~ Pergamon

PH: 50273-1223(97)00197-2

Wal. Sci. T~ch. Vol. 35. No.9, pp. 191-198, 1997.C 1997 IAwQ. Published by Elsev,er Science Ltd

Pnnled in Great Bntain0273-1223197 $17'00 + 0'00

A SUSTAINABLE MUNICIPALWASTEWATER TREATMENT PROCESSFOR TROPICAL AND SUBTROPICALREGIONS IN DEVELOPING COUNTRIES

Hanqing Yu, Joo-Hwa Tay and Francis Wilson

School o/Civil & Structural Engineering. Nanyang Technological University, 639798,Singapore

ABSTRACf

In this paper. a feasible municipal wastewater treatment process. using the upnow anaerob,c sludge blanket(UASB) or the anaerobic barned reactor (ABR) as an anaerobic pre-treatment system. and the reed bed or thestabilization pond with supporting media as a post-treatment system. is presented and discussed. Resultsobtained in pilot- and full-scale treatment plants clearly reveal that the anaerobic treatment is indeed a veryattractive option for muniCipal wastewater pre-treatment at temperatures exceeding 20C in tropical and sub­tropical regions. The UASB system has been commonly employed as an anaerobic pre-treatment system. TheABR provides another potential for the anaerobic pre-trealment. The effluents from the anaerobic treatmentsystem should be post-treated to meet discharge standards. Because of the advantages of the reed bed systemwhen il is employed for tertiary treatment. this system could be conSidered as a post-treatment system.Another cost-effective system. the stabilization pond packed with attached-growth media. is also a potentialpost-treatment system. @ 1997 IAWQ. Published by Elsevier Science Ltd

KEYWORDS

ABR; anaerobic; municipal wastewater; post-treatment; reed bed; stabilization pond: temperature; UASa.

INTRODUCTiON

Rapid urbanization in developing countries has led to very high population densities, and stressed theaccommodation capacity of the surrounding environment in urban areas. The main objectives of sanitationstrategies in these growing cities are to improve public health, and to avoid surface water and groundwaterpollution (Alaerts et al., 1993). This results in the necessity to implement appropriate sanitation approachesto the specific conditions for removing oxygen-consuming organic matters and nutrients from municipalwastewater. Therefore, it becomes more and more important to develop appropriate wastewater treatmentprocesses which combine a high treatment efficiency with low capital investment, maintenance costs, andsimple operation requirements. At the present time, aerobic processes are widely used for municipalwastewater treatment. However, aerobic processes have serious drawbacks including considerableinvestment, operation and maintenance costs and high sludge production (Lettinga et aI" 1993), Because oflack of enough financial source and shortage of skilled manpower for facility operation, most developingcountries can not afford to construct and operate enough municipal wastewater treatment plants (lza et al.,1991). Fortunately, anaerobic treatment processes provide another option for municipal wastewater

191

192 H. YU erat.

treatment. Anaerobic treatment processes not only consume lower grade energy, but also produce usefulenergy in the form of biogas (Henze and Harremoes, 1983). As a consequence, anaerobic treatment ofmunicipal wastewater seems to be an ideal and sustainable solution for water environmental protection.However, post-treatment is necessary if the effluents from the municipal wastewater anaerobic treatmentsystem is discharged into surface waters because the anaerobic treatment alone is not able to produceeffluents that can meet the discharge standards.

In this paper, based on the relevant research results, a feasible municipal wastewater treatment process,using the VASB or the ABR as anaerobic pre-treatment system, and the reed bed or the stabilization pondwith supporting media as a post-treatment system, is presented and discussed.

ANAEROBIC TREATMENT OF MUNICIPAL WASTEWATER

Over the last decade, new advanced anaerobic reactor systems, such as VASB, anaerobic filter (AF),anaerobic fluidized bed (FB) and expanded bed (EB) reactors, with a significant difference between thehydraulic retention time (HRT) and solids retention time (SRT) have been developed (Henze andHarremoes, 1983). These systems offer good opportunities for the treatment of a large variety of high andmedium strength industrial wastewaters, and also enable treatment of low strength wastewaters such asmunicipal wastewater at ambient temperatures (Iza et al., 1991).

The concept of anaerobic treatment of municipal wastewater was first developed in the early 1980s fromresearch work by Lettinga and his co-workers, who were looking at the feasibility of anaerobic treatmentusing VASB at ambient temperatures (Lettinga et aI., 1983). Since then, a considerable amount of researchhas been carried out on municipal wastewater treatment using various anaerobic reactor types. Among thedifferent high-rate reactors, the FBIEB reactor proved to be the most efficient with excellent effluent qualityat a very short HRT (Maragno and Campos, 1992). A slightly lower efficiency results from with the use ofVASB reactors and AFs (Mergaen et al., 1992). So far, few full-scale FBIEB systems have beencommissioned because of high energy demand for fluidization and the difficulty in construction andoperation (lza et al., 1991). The VASB design has been considered to be the most attractive reactor systemdue to its simplicity, low investment and operation cost as well as long experience in the treatment of a widerange of industrial wastewaters (Lettinga and Hulsffof, 1991).

Compared to the conventional aerobic process for municipal wastewater treatment, the anaerobic processhas the following main advantages (Alaerts et al., 1993; Henze and Harremoes, 1983; Iza et al., 1991;Mergaen et aJ., 1992):

- simple in construction, operation and maintenance.- less energy requirements- less excess sludge production- the sludge is well stabilized- removal of some compounds which the aerobic process does not remove.

Anaerobic process has the following main disadvantages (Lettinga etaJ., 1993; Mergaert etai., 1992):

- it leaves a higher residual biological oxygen demand (BOD), chemical oxygen demand (COD), nitrogenand phosphorus- ~t IS very sensitive to lower ambient temperatures- It recovers very slOWly after a toxic shock- design and operation experience of full-scale treatment is still very limited.

The efficient treatment of municipal wastewater by anaerobic systems depends largely on the followingfactors (Kooijrnans et al., 1986; Lettinga et aJ., 1993; Mergaert et ai., 1992):

Municipal wastewaler trealment process

- the characteristics (organic concentration. temperature, etc.) of municipal wastewater- the fluctuations in the composition and in the flow of municipal wastewater- the amount and activity of anaerobic biomass- the intensity of contact between the organic matters and the biomass- the HRT of municipal wastewater in the reactor.

Table I. Summary of pilot-scale uasb reactor experimental results with municipal wastewater

Reactor T HRT CODiol' CODcII' Reduction (%1 ReferenceVolume (I) ("C) (h) (mgl\) (mgl\) COD BOD TSS

120 12-16 24 688 150 55-75 55-80 I160 20 6 467 245 50 54 2120 16·18 7 181 72 89 3120 8-10 8 154 55 50 3106 J5 4 300 120 65 72 61 4110 12·18 18 465 163 (i5 73 5120 19-28 4 627 163 74 78 72 6160 20 6 1076 310 64 88 7

I. Lettinga et al. (1983). 2. Deryck. Ind Ventnlele (1986); 3. LeUmga elll (1993). 4 V,e,ra (1992); 5.Monroyet al. (1988); 6 Barbosa and Sanl'AnnI (1989), 7. Mergaert eill. (1992).

Table 2. Performance results of full-scale uasb reactors treating municipal wastewater

Bergambocht Bergambachl Sao Bn<lmllnongl Cah Bombay Klnpurnle nle Plulo

NetlleTlands Netherlands Bruzil Colomb,. Colomb," b,di. hld,"(I) (2) (J) (4) (5) (6) (7)

Reoctor volwne(m') 20 6 120 35 64 200 1200

HRT(h) 10 8 4.7·9.0 5 6 12 6T("C) 15-16 15-19 21·25 23-27 25 15-25 20-30Loadmg rale(lcgCOOiln'd) 0.4-0.9 0.5-1.0 07

ReJuctiOltCOD (0/0) 49 55 70 66 78 49-78 62-70BOD(%) 80 80 66 69.83 65·75TSS(%) 60 60 79 70 75 68 67.79

Efflu,nl92-198 91·103COD (mgll) 170·303 220 96 145 120

BOD (mgll) 40-110 31 39 25 22·55 50-56TSS (mgll) 51 43·80 3S 70 30 111 134

SludgeProdllclion(lcglkgC0D;,J 0.17-0.34 0.18-0.36 0.15-0.2 0.1 0.43

I. Letting. el at. (1983); 2. Lelling. el al. (1983); 3. Vieira (1988); 4. SchelJinkhoul el 81. (1988); 5.Kooijmans cl al. (1986); 6. Joshi CI al. (1987); 7. Draaijcr el al. (1992).

Specific attention should be paid to the temperature of municipal wastewater. The anaerobic process issensitive to low temperatures (Henze and Harremoes, 1983). The removal and degradation of the colloidalfraction of the organic pollution. which may account for 25-30% of the total COD in municipal wastewater.appear to become limiting at lower temperatures (Mergaert tt al., 1992). Besides, when municipalwastewater is treated, the rate of hydrolysis is a crucial factor, because it is the rate-limiting step inanaerobic digestion at temperatures below 20·C. Therefore, the anaerobic process is appropriate for thetreatment of municipal wastewater in tropical or sub-tropical regions.

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194

UASB system

H. YU ~Iat.

Originally the UASB reactor was developed for industrial wastewaters with a medium and high organicmaterial concentration (Lettinga and Hulsffof, 1991). There are tremendous applications of UASB systemsto various industrial wastewaters. The cornmon application of UASB system is due to its high organicremoval efficiency. its low capital investment and land requirement, and its very simple operation (Lettingaand Hulsffof, 1991). The UASa is also the most widely used high-rate anaerobic municipal wastewatertreatment. A summary of pilot-scale UASa reactor experiment results is given in Table 1. Most UASBreactors were inoculated with granular sludge from other UASB reactors or digested activated sludge. Allthe investigations show fair removal efficiencies of COD (50-75%), BOD (54-78%) and TSS (50-89%).

Since 1986, several full-scale UAsa systems have been employed for the treatment of municipalwastewater. At present a couple of UASB systems are in full operation and many more are underconstruction in Brazil (Vieira and Garcia, 1992), Colombia (Schellinkhout and Collazos, 1992), India(Draaijer et al., 1992) and Indonesia etc. (Alaerts et al., 1993). The largest UASa units so far are 2200 m3reactors, treating 5000 m3 municipal wastewater per day with a HRT of 8 h and a removal of 60-80% BOD(Draaijer et ai., 1992). The performance results of some of these full-scale UASa reactors are summarizedin Table 2.

On the basis of the investigation results from the pilot- and full-scale UASB systems, some operationalexperiences have been summarized by Lettinga et ai. as below (1993).

- The start-up of a UASB reactor treating municipal wastewater can be accomplished at an HRT of about 6h with a period of 6-12 weeks without using any seed sludge.- The treatment efficiencies at a HRT of 5-6 b that can be achieved after completion of start-up are: 55-75%COD, 65-80% BOD, 67-81 % TSS and over 70% pathogens.- The CH4·gas production amounts to 0.19 m3/kg CODremova!' Over 50% of the methane actually producedleaves the reactor with the effluent solution because of the diluted character of municipal wastewater.- The excess sludge production amounts to 0.4 - 0.6 kg TSS/kg TSSin. The excess sludge is very wellstabilized and exerts excellent drying characteristics.

Anaerobic baffled reactor

Among the modem high-rate anaerobic reactors developed, the ABR looks prorrusmg for municipalwastewater. The ABR was first developed by Bachmann and McCarty (1983), it was described as a series ofUASBs. This design consisted of a series of vertical baffles to force wastewater to flow upwards through aseries of compartments containing the mixed anaerobes as they passed from the influent to the effluent. Thebench-scale ABR has been found to be effective for the treatment of high as well as low strength solublewastewaters (Grobicki and Stuckey, 1991). The results published have demonstrated that the ABR has manyadvantages. such as: very low concentrations of volatile fatty acid (YFA) in the effluent and lower bedbypass (Garuti et ai., 1992); a high degree of sludge retention; a higher tolerance to shock loads and stablereactor performance (Bachmann et al., 1983); and is simple in construction and requiring maintenance andoperation allention requirement (Polprasert, 1992).

Considering the advantages of the ABR and the characteristics of municipal wastewater, a modified ABRmay be an economical anaerobic system for municipal wastewater treatment for tropical and subtropicalareas of developing countries. An investigation was carried out at lab-scale to explore the feasibility of theuse of such a modified ABR for municipal wastewater treatment at ambient temperatures (Yu and Anderson,1996).

The reactor consists of three 3.6-1 chambers ( O.lm x O.lm x 0.36m ). The first chamber is a UASa without agas-solid-liquid separator, the second one is a down flow fixed film reactor filled with plastic media, whilethe third one is a hybrid UASB-AF with plastic Paul ring media located in the top 3/5 of it. Raw municipalwastewater from Durham City, Britain, was used for this lab-scale experiment. The raw wastewater was

Municipal wastewater treatment process 19S

collected from a municipal wastewater lreatment plant and brought to the laboratory. The raw wastewaterwas pre-settled and then pumped into the reactor. The characteristics of the settled wastewater are presentedin Table 3.

Table 3. The characteristics of the settled wastewater

COO(mg/l)

338·S16

COO,(mg/l)

2S7-338

NH.-N(mg/l)

22-38

VFA(mg/l)

20-53

SS(mg/l)

21-41

Alkalinity(mgtl)

320-410

SO,,- pH(mg/l)60-85 6.7-7.3

The performance of the reactor for municipal wastewater treatment at ambient temperatures was evaluatedin terms of substrate removal, biogas and methane production, VFA profiles, sludge production and effluent55, etc. The conclusions drawn from this investigation are as follows:

- When pre-settled wastewater is used as influent with digested sludge as seed, at a HRT 11-16 h andtemperature 18'C-28'C, the start-up of the reactor could be completed within two months.- In the course of steady-state operation, the COD and CODf removal efficiencies varied from 83.5 % and82.8 % at a HRT of 4 h to 67.8 % and 61.9 % at 4 hL the observed biogas production and the methaneconversion rate were 0.09-0.30 m3/m3 d and 0.09-0.12 m" CH4/kg COD removed.- The effluent 55 concentration increased with the decrease in HRT. When the HRT was greater than 5 h,the effluent 55 concentration was less than 40 mgll.- The granular sludge at the bottom of the first chamber played a major role in removing influent substrate,but as the HRT fell, more and more bacteria in the biofilm of the second and third ones were involved inremoving substrate.- After the HRT decreased from 4 h to 2 h, the reactor's performance deteriorated, and the hydraulic loadingwas the dominating factor for the reactor operation.

The above results are similar or compare favorably with other anaerobic reactor systems for municipalwastewater treatment at ambient temperature (Derycke and Verstraete, 1986; Kooijmans et al., 1986;Maragno and Campos, 1992; Monroy et al., 1988; Schellinkhout et al., 1988; Vieira and Garcia. 1992).However, further research is required on the hydraulic characteristics of the reactor, frequency of biologicalsolids wasting and the effect on the reactor's performance at low temperatures. Also requiring evaluation arefeasibility of intermittent and seasonal operation, and system economics. Most important, a field-scaleoperation is required to evaluate the potential of this modified ABR in some tropical and sub-tropicalregions.

POST-TREATMENT OF THE EFFLUENT FROM AN ANAEROBICSYSTEM TREATING MUNICIPAL WASTEWATER

The anaerobic process is efficient for the removal of organic material and suspended solids from municipalwastewater. However, the anaerobic process has little effect on the concentrations of nitrogen andphosphorus, whereas pathogenic organisms are only partially removed (CoIlivignareIli, 1990).Consequently, as anaerobic effluent is unlikely to meet future effluent requirements of many developingcountries (BOD < 20 or 30 mgll), anaerobic lreatment is only to be considered a very effective pre­treatment. Depending upon the local legislation concerning the effluent quality, post-treatment may berequired for removing residual COD and total suspended solids, and to reduce the concentrations of nutrientsand pathogens.

Some biological. chemical. physical-chemical processes or a combination of these are able to be employedas a post-treatment method (Garuti et al., 1992). From an economic point of view, biological processes arestill an ideal option for post-treatment in developing countries. Among the biological processes, theactivated slUdge process is currently the most widely utilized municipal wastewater treatment process.

\96 H. YUetal.

However, considering its high construction and operational costs, and large amount of excess sludge, theactivated sludge process is not generally considered as a post-treatment method for developing countries.

There is a growing interest in natural systems for wastewater treatment. Natural systems have a minimumdependence on mechanical elements, hence, they are very low cost and low maintenance, and particularlysuitable for developing countries where money and skilled manpower are lacking (Conley et aI., 1991).Stabilization pond and constntcted wetland are two typical kinds of natural systems for wastewatertreatment. Then, they are recommended as prospective post-treatment methods by the authors.

Reed bed

Constructed wetlands have been in use for a few decades for the treatment of municipal wastewater. Theyconsist of soil filled beds planted with various aquatic plants (Emergent macrophytes). In Europe commonreed (Phragmites) are found in most systems, whereas in the United States bulrush (Scirpus) and catail(Typha) are the most common plant species (Conley et at., 1991).

Table 4. Performance data for reed beds used for the tertiary treatment at five treatment plants

Works I!QQ ~ ~ IIQ!: ammN199\ 1992 1991 1992 1991 1992 1991 1992 1991 1992

MiddlelonInlet 1\6 \33 72.7 87.3 25.5 26.2 6.1 9.3 3.4 5.1RB effluent 27 1.6 36.6 35.3 5.7 46 1.8 1.8 0.7 0.5

Leek WoottonInlet 13.2 12.\ 94.3 106.1 20.2 20.6 8.6 9.9 5.0 5.8RBefflllent 3.6 2.1 55.2 54.3 4.1 4.8 5.2 4.0 3.2 1.9

HimleyInlet 13.5 27.3 79.8 119.2 38.9 39.0 6.6 7.1 2.5 1.4RBefflllent 1.5 3.7 31.7 50.2 5.1 5.0 2.5 3.3 0.9 1.5

ThOIpC SatchvilleInlet 15.4 14.9 100.2 116.4 35.3 33.8 8.7 12.6 5.1 8.3RBemuent 4.0 3.4 50.6 53.2 5.5 5.8 6.0 8.3 4.3 6.4

Ashby FolvilleInlet 14.5 17.7 99.1 83.3 60.5 25.7 6.9 11.3 3.2 8.0RB effluent 3.0 2.8 46.8 50.6 7.5 3.7 5.5 8.6 4.1 7.0

Since the late 1980s, the reed bed system has been tried in the UK and found very successful (Green andUpton, 1994). It was proposed that they should be used as a tertiary treatment units for small plants andwould be constructed to follow the secondary bio-oxidation systems of either trickling filters or rotatingbiological contactors (RBCs). The standard tertiary treatment reed beds comprise gravel filled tanks plantedwith Phragmites australis. The gravel is usually confined in bound walls lined with an impermeablepolyethylene or bentonite liner. The process requires that effluent flows through the gravel matrix in a fairlyuniform manner. Humus solids are deposited mostly near the inlet and soluble and colloidal organics areoxidized on contact with microbial growths on the surface of the gravel and on the roots and rhizomes ofPhragmites. The reed bed systems used by Severn Trent closely follow design concepts outlined by Crabtree& Rowell (1993) and Green & Upton (1994).

The performance of reed bed systems is well reported and most of them show consistent effluent quality intenus of BOD, TSS and ammonia-No Table 4 summarizes the performance data for reed beds used fortertiary treatment at five municipal wastewater treatment plants in Severn Trent Water (Green & Upton,1994).

Municipal wastewater treatment process 197

It should be noticed that the reed bed system mentioned above has only been used to polish the effluentsfrom aerobic processes. Unfortunately, there is no report about employing the reed bed to post-treat theeffluent from an anaerobic treatment system for municipal wastewater. The effluent quality of an anaerobictreatment system for municipal wastewater has some differences with that of an aerobic treatment system.So, if the reed bed is designed and constructed to follow an anaerobic system for municipal wastewatertreatment, comprehensive studies are required to get appropriate design and operational parameters.

Stabilization pond with sU12portini media

A stabilization pond (SP) is a shallow earth basin where wastewater is treated by biological processes. SPhas a great reputation for its low construction investment. less operation costs, and therefore has foundcommon application in raw municipal wastewater treatment. with the main goals to remove organic materialand suspended solids. SP can also be used as a post-treatment system for municipal wastewater.

Because of the main shortcomings of SP such as very long HRT and low biomass concentration. large landarea requirements for pond construction are needed. This limits its further utilization. It is estimated thatponds are suitable for post-treatment only if land is relatively cheap «US$ 151m2). Besides, there has been agreat need for the improvement of pond performance for organic matter removal. An attached-growth pondsystem developed by Zhao and Wang (1996) may partially overcome these drawbacks and provide apotential post-treatment system.

In the pond modified by Zhao and Wang (1996), attached-growth media (AGM) or the so-called artificialfibrous carriers were installed. This type of media consists of fine strings of polyvinyl acetate. with specificsurface area of 1236 m2/m3 and cost only US$ 51m3. A pilot-scale investigation has been conducted bythem. using three ponds with working dimensions of 4.0 m in depth. 1.2 m in width and 1.1 m in depth. Thisstudy has conftrmed that the incorporation of AGM enhanced the performance of conventional SPs byformation of a great number of small stable ecological systems around AGM. being abundant in biospeciesfrom bacteria and algae to protozoa. increasing the biomass concentration. improving the biologicaldistribution. Better removal efficiencies of COD (75.6%). BOD5 (90.2%) and NH4-N (68.5%) had beenachieved in the SPs with AGM than in the conventional SPs. although the total HRT of the former had beenshortened to 7.5 days. On the basis of these results. a full-scale SP system with AGM has been constructed(Wang. 1996).

It should be pointed out that the incorporation of AGM must increase the capital investment of the SPconstruction. This should be the major drawback of the modified SP system. It should also be cautioned thatthere is no investigation into the use of the SP system with SGM for post-treating the effluent fromanaerobic systems for municipal wastewater treatment. Therefore. there is a long way to go before such apost-treatment process becomes practical.

CONCLUSIONS

Anaerobic treatment of municipal wastewater would represent a sustainable solution for waterenvironmental protection. particularly for developing countries. Results obtained in pilot- and full-scaletreatment plants clearly reveal that the anaerobic treatment is indeed a feasible and very attractive option formunicipal wastewater at temperatures exceeding 20'C in tropical and sub-tropical regions. The UASBsystem has been commonly employed as an anaerobic pre-treatment system because of its inherent merits.The ABR system provides another potential for the anaerobic pre-treatment. But it requires furtherinvestigations.

The effluents from the anaerobic treatment systems should be post-treated to meet more and more strictdischarge standards. Considering the many advantages of the reed bed system when it is employed fortertiary treatment. we suggest utilizing this system as a post-treatment system. Another cost-effectivetreatment system. the stabilization pond. can also be considered as a post-treatment system. A pilot-scale

198 H. YV eraJ.

study on a stabilization pond packed with attached-growth media has demonstrated that this modified systemis effective for the treatment of municipal wastewater. providing a potential post-treatment system.

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