Association of a UASB reactor and a submerged aerated biofilter for domestic sewage treatment

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Pergam on

PU: S0273-1223(98)00693-3

Wat. Sci. Tech. Vol. 38, No. 8-9, pp. 189-195,1998. lAWQ © 1998 Published by Elsevier Science Ltd. Printed in Great Britain. All rights reserved 0273-1223/98 $19'00 + 0'00

ASSOCIATION OF A UASB REACTOR AND A SUBMERGED AERATED BIOFILTER FOR DOMESTIC SEWAGE TREATMENT Ricardo Franci Gon~alves*, Vera Lucia de Araujo* and Carlos Augusto L. Chemicharo** CT, Federal University of Espirito Santo, * ~ep~rtment of Hydraulics and Sanitation,Vitoria (ES), Brazil

AgenclQ FCAA, c.P. 01-9011, 29.060-970, sity of **. Departm~nt ofSanitary and Environmental Engineering, Federal Univer Brazil (MG), nte Horizo Mmas Germs, Av. Contorno 841701,30110·060, Belo

ABST RACT Anaerobic Sludge Blanket - DASB This paper presents exploratory results on the association of an Dptlow domestic sewage treatment. The for L) reactor (46 L) and a submerged aerated biofilter - BF (6.3 and organic loads were gradually ic hydraul experimental period extended for 322 days, during which the hydraulic loads were tested: 0.4 g followin increased in both reactors. Having the DASB as a reference, the 2 3 2 3 3/m 2 .h (8 .h (8 6 h) and 1.45 m 3/m 2.h (9 /m m 3 2 1.0 IOh); 0.8 m /m .h (8 8 h); m /m .h (8 ::: 16 h); 0.6m a hydraulic detention time of 6 at g ::: 4h). During the experiments carried out with the DASB reactor operatin cy in terms of 5S, BOD 5 efficien l remova hours, related to a 8 < 11' in the granular media of the BF, the mean , related to the BF effluent final The %. and COD, in both reactors, were respectively 94%, 96% and 91 COD::: 38 mg/L. and mglL 9 = BODS effluent, presented the following mean characteristics: 5S ::: 10 mglL, DASB reactor the in load ic hydraul the The results obtained in the last phase of the experiments, when These results phase. s 2 3 previou the in d reached 1.45 m /m .h (8 = 4h), were similar to those obtaine atment of post-tre the for ive alternat demonstrate that submerged aerated biofilters can be considered a viable d with operate being of capable are reactors effluents from DASB reactors treating domestic sewage. These reserved rights All Ltd. Science r very short hydraulic detention times. © 1998 Published by Elsevie

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KEYW ORDS Biofilm~ domestic

reactor. sewag e treatment; post-treatment; submerged aerated biofilter; VASB

INTR ODVC TION as low cost, operational simplicity and The several favourable characteristics of anaerobic processes, such Brazil, where the temperature is usually low solids production, together with the environmental conditions in ent of domestic sewage, especially high, have contributed to highlighting anaerobic systems for the treatm for wastewater treatment includes these through VASB reactors. Nowadays, almost all alternative analysis VASB reactors have difficulties in reactors as one of the main options. Despite these advantages, the al standards. Therefore, it is of great producing effluents that can comply with the Brazilian environment r of adapting the treated effluent to importance to consid er the post-treatment of VASa reactors, as a manne 189

190

R. F. GON<;::ALYES el at.

these environmental discharge standards. The main objective of the post-treatment is to complement the organic matter removal, as well as to promote the removal of certain components which are barely affected by the anaerobic treatment (nutrients and pathogens). Even at international level there are inconsistent experiences related to this important stage of post-treatment. The existing knowledge is fairly sufficient when each treatment unit is analysed individually, but not when the anaerobic reactor and the post-treatment system are treated in a combined fashion. Also in Brazil the perfonnance of post-treatment units has not been investigated in a systematic way. On the other hand, the importance of combining aerobic and anaerobic reactors for wastewater treatment is well known by many researchers (Mergaert et ai, 1992; Van Haandel and Lettinga, 1994). Compactness, energy saving and low sludge production are some advantages of this association, when compared to conventional treatment facilities. Iwai and Kitao (1994) mentioned several small scale domestic wastewater treatment facilities, combining septic tanks and biofilm aerated reactors that are actually in operation in Japan. The association of UASB and biofilm reactors is a subject of recent research, involving mainly percolating biological filters in the aerobic treatment step (Kitao et ai., 1986, Collivignarelli et ai., 1990). Few experiences, restricted to laboratory scale, were elaborated with BF's functioning as an aerated treatment step. Biofilters can constitute an excellent option for the post-treatment of UASB reactor effluents, due to their capacity to degrade the soluble compounds and filter the suspended particulate in the same reactor. In the majority of facilities operating with BF's as a secondary treatment step, the clogging control of the filter media is assured by a primary sedimentation basin located upstream the BF's. Recently, the association of an anoxic UASB with a BF was tested in the mineralization and the nitrification/ denitrification of wastewater (Gon~alves et ai., 1993). A great instability of the sludge blanket was observed in the UASB reactor, originated from the excessive hydraulic loads applied, which reached up to 10 m3/m 2 .h, and specially due to an intensive denitrification activity in this reactor. Chernicharo et ai. (1996a) used successfully an anaerobic filter associated with a BF for the treatment of tannery effluents in Brazil. Preliminary results regarding secondary domestic sewage treatment through an association of UASB + BF, obtained in a 60 day battery of tests, were reported by Chernicharo et ai. (1996b). The present study shows the results obtained through the association of these two high rate reactors for secondary treatment, in a battery of tests with a duration of approximately one year. The main objective of this study was the development of a compact wastewater treatment plant, appropriate to Brazilian urban areas, joining the main positive characteristics which are common in both processes: compactness, high concentration of active biomass in the reactor's volume, high sludge detention times (resulting in low sludge production), resistance to hydraulic and organic shock loads and cover possibility (avoiding odour and visual impact problems). From the operational point of view, the integration of these two reactors is meant to produce a secondary level effluent (SS < 30 mglL, BOD < 25 mglL and COD < 90 mglL). MATERIAL AND METHODS The pilot scale UASB and biofilter reactors used in the experiments presented the following main characteristics, as shown in Table I and in Figure I. Table I. Characteristics of the pilot reactors Characteristic Material Internal diameter (mm) Total high (m) Total volume (L) Packing material high (m) Packing material volume (L)

UASB reactor PVC 100 4.6 46

Submerged Aerated Biofilter PVC 100 0.8 6.3 0.5 3.9

Association of a reactor and a biofilter

191

As shown in Figure 1, the BF granular medium was floating and totally submerged, composed by S5 type polystyrene spheres with 3mm diameter, 1200 m2/m 3 specific surface and 0.04 density, 0.50m height. The air supply was assured by a SCHULZ NISI compressor model 26 VLl60 (1/2 hp) . The air was injected in the BF bottom and crossed the bed in upstream flow, wastewater co-current. The air flow in the BF was controlled through a flowmeter made by Cole Parmer, with a reading scale from 20 I to 1682 mllmin. The VASB reactor was fed with raw wastewater taken downstream ofthe pre-treatment unit of the Camburi Municipal Wastewater Treatment Plant (160,000 p.e.) - Vit6rialBrazii. The effluent of the UASB reactor was continuously submitted to a polishing stage in the BF (Figure 1). The routine operation of the experimental apparatus included sludge removal from the UASB reactor and BF backwash for excess biofilm removal. The backwashes were frequently proceeded, at least once every 72 hours, constituted by several intense discharges of the liquid phase followed by air injection under very high hydraulic rate (>45 m3/m 2 .h). The experimental period had a total duration of 322 days during which the hydraulic (and organic) load were gradually incremented in both reactors (Table 2). The performance of both reactors was monitored through 24-hour composite samples, taken from the inlet and outlet of the UASB reactor and from the outlet of the BF. The routine analysis were carried out three times per week, and included the following monitoring parameters: COD, BODs, SS, pH, alkalinity, KTN, N-NH 4+, N-NOx and DO. Sludge production was evaluated in both reactors during the whole experimental period and biogas production was measured in the last two phases of the research. ..................

UASB

biogas

anaerobic ,...--.......---, effl uent t--~~~

.......

BIOFILTER ................... n n

1

final effluent

n

1

raw sewage

compressor ........ ................. '-----J---,

sludge

o~ ~i~~_g __

sludge

Figure 1. Pilot plant scheme.

Table 2. Operational Conditions HDT (h) DASB BF 0.46 16 0.28 10 0.23 8 0.17 6 0.11 4 UASB and BF

Phase 1 2 3 4 5 (*)

Flowrate (l/h) 2.8 4.6 5.7 7.6 11.4

Hydraulic load * (m3/m2 .h) 0.36 0.58 0.73 0.97 1.45

Duration (days) 65 74 30 33 45

192

R. F. GONl;ALYES et ai.

RESULTS AND DISCUSSION Reactors start-up A good performance in terms of COD and SS removal was noticed in the first 15 days of reactor operation (Figs 2 and 3). The UASB was capable of producing an effluent with approximately 50 mgSSIL and 200 mgCODIL, while the concentrations in the final effluent of the BF were 15 mgSSIl and 70 mgCODIL. The organic loading rates applied were around 0.6 kgCOD/m 3.d in the UASB reactor and of 3.4 kgCOD/m 3.d in the BF. The steady state condition was rapidly reached in the BF. This very positive characteristic of the BF (Guiliano and Joret, 1988) compensated for the slow acclimatisation in the UASB reactor. The development of a thin, homogenous and very active biofilm layer in the BF polystyrene beads was observed, which was not removed during backwash operations. The overall SS and COD removal efficiencies were around 91 % and 88%, respectively, during the first three weeks. -+-RW ...-UASB

o 0 I I

450 400

1000

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350

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300

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150

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900 800

700 600 500 400 300 200

100 50

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57

82

107 132 157 182 207 232 Time(d)

Figure 2. Results of SS.

10:

~~~~~~~~~~~~~ 7

32

57

82

107 132 157 182 207 Time (d)

Figure 3. Results of COD.

Carbonaceous material removal The UASB performance in terms of SS and COD removal indicated a good improvement during the experiments, reaching a steady state condition around the BOth day. At this time, the UASB was submitted to a hydraulic load of 0.8 m3/m 2.h (8 = 8 h), and the anaerobic effluent presented the following mean characteristics during this phase: SS = 37 mgIL, COD = 112 mglL and BOD = 36 mglL. The UASB clarification efficiency (73%) concentrated the BF action almost exclusively in the removal of the remaining soluble compounds present in the anaerobic effluent. The organic loads reached values around 4.0 kg COD/m 3.d or 1.2 kg BODs/m3 .d in the BF, constituted by organic substrate with reduced biodegradability. Even so, this reactor presented removal efficiency of 56% (COD) and 72% (BODs), producing an effluent with 10 mgSS/l, 10 mgBODslL and 49 mg CODIL. In the experiments carried out with the UASB reactor operating at a hydraulic detention time of 6 hours, corresponding to a 8 < II' in the BF granular medium, the mean organic loads applied to the UASB reactor reached 1.84 kgCOD/m3 .d and 0.92 kg BOD s/m 3.d. Its performance was very similar to the previous phase, with removal efficiencies reaching 78% (SS), 76% (COD) and 87% (BODs). The BF was submitted to mean organic loads of 4.9 kgCOD /m 3.d and 1.35 kgBOD s/m3.d, producing an effluent with the following

Association of a reactor and a biofilter

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193

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characteristics: SS 10mg /L, BOD s -- 9 mg/L and COD 38 mgIL. The mean efficiencies of SS BOD and COD 's removal of both reactors were, respectively, 94%, 96% and 91 % in this phase.

~inall~, the tests carried out with hydraulic loads of 1.45 m 3/m 2.h (8 = 4h) in the VASB were intended

mvestJga~e the reactor's behaviour in a breakdown situation in the anaerobic step A 8 <5 h k to a determmant . d . ours was ta en as d param~ter: appOl~t~ b.y several researchers as the recommendable minimum in order to kee an a equate mechanIsatIOn actlvlt~ 10 the VASB (Vieira & Garcia Jr., 1992; Van Haandel & Lettin : the VASB behavIOur was noticed to be stable and the results were very similar to the o alOe 10 phases 3 and 4 (Table 3). Even with the BF mean organic loads reaching 6.15 kgCOD/m 3 d d 1.47 kgB?Ds /m 3 .d, the removal efficiencies remained very high and the final effluent pre~en~~d concentrations of 10 mgSS/L, 49 mgCOD/L and 10 mgBODs/L.

I~~~). ~~wever,

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Table 3. Mean influent and effluent characteristics Phase Parameter

3

SS COD

BODs 4

KTN SS COD

BODs 5

KTN SS COD

BODs KTN

n

14 14 14 14 12 12 12 12 18 18 18 18

Raw wastewater Mean S.D. 174 92 463 183 214 80 38.8 3.7 59 171 95 459 229 58.4 42.3 5.2 46 127 297 99 125 43 4.2 34.4

VASB Mean 37 112 36 35 35 105 29 32.7 30 88 21 32.5

S.D. 14.8 23.3 10 4.1 6.5 14.4 2.5 2.4 9.3 23 7 4.9

BF

% rem Mean 73 10 73 49 82 9.7 10 25.8 78 9.8 76 37.8 87 9.4 27 23 74 10.3 68 49 9.6 83 27.6 6

S.D. 3.4 13.5 3.3 2.5 4.7 9.0 2.1 3.2 4.6 13.5 3.4 4.7

Overall

% rem 69 56 72 26 72 64 68 17 66 44 54 15

% rem. 92 88 95 34 94 92 96 36 92 82 92 20

Intensive nitrification activity was observed in the BF during the first experimental phase, when N-NH 4+ removal efficiency was over 90%. However, a significant decrease in nitrification was noticed during the second experimental phase, through nitrite ions accumulation in the effluent. The association of a high organic load and high nitrite concentration in the liquid phase of the BF (>5 mg N-NOf/L) inhibited definitely the nitrification process thereafter. The backwashes were performed at least once every 72 hours, through several intensive discharges of the 2 liquid phase, followed by air and water injection under elevated hydraulic load (>45 m 3/m .h). The headloss evolution rate was 1.5 em/day, which is very low when compared to those observed in wastewater treatment plants with primary sedimentation tanks before BF's (5 cmlh) (Gon~alves et aI., 1992). Stability performance In spite of variations in the performance of the VASB reactor, the combination VASB + BF presented a remarkable stability in terms of SS and organic material removal. The BF operated as a compensation unit on occasions when the anaerobic phase presented significant efficiency loss. This fact can be visualised through Figures 4 and 5, which present frequency distribution of the final effluent quality. In experimental periods when the VASB reactor operated under 4 and 6 hours retention times, the applied 3 organic loads ranged from 2.0 to 4.0 kgCOD/m 3.d in the VASB reactor and from 5.0 to 9.0 kgCOD/m .d in the BF. The corresponding loads in terms of BODs varied from 1.0 to 2.0 in the VASB reactor and from 1.2 to 2.2 kgBOD /m3.d in the BF. From the frequency distribution related to the final effluent quality, it can be S observed that all samples of the two last phases presented concentrations lower than 20 mgSS/l, 15

194

R. F. GON<;ALYES et al.

mgBODS/L and 70 mgCOD/L. Around 70% of these samples presented effluent characteristics equal or better than 10 mg SSIl, 10 mgBODs/L and 50 mgCOD/L. . ·0 - . 0,36m1h -

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Figure S. Frequency distribution of COD concentration in the final effluent (BF). Slud~e

production

Sludge specific production in the UASB reactor varied between 0.14 and 0.16 kgTS/kgCODremoved' which are figures very similar to the ones reported by Haandel &t Lettinga (1994). In the BF, the sludge specific production in phase 4 was 0.37 kgTS/kgCODremoved' slightly inferior to the usual values reported to BF's following primary sedimentation tanks (Pujol et al. 1992). At least 70% of the incoming carbonaceous material was anaerobically metabolised in the UASB reactor. Consequently, low sludge production and significant energy savings are advantages that can be attributed to the UASB + BF association, in comparison to the BF's original configuration. Another important aspect to be emphasised is the solids concentration in excess sludge discharged from the bottom of the UASB reactor (>100 SSTIl), much higher than the sludge concentration in primary sedimentation tanks. In tests in progress, the BF backwash sludge is being recycled directly into the UASB compartment for complementary anaerobic stabilisation. In this way, it is expected to be possible to reduce the sludge withdraw1 only to the UASB reactor, which is capable of producing a highly concentrated and mineralised sludge.

Association of a reactor and a biofilter

195

CONCLUSION

T~e results of the UASB and BF association for the treatment of domestic sewage indicate the capability of thls~ystem to. produce an excellent quality final effluent. In tests conducted with a UASB reactor operating

at . .DT. varymg from 16 to 4 hours and the biofilter from 0.46 to 0.11 hours, the average removal effJcle~cles of. ~S, .BOD5 and ~OD were 95:0' 95% and 88%, respectively. The final effluent presented a 10 very hIgh stabIlIsatIOn stage, wIth ~he. followmg characteristics being observed: SS 10 mg/L, BOD 5 mg/L and COD ~ 50 mg/L. Ve~ ~lmJ1ar results were obtained by Chemicharo et al (1996b) in carrying out exploratory studIes on the aSSOCIatIOn UASB+BF, also treating domestic sewage.

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o~ the UASB reactor in. terms of SS removal, in reducing it to less than 20% of the original mflu~nt concentratlo~,. al~owed an effectIve control of biofilm growth in the biofilter, resulting in optimised

!he performance

reactIOn ra~es and mmlmlse~ backwashing operations. The BF backwash frequency was once every 3 days, correspondmg to a headloss mcrease rate of only 1.5 em/day. The ~r~sented results confirmed the vi~bili~y of having an excellent quality final effluent through the assOCIatIOn of UASB .and BF reactors. ThIS high rate reactor association resulted in a compact, efficient and low energy consumption system which can constitute a desirable solution for domestic wastewater treatment in Brazilian metropolitan regions. . ACKNOWLEDGEMENTS The authors thank CESAN (Espfrito Santo Sanitation Company) for its support during the set up and operation of the experimental apparatus. We also thank the Centro Cultural Americano IdiomaslES for the translation of this text. REFERENCES Chernicharo, C. A. L., Von Sperling, M., Silva, P. c., Gon~alves and R. F. (1996a). Tratamento de efluente de curtume atraves de filtros anaer6bio seguido por biofiltro aerado. In: Proceedings of the XXV Congreso Panamericano de Ingenieria Sanitaria y Ambiental (AlDIS), Mexico - DF, Torno I, Vol. 2, pp. 715-724. Chernicharo, C. A. L., Araujo, V. L. and Gon~alves, R. F. (1996b). Estudos sobre 0 p6s-tratamento de reatores UASB atraves de biofiltros aerados submersos. In: Proceedings of the XXV Congreso Panamericano de Ingenieria Sanitaria y Ambiental (AlDIS), Mexico - DF, Torno I, Vol. I, pp. 360-367. Collivignarelli, C., Urbini, G., Farneti, A., Bassetti, A. and Barbaresi, U. (1990). Anaerobic-Aerobic treatment of municipal wastewater with full-scale upflow anaerobic sludge blanket and attached biofilm reactors. Wat. Sci. Tech., 22(1/2), 475• 482. Giuliano, C. and Joret, J. C. (1988). Distribution, characterization and activity of microbial biomass of an aerobic fixed-bed reactor. In: IA WPRC Conference on Microbiology of Waters and Wastewater, Irvine (USA), Feb. 8-11. Gon~alves, R. F., Sammut, F. and Rogalla, F. (1992). High rate biofilters: simultaneous phosphorus precipitation and nitrogen removal. In: Chemical Water and Wastewater Treatment II - Proceedings of the 5th Gothenburg Symposium. R. Klute and H. H. Hahn (eds). Springer Verlag Berlin Heidelberg, New York, pp. 357-372. Gon~alves, R. F., Zeghal, S., Sammut, F. and Rogalla, F. (1993). Rem~ao de nutrientes de aguas residuarias atraves de biofiltros submersos de alta taxa. In: /70 Congresso Brasileiro de Engenharia Sanitaria e Ambiental, Natal, Set. 19-23,3-20. Iwai, S. and Kitao, T. (1994). Wastewater treatment with microial films. Technomic Publishing Co. Inc., 184. Kitao, T., Iwai, S., Ohmori, H., Yamamoto, Y. and Fujii, M. (1986). Anaerobic Submerged Biofilter for the Treatment of Domestic Wastewater. In: EWPCA Con!, Amsterdam, pp. 736-739. Mergaert, K., Vanderhaegen, B. and Verstraete, W. (1992). Applicability and trends of anaerobic pre-treatment of municipal wastewater. Wat. Res., 26(8), 1025-1033. Pujol, R., Canler, J. P. and Iwema, A. (1992). Biological aerated filters: an attractive and alternative biological process. Wat. Sci. Tech., 26(3-4), 693-702. Van Haandel, A. C. and Lettinga, G. (1994). Anaerobic sewage treatment: a practical guide for regions with a hot climate. John Wiley and Sons, 222. Vieira, S. M. M. (1988). Anaerobic treatment of domestic wastewater in Brazil: research and full-scale experience. Adv. Wat. Poll. Control, no. 5, 185-196. Vieira, S. M. M. and Garcia Jr, A. D. (1992). Sewage treatment by UASB-reactor. Operation results and recommendations for design and utilization. Wat. Sci. Tech., 25(7), 143-157.