generation; (3) produced solids can be discharged from the UASB reactor; (4) the pH increase in post- treatment ponds allows the removal of nutrients via ammonia desorption and phosphate precipitation; and (5) since there is no odour problem, the novel system can be built near or even within urban areas, thus reducing construction costs. This paper’s aim is to quantify these advantages. Post-treatment after efficient anaerobic pre-treatment may be carried out in polishing ponds (PPs), which differ from the conventional stabilization ponds (WSPs) used for the treatment of raw sewage. Since UASB effluents are of better quality than raw sewage, the configuration and operation, as well as the objectives, of a PP will be quite different from those of a conventional WSP system for treating raw sewage. Table1 shows the differences between conventional stabilization ponds (WSPs) treat- ing raw sewage and polishing ponds (PPs) treating effluents from the UASB reactor. The advantages of PPs are not limited to aspects of the treatment system itself (smaller area, less evaporation, lack of odour, use of sludge as fertilizer, possibility of nutrient removal, etc.) but also in terms of the sewage collection system (shorter outfall, the possibility of segmen- tation of the network in large cities, etc.). The reduced cost due to the shorter outfall and segmentation of the network is great, and often investment in the collection and treatment of sewage with a system composed of a network + UASB + PP may be smaller than the investment in the network alone for a conventional WSPs system due to a reduction in sewage network costs.

Table 1. Differences between conventional waste stabilization ponds (WSPs) and polishing ponds.

Parameter Waste Stabilization Pond UASB + Polishing Pond Influent Raw sewage Digested sewage Main objective(s) BOD and Pathogen removal Pathogen and nutrient removal Series of ponds Series of sequential batch polishing Configuration (AP-FP-MP) ponds (SBPPs), operated in parallel Rapid accumulation in AP Slow accumulation, mostly algae Bottom sludge (250 mg·L−1) (70 mg·L−1) Continuous flow with completely Ponds with a sequential batch Desired flow regime mixed ponds regime (SBPPs) Retention time On the order of 1 month: organic material Less than 1 week: thermotolerant (in hot climate) removal is limiting coliform removal is limiting Biogas released to the atmosphere Biogas is generated in the UASB reactor Gas emissions causing bad odours and can be flared off Methane released contributes to GHG emissions are at the lowest possible Methane generation GHG emission level for sewage treatment Water 2021, 13, x FOR PEER REVIEW 4 of 15

Far from urban regions, requir- Proximity of the population is not Areas of application ing long outfalls a problem Virtually complete N and P re- Nutrient removal Little (<20%) moval is feasible Sewage treatment “Segmented” network with sepa- Centralized system rate treatment systems is feasible Reuse Limited to agricultural reuse Reuse in agriculture and industry Almost complete nutrient removal Protection-receiving No protection: Nutrients can is possible, avoiding eutrophica- water body cause extensive eutrophication Water 2021, 13, 1193 tion. 4 of 15 Legend: AP = anaerobic pond; FP = facultative pond; maturation pond; UASB = upflow anaerobic sludge blanket; SBPP = sequencing batch polishing pond; BOD = biological oxygen demand; GHG = greenhouse gas. Table 1. Cont. 2.1. Process Development in Polishing Ponds Parameter Waste Stabilization Pond UASB + Polishing Pond In treatment ponds, several biochemical, physical, and chemical processes develop, Far from urban regions, requiring Proximity of the population is not Areas of application some of which result in a reduction in the sewage’s undesirable constituents. Figure 1 long outfalls a problem schematically shows the three biochemical processes that can develop and their products. Virtually complete N and P removal Nutrient removal(1) In the anaerobic Littleenvironment (<20%) (the AP and lower part of the FP), anaerobic diges- tion develops and results in biogas production from the digestionis feasible of organic material “Segmented” network with separate Sewage treatment system(OM): Centralized treatment systems is feasible OM → CH4 + CO2 (1) Reuse Limited to agricultural reuse Reuse in agriculture and industry (2) In the aerobicNo protection: environment, Nutrients oxidation can cause of the organicAlmost material complete occurs, nutrient which removal results is Protection-receiving water body in oxygen consumptionextensive and eutrophicationthe generation of carbon dioxide:possible, avoiding eutrophication. Legend: AP = anaerobic pond; FP = facultative pond; maturation pond; UASB = upflow anaerobic sludge blanket; SBPP = sequencing OM + O2 → CO2 + H2O (2) batch polishing pond; BOD = biological oxygen demand; GHG = greenhouse gas. (3) At the same time, photosynthesis occurs, which is essentially the inverse process of2.1. oxidation Process Developmentinvolving the in production Polishing Ponds of organic material and oxygen from carbon dioxide and water:In treatment ponds, several biochemical, physical, and chemical processes develop, some of which result in a reduction in the sewage’s undesirable constituents. Figure1 CO2 + H2O → OM + O2 (3) schematically shows the three biochemical processes that can develop and their products.

Anaerobic digestion Oxidation

CH4 Organic C - (CH2O)n CO2 (N – ox = – 4) (N – ox = +4)

Photosynthesis

Figure 1. Schematic representation of biological processes in sewage treatment ponds. Figure 1. Schematic representation of biological processes in sewage treatment ponds. (1) In the anaerobic environment (the AP and lower part of the FP), developsThere and are resultsalso physical in biogas and production chemical processes from the that digestion develop of organicin the pond material parallel (OM): with the biochemical processes, as indicated in Figure 2. Physical processes are the processes of desorption or absorption of volatileOM compounds→ CH4 + CO that2 are present in the UASB effluent(1) or are generated in the ponds. These compounds are ammonia, carbon dioxide, dissolved oxygen(2) (DO), In the methane, aerobic environment, and hydrogen oxidation sulphide. of In the the organic case of material H2S, NH occurs,3, and CH which4, desorp- results tionin oxygen occurs consumptionbecause these and gases the have generation no appreciable of carbon concentration dioxide: in the air. In the case of CO2 and O2, desorption or absorption may occur depending on whether the concentration → of the compounds in the liquid phaseOM +is O greater2 CO or2 +smaller H2O than the saturation concentra- (2) tion. (3) At the same time, photosynthesis occurs, which is essentially the inverse process of oxidation involving the production of organic material and oxygen from carbon dioxide and water:

CO2 + H2O → OM + O2 (3) There are also physical and chemical processes that develop in the pond parallel with the biochemical processes, as indicated in Figure2. Physical processes are the processes of desorption or absorption of volatile compounds that are present in the UASB effluent or are generated in the ponds. These compounds are ammonia, carbon dioxide, dissolved oxygen (DO), methane, and hydrogen sulphide. In the case of H2S, NH3, and CH4, desorption occurs because these gases have no appreciable concentration in the air. In the case of CO2 and O2, desorption or absorption may occur depending on whether the concentration of the compounds in the liquid phase is greater or smaller than the saturation concentration. Water 2021, 13, x FORWater PEER2021 REVIEW, 13, 1193 5 of 15 5 of 15

Sunlight

NH3 CO2 O2 CH4

Aerobic: oxidation + photosynthesis Influent Oxypause DO = 0 Anaerobic: anaerobic digestion 2 1 Effluent Sludge layer

FigureFigure 2. 2. SchematicSchematic representation representation of of physical physical processes processes that that develop develop in in sewage sewage treatment treatment ponds. ponds. Anaerobic digestion Oxidation Absorption andAbsorption desorption and processes desorption can processesbe described can beby describedFick’s Equation by Fick’s (Equation Equation (Equation (4)), (4)), which stateswhich that the states desorption that the desorption rate of a volatile rate of compound a volatile compound in a liquid in is aproportional liquid is proportional to the degree of supersaturation that exists between the current concentration of the compound to the degree of supersaturationCH4 thatOrganic exists C between- (CH2O) then) current concentrationCO2 of the com- pound and the(N andconcentration-ox the = -4) concentration saturation: saturation:(N -ox = 0) (N-ox = +4)

rd = kd (Cl−Csr)d = kd (Cl−Cs)(4) (4) Photosynthesis where rd is the desorption of the volatile compound; kd is the desorption constant; Cs is where rd is the desorption of the volatile compound; kd is the desorption constant; Cs is the the saturation concentration of the volatile compound; Cl is the concentration of the vola- saturation concentration of the volatile compound; Cl is the concentration of the volatile tile compound compoundin the liquid in phase. the liquid phase. Thus, the rates Thus,of transfer the rates can be of transferexpressed can as be expressed as

rdN = kdN [NH3] (5) rdN = kdN [NH3] (5) rdO = kdO ([DO] − [DO]s), (6) rdO = kdO ([DO] − [DO]s), (6) rdC = kdC ([CO2] − [CO2]s) (7) rdC = kdC ([CO2] − [CO2]s) (7) The desorption of DO and CH4 does not affect alkalinity or acidity; therefore, it also The desorption of DO and CH does not affect alkalinity or acidity; therefore, it also does not affect pH. H2S desorption, although noticeable,4 is very small (<0.5 meq/L) and does not affect pH. H S desorption, although noticeable, is very small (<0.5 meq/L) and also does not affect pH. Desorption of2 the acid CO2 decreases acidity; therefore, it increases the pH. The desorptionalso does notof ammonia affect pH. increases Desorption the of acidity the acid and CO 2decreasesdecreases the acidity; alkalinity therefore,; it increases therefore, it tendsthe to pH. decrease The desorption the pH. of ammonia increases the acidity and decreases the alkalinity; Other physicaltherefore, processes it tends that to develop decrease in the the pH. ponds include the sedimentation of sol- Other physical processes that develop in the ponds include the sedimentation of solids ids that can enter with the influent or form in the PP due to the flocculation of the algae that can enter with the influent or form in the PP due to the flocculation of the algae growth. growth. These solids are partially transformed into biogas (methane) via anaerobic diges- These solids are partially transformed into biogas (methane) via anaerobic digestion, with tion, with the unbiodegradable part accumulating at the bottom of the pond as a sludge the unbiodegradable part accumulating at the bottom of the pond as a sludge layer. An layer. An important chemical process is the precipitation of phosphate salts. These salts important chemical process is the precipitation of phosphate salts. These salts only develop only develop when there is a substantial pH increase in the pond, which can happen only when there is a substantial pH increase in the pond, which can happen only in SBPP-type in SBPP-type ponds. Under suitable conditions (pH ≈ 10), calcium carbonate can also pre- ponds. Under suitable conditions (pH ≈ 10), calcium carbonate can also precipitate. cipitate. 2.2. Treatment Objectives of Polishing Ponds 2.2. Treatment Objectives of Polishing Ponds The objectives of the PPs in treating effluent from a UASB reactor or other efficient The objectivesanaerobic of the sewagePPs in treating digestion effluent systems from depend a UASB on thereactor destination or other that efficient will be given to the anaerobic sewagefinal digestion effluent. systems For agricultural depend reuse,on the a destination reduction in that the suspendedwill be given solids to the and residual BOD final effluent. Forand agricultural the efficient reuse, removal a ofreduction helminth in eggs the and suspended thermotolerant solids and coliforms residual are important. In BOD and the efficientcase of reuse removal of the of final helminth effluent eggs in industryand thermotolerant or its discharge coliforms into surface are im- waters, additional portant. In casenutrient of reuse removal of the final is of effluent fundamental in industry importance. or its discharge into surface wa- ters, additional nutrientThe removal ofis thermotolerantof fundamental coliformsimportance. (TTC) was described as a first-order process The removalby of Marais thermotolerant (1974) and coliforms Van Haandel (TTC) and was Van described der Lubber as a (2019) first-order [3,11]. process Levenspiel (2003) [13] by Marais (1974)showed and Van that, Haandel in this case, and itVan is very der advantageousLubber (2019) to[3,11 use]. reactorsLevenspiel operating (2003) in a sequential [13] showed that,batch in regime.this case, Depending it is very advantageous on the depth, a to value use inreactors the range operating of 3 to 7in days a se- can be expected quential batch regime. Depending on the depth, a value in the range of 3 to 7 days can be

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expectedfor TCC for removal, TCC removal, which which entails entails a large large decrease decrease compared compared to the retention to the retentiontime time of WSPs, ofwhich WSPs, iswhich approximately is approximately 30 30 d. d. If Ifthe the objectives objectives of the PP of include the PP nutrient include removal, nutrient the retention removal, time the in the retention PP will time in the PP will bebe longer longer because because it will itbe will necessary be necessary to increase tothe increasepH before theammonia pH beforedesorptio ammonian and desorption and subsequent phosphate precipitation. subsequent phosphate precipitation. 3. Materials and Methods 3. MaterialsThe experimental and Methods systems were installed and monitored in the City of Campina Grande,The Brazil. experimental The utilized wastewater systems was were raw municipal installed sewage and from monitored the city. in the City of Campina Polishing ponds: Grande,The wastewater Brazil. Thewas treated utilized anaerobically wastewater in a UASB was rawreactor municipal installed at sewagethe experi- from the city. mentalPolishing site. The reactor ponds: has a volume of 2.5 m3 and a height of 1.7 m with a treatment capacityThe of 10 wastewater m3/d (Santos waset al., treated2016). After anaerobically digestion, part inof the a UASB UASB effluent reactor was installed at the exper- usedimental to feed site. the polishing The reactor ponds. has a volume of 2.5 m3 and a height of 1.7 m with a treatment The polishing ponds3 used in the experimental investigation were made of fibreglass, withcapacity a diameter of 10 of 0.5 m m/d and (Santos depths of et 0.2, al., 0.4, 2016). 0.6, and After 1.0 m. digestion,Figure 3 shows part a schematic of the UASB effluent was representationused to feed and the photo polishing of the polishing ponds. ponds used in the investigation. Shallow ponds were used,The as polishing Cavalcanti ponds(2003) showed used inthat the shallow experimental ponds operate investigation with high efficiency were made of fibreglass, [1with2]. These a diameter pond models of 0.5were m operated and depths with very of 0.2,gentle 0.4, superficial 0.6, and stirring 1.0m. using Figure a shal-3 shows a schematic low superficial metal bar (1 cm width) attached to a small motor (6 rpm) to resuspend any algaerepresentation floating within and bubbles photo of ofdissolved the polishing oxygen that ponds emerged used from in the the ponds investigation. when Shallow ponds theywere were used, supersaturated as Cavalcanti with (2003)DO. At showedthe same thattime, shallowthe agitation ponds served operate to even with out high efficiency [12]. stratificationThese pond in the models liquid phase. were In operated practice, this with gentle very agitation gentle might superficial not be necessary stirring using a shallow forsuperficial full-scale ponds metal because bar (1 factors cm width) such as attached wind and to sun a- smallbased thermal motor mixing (6 rpm) could to resuspend any algae introduce enough mixing for a uniform liquid phase. floatingThe ponds within were bubbles operated ofunder dissolved a sequential oxygen batch regime that emerged since experiments from the devel- ponds when they were opedsupersaturated by Albuquerque with et al. DO. (2020) At showed the same that SBPP time, the agitation were more served advantageous to even out stratification thanin the CFPPs liquid for all phase. treatment In objectives. practice, this gentle agitation might not be necessary for full-scale pondsThe becauseexperiments factors were conducted such as windoutdoors and for sun-based a period of nine thermal months. mixing In Campina could introduce enough Grande, sunlight is abundant, and the sewage temperature remains at 25 °C, all year long. However,mixing forduring a uniform winter, there liquid is a greater phase. incidence of .

Plan Sequential batch polishing ponds (in the open air)

UASB 0.5 m 0.5 m 0.5 m 0.5 m effluent

UASB Reactor Transfer pond

(optional))

m

1.0 1.0

m 0.6 0.6

Raw m m

sewage 0.4 0.2 0.2 Final Final Final Final Water 2021, 13, x FOR PEER REVIEW 7 of 15 effluent effluent effluent effluent Anaerobic pre-treatment Sequential batch polishing ponds for post treatment

Figure 3. Flow sheet and photograph of an SBPP system with depths of 0.2, 0.4, 0.6, and 1.0 m with Figuremild stirring 3. Flow of the sheetpond contents. and photograph of an SBPP system with depths of 0.2, 0.4, 0.6, and 1.0 m with mild stirring of the pond contents. Monitoring and evaluation of the system performance: The SBPPs were operated without inocula. They were filled with effluent from the UASB reactor, and the operational variables of temperature, pH, dissolved oxygen (DO), biochemical oxygen demand (BOD), ammonia, and phosphate were assessed daily. Test completion was determined by two criteria: (i) when a phosphate concentration lower than 1 mg/L was reached or (ii) when an operation time of 30 d was reached. Laboratory analyses were carried out to characterize the effluent of the UASB reactor and that of each of the lagoons. SBPPs were monitored daily since the operational regime caused rapid changes in the variables. Samples were always collected in the morning. Sampling was carried out by taking grab samples from the liquid phase. A total of 93 batches were carried out and distributed as follows: L1 (0.2 m): 45 batches in summer and 11 in winter; L2 (0.4 m): 30 batches in summer and 05 in winter; L3 (0.6 m): 10 batches in summer and 03 in winter; L4 (1.0 m): 08 batches in summer and 03 in winter. The following variables were evaluated: DO, pH, temperature, BOD, COD (), ammoniacal nitrogen, and phosphate. For the measurement of DO, tem- perature, and pH, a multiparametric probe (Hanna, model HI 98196) was used for online measurements, while for the other parameters, daily samples were analysed, following the procedures of the Standard Methods for the Examination of Water and Wastewater (APHA; AWWA; WEF, 2017).

4. Results and Discussion Figure 4 shows the evolution as a function of time for the important SBPP variables: DO concentration, pH, COD, and ammonium and phosphate concentrations at different pond depths and under summer (left) and winter conditions (right) at Campina Grande. The figures show the trends of the variables and demonstrate the feasibility of applying the SBPP, at least under the climatic conditions of Campina Grande.

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The ponds were operated under a sequential batch regime since experiments devel- oped by Albuquerque et al. (2020) showed that SBPP lagoons were more advantageous than CFPPs for all treatment objectives. The experiments were conducted outdoors for a period of nine months. In Campina Grande, sunlight is abundant, and the sewage temperature remains at 25 ◦C, all year long. However, during winter, there is a greater incidence of rain. Monitoring and evaluation of the system performance: The SBPPs were operated without inocula. They were filled with effluent from the UASB reactor, and the operational variables of temperature, pH, dissolved oxygen (DO), biochemical oxygen demand (BOD), ammonia, and phosphate were assessed daily. Test completion was determined by two criteria: (i) when a phosphate concentration lower than 1 mg/L was reached or (ii) when an operation time of 30 d was reached. Laboratory analyses were carried out to characterize the effluent of the UASB reactor and that of each of the lagoons. SBPPs were monitored daily since the operational regime caused rapid changes in the variables. Samples were always collected in the morning. Sampling was carried out by taking grab samples from the liquid phase. A total of 93 batches were carried out and distributed as follows: L1 (0.2 m): 45 batches in summer and 11 in winter; L2 (0.4 m): 30 batches in summer and 05 in winter; L3 (0.6 m): 10 batches in summer and 03 in winter; L4 (1.0 m): 08 batches in summer and 03 in winter. The following variables were evaluated: DO, pH, temperature, BOD, COD (chemical oxygen demand), ammoniacal nitrogen, and phosphate. For the measurement of DO, tem- perature, and pH, a multiparametric probe (Hanna, model HI 98196) was used for online measurements, while for the other parameters, daily samples were analysed, following the procedures of the Standard Methods for the Examination of Water and Wastewater (APHA; AWWA; WEF, 2017).

4. Results and Discussion Figure4 shows the evolution as a function of time for the important SBPP variables: DO concentration, pH, COD, and ammonium and phosphate concentrations at different pond depths and under summer (left) and winter conditions (right) at Campina Grande. The figures show the trends of the variables and demonstrate the feasibility of applying the SBPP, at least under the climatic conditions of Campina Grande. WaterWater 20212021, ,1313,, x 1193 FOR PEER REVIEW 8 8of of 15 15

20 20 18 0.40 m L1 0.20m (L2) 0.60m 16 (L1) (L3) 15 L2 14

) L3

1 )

- 12 1 10 - 10 L4 8 1.0 m 6 5

4 (L4) DO (mg.L DO

2 (mg.L DO 0 0 0 5 10 15 20 25 30 0 5 10 15 20 25 30 Time (d) 12 12 Time (d) 11 11 10 10 9 9

8 pH pH 8 L1 L1 7 L2 7 L2 6 L3 6 L3 L4 L4 5 5 0 5 10 15 20 25 30 0 5 10 15 20 25 30 Time (d) Time (d) 100 300 90 0.2 m 0.4 m 80 250

) 0.6 m 1.0 m

)

1

1 - 70 - 200 60 50 150 40 100

30 0.2 m 0.4 m BOD (mg.L BOD 20 (mg.L COD 50 0.6 m 1.0 m 10 0 0 0 1 2 3 4 5 0 1 2 3 4 5 Time (d) Time (d)

)

) 100 50

1

1

- - 80 L1 40 L1 60 L2 30 L2

L3 .(mgN.L . (mgN.L . L3 40 L4 20 L4

20 10 concentr concentr 0

0 N N N 0 5 10 15 20 25 30 0 5 10 15 20 25 30

time (d) time (d) )

1 10 - L1 6

) L1 1 - 8 L2 - 5 L2 L3 L3 6 L4 4 . (mgP.L . L4

. (mgP.L . 3 4 2 2

concentr 1 concentr

P P 0 0 P P 0 5 10 20 25 30 0 5 10 20 25 30 time (d) time (d)

Figure 4. Experimental results of DO, pH, organic material, ammonium, and phosphate concentrations (L1 = 0.2 m; FigureL2 = 0.4 4. m;Experimental L3 = 0.6 m; L4results = 1.0 of m). DO, pH, organic material, ammonium, and phosphate concentrations (L1 = 0.2 m; L2 = 0.4 m; L3 = 0.6 m; L4 = 1.0 m).

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4.1. DO Profiles Figure4 shows typical profiles of the DO concentration as a function of batch time for different depths in the SBPP. Figure4 illustrates the following: (1) In all SBPPs, the DO concentration started at DO ≈ 0 and increased with time. (2) For any retention time, the DO increased faster as the pond became shallower, which is expected given that photosynthetic production and oxygen transfer are faster in shallow ponds with relatively large areas. (3) All profiles showed large daytime oscillations, which are attributable to the absence of photosynthetic DO production at night. (4) The DO concentration trended towards its maximum and then decreased over time despite algae generation in the ponds, which may possibly be attributable to a decrease in activity due to a pH increase.

4.2. pH Variation The processes that affect the pH variation in PP are photosynthesis and ammonia desorption [14]. Photosynthesis decreases acidity but does not affect alkalinity, so pH tends to increase. Ammonia desorption increases acidity and decreases alkalinity, resulting in an increase in pH. As shown in Figure4, the pH increased in all ponds over time. However, the rate of change depended on the depth: While in the SBPP with a 0.2 m depth, a pH above 9.5 was obtained in a few days, in the deeper ponds, this high pH was not reached after 30 d. Due to the higher rate of increase in the pH in the shallow pond, it can be concluded that photosynthesis is faster, which is corroborated by the rapid increase in DO concen- tration. The reason for this difference is that in the shallow ponds, the area was relatively large, so more sunlight entered the liquid phase.

4.3. Organic Material Figure4 shows that the concentration of organic material can reach very low levels of BOD ≈ 20 mg/L and COD ≈ 120 mg/L. However, both BOD and COD increased with time, which can be attributed to the algae growth in the ponds, stimulated by the high transparency of the UASB effluent. In all cases, the increase in COD was much smaller than expected based on stoichiometry for the increase of oxygen in the pond (Equation (3)). This can be attributed to the algae flocculation and sedimentation in the pond. It can also be observed that the COD in shallow ponds (0.2 m) tended to be higher than the COD in deeper ponds.

4.4. Ammonium Removal Efficiency According to Equation (5), the rate of ammonia desorption is proportional to the union- ized ammonia concentration, which in turn depends on the total ammonia concentration and the pH: + + NH4 ↔ NH3 + H or + + Ka = [NH3] [H ]/[NH4 ] (8) thus, (pH-pKa) [NH3]/Nt = 1/(1 + 10 ) ◦ Emerson (1975) [15] determined the dissociation constant pKa = 9.1 at 25 C, which means that at pH = 9.1, the fraction of unionized ammonia is 50%, but at a neutral pH (pH = 7.1), the fraction is low (1%). Therefore, at a neutral pH, the rate of ammonia desorption is very low. As the pH increases, unionized ammonia and the desorption rate increase. The rate will decrease again when the total ammonia concentration becomes small.

4.5. Phosphate Removal Efficiency The data in Figure 6 show that the SBPPs offered a high efficiency of P removal even under short retention times—5 and 10 days for ponds 0.20 and 0.40 m deep, respectively. Water 2021, 13, 1193 10 of 15

The phosphate removal efficiency is closely related to the pH value in the pond. In a subsequent paper, it will be shown that P removal was, in fact, due to hydroxyapatite precipitation and that for efficient P removal, a pH in the range of 9.5 to 9.7 is required. This high pH could not be reached in the FTPP (flow-through polishing pond), and for that reason, the phosphorus removal was poor (Cavalcanti (2003) [12] and Albuquerque et al. (2021)) [14]. Partial phosphate removal is not necessarily a problem for water reuse in industry. In this case, clarification or flotation is necessary to remove algae and other suspended solids. As ferric or aluminium salts are normally used, phosphate precipitation will occur in parallel and will leave a very low residual concentration of less than 0.1 mgP.L−1. Naturally, the removal of suspended solids to produce a high-quality water industry generates the problem of determining what to do with the suspension that results from clarification. One solution is anaerobic digestion, where the digester could be the UASB reactor or a dedicated sludge digester.

4.6. Thermotolerant Coliform Removal Several researchers have shown that the decay of thermotolerant coliforms in PPs is a first order process (Marais (1974) [3] and Van Haandel and Van der Lubbe [11]):

dN/dt = kbN (9)

where N is the number of thermotolerant coliforms present at time t; kb is the decay constant. Marais (1974) showed that the basic differential equation can be solved for different hydrodynamic reactors and proposed the following solutions for the removal efficiency of Ne/Ni [3]: (1) Sequential batch reactor: An exponential relationship between the removal efficiency and the retention time:

Ne/Ni = exp(−kbHRT) (10) (2) Complete mix continuous flow reactor: A hyperbolic relationship between the re- moval efficiency and the retention time:

Ne/Ni = 1/(1 + kbHRT) (11) (3) Complete mix series reactor systems: For a series N of equal reactors, one has:

N Ne/Ni = 1/(1 + kbHRT/N) (12) These solutions indicate the following: (1) For any retention time, the most efficient solution for bacterial removal is to operate a sequential batch reactor; (2) A series of complete mix flow through the reactors is more efficient than a single reactor with an equal volume, and this difference increases with the number of reactors in the series. In Figure5, the ratio of the retention times in a completely mixed flow series through ponds and a sequential batch polishing pond is plotted as a function of the number of ponds in the series. The difference in retention time (and, hence, in the pond area) is very large, especially if the number of ponds is not great. Thus, for a removal efficiency of 99.99% of thermotolerant coliforms, the retention time in a series of four FTPP is 4 times longer than that in an SBPP, which means that the size of each of the four flows through the ponds is equal to that of the single sequential batch pond. Water 2021, 13, x FOR PEER REVIEW 11 of 15

Water 13 2021, , 1193 It is concluded that the operation of the SBPP is advantageous and reduces the 11size of 15 of the post-treatment unit significantly.

10 9 Temp = 25 oC 8 7

Effi- Efficiency )

- 6 ( ciency = 99.99%

5 = 99.9% SBPP

/R 4

FTPP 3 R 2 1 0 2 4 6 8 10 Number of pond in series (-)

Figure 5. Ratio of retention times in a series of FTPPs (complete mix) and in an SBPP for removal Figureefficiencies 5: Ratio of of 99.9% retention (blue times curve) in and a series 99.99% of ofFTPPs the influent (complete thermotolerant mix) and in coliformsan SBPP for (red removal curve) as a efficiencies of 99.9% (blue curve) and 99.99% of the influent thermotolerant coliforms (red curve) function of the number of FTPPs. as a function of the number of FTPPs. It is concluded that the operation of the SBPP is advantageous and reduces the size of theIn post-treatment practice, the most unit important significantly. variable for pond design is the required area for treat- ment. ThisIn practice, area is related the most to the important retention variable time, as forshown pond by design Equation is the13: required area for

treatment. This area is related to theAIE retention = VIE/H = time, qIERh as/H shown by Equation (13): (13)

where AIE and VIE are the area and volumeAIE = V ofIE the/H =polishing qIERh/H pond per inhabitant equivalent(13) (IE), respectively; qIE is the contribution per inhabitant equivalent; H is the pond depth; Rh is wherethe retentionAIE and timeVIE arein the the pond. area and volume of the polishing pond per inhabitant equivalent (IE),The respectively; data in FigureqIE is 3 theand contribution Equation (13 per) were inhabitant used to equivalent; construct FigureH is the 6, pond in which depth; theR h requiredis the retention area per timeinhabitant in the pond.equivalent for the different treatments (removal of COD, ni- trogen,The and dataphosphorus) in Figure is3 plottedand Equation as a function (13) were of the used pond to depth construct for summer Figure 6and, in winter which conditionsthe required at Campina area per inhabitantGrande (7° equivalent South). Additionally, for the different the per treatments capita area (removal for TTC of COD,re- movalnitrogen, is plotted and phosphorus)in Figure 6 (Eq isuation plotted (10 as)). a A function decay constant of the pond of k depthb = 1.6/H*1.07 for summer(t−20) was and ◦ determinedwinter conditions by Medeiros at Campina et al. (2021) Grande [16 (7] forSouth). the conditions Additionally, in Campina the per capitaGrande. area Th fore Fig- TTC (t−20) ureremoval assumes is plotteda per capita in Figure contribution6 (Equation of 100 (10)). L/d A but decay can constantbe adapted of kforb =any 1.6/H other∗1.07 value sincewas the determined area is proportional by Medeiros to etthe al. contribution. (2021) [16] for the conditions in Campina Grande. The FigureThe assumesresults plotted a per capita in Figure contribution 6 indicate of the 100 following: L/d but can be adapted for any other value since the area is proportional to the contribution. (1) The area required for SBPP to remove BOD and coliforms is much smaller than the The results plotted in Figure6 indicate the following: value required for conventional stabilization ponds, which is about 3 m2 per inhabit- (1) ant.The The area data required indicate for that SBPP with to removeSBPPs, for BOD the and BOD coliforms and thermotolerant is much smaller coliforms than the 2 (TTCs),value the required area is for reduced conventional by a factor stabilization of 4 to ponds,5 compared which to is that about of 3 conventional m per inhabi- WSPs.tant. The data indicate that with SBPPs, for the BOD and thermotolerant coliforms (2) If nitrogen(TTCs), the removal area is reducedis required, by a the factor SBPP of 4area to 5 comparedis reduced to by that a factor of conventional of 2, approxi- WSPs. (2) mately.If nitrogen removal is required, the SBPP area is reduced by a factor of 2, approximately. (3)(3) If phosphorusIf phosphorus removal removal is isdesired, desired, the the SBPP SBPP area area is ismore more or or less less equivalent equivalent to tothat that of of a WSP;a WSP; (4) The SBPP area for organic material and CTT removal is only marginally affected (4) The SBPP area for organic material and CTT removal is only marginally affected by by climate and depth. In contrast, the area for removing nutrients depends on climate and depth. In contrast, the area for removing nutrients depends on these fac- these factors; tors; (5) The depth of SBPP has an optimal value of about 0.5 m for nutrient removal, which is (5) The depth of SBPP has an optimal value of about 0.5 m for nutrient removal, which a value much smaller than that normally applied to WSPs; is a value much smaller than that normally applied to WSPs; (6) The removal of phosphorus requires about twice the area required for the removal (6) The removal of phosphorus requires about twice the area required for the removal of ammonia. of ammonia.

Water 2021, 13, x FOR PEER REVIEW 12 of 15

Water 2021, 13, 1193 12 of 15

4 4 Conventional WSP system Conventional WSP system 3.5 3.5 for BOD and coliformremoval for BOD and coliform removal

3 3

E I E 2.5 I 2.5 P < 1 mg/L P < 1 mg/L 2 2 per per N < 1 mg/L 1.5 N < 1 mg/L 1.5

Area 3 Area 1 1 TTC < 10 0.5 0.5 BOD < 20 mg/L BOD < 20 mg/L 0 0 0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1 Pond depth (m) Pond depth (m)

Figure 6. Area per inhabitant equivalent SBPP for the removal of BOD, thermotolerant coliforms, nitrogen, and phosphorus Figure 6. Area per inhabitant equivalent SBPP for the removal of BOD, thermotolerant coliforms, nitrogen, and phospho- as functions of the pond depth under summer (left) and winter conditions (right) in the tropics (per capita contribution of rus as functions of the pond depth under summer (left) and winter conditions (right) in the tropics (per capita contribution · −1 of100 100 L Ld·d−1).). 4.7. Results of the Experimental Investigation 4.7. Results of the Experimental Investigation The most important results of the experimental investigation can be resumed as follows: The most important results of the experimental investigation can be resumed as fol- (1) SBPP can be operated with a much shallower depth than conventional maturation lows: ponds (MPs) (Figure4).

(1)(2) SBPPThe can area be required operated forwith SBPP a much is muchshallower smaller depth than than conventional conventional maturation MPs (Figure ponds6). (MPs) (Figure 4). (3) There is a considerable advantage in using SBPP rather than FTPP. (2) The area required for SBPP is much smaller than conventional MPs (Figure 6). (4) Foul odours are avoided in SBPP; thus, they can be applied near urban regions. (3) There is a considerable advantage in using SBPP rather than FTPP. (4) Foul odours are avoided in SBPP; thus, they can be applied near urban regions. 4.8. Reengineering of Waste Stabilization Pond Systems 4.8. ReengineeringThe presented of dataWaste show Stabilization that the inclusion Pond Systems of a UASB reactor as a pre-treatment unit enablesThe presented a radical modificationdata show that in the the inclusion size, operation, of a UASB and reactor configuration as a pre- oftreatment the ponds unit as post-treatment units, taking into account the gaps still present in pond research, especially enables a radical modification in the size, operation, and configuration of the ponds as with regard to biochemical aspects (Espinoza et al., 2020 [17]; Verbijla et al., 2015 [18]; post-treatment units, taking into account the gaps still present in pond research, especially Ho et al., 2019 [19]). The traditional flow-through ponds can be substituted with great ad- with regard to biochemical aspects (Espinoza et al., 2020 [17]; Verbijla et al., 2015 [18]; Ho vantage via sequential batch ponds, which have a much smaller area and allow significant et al., 2019 [19]). The traditional flow-through ponds can be substituted with great ad- improvements in final effluent quality, with the possibility of efficient nutrient removal in vantage via sequential batch ponds, which have a much smaller area and allow significant the ponds. The area required for the SBPP depends on the SBPP’s function—the removal of improvements in final effluent quality, with the possibility of efficient nutrient removal residual organic material, pathogens, or nutrients. There is an optimal depth in the range in the ponds. The area required for the SBPP depends on the SBPP’s function—the re- of 0.4 to 0.6 m for the SBPP that would allow operation of the smallest per-capita area for moval of residual organic material, pathogens, or nutrients. There is an optimal depth in SBPP. This value is much smaller than the depths used in WSP maturation ponds, which the range of 0.4 to 0.6 m for the SBPP that would allow operation of the smallest per-capita are usually 1–1.5 m. area for SBPP. This value is much smaller than the depths used in WSP maturation ponds, The SBPP produces final effluent quality equal to the WSPs in terms of the BOD and whichTTC removalare usually efficiency 1–1.5 m. but in a much smaller area. The SBPP has the great advantage of offeringThe SBPP the possibility produces offinal removing effluent nitrogen quality equal and phosphorus, to the WSPs whichin terms cannot of the be BOD achieved and TTCin WSPs. removal efficiency but in a much smaller area. The SBPP has the great advantage of offeringFor the the possibility configuration, of removing the conventional nitrogen and system phosphorus, of ponds which in series cannot and be continuous achieved inflow WSPs. (AP + FP + MP) is replaced by a series of PPs operating in parallel independent of eachFor other the under configuration, a sequential the conventional batch regime. system Thisregime of ponds is possiblein series thanks and continuous to the low flowconcentration (AP + FP + of MP) biodegradable is replaced materialby a series in theof PPs UASB operating reactor in effluent. parallel It independent was shown that of eachfor allother studied under depths a sequential (range 0.2batch to 1.0regime. m), the This environment regime is possible in the SBPP thanks was to always the low aerobic, con- ceandntration the DO of concentrationbiodegradable rapidlymaterial increased in the UASB from reactor a value effluent. of about It 0 was mg/L shown at the that start for of allthe studied batch todepths a value (range above 0.2 the to saturation1.0 m), the concentration environment inin the the shallow SBPP was ponds always (0.2 aerobic, to 0.4 m).

Water 2021, 13, x FOR PEER REVIEW 13 of 15

and the DO concentration rapidly increased from a value of about 0 mg/L at the start of the batch to a value above the saturation concentration in the shallow ponds (0.2 to 0.4 m). Having defined the area and depth of the UASB–SBPP system, it is necessary to de- termine the number of ponds to be constructed. There are essentially two possible config- urations [11]: Water 2021, 13, 1193 13 of 15 (1) A series of ponds receiving the UASB effluent sequentially in each pond (Figure 7a). In this configuration, the retention time for the TTC decay is counted from the mo- ment that the UASB effluent fills the pond and starts to feed the next one in the series. For Having defined the area and depth of the UASB–SBPP system, it is necessary to operational convenience, one can choose the volume of the pond such that the time to fill determine the number of ponds to be constructed. There are essentially two possible the pond is one day. Thus, the total time spent in the pond will be the filling time (1 day) configurations [11]: plus the retention time for the chosen objective of the SBPP. The number of ponds for the (1)treatmentA series is the ofponds retention receiving time plus the UASBthe time effluent to fill the sequentially pond. in each pond (Figure7a).

(a) (b)

FigureFigure 7. 7.Schematic Schematic representation representation of of the the flow flow sheet sheet and and operation operation of of polishing polishing ponds ponds fed fed with with sequential sequential batches batches without without (left,(left, ( a())a)) and and with with (right, (right, (b (b))) an) an intermediate intermediate transfer transfer pond. pond.

(2) InA thisseries configuration, of ponds receiving the retention the effluent time for from the TTCan intermediate decay is counted transfer from pond the moment be- that thetween UASB the effluentUASB and fills LPBS the pond(Figure and 7b). starts to feed the next one in the series. For operationalIn this convenience, configuration, one the can transfer choose thepond volume has the of the function pond suchof anthat equalization the time to tank, fill thewhich pond receives is one day.a continuous Thus, the flow total of time effluent spent infrom the the pond UASB will while be the the filling batches time (1are day) dis- pluscharged the retention into the SBPPs, time for allowing the chosen for the objective sequential of the batch SBPP. regime. The number The transfer of ponds pond for is the not treatmentonly an equalization is the retention tank time but also plus functions the time toas filla settler the pond. for settleable solids that will even- (2)tuallyA be series discharged of ponds from receiving the UASB the effluent effluent from, which an intermediatemay be a fraction transfer of 30 pond%–50% between of the producedthe UASB solids and [11 LPBS]. If photosynthesis (Figure7b). is sufficient in the transfer pond, sulphide removal may be possible if the oxygen production rate is compatible with the sulphide load on the In this configuration, the transfer pond has the function of an equalization tank, which transfer pond. In this way, it becomes possible to remove sulphate in industrial receives a continuous flow of effluent from the UASB while the batches are discharged wastewater by reducing it to sulphide in the UASB reactor, followed by the oxidation of into the SBPPs, allowing for the sequential batch regime. The transfer pond is not only an sulphide to sulphur in the transfer pond. equalization tank but also functions as a settler for settleable solids that will eventually be discharged from the UASB effluent, which may be a fraction of 30–50% of the produced 4.9. Applications of the Proposed Sewage Treatment System solids [11]. If photosynthesis is sufficient in the transfer pond, sulphide removal may be possibleThe if most the oxygen obvious production and immediate rate is application compatible of with the the proposed sulphide UASB load– onSBPP the system transfer is pond.the transformation In this way, itof becomes the numerous possible conventional to remove stabilization sulphate in pond industrial systems wastewater into systems by reducingwith efficient it to sulphide pre-treatment in the (UASB UASB reactor, reactors) followed followed by by the sequential oxidation batchof sulphide-polishing to sulphurponds. Not in the only transfer will the pond. population surrounding the WSP systems benefit by removing uncomfortable odours, but there will also be significant environmental gains, such as 4.9.burning Applications biogas ofinstead the Proposed of releasing Sewage it Treatmentinto the atmosphere System and the possibility of protecting the receivingThe most obviousbodies, thereby and immediate avoiding application eutrophication. of the proposedIn regions UASB–SBPP with scarce systemwater re- is thesources, transformation one of the ofmost the important numerous improvements conventional stabilization yielded by the pond new systems treatment into system systems is withthe possibility efficient pre-treatment of generating (UASB a new reactors) source of followed water for by use sequential in industries, batch-polishing thus reducing ponds. the Notdemand only will for the public population supply surrounding water and unlocking the WSP systems the chains benefit of by economic removing development uncomfort- able odours, but there will also be significant environmental gains, such as burning biogas instead of releasing it into the atmosphere and the possibility of protecting the receiving bodies, thereby avoiding eutrophication. In regions with scarce water resources, one of the most important improvements yielded by the new treatment system is the possibility of generating a new source of water for use in industries, thus reducing the demand for public supply water and unlocking the chains of economic development caused by a lack of water. All benefits that can be materialized via the application of the novel system are in Water 2021, 13, 1193 14 of 15

line with the Sustainable Development Goals (SDGs), which will undoubtedly contribute significantly to a more sustainable environment, Ho et al. (2017) [20]. Systems composed of UASB–SBPP reactors can be much cheaper than conventional WSP systems for three reasons: (1) they can be built close to urban regions (or even within these regions), thus reducing the costs of the final outfall; (2) they can reduce costs due to a reduction in the area needed for implementing the system and in the height of the slopes of the ponds, which will be much smaller; (3) as the various SBPPs operate independently of each other, they can be built at different levels on terrain with rugged topography, thus reducing earthmoving costs. The UASB–SBPP system is applicable at any scale, although it will generally not be used in large urban agglomerations because it requires a considerable area, despite being much smaller than that of a conventional WSP system. This system can also be applied to small flows. In rural areas, in the absence of a sewage collection network, the possibility exists to build single-family systems that reuse the effluent for food production on the properties.

5. Conclusions (1) A novel sewage treatment system composed of a UASB reactor and a series of se- quential batch polishing ponds was proposed (UASB–SBPP) as a substitute for the conventional waste stabilization pond system (WSPs). The most important advan- tages of the new system are (1) the possibility to remove nitrogen and phosphorus, (2) a reduction of the required area, and (3) elimination of the vile odours emitted by collecting and burning the biogas. (2) Treatment in WSPs is limited to a reduction of organic materials and thermotolerant coliforms such that the effluent quality is not compatible with legal standards and, strictly speaking, cannot be discharged into surface waters. The UASB–SBPP system also removes organic material and thermotolerant coliforms but does so much more efficiently and is also able to remove the nutrients, nitrogen, and phosphorus. (3) The predominance of photosynthesis over the oxidation of organic material in polish- ing ponds leads to the consumption of carbon dioxide and increases pH. This high pH permits the removal of nitrogen due to the desorption of ammonia and phosphorus due to the precipitation of phosphate. In this way, a final effluent quality compatible with legal standards can be produced. (4) If the UASB–SBPP system is used only for organic material and coliform removal, the area is reduced by a factor of 4–5 compared to the WSP system. If nitrogen removal is also an objective, the area is reduced by a factor of 2 compared to the WSP system. (5) Costs of the UASB–SBPP are reduced because there is no need for a long outfall; further, the polishing ponds are much smaller and shallower and do not need to be levelled as they operate independently. (6) Due to the virtual absence of odour problems, the UASB–SBPP system can be con- structed near or even within urban regions, avoiding high costs for the collection system. Moreover, the steep reduction of the PP area itself leads to an important cost reduction and can augment the system’s applicability. Another factor related to cost reduction is that the pond depth of PP is much smaller than that of WSP, which reduces excavation costs. (7) Transfer ponds function not only as equalization tanks but also act as settlers, facilitate the retention of helminth eggs, and enable CO2 desorption and sulphide’s oxidation to sulphur.

Author Contributions: A.v.H.: Conceptualization, Methodology, Resources, Writing. S.L.d.S.: Vali- dation, formal analysis, investigation, supervision. All authors have read and agreed to the published version of the manuscript. Funding: The authors thank the support of the National Research Council (CNPq). Conflicts of Interest: The authors declare no conflict of interest. Water 2021, 13, 1193 15 of 15

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