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Post-treatment of UASB reactor effluent in an integrated duckweed and stabilization pond system
PII: S0043-1354(98)00270-X
Wat. Res. Vol. 33, No. 3, pp. 615±620, 1999 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/98 $19.00 + 0.00
POST-TREATMENT OF UASB REACTOR EFFLUENT IN AN INTEGRATED DUCKWEED AND STABILIZATION POND SYSTEM M M PETER VAN DER STEEN1, ASHER BRENNER1* , JOOST VAN BUUREN2* and M GIDEON ORON1** 1 Water Resources Environmental Center, Jacob Blaustein Institute for Desert Research, 84990 Kiryat Sde Boker, Israel and 2Department of Environmental Technology, Wageningen Agricultural University, Wageningen, The Netherlands
(First received October 1997; accepted in revised form June 1998) AbstractÐPost-treatment of euent from an Up¯ow Anaerobic Sludge Blanket (UASB) reactor, that was fed with domestic sewage, was conducted in an integrated pond system. The system consisted of a series of shallow duckweed and stabilization ponds. The main objective of post-treatment is removal of bacterial pathogens and further polishing of euent quality. Rapid and ecient pathogen removal can be achieved in shallow stabilization ponds but their euent BOD and TSS is relatively high, due to presence of algae. Passing stabilization pond euent through duckweed ponds was expected to remove algae due to reduced light penetration. Duckweed ponds have revenue generating potential since the produced biomass can be used as animal fodder. However, when applied separately, their pathogen removal is poor. A pilot plant system with an overall retention time of 4.2 days, was tested for this purpose. This system consisted of 10 ponds in series, arranged in 3 stages. The ®rst stage consisted of 2 duckweed ponds the second stage of 3 stabilization ponds and the third stage of 5 duckweed ponds. The system's euent median fecal coliform count in two experimental periods of 6 months was 3.3*102±5.0*103 per 100 ml. Increasing the retention time of the stabilization ponds to 3±4 days is suggested for consistently satisfying the WHO criterion for unlimited irrigation. Rapid removal took place in the stabilization ponds. A ®rst order fecal coliform decay constant Kd was calculated for each of the three stages. The values obtained were 0.7±3.2, 4.0±5.9 and about 1.4 dÿ1, respectively. The shading by the duckweed cover in the last stage proved to be able to remove practically all algae. Therefore, an excellent euent quality with respect to TSS was achieved (11 mg/l). It was demonstrated that duckweed biomass-production and wastewater treatment for reuse in irrigation can be achieved in one simple system. # 1998 Elsevier Science Ltd. All rights reserved Key wordsÐstabilization ponds, duckweed ponds, fecal coliforms, pathogens, wastewater treatment, euent reuse, UASB reactor
INTRODUCTION
Stabilization ponds for post-treatment Several studies carried out in both pilot-scale and full-scale systems have demonstrated that the Up¯ow Anaerobic Sludge Blanket (UASB) reactor is a reliable and simple technology for treatment of domestic sewage (Lettinga et al., 1993; Van Haandel and Lettinga, 1994). Therefore, this technology could be applied in wastewater treatment and reuse schemes in arid and semi-arid regions in developing countries. However, the UASB euent still contains high counts of fecal micro-organisms. In order to achieve the euent quality required by
*Author to whom all correspondence should be addressed; also at the Department of Industrial Engineering and management, Ben-Gurion University. [Fax: +972-7659-6909; E-mail: [email protected]]. {Less than 1000 fecal coliforms per 100 ml and less than 1 helminth egg per l. 615
the World Health Organization (WHO, 1989) for unlimited irrigation{, the UASB euent should undergo post-treatment. It has been shown that post-treatment of UASB euent in a series of shallow Stabilization Ponds (SP) can reduce the concentration of both helminth eggs and fecal coliforms below the guideline-concentrations of the WHO for unlimited irrigation (Van Haandel and Lettinga, 1994; Dixo et al., 1995). Systems with a plug-¯ow hydraulic regime were found to be more ecient in pathogen removal (Marais, 1974; Catunda and Van Haandel, 1996). However, conventional SP cannot usually be designed as a plug-¯ow system because the excessive loading of the ®rst pond(s) will cause anaerobiosis. Anaerobiosis is associated with odor generation and poor bacterial-pathogen removal. Introduction of preliminary UASB treatment enables implementation of plug-¯ow conditions in pond systems. Thus, the overall retention time in the treatment system can also be reduced. Van Haandel and
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Peter van der steen et al.
Lettinga (1994) calculated, on the basis of experimental results, that a plug-¯ow system consisting of shallow SP and having a retention time of only 3 days could meet the WHO euent quality criteria. Bacterial decay in stabilization ponds The decay of bacterial pathogens in SP systems has been the subject of extensive research during the last decades. It was shown that bacterial pathogen decay results from complex interactions of several factors such as light radiation, depletion of nutrients, microbial antagonism, presence of antibacterial substances produced by algae and high oxygen concentrations (Polprasert et al., 1983; Pearson et al., 1987; Saqqar and Pescod, 1992). Another important factor is the high pH that occurs during daytime, especially in shallow SP with intensive algae-growth (Parhad and Rao, 1974). Curtis et al. (1992a,b) partly elucidated the mechanism by which light, in combination with high pH values and high oxygen concentrations, accelerates the decay of bacterial pathogens. Integration of stabilization and duckweed ponds A drawback of shallow SP is the low eciency of TSS and BOD removal, due to the presence of algae in the euent (Dixo et al., 1995). This could result in diculties to satisfy discharge criteria for BOD or in reuse applications for drip-irrigation. This work intends to show that algae can be removed from SP euent by passing the SP euent through a stage with reduced illumination. This shading is expected to cause the algae to die and to settle or disintegrate. The shading can be realized in a series of Duckweed Ponds (DP). Duckweed ponds are covered by a ¯oating mat of macrophytes, thus preventing light penetration into the pond.
DP have been applied as a polishing treatment stage to remove nutrients from wastewater. The high growth rates of the macrophyte permits regular harvesting of the biomass and hence nutrients are removed from the system. The produced biomass has an economic value because it can be applied as fodder for poultry and ®sh (Oron et al., 1987; Skillicorn et al., 1993; Oron, 1994). The duckweed production and nutrient removal potential of the system are reported elsewhere (Van der Steen et al., 1998). An important sanitary disadvantage of DP is their poor performance with respect to bacterial pathogen removal due to the reduced light penetration into the water. This problem can be simply solved by combining DP with SP in an integrated pond system, to bene®t from their respective advantages. MATERIALS AND METHODS
The experimental pond system A pilot-scale research in a pond system consisting of three stages is in progress (Fig. 1). The ®rst stage consists of 2 DP. This stage is included to bene®t from the high nutrient concentration in the UASB euent for duckweed production. The second stage consists of 3 SP, especially for the removal of fecal coliforms. The third stage consists of 5 DP for the removal of algae and for further nutrient conversion and euent polishing. The in¯uent to the pond system is euent of a UASB reactor, fed with domestic sewage of the Sde Boker Campus (Negev Desert, Israel). The pond system comprises 10 mini-ponds in series, in order to create a plug¯ow pattern. The volume of each mini-pond is 63 l, the depth is 0.29 m and the surface area is 0.24 m2. The mean ¯ow rate is 6.2 l/h and the overall retention time is 4.2 days. During the 1996 experimental period, once a week 50% of the duckweed cover was harvested from each duckweed pond. It was expected that in this way the duckweed mat would be dense enough to prevent growth of algae. During the 1997 experimental period, the
Fig. 1. Schematic description of the integrated pond system consisting of duckweed and stabilization ponds for post-treatment of UASB-euent.
Post-treatment of UASB reactor euent harvesting was reduced to 50% of the duckweed cover once in 2 weeks. The ponds are located in an open ®eld exposed to local weather conditions, characterized by high summer temperatures (30±358C in the afternoon), mild winter temperatures (about 208C in the afternoon), annual precipitation of about 80 mm, occasionally strong winds and continuous strong sunlight radiation. The average intensity of global radiation during daytime ranges from 300 to 600 W/m2. The results presented in this paper were obtained during the period March to December 1996 and March to August 1997. The main characteristics of the UASB reactor euent that was applied as the pond system in¯uent, are given in Table 1. Once a week, a composite sample (24 h) of the pond system in¯uent and a grab sample of the pond system euent were collected for chemical analysis. Pond system euent samples were taken 10 cm below the duckweed cover of the last pond, to prevent duckweed withdrawal. All samples were analyzed for Chemical Oxygen Demand (COD), Total Suspended Solids (TSS), pH, NH4-N and PO4-P according to APHA (1995). The pH and oxygen concentration in each pond were monitored routinely several times during the day, from just after sunrise until sunset. Grab samples of the pond system in¯uent and of ponds 2 (DP), 5 (SP) and 10 (DP) were taken every second week (15 samples in duplicate for 1996, 17 samples for 1997). The samples of those ponds represent the euent of the ®rst DP stage, the SP stage and the second DP stage, respectively. The samples were analyzed for fecal coliforms using the membrane ®ltration method (APHA, 1995). Tracer studies for hydraulic ¯ow pattern characterization Rebhun and Argaman (1965) proposed to characterize the hydraulic ¯ow pattern in sedimentation basins by the relative volume fraction of plug ¯ow and CSTR (Continuous Stirred Tank Reactor) regime. For theoretical calculations, any reactor system can therefore be considered as a plug ¯ow reactor and a CSTR in series, each having a fraction of the overall retention time. It was decided to use this method for the characterization of the hydraulic ¯ow pattern in the pond system, rather than the mixers-in-series model or the dispersion model (Levenspiel, 1972). The mixers-in-series model is especially suitable for situations with intense mixing, that do not occur in our ponds. The dispersion model is recommended for describing bacterial die-o kinetics in single-cell pond systems (Polprasert et al., 1983). Analyzing the mixing conditions in the pond system showed that the model proposed by Rebhun and Argaman (1965) is preferable.
617
A tracer experiment for the determination of the ¯ow pattern in each stage of the pond system was conducted. The tracer (6 g LiCl) was added in one pulse to the pond system in¯uent. Three or four samples were collected daily from the euent of pond 2, 3 and 5, during three retention times. In this way the fraction of plug ¯ow could be determined for a series of 2, 3 and 5 ponds. The samples were analyzed for Li content by atomic emission measurements Rebhun and Argaman (1965) de®ned a function F(t), being the fraction of ¯uid with detention time less than time parameter ``t''. This function can be determined from the euent tracer concentration: 1 t c t dt c t dt 1 F t 0
0
where c(t) = euent tracer concentration (mg/l) and t = time (days). For a sedimentation basin, F(t) was found to be described by the following equation (Rebhun and Argaman, 1965): ÿ1 t F t 1 ÿ exp ÿ p 1 ÿ m 2 1 ÿ p 1 ÿ m y where p = fraction of plug-¯ow (0Rp R1) and m = fraction of dead-volume (0RmR 1). After F(t) has been determined by the tracer experiment, [1 ÿ F(t)] vs [t/y] can be plotted on semi-log paper and this gives a straight line. Because equations 1 and 2 describe the same function, the slope of the curve is equal to [ÿ(log e)/((1 ÿ p)(1 ÿ m))] and for F(t) = 0 the value of p(1 ÿ m) is obtained. The fraction plug ¯ow ( p) and dead volume (m) can then be calculated. Mathematical model for pathogen decay Marais (1974) proposed to use Chick's law for fecal coliform decay in stabilization ponds. A mass balance for fecal coliforms over a CSTR and plug ¯ow, respectively, gives the following equations: for CSTR: Ne N0 = 1 Kd y
3
for Plug flow: Ne N0 exp ÿKd y
4
where N0=in¯uent fecal coliform count (#/100 ml); Ne=euent fecal coliform count (#/100 ml) and Kd=®rst order decay constant (dayÿ1). After the percentage of CSTR and plug ¯ow have been determined the Kd value can be determined by solving the
Table 1. Characteristics of pond system in¯uent (UASB reactor euent) and euent, and removal eciencies (1996 and 1997 period) Parameter
BOD-total BOD-®ltered BOD-suspendeda COD-total COD-®ltered COD-suspendeda TSS NH4-N NO3-N N-organic PO4-P pH a
Suspended = total-®ltered.
Concentration (mg/l, unless otherwise indicated) in¯uent
euent
23 213 13 26 10 28 126 281 54 237 72 262 35 230 48 218 negligible 6 29 16 23 7.4±7.9
8 25 4 22 4 24 49 220 29 220 20 220 11 24 26 212 2 21 negligible 11 24 7.4±8.3
Removal eciency (%)
No. of samples
60 232 65 225 67 236 54 224 41 237 65 233 57 229 46 226 ÿ 100 33 229 ÿ
21 21 21 43 43 43 43 51 7 7 11 30
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Peter van der steen et al.
The median pond system euent count was 3.3*102 per 100 ml in the 1996 period and 5.0*103 in the 1997 period. The median count satis®ed the WHO (1989) criteria for unlimited irrigation in the 1996 period, but it did not in the 1997 period. Fecal coliforms removal in the pond system was approximately 99.6±99.95%, thus raising the overall fecal coliforms removal potential of the combined system (UASB reactor and pond system) to 99.9±99.99%. The removal of fecal coliforms did not dier signi®cantly between the summer and winter months. The mean water temperature in the integrated pond system ranged during the summer months from 228C in the morning to 318C in the afternoon and during the winter months from 128C in the morning to 188C in the afternoon. The tracer study and data analysis using equations 1±4 showed that the ®rst DP stage, the SP stage and the second DP stage comprised of 14, 27 and 37% plug ¯ow, respectively (Table 2). As expected the plug ¯ow fraction increases along with the number of ponds per stage. The Kd values for the three stages, as calculated by equation 5, were 0.7±3.2, 4.0±5.9 and about 1.4 dayÿ1, respectively. The Kd values that are reported in the literature (Table 3) show a large variation, even for the same type of pond. Dierences in factors such as temperature, pond depth, organic loading, pH and oxygen levels might cause this variation. The Kd value in the second DP stage was similar to the Kd value reported by Van Buuren and Hobma (1991), who found values of 0.3 to 1.2 dayÿ1 for DP treating algae pond euent, operated indoor at 208C. The Kd value obtained for the ®rst DP stage during 1997 (0.7) was also in that range, however, the Kd value for the same stage during 1996 was considerably higher (3.2). This might be explained by the UASB reactor still being in the start-up phase during 1996. Therefore there was still a considerable fraction of the fecal coliforms in the UASB euent that could be removed by sedimentation processes. The Kd for the ®rst DP stage during that period was therefore more similar to Kd values reported for anaerobic ponds that treat raw sewage (Table 3).
equation for fecal coliform decay in a system of a CSTR and a plug ¯ow in series: 1 Ne N0 fexp ÿKd pyg 5 1 Kd 1 ÿ py where p = fraction of plug ¯ow (0RpR1); (1 ÿ p) = fraction of CSTR. For each of the three treatment stages (®rst DP stage, SP stage, second DP stage) the ®rst order decay constant Kd was calculated accordingly. RESULTS AND DISCUSSION
Removal of BOD and TSS Post-treatment in the combined duckweed and algae pond system was able to remove 60 2 32% of UASB euent BOD (Table 1). The overall BOD removal potential of the combined system, consisting of an UASB reactor and followed by a pond system is therefore about 90%: approximately 75% BOD removal in the UASB reactor (Lettinga et al., 1993) and an additional 60% (from the UASB euent) in the integrated pond system. Mean euent TSS concentration was 11 2 4 mg/l, which is considerably lower than commonly achieved in high rate algae ponds (Oron and Shelef, 1982). Dixo et al. (1995) even observed a slight increase in TSS when UASB euent was treated in a series of shallow maturation ponds, due to the presence of algae in the euent. Actually, in the present study, euent of pond No. 5 (last SP) contained small algae¯ocs leading to TSS concentrations often higher than in the pond system in¯uent. The duckweed mat on the second DP stage facilitated the decay of most algae by reducing the penetration of sunlight into those ponds. Apparently, the algae died, settled and disintegrated. The ®nal euent had a light green color, however, no algae ¯ocs were observed. During a few summer weeks, the duckweed density in the second DP stage decreased and the pond euent concentration immediately increased to about 50 mg/l due to algae development. Pathogen removal Median fecal coliforms count in the pond system in¯uent was 6.3*105±1.3*106 per 100 ml, typical for UASB euent (Van Buuren and Hobma, 1991).
Table 2. Fecal coliforms count, the volume fraction of plug ¯ow and the ®rst order coliform decay constant Kd for each stage of the integrated pond system Median fecal coliform count (#/100 ml) (year)
In¯uent Euent First duckweed pond stage (DP1) Stabilization pond stage (SP) Second duckweed pond stage (DP2)
Range of fecal coliform count (#/100 ml)
1996
1997
6.3*105
1.3*106
5
5
1.3*10 2.7*103 3.3*102
7.8*10 4.3*104 5.0*103
105 ±106 4
6
10 ±10 102 ±104 102 ±104
Volume fraction plug ¯ow
First order coliform decay constant Kd (d-1) (year) 1996
1997
ÿ
ÿ
ÿ
0.14 0.27 0.37
3.2 5.9 1.4
0.7 4.0 1.4
Post-treatment of UASB reactor euent
619
Table 3. First order coliforms decay constant Kd as reported in literature Pond type
Temperature (8C)
Depth (m)
Kd (dayÿ1)
Refs.
Anaerobic
25 23
2.5 1.5
2.7 1.9
Arridge et al. (1995); Pearson et al. (1995) de Oliveira et al. (1996)
Facultative
25 12±25 12±25
1±2 2.2 1.6
1.2±1.9 0.3±0.5 0.4±0.8
Arridge et al. (1995); Pearson et al. (1995) Saqqar and Pescod (1992) Saqqar and Pescod (1992)
Maturation
25 12±25
0.4±1.0 1.2
6±13 0.4±0.9
Arridge et al. (1995); Pearson et al. (1995) Saqqar and Pescod (1992)
Mixed high rate algae pond
12 28
0.4 0.4
1 25
El Hamouri et al. (1995) El Hamouri et al. (1995)
Shallow stabilization ponds
25 25 25 25 25
0.3 0.7 1.0 2.2 3.0
7.3 4.3 3.0 1.3 0.8
Duckweed pond
20
0.4
0.3±1.2
The Kd value for the SP stage was notably higher than in the DP stages, but lower than reported for shallow stabilization ponds (Table 3). Environmental conditions in the SP were favorable for pathogen decay, due to unhindered light penetration and algae growth. Algal photosynthesis caused oxygen oversaturation and pH increase during daytime (Fig. 2). In combination with the intense radiation it resulted in rapid decay of fecal coliforms (Curtis et al., 1992a,b). The dierence in Kd value for the SP stage in the two periods is considerable. This corresponds with the observations that the mean pH in the afternoon during the 1997 period was consistently lower than in the 1996 period (8.7 in 1996, 8.2 in 1997). The pH was
Fig. 2. Oxygen concentrations (a) and pH (b) in the pond system during a typical day (sunrise about 6.00, sunset about 19.30; SP-stabilization pond, DP-duckweed pond).
Van Van Van Van Van
Haandel Haandel Haandel Haandel Haandel
and and and and and
Lettinga Lettinga Lettinga Lettinga Lettinga
(1994) (1994) (1994) (1994) (1994)
Van Buuren and Hobma (1991)
shown to relate linearly to the Kd (Dixo et al., 1995). The pH in the SP stage was almost never above 9, in both experimental periods. This con®rms the observation by Curtis and co-workers that it is not necessary to reach a pH value between 9.0 and 9.3 to cause rapid pathogen decay, as was suggested by other researchers (Parhad and Rao, 1974; Pearson et al., 1987). Rapid pathogen decay took place even without extreme pH values, due to intense solar radiation and high oxygen concentrations. CONCLUSIONS
The proposed integrated duckweed and stabilization pond system proved to be a simple and eective method for post-treatment of UASB reactor euent. Fecal coliforms count in the pond system euent met the WHO (1989) criterion for unlimited irrigation mainly during the 1996 experimental period. Increasing the retention time in the SP stage to 3±4 days is suggested, for consistently satisfying the WHO (1989) criterion for unlimited irrigation. However, preliminary treatment in the UASB reactor reduced signi®cantly the required retention time of the pond system, when compared to conventional stabilization ponds that produce the same microbiological quality (20±25 days retention time). It was shown that integrating duckweed and stabilization ponds in one system neutralized their respective disadvantages. Ecient removal of both pathogens and suspended solids was achieved. Duckweed production and wastewater treatment for safe reuse in irrigation can be achieved in one simple system. Main conclusions derived from this study may be summarized as follows: . Fecal coliforms removal in the integrated pond system was 99.6±99.95%, resulting in a median euent count of 3.3*102±5.0*103 per 100 ml. Most
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removal took place in the SP. The fecal coliform decay constant Kd of the ®rst DP stage, the SP stage and the second DP stage were 0.7±3.2, 4.0±5.9 and about 1.4 dÿ1, respectively. . Passing the SP euent through a stage with reduced solar radiation, having a retention time of 2 days, proved sucient to remove practically all algae. Euent quality with respect to TSS was excellent (11 mg/l). AcknowledgementsÐThe work was partially supported by the European Union AVICENNE program, research project number 93AVI076 on ``Integrated Management of Reclaimed Wastewater Resources in the Mediterranean Region'' and by the Midbar Foundation, The Netherlands. The authors are very grateful for assistance in the work by Christopher Nganga, Babatunde O. Adekanbi and Dr Ludmilla Katz from the Water Resources Environmental Center. REFERENCES
APHA (1995) Standard Methods for the Examination of Water and Wastewater, 19th edn. Washington D.C., U.S.A. Arridge H., Oragui J. I., Pearson H. W., Mara D. D. and Silva S. A. (1995) Vibrio cholerae 01 and Salmonellae removal compared with the die-o of faecal indicator organisms in waste stabilization ponds in Northeast Brazil. Water Sci. Technol. 31(12), 249±256. Catunda P. F. C. and Van Haandel A. C. (1996) Improved performance and increased applicability of waste stabilisation ponds by pretreatment in a UASB reactor. Water Sci. Technol. 33(7), 147±156. Curtis T. P., Mara D. D. and Silva S. A. (1992a) In¯uence of pH, oxygen and humic substances on ability of sunlight to damage fecal coliforms in waste stabilization pond water. Appl. Environ. Microbiol. 58(4), 1335±1343. Curtis T. P., Mara D. D. and Silva S. A. (1992b) The eect of sunlight on faecal coliforms in ponds: implications for research and design. Water Sci. Technol. 26(7±8), 1729±1738. de Oliveira R., Silva S. A., Araujo A. L. C., Soares J., Mara D. D. and Pearson H. W. (1996) The performance of a pilot-scale series of ten ponds treating municipal sewage in Northeast Brazil. Water Sci. Technol. 33(7), 57±61. Dixo N. G. H., Gambrill M. P., Catunda P. F. C. and Van Haandel A. C. (1995) Removal of pathogenic organisms from the euent of an up¯ow anaerobic digester using waste stabilization ponds. Water Sci. Technol. 31(12), 275±284. El Hamouri B., Jellal J., Outabiht H., Nebri B., Khallayoune K., Benkerroum A., Hajli A. and Firadi R. (1995) The performance of a high-rate algal pond in the Moroccan climate. Water Sci. Technol. 31(12), 67± 74.
Lettinga G., DeMan A., VanderLast A. R. M., Wiegant W., VanKnippenberg K., Frijns J. and Van Buuren J. C. L. (1993) Anaerobic treatment of domestic sewage and wastewater. Water Sci. Technol. 27(9), 67±73. Levenspiel, O. (1972) Chemical Reaction Engineering, 2nd edn. John Wiley and Sons, New York. Marais G. v. R. (1974) Faecal bacterial kinetics in stabilization pond. J. Env. Eng. Div. Am. Soc. Civil Eng. 100(1), 119±139. Parhad N. M. and Rao N. U. (1974) Eect of pH on survival of Escherichia coli. J. Water Pollut. Control Fed. 46(5), 980±986. Pearson H. W., Mara D. D., Mills S. W. and Smallman D. J. (1987) Physico±chemical parameters in¯uencing faecal bacterial survival in waste stabilization ponds. Water Sci. Technol. 19(12), 145±152. Pearson H. W., Mara D. D. and Arridge H. A. (1995) The in¯uence of pond geometry and con®guration on facultative and maturation waste stabilisation pond performance and eciency. Water Sci. Technol. 31(12), 129±139. Polprasert C., Dissanayake M. G. and Thanh N. C. (1983) Bacterial die-o kinetics in waste stabilization ponds. J. Water Pollut. Control Fed. 55, 285±296. Oron G. and Shelef G. (1982) Maximizing algal yield in high rate oxidation ponds. J. Environ. Engng ASCE 108(EE4), 730±738. Oron G., Porath D. and Jansen H. (1987) Performance of the duckweed species Lemna gibba on municipal wastewater for euent renovation and protein production. Biotechnol. Bioeng. 29, 258±268. Oron G. (1994) Duckweed culture for wastewater renovation and biomass production. Agric. Water Manage. 26, 27±40. Rebhun M. and Argaman Y. (1965) Evaluation of hydraulic eciency of sedimentation basins. J. San. Eng. Division Am. Soc. Civil Eng. 91(SA5), 37±45. Saqqar M. M. and Pescod M. B. (1992) Modelling coliform reduction in wastewater stabilization ponds. Water Sci. Technol. 26(7±8), 1667±1677. Skillicorn P., Spira W. and Journey W. (1993) Duckweed Aquaculture. World Bank publication, Washington. Van Buuren J. C. L. and Hobma S. J. (1991) The Fecal Coliform Die-o at Posttreatment of Anaerobically Pretreated Wastewater. Internal report. Department of Environmental Technology, Wageningen Agricultural University, The Netherlands. Van der Steen P., Brenner A. and Oron G. (1998) Integrated duckweed and algae pond system for removal and conversion of nitrogen from wastewater into valuable products. Presented at the IAWQ Water Quality International `98 Conference, Vancouver, 1998. Van Haandel A. C. and Lettinga G. (1994) Anaerobic Sewage Treatment; A Practical Guide for Regions with a Hot Climate. John Wiley and Sons, Chichester, England. WHO (1989) Health Guidelines for the Use of Wastewater in Agriculture and Aquaculture. Technical report series No. 778. Geneva, World Health Organization.
Wat. Res. Vol. 33, No. 3, pp. 615±620, 1999 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/98 $19.00 + 0.00
POST-TREATMENT OF UASB REACTOR EFFLUENT IN AN INTEGRATED DUCKWEED AND STABILIZATION POND SYSTEM M M PETER VAN DER STEEN1, ASHER BRENNER1* , JOOST VAN BUUREN2* and M GIDEON ORON1** 1 Water Resources Environmental Center, Jacob Blaustein Institute for Desert Research, 84990 Kiryat Sde Boker, Israel and 2Department of Environmental Technology, Wageningen Agricultural University, Wageningen, The Netherlands
(First received October 1997; accepted in revised form June 1998) AbstractÐPost-treatment of euent from an Up¯ow Anaerobic Sludge Blanket (UASB) reactor, that was fed with domestic sewage, was conducted in an integrated pond system. The system consisted of a series of shallow duckweed and stabilization ponds. The main objective of post-treatment is removal of bacterial pathogens and further polishing of euent quality. Rapid and ecient pathogen removal can be achieved in shallow stabilization ponds but their euent BOD and TSS is relatively high, due to presence of algae. Passing stabilization pond euent through duckweed ponds was expected to remove algae due to reduced light penetration. Duckweed ponds have revenue generating potential since the produced biomass can be used as animal fodder. However, when applied separately, their pathogen removal is poor. A pilot plant system with an overall retention time of 4.2 days, was tested for this purpose. This system consisted of 10 ponds in series, arranged in 3 stages. The ®rst stage consisted of 2 duckweed ponds the second stage of 3 stabilization ponds and the third stage of 5 duckweed ponds. The system's euent median fecal coliform count in two experimental periods of 6 months was 3.3*102±5.0*103 per 100 ml. Increasing the retention time of the stabilization ponds to 3±4 days is suggested for consistently satisfying the WHO criterion for unlimited irrigation. Rapid removal took place in the stabilization ponds. A ®rst order fecal coliform decay constant Kd was calculated for each of the three stages. The values obtained were 0.7±3.2, 4.0±5.9 and about 1.4 dÿ1, respectively. The shading by the duckweed cover in the last stage proved to be able to remove practically all algae. Therefore, an excellent euent quality with respect to TSS was achieved (11 mg/l). It was demonstrated that duckweed biomass-production and wastewater treatment for reuse in irrigation can be achieved in one simple system. # 1998 Elsevier Science Ltd. All rights reserved Key wordsÐstabilization ponds, duckweed ponds, fecal coliforms, pathogens, wastewater treatment, euent reuse, UASB reactor
INTRODUCTION
Stabilization ponds for post-treatment Several studies carried out in both pilot-scale and full-scale systems have demonstrated that the Up¯ow Anaerobic Sludge Blanket (UASB) reactor is a reliable and simple technology for treatment of domestic sewage (Lettinga et al., 1993; Van Haandel and Lettinga, 1994). Therefore, this technology could be applied in wastewater treatment and reuse schemes in arid and semi-arid regions in developing countries. However, the UASB euent still contains high counts of fecal micro-organisms. In order to achieve the euent quality required by
*Author to whom all correspondence should be addressed; also at the Department of Industrial Engineering and management, Ben-Gurion University. [Fax: +972-7659-6909; E-mail: [email protected]]. {Less than 1000 fecal coliforms per 100 ml and less than 1 helminth egg per l. 615
the World Health Organization (WHO, 1989) for unlimited irrigation{, the UASB euent should undergo post-treatment. It has been shown that post-treatment of UASB euent in a series of shallow Stabilization Ponds (SP) can reduce the concentration of both helminth eggs and fecal coliforms below the guideline-concentrations of the WHO for unlimited irrigation (Van Haandel and Lettinga, 1994; Dixo et al., 1995). Systems with a plug-¯ow hydraulic regime were found to be more ecient in pathogen removal (Marais, 1974; Catunda and Van Haandel, 1996). However, conventional SP cannot usually be designed as a plug-¯ow system because the excessive loading of the ®rst pond(s) will cause anaerobiosis. Anaerobiosis is associated with odor generation and poor bacterial-pathogen removal. Introduction of preliminary UASB treatment enables implementation of plug-¯ow conditions in pond systems. Thus, the overall retention time in the treatment system can also be reduced. Van Haandel and
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Lettinga (1994) calculated, on the basis of experimental results, that a plug-¯ow system consisting of shallow SP and having a retention time of only 3 days could meet the WHO euent quality criteria. Bacterial decay in stabilization ponds The decay of bacterial pathogens in SP systems has been the subject of extensive research during the last decades. It was shown that bacterial pathogen decay results from complex interactions of several factors such as light radiation, depletion of nutrients, microbial antagonism, presence of antibacterial substances produced by algae and high oxygen concentrations (Polprasert et al., 1983; Pearson et al., 1987; Saqqar and Pescod, 1992). Another important factor is the high pH that occurs during daytime, especially in shallow SP with intensive algae-growth (Parhad and Rao, 1974). Curtis et al. (1992a,b) partly elucidated the mechanism by which light, in combination with high pH values and high oxygen concentrations, accelerates the decay of bacterial pathogens. Integration of stabilization and duckweed ponds A drawback of shallow SP is the low eciency of TSS and BOD removal, due to the presence of algae in the euent (Dixo et al., 1995). This could result in diculties to satisfy discharge criteria for BOD or in reuse applications for drip-irrigation. This work intends to show that algae can be removed from SP euent by passing the SP euent through a stage with reduced illumination. This shading is expected to cause the algae to die and to settle or disintegrate. The shading can be realized in a series of Duckweed Ponds (DP). Duckweed ponds are covered by a ¯oating mat of macrophytes, thus preventing light penetration into the pond.
DP have been applied as a polishing treatment stage to remove nutrients from wastewater. The high growth rates of the macrophyte permits regular harvesting of the biomass and hence nutrients are removed from the system. The produced biomass has an economic value because it can be applied as fodder for poultry and ®sh (Oron et al., 1987; Skillicorn et al., 1993; Oron, 1994). The duckweed production and nutrient removal potential of the system are reported elsewhere (Van der Steen et al., 1998). An important sanitary disadvantage of DP is their poor performance with respect to bacterial pathogen removal due to the reduced light penetration into the water. This problem can be simply solved by combining DP with SP in an integrated pond system, to bene®t from their respective advantages. MATERIALS AND METHODS
The experimental pond system A pilot-scale research in a pond system consisting of three stages is in progress (Fig. 1). The ®rst stage consists of 2 DP. This stage is included to bene®t from the high nutrient concentration in the UASB euent for duckweed production. The second stage consists of 3 SP, especially for the removal of fecal coliforms. The third stage consists of 5 DP for the removal of algae and for further nutrient conversion and euent polishing. The in¯uent to the pond system is euent of a UASB reactor, fed with domestic sewage of the Sde Boker Campus (Negev Desert, Israel). The pond system comprises 10 mini-ponds in series, in order to create a plug¯ow pattern. The volume of each mini-pond is 63 l, the depth is 0.29 m and the surface area is 0.24 m2. The mean ¯ow rate is 6.2 l/h and the overall retention time is 4.2 days. During the 1996 experimental period, once a week 50% of the duckweed cover was harvested from each duckweed pond. It was expected that in this way the duckweed mat would be dense enough to prevent growth of algae. During the 1997 experimental period, the
Fig. 1. Schematic description of the integrated pond system consisting of duckweed and stabilization ponds for post-treatment of UASB-euent.
Post-treatment of UASB reactor euent harvesting was reduced to 50% of the duckweed cover once in 2 weeks. The ponds are located in an open ®eld exposed to local weather conditions, characterized by high summer temperatures (30±358C in the afternoon), mild winter temperatures (about 208C in the afternoon), annual precipitation of about 80 mm, occasionally strong winds and continuous strong sunlight radiation. The average intensity of global radiation during daytime ranges from 300 to 600 W/m2. The results presented in this paper were obtained during the period March to December 1996 and March to August 1997. The main characteristics of the UASB reactor euent that was applied as the pond system in¯uent, are given in Table 1. Once a week, a composite sample (24 h) of the pond system in¯uent and a grab sample of the pond system euent were collected for chemical analysis. Pond system euent samples were taken 10 cm below the duckweed cover of the last pond, to prevent duckweed withdrawal. All samples were analyzed for Chemical Oxygen Demand (COD), Total Suspended Solids (TSS), pH, NH4-N and PO4-P according to APHA (1995). The pH and oxygen concentration in each pond were monitored routinely several times during the day, from just after sunrise until sunset. Grab samples of the pond system in¯uent and of ponds 2 (DP), 5 (SP) and 10 (DP) were taken every second week (15 samples in duplicate for 1996, 17 samples for 1997). The samples of those ponds represent the euent of the ®rst DP stage, the SP stage and the second DP stage, respectively. The samples were analyzed for fecal coliforms using the membrane ®ltration method (APHA, 1995). Tracer studies for hydraulic ¯ow pattern characterization Rebhun and Argaman (1965) proposed to characterize the hydraulic ¯ow pattern in sedimentation basins by the relative volume fraction of plug ¯ow and CSTR (Continuous Stirred Tank Reactor) regime. For theoretical calculations, any reactor system can therefore be considered as a plug ¯ow reactor and a CSTR in series, each having a fraction of the overall retention time. It was decided to use this method for the characterization of the hydraulic ¯ow pattern in the pond system, rather than the mixers-in-series model or the dispersion model (Levenspiel, 1972). The mixers-in-series model is especially suitable for situations with intense mixing, that do not occur in our ponds. The dispersion model is recommended for describing bacterial die-o kinetics in single-cell pond systems (Polprasert et al., 1983). Analyzing the mixing conditions in the pond system showed that the model proposed by Rebhun and Argaman (1965) is preferable.
617
A tracer experiment for the determination of the ¯ow pattern in each stage of the pond system was conducted. The tracer (6 g LiCl) was added in one pulse to the pond system in¯uent. Three or four samples were collected daily from the euent of pond 2, 3 and 5, during three retention times. In this way the fraction of plug ¯ow could be determined for a series of 2, 3 and 5 ponds. The samples were analyzed for Li content by atomic emission measurements Rebhun and Argaman (1965) de®ned a function F(t), being the fraction of ¯uid with detention time less than time parameter ``t''. This function can be determined from the euent tracer concentration: 1 t c t dt c t dt 1 F t 0
0
where c(t) = euent tracer concentration (mg/l) and t = time (days). For a sedimentation basin, F(t) was found to be described by the following equation (Rebhun and Argaman, 1965): ÿ1 t F t 1 ÿ exp ÿ p 1 ÿ m 2 1 ÿ p 1 ÿ m y where p = fraction of plug-¯ow (0Rp R1) and m = fraction of dead-volume (0RmR 1). After F(t) has been determined by the tracer experiment, [1 ÿ F(t)] vs [t/y] can be plotted on semi-log paper and this gives a straight line. Because equations 1 and 2 describe the same function, the slope of the curve is equal to [ÿ(log e)/((1 ÿ p)(1 ÿ m))] and for F(t) = 0 the value of p(1 ÿ m) is obtained. The fraction plug ¯ow ( p) and dead volume (m) can then be calculated. Mathematical model for pathogen decay Marais (1974) proposed to use Chick's law for fecal coliform decay in stabilization ponds. A mass balance for fecal coliforms over a CSTR and plug ¯ow, respectively, gives the following equations: for CSTR: Ne N0 = 1 Kd y
3
for Plug flow: Ne N0 exp ÿKd y
4
where N0=in¯uent fecal coliform count (#/100 ml); Ne=euent fecal coliform count (#/100 ml) and Kd=®rst order decay constant (dayÿ1). After the percentage of CSTR and plug ¯ow have been determined the Kd value can be determined by solving the
Table 1. Characteristics of pond system in¯uent (UASB reactor euent) and euent, and removal eciencies (1996 and 1997 period) Parameter
BOD-total BOD-®ltered BOD-suspendeda COD-total COD-®ltered COD-suspendeda TSS NH4-N NO3-N N-organic PO4-P pH a
Suspended = total-®ltered.
Concentration (mg/l, unless otherwise indicated) in¯uent
euent
23 213 13 26 10 28 126 281 54 237 72 262 35 230 48 218 negligible 6 29 16 23 7.4±7.9
8 25 4 22 4 24 49 220 29 220 20 220 11 24 26 212 2 21 negligible 11 24 7.4±8.3
Removal eciency (%)
No. of samples
60 232 65 225 67 236 54 224 41 237 65 233 57 229 46 226 ÿ 100 33 229 ÿ
21 21 21 43 43 43 43 51 7 7 11 30
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Peter van der steen et al.
The median pond system euent count was 3.3*102 per 100 ml in the 1996 period and 5.0*103 in the 1997 period. The median count satis®ed the WHO (1989) criteria for unlimited irrigation in the 1996 period, but it did not in the 1997 period. Fecal coliforms removal in the pond system was approximately 99.6±99.95%, thus raising the overall fecal coliforms removal potential of the combined system (UASB reactor and pond system) to 99.9±99.99%. The removal of fecal coliforms did not dier signi®cantly between the summer and winter months. The mean water temperature in the integrated pond system ranged during the summer months from 228C in the morning to 318C in the afternoon and during the winter months from 128C in the morning to 188C in the afternoon. The tracer study and data analysis using equations 1±4 showed that the ®rst DP stage, the SP stage and the second DP stage comprised of 14, 27 and 37% plug ¯ow, respectively (Table 2). As expected the plug ¯ow fraction increases along with the number of ponds per stage. The Kd values for the three stages, as calculated by equation 5, were 0.7±3.2, 4.0±5.9 and about 1.4 dayÿ1, respectively. The Kd values that are reported in the literature (Table 3) show a large variation, even for the same type of pond. Dierences in factors such as temperature, pond depth, organic loading, pH and oxygen levels might cause this variation. The Kd value in the second DP stage was similar to the Kd value reported by Van Buuren and Hobma (1991), who found values of 0.3 to 1.2 dayÿ1 for DP treating algae pond euent, operated indoor at 208C. The Kd value obtained for the ®rst DP stage during 1997 (0.7) was also in that range, however, the Kd value for the same stage during 1996 was considerably higher (3.2). This might be explained by the UASB reactor still being in the start-up phase during 1996. Therefore there was still a considerable fraction of the fecal coliforms in the UASB euent that could be removed by sedimentation processes. The Kd for the ®rst DP stage during that period was therefore more similar to Kd values reported for anaerobic ponds that treat raw sewage (Table 3).
equation for fecal coliform decay in a system of a CSTR and a plug ¯ow in series: 1 Ne N0 fexp ÿKd pyg 5 1 Kd 1 ÿ py where p = fraction of plug ¯ow (0RpR1); (1 ÿ p) = fraction of CSTR. For each of the three treatment stages (®rst DP stage, SP stage, second DP stage) the ®rst order decay constant Kd was calculated accordingly. RESULTS AND DISCUSSION
Removal of BOD and TSS Post-treatment in the combined duckweed and algae pond system was able to remove 60 2 32% of UASB euent BOD (Table 1). The overall BOD removal potential of the combined system, consisting of an UASB reactor and followed by a pond system is therefore about 90%: approximately 75% BOD removal in the UASB reactor (Lettinga et al., 1993) and an additional 60% (from the UASB euent) in the integrated pond system. Mean euent TSS concentration was 11 2 4 mg/l, which is considerably lower than commonly achieved in high rate algae ponds (Oron and Shelef, 1982). Dixo et al. (1995) even observed a slight increase in TSS when UASB euent was treated in a series of shallow maturation ponds, due to the presence of algae in the euent. Actually, in the present study, euent of pond No. 5 (last SP) contained small algae¯ocs leading to TSS concentrations often higher than in the pond system in¯uent. The duckweed mat on the second DP stage facilitated the decay of most algae by reducing the penetration of sunlight into those ponds. Apparently, the algae died, settled and disintegrated. The ®nal euent had a light green color, however, no algae ¯ocs were observed. During a few summer weeks, the duckweed density in the second DP stage decreased and the pond euent concentration immediately increased to about 50 mg/l due to algae development. Pathogen removal Median fecal coliforms count in the pond system in¯uent was 6.3*105±1.3*106 per 100 ml, typical for UASB euent (Van Buuren and Hobma, 1991).
Table 2. Fecal coliforms count, the volume fraction of plug ¯ow and the ®rst order coliform decay constant Kd for each stage of the integrated pond system Median fecal coliform count (#/100 ml) (year)
In¯uent Euent First duckweed pond stage (DP1) Stabilization pond stage (SP) Second duckweed pond stage (DP2)
Range of fecal coliform count (#/100 ml)
1996
1997
6.3*105
1.3*106
5
5
1.3*10 2.7*103 3.3*102
7.8*10 4.3*104 5.0*103
105 ±106 4
6
10 ±10 102 ±104 102 ±104
Volume fraction plug ¯ow
First order coliform decay constant Kd (d-1) (year) 1996
1997
ÿ
ÿ
ÿ
0.14 0.27 0.37
3.2 5.9 1.4
0.7 4.0 1.4
Post-treatment of UASB reactor euent
619
Table 3. First order coliforms decay constant Kd as reported in literature Pond type
Temperature (8C)
Depth (m)
Kd (dayÿ1)
Refs.
Anaerobic
25 23
2.5 1.5
2.7 1.9
Arridge et al. (1995); Pearson et al. (1995) de Oliveira et al. (1996)
Facultative
25 12±25 12±25
1±2 2.2 1.6
1.2±1.9 0.3±0.5 0.4±0.8
Arridge et al. (1995); Pearson et al. (1995) Saqqar and Pescod (1992) Saqqar and Pescod (1992)
Maturation
25 12±25
0.4±1.0 1.2
6±13 0.4±0.9
Arridge et al. (1995); Pearson et al. (1995) Saqqar and Pescod (1992)
Mixed high rate algae pond
12 28
0.4 0.4
1 25
El Hamouri et al. (1995) El Hamouri et al. (1995)
Shallow stabilization ponds
25 25 25 25 25
0.3 0.7 1.0 2.2 3.0
7.3 4.3 3.0 1.3 0.8
Duckweed pond
20
0.4
0.3±1.2
The Kd value for the SP stage was notably higher than in the DP stages, but lower than reported for shallow stabilization ponds (Table 3). Environmental conditions in the SP were favorable for pathogen decay, due to unhindered light penetration and algae growth. Algal photosynthesis caused oxygen oversaturation and pH increase during daytime (Fig. 2). In combination with the intense radiation it resulted in rapid decay of fecal coliforms (Curtis et al., 1992a,b). The dierence in Kd value for the SP stage in the two periods is considerable. This corresponds with the observations that the mean pH in the afternoon during the 1997 period was consistently lower than in the 1996 period (8.7 in 1996, 8.2 in 1997). The pH was
Fig. 2. Oxygen concentrations (a) and pH (b) in the pond system during a typical day (sunrise about 6.00, sunset about 19.30; SP-stabilization pond, DP-duckweed pond).
Van Van Van Van Van
Haandel Haandel Haandel Haandel Haandel
and and and and and
Lettinga Lettinga Lettinga Lettinga Lettinga
(1994) (1994) (1994) (1994) (1994)
Van Buuren and Hobma (1991)
shown to relate linearly to the Kd (Dixo et al., 1995). The pH in the SP stage was almost never above 9, in both experimental periods. This con®rms the observation by Curtis and co-workers that it is not necessary to reach a pH value between 9.0 and 9.3 to cause rapid pathogen decay, as was suggested by other researchers (Parhad and Rao, 1974; Pearson et al., 1987). Rapid pathogen decay took place even without extreme pH values, due to intense solar radiation and high oxygen concentrations. CONCLUSIONS
The proposed integrated duckweed and stabilization pond system proved to be a simple and eective method for post-treatment of UASB reactor euent. Fecal coliforms count in the pond system euent met the WHO (1989) criterion for unlimited irrigation mainly during the 1996 experimental period. Increasing the retention time in the SP stage to 3±4 days is suggested, for consistently satisfying the WHO (1989) criterion for unlimited irrigation. However, preliminary treatment in the UASB reactor reduced signi®cantly the required retention time of the pond system, when compared to conventional stabilization ponds that produce the same microbiological quality (20±25 days retention time). It was shown that integrating duckweed and stabilization ponds in one system neutralized their respective disadvantages. Ecient removal of both pathogens and suspended solids was achieved. Duckweed production and wastewater treatment for safe reuse in irrigation can be achieved in one simple system. Main conclusions derived from this study may be summarized as follows: . Fecal coliforms removal in the integrated pond system was 99.6±99.95%, resulting in a median euent count of 3.3*102±5.0*103 per 100 ml. Most
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removal took place in the SP. The fecal coliform decay constant Kd of the ®rst DP stage, the SP stage and the second DP stage were 0.7±3.2, 4.0±5.9 and about 1.4 dÿ1, respectively. . Passing the SP euent through a stage with reduced solar radiation, having a retention time of 2 days, proved sucient to remove practically all algae. Euent quality with respect to TSS was excellent (11 mg/l). AcknowledgementsÐThe work was partially supported by the European Union AVICENNE program, research project number 93AVI076 on ``Integrated Management of Reclaimed Wastewater Resources in the Mediterranean Region'' and by the Midbar Foundation, The Netherlands. The authors are very grateful for assistance in the work by Christopher Nganga, Babatunde O. Adekanbi and Dr Ludmilla Katz from the Water Resources Environmental Center. REFERENCES
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