Removal of indicator bacteria from municipal wastewater in an experimental two-stage vertical flow constructed wetland system

Transcription

1 Removal of indicator bacteria from municipal wastewater in an experimental two-stage vertical flow constructed wetland system C.A. Arias*, A. Cabello*, H. Brix* and N.-H. Johansen** * Department of Plant Ecology, University of Aarhus, Nordlandsvej 68, 820 Risskov, Denmark ( Hans.brix@biology.au.dk) ** ENVICARE Aps., Skodshøj 16, 950 Støvring, Denmark Abstract The removal of sanitary indicator bacteria (total coliforms, faecal coliforms, and faecal streptococci) was studied in an experimental constructed wetland system consisting of (1) a 2-m threechamber sedimentation tank, (2) a 5 m 2 vertical flow constructed wetland, () a filter-unit with calcite aimed at removing phosphorus, and () a 10 m 2 vertical flow constructed wetland. The indicator bacteria were enumerated before and after each unit of the wetland system during four monitoring episodes with different loading conditions. At a hydraulic loading rate of 520 1,70 mm/d, the first-stage vertical flow beds removed about 1.5 log-units of total coliforms, 1.7 log-units of faecal coliforms and 0.8 log-units of faecal streptococci. In the second stage bed receiving lower loadings both in term of concentration and quantity ( mm/day), the eliminations were lower. It was not possible in the present study to identify any seasonal effects, but no measurements were done during summer. Recycling of treated effluent back to the sedimentation tank did not affect elimination. Area-based rate constants for the vertical flow wetland receiving effluent from the sedimentation tank averaged.2 m/d for total coliforms,. m/d for faecal coliforms and 2.1 m/d for faecal streptococci. The rate constants depended on loading rates. It is suggested that filtration is a major removal mechanism for bacterial indicator organisms in vertical flow constructed wetland systems. Keywords Coliform bacteria; constructed wetlands; indicator bacteria; pathogens; rate constant; vertical flow Introduction Human pathogens are a normal component of domestic sewage and their control is one of the fundamental reasons for wastewater treatment. Constructed wetland systems remove pathogens by factors such as natural die-off, low temperatures, ultraviolet radiation, unfavourable water chemistry, predation, and sedimentation. In general water residence time in wetland systems is long and therefore the removal processes appear to be effective (e.g. Hill and Sobsey, 2001; Steer et al., 2002). Single-house systems may be located close to inhabited houses increasing the risk of people getting in physical contact with the effluent, and in arid-climate areas, where water is sparse, there might be a wish to use the effluent water for irrigation. Therefore it is important that the sanitary quality of the effluent from wastewater treatment systems is safe. The removals of pathogenic organisms from wastewater in constructed wetlands appear to be very effective (e.g. Khatiwada and Polprasert, 1999; Karpiscak et al., 2000; Hagendorf et al., 2000). The research documenting this has been focused on surface flow and horizontal subsurface flow constructed wetlands where the residence time is usually long. In vertical flow systems, however, the residence time might be very short, and little data are available about the fate of sanitary indicator bacteria in these systems. In order to document the performance of vertical flow systems for effective removal of suspended solids, biochemical oxygen demand, phosphorus and nitrification of domestic sewage, an experimental vertical flow constructed wetland system was constructed in 2001 Water Science and Technology Vol 8 No 5 pp 5 1 IWA Publishing 200 5

2 under the funding of the Danish EPA. Taking into account that constructed wetlands are generally chosen as a solution for autonomous wastewater treatment and that commonly the treated wastewater is returned to the environment without bacterial evaluation, we extended the objectives of the project to evaluate the removal of traditional sanitary indicator bacteria in the system. This paper presents the removal of total coliforms (TC), faecal coliforms (FC) and faecal streptococci (FS) during the passage of settled domestic sewage through the two-stage vertical flow constructed wetland system. Materials and methods Pilot plant The study took place at an experimental two-stage constructed wetland system located in the vicinity of Århus, Denmark. The plant was constructed during the spring of 2001 in the grounds of the municipal wastewater treatment plant of Trige (WWTP) (Johansen et al., 2002). The plant consists of a prefabricated 2-m three-chamber polyethylene sedimentation tank, a 5-m 2 first stage vertical flow bed, a battery of three consecutive calcite filled filters for removal of phosphorus, a second stage 10-m 2 vertical flow bed, and the necessary wells, pumps, pipes, valves and sensors to permit a diversified and controlled operation of the system. The two beds are constructed in a HDPE lined metal container divided into two sections. Each bed is filled with two layers of different diameter gravel. The bottom layer is 20 cm high 8 16 mm gravel and houses the drainage system. On top of this layer lies a 0.8 m high 0 2 mm gravel section. The wastewater distribution system consists of four 0 mm perforated polyethylene pipes in each bed that lie across the bed s surface. The drainage systems consist of three 50 mm drainage pipes placed transversally on the bottom of the beds and which drain to a 110 mm pipe, which evacuates the water from the beds. Each pipe of the drainage system is connected to a 50 mm vertical pipe, which stands perpendicularly to the bed to facilitate the flow of convective atmospheric air to the bottom of the bed. The beds are planted with potted seedlings of common reeds (Phragmites australis) at a density of plants per m 2. The plant is fed with raw wastewater taken from a well, located just before the WWTP s inlet. The raw wastewater is pumped into the sedimentation tank. After the sedimentation tank the water drains to a level-controlled pumping well, where the water is pumped and distributed on the surface of the first vertical bed. The water percolates through the bed and thereafter it is conducted by gravity to and through the phosphorus filtering units. Thereafter, the water is pumped to the second vertical bed. After percolating through the second bed, the treated water drains to an exit well, where it can either be recycled to the sedimentation tank or can be finally disposed of. At different points along the treatment s units series of probes are installed. These probes include: ph meters, 2 thermometers, oxygen probes, 2 redox potential probes, 2 vane meters and one ultrasound wastewater flow meter. All the information gathered by the probes is saved in a logger, which can be locally or remotely accessed. Programming either the frequency of pumping pulses and/or the pulse s time length controls the wastewater flow into the pilot plant. Pumping times control the recirculation flow from the exit well to the sedimentation tank. 6 Sampling and methods The sampling took place during November 2001 and February 2002 during four sampling campaigns at an intended wastewater inflow rate of 2 m d 1. Every other campaign the operational mode of the plant was changed so that there was no wastewater recycling, while in the following campaign there was a 100% water recycling, i.e. a recycling rate of 2 m d 1. Each sampling campaign consisted of daily grab samples for five consecutive days

3 and taken at five different places along the flow, as follows: (1) at the inlet to the sedimentation tank; (2) at the outlet of the sedimentation tank; () at the outlet of the first bed; () at the outlet of the P-filtering unit, and (5) at the outlet of the second vertical bed. The bacteriological indicators were evaluated for total coliforms (TC), faecal coliforms (FC) and faecal streptococci (FS) according to the protocols for membrane filter procedures of Standard Methods 9222 B (total coliforms), 9222 D (faecal coliforms), and 920 C (faecal streptococci) (Standard Methods, 1995). Results Hydraulic loading rates and temperature Table 1 presents the measured flow, hydraulic loading rates and the average temperatures of the plant during the sampling campaigns. As a result of the experimental plant s dependency on the municipal WWTP inflow, the planned flow of 2 m d 1 during the last two campaigns was not achieved. This was due to the fact that heavy rain occurred in the area during these sampling campaigns. The hydraulic overloading during the campaigns resulted in flooding of the first vertical bed during February. Removal of indicator bacteria The average overall removal of indicator bacteria was % for the three groups of bacteria analysed, suggesting a good capacity of the system to remove pathogenic bacteria (Figure 1). The bacteria concentration in the outlet of the sedimentation tank was generally at the same level as the raw inlet concentration showing that no removal occurs in the sedimentation tank. After the water percolated through the first vertical bed, the bacteria concentration was reduced by 1.5 log units for TC, 1.7 log units for FC and 0.8 log units for FS. Table 1 Average hydraulic loading rates and mean water temperatures during the four sampling campaigns (n = 5). Campaigns 1 and 2 were carried out in November 2001; campaigns and in February 2002 Flow Hydraulic loading rate of vertical beds Temperature Campaign Raw Recycling Bed No 1 Bed No 2 input (m d 1 ) (m d 1 ) (m d 1 ) (m d 1 ) C C Total coliforms Faecal coliforms Faecal streptococci log cfu/100 ml Sampling location Figure 1 Average bacterial counts (± 1 standard deviation) at the five sampling points along the system. 1: Raw inflow; 2: outflow sedimentation tank; : outflow bed No. 1; : outflow P-filter; 5: outflow bed No. 2 7

4 The P-filtering unit reduced the concentrations by approximately a log unit, and finally, the second vertical bed reduced bacteria numbers by log units. Seasonal effects The campaigns were carried out only during late autumn (November) and winter (February) and the water temperatures did not differ much between the campaigns (Table 1). It is therefore not possible, based on the present data, to evaluate any seasonal effect on treatment performance as we lack data for summer conditions. In addition, the hydraulic loading rate as well as the composition of the wastewater differed between the two periods because of heavy rain in February (Table 2); any differences in treatment performance observed between the two sampling periods can therefore not be ascribed solely to seasonal effects. An analysis of variance for bacteria removal in the two vertical beds shows that season has no significant effect on removal rate for any of the three indicator bacteria studied (Table ). Except for the difference in inlet concentration being more dilute in February, the removal pattern did not differ significantly between the two seasons. Table 2 Average bacterial concentrations at the five sampling points along the flow path in the two-stage vertical flow constructed wetland system. Values given are average ± 1 standard deviation (n = 5). Campaigns 1 and 2 were carried out in November 2001; campaigns and in February 2002 Inlet Outlet sed. tank Outlet Bed 1 Outlet P-filter Final effluent Total coliforms (10 6 cfu/100 ml): Campaign 1 5 ± ± ± ± ± 0.01 Campaign 2 8 ± 5 27 ± ± ± ± 0.01 Campaign 8.8 ± ± ± ± ± Campaign.0 ± ± ± ± ± Faecal coliforms (10 6 cfu/100 ml): Campaign 1 27 ± 15 9 ± ± ± ± 0.05 Campaign 2 56 ± 6 19 ± ± ± ± 0.01 Campaign.5 ± ± ± ± ± Campaign.0 ± ± ± ± ± Faecal streptococci (10 cfu/100 ml): Campaign ± 50 ± ± 16 2 ± 6 ± 1 Campaign 2 27 ± ± 2 26 ± 15 7 ± 1.0 ± 0.7 Campaign 210 ± ± 5 1 ± ± ± 0.2 Campaign 190 ± ± 62 2 ± 7 0. ± ± 0.2 Table Three-way analysis of variance (F-ratios) for removals (log-units) of total coliforms, faecal coliforms and faecal streptococci in the two-stage vertical flow constructed wetland system. The factors tested are: Bed (first or second bed), Season (November and February) and Recycling (with and without recycling of effluent to the sedimentation tank). F-ratios marked with *** are statistically significant at the 5%-significance level (n = 6) F-ratios Total coliforms Faecal coliforms Faecal streptococci 8 Main effects: A: Bed 16.7*** 20.96*** 8.62*** B: Season C: Recycling Interactions: A B A C B C A B C

5 Effects of recycling The effect of wastewater recycling of final nitrified effluent to the sedimentation tank was also evaluated. During loadings without recycling the TC and FC counts tended to increase in the sedimentation tank, whereas the recycling diluted the effluent from the sedimentation tank and hence the bacteria counts. For FS the counts in the effluent from the sedimentation tank were lower than in the inlet irrespective of recycling. The analysis of variance showed that recycling did not significantly affect the bacteria removal in the vertical beds (Table ). Removals in the two vertical flow beds The removal of all studied indicator bacteria was significantly higher in the first vertical bed than in the second vertical bed as shown by the highly significant effect of bed in the analysis of variance (Table ). In the first bed the bacteria were reduced log units, whereas the reduction in the second vertical bed was log units. The elimination was greater at higher inlet concentrations, i.e. higher eliminations were observed in the first bed, which received effluent from the sedimentation tank, than in the second bed, which received water treated in the first bed and the P-filter (Figure 2). The hydraulic loading rate of the second bed was only half that of the first bed, so concentration rather than hydraulic loading rate seems to affect removal. Removal kinetics The average inflow and outflow indicator bacteria counts permit the calculation of areabased first order bacteria decay constants (k, m/d) using the following formula (Kadlec and Knight, 1996). C o /C i = exp( k/q) (1) 1e+7 (A) Total coliforms 1e+7 (B) Faecal coliforms log output (cfu/100 ml) 1e+6 1e+5 1e+ 1e+ log output (cfu/100 ml) 1e+6 1e+5 1e+ 1e+ 1e+2 1e+2 1e+ 1e+ 1e+5 1e+6 1e+7 1e+8 1e+9 log input (cfu/100 ml) 1e+2 1e+2 1e+ 1e+ 1e+5 1e+6 1e+7 1e+8 log input (cfu/100 ml) 1e+6 (C) Faecal streptococci log output (cfu/100 ml) 1e+5 1e+ 1e+ 1e+2 1e+2 1e+ 1e+ 1e+5 1e+6 log input (cfu/100 ml) Figure 2 Average counts of bacterial indicator organisms in the effluent from the vertical beds plotted against the counts in the influent to the beds. Filled points are data from the first bed; open points are data from the second bed 9

6 5 (A) Total coliforms 6 5 (C) Faecal coliforms K tc (m/d) 2 K tc (m/d) e+6 1e+7 1e+8 1e+9 1e+10 1e+11 1e+12 log input (cfu/m 2 /d) 1 0 1e+6 1e+7 1e+8 1e+9 1e+10 1e+11 1e+12 log input (cfu/m 2 /d) (C) Faecal streptococci K tc (m/d) e+6 1e+7 1e+8 1e+9 1e+10 log input (cfu/m 2 /d) Figure Calculated area-based rate constants (m/d) for the bacterial indicator organisms plotted against the mass loading rates of the beds. Filled points are data from the first bed; open points are data from the second bed Where C o = final bacteria concentration in cfu/100 ml C i = initial bacterial concentration in cfu/100 ml q = hydraulic loading rate (m/d) Isolating k in Eq. (1) yields: k = q ln (C i /C o ) (2) Based on the average inlet and outlet indicator bacteria counts and hydraulic loading rates of each individual bed, the area-based rate constants, k TC, k FC and k FS (m/d) were estimated for each campaign. The calculated rate constants for the three bacterial indicators depend on the loading rate (Figure ). The average k TC was.2 m/d for the first bed and 0.58 m/d for the second bed. Average k FC was. m/d for the first bed and 0.8 m/d for the second bed, and average k FS was 2.1 m/d for the first bed and 0. m/d for the second bed. 0 Discussion Removal of pathogenic organisms from wastewater should be one of the concerns when establishing a wastewater treatment system. The data presented show that vertical flow constructed wetland systems have the capacity to remove indicator bacteria from wastewater, in spite of the short residence time in vertical flow filters. The results also show that most of the elimination occurs while the wastewater percolated through the vertical beds and/or the phosphorus filter, and not in the sedimentation tank, suggesting that the removal occurred principally by mechanisms associated with filtration. We could not identify any seasonal dependency in elimination, as we did not carry out measurements during summer. We did, however, monitor performance during winter with low temperatures, and it might be expected that performance is better during summer with higher temperatures, even

7 though others have suggested no seasonal dependency (Rivera et al., 1995). The recycling of treated and nitrified effluent back to the sedimentation tank did not significantly affect elimination. This should be evaluated in greater detail, since the bacterial removal process is one that combines mechanisms occurring simultaneously in and outside of the system (Khatiwada and Polprasert, 1999) and since recycling affects hydraulic loading rate and residence time in the system. The estimated area-based rate constants suggest that the bacterial removal process depends on the loading rate and strength of the wastewater. Conclusions Indicator bacteria are removed effectively by vertical flow constructed wetland systems in spite of the short residence time of the water. The loading rates tested in the present study are high compared to the loading rates that will be used in operational full-scale systems. Therefore, it can be expected that the number of indicator bacteria in effluents from fullscale systems will be lower than in this study. It still remains to be resolved if the outlet concentrations of indicator bacteria can meet the criteria for reuse of effluent for irrigation purposes. Acknowledgements The study was funded by the Danish EPA, projects and We thank the municipality of Aarhus for hosting the experimental system. We also thank the following companies for supplying various materials for the system: Uponor A/S, Danfoss A/S, Pumpex A/S, Damolin A/S, BV Electronic A/S, MJK Automation A/S and Proagria A/S. References Hagendorf, U., Diehl, K., Feuerpfeil, I., Hummel, A. and Szewzyzk, R. (2000). Retention of microbial organisms in constructed wetlands. Preprints 7th Int. Conf. on Wetland Systems for Water Pollution Control, Florida, USA, 1, Hill, V.R. and Sobsey, M.D. (2001). Removal of Salmonella and microbial indicators in constructed wetlands treating swine wastewater. Wat. Sci. Tech., (11 12), Johansen, N.-H., Brix, H. and Arias, C.A. (2002). Design and characterization of a compact constructed wetland system removing BOD, nitrogen and phosphorus from single household sewage. In Preprints of the 8th International Conference on Wetland Systems for Water Pollution Control. Vol 1, Comprint International Limited, Dar es Salam, pp Kadlec, R.H. and Knight, R.L. (1996). Treatment Wetlands. Lewis Publishers, Boca Raton, New York, London, Tokyo. Karpiscak, M., Sanchez, L., Freitas, R. and Gerba, C. (2001). Removal of bacterial indicators and pathogens from dairy wastewater by a multi-component treatment system. Wat. Sci. Tech., (11 12), Khatiwada, N. and Polprasert, C. (1999). Kinetics of faecal coliform removal in constructed wetlands. Wat. Sci. Tech., 0(), Rivera, F., Warren, A., Ramirez, E., Decamp, O., Bonilla, P., Gallegos, E., Calderón, A. and Sanchéz, J. (1995). Removal of pathogens from wastewaters by root zone method (RZM). Wat. Sci. Tech., 2(), Standard Methods for the Examination of Water and Wastewater (1995). 19th edition. American Public Health Association. American Waterworks Association and Water Environment Federation, Washington D.C, USA. Steer, D., Fraser, L., Boddy, J. and Seibert, B. (2002). Efficiency of small constructed wetlands for subsurface treatment of single-family domestic effluent. Ecol. Eng., 18,