Removal of fecal coliforms, somatic coliphages and F-specific bacteriophages in a stabilization pond and reservoir system in arid regions

Transcription

1 Removal of fecal coliforms, somatic coliphages and F-specific bacteriophages in a stabilization pond and reservoir system in arid regions L. Alcalde*, G. Oron**, ***, L. Gillerman**, M. Salgot* and Y. Manor**** * Soil Sci. Unit, Faculty of Pharmacy, Univ. of Barcelona, Joan XXIII, s/n Barcelona, Spain ( laur_ab@yahoo.es) ** Env. Wat. Resources, Ben-Gurion Univ. of the Negev, The Inst. for Desert Res., Kiryat Sde-Boker 84990, Israel, and the Dept. of Industrial Eng. and Management, Beer-Sheva 84105, Israel *** The Grand Wat. Res. Inst., Technion, Haifa 32000, Israel **** Sheba Medical Center, Tel-HaShomer 52521, Israel Abstract The contents of FC, somatic coliphages and F-specific bacteriophages were measured in the stabilization pond and stabilization reservoir system of the City of Arad (Israel) in order to determine the efficiency of the treatment process regarding the microorganisms removal. Monitoring was conducted close to one year. Physical and chemical parameters (temperature, ph, BOD 5, COD, SS) were also analyzed with the aim of finding factors that affect the microorganisms removal. The field results prove a very good performance of this treatment system. FC, somatic coliphages and F-specific bacteriophages were removed between 4.16 and 5.76 log units, during winter and in between 6.02 and 6.47 log units during summer. The microbial quality of the final effluent complies with the WHO guidelines for unrestricted irrigation. The results also indicate that retention time and temperature seem to be the most important factors for microorganisms removal. F-specific bacteriophages were removed at higher rates than FC and somatic coliphages. Consequently, it is suggested that F-specific bacteriophages might be less adequate viral indicators for this treatment system. Keywords FC; F-specific bacteriophages; reservoirs; rock filter; somatic coliphages; stabilization ponds Introduction Waste stabilization ponds (WSP) are one of the most appropriate extensive wastewater treatment methods to reduce pathogens. Low operation and maintenance costs coupled with effective pathogen removal have made WSP widely employed all over the world, particularly in developing countries where sufficient land is normally available and the climate is more favourable for their operation (Mara, 1976; Mara and Pearson, 1998). Despite their operational simplicity, the pathogen removal mechanisms in WSP are not well established yet and there is still a risk of contamination of crops and soil irrigated with this kind of effluent (Maynard et al., 1999). Therefore, it is important to investigate the behaviour of microbial indicators in wastewater treatment processes in order to predict the inactivation of infectious microorganisms and to evaluate their efficiency. FC are the principal microbial indicator organisms used for wastewater treatment system monitoring. However, the use of FC does not always reflect the viral pollution of the effluent due to the poor correlation with virus occurrence and persistence in treatment processes. Bacteriophages have been proposed as indicators for human pathogenic viruses due to their similar morphology and survival in aquatic environments. Somatic coliphages, F-specific bacteriophages and phages of Bacteroides fragilis are so far the more suitable viral indicators (Kott et al., 1974; Jofre et al., 1986; Havelaar et al., 1993). However, information on the concentration of these indicator organisms in sewage and their behaviour in treatment processes is needed to evaluate their reliability as model organisms. Water Science and Technology: Water Supply Vol 3 No 4 pp IWA Publishing

2 The purpose of this work was to examine the fate of bacterial and viral indicators (FC, somatic coliphages and F-specific bacteriophages) in a full-scale advanced integrated system of stabilization ponds and stabilization reservoirs in order to determine its efficiency regarding the microorganisms removal and to identify the main physico-chemical parameters involved. It is for further consideration of utilizing bacteriophages as microbial indicators for monitoring of water quality in waste stabilization systems. Material and methods Site description The study was performed in a full-scale advanced integrated pond and reservoir system located near the City of Arad, in the Judean Desert of Israel, at 525 m. above the sea level. The conditions of this area reflect an arid or semi-arid climate. The mean annual precipitation during the rainy season (October April) is around 150 mm. The solar radiation is high along the entire year being the maximum monthly mean of approximately 1,000 W/m 2. The plant treats the wastewater of the city that has an stable population of around 22,000 inhabitants. The wastewater is mainly domestic and the daily flow rate mean is about 5,000 m 3 /d. The final effluent is used for irrigation of a variety of crops such as almond trees, wheat, barely, sunflower and alfalfa, under onsurface and subsurface drip irrigation systems. This integrated pond system is in operation since 1999 and consists of different units. The design characteristics of the different components are shown in Table 1. After a pretreatment stage in a grit chamber the effluent flows into three anaerobic ponds operating in parallel, followed by a facultative pond, a maturation pond, a two stage rock filter, three stabilization reservoirs disposed in parallel operating in a cycle of fill-rest-use and a large seasonal storage reservoir (Figure 1). The rock filter has two dikes consisting of gravels of cm in diameter and actually performs as an horizontal trickling filter. Sample collection and experimental methods Wastewater samples were collected three to four times per month from June 2000 to March Sample collection dates were randomly chosen and all collections were done between 09:00 and 11:00. Sampling locations were at the main inlet (raw wastewater) and pond outlets (Figure 1). Raw wastewater samples were 24-hour composite samples and pond effluent samples were grab samples. Sample collection and handling procedure were performed according to the standard proceedings recommended by APHA (1998), for microbiological and physico-chemical analysis. The physico-chemical parameters analyzed included the BOD 5 (method 5210 B, Standard Methods (APHA, 1998), COD (method 5220 C), SS (method 2540 D) and ph and temperature were measured in situ at 25 cm from the surface of the ponds with a portable device model Schott, Handylab 1. The microbiological parameters analyzed were FC (method 9222 D), somatic coliphages (ISO/DIS /1999) and F-specific Table 1 Design characteristics of the treatment system components Components of the system Volume (m ) Surface area (m ) Depth 1 (m) HRT 2 (d) Anaerobic ponds Facultative pond Maturation pond Rock filter Stabilization reservoirs Seasonal storage reservoir Maximal depth of the water in the related pond/reservoir 2 Theoretical hydraulic retention time (HRT)

3 St To irrigation Ring filter Rs Ft Mt Rs An Rs Fc An An Rw (after a grit Not operating Figure 1 Flow diagram of the treatment system and sampling points. Rw = raw wastewater, An = anaerobic pond, Fc = facultative pond, Mt = maturation pond, Ft = rock filter, Rs = stabilization reservoir, St = seasonal storage reservoir bacteriophages (ISO/CD /1996). All data from the microbial analyses was converted into log 10 scale. Therefore, values below the detection limit were assumed as 0 value in order to be able to convert it to log units. The detection limit for the FC assay was 10 CFU/100 ml and for the bacteriophage assay was 10 PFU/100 ml. Results and discussion Results are presented separetely for the summer period (June 2000 October 2000) and the winter period (November 2000 March 2001) in order to detect any influence of climate factors on the microorganisms removal. Removal of bacterial and viral indicators through the treatment system The FC, somatic coliphage and F-specific bacteriophage content in the raw wastewater and the effluent of each system s component are presented in Figure 2. The numbers of the three pathogen indicators in the raw wastewater are quite similar for the two seasons with a slightly increase during summer. The average content of FC in the final effluent of the treatment system (seasonal storage reservoir effluent) never exceeded from 3.00 log units/100 ml along the period of study. Some of the values were below the detection limit. Therefore, the final effluent of this treatment system complies with the WHO guidelines for unrestricted irrigation regarding the FC content ( 1,000 CFU/100 ml or 3 log units/100 ml). Consequently the effluent can be used for unrestricted irrigation of several crops during the entire year without any additional disinfection treatment. According to all the data processed during this study the highest value for the average 179

4 180 content of somatic coliphages in the final effluent was 3.67 log units/100 ml and some of the values were below the detection limit. The highest value for F-specific bacteriophages was 1.37 log units/100 ml and most of the values were below the detection limit. Although there are no guidelines from the WHO or other institutions regarding maximal content of these phages in reclaimed wastewater for irrigation, due to the low numbers found in this study it can be concluded that the concentration of pathogenic viruses will be also low. The average removal of FC through the whole treatment train was 6.02 log units/100 ml during summer and 4.56 log units/100 ml during winter. The average removal of somatic coliphages and F-specific bacteriophages in the treatment system was 6.03 and 6.47 log units/100 ml, respectively, during summer and 4.16 and 5.76 log units/100 ml, respectively, during winter. Taking into account that there are no significant differences in the concentrations of the three pathogen indicators in the system s influent, it can be claimed that the capability of the treatment system for FC, somatic coliphage and F-specific bacteriophage removal is higher during summer than during winter. The average removal of FC, somatic coliphages and F-specific bacteriophages for summer and winter in each component of the treatment system is presented in Figure 3. All the components have an average removal of the three indicators monitored higher during summer than during the winter, except the seasonal storage reservoir and, in the case of the F-specific bacteriophages, also the stabilization reservoirs. These exceptions are probably due to the low numbers of the three pathogen indicators found in these two last components of the system, being difficult to determine significant improvement in the removal efficiencies. It has to be noticed that those microorganisms still remaining viables are more resistant to the ambient conditions, so their elimination is more difficult. The higher removal rates during summer for the three indicators monitored can be explained by the major environmental factors effects. These include the high ambient temperature, solar radiation and ph which cause microorganisms content reduction (Saqqar and Pescod, 1992; Davies-Colley et al., 1999). The stabilization reservoirs and the seasonal storage reservoir were the components of the treatment system with the highest reduction rates of FC and somatic coliphages, independently of the period of study (summer and winter). It is probably due to the extended hydraulic retention time in the last components of the treatment system which is much higher than in the rest of the components. The stabilization reservoirs have a theoretical hydraulic retention time of 40 days and in the seasonal storage reservoir the retention time is about 150 days (Table 1). The hydraulic retention time is considered to be similiar for summer and winter due to the fact that, in this case, the irrigation season lasts the whole year. These results emphasize the major importance of the hydraulic retention time in pond disinfection and are in agreement with other studies (Polprasert et al., 1983; Oragui et al., 1987; Rangeby et al., 1996). The extended retention time in ponds allows other factors in the lagoon environment, such as ph, temperature, solar radiation, etc. to affect microorganisms die-off. The stabilization reservoirs and the seasonal storage reservoir had also the highest reduction rates of F-specific bacteriophage removal, however, primarily for the winter period. During the summer period, the maturation pond and the rock filter had the highest reduction rate for F-specific bacteriophage removal. with values of 1.69 and 1.72 log units/100 ml, respectively (Figure 3). These results indicate that F-specific bacteriophages are removed during summer more efficiently in components of the treatment system located before the stabilization reservoirs and the seasonal storage reservoir. This behaviour differs from the one observed for FC and somatic coliphages.

5 Fecal coliforms (Log cfu/100 ml) Rw An Fc Mt Ft Rs St Summer Winter (a) 8.00 Somatic coliphages (Log pfu/100 ml) F-specific bacteriophages (Log pfu/100 ml) Rw An Fc Mt Ft Rs St Summer Winter (b) Rw An Fc Mt Ft Rs St Summer Winter (c) Figure 2 Mean and standard deviation of FC (a), somatic coliphage (b) and F-specific bacteriophage (c) content in the raw wastewater and the effluent of each component of the treatment system for summer and winter periods. Rw = raw wastewater, An = anaerobic pond, Fc = facultative pond, Mt = maturation pond, Ft = rock filter, Rs = stabilization reservoir, St = seasonal storage reservoir The removal rates for FC and somatic coliphages in the different components of the treatment system are similar. However, it is noted that the removal of somatic coliphages and F-specific bacteriophages in the anaerobic ponds is higher than for FC (Figure 3). This result indicates that phages are removed more efficiently than FC in this type of ponds. The main mechanism for microorganisms removal in anaerobic ponds is the adsorption onto settling solids (Bitton, 1975). The rock filter presents a good reduction of the three pathogen indicators content monitored. The less land requirements for the rock filters would minimize this disadvantage of the classical stabilization pond systems. F-specific bacteriophages were removed in a higher rate than FC and somatic coliphages in all the components of the treatment system, where their concentrations in the effluent of 181

6 2.50 FC Log (Ni/Ne)/100 ml An Fc Mt Ft Rs St Summer Winter (a) F-specific bacteriophages Log (Ni/Ne)/100 ml Somatic coliphages Log (Ni/Ne)/100 ml Summer Winter An Fc Mt Ft Rs St (b) An Fc Mt Ft Rs St Summer Winter (c) Figure 3 Average removal of FC, somatic coliphages and F-specific bacteriophages in each component of the treatment system for summer and winter. Ni = microorganisms concentration in the affluent of each component, Ne = microorganisms concentration in the effluent of each component. Rw = raw wastewater, An = anaerobic pond, Fc = facultative pond, Mt = maturation pond, Ft = rock filter, Rs = stabilization reservoir, St = seasonal storage reservoir the stabilization reservoirs were below the detection limit (Figures 2 and 3). Turner and Lewis (1995), in their evaluation of a stabilization pond treatment system in New Zealand, also found that F-specific bacteriophages were reduced at a higher rate than FC. These results indicate that F-specific bacteriophages may not be adequate as viral indicators for this treatment system, due to their rapid elimination. However, more work is needed to confirm this tendency. 182

7 Influence of physico-chemical parameters in microorganisms removal Average values for summer and winter periods of the different physico-chemical parameters in the various systems components were evaluated. The difference between the summer and winter periods for ph, BOD 5, COD, and SS are less important and hardly explain the capability of the treatment system for a higher FC, somatic coliphage and F-specific bacteriophage removal rates during summer than during winter. Almost all the components have an average removal of the three indicators monitored higher during summer than in winter (Figures 2 and 3). However, a good performance of the treatment system with regards to the various conventional parameters was noticed. BOD 5 removal was around 95% during summer and 98% during winter, COD removal was 87% during summer and 90% during winter and SS removal was approximately 92% during summer and 94% during winter. Temperature is the parameter that better explains the main differences between summer and winter and seems to be the most important factor influencing the performance of the system during the two periods. During summer, when the temperature is higher, the removal rates for the three pathogen indicators are higher than during the winter. This result is in agreement with other studies (Saqqar and Pescod, 1992; Mills et al., 1992) and emphysizes the importance of the temperature in pond disinfection. Conclusions Field experiments were conducted in order to evaluate the efficiency of domestic wastewater treatment in an integrated stabilization pond and reservoir system. Subject to the work several conclusions can be drawn. (a) The FC content of the final effluent of the treatment system complies with the WHO guidelines (1989) for unrestricted irrigation with reclaimed wastewater. (b) The stabilization reservoirs and the seasonal storage reservoir allow a significant reduction of FC, somatic coliphage and F-specific bacteriophage concentration in the final effluent, confirming that the additional hydraulic retention time of these components improves the microbial quality of the wastewater. (c) The rock filter presents a good reduction of the three pathogen indicators content monitored. The reduced land requirements for rock filter ponds would minimize this disadvantage of the classical waste stabilization pond systems and make the rock filters an upgrading phase. (d) Anaerobic ponds seem to be more efficient for the removal of somatic coliphages and F-specific bacteriophages than for FC. (e) F-specific bacteriophages are removed more efficiently than FC and somatic coliphages in this treatment system. This can imply that F-specific bacteriophages would not be good indicators for pathogenic viruses behaviour in this kind of systems due to their rapid elimination. However, more work is needed to confirm this tendency. (f) Temperature seems to be one of the more influential parameters in microorganisms removal. The performance of the system regarding BOD 5, COD, and SS removal is very good, yielding removal rates of around 90% or above with negligible differences between summer and winter periods. Acknowledgements The partial financial support for this study by EC COPERNICUS research fund, project number IC15-CT , the EC INCO-MED (DGXII) research fund, contract ICA3-CT , the Beracha Foundation of the Grand Water Institute, The Technion, Haifa, and the Ministry of Environment Quality of the State of Israel is greatly acknowledged. 183

8 References APHA (1998). Standard Methods for the Examination of Water and Wastewater. 19th edition, American Public Health Association, Washington DC., USA. Bitton, G. (1975). Adsorption of viruses onto surfaces in soil and water. Wat. Res., 9, Davies-Colley, R.J., Donnison, A.M., Speed, D.J., Ross, C.M. and Nagels, J.W. (1999). Inactivation of faecal indicator microorganisms in waste stabilization ponds: interactions of environmental factors with sunlight. Wat. Res., 33(5), Havelaar, A.H., Vanolphen, M. and Drost, Y.C. (1993). F-specific RNA bacteriophages are adequate model organisms for enteric viruses in fresh water. Appl. Env. Microbiology, 59(9), ISO (1996). Water Quality-Detection and Enumeration of Bacteriophages. Part 1: Enumeration of F- Specific RNA Bacteriophages. ISO/CD , International Organization for Standardization, Geneva, Switzerland. ISO (1999). Water Quality-Detection and Enumeration of Bacteriophages. Part 2: Enumeration of Somatic Coliphages. ISO/DIS , International Organization for Standardization, Geneva, Switzerland. Jofre, J., Bosch, A., Lucena, F., Girones, R. and Tartera, C. (1986). Evaluation of Bacteroides fragilis bacteriophages as indicators of the virological quality of water. Wat. Sci. Tech., 18(10), Kott, Y., Roze, N., Sperber, S. and Betzer, N. (1974). Bacteriophages as viral pollution indicators. Wat. Res., 8, Mara, D.D. (1976). Sewage Treatment in Hot Climates. John Wiley and Sons, Ltd., London, UK. Mara, D. and Pearson, H. (1998). Design Manual for Waste Stabilization Ponds in Mediterranean Countries European Investment Bank, Mediterranean Environmental Technical Assistance Programme, Lagoon Technology International, Leeds, UK. Maynard, H.E., Ouki, S.K. and Williams S.C. (1999). Tertiary lagoons: a review of removal mechanisms and performance. Wat. Res., 33(1), Mills, S.W., Alabaster, G.P., Mara, D.D., Pearson, H.W. and Thitai, W.N. (1992). Efficiency of faecal bacterial removal in waste stabilization ponds in Kenya. Wat. Sci. Tech., 26(7 8), Oragui, J.I., Curtis, T.P., Silva, S.A. and Mara, D.D. (1987). The removal of excreted bacteria and viruses in deep waste stabilization ponds in Northeast Brazil. Wat. Sci. Tech., 19, Polprasert, C., Dissanayake, M.G. and Than, N.C. (1983). Bacterial die-off kinetics in waste stabilization ponds. JWPCF, 55(3), Rangeby, M., Johansson, P. and Pernrup, M. (1996). Removal of faecal coliforms in a wastewater stabilization pond system in Mindelo, CapeVerde. Wat. Sci. Tech., 34(11), Saqqar, M.M. and Pescod, M.B. (1992). Modeling coliform reduction in wastewater stabilization ponds. Wat. Sci. Tech., 26(7 8), Turner, S.J. and Lewis, G.D. (1995). Comparison of F-specific bacteriophage, enterococci and faecal coliform densities through a wastewater treatment process employing oxidation ponds. Wat. Sci. Tech., 31(5 6), World Health Organisation (WHO) (1989). Health guidelines for the use of wastewater in agriculture and aquaculture. Technical Report, Series No 778. World Health Organization, Geneva, Switzerland. 184