Anaerobic Processes As the Core Technology for Sustainable Domestic Wastewater Treatment: Consolidated Applications, New Trends, Perspectives, and Challenges

Reviews in Environmental Science and Bio/Technology (2006) 5:3–19 Ó Springer 2006 DOI 10.1007/s11157-005-4630-9

Anaerobic processes as the core technology for sustainable domestic wastewater treatment: Consolidated applications, new trends, perspectives, and challenges

Eugenio Foresti*, Marcelo Zaiat & Marcus Vallero Departamento de Hidraúlica e Saneamento, Escola de Engenharia de Saõ Carlos, Universidade de Saõ Paulo, Av. Trabalhador Saõ-Carlense, 400 Centro, 15566-590, Saõ Carlos, SP, Brazil (*author for correspondence: e-mail: [email protected]; phone: +55 16 3373 9671; fax: +55 16 3373 9550)

Key words: anaerobic processes, domestic sewage, improved anaerobic reactor design, post-treatment, resources recovery, sustainability

Abstract Anaerobic digesters have been responsible for the removal of large fraction of organic matter (minerali- zation of waste sludge) in conventional aerobic sewage treatment plants since the early years of domestic sewage treatment (DST). Attention on the anaerobic technology for improving the sustainability of sewage treatment has been paid mainly after the energy crisis in the 1970s. The successful use of anaerobic reactors (especially up-flow anaerobic sludge blanket (UASB) reactors) for the treatment of raw domestic sewage in tropical and sub-tropical regions (where ambient temperatures are not restrictive for anaerobic digestion) opened the opportunity to substitute the aerobic processes for the anaerobic technology in removal of the influent organic matter. Despite the success, effluents from anaerobic reactors treating domestic sewage require post-treatment in order to achieve the emission standards prevailing in most countries. Initially, the composition of this effluent rich in reduced compounds has required the adoption of post-treatment (mainly aerobic) systems able to remove the undesirable constituents. Currently, however, a wealth of information obtained on biological and physical-chemical processes related to the recovery or removal of nitrogen, phosphorus and sulfur compounds creates the opportunity for new treatment systems. The design of DST plant with the anaerobic reactor as core unit coupled to the pre- and post-treatment systems in order to promote the recovery of resources and the polishing of effluent quality can improve the sustainability of treatment systems. This paper presents a broader view on the possible applications of anaerobic treatment systems not only for organic matter removal but also for resources recovery aiming at the improvement of the sustainability of DST.

1. The emergence of anaerobic process The application of anaerobic processes, however, for domestic sewage treatment can be traced to the time of the first engineered wastewater treatment plants (McCarty 1982). For The proposition of anaerobic processes as the instance, anaerobic processes have been exten- core technology for domestic sewage treatment sively applied for the digestion of primary and (DST) is quite recent. This proposition derives secondary sludge in wastewater treatment plants from a more general concept of sustainable envi- based on conventional aerobic systems such as ronmental protection and resource conservation the activated sludge and trickling filter systems. (EPRC) applied for wastewaters and solid The settleable solids fraction in the raw sewage wastes, as proposed by Lettinga et al. (1997). (separated in primary settlers) corresponds to 4 about 40–50% of the total influent organic mat- obic filter, that is an up-flow reactor with a fixed ter, whereas the organic-rich supernatant is trea- bed for anaerobic biofilm attachment (Young & ted in aerobic units, where a considerable McCarty 1969); (4) the anaerobic expanded/fluid- fraction of dissolved organics is converted into ized bed reactor, that is an up-flow reactor with biological solids. A fraction of produced biologi- expanded/fluidized bed for anaerobic biofilm cal solids is returned to the aeration units (acti- attachment (Switzenbaun & Jewell 1980); (5) the vated sludge systems) but ultimately most of it is up-flow anaerobic sludge blanket (UASB) reac- discharged from secondary settlers as excess tor, that is an up-flow reactor equipped with an sludge. Therefore, the suspended solids fraction internal solid/liquid/gas (SLG) separation device (sum of primary and secondary sludges) may ac- to keep the biological solids inside the reaction count for about 40–60% of the total organic zone (Lettinga et al. 1980); (6) the anaerobic matter (present in raw sewage) to be treated in rotating biological reactor, that contains rotating anaerobic digesters before final disposal. Thus, it biodiscs in a air-tight tank (Blanc et al. 1983; is evident that anaerobic processes have played Tait & Friedman 1980); (7) and the anaerobic an important role in the organic load abatement baffled reactor, that is a series of up- and down- and sludge manageability, mainly in large flow chambers separated by baffles to provide the conventional aerobic treatment plants. SLG separation along the reactor length (Bach- Since about 1882 experiments had been car- mann et al. 1985). ried out on the aeration of settled sewage with In any case, anaerobic digestion was consid- research efforts in the last two decades of the ered to be feasible for high-strength wastewater nineteenth century concentrated on wastewater and only for temperature conditions above treatment by the promising biological filtration 20–25 °C (Kalogo & Verstraete 2001) so that the theories (Cooper 2001). Important developments first anaerobic reactor configurations were of the activated sludge process and its many vari- designed for industrial wastewater treatment. The ants (including anaerobic sludge digesters) has use of anaerobic processes for the treatment of turned it into the main technique for secondary high-strength industrial wastewater, especially sewage treatment and has had probably the big- those from food processing and pulp and paper gest impact of all processes on environmental industries, became a very attractive option be- improvement in the past century. In the last half cause expensive aeration equipment (used in aer- of the past century, however, economical con- obic processes) are not needed, resulting in lower straints, due to the sharp increase in energy pri- investment and energy costs. In addition, the net ces and environmental sustainability awareness, production of excess sludge is also lower and the stimulated an impressive development of anaero- methane gas produced can be used as energy bic processes for wastewater treatment. Unques- source. The anaerobic digestion of high strength tionably the greatest development for success of wastewater in temperate climate countries is the the anaerobic digestion was the design of new appropriate choice because the great volume of reactor concepts (as compared to the aerated methane gas produced is used to heat the reactor units) that allowed the retention of a high con- to a desired operational temperature (30–35 °C). centration of biomass, enabling a sharp decrease More recently, however, the anaerobic pro- in the hydraulic retention time (HRT) needed to cesses proved also to be suited for the treatment achieve acceptable waste removal efficiencies un- of low-strength domestic sewage, notwithstand- der anaerobic conditions (McCarty 1982). ing that the successful full-scale application and From 1950 to 1980, several anaerobic reactor operation of anaerobic reactors is still restricted configurations were developed for industrial to tropical regions (no need for heating), where wastewater treatment (McCarty 1982): (1) the sewage temperatures generally exceed 20 °C anaerobic clarigester, that couples a sedimenta- (Lettinga 2001). Nevertheless, the presence of tion zone on top of the reaction zone; (2) the suspended solids (including fats) in raw sewage is anaerobic contact process, that incorporates a a drawback for its treatment in anaerobic reac- separate settler for retaining and returning the tors. The suspended solids correspond to about biomass to the anaerobic reactor (Seyfried et al. 50% of the total chemical oxygen demand 1984; Anderson & Donnelly 1977); (3) the anaer- (COD) of sewage, making hydrolysis the limiting 5 step. Therefore, some anaerobic reactor configu- phosphorus (as phosphate) and sulfur (as S0). rations successfully applied for industrial waste- As such, in terms of sustainability the use of water treatment may not be applied to raw anaerobic reactors as the core unit of a DST sewage treatment, as the suspended solids in sew- system is most suited for this purpose. In addi- age clog fixed beds (Zaiat et al. 2000; Jawed & tion to the removal of organic matter with low Tare 2000), or they cannot be properly retained (if any) energy consumption and with a net pro- (and therefore digested) in anaerobic reactors duction of methane gas, the presence of phos- operated at high up-flow velocities (Mahmoud phate, nitrogen and sulfur reduced compounds et al. 2003). in the effluent opens the opportunity for the Research and application on the anaerobic development of economically feasible processes treatment of DST have put little emphasis on the to recover these compounds of interest. In fact, possibilities of improving bioreactor design and the development of post-treatment units of treatment systems in order to further improve anaerobic reactors is not only important to their performance in comparison to existing improve the effluent quality for environmental anaerobic reactors configurations. This paper pre- protection, but also to achieve the recovery of sents the possibilities of pre- and post-treatment resources. and new reactor configurations both to enhance It is well known that existing anaerobic reac- organic matter conversion and to recover energy tors applied to DST produce effluents that can- and resources, viz. nitrogen, phosphorus and sul- not be directly discharged in superficial water fur compounds. bodies (Figure 1). The effluent quality from Specific aspects of reactor configurations for anaerobic reactors treating domestic sewage can DST including post-treatment options are pre- vary widely depending on several factors, sented in other sections of this special issue of including: local conditions, influent characteris- Re/Views in Environmental Science and Bio/Tech- tics, reactor design, operational parameters, etc. nology. As such, a general anaerobic reactor effluent Thus, a broader view on the possible applica- quality cannot be defined strictly. From most tions of anaerobic technology to promote a more data available, however, anaerobic effluents are sustainable DST is presented. In this paper, sus- normally launched with a COD from 100 to tainability in DST is based on at least three 200 mg L)1, total suspended solids (TSS) from important issues: (1) public health protection, (2) 50 to 100 mg L)1 (Passig et al. 2000; Vieira environmental protection, and (3) resource recov- 1998; van Haandel & Lettinga 1994;); ammonia ery. In addition, economical limitations play a from 30 to 50 mg L)1 (Kobayashi et al. 1983; key role on what sustainable DST is, as develop- Torres & Foresti 2001), and phosphorus from ing countries cannot afford the complete attain- 10 to 17 mg L)1 (Torres & Foresti 2001). ment of these three issues, because construction Sulfide concentrations depend on the influent and maintenance of high-tech DST are very sulfate concentration and on the extension of expensive. In this paper, a sustainable system is the prevalence of sulfide generation over metha- considered as the best affordable (low-cost) pro- nogenesis, as sulfate reduction occurs preferably cess for public health and environmental protec- over methanogenesis when organic carbon is tion as well as resource recovery. available in the influent (Lens et al. 2000). In addition, it is well known that biological processes, either aerobic or anaerobic, are ineffec- 2. Anaerobic technology as a tool to promote tive for appropriated pathogens removal, except a sustainable DST in stabilization ponds (von Sperling 1996). Therefore, anaerobic reactor effluents still repre- The ideal situation for DST systems would be sent a real risk to health (presence of patho- the complete removal of pathogens (health pro- gens) and environment (high-residual COD and tection) and the highest removal of COD (envi- nutrients). Consequently, anaerobic reactors ronmental protection) with recovery of energy must be combined with other technologies in (methane or hydrogen) and compounds of inter- order to pursue the presented ideal situation for + ) ) est: nitrogen (as NH4 ,NO2 , and NO3 ), DST systems. 6

CH4

Anaerobic CO2 Output of H S compounds in reactor 2 H anaerobically 2 treated sewage Input of compounds in raw sewage NH + Organic Carbon (OC) 4 OC → CH / CO / OC Organic Nitrogen (ON) 4 2 effluent HS- / H S → + 2 dissolv. Organic Sulfur (OS) ON NH4 Ammonia nitrogen (NH +) 4 OS → HS- / H S 2 PO 3- 2- 4 Sulfate (SO4 ) 2- → - 3- SO4 HS / H2S Phosphate (PO4 ) OCe

Sludge ready for dewatering

Figure 1. Input and output of compounds in an anaerobic reactor treating domestic sewage.

3. Application of anaerobic reactors for sewage HRT in the range of 6–10 h, removal efficiencies treatment from 65% to 80% for COD and BOD, and from 67% to 90% for TSS have been obtained with 3.1. Successes and constraints in the use UASB reactors (Wiegnant 2001; Foresti 2002). of anaerobic reactors for DST Despite the recognized success of the UASB reactor as the most efficient and used anaerobic The increasing use of anaerobic reactors as the unit for the treatment of raw sewage, some limi- first unit of DST systems is mainly due to tation of this reactor configuration are evident. the successful use of UASB reactors for indus- Scum formation inside the GLS separator and trial wastewater treatment. The earlier reports on high losses of volatile suspended solids (VSS) in the application of UASB reactors for DST are the effluent are serious issues that deserve proper from the beginning of the eighties (Lettinga et al. attention for the improvement of reactor design, 1982). At the moment, the UASB reactor is thus presumably enabling enhanced reactor per- undoubtedly the most successful reactor for the formance. treatment of raw domestic sewage, due to both the absence of a fixed bed (avoiding clogging) and the presence of a SLG separator on the reac- 3.2. Options of pre-treatment tor’s top that prevents excessive solid losses. Even so, this application of the UASB reactor is The negative effects of poorly biodegradable sus- still confined to tropical and sub-tropical coun- pended solids on the methanogenic activity of the tries, where most developing countries are sludge were observed since the first experiences located. In fact, these countries constitute a privi- using the UASB reactor for the treatment of leged niche for the advantageous application of DST (Lettinga et al. 1982). Many alternatives anaerobic process as the core technology for were suggested to improve the digestion of DST. At temperatures higher than 20 °C and suspended solids. 7

3.2.1. Two-stage anaerobic process pre-screened sewage contains a substantial frac- In order to overcome this problem, two-stage tion of suspended solids and fatty matter. Cur- anaerobic processes have been proposed to retain rently, experiments on the use of forced screening and degrade suspended solids from sewage (van to reduce the size of suspended solids from raw Haandel & Lettinga 1994; Zeeman et al. 1997). sewage for the improvement of hydrolysis in an In the first stage, the particulate organic matter UASB reactor are under investigation at the is entrapped and partially hydrolyzed into solu- Federal University of Minas Gerais, Brazil. ble compounds, which are then presumably digested in the second stage. According to Zeeman et al. (1997), the combined system (two-stage 3.2.4. Two-step (sedimentation + digestion) anaerobic process) resulted in high removal effi- process ciency as compared to a single anaerobic reactor The first researchers that inferred about the need for (one phase UASB system). a pre-treatment for anaerobic DST were Lettinga et al. (1982), who suggested the use of a two-step 3.2.2. Chemically assisted sedimentation process: primary sedimentation for suspended solids Another option is the use of chemically assisted separation – and separate digestion of sludge – fol- sedimentation of raw domestic sewage followed lowed by a methanogenic step. To the best of our by anaerobic reactors. For instance, raw domes- knowledge, there are no reports on further devel- tic wastewater was treated by the combination of opments of two-steps processes for DST, probably a chemically enhanced primary treatment because practitioners and designers of the coun- (CEPT) followed by an UASB reactor (Kalogo tries where anaerobic reactors are applied look & Verstraete 2000). The CEPT (fed with either mainly for low-cost and easy operation DST FeCl3 or moringa oleifera seeds) increased the systems. It is worth noting that the separation of soluble COD/VSS ratio of the supernatant and a suspended solids in a primary sedimentation step net COD removal rate of 71% was achieved is a common practice in the design of aerobic when a HRT of 2 h and volumetric loading rate treatment systems. However, in a scenario where ) ) of 4 g COD L 1 day 1 was applied. In another the same primary settlers used in conventional aer- work integrating CEPT + UASB + zeolite obic systems are adopted (that means, primary cartridge, a CEPT (fed with FeCl3 and an anio- sedimentation and heated digesters), the problems nic organic flocculant) removed an average 73% due to hydrolysis (known as the limiting step) in of the COD, 85% of the total suspended solids anaerobic reactors can be minimized. In addition, 3) and 80% of PO4 present in the wastewater the adoption of pre-treatment units for solids (Aiyuk et al. 2004). The UASB system received a separation would allow the use of reactor configu- ) low COD (140 mg L 1) and with an HRT of 5 h rations other than the UASB reactor, allowing a very low effluent COD of approximately thereby the use of attached growth biomass reac- ) 50 mg L 1 was produced. Finally, the regenera- tors for sewage treatment, such as anaerobic filters ble zeolite cartridge removed almost 100% of (AF) and horizontal-flow anaerobic immobilized NH4 from the UASB effluent. The authors claim biomass (HAIB) reactors (Zaiat et al. 2000). As low construction and operating costs (estimated mentioned before, fixed-film reactors are especially at e 0.07–0.1 per m3 wastewater treated), pro- sensitive to influent suspended solids that cause posing this treatment system for developing bed clogging. On the other hand, the previous re- countries. moval of suspended solids would probably allow expanded/fluidized bed (Switzenbaun & Jewell, 3.2.3. Forced screening 1980) and expanded granular sludge bed (EGSB) Alternatively to two-stage or CEPT units, the (Kato 1994) reactors to operate at higher superfi- adoption of screens to reduce the size of influent cial velocities, increasing mass transfer and the particulate matter (suspended solids) has been overall kinetic. used (mainly in lab-scale and pilot plants) for the Therefore, the prior removal of suspended removal of a fraction of suspended solids that solids seems to be an efficient way to either interfere in the operation of pumps and other enhance the performance of UASB reactors or to equipments (Zaiat et al. 2000). Nevertheless, even allow the use of fixed bed anaerobic reactors. 8

3.2.5. Pre-treatment as a tool to allow anaerobic 4.1. Tilted plates DST in temperate climate countries It is well known that low temperatures restrict Tilted plates were installed on top of the SLG the anaerobic sewage treatment in temperate cli- separator of an UASB reactor in order to pro- mate countries, mainly because the hydrolysis mote the retention of flocculant biomass prone to step occurs at very low rates. This is particularly be washed-out from conventional UASB reactors true for suspended solids, oil and lipids. It is be- (Cavalcanti 2003). The results obtained in this lieved, however, that low temperatures would not work showed that indeed the retention of solids be a hindrance for the anaerobic treatment of is improved with a better design of the settling pre-treated domestic sewage. If a primary solid zone resulting in enhanced reactor performance, separation step is designed (as in aerobic reac- mainly when operating at short HRT (<8 h). tors) the problems due to hydrolysis (known as Under shorter HRTs, effluent COD values ob- the limiting step at low temperatures) in anaero- tained in a UASB reactor with tilted plates were bic reactors can be minimized. Although the about half of those obtained with no tilted plates activity of the biomass would remain low at psy- (Cavalcanti 2003). chrophilic conditions, the amount of biomass could be conceivably increased in reactor setups 4.2. Hybrid UASB reactor in order to achieve the needed overall removal rates of dissolved organic matter. It is worth not- Another interesting example of improved reactor ing that methanogenesis at very low temperatures design for the treatment of domestic wastewater (<10 °C) has been reported (Nozhevnikova et al. is the incorporation of a fixed bed (e.g., foam 2000). There are even reports on successful meth- matrices) filling the outer layer of the SLG sepa- anogenesis of low-strength wastewater at psy- rator of an UASB reactor (Elmitwalli et al. 2002; chrophilic conditions (10–12 °C) in an EGSB Passig & Campos 2004). Recent results at our reactor with volatile fatty acids (VFA) removal laboratories showed that an almost solid-free efficiencies exceeding 90% (Rebac et al. 1999), effluent was produced in such a unit (data not encouraging for positive expectations for the shown). In addition, if this chamber is aerated, it application of anaerobic reactors for sewage would be possible to proceed with nitrification treatment in temperate climate regions. A two- and sulfide oxidation, eliminating one of the step system (anaerobic filter followed by an important sources of odor in the UASB effluent. anaerobic hybrid reactor) provided suitable COD removal (71% for total COD) in the treatment of 4.3. Perforated submerged outlet devices domestic sewage at 13 °C (Elmitwalli et al. 2002). Thus, efficient pre-treatment units would allow a Generally the effluent liquid is collected superfi- broader range of temperatures for the efficient cially in open channels, causing problems of odor use of anaerobic reactors for DST. nuisance and, if poorly designed or constructed, preferential flow patterns may occur, resulting in reactor bulk short-circuits. The use of perforated 4. Improvement of UASB reactor design for DST submerged tubes discharging in flooded channels is a promising alternative because: (1) it de- As already pointed out, anaerobic reactor de- creases the losses in suspended solids in UASB signs were conceived primarily for the treatment effluents, and (2) minimizes the emission of H2S of high-strength wastewater. For instance, the to the atmosphere caused by the turbulence in UASB reactor was not designed for wastewaters the existing effluent collecting channels. With where the composition does not favor the forma- well-designed flooded outlet devices, a consider- tion of granules, despite the success of this appli- able fraction of the suspended solids can be re- cation for domestic wastewater treatment. As tained inside the reactor, reducing the effluent such, there is so far no reactor concepts specially TSS concentration (and accordingly, the raw designed for the treatment of sewage, although COD). In this case, a significant increase of some adaptations in established reactors concepts floated material on the reactor surface may were proposed: occur. To solve that, the use of appropriate 9 devices must be designed in order to remove (DAF) systems after an anaerobic reactor (Penetra floated solids from inside the reactor, so this et al. 1999). Reali et al. (2001) obtained 73% COD material can be directed to drying beds or other removal, 86% phosphorus removal and 98% tur- convenient units of the sludge management bidity removal in a DAF with FeCl3 as coagulant sub-system. (dosages from 65 to 30 mg L)1) plus 0.4 mg L)1 of nonionic polymer. Indeed, excellent effluent quality was achieved in this system, with residual 5. Post-treatment: COD polishing technologies COD of 23 mg L)1 and residual phosphate of 0.9 mg L)1 (Reali et al. 2001). Phosphate enriched 5.1. Biological post-treatment systems sludge is obtained from the flotation chamber surface, allowing the recovery of an important Additional COD (and BOD) and VSS removal resource. However, the removal of nitrogen is very have been obtained by coupling aerobic processes poor. In addition, the use of chemicals represents after the anaerobic reactor. Polishing ponds (PP; a drawback in respect to costs and sustainability Cavalcanti 2003), trickling filter (TF)systems issues when using DAF for the post-treatment of (Chernicharo & Nascimento 2001), submerged anaerobic effluents. aerated filters (SAF; Gonçalves et al. 1998), acti- Finally, it must be pointed out that most of vated sludge systems (ASS; Passig et al. 2000; von these COD polishing units only allow the recov- Sperling et al. 2001), rotating biological contac- ery of compounds of interest such as nitrogen, tors (RBC; Tawfic et al. 2002, 2003, 2004; phosphorus and sulfur if the post-treated efflu- Castilho et al. 1997), wetlands (Kaaseva 2004; ents are used for irrigation. Mbuligue 2004, de Sousa et al. 2001), radial-flow aerobic immobilized biomass reactor (RAIB; Vieira et al. 2003) and sequencing batch reactors 6. Biogas production and recovery (SBR Torres & Foresti 2001; Sousa & Foresti 1996) are among the most used post-treatment Considerable losses of methane observed in units or systems for anaerobic effluents treatment. UASB reactors treating sewage are mainly due to In most cases, any of these alternatives produce inappropriate GLS separator device design, con- effluents low in COD and TSS. In comparison to struction, and operation. Moreover, the inevita- these alternatives, multifunctional SBR, which ble loss of dissolved methane in the effluent operates in alternating anaerobic–aerobic condi- represents a considerable fraction of the total tions in the same batch cycle, are more compact methane produced from low-strength wastewa- and allows the removal of not only the remaining ters (Kobayashi et al. 1983). Apart from modifi- COD but also nutrients (Callado & Foresti 2001). cations in reactor setup, research is oriented to Although not extensively reported in the litera- the improvement of the hydrolysis of particulate ture, most of the existing full-scale DST plants in matter in order to improve the production and Brazil include an UASB reactor followed by acti- recovery of methane (Zeeman & Sanders 2001). vated sludge systems, submerged aerated filters or As stated before, the adoption of proper pre- stabilization ponds. These systems attain effluents treatment systems for solids retention and diges- with low-residual COD (<50 mg L)1) and the tion would improve the performance of the produced methane is normally burned in flares anaerobic reactor and also allow a more efficient due to the low biogas production. Therefore, a recovery of methane. In fact, primary sludge better utilization of the biogas (e.g., as electron digesters have been successfully used for organic donor in reductive processes) remains an impor- matter removal and methane recovery (Speece tant issue for its wise use in sustainable DST. 1988). Therefore, more efficient SLG separators, 5.2. Physical-chemical post-treatment systems modifications in the design of anaerobic reactors for improved particulate matter hydrolysis, or Additional COD and SSV (and phosphorus) modifications in the DST system by the adoption removal can be obtained also by the use of physi- of primary suspended solids separation and diges- cal-chemical treatment in dissolved air flotation tion would conceivably improve the treatment 10 system performance in terms of the production 7.2. Advances in nitrogen removal and recovery of methane. The few alternatives already proved to be efficient at lab-scale for nitrogen recovery in the + ) 7. Opportunities for nutrients (N, S, P) recovery form of dissolved nitrogen (NH4 ,NO2 , ) and removal NO3 ), such as adsorption columns (Aiyuk et al. 2004), would be costly for developing countries 7.1. Nitrogen and their adoption may constitute a drawback at the current stage of development. Therefore, the The recovery of nitrogen is quite complex due to formation of N2 obtained by the biological con- the high solubility of all nitrogen forms of inter- version of both reduced and oxidized nitrogen + ) ) est (NH4 ,NO2 ,NO3 ). It must be remem- forms seems to be the most economical way to bered that N2 is not a useful end product, remove nitrogen from domestic sewage. although this gaseous form is easily and safety Conventional nitrogen removal processes are removed from an environmental standpoint in based in two distinct biological processes: nitrifica- the liquid phase (Figure 2). Currently, scientists tion and denitrifrication. Initially, nitrification and and practitioners pursue ways to remove nitro- denitrification occurred in separate tanks, as it was gen in its dissolved form from anaerobic efflu- believed that specific environmental conditions for ents. Thus, adsorption seems to be the most each process were of ultimate importance for appropriated method for nitrogen recovery (Fig- successful conversion to gaseous N2 (Metcalf & ure 2). Hence, adsorption of nitrogen in its more Eddy 2002). The first progress towards a unique + reduced form (NH4 ) is advantageous because treatment unit for nitrogen removal was through this form predominates in anaerobic reactor the development of a modified activated sludge effluents. Recent publications present the use of process able to promote organic carbon, nitrogen + zeolite column for NH4 adsorption followed by and phosphorus removal in the same system setup, recovery of nitrate during column regeneration. in the so-called Bardenpho process (Barnard This seems to be a promising method for full- 1984). In the Bardenpho process, nitrification and scale applications (Aiyuk et al. 2004), including phosphorus uptake occur in aerated zones of a the possibility of nitrogen recovery use as a soil reactor, whereas denitrification occurs in anoxic fertilizer. zones of the same reactor. Variations of the

Nitrogen recovered as a concentrated + H S or CH - NH4 entrapped 2 4 stream of NO3 - and concentrated NO3 in a column (ex. 6 zeolite) 0 5 N2 /S O 2 O biogas 2 O2 Electron donor 1 + - - NH4 NO2 NO3 N2 3 4 2 Electron donor N /S0 2 - N2 /NO3

2- SO4

Figure 2. Possible ways for nitrogen removal/recovery from anaerobically treated sewage. Compounds in bold boxes are present in anaerobic reactor effluents. Dotted lines refer to processes where the recovery of the compound of interest is possible. (1) Conven- tional nitrification + denitrification for N removal. (2) Partial nitrification + denitrification (e.g., SHARON + denitrification with methanol). (3) Chemolithotrophic denitrification of nitrite with ammonia as e-donor (ANAMMOX). (4) Sulfate reduction via ammonia oxidation (SURAMMOX). (5) Chemolithotrophic denitrification with sulfide or methane. (6) Regenerable zeolite column to entrap and concentrate ammonia, followed by N recovery as nitrate-rich stream – Aiyuk et al. 2004. 11

Bardenpho process setup segregate aerated from by Banister and Pretorius (1998). Therefore, the anoxic tanks, as reviewed in Metcalf and Eddy need for adding a complementary amount of an (2002). In one of the various possible system con- external carbon source will probably remain, un- figurations, raw sewage (as the carbon source) is less ammonia is removed (for instance, by added in the first anoxic zones, avoiding the need adsorption) from the VFA rich stream. of addition of an external electron donor. The use of conventional nitrification and 7.2.2. Methane as e-donor denitrification processes without the addition of Although methane can be used as an electron an external electron donor for nitrogen removal donor for denitrification, the metabolic routes in- is not wise when an anaerobic reactor is the first volved in the process are still not completely biological unit in DST. This is because the understood. In the presence of low oxygen con- amount of organic matter in the effluent from centrations, the use of methane in the denitrify- the anaerobic reactor is normally lower than that ing process may occur according to two main needed for denitrification. Moreover, the organic mechanisms: (1) denitrifying organisms use meth- fraction of the effluent from an anaerobic reactor ane as electron donor and nitrate/nitrite as elec- is not readily biodegradable. This could be cir- tron acceptors, and oxygen does not participate cumvented by the use of raw sewage. However, in the process; and, (2) methanotrophic bacteria in order to provide enough organic matter read- produces intermediate organic compounds (e.g., ily available for denitrification, a big fraction of methanol) under low partial oxygen pressure that raw sewage must be derived for the nitrification/ are utilized by aerobic, anaerobic or facultative denitrification tank. Such a concept is similar to denitrifying bacteria. Both mechanisms have been the aforementioned Bardenpho process based on demonstrated in lab-scale experiments (Thalasso conventional aerobic technology, making unnec- et al. 1997; Costa et al. 2000; Islas-Lima et al., essary the anaerobic step, as it would treat only 2004). Denitrification using methane as the sole a small fraction of the raw sewage. electron donor under anoxic denitrifying condi- Thus, for nitrogen removal from anaerobic tions is reported to be dependent on the methane effluents of DST in the traditional nitrification/ partial pressure (Islas-Lima et al. 2004). denitrification design, there is a need for an exter- In our lab, experiments using methane as an nal electron donor. This would open the opportu- electron donor in a bench scale reactor operating nity to explore the use of electron donors in alternating aerated and non aerated steps produced in anaerobic reactors. Among such elec- showed that denitrification with methane pro- tron donors, VFA, methane, ammonia and sul- ceeds at similar rates as that obtained when fide are the natural candidates (Figures 1 and 2). using methanol or ethanol as electron donors (data not shown). As such, small concentrations 7.2.1. VFA as e-donor of oxygen were presumably present at the begin- VFA can be produced in hydrolytic reactors ning of the anoxic step. Interestingly, the denitri- receiving domestic sewage. In this way, only a fication with methane in experiments with the part of the influent goes to the methanogenic absence of oxygen was much slower than that reactor, but the very diluted nature of the efflu- obtained with methanol and ethanol (Santos ent makes such an alternative unpractical. On the et al. 2004). At first sight, this would suggest that other hand, concentrated streams of VFA (higher methane is firstly converted to methanol, al- than 1000 mg L)1) can be produced by the though this has not been proved so far. In con- hydrolysis and fermentation of the settleable sol- trast, other researchers affirm that denitrification ids separated in a primary sedimentation tank with methane in the absence of oxygen proceeds (Banister & Pretorius 1998; Ferreiro & Soto satisfactorily (Islas-Lima et al. 2004). Different 2003; A´lvarez et al. 2003). The preference for experimental setup might explain the different re- VFA is justified because they are among the sults, as Islas-Lima et al. (2004) found that the most appropriate electron donors for denitrifica- denitrification rate was independent of the meth- tion (Jonsson et al. 1996). Effluents from such ane partial pressure when superior or equal to hydrolytic-fermentative units, however, may also 8.8 kPa. As such, further research is warranted in contain high ammonia concentration, as found order to understand the use of methane as an 12 electron donor. In practice, the nitrification and et al. 1999). Although it is an intrinsically biolog- denitrification can be carried out in the same ical process, SND can be explained considering reactor unit (e.g., SBR reactors), so that there the nature of the phenomena that cause it to be will always have low amounts of oxygen remain- physical or biochemical. For some authors, SND ing from the former nitrification step, guarantee- occurs as a consequence of the existence of oxy- ing enough amounts of oxygen to promote the gen gradient concentrations inside granules and methanol formation. Nevertheless, the use of biofilms due to diffusional limitations. Nitrifying methane for denitrification has not been tested in microorganisms are located at regions of high full-scale plants so far, despite the availability of oxygen concentration while denitrifying microor- methane from anaerobic reactors treating domes- ganisms would be located at regions of low oxy- tic sewage. Currently the biogas is usually burned gen concentration. Other authors indicate there in flares. are microorganisms able to nitrify and denitrify under different environmental conditions in re- 7.2.3. Reduced sulfur compounds as e-donor spect to oxygen and carbon source (Zhao et al. Interactions between the sulfur and nitrogen 1999). Some claimed advantages of SND are: (1) cycles represent a real possibility of promoting for continuously fed reactors, SND eliminates the removal of both compounds from wastewa- the need of another reactor; (2) there is no need ters (Figure 2). Denitrification using sulfide as to change the operating conditions to provide a electron donor has already been suggested by suitable environment for the biomass either of Hulshoff Pol et al. (1998), whereas the simulta- the desired processes (nitrification or denitrifica- neous organic nitrogen and sulfate removal in an tion); (3) the time needed for full nitrification anaerobic reactor was reported by Fdz-Polanco and denitrification in SBR is shorter; and (4) et al. (2001). Liquid effluents of anaerobic reac- there is a considerable reduction in costs in view tors contain both sulfur and nitrogen in their of the fact that the requirement of oxygen and most reduced forms (ammonia and sulfide, alkalinity is lower (Mu¨nch et al. 1996). These au- Figure 1). The autotrophic oxidation of ammo- thors report on the application of bench-scale nia to nitrite and nitrate occurs mainly in aero- sequencing batch reactors for the complete treat- bic environments. Therefore, any sulfide present ment of pre-screened domestic sewage. They ob- in the liquid is also oxidized. However, the served the occurrence of aerobic denitrification at advantageous use of the interactions between the beginning of the aerobic step. They also veri- sulfur and nitrogen requires one of them to be in fied that nitrite predominates as the oxidized a reduced form (electron donor) and the other to nitrogen species. be in an oxidized form (electron acceptor). Theo- However, most of the information available retically, it is possible to derive part of the on SND refers to raw domestic sewage or syn- anaerobic effluent that is rich in reduced forms thetic substrate treatment. There is no report so of both sulfur and nitrogen to promote denitrifi- far on the application of SND for anaerobic cation and sulfate reduction in a separate tank effluents from full-scale DST plants. or in the anoxic zone or stage of a biological reactor. Another possibility is to return the bio- 7.2.5. Denitrification over nitrite gas that contains methane and sulfide to the The single-reactor high-activity ammonium denitrification reactor, which is currently being removal over nitrite (SHARON) process is an researched in our labs. alternative to conventional nitrification based on the fact that nitrification can be shortened if 7.2.4. Simultaneous nitrification–denitrification nitrite-oxidizing organisms are eliminated from Recent studies have demonstrated the possibility the reactor (Figure 2). In this case, the second of the occurrence of simultaneous nitrification– step (nitrate production) does not occur and ni- denitrification (SND). The SND process requires trite is the predominant oxidized form of nitro- smaller installations than the conventional pro- gen (Hellinga et al. 1998). The process operates cess and can represent a considerable economy, at high temperature (30–40 °C) and pH (7–8) mainly due to the lower oxygen consumption thus requiring strict operational conditions. The (Watanabe et al. 1995; Munch et al. 1996; Zhao ANAMMOX (anaerobic ammonia oxidation) 13 process involves the oxidation of ammonia to concentration of 820 mg L)1. The anaerobic SBR nitrogen gas using nitrite as the electron acceptor operated in cycles of 12 h and delivered an effluent (Schmidt et al. 2003; Mulder et al. 1995). Both with a COD concentration of about 240 mg L)1,a + )1 processes can be combined in a so-called com- NH4 concentration of about 35 mg L and pletely autotrophic nitrogen removal over nitrite phosphate concentration of about 25 mg L)1. The (CANON) process. In this process, a small frac- second SBR operated in 4 steps: 3-h aeration/3-h tion of the nitrogen involved in the reaction is anoxic/3.5-h aeration/2-h sedimentation with the released as nitrate (Khin & Annachhatre 2004; addition of 500 mg L)1 of acetate in the anoxic Pynaert et al. 2002). step (for denitrification and phosphorus removal). These intercalating aerobic/anoxic steps produced 7.3. Phosphorus a high effluent quality, with effluent COD concen- )1 + tration of about 45 mg L ,NH4 concentration 7.3.1. Enhanced biological phosphorus removal of about 2 mg L)1 and phosphate concentration Enhanced biological phosphorus removal (EBPR) of about 1 mg L)1 (Callado & Foresti 2001). has been achieved under alternating anaerobic- Thus, the SBR looks very attractive for simulta- aerobic conditions using polyphosphate-accumu- neous carbon polishing and nitrogen and phos- lating organisms (PAO) in the presence of an phorus removal for anaerobically treated sewage. easily assimilating organic carbon source (Comeau The drawback of this process is the need of an eas- et al. 1986). Most of the knowledge on the EBPR ily degradable carbon source at the beginning of process derived from modifications on activated the anoxic cycle phase. Therefore, research must sludge systems aiming to improve nutrient be oriented into the substitution of external car- removal. According to Zeng et al. (2004), the prin- bon source by soluble organic matter released in ciples of nutrient removal (nitrogen and phospho- other units of a treatment system (e.g., methane or rus) are well known, but recent findings have VFA from an anaerobic effluent; Figure 1). The revealed the existence of a much more intricate use of carbon sources produced in the DST plant system of biochemical processes involving simulta- would then increase the sustainability of this neous nitrogen and phosphorus removal. SBR technology for nutrients removal. reactors appear to be the most appropriate technology to provide biological nutrient removal 7.3.2. Physical-chemical methods (Surampalli et al. 1997). The imposition of a series In addition to biological processes for phospho- of aerobic intercalated by anoxic steps in the same rus removal, promising physical-chemical technol- batch cycle of a SBR causes the gradual selection ogies such as the DAF with the addition of of a specialized biomass able to promote the phos- coagulants and polymers were developed, as phorus luxury up-take, with the result that phos- already presented in this text (Figure 3). High- phorus is almost completely removed from the phosphate removal using physical-chemical units liquid (Kuba et al. 1996). In addition, nitrogen can is also reported for a CEPT (before an UASB be also removed by SND with phosphorus reactor) also using ferric chloride as the coagulant removal (Kuba et al. 1996). Despite the great (Aiyuk et al. 2004). Therefore, phosphate can be number of reports on the use of SBR systems for easily removed in physical-chemical methods be- integrated carbon, nitrogen and phosphorus re- cause phosphate precipitates are formed when a moval, there are only a few reports on the use of coagulant is added. Thus, the obvious drawback SBR for carbon polishing and nutrients removal of such technologies is that the addition of costly when an anaerobic reactor is installed as a pre- coagulant is needed. In addition, nitrogen com- treatment unit for DST. For instance, Callado and pounds are not removed in these systems. Foresti (2001) operated a system composed by an anaerobic SBR followed by another SBR operat- 7.3.3. Struvite formation ing under alternating anoxic/aerobic conditions. Finally, the controlled formation of struvite The synthetic influent (composed of carbohy- (MgNH4PO4 Æ 6H2O) in high pH environments drates: glucose, starch and cellulose; protein: meat (Figure 3) opens the opportunity for the simulta- extract; lipids: soy bean oil, and drops of deter- neous removal of ammonia and phosphate (in gent) simulated domestic sewage and had a COD the form of crystals). Formation of struvite has 14

Coagulant Electron donor ex. VFA e.g. Separation of ex. FeCl3 phosphate precipitate by DAF (Penetra et al., 1999) Poliphosphate precipitation Phosphate precipitation PO 3- 4 (luxury phosphorus uptake)

All phosphate precipitates, if Struvite formation properly enriched (e.g. K+), can be used as fertilizer, thus being a path for P recovery 2+ + Mg / NH4 / high pH

Figure 3. Possible ways for phosphorus removal/recovery from anaerobically treated sewage. Compounds in bold boxes are present in anaerobic reactor effluents. Dotted lines refer to processes where the recovery of the compound of interest is possible. already been demonstrated in a fluidized bed 7.4. Sulfur reactor using dewatered filtrate anaerobic sludge digestion (sewage treated in activated sludge sys- 7.4.1. Partial sulfide oxidation to elemental sulfur tems and the produced sludge is digested) where The conversion of soluble sulfide (HS)) into col- magnesium hydroxide is added in a magnesium loidal elemental sulfur in the liquid phase is possi- to phosphate ratio of 1:1 and the pH is adjusted ble in micro-aerated reactors and this technology to between 8.2 and 8.8 with the addition of so- is already available (Vallero et al. 2003; Figure 4). dium hydroxide (Ueno & Fuji 2001). A retention Based on the ability of colorless bacteria to oxi- time of 10 days allowed the growth of pellets be- dize sulfide partially to elemental sulfur, an aero- tween 0.5 and 1.0 mm in size. The recovered bic biotechnological sulfide- removing method struvite contained only minute traces of toxic was developed (Buisman et al. 1990). In order to substances and was used to enhance existing fer- obtain S0 as a product, sulfide oxidation must be tilizers, which are widely used on paddy rice, terminated at the sulfur formation step. This can vegetables and flowers (Ueno & Fuji 2001). The be accomplished, for example, by applying high formation of struvite from anaerobic reactor sulfide loads or low oxygen concentrations (Ste- effluents, however, has so far not yet been dem- fess et al. 1996). The application of such technol- onstrated, probably because the diluted stream ogy for the low sulfide containing anaerobic would make the process costly. effluents, however, seems doubtful. Nevertheless,

O2 O2

Heavy Metal (ex. Fe2+) 0 2- S SO4 HS- dissolved in the liquid S-free stream settler - MeS HS / H2S HS- dissolved in S0 rich sludge the liquid H2S present in the biogas S0 + Fe2+ settler

S0 rich sludge N /S0 2 Fe3+

- NO3 O2

Figure 4. Possible ways for sulfur removal/recovery from anaerobically treated sewage. Compounds in bold boxes are present in anaerobic reactor effluents. Dotted lines refer to processes where the recovery of the compound of interest is possible. 15 the post-treatment of anaerobic effluents are car- uble elemental sulfur. The formed ferrous iron is ried out in aerobic reactors, so that necessarily then directed to an oxygen-fed column for the sulfide is re-oxidized back to sulfate, which does regeneration to ferric iron (Figure 4). not offer any environmental risk at low concentrations. A possible option would be to introduce modifications in the outlet devices of the existing 8. Development of new reactor configurations anaerobic reactor configurations in order to promote oxygen limited regions so that the forma- Attempts to develop new reactor configurations tion of S0 is favored, thus, enabling its separation or to redesign the existing anaerobic reactors for from the liquid phase before post-treatment units. DST are difficult due to the complex characteris- tic of domestic sewage. As stated before, sus- 7.4.2. Formation of metal-sulfide precipitates pended solids and lipids (oil and grease) are the The soluble sulfide (HS)) can be easily removed main constituents of domestic sewage that hin- by means of the formation of insoluble metal ders the anaerobic process in high rate reactors. precipitates (Johnson & Hallberg 2005). This They are normally present in higher proportion would require, however, the addition of an exog- (in respect to the easily biodegradable organic enous source of heavy metals (e.g., Fe2+ ), so that matter fraction) than in most industrial wastewa- this process is economically and environmentally ters where anaerobic processes are successful. not viable. Nevertheless, there are alternative Thus, anaerobic reactors in DST are hindered by ways to promote the formation of metal precipi- the hydrolysis rather than by the cellular reten- tates, thereby removing sulfide from anaerobic tion time which is true for most industrial waste- effluents. For instance, one could consider stuff- waters. ing the channel receiving anaerobic effluent with The first emerging question refers in the scrap iron fillings, so that the effluent could flow improvement of existing reactor configurations in through this material provoking sulfide entrap- order to improve their performance, as already ment. To the best of our knowledge, however, presented for the UASB reactor (see item 4). In this has not been tested for domestic sewage fact, the best use of UASB reactors is for high anaerobic treatment so far. strength wastewaters and its successful use for DST is associated with their capacity to retain 7.4.3. Gaseous sulfide scrubbing and subsequent and digest a significant fraction of the influent elemental sulfur formation TSS. One of the most promising technologies for sulfide A second emerging question is whether pre- removal from biogases is a two-step process where treatment systems are to be adopted to allow gaseous sulfide is dissolved into the liquid in the anaerobic fixed-film reactors for DST. It is well first step, followed by sulfide oxidation to elemen- known that fixed film reactors are particularly tal sulfur (Figure 4). An alternative to the biologi- sensitive to suspended solids that tend to accu- cal oxidation of sulfide is the chemical oxidation mulate on the bed, deteriorating the reactor per- of aqueous sulfide to elemental sulfur by ferric sul- formance. Therefore, the previous removal of fate at low pH, which yields elemental, ortho- TSS is mandatory if fixed-bed anaerobic reactors rhombic a-sulfur (de Smul & Verstraete, 1999). are to be used as the core unit in DST. The process can be coupled to a membrane-as- As fixed-bed reactors are appropriated for the sisted extraction (e.g., permeable silicon) of H2S treatment of low strength wastewaters with low out of the liquid. After the removal of the sulfur concentration of TSS, they seem to be suitable from the ferric solution, the ferric solution can be for treatment of solid-free sewage. This would regenerated by aeration (de Smul & Verstraete open the possibility for the application of anaer- 1999). Currently experiments are being carried out obic technology to the removal of a fraction of in our laboratories feeding biogas from a UASB influent organic matter from sewage in temperate reactor treating sewage into a Fe3+-fed scrubber climate countries, substituting partially the role based on a previous work from Oprime et al. of the activated sludge system. In any case, post- (2001). Excellent sulfide removal (higher than 99%) treatment units have to be adopted. However, it is obtained due to the immediate formation of insol- is worthy of note that excess sludge produced in 16 activated sludge systems is one of the main prob- tended to deteriorate the reactor performance in lems that wastewater treatment plants face nowa- a way that was not predictable by the model. Al- days, so that the substitution of high sludge though not tested so far, the application of fixed- producing aerobic technology for low sludge pro- bed reactors for pre-treated solid-free sewage, ducing anaerobic technology must be pursued. using the same design procedures used by Zaiat Information on the performance of fixed film et al. (2000), seems to be feasible. reactors for the anaerobic treatment of sus- Attention must also be paid to the develop- pended solids free domestic wastewater at low ment of post-treatment units that incorporate the temperatures (lower than 15 °C) are scarce, prob- recent advances in nitrogen, phosphorus and sul- ably because temperate climate countries have fur removal/recovery. Despite the wealth of adopted the aerobic technology for soluble information recently gathered on nutrients organic matter removal. The development of removal/recovery, little is known about funda- fixed-film reactors operating at high superficial mentals and applications for the case of anaero- velocities and high cellular retention times seems bic reactor effluents treating domestic sewage. to be a proper way to overcome kinetic and mass Insights on fundamentals such as the physiology transfer limitations imposed by low ambient tem- and ecology of involved microbial population peratures. growing on anaerobically treated sewage will Current research pays attention to the selec- surely improve the design of post-treatment units tion of packing material for the fixed bed in or- for nutrients removal/recovery. der to not only seek the highest cellular retention The biggest constraint for the development of time possible but also favor the best arrangement new reactor configurations for DST on the basis of microorganisms, resulting in an organization of rational criteria are: (1) the complexity and in the most favorable way to enhanced mass and variability of domestic sewage; (2) the use of energy fluxes and turnovers (Picanço et al. 2001). mixed authochtonous microorganisms; (3) the complexity of some operating parameters; (4) 8.1. Rational basis to develop new anaerobic the poor quantification of active biomass in the reactors reactor, currently expressed as VSS; and (5) the parameters (COD, BOD) used to quantify or- A desirable situation for the development of new ganic matter, which simplifies excessively the reactor configurations for DST would be the mathematical model, possibly masking important establishment of rational criteria to design and information about organic matter degradation in scale-up anaerobic reactors based on mathemati- the reactor (such as hydrolysis). A rational model cal models extracted from bench or pilot-scale that considers all the pathways and microorgan- plants. The estimated kinetic parameters for isms involved in the process is difficult to obtain complex sewage degradation and mass transfer making the determination of all parameters with parameters would be used for design purpose. As precision troublesome. Moreover, the application mass transfer phenomena interfere with biochem- of an extremely complex model can be impracti- ical kinetic, most of the data available in the lit- cal for design proposals. Thus, the challenge is erature are not useful for scale-up reactors since the development of rational models that consider they are apparent and not intrinsic kinetic the complexity of anaerobic digestion, the physi- parameters (Zaiat et al. 1997). To obtain intrinsic cal and chemical interactions in the reactor and, parameters, the knowledge of the hydrodynamic at the same time, be practical and applicable for behavior of the reactor is essential. design and scale-up purposes. In this sense, Zaiat et al. (2000) developed It is obvious that it is impossible to consider and applied the HAIB reactor for the anaerobic all the variables related to anaerobic conversion treatment of pre-screened domestic sewage. 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