Post-treatment options for the anaerobic treatment of domestic wastewater

C. A. L. ChernicharoDepartment of Sanitary and Environmental Engineering, Federal University of Minas Gerais, Av. do Contorno842/701, 300110-060, Belo Horizonte, Brazil (e-mail: calemos@desa.ufmg.br; phone: +55-31-3238-1020;fax: +55-31-3238-1879)

Key words: anaerobic digestion, domestic wastewater, emerging technologies, new developments, post-treatment, UASB reactors, wastewater treatment

Abstract

This paper focuses on the post-treatment options for the anaerobic treatment of domestic wastewater.Initially, the main limitations of anaerobic systems regarding carbon, nutrients and pathogen removalare presented. In sequence, the advantages of combined anaerobic/aerobic treatment and the mainpost-treatment options currently in use are discussed, including the presentation of flowsheets and acomparison between various post-treatment systems. Lastly, the paper presents a review of emergingoptions and possible improvements of current post-treatment alternatives.

1. Introduction

A deep discussion on the evolution and applica-bility of the anaerobic technology for the treat-ment of domestic sewage is presented elsewhere(Lettinga et al., 1993; Seghezzo et al., 1998; vonSperling and Chernicharo, 2005), where the sev-eral favourable characteristics of the anaerobicprocesses are highlighted, such as low cost, oper-ational simplicity, no energy consumption andlow production of solids. These advantages, asso-ciated with the favourable environmental condi-tions in warm-climate regions, where hightemperatures prevail practically throughout theyear, have contributed to establish the anaerobicsystems, particularly the UASB reactors, in anoutstanding position.

Nowadays, it can be said that the high-rateanaerobic reactors used for treatment of domesticsewage are a consolidated technology in somewarm-climate countries, especially in Brazil, Colombiaand India, with several treatment systems operating infull scale (population equivalents from a few thou-sand up to around one million inhabitants). InBrazil, practically all the wastewater treatment

feasibility studies include anaerobic reactors asone of the main options. Undoubtedly, a greatcontribution to the consolidation and dissemina-tion of the anaerobic technology for the treatmentof domestic sewage in Brazil came from theNational Research Programme on Basic Sanita-tion – PROSAB, which has been carried out since1997 (Chernicharo et al. 2001a). Therefore, for thepurpose of this paper, the advances on post treat-ment of anaerobic effluents will be mainly fo-cussed on the Brazilian experience, that is believedto reflect also the reality of other warm climatecountries.

2. Main limitations of anaerobic systems

In spite of their great advantages, anaerobic reac-tors hardly produce effluents that comply withusual discharge standards established by environ-mental agencies. Therefore, the effluents fromanaerobic reactors usually require a post-treat-ment step as a means to adapt the treated effluentto the requirements of the environmental legisla-tion and protect the receiving water bodies.

Reviews in Environmental Science and Bio/Technology (2006) 5:73–92 � Springer 2006DOI 10.1007/s11157-005-5683-5

The main role of the post-treatment is tocomplete the removal of organic matter, as wellas to remove constituents little affected by theanaerobic treatment, such as nutrients (N and P)and pathogenic organisms (viruses, bacteria, pro-tozoans and helminths).

2.1. Limitations regarding organic matter

Limitations imposed by environmental agenciesfor BOD are usually expressed in terms of efflu-ent discharge standards and minimum removalefficiencies. These constraints are probably thecause that has mostly limited the use of anaero-bic systems (without post-treatment) for sewagetreatment (see typical values in Table 1).

In view of the limitations imposed by the envi-ronmental legislation for the effluent BOD concen-tration, or also when the receiving body haslimited capacity for assimilating the effluent fromthe treatment plant (which is frequently the case),it is usually necessary to use aerobic treatment tosupplement the anaerobic stage. However, thereare situations in which the combination of differ-ent anaerobic processes can meet less restrictiverequirements regarding efficiency and concentra-tion of the final effluent (e.g. 80% and60 mgBOD/l, respectively). This is the case of sys-tems consisting of a septic tank followed by ananaerobic filter (usually feasible for small popula-tions, generally fewer than 1000 inhabitants) or ofa UASB reactor followed by an anaerobic filter.Obviously, the application of these combinedanaerobic systems is conditioned to an appropri-ate dilution capacity of the receiving body.

In this sense, in situations in which the receiv-ing body presents a good dilution capacity, theadoption of less restrictive discharge standardscould enable the construction of simpler andmore economical treatment plants in severalsmall cities by means of a more intensive use ofanaerobic reactors, particularly UASB reactors.At a later stage, if it becomes necessary to pro-duce a better quality effluent, a complementarytreatment unit can be built after some years. Thehigh costs of sophisticated treatment systems, de-signed exclusively to meet BOD discharges stan-dards, make their construction at a single stageunfeasible for most cities located in developingcountries. On the other hand, the construction instages could be decisive and that systems consist-ing of UASB reactor and a post-treatment unitbecome the most feasible ones regarding techni-cal and economical criteria.

2.2. Limitations regarding nitrogenand phosphorus

The discharge of nutrients into surface waterbodies may cause increased algal biomass as a re-sult of the eutrophication process (abnormal al-gae growth due to the nutrients discharged). It isknown that 1.0 kg of phosphorus can result inthe reconstruction of 111 kg of biomass, whichcorresponds to approximately 138 kg of chemicaloxygen demand in the receiving body (Randallet al. 1992). Similarly, the discharge of 1.0 kg ofnitrogen can result in the reconstruction ofapproximately 20 kg of chemical oxygen demandunder the form of dead algae (Randall et al.1992). The problem can be even worsened due tothe decreased oxygen levels, by means of thenitrification processes, when at least 4.0 kg ofdissolved oxygen are consumed for each kg ofammonia discharged into the receiving body(Grady & Lim 1980).

In cases in which nutrient removal is requiredto meet the quality standards of the receivingwater body, the use of anaerobic processes pre-ceding a complementary aerobic treatment forbiological nutrient removal should be analysedvery carefully, once anaerobic systems presentgood biodegradable organic matter removal, butpractically no N and P removal efficiency. Thiscertainly causes a negative effect on biologicaltreatment systems aiming at good nutrient

Table 1. Usual effluent BOD and removal efficiencies in anaerobicsystems treating domestic sewage

Anaerobic system Effluent

BODa (mg/l)

BOD removal

efficiencya (%)

Anaerobic pond 70–160 40–70

UASB reactor 60–120 55–75

Septic tank 80–150 35–60

Imhoff tank 80–150 35–60

Septic tank followed

by anaerobic filter

40–60 75–85

aRanges of effluent concentration and typical removal efficien-cies based on Brazilian experience. Lower efficiency limits areusually associated with poorly operated systems (Chernicharoet al. 2001a).

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removal, because the effluent from the anaerobicreactor will have N/COD and P/COD ratios muchhigher than the values desired for the good perfor-mance of biological nutrient removal processes.

When the purpose of the treatment plant isalso a good nitrogen removal, the anaerobicreactor should be used to treat initially only apart of the influent raw sewage (possibly nomore than 50–70%), and the remaining part (30–50%) should be directed to the complementarybiological treatment, aiming at nitrification anddenitrification, so that there is enough organicmatter for the denitrification step. In this case,the great advantage of the use of the anaerobicreactor is to receive and stabilise the sludge gen-erated in the complementary treatment, eliminat-ing the need for an anaerobic sludge digester.

On the other hand, when the purpose is thebiological phosphorus removal, the use of ananaerobic reactor is not advisable for two mainreasons: (i) the effluent from the anaerobic reac-tor presents a P/COD ratio higher than that ofthe raw sewage, which harms the performance ofthe biological phosphorus removal system; and(ii) if the phosphorus-rich sludge generated in thebiological phosphorus removal treatment is direc-ted to the anaerobic reactor for stabilisation, thephosphorus incorporated to this sludge will bereleased under anaerobic conditions and leavewith the effluent from the anaerobic reactor. Thisfact makes an efficient phosphorus removalunfeasible in a treatment plant with an anaerobicreactor followed by complementary treatmentwith biological phosphorus removal.

Phosphorus removal in treatment plants usinganaerobic reactor will only be effective if chemi-cal products are used for P precipitation (iron oraluminium salts). In this case, the anaerobicreactor has the advantage of stabilising thesludge generated in the complementary biologicalaerobic treatment.

2.3. Limitations regarding microbiologicalindicators

Regarding the microbiological indicators, lowfaecal coliform removal efficiencies have beenreported in anaerobic reactors, usually amount-ing to around only 1 log-unit (Chernicharoet al. 2001c; von Sperling et al. 2002, 2004; vonSperling & Mascarenhas 2004). Regarding other

types of microorganisms, such as viruses andprotozoans (mainly Giardia and Cryptosporidi-um), there are few references covering theirreduction or elimination in anaerobic reactors.The removal of helminth eggs in anaerobicreactors, particularly in UASB reactors, hasbeen reported as amounting to 60–90%(Chernicharo et al. 2001c; von Sperling et al.2002, 2004), being therefore insufficient to pro-duce effluents that may be used in irrigation.However, it should be mentioned that theselimitations are not exclusive of anaerobic reac-tors, but are a characteristic of most compactwastewater treatment systems.

As the risk of human contamination by inges-tion or contact with water containing pathogenicorganisms is high, many times it may be neces-sary to disinfect the effluents. This fact becomeseven more serious due to the poor sanitary con-ditions in developing countries. On the otherhand, the low investments in health and sanita-tion make the population of these countries bear-ers of several diseases that can be transmitted byfaeces and, consequently, by the sewage gener-ated by this population.

Although the domestic sewage is an unques-tionable source of contamination by pathogenicorganisms, it is worth of mention that the agentsused in the disinfection processes can also causeharm to human health and the aquatic environ-ment. It is then concluded that the decision toeither disinfect or not sewage should be takenfrom a careful evaluation, based on the specificcharacteristics of each situation. In other words,there are no universal guidelines ruling sewagedisinfection requirements. The decision on theneed to disinfect the sewage of a certain localityinvolves (USEPA 1986):• an investigation on the uses of the water down-stream the discharge point, and on the publichealth risks associated with that water;

• an evaluation of the alternatives available forpathogens removal from sewage;

• an evaluation of the environmental impacts thecontrol measures may cause.Figure 1 presents a flowsheet that can aid the

decision making on the implementation need andrequirements of a sewage disinfection system, tak-ing into account the public health risks involvedand the possibility to either reduce or eliminatethese risks. Once the risks involved are identified,

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the environmental aspects start to determine theapplicability of the control alternative.

In cases where disinfection proves to be nec-essary, a series of processes for the removal ofpathogenic organisms can be used as listed inTable 2. Only short comments are made for eachprocess, since the removal of pathogenic organ-isms, especially by artificial methods, is outsidethe scope of this paper.

The processes listed in Table 2 are capableof reaching a coliform removal of 99.99% ormore. Regarding pathogenic organisms, bacteriaremoval efficiency is very high (equal to orhigher than coliform removal), and the otherpathogens (protozoa, virus, helminths) areusually high, but variable, depending on the

removal mechanism and the resistance of eachspecies (USEPA 1986).

3. Advantages of the combined (anaerobic/aerobic)

systems

In comparison with a conventional wastewatertreatment plant consisting of primary sedimenta-tion tank followed by aerobic biological treat-ment (activated sludge, trickling filter, submergedaerated biofilter or biodisc), with the primaryand secondary sludge passing through sludgethickeners and anaerobic digesters prior to dewa-tering, a treatment consisting of a UASB reactorfollowed by aerobic biological treatment (with the

Figure 1. Flowsheet for local evaluation of the need for and requirements of sewage disinfection (adapted from USEPA 1986).

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secondary sludge directed to thickening and diges-tion in the UASB reactor itself and then straightto dewatering), can present the following advanta-ges (Alem Sobrinho & Jordao 2001):• the primary sedimentation tanks, sludge thick-eners and anaerobic digesters, as well as alltheir equipment, can be replaced with UASBreactors, which do not require the use of

equipment. In this configuration, besides theirmain sewage treatment function, the UASBreactors also accomplish the aerobic sludgethickening and digestion functions, requiringno additional volume;

• power consumption for aeration in activatedsludge systems preceded by UASB reactorswill be substantially lower compared to

Table 2. Main processes for the removal of pathogenic microorganisms in sewage treatment

Type Process Comment

Natural Maturation ponds • Shallow ponds, where the penetration of solar ultraviolet radiation and unfavourable

environmental conditions causes a high mortality of the pathogens.

• The maturation ponds do not need chemical products or energy, but require large areas.

• They are highly recommended systems (if there is area available), due to their great

simplicity and low costs.

Land treatment

(infiltration in soil)

• The unfavourable environmental conditions in the soil favour the mortality of the

pathogens.

• In slow-rate systems, there is the possibility of vegetable contamination, depending on the

type of application.

• Chemical products are not needed.

• Requires large areas.

Artificial Chlorination • Chlorine kills pathogenic microorganisms (although protozoan cysts and helminth eggs

are not much affected).

• High dosages are necessary, which may increase operational costs. The larger the previous

organic matter removal, the lower the chlorine dosage required.

• There is a concern regarding the generation of toxic by-products to human beings.

However, the great benefit to public health in the removal of pathogens must be taken into

consideration.

• The toxicity caused by the residual chlorine in the water bodies are also of concern. The

residual chlorine must have very low levels, frequently requiring dechlorination.

• There is large experience with chlorination in the area of water treatment in various

developing countries.

Ozonisation • Ozone is a very effective agent for the removal of pathogens.

• Ozonisation is usually expensive, although the costs are reducing, making this alternative a

competitive option in certain specific circumstances.

• There is less experience with ozonisation in most developing countries.

Ultraviolet radiation • Ultraviolet radiation, generated by special lamps, affects the reproduction of the patho-

genic agents.

• Toxic by-products are not generated.

• Ideally, the effluent must be well clarified for the radiation to penetrate well in the liquid

mass.

• This process has recently shown substantial development, which has made it more com-

petitive or more advantageous than chlorination in various applications.

Membranes • The passage of treated sewage through membranes of minute dimensions (e.g. ultrafil-

tration, nanofiltration) constitutes a physical barrier for the pathogenic microorganisms,

which have larger dimensions than the pores.

• The process is highly interesting and does not introduce chemical products into the liquid.

• The costs are still high, but they have been reducing significantly in recent years.

(Adapted from von Sperling and Chernicharo 2005).

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conventional activated sludge systems, andespecially extended aeration systems;

• thanks to the lower sludge production inanaerobic systems and to their better dewater-ability, sludge volumes to be disposed of fromanaerobic/aerobic systems will be much lowerthan those from aerobic systems alone.According to studies carried out by Pontes(2003), a 30% VSS destruction can be reachedwhen secondary sludge produced in a tricklingfilter is returned to a UASB reactor. When themass balance is performed, the total sludgeproduction in a combined UASB/Trickling Fil-ter system can be 30–50% lower than in a con-ventional trickling filter system.

• the construction cost of a treatment plant withUASB reactor followed by aerobic biologicaltreatment usually amounts 50–80% of the costof a conventional treatment plant (20–50%investment savings). In addition, due to the sim-plicity, smaller sludge production and lowerpower consumption of the combined anaerobic/aerobic system, the operational costs also repre-sent an even greater advantage. Savings onoperation and maintenance costs are usually inthe range of 40–50% in relation to a conven-tional treatment plant (see Table 4 and vonSperling & Chernicharo 2005).

4. Main post-treatment options currently in use

Taking into consideration the intrinsic limitationsassociated with the anaerobic systems and theneed to develop technologies that are moreappropriate to the reality of developing coun-tries, it is important to include a post-treatmentstage for the effluents generated in anaerobicreactors. This stage has the purpose of polishingnot only the microbiological quality of the efflu-ents, in view of the public health risks and limi-tations imposed on the use of treated effluents inagriculture, but also the quality in terms of or-ganic matter and nutrients, in view of the envi-ronmental damages caused by the discharges ofthe remaining loads of these components into thereceiving bodies.

Some of the main possible combinations ofUASB reactors with post-treatment systems arediscussed in the following items. It can be ob-served that in the UASB+activated sludge and

UASB+biofilm aerobic reactor systems, the aer-obic biological excess sludge is simply returnedto the UASB reactor, where it undergoes diges-tion and thickening with the anaerobic sludge,dispensing separate digestion and thickeningunits for the aerobic sludge. Hence, the overallsludge production of the combined system iswasted only from the UASB reactor. Since it isalready thickened and stabilised, and can be di-rectly sent for dewatering and final disposal.Sludge drying beds have been frequently used insmall-sized plants. Thus a large simplification inthe overall flowsheet is obtained, including the liquid(sewage) and solid (sludge) phases.

4.1. Polishing pond

Facultative ponds are largely used for post-treat-ment of effluents from anaerobic ponds. Thesesystems have the advantage of removing at ahigher efficiency the pathogenic organisms pres-ent in the sewage, but their main disadvantagesare excessive land requirement and the high con-centration of algae in the final effluent, whichleads to serious restrictions by some environmen-tal agencies.

When an efficient anaerobic pre-treatment isapplied prior to the sewage discharge into apond, the concentrations of organic matter andsuspended solids are largely reduced, and conse-quently it will be required only a complementaryremoval of these two constituents, needing muchlower hydraulic detention times. In these condi-tions, the limiting factor that determines the min-imum detention time (and, therefore, the volumeand the area of a pond system) will usually bethe removal of pathogenic organisms, and notthe stabilisation of the organic matter. For thisreason, the nomenclature polishing pond has beenadopted to name those ponds intended forthe post-treatment of effluents from efficientanaerobic systems, thus distinguishing them fromthe stabilisation pond, which treats raw sewage(Cavalcanti et al. 2001a, b).

The UASB reactor+polishing pond configu-ration is a very interesting alternative fromthe technical–economical–environmental point ofview, mainly when there are area limitations forthe construction of only stabilisation ponds. Inaddition, the problems related to odours fromanaerobic ponds can be avoided in plants utilising

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UASB reactor and polishing pond, since theanaerobic reactor can be installed with odourcontrol (Cavalcanti et al. 2001a, b). This alterna-tive is even more attractive when the effluentfrom the pond can be used for agricultural pur-poses, since the polishing ponds aim mainly at theremoval of pathogenic organisms. Because of itsadvantages, the post-treatment of effluents fromanaerobic reactors through ponds has been com-mon in developing countries.

Wastewater treatment plants using UASBreactors followed by polishing ponds also have avery simplified flowsheet (Figure 2). Besides thepreliminary treatment units (screen and gritchamber), the flowsheet comprises the anaerobictreatment unit, the polishing pond (either a singlebaffled pond or ponds in series), and the dewa-tering unit for the sludge produced in the UASBreactor which is already thickened and stabilised.Thus, dewatering units using drying beds are alsousual in smaller plants.

Long term studies conducted by von Sperlingand Mascarenhas (2004) have shown that adomestic sewage treatment system comprised of aUASB reactor followed by four very shallow(0.40 m-depth) polishing ponds in series, oper-ated with very low detention times (1.4–2.5 daysin each pond), was able to achieve excellent re-sults in terms of BOD and E. coli removal, andalso good results in terms of ammonia removal.The average concentrations observed in the finaleffluent were 44 mgBOD l)1, 3.8�102 MPN100 ml)1 and 7.3 mgN–NH4 l

)1. In relation tohelminth eggs, other studies have shown thatpolishing pond systems are capable to produceeffluents with helminth eggs concentrations pre-dominantly equal to zero, and satisfying the

WHO guidelines for unrestricted and restrictedirrigation (von Sperling et al. 2002, 2004).

Since polishing ponds are designed with lowdepths (0.40–1.00 m) and relatively short HDT(usually between 9 and 12 days in a series of 3–4ponds), care should be taken to the operation ofthe UASB reactor in order to avoid excessivewash out of solids.

4.2. Overland flow system

Sewage treatment by the overland flow methodis the one that presents the lowest relationshipwith the type of soil. In this method, the vege-tation, associated with the top soil layer, acts asa filter, removing the nutrients and providingconditions for the retention and transformationof the organic matter contained in the sewage.Besides that, it protects the soil against erosionand creates a support layer on which the micro-organisms settle. The main mechanisms throughwhich organic matter and solids are removedare biological oxidation, sedimentation and fil-tration (USEPA 1981, 1984; Metcalf & Eddy1991). The main characteristic that differentiatesthis method from the others is the fact that theeffluent flows downward on a slightly inclinedvegetated ramp and the remaining water (efflu-ent), which is neither absorbed nor evaporated,is collected downstream and directed for dis-posal. For more permeable soils, the process issimilar to that of irrigation, but with the gener-ation of effluent.

Therefore, the method consists in applyingthe liquid in the highest part of the ramp. Theeffluent then drains all over the slope by gravity,where part of it is lost by evapotranspiration and

Figure 2. Typical configuration of a treatment plant with UASB reactor and polishing ponds (von Sperling & Chernicharo 2005).

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the remaining part is collected on the base of theramp. Percolation can be insignificant becausethis system is initially conceived for low-perme-ability soils. In spite of that, its use has been alsoreported in soils with medium permeability andimpermeable underground (USEPA 1981). Sew-age application is intermittent and the followingtypes of feeding can be adopted: (i) high pressuresprinklers; (ii) low pressure sprinklers; (iii) distri-bution piping or channels with spaced openings.

The typical configuration of a wastewatertreatment plant consisting of a UASB reactorand post-treatment by overland flow has a verysimple flowsheet (Figure 3). Besides the pre-liminary treatment units, the flowsheet comprisesthe anaerobic treatment unit, the land treatmentsystem and the dewatering unit for the sludgeproduced in the UASB reactor. The same consid-erations made for the polishing ponds, regardingthe characteristics of the anaerobic sludge, that isalready thickened and stabilised, are also validhere. Dewatering units using drying beds can beused in small-sized plants.

Studies carried out by Coraucci Filho et al.(2000) and Chernicharo et al. (2001c) with overlandflow systems following anaerobic filters and UASBreactors, respectively, operated under applicationrates varying from 0.10 to 0.50 m3 m)1 h)1, haveconducted to average concentrations in the finaleffluent ranging from 98 to 119 mgCOD l)1,48 to 62 mgBOD l)1 and 17 to 57 mgTSS l)1,respectively). In relation to the microbiologicalquality of the final effluent, an excellent removalof helminth eggs in the UASB/overland flow sys-tem was observed, with an average counting of0.2 egg l)1 in the final effluent. Regarding faecalcoliforms, the removal was only satisfactory,

with the whole treatment system removing 2–3log-units.

4.3. Activated sludge

The essence of the continuous flow activated sludgeprocess is the integration of the aeration tank(aerobic biological reactor), secondary sedimenta-tion tank and sludge recirculation line. Thesethree components are maintained in the alterna-tive of activated sludge systems acting as post-treatment of effluents from anaerobic reactors.

The intermittent flow activated sludge system(sequencing batch reactors) can also be adoptedas post-treatment, requiring, in this case, only thetanks that alternate in the functions of reactionand sedimentation. Recent developments regard-ing the application of such system for the post-treatment of anaerobic effluents are discussed inthe last item of this paper.

When the activated sludge system acts aspost-treatment of anaerobic effluents, the anaero-bic reactor is used instead of the primary sedi-mentation tank (which is an integral part of theconventional activated sludge system). The aero-bic sludge is recirculated in the usual manner,that is, from the bottom of the secondary sedi-mentation tank to the entrance of the aerobicreactor (aeration tank).

The excess aerobic sludge generated in theactivated sludge stage, not yet stabilised, is sentto the UASB reactor, where it undergoes thick-ening and digestion, together with the anaerobicsludge. As the return flow of the excess aerobicsludge is very low compared with the influentflow, there are no operational disturbances in theUASB reactor. The sludge treatment is largely

Figure 3. Typical configuration of a treatment plant with UASB reactor and overland flow system (von Sperling & Chernicharo2005).

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simplified: there is no need for separate thicken-ers and digesters, and just the dewatering stage isnecessary. The mixed sludge removed from theanaerobic reactor is digested, has solids concen-trations similar to those from sludge thickenersand presents good dewaterability. Figure 4 pre-sents the flowsheet of this configuration.

4.4. Submerged aerated biofilter

A submerged aerated biofilter consists of a tankfilled with porous material, through which sew-age and air flow permanently. In almost all theexisting processes, the porous medium is main-tained totally submerged by the hydraulic flow.The biofilters are characterised as three-phasereactors consisting of:• solid phase: consisting of the support mediumand colonies of microorganisms present in theform of a biofilm;

• liquid phase: consisting of the liquid in perma-nent flow through the porous medium;

• gas phase: formed by the artificial aerationand, in a reduced scale, by the gases derivingfrom the biological activity.Sewage treatment plants that use UASB reac-

tors followed by submerged aerated biofilters alsopresent a simple flowsheet (Figure 5). Besides thepreliminary treatment units, the flowsheet comprisesthe sequential anaerobic and aerobic biological treat-ment units (UASB reactor and submerged aeratedbiofilter), as well as the aeration, sludge accumula-tion, and dewatering units. Also in this configura-tion, the excess aerobic sludge removed from thebiofilter is returned to the UASB reactor for thicken-ing and anaerobic digestion. Therefore, with thisflowsheet, primary sedimentation tanks and separateunits for thickening and anaerobic digestion of theexcess aerobic sludge are avoided, differently fromthe conventional treatment plants that use sub-merged aerated biofilters.

Studies conducted by Goncalves et al. (2000)have shown that UASB/submerged aerated bio-filter systems are capable of maintaining stable

Figure 4. Typical configuration of a treatment plant with UASB reactor and activated sludge system (von Sperling & Chernicharo2005).

Figure 5. Typical configuration of a treatment plant with UASB reactor and submerged aerated biofilters (von Sperling &Chernicharo 2005).

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operational conditions despite influent load vari-ations and recycle of aerobic sludge dischargedfrom the BF. The average concentrations ofCOD and TSS in final effluent are usually keptbelow 90 mgCOD l)1 and 30 mgTSS l)1.

4.5. Trickling filter

A trickling filter consists basically of a tank filledwith a highly-permeable material, onto whichwastewater is loaded under the form of drops orjets. Wastewater percolates towards the bottomdrains, allowing bacterial growth on the surfaceof the packing material, under the form of afixed film (biofilm). Wastewater passes over thebiofilm, allowing a contact between the microor-ganisms and the organic matter.

Although the trickling filters (TF) are waste-water treatment systems with great potential andnumerous advantages, mainly because of theirsimplicity and low operational cost, few unitshave been implemented so far with the purposeof performing the post-treatment of effluentsfrom anaerobic reactors (von Sperling and Cher-nicharo 2005).

The main and innovative purpose of the re-searches developed in the last years was to evaluatethe applicability and behaviour of the tricklingfilters, when used for polishing of effluents fromanaerobic reactors, particularly UASB reactors.This association (UASB reactor+TF) may contrib-ute significantly to the reduction of the power andoperational costs of the treatment plant.

Wastewater treatment plants that use UASBreactors followed by trickling filters present asimple flowsheet (Figure 6). Basically, besides thepreliminary treatment units, the flowsheetcomprises the sequential anaerobic and aerobic

biological treatment units (UASB reactor, trick-ling filter and secondary sedimentation tank), aswell as the dewatering unit. Therefore, with thisflowsheet, primary sedimentation tanks and sepa-rate units for thickening and anaerobic digestionof the excess aerobic sludge are avoided, differ-ently from the conventional treatment plants thatuse trickling filters.

Results of the researches developed by Aisseet al. (2000), Chernicharo and Nascimento (2001),and Pontes et al. (2003) indicated that thattrickling filters as post-treatment unit of UASBreactors can be satisfactory operated at OLRup to 1.5 kgBOD m)3 day)1 and HLR up to20 m3 m)2 day)1. Under those operating condi-tions, UASB+TF systems are usually capable ofproducing a final effluent with average COD, BODand TSS concentrations around 120 mgCOD l)1,40 mgBOD l)1 and 30 mgTSS l)1. If nitrification isdesired, much lower OLR should be used.

4.6. Anaerobic filter

Until recently, the anaerobic filters were limitedto small populations, usually treating effluentsfrom septic tanks. Nowadays, anaerobic filtersafter UASB reactors are being used even in citieswith population larger than 50,000 inhabitants(von Sperling & Chernicharo 2005). The comple-mentary organic matter removal achieved in thesecond anaerobic reactor (anaerobic filter) occursby:• the retention of solids in the anaerobic filter,reflecting on the removal of particulate matter.In this case, physical removal mechanismsprevail through the combined effects of coarsefiltration in the packing medium and sedimen-tation along the column;

Figure 6. Typical configuration of a treatment plant with UASB reactor and trickling filter (von Sperling & Chernicharo 2005).

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• the formation of biofilm on the packing med-ium and removal of the remaining soluble or-ganic matter. In this case, the formation ofbiofilm and the removal of carbonaceous mat-ter by biochemical means depend on theamount of organic matter present in the efflu-ent from the UASB reactor.This association of anaerobic processes con-

tributes greatly to the reduction of power andoperational costs of the treatment plant. Waste-water treatment plants using UASB reactors fol-lowed by anaerobic filters represent a very simpleflowsheet (Figure 7). Besides the preliminarytreatment units (screen and grit chamber), theflowsheet comprises basically the two sequentialanaerobic treatment units (UASB reactor andanaerobic filter) and the dewatering unit. This isbecause the sludge produced in the anaerobicunits is already thickened and stabilised.

Chernicharo and Machado (1998) and And-rade Neto et al. (2000) have been evaluatingthe use of pilot and demonstration scale anaer-obic filters (AF) for the post-treatment ofanaerobic effluents from septic tanks andUASB reactors. Different packing materialshave been investigated, such as blast furnaceslag (40–60 mm), perforated construction bricks,granite stones (50–75 mm) and pieces of wastedelectrical tubing (cut in small pieces). The re-sults of these researches indicate that anaerobicfilters with packing medium height between0.80 and 3.0 m and operated at HDT rangingfrom 5 to 10 h can further reduce particulateand soluble organic matter of the incominganaerobic effluent, being able to maintain thefinal concentrations of COD, BOD and TSSusually below 120 mgCOD l)1, 60 mgBOD l)1

and 30 mgTSS l)1, respectively.

The combination of anaerobic filters (AF) andanaerobic hybrid (AH) reactors have been re-searched by Elmitwalli et al. (2002a, b), aiming atto investigate the treatment of domestic sewage atlow temperature of 13�C. The AF/AH systemoperated at a HRT of 4+8 h, respectively, pro-vided high removal efficiencies for all COD frac-tions, reaching a total COD removal efficiency ashigh as 71%, with 60% of the removed CODbeing converted to methane.

4.7. Dissolved air flotation

The post-treatment of anaerobic effluents by dis-solved-air flotation (DAF) was investigated usingbatch flotation test equipment (Reali et al. 2001)and also in a demo-scale (240 m3 day)1) continu-ous flow system composed by an expanded bedanaerobic reactor followed by a DAF unit treat-ing domestic sewage (Penetra et al. 2002). In thelatter experiment, the use of 50 mg l)1 FeCl3 ascoagulant and flocculation under the gradient of80 s)1 associated with a retention time of 20 minconducted to the best results: 94.4% CODremoval (53 mgCOD l)1 residual), 87% phos-phorus removal (0.80 mgP l)1 residual), 96.7%TSS removal (9 mgTSS l)1 residual).

The use of DAF units for post-treatment ofanaerobic effluents results in a very compacttreatment system (Figure 8) that is capable ofproducing very high quality effluents in terms ofCOD, TSS and phosphorus. However, the re-moval of ammonia nitrogen and faecal coliformsis poor. In relation to the sludge produced inDAF units, the amount tends to be higher thanthe ones observed in biological post-treatmentsystems, but it usually presents higher solidscontent, favouring its final disposal in landfills.

Figure 7. Typical configuration of a treatment plant with UASB reactor and anaerobic filter (von Sperling & Chernicharo 2005).

83

4.8. Constructed wetlands

Constructed wetlands are purposely built waste-water treatment processes, which consist ofponds, basins or shallow canals (usually with adepth of less than 1.0 m) that shelter aquaticplants, and use biological, chemical and physicalmechanisms to treat the sewage. The constructedwetlands usually have an impermeable layer ofclay or synthetic membrane, and structures tocontrol the flow direction, hydraulic detentiontime and water level. Depending on the system,they can contain an inert porous medium such asstones, gravel or sand.

The subsurface flow wetlands seem to bemore appropriate to receive effluents from septictanks and anaerobic reactors because of its lowerpotential for the generation of odours and theappearance of mosquitoes and rats. For effluentsfrom anaerobic reactors the land requirementsare around 2.5–4.0 m2/hab (von Sperling &Chernicharo 2005).

The association of anaerobic reactors and con-structed wetlands contributes greatly to the reduc-tion of power and operational costs of the

treatment plant. Besides, the wastewater treatmentplant represents a very simple flowsheet (Figure 9).

Results of the 1-year research developedby Sousa et al. (2001), with a UASB/constructedwetland system, showed average COD removalefficiencies in the range of 79–85%, suspendedsolids in the range of 48–71% and faecal coli-forms around 4 log-units. Phosphorus was alsoefficiently removed (average of 90% for the low-est hydraulic load) but nitrogen removal wasonly partial (45–70% for ammonia and 47–70%for TKN).

5. Comparison between various post-treatment

options

Tables 3–7 (adapted from von Sperling & Chernicharo2005) present a comparative analysis between themain systems applied to the post-treatment ofeffluents from UASB reactors, as follows:• Quantitative comparison (Table 3): averageeffluent concentrations and typical removal effi-ciencies of the main pollutants of interest indomestic sewage

Figure 8. Typical configuration of a treatment plant with UASB reactor and DAF (von Sperling & Chernicharo 2005).

Figure 9. Typical configuration of a treatment plant with UASB reactor and constructed wetland (von Sperling & Chernicharo 2005).

84

Table

3.Averageeffl

uentconcentrationsandtypicalremovaleffi

ciencies

ofthemain

pollutants

ofinterest

indomesticsewage

System

Averagequality

oftheeffl

uent

Averageremovaleffi

ciency

BOD

5

(mg/l)

COD

(mg/l)

TSS

(mg/l)

Ammonia

(mg/l)

TotalN

(mg/l)

TotalP

(mg/l)

FC

(FC/100ml)

Helminth

eggs(eggs/l)

BOD

5

(%)

COD

(%)

TSS

(%)

Ammonia

(%)

TotalN

(%)

TotalP

(%)

FC

(logunits)

UASB

reactor

70–100

180–270

60–100

>15

>20

>4

106–107

>1

60–75

55–70

65–80

<50

<60

<35

1–2

UASB+

activated

sludge

20–50

60–150

20–40

5–15

>20

>4

106–107

>1

83–93

75–88

87–93

50–85

<60

<35

1–2

UASB+

submerged

aeratedbiofilter

20–50

60–150

20–40

5–15

>20

>4

106–107

>1

83–93

75–88

87–93

50–85

<60

<35

1–2

UASB+

highrate

tricklingfilter

20–60

70–180

20–40

>15

>20

>4

106–107

>1

80–93

73–88

87–93

<50

<60

<35

1–2

UASB+

anaerobic

filter

40–80

100–200

30–60

>15

>20

>4

106–107

>1

75–87

70–80

80–90

<50

<60

<35

1–2

UASB+

dissolved-air

flotation

20–50

60–100

10–30

>20

>30

1–2

106–107

>1

83–93

83–90

90–97

<30

<30

75–88

1–2

UASB+

polishing

ponds

40–70

100–180

50–80

10–15

15–20

<4

102–104

<1

77–87

70–83

73–83

50–65

50–65

>50

3–5

UASB+

overland

flow

30–70

90–180

20–60

10–20

>15

>4

104–106

<1

77–90

70–85

80–93

35–65

<65

<35

2–3

Notes:

1.Ranges

ofeffl

uentconcentrationandtypicalremovaleffi

ciencies

consideringsystem

sproperly

designed

andoperated.

2.Chem

icalprecipitationofphosphoruswithanyofthetechnologiesabove:

P<

1mg/l.

3.Disinfection:e.g.chlorination,ozonisation,UV

radiation;Barrier:e.g.mem

branes

(provided

thedisinfection/barrierprocess

iscompatiblewiththequality

oftheeffl

uentfrom

theprecedingtreatm

ent):CF<103FC/100ml;helminth

eggs:variable.

(Adaptedfrom

vonSperling&

Chernicharo

2005).

85

Table

4.Typicalcharacteristics

ofUASBreactorandvariouspost-treatm

entsystem

s,expressed

asper

capitavalues

System

Landrequirem

ents

(m2/inhab)

Power

foraeration

Sludgevolume

Costs

Installed

power

(W/inhab)

Consumed

power

(kWh/inhab.year)

Liquid

sludgeto

be

treated(l/inhab.year)

Dew

ateredsludge

tobedisposedof

(l/inhab.year)

Construction

(US$/inhab)

Operationand

maintenance

(US$/inhab.year)

UASBreactor

0.03–0.10

00

70–220

10–35

12–20

1.0–1.5

UASB+

activatedsludge

0.08–0.2

1.8–3.5

14–20

180–400

15–60

30–45

2.5–5.0

UASB+

submerged

aerated

biofilter

0.05–0.15

1.8–3.5

14–20

180–400

15–55

25–40

2.5–5.0

UASB+highrate

tricklingfilter

0.1–0.2

00

180–400

15–55

25–35

2.0–3.0

UASB+

anaerobic

filter

0.05–0.15

00

150–300

10–50

20–30

1.5–2.2

UASB+

dissolved-airflotation

0.05–0.15

1.0–1.5

8–12

300–470

25–75

25–35

2.5–3.5

UASB+

polishingponds

1.5–2.5

00

150–250

10–35

15–30

1.8–3.0

UASB+

overlandflow

1.5–3.0

00

70–220

10–35

20–35

2.0–3.0

Notes:

1.CostsbasedonBrazilianexperience

(basis:year2002–US$1.00=R$2.50).

2.Per

capitacostsare

applicableinsidethetypicalpopulationranges

within

whicheach

treatm

entsystem

isusuallyapplied

(usually,foracertain

system

,thelower

thepopulation,

thegreatertheper

capitacosts).

3.Additionaldisinfection:constructioncosts–increase

US$2.0

to4.0/inhab;operationalandmaintenance

costs:increase

US$0.2

to0.6/inhab.year.

4.In

compact

aeratedsystem

s(e.g.:activatedsludge,submerged

aeratedbiofilters)orafter

treatm

entwithaUASBreactor,aerationcontrolallowsacertain

economy(notallthe

installed

power

isconsumed).

(Adaptedfrom

vonSperling&

Chernicharo

2005).

86

• Quantitative comparison (Table 4): typical char-acteristics of the main sewage treatment sys-tems, expressed in per-capita values

• Diagrammatic comparison (Tables 5 to 7):capacity of the various sewage treatment sys-tems in consistently reaching different qualitylevels in terms of BOD, COD, TSS, ammonia,total nitrogen, total phosphorous, faecal coli-forms and helminth eggs

6. Emerging options and possible improvements

of current post-treatment alternatives

Aerobic post-treatment of anaerobic effluentprovides further reduction of residual organicsand nitrification of ammonia. A denitrification

step is necessary in order to remove nitrate andnitrite produced from nitrification. The chal-lenge of adopting a post-treatment system totreat an anaerobic effluent is to find a proper,reliable and efficient process that is simple inconstruction, operation and maintenance; havelower capital costs; have capacity to remainstable under both hydraulic and organic shockloads; and is energy efficient (Tai et al. 2004).Although researches have contributed a lot forthe understanding, improvement and develop-ment of post-treatment processes in the lastdecade, the main contributions were related toorganic matter and pathogen removal. How-ever, due to the substantial organic material re-moval reached in the anaerobic step, biological

Table 5. Capacity of UASB reactors and various post-treatment systems in consistently achieving the indicated levels of effluentquality in terms of BOD, COD and TSS

Table 6. Capacity of UASB reactors and various post-treatment systems in consistently achieving the indicated levels of effluentquality in terms of Ammonia, total N and total P

Table 7. Capacity of UASB reactors and various post-treatment systems in consistently achieving the indicated levels of effluentquality in terms of Faecal (thermotolerant) coliforms and Helminth Eggs

87

removal of nitrogen (by nitrification/denitrifica-tion) and phosphorus (by luxury uptake)becomes problematic due to the lack of biode-gradable organic carbon. Hence, the maindrawbacks of current technologies are stillrelated to nutrient removal and that is thereason why most recent studies have beenfocusing on this subject as discussed herein.

6.1. Sequence batch reactors (SBR)

In studies developed by Callado and Foresti(2001), almost complete removal of COD, nitro-gen and phosphorus were accomplished in anaer-obic–aerobic systems composed of SBRs inseries. Nitrification, denitrification and biologicalphosphorus removal may occur in the secondSBR treating the effluent of the first anaerobicSBR supplemented with acetate, operating underaerobic–anoxic cycles. The results obtained inbench-scale unit opens the possibility of usingvery simple systems to promote the completetreatment of domestic wastewater. The first unitcan be an anaerobic SBR or any other configura-tion, since the effluent quality of anaerobic reac-tors is not expected to change drastically fromone configuration to another. However, the needof supplementary addition of an external carbonsource for denitrification and biological phos-phate removal makes this alternative inconve-nient from the sustainability point of view. Onthe other hand, the external carbon source canbe alternatively produced from the controlleddigestion of sludge and domestic solid wastes.These considerations imply in changes on theconception of environmental control systems tointegrate solid and liquid wastes treatment.

Van Haandel and Guimaraes (2000) evaluatedthe sequencing batch reactor as an alternative foraerobic post-treatment. Even though the requiredretention time was very low (an anaerobic HDTof 5 h and an aerobic HDT of 2.4 h) proved tobe sufficient to produce consistently a very higheffluent quality (BOD and TSS <20 mg l)1). Theproduced activated sludge maintained fair togood settling properties and no bulking was ob-served. The authors showed that nitrification athigh sludge age did not cause problems duringthe settling period: during settling the denitrifica-tion rate was low (no extra cellular material), so

that not enough nitrogen was formed to causesludge flotation.

6.2. Hybrid systems

An alternative to the treatment in activated sludgesystem is the utilization of high capacity hybridsuspended biomass-biofilm systems. These systemshave been successfully employed not only to up-grade low nitrifying capacity wastewater treatmentplants, but also as a new technology to developcompact systems for simultaneous nitrogen andorganic matter removal. Further improvement inthese systems include the replacement of the finalsettler by membrane filtration units, as in the con-figuration proposed by Oyanedel et al. (2002) thatincludes an anoxic chamber with suspendedbiomass, followed by an aerobic circulating bedreactor (CBR) which contains biofilm and sus-pended biomass. The aerobic reactor is coupled toa vessel containing a hollow fibre ultrafiltrationmembrane module that allows the separation ofthe permeate (effluent) from a retentate (sludge)that is recycled to the anoxic chamber. With thissystem it was possible to reach high COD andnitrogen removals and no solids in the final effluent.

6.3. Rotating biological contact (RBC)

Tawfik et al. (2002) tested a three stage pilot-scaleRBC for removal of E. coli, COD and ammoniafrom anaerobically pre-treated domestic sewage.The three stage RBC system operated at a HDTof 10.0 h and an OLR of 5.3 gCOD m)2 day)1

was capable of producing a final effluent containing43 mgCOD l)1, 3.3 mgNH4-N l)1 and 2.0�103E. coli/100 ml. The authors pointed out that theremoval of E. coli in a RBC system comprises: (i)sedimentation of coarse particles; (ii) adsorptiononto the biofilm; and (iii) predation by ciliatedprotozoa.

Also working with a RBC system, but forthe post-treatment of an ammonium rich anaer-obic effluent, Pynaert et al. (2002) reported thatthe inoculation of the RBC with methanogenicsludge favoured nitrogen removal via oxygen-limited oxidation of ammonium with nitrite asthe electron acceptor. The authors state that

88

the experiment confirms the property of thissystem to remove ammonium to nitrogen gaswithout the use of heterotrophic carbon source.

6.4. Expanded granular sludge bed reactors

The feasibility of using EGSB reactor for thepost-treatment of very low concentration anaero-bic effluents was evaluated by Kato et al. (2002). Apilot-scale 157.5-L EGSB reactor treating the efflu-ent from a full scale UASB reactor and operatingat 4-h HDT and upflow velocity of 3.75 m h)1

was capable of producing a final effluent with to-tal and soluble COD concentration below 87 and55 mg l)1, respectively, and TSS below 32 mg l)1.The authors also reported stable operation duringall experimental period.

6.5. Fixed bed reactors and alternative supportmaterials

Daniel and Foresti (2004) carried out studies in afixed bed reactor filled of polyurethane foam, fedwith synthetic substrate simulating an anaerobiceffluent with a high concentration of ammonium.The results obtained with the reactor operated ina sequential batch mode and each cycle com-posed by aerobic and anoxic periods indicated astable process of nitrogen removal. It was possi-ble to establish partial nitrification to nitrite andcomplete denitrification.

Machdar et al. (1997, 2000), Araki et al.(1999) and Uemura et al. (2002) present thedevelopment of the downflow hanging sponge –DHS – reactor, tested for the aerobic post-treat-ment of effluents from UASB reactors treatingmunicipal wastewater. In its fourth generation(Tandukar et al. 2005), the DHS reactor wasconstituted of slabs containing long sponge stripsmeasuring 2.5�2.5�50 cm, which were thenstacked one above another but in direction 90�to each other to make 20 rows. This was consid-ered a module with 300 sponge units and 39%occupancy of the sponge by volume. Four suchmodules were put one above the other with acertain gap in between for the construction ofthe whole reactor, giving a height of 4 m. Themain improvements in the reactor were related tothe enhancement of air dissolution into thewastewater and to avert the possible clogging ofthe reactor. The whole system was operated at a

total hydraulic retention time of 8 h (UASBreactor: 6 h; DHS: 2 h) being capable of remov-ing 96% of unfiltered BOD and 3.45 log-units offaecal coliforms. The authors also reported ahigh nitrification degree during the start-up peri-od, with NH4–N removal over 56%. As HDTwas decreased and organic and hydraulic loadsincreased after the start-up period, the NH4–Nremoval efficiency dropped to less than 30%.The DHS reactor accommodates both nitrifiersand denitrifiers giving way to simultaneous nitri-fication and denitrification.

A novel radial anaerobic/aerobic immobilizedbiomass reactor using biogas constituents as elec-tron donor was investigated by Garbossa et al.(2005), aiming at simultaneous carbon and nitro-gen removal from municipal wastewater. Theresearch meant to confirm that methane andsulphide present in the biogas could be used aselectron donors for denitrification, as previouslysuggested by other authors (Thalasso et al. 1997;Islas-Lima et al. 2004). The bench-scale reactorwas divided into five concentric chambers withthe second and fourth chambers filled with poly-urethane foam matrices for biomass immobiliza-tion. Promising results were obtained, with meanCOD and TKN removal efficiencies of 90% and92% being observed. Average COD, N-TKN andN–NO3 were 44, 3.2 and 1.9 mg l)1, respectively.

6.6. Jet loop reactor (JLR)

A novel two-stage anaerobic/aerobic integratedsystem consisting of an UASB reactor and jetloop reactor (JLR) was developed by Tai et al.(2004) aiming at complementary removal oforganics and simultaneous removal of TotalKjeldahl Nitrogen and Total Nitrogen. The JLRis a sort of aeration tank that incorporates a re-cycle line with a venture and a draft tube, thusallowing the introduction of air drawn from theatmosphere and mixing of the reactor contents.In this system configuration the UASB reactor isused to achieve denitrification and methanogene-sis, by means of recycling the nitrified effluent(taken from the secondary clarifier) jointly withthe influent wastewater to the UASB reactor.The authors claims that the combined systemwas capable of removing more than 85% of totalBOD and COD and more than 95% of solubleBOD and COD. The JLR presented average

89

removals of 94.0 and 95.4% of TKN at HDTs of 8and 5 h, respectively. The UASB-JLR achieve anaverage of 78.1% TN removal with a 4:1 recycleratio at a combined system HDT of 58.8 h.

6.7. Membrane bioreactors (MBR)

Another interesting post-treatment option is theuse of micro and ultrafiltration membranesassociated with anaerobic reactors, aiming at toincrease the quality of the final effluent and tomaintain biomass inside the anaerobic reactorwith greater efficiency. In Membrane AnaerobicBioreactors (AnMBR), enhanced biomass reten-tion can be accomplished by membrane-basedseparation techniques (Jeison & van Lier 2005),favouring the increase of the mean cell residencetime and improving the conditions for the degra-dation of low degradable compounds. In lastyears, important advances have been made in thedevelopment of new types of membranes withreduced costs, and research is being carried outin order to find better reactor configurations andoperational procedures that decrease energyconsumption and fouling (Hernandez et al. 2002;Beal & Monteggia 2004; Fitzke et al. 2004;Jeison & van Lier 2005).

6.8. Advanced oxidation processes (AOP’s)

The use of advanced oxidation processes can alsobecome an interesting alternative for post-treat-ment of anaerobic effluents. The AOP’s involvethe generation of hydroxyl radicals which have ahigh oxidation potential and attacks organicmolecules by either abstracting a hydrogen atomor by adding to double bonds, thus allowing itsmineralization to non-toxic forms such carbondioxide or water. Studies carried out by Siggeet al. (2002) demonstrated the feasibility of thisprocess in further reducing the COD contents ofanaerobic effluents, when using ozone and ozone/hydrogen peroxide in combination with a granu-lar activated carbon contacting column. Colourand COD reductions ranged from 66 to 90% andfrom 27 to 55%, respectively.

6.9. Two stage flotation

Tessele et al. (2004) presented an importantalternative for the improvement of conventional

dissolved air flotation (DAF) process, proposinga two stage flotation unit. In this configuration,the first flotation stage is intended to remove sus-pended solids by the flocculation–flotation pro-cess. This flotation technique was originallydeveloped for oil removal and is based on theformation of aerated flocs, in the presence ofhigh molecular weight polymer under high shear.The second stage flotation removes phosphate byprecipitation and coagulation with Fe(FeCl3) andalso acts as a polishing step, separating the resid-ual fine solids. In trial studies, the application of5.0–7.5 mg l)1 cationic flocculant was capable toseparate off more than 99% of the suspendedsolids, while phosphate ions were completelyrecovered using carrier flotation with 5–25 mg l)1

of Fe(FeCl3) at pH varying from 6.3 to 7.0. Thestaged flotation leads to high recoveries of waterand allows separating organic matter and phos-phate bearing sludges. In contrast, the conven-tional DAF produces significant volumes ofmixed organic and inorganic sludge which maylead to complex post sludge treatment either toreuse or to dispose of.

7. Conclusion

The fundamental and practical results obtainedso far have effectively contributed to consolidatethe anaerobic technology as the first stage treat-ment for domestic and municipal sewage, andalso to offer a series of post-treatment alterna-tives that take into account the social, economi-cal and environmental aspects of mostdeveloping countries. Recent developments andfurther research on nutrient removal will soonovercome the few drawbacks that still remain,which are challenging a wider application ofcombined anaerobic/aerobic, anaerobic/anaero-bic and anaerobic/physico-chemical systems fordomestic sewage treatment.

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