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versión On-line ISSN 1870-0462
Trop. subtrop. agroecosyt vol.14 no.2 Mérida may./ago. 2011
Artículos de investigación
Performance of a surface flow constructed wetland system used to treat secondary effluent and filter backwash water
Comportamiento de un sistema de pantanos construidos de flujo superficial para el tratamiento de efluente secundario y de retrolavado
J. A. Vidales-Contreras1*, C. P. Gerba2, M. M. Karpisack2, H. Rodríguez-Fuentes1, C. Chaidez-Quiroz3, E. Olivares-Sáenz
1 Facultad de Agronomía, Universidad Autónoma de Nuevo León; Campus de Ciencias Agropecuarias, calle Francisco Villa al norte s/n, Exhacienda el Canadá, Escobedo, Nuevo León, México. * Corresponding Author Email: firstname.lastname@example.org
2 Department of Soil, Water and Environmental Science, University of Arizona. USA.
3 Centro de Investigación en Alimentación y Desarrollo, Culiacán, Sin. México.
The performance of a surface flow wetland system used to treat activated sludge effluent and filter backwash water from a tertiary treatment facility was evaluated. Samples were collected before and after vegetation removal from the system which consists of two densely vegetated settling basins (0.35 ha), an artificial stream, and a 3-ha surface flow wetland. Bulrush (Scripus spp.) and cattail (Typha domingensis) were the dominant plant species. The average inflow of chlorinated secondary effluent during the first two months of the actual study was 1.9 m3 min-1 while the inflow for backwash water treatment ranged from 0.21 to 0.42 m3 min-1. The system was able to reduce TSS and BOD5 to tertiary effluent standards; however, monitoring of chloride concentrations revealed that wetland evapotranspiration is probably enriching pollutant concentrations in the wetland outflow. Coliphage removal from the filter backwash was 97 and 35% during 1999 and 2000, respectively. However, when secondary effluent entered the system, coliphage removal averaged 65%. After vegetation removal, pH and coliphage density increased significantly (p<0.05) at the outlet of the wetland. This study showed that surface flow wetlands are an alternative technology for TSS, BOD5, and turbidity removal from both secondary or backwash water. However, growth of bacteria populations or recovering of injured bacteria may occur.
Key words: TSS; BOD5; constructed wetlands; wastewater; backwash water; microbial indicators.
El comportamiento de un sistema de pantanos construidos de flujo superficial para el tratamiento de efluente secundario y de retrolavado proveniente de una planta de tratamiento terciario fue evaluado. Muestras de agua fueron colectadas antes y después de la remoción de la vegetación del sistema el cual consiste de dos cuencas de sedimentación (0.35 ha), un manantial artificial, y un pantano construido de flujo superficial de 3 ha. Las especies de plantas dominantes fueron Junco (Scripus spp.) y Typha (Typha domingensis). La tasa de flujo promedio de efluente secundario durante los primeros dos meses del estudio fue 1.9 m3 min-1 mientras que la tasa de flujo de efluente de retrolavado fue de 0.21 a 0.42 m3 min-1. El sistema fue capaz de reducir SST y DBO5 a estándares de calidad de efluente terciario; sin embargo, el monitoreo de la concentración de ion cloruro reveló que la evapotranspiración esta probablemente enriqueciendo las concentraciones de contaminantes en la descarga del sistema. La remoción de colifagos nativos presentes en el efluente de retrolavado fue de 97 y 35% durante el periodo de muestreo de 1999 y 2000, respectivamente. Sin embargo, cuando efluente secundario fue introducido al sistema, la remoción de colifagos promedio 65%. Los resultados del presente estudio muestran que los pantanos construidos de flujo superficial son una tecnología alternativa para remover SST, DBO5 y la turbidez de efluente secundario y de retrolavado. Sin embargo, el crecimiento o el recobramiento de poblaciones bacteriales dañadas por cloración puede ocurrir.
Palabras clave: SST; DBO5; pantanos construidos; agua residual; agua de retrolavado; indicadores microbiológicos.
In conventional wastewater or drinking water treatment plants, backwash water results from periodic backwashing of single or mixed media filters used for removing organic matter, enteric pathogens, and other paniculate matter from raw drinking water or activated sludge secondary effluent (Steven and Lester, 1994; Laurent et al, 1999; Persson et al, 2005; Khan and Subramania, 2007; Horan and Lowe, 2007). Consequently, backwash water without efficient treatment or recycling in treatment facilities may represent a public health risk (Koivunen et al, 2003). In order to protect public health, environmental protection agencies have regulated its recycling in conventional drinking water treatment plants to control disinfection resistant microbial pathogens and consequently waterborne diseases (USEPA 2002). In the wastewater treatment industry, constructed wetlands are considered an attractive technology to treat low strength domestic sewage and secondary wastewater effluents. Wetland technology has also offered an innovative approach for reduction, with different degrees of success, of a wide range of chemical pollutants (Grove and Stein, 2005; Matamoros et al, 2007; Winter and Kickuth, 1989). However, until recently few or not data existed on the efficacy of constructed wetland systems to treat backwash water; even though, one of the most mmon methods to treat backwash water has been settling in lagoon facilities (Montgomery, 1985). In 1997, two wetland systems were constructed at the Sweetwater Recharge Facility to treat backwash water from a tertiary wastewater treatment plant in Tucson AZ, USA. After soil aquifer treatment, wastewater is recovered from the aquifer at 1.6 x 107 m3 year-1 extraction rate to be deliver in golf course facilities, parks, schools, and residential sites. The objective of the actual study was to assess wastewater quality performance in the wetland system before aquifer recharge. Hence, physical, chemical, and microbial indicators for wastewater quality were evaluated in the wetland during secondary effluent and backwash water treatment from February to September 1999 and 2000. The actual paper presents the results of this monitoring study.
MATERIALS AND METHODS
The Research Site and Sampling
The research was conducted at the Sweetwater Wetland and Recharge Facility (SWRF) in Tucson, AZ. In this site (Figure 1), two polishing wetland systems referred to as East and West were designed in about 12.46 ha to reclaim backwash water from the City of Tucson Reclamation Plant. At this site, residual chlorine in secondary effluent at the pressure mixed media filters was on average 1 mg L-1. After backwashing mixed media filters, the backwash effluent was kept free of chlorine additions. At the wetland facility, both the East or West polishing systems consist of a 3-ha wetland cell and a pair of settling basins vegetated with bulrush species (Scirpus spp.) and cattail (Typha domingenses). However, in the East Polishing System (EPS), wastewater flows briefly through an artificial stream before entering the 3-ha wetland cell for additional wastewater treatment. A sequence of islands of different sizes, shallow vegetated zones, and 1.2-m deep open water areas is the geometric configuration of the wetland to provide tertiary treated wastewater. After wetland treatment, polishing wastewater goes by gravity to four recharge basins, located at the vicinity area of The Santa Cruz River, for soil aquifer treatment. Eventually, drilling wells pump reclaimed water from the aquifer to the Pima County Roger Road Wastewater Treatment Facility (RRWTF) for chlorine disinfection and deliver in parks, schools, and golf course fields. In spite of both polishing systems were designed to treat backwash water, chlorinated secondary effluent from RRWTF was introduced for starting wetland operation in October 1997; on April 1998, the polishing wetland systems began to treat backwash water. In the winter 1999, wetland vegetation was harvested from the EPS; however, by the end of Spring a new complete plant canopy had taken its place. From February to September of 1999 and 2000, water samples were collected monthly from the EPS at the backwash splitter box (1), outlet of the south settling basin (2), both ends of the stream (3 and 4), and outlet of the wetland cell (5). Concurrently, measurements of water temperature (T°), biochemical oxygen demand (BOD5), total suspended solids (TSS), SO4, C1-, total and free chlorine (C12), turbidity, pH, native coliphages (NC), and total (TC) and fecal coliforms (FC) were conducted by using standard methods for water and wastewater analysis (APHA/AWWA/WEF, 1998).
Physical and Chemical Analysis
The 5-day incubation method (APHA/AWWA/WEF, 1998) was used for BOD5 analysis with three replicates. Determination of TSS was conducted by filtering a known volume of sample through a pre-cleaned and pre-weighed glass fiber filter. TSS concentration was estimated reweighing the filter after a 24-h drying period at 100 °C. Sulfate was assessed by adding BaC12 to a known volume of sample and measuring the absorbance at 420 nm in a HACH DR/2000 spectrophotometer (Loveland, CO). Chloride was quantified by a chloride-specific electrode, a standard solution of a known amount of Cl and SO4 was used as a sample to ensure that both analytical methods were working properly. Turbidity was measured with a portable turbidimeter (HACH, model 21 OOP, Loveland, CO) reading as Nephelometric Units (NTU), previously calibrated with formazine standard solutions. A calibrated pH meter (model 8005, West Chester, PA) quantified water pH whereas the DPD (N,N-diethyl-p-phenylenediamine) indicator method (HACH Spectrophotometer, model DR/2000, Loveland, CO) was chosen for total and free chlorine (C12) determination by using a distiller water blank like reference reading at 515 nm.
Coliforms and Coliphages
Total and fecal coliforms were analyzed within 4 h of sampling by membrane filtration using mEndo Agar Les and mFC culture media (DIFCO, Detroit, MI), respectively. The membrane filters were 47 mm diameter with a porosity of 0.45 um (Millipore, Molsheim, France). Sample volumes of 0.1, 1, and 10 mL with two replicates per volume were assayed and incubated at 37°C for total coliforms and 44.5°C for fecal coliforms, results are reported as colony forming units (CFU). Native coliphages were quantified by the double layer agar method described by Adams (1959). A 1-mL aliquot from Escherichia coli ATCC 15597 (ATCC) culture, previously incubated at 37°C for 24 h in trypticase soy broth (DIFCO, MI), was combined with one mL of sample in a test tube containing molten overlay agar. This suspension was poured onto a layer of triplicase soy agar (DIFCO, MI), and incubated at 37°C for 18 h in order to enumerate the coliphage as plaque forming units (PFU) in two replicates. This method detects both somatic and male specific coliphages.
The Statistical Package for Social Science 12 (SPSS Inc., Chicago ILL) was used to conduct the statistical analysis for physical, chemical and microbial data sets. Tests to determine significant differences between sampling periods and monitoring sites were conducted by two way ANOVA analysis. Because of extreme concentration values and high variability into microbial data sets, ANOVA analysis was conducted transforming microbial concentrations to base 10 logarithmic units (log10). The geometric mean was used as a centiality measure for observed microbial indicator distributions. Extent of data dispersion was represented by geometric coefficient of variation, (10^ (σ) - 1) × 100 where σ is the standard deviation of log10 transformed microbial concentration values. The physical/chemical data sets were analyzed without transformation.
Standard Methods for Water and Wastewater Analysis (APHA/AWWA/WEF, 1998) reports the coefficient of variation (CV) per water or wastewater physical or chemical parameter analyzed per sample. For instance, CV for TSS should be lower than 33% at 15 mg L-1 concentration, however, if TSS concentration is about 242 mg L-1, a CV lower than 10% is recommended. For coliforms enumeration, their analysis was conducted with three different volumes, accepting up to 15% difference between replicates, and contrasting CFU enumeration between analyzed volumes. For example, if at 0.1 ml TC average quantification was 10 CFU per mL, at 1 mL volume, the expected concentration should be 100 ± 15% CFU mL-1. These quality criteria were revised for the analyzed parameters before conducting statistical determinations.
An actual hydraulic residence time of 7.2 days was estimated by a tracer study in the East wetland cell (Vidales et al, 2006). During this study, February 12 to March 18, 1999, the wetland was receiving chlorinated secondary effluent at an average rate of 1.84 m3 min"1. From March 19 to 22, a mixture of chlorinated secondary effluent and backwash water was introduced into the East and West system changing to 100% backwash water at 0.42-m3 min-1 flow rate, on March 23. This hydraulic condition was changed to 0.25 m3 min-1 on June 30 and was held until September 21 when the EPS started to be drained for vegetation harvesting in the winter of 1999. The EPS returned to normal operation in February 2000 at an average inflow rate of about 0.32 m3 min-1 of backwash water (Tucson Water, 1999 and 2000).
Sampling of Chlorinated Secondary Effluent
Two water samples were collected per sampling site during February and March, 1999. Influent BOD5 and TSS concentrations were 29 and 21 mg L-1, respectively, decreasing about 69% at sampling location 2 (Table 1). Turbidity reduction was very similar to BOD5 and TSS performance with a 56% decrement from location 1 to 2. At sampling site 2, a significant increase of indicator bacteria was observed; in fact, TC inflow concentrations increased by a forty five-fold factor, approximately, at this sampling site. In contrast, the East wetland cell noticeably removed TC, FC, and NC reaching reductions about 91,81 and 72%, respectively, from end to end of the 3-ha wetland.
Chloride was practically constant in the wetland system showing the lowest concentration at site 5. Sulfate revealed a greater variability than C1- ranging its concentration between 122 and 144 mg L-1. ForpH, the lowest value was observed in the settling basin and the highest at splitter box. An average temperature of 22.75 °C was recorded at site 1 decreasing to 10.5 °C at wetland outlet, 3.66 °C below the average temperature for February and March 1999 recorded at the Tucson Meteorological Station (The Arizona Meteorological Network, AZMET, 2008). On February 19, total and free C12 concentrations were 1.19 and 0.14 mg L-1, respectively, in splitter box water. Both concentrations were below the method detection limit thereafter. On March 20, C12 was undetected at any sampling point in the EPS.
Sampling of Backwash Effluent
Geometric mean concentrations for total coliforms, fecal coliforms, and coliphage observed at monitoring sites during backwash water study are presented in Figure 2. It can be clearly seen that TC and FC average concentrations increased in the settling basin. The remaining wetland treatment units resulted in further bacterial removal. At the system outflow, inflow NC concentrations decreased by about 97 and 35% in 1999 and 2000, respectively. Table 2 illustrates the high variability observed in microbial concentrations particularly in the first sampling period when geometric coefficient of variation (CV) ranged between 90 and 633% for TC and from 120 to 526% for FC. The ANOVA analysis revealed a significant difference for microbial indicator concentrations between sampling sites (p<0.05) but not for sampling periods (p>0.05).
BOD5, TSS, Temperature, and Turbidity
Table 3 shows BOD5, TSS, and turbidity descriptive statistics for 1999 and 2000 backwash water treatment. The CV values for those water quality parameters varied from 14.33 to 127.66 %; however, CV estimates during 2000 ranged between 19 to 76.10% for all the sampling sites. In contrast to the artificial stream and wetland cell, the settling basins were highly efficient for backwash water treatment. At this site, the average TSS, BOD5 and turbidity were significantly reduced to 91-93%, 60-74%, and 65-78 %, respectively. These results are fairly lower than the estimates for the entire polishing system during both sampling periods, 90-96% for TSS; 84-89% for BOD5; and 77-79% for turbidity, suggesting that TSS are more efficiently removed than BOD5 and turbidity.
C1-, SO42-, and pH
From April to September 1999, ANOVA analysis indicated that Claverage influent concentration was 141 mgL-1 increasing significantly (p<0.05) to 164 mg L-1 at site 2 (Figure 3). Thereafter, its concentration was 142 and 155 mg L-1 at site 3 and 4, respectively. At 5, a significant increase (p<0.05) of 40 mg L-1 above site 4 concentration was recorded. During the 2000 backwash sampling period, lower C1- concentrations than in 1999 were observed at monitoring sites. ANOVA analysis revealed a statistical difference (p<0.05) between both backwash water periods for C1-, in fact, a 16% difference between sampling periods for influent C1- concentrations at site 1 was estimated; for SO4, this estimate was 11 %. Table 4 gives coefficients of variation for Cl" and SO4; it seems clear that observed concentrations for both parameters showed a lower variation than the other water quality indicators (TC, FC, NC, TSS, BOD5 and turbidity). For example, in 2000, the calculated CV at the system outflow was about 19 % for both C1- and SO4 which is similar to the lowest CV estimated for TSS, BOD5 and turbidity in the polishing system during backwash water treatment. In the same effluent, water pH ranged from 7.4 to 7.5 at sampling sites 1 to 3; whereas, at the wetland outflow pH decreased, no significatively (p>0.05), to 7.35. Regarding residual C12, its concentration was below the method detection limit at the sampling sites during the backwash water treatment study.
Physical/chemical water quality indicators
For secondary effluent treatment, outflow water in the wetland met on average the 10 mg L-1 tertiary standard required by the Arizona Department of Environmental Quality (ADEQ) for BOD5 and TSS. A significant increase of turbidity, BOD5, and TSS occurred when secondary effluent was switched to backwash water at site 1. At the wetland outlet, removal of BOD5 was comparable to the 89 % reduction reported by Vrhovsek et al (1996) in a subsurface flow wetland operated at 962 mg L-1 BOD5 loading rate. Overall, BOD5, and TSS removal in the East Polishing System was according to reported values for constructed wetlands operating across USA (Kadlec and Knight, 1996). In fact, the average TSS and BOD5 at the outlet end of the system were lower than the 30 mg L-1 secondary standard limit established by the ADEQ for wastewater treatment.
Chloride is considered highly stable in most terrestrial environments. In wetlands, its total mass is approximately constant (Kadlec and Knight, 1996) because its incorporation in plant tissues is negligible (Hayashi et al., 1998). Consequently, C1- has been used as a conservative tracer to estimate evapotranspiration in wetland ecosystems (Hayashi et al, 1998). In the 3-ha polishing wetland, evapotranspiration may be a suitable mechanism for Cl" augmentation during both backwash sampling periods when water flow rate was below 0.42-m3 min-1. Concentrations of C1- at both ends of the polishing wetland increased 25 and 19 % during backwash operation, before and after vegetation removal, respectively. ANOVA analysis indicated that only in 1999 was there a significant (p<0.05) statistical difference between inflow and outflow concentration from the 3-ha wetland cell. Sulfate is an essential nutrient for plants; thus, it can be retained by plant uptake in terrestrial environments; however, it is rarely a limiting factor for plant growth in wetlands (Kadlec and Knight, 1996). Its presence in high organic content environments induces production of hydrogen sulfide because SO4-2 is an electron acceptor for sulfur-reducing bacteria (Maier, 2000). This microbiological mechanism probably was responsible for reduction of SO42- in the settling basin, mainly observed during 1999 backwash water treatment. Similar to Cl", an increase of SO4 2- concentration occurred at the outflow of the wetland.
Removal efficacies greater than 90 % for FC in surface flow wetlands receiving 104-106 UFC/100 inlet concentration loads have been reported (Kadlec, 2005). It appears that the amount of organic matter introduced into the settling basins is playing an important role for regrowth or recovery of injured conform bacteria (Gerba, 2000; Bucklin et al, 2003; Bolster et al., 2005). Coliform bacteria such as Klebsiella, Enterobacter, and Citrobacter have shown ability to proliferate during wastewater treatment. For example, Klebsiella was found at high densities in the outflowing water from a treatment facility receiving municipal wastewater (Elmund et al, 1999) apparently because of an increase of carbohydrates in the wastewater influent. F-specific RNA bacteriophages have been used as potential indicator for human enteroviruses instead of fecal coliforms and fecal streptococci (Stetler, 1984; Havelaar et al, 1993). A 90% removal of coliphage has been previously observed in constructed wetlands (Gersberg et al, 1987; Chendorain et al, 1998); however, removals lower than 90% were reported by Karpiscak et al. (1995) in a duckweed (Lemna spp.) pond and by Gersberg et al. (1989) in non-vegetated wetlands. The extent of somatic and F-specific RNA coliphage replication in water has been discussed by several researchers (Jofre, 2009; Muniesa and Jofre, 2004; Woody and Cliver, 1997). Their findings suggest that coliphage replication is possible at host bacteria and virus concentrations uncommonly found in water environments. However, threshold concentrations may emerge because of bacteria growth. In the present study, coliphage removal in the settling basin, site 2, showed a decrease from 98 to 65 % after vegetation harvesting in 1999. Probably, some mechanism associated to vegetation density or phage replication was responsible for undetectable coliphage removal from site 3 to 5 during the second backwash sampling period.
Apparently, the high organic removal and posterior mineralization in the settling basin promoted growth of TC and FC. Probably, repair of both groups of injury bacteria, after exposure to chlorination, was also occurring. Even, after vegetation removal, the surface flow constructed wetland system was able to treat filter backwash water to secondary effluent standards required by the Arizona Department of Environmental Quality for TSS and BOD5. However, little removal of these parameters was observed in both the artificial stream and the 3-ha surface flow wetland cell. Similar to TSS, BOD5 and turbidity, native coliphage concentrations showed a remarkable decrease at the settling basin apparently due to sedimentation of associated suspended solids. Interestingly, C1- and SO4 increased their concentration in the wetland cell suggesting that some additional mechanism probably related with wetland evapotranspration is promoting pollutant enrichment. The findings of this study showed that settling basins are an acceptable facility for TSS, BOD5, and turbidity removal from both secondary or backwash water; however, growth of bacteria populations or recovering of injured bacteria also may occur. The actual study has showed the complexity of a wetland environment where biological, physical, and hydrological conditions may explain pollutant performance during wastewater treatment.
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