PERFORMANCE EVALUATION AND DETERMINATION OF E. coli REMOVAL COEFFICIENTS IN TWO BAFFLED AND UNBAFFLED PONDS IN PARALLEL TREATING ANAEROBIC EFFLUENT

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1 PERFORMANCE EVALUATION AND DETERMINATION OF E. coli REMOVAL COEFFICIENTS IN TWO BAFFLED AND UNBAFFLED PONDS IN PARALLEL TREATING ANAEROBIC EFFLUENT Marcos von Sperling ( ) Civil engineer. PhD in Environmental Engineering (Imperial College UK). Senior lecturer at the Department of Sanitary and Environmental Engineering Federal University of Minas Gerais - Brazil. Carlos Augusto de Lemos Chernicharo Civil engineer. PhD in Environmental Engineering (Imperial College UK). Senior lecturer at the Department of Sanitary and Environmental Engineering Federal University of Minas Gerais - Brazil. América Maria Eleuthério Soares Civil engineer. MSc in Sanitation, Environment and Water Resources (Federal University of Minas Gerais - Brazil). Adriana Molina Zerbini Biologist. MSc in Sanitation, Environment and Water Resources (Federal University of Minas Gerais Brazil) (*) Departamento de Engenharia Sanitária e Ambiental UFMG; Av. Contorno 84 7o andar; Fone: (3) ; Fax: (3) ; CEP Belo Horizonte; Minas Gerais; Brasil. marcos@desa.ufmg.br RESUMO The paper evaluates the removal efficiency and derives the decay coefficients of Escherichia coli in a combined UASB (Upflow Anaerobic Sludge Blanket) reactor maturation pond system. The system is comprised of demonstration-scale units, treating actual domestic sewage from a town in Brazil. The UASB reactor (9 m 3 ) is a partitioned reactor, representing a novel configuration, constituted of three digestion compartments, three gas separation devices and a single settler compartment for solids separation (Chernicharo & Cardoso, 999). Two maturation (polishing) ponds in parallel (baffled and unbaffled), each with 8 m x 4 m external dimensions, are included in the system, with the main objective of removing pathogens. The UASB pond systems showed very good E. coli removal efficiencies, especially considering the low overall HDT (between 6 and 9 days in the ponds). The UASB reactor removed around log unit of E. coli, whereas the ponds showed removal efficiencies varying from to 4 log units in the four operating phases. The best operating conditions were with an average HDT of around 9 days in each pond and a liquid depth of only 0.6 m. Although there was no substantial difference in the effluent E. coli concentrations, the unbaffled pond had higher K b values (varying from 0.96 to.4 d - ) than the baffled one (varying from 0.55 to 0.95 d - ). In the unbaffled pond, the observed K b values are greater than those predicted according to von Sperling s (999) model, while in the baffled pond there is a reasonable agreement. Keywords: UASB reactor, maturation ponds, E. coli, decay coefficient INTRODUCTION The UASB (Upflow Anaerobic Sludge Blanket) maturation pond system plays a very important role in warmclimate countries, due to its satisfactory efficiency, simplicity, economy, low sludge production and no energy requirements. However, there are very few data on the international literature on this relatively new configuration (UASB maturation ponds), and the present paper aims at contributing to the understanding of such an important system. The investigation on the removal of faecal indicators is extremely important for all countries, especially the developing ones.

2 The paper evaluates the removal efficiency and derives the decay coefficients of Escherichia coli in a combined UASB reactor maturation pond system. The system is comprised of demonstration-scale units, treating actual domestic sewage from a town in Brazil. The UASB reactor is a partitioned reactor, representing a novel configuration, constituted of three digestion compartments, three gas separation devices and a single settler compartment for solids separation (Chernicharo & Cardoso, 999). Two maturation (polishing) ponds in parallel (baffled and unbaffled) are included in the system, with the main objective of removing pathogens. METHODOLOGY The research was conducted at Nova Vista WWTP, located in the town of Itabira, Brazil, latitude 9º37 00 south, longitude 43º3 40 west, altitude of 78 m above sea level, average liquid temperature between 7 and 6 o C. Fig. provides a view of the ponds, while Figure shows the system flowsheet, including the sampling points investigated. Each pond has 8 m x 4 m external dimensions. One of the ponds is baffled and the other is unbaffled. The UASB reactor (9 m 3 ), operated with a low HDT (hydraulic detention time), 5.0 h and 7,5 h in different phases, while the polishing ponds (3 m each) operated with different hydraulic detention times, length/breadth (L/B) ratios and depths (H), as presented in Table. The work comprised four different operational phases and covered almost two years (648 days) of intensive monitoring of various water quality parameters. UNBAFFLED POND BAFFLED Fig.. View of the experimental ponds used in the research sewage Anaerobic Reactor UASB Baffled Pond Unbaffled Pond Preliminary treatment,6 m Pond effluent m 4 m Fig.. Flowsheet of the UASB ponds system, showing sampling points Pond effluent Table. Operational phases during the research period Phase Duration HDT in the Baffled pond Unbaffled pond UASB reactor (h) L/B ratio H (m) Mean HDT (d) L/B ratio H (m) Mean HDT (d) Aug 999 Nov Jan 000 Sep Sep 000 Dec Jan 00 Jun

3 During Phase the baffled pond had 7 internal baffles, which led to an L/B ratio of 3.0. After that, the number of baffles was decreased to 4, leading to an L/B ratio of.5. A large array of physical-chemical parameters was included, but the paper concentrates on E. coli. Each sample collected a liquid from the whole depth of the pond (excluding the sludge layer in the bottom 0 cm). Samples were collected weekly, between 9 am and 0 am. E. coli was analysed according to the Defined Substrate / Quantitray 000 Colilert (AWWA/APHA/WEF, 998). Further details can be found in Soares (00). RESULTS AND DISCUSSION Descriptive statistics Table presents the descriptive statistics of E. coli in the four operational phases. Figure 3 presents the boxand-whisker plot (min, max, 5%ile, 50%ile, and 75%ile) of E. coli in the four phases, in the four sampling points. The very high concentrations in the raw sewage may be observed (median values between 0 8 and 0 0 MPN/00mL). The decrease along the system may also be seen, with a reduction between -3 log units in Phases to 3, and 3 to 5 log units in Phase 4. The phase with the best overall efficiency was Phase 4 (liquid depth of only 0.6 m), in which both ponds sustained a high photosynthetic activity and allowed a good penetration of UV radiation, which were responsible for the high E. coli removal efficiency. Figure 4 presents the time series of E. coli along the system, whereas Fig. 5 presents the time series of the overall removal efficiency, expressed in terms of log units removed. Contrary to what could be expected in terms of reactor hydraulics, both baffled and unbaffled ponds showed very similar performances. Again, it can be clearly seen that the lowering of the pond depths (Phase 4) had a beneficial effect in terms of E. coli removal. Table. Descriptive statistics of E. coli in the four operational phases (August/999 a June/00) Phase Statistics Escherichia coli concentration (NMP/00ml) Removal efficiency in each unit (%) Removal efficiency in the system (%) UASB Ponds effluent UASB Ponds UASB/Ponds Influent Effluent Unbaffled Baffled Unbaffled Baffled Unbaffled Baffled No. data Minimum,00E+07 9,80E+06 5,0E+04 4,0E+04 0,000 48,78 98,000 9,837 99,67 Mean 3,45E+08 6,94E+07,38E+06 3,E+05 64,86 90,55 98,9 97,799 99,568 Maximum 9,30E+09 5,80E+08 4,00E+07 8,0E+06 97,647 99,734 99,988 99,97 99,993 Stand. dev. 3,3E+09,06E+08,48E+07 4,03E+06 3,548 8,456 0,844,977 0,445 Median 4,50E+08 7,90E+07 9,30E+05,80E+05 7,07 96, 98,849 99,500 99,556 No. data Minimum 4,00E+08,0E+07 6,30E+05 6,30E+05,49-4,634 4,878 78,537 90,488 Mean,0E+09,78E+08 4,54E+06 5,74E+06 83,474 87,079 89,53 98,677 99,75 Maximum,30E+0 8,70E+08 8,80E+08 3,90E+08 99,65 99,9 99,897 99,986 99,978 Stand. dev. 4,73E+09,45E+08,9E+08 8,E+07 9,03 46,79 0,90 4,630,993 Median,85E+09,85E+08 5,0E+06 4,75E+06 89,4 98,73 97,300 99,793 99,707 No. data Minimum,00E+07 7,40E+06,00E+05,00E ,000 6,88 54,054 85,500 8,000 3 Mean,03E+08 4,E+07 7,94E+05,0E+06 3,47 94,44 9,830 97,068 97,477 Maximum,0E+09 7,70E+08,0E+07,50E+07 98,64 99,909 99,900 99,99 99,976 Stand. dev. 3,64E+08,E+08 6,45E+06 4,5E+06 3,59 0,960 4,07 5,446 5,086 Median,45E+08 4,30E+07 5,0E+05 8,0E+05 79,74 98,7 96,60 99,79 99,490 No. data Minimum,00E+07 9,80E+06 7,30E+03 6,30E+03-90,000 99,659 98,385 99,000 93,700 4 Mean 3,4E+08,6E+08 5,66E+04,34E+05 9,895 99,887 99,564 99,878 99,49 Maximum,70E+09 5,50E+08 9,0E+05,70E+06 94,84 99,996 99,999 99,999 99,999 Stand. dev. 7,07E+08,73E+08 3,4E+05 7,63E+05 3,83 0,9 0,556 0,9,805 Median 3,50E+08,0E+08 4,45E+04,85E+05 76,04 99,96 99,854 99,990 99,95 Note: Mean = Geometric mean

4 e e0 Escherichia coliin UASB - POND SYSTEM PHASE PHASE PHASE 3 PHASE 4 e9 e8 e7 e6 e Max Min 75% 5% Median Fig. 3. Box-and-whisker plot of E. coli along the treatment system, in the four different phases E. COLI (UASB-POND SYSTEM) E. coli (MPN/00mLL),0E+,0E+,0E+0,0E+09,0E+08,0E+07,0E+06,0E+05,0E+04,0E+03 Phase Phase Phase 3 Phase TIME (d) Effl.UASB Fig. 4. Time series of E. coli concentration along the system 6,00 5,00 E. COLI REMOVAL EFFICIENCY (LOG UNITS) Phase Phase Phase 3 Phase 4 LOG UNITS 4,00 3,00,00,00 0, TIME (d) UASB - unbaffled pond UASB - baffled pond Fig. 5. Time series of E. coli overall removal efficiency (log units) in the UASB - pond systems Bacterial decay coefficient K b

5 The values of the coliform decay coefficient K b (for dispersed flow) calculated based on the geometric means of E. coli concentration in each phase are presented in Table 3 and Figure 6. Table 3 also presents the mean liquid temperature and the mean hydraulic detention time in each phase, which allowed calculation of K b. All observed K b values in the unbaffled pond are higher that in the baffled one. The observed K b value in the unbaffled pond in Phase 4 is very high, in accordance with the excellent performance in this period. The observed K b values are compared with those estimated according to von Sperling (999) (see equation in note of Table 3). It is seen that in the unbaffled pond the observed K b values are higher than those estimated according to von Sperling, whereas in the baffled pond there is a reasonable agreement. Figure 7 presents a comparison between observed and estimated values of K b. Table 3. Comparison of observed and estimated values of K b coefficient in the ponds Phase Mean liquid temperature Mean pond HDT (d) K b coefficient (0 o C) Unbaffled pond K b coefficient (0 o C) Baffled pond ( o C) Unbaffled Baffled Observed Estimated Observed Estimated (*) (*) 7.3 8,00 8,00,39 0,46 0,95 0,46.9 6,4 6,4 0,96 0,50 0,55 0, ,7 6,0,0 0,47 0,60 0, ,95 9,33,4 0,70 0,8 0,69 HDT = Hydraulic Detention Time For all calculations: Dispersion Number: d=/(l/b) (von Sperling, 999); temperature coefficient θ=.07 (*) K b estimation according to von Sperling (999): K b =0.97.H HDT VALUES OF Kb COEFFICIENT (DISPERSED FLOW),4,5 Kb (/d),5 0,5,39 0,95 0,96,0 0,55 0,6 0,8 0 Phase Phase Phase 3 Phase 4 Unbaffled Baffled Fig. 6. Observed values of K b coefficient for dispersed flow (0 O C)

6 Unbaffled pond Baffled pond,5,5 Kb (/d),5 Kb (/d),5 0,5 0,5 0 Phase Phase Phase 3 Phase 4 0 Phase Phase Phase 3 Phase 4 Observed Estimated Observed Estimated Fig. 7. Observed and estimated values of K b coefficient for dispersed flow (0 O C). Estimated values based on von Sperling (999) CONCLUSIONS The UASB pond systems showed very good E. coli removal efficiencies, especially considering the low overall HDT. The UASB reactor removed around log unit of E. coli, whereas the ponds showed removal efficiencies varying from to 4 log units in the four operating phases. The best operating conditions were with an average HDT of around 9 days in each pond and a liquid depth of only 0.6 m. Although there was no substantial difference in the effluent E. coli concentrations, the unbaffled pond had higher K b values (varying from 0.96 to.4 d - ) than the baffled one (varying from 0.55 to 0.95 d - ). In the unbaffled pond, the observed K b values are greater than those predicted according to von Sperling s (999) model, while in the baffled pond there is a reasonable agreement. REFERENCES AWWA/APHA/WEF (998). Standard methods for the examination of water and wastewater. 0th edition. Washington. Chernicharo, C.A.L & Cardoso, M.R. (998). Development and evaluation of a partitioned UASB reactor for the treatment of domestic sewage from small villages. Water Science and Technology (40), 8, pp Soares, A. M. E. (00). Avaliação experimental de um sistema reator UASB/lagoas de polimento com e sem chicanas em escala de demonstração. Dissertação de mestrado. Escola de Engenharia da Universidade Federal de Minas Gerais, Brasil. 77 p. (in Portuguese). Von Sperling, M. (999). Performance evaluation and mathematical modelling of coliform die-off in tropical and subtropical waste stabilisation ponds, Water Research 33 (6), pp