Fate of viruses in artificial wetlands

Understanding Viral Impacts in Soil Microbial Ecology Through the Persistence and Decay of Infectious Bacteriophages

Marine bacteriophages have been well characterized in terms of decay rates, population dynamics in relation to their hosts, and their impacts on biogeochemical cycles in the global ocean. Knowledge in soil bacteriophage ecology lags considerably behind, with few studies documenting population dynamics with hosts and even fewer reporting phage decay rates. By using sterile soil or aquatic microcosms inoculated with single bacteriophage isolates, phage decay rates (loss of infectivity over time) were determined, independent of host interactions, for 5 model phage isolates. Decay rates varied by phage from 0.11-2.07% h-1 in soils to 0.07-0.28% h-1 in aquatic microcosms. For phages incubated in both soil and aquatic microcosms, the observed decay rate was consistently higher in soil microcosms than in aquatic microcosms by at least a factor of two. However, when decay rates for soil phage isolates in the present study were compared to those reported for marine and freshwater phage isolates from previous studies, the decay constants for soil phages were, on average, 4 times lower than those for aquatic phages. Slower rates of phage decay in soils indicate a lower turnover rate, which may have subsequent and potentially far-reaching impacts on virus-mediated mortality and bacterial activity. The wide range of decay rates observed in the present study and the lack of information on this critical aspect of virus-host dynamics in soil emphasizes the need for continued research in this field.

The Historical Development of Constructed Wetlands for Wastewater Treatment

Constructed wetlands (CWs) for wastewater treatment are engineered systems that are designed and operated in order to use all natural processes involved in the removal of pollutants from wastewaters. CWs are designed to take advantage of many of the same processes that occur in natural wetlands, but do so within a more controlled environment. The basic classification is based on the presence/absence of wastewater on the wetland surface. The subsurface flow of CWs can be classified according to the direction of the flow to horizontal and vertical. The combination of various types of CWs is called hybrid CW. The CWs technology began in the 1950s in Germany, but the major extension across the world occurred during the 1990s and early 2000s. The early CWs in Germany were designed as hybrid CWs; however, during the 1970s and 1980s, horizontal subsurface flow CWs were mostly designed. The stricter limits for nitrogen, and especially ammonia, applied in Europe during the 1990s, brought more attention to vertical subsurface flow and hybrid systems. Constructed wetlands have been used to treat various types of wastewater, including sewage, industrial and agricultural wastewaters, various drainage and runoff waters and landfill leachate. Recently, more attention has also been paid to constructed treatment wetlands as part of a circular economy in the urban environments: it is clear that CWs are a good fit for the new concept of sponge cities.

Sustainable, Decentralized Sanitation and Reuse with Hybrid Nature-Based Systems

Nature (ecosystem) based processes for wastewater treatment include constructed wetlands (CWs), waste stabilization ponds, vegetated drainage ditches, buffer zones, instream or bankside river techniques, and mixotrophic systems, where light and CO2 are utilized, in addition to organic carbon compounds, by algal cultures. Algae-based systems can simultaneously remove organic matter, N, and P and may offer substantial energetic advantages compared to traditional biological treatment systems, require small spatial footprint, and contribute to biofuels production and CO2 emissions mitigation. Bioelectrochemical systems (BES) such as microbial fuel cells (MFCs) present characteristics compatible with the use in isolated realities for water and wastewater treatment with contextual energy recovery and may be combined with other nature-based process technologies to achieve good treatment and energy efficiencies. Despite that their application in real-scale plants has not been assessed yet, the most probable outcome will be the in situ/on site treatment (or pretreatment) of wastes for small “in house” plants not connected to the sewerage network. This paper focuses on the current practices and perspectives of hybrid nature-based systems, such as constructed wetlands and microalgae integrated phytoremediation plants, and their possible integration with microbial electrochemical technologies to increase recovery possibilities from wastes and positively contribute to a green economy approach.

Constructed wetland for wastewater reuse: Role and efficiency in removing enteric pathogens

Water stress has become a perennial concern in most of the developing countries due to rapid urbanization and population growth. As the growing population requires more fresh water and better ways for wastewater disposal, the demand for wastewater reclamation has increased drastically in recent years. Wastewater, either raw or treated, is being widely used for agricultural irrigation in developing countries, which cause a serious threat to human health mainly because of its pathogenic content. One of the alternative methods to treat wastewater and make it reusable for agricultural irrigation is to implement constructed wetland (CW); a sustainable and cost-effective technology that is applicable for the elimination of both pollutants and pathogens from wastewater. Despite its wide application, the role of macrophytes that form an integral part of CW and specific mechanisms involved in pathogen removal by them is still barely understood due to complexities involved and influencing factors. This has, therefore, attracted various scientific studies to reveal further functional mechanisms involved in vegetated CW to increase its proficiencies. This review paper illustrates the comparative studies of different CW and their pathogen removal efficiencies with major emphasis on macrophytes involved and factors influencing related mechanism. Further, the paper also covers detailed information on the enteric pathogens present in wastewater and the associated health risks involved in its reuse. The ultimate objective is to further clarify the role of CW in enteric pathogen removal and its efficiency for wastewater purification in perspective with safe reuse in agriculture.

Constructed wetlands for greywater recycle and reuse: A review

Concern over dwindling water supplies for urban areas as well as environmental degradation from existing urban water systems has motivated research into more resilient and sustainable water supply strategies. Greywater reuse has been suggested as a way to diversify local water supply portfolios while at the same time lessening the burden on existing environments and infrastructure. Constructed wetlands have been proposed as an economically and energetically efficient unit process to treat greywater for reuse purposes, though their ability to consistently meet applicable water quality standards, microbiological in particular, is questionable. We therefore review the existing case study literature to summarize the treatment performance of greywater wetlands in the context of chemical, physical and microbiological water quality standards. Based on a cross-section of different types of wetlands, including surface flow, subsurface flow, vertical and recirculating vertical flow, across a range of operating conditions, we show that although microbiological standards cannot reliably be met, given either sufficient retention time or active recirculation, chemical and physical standards can. We then review existing case study literature for typical water supply disinfection unit processes including chlorination, ozonation and ultraviolet radiation treating either raw or treated greywater specifically. An evaluation of effluent water quality from published wetland case studies and the expected performance from disinfection processes shows that under appropriate conditions these two unit processes together can likely produce effluent of sufficient quality to meet all nonpotable reuse standards. Specifically, we suggest that recycling vertical flow wetlands combined with ultraviolet radiation disinfection and chlorine residual is the best combination to reliably meet the standards.

Stormwater constructed wetlands: A source or a sink of Campylobacter spp

Stormwater constructed wetlands are not well characterised for their ability to remove pathogens which can pose public health risks during stormwater harvesting activities. This study investigated the behaviour of faecal indicator organism Escherichia coli (E. coli) and reference pathogen Campylobacter spp. in stormwater constructed wetlands, using a case study system located in Melbourne, Australia. Grab sampling and event-based monitoring revealed influent concentrations of E. coli were typical of other urban stormwater studies, yet Campylobacter concentrations were orders of magnitude above those urban stormwater studies used to develop the Australian Guidelines for Water Recycling, reached levels typical of raw domestic wastewater. The wetland consistently removed E. coli from stormwater (mean log removal 0.96, range 0.19-1.79), while Campylobacter spp. concentrations were often higher in outflow than inflow (mean log removal 0.05, range -0.9-1.25). These results indicate that E. coli is a poor indicator for this reference pathogen. The log reductions of both organisms also failed to meet the criteria specified for any end-use, as listed in the Australian Guidelines for Water Recycling, suggesting further treatment is required prior to harvesting. Finally, this study proposed that direct faecal deposition by waterfowl faeces was a microbial source to stormwater wetlands and that this was partly responsible for the varied microbial removal rates observed. Overall, this work validates the need for further characterisation of pathogens in raw urban stormwater, and the ability for water sensitive urban design features, such as wetlands, to remove both indicator and pathogenic microorganisms.

Phytoremediation of domestic wastewater using a hybrid constructed wetland in mountainous rural area

The purpose of this study is to evaluate the efficiency of hybrid constructed wetlands (HCWs) in a rural mountainous area. The experiment was set up in small rural community named Tidili within the region of Marrakech, Morocco. The wastewater treatment plant was composed of three vertical flow constructed wetlands (VFCWs) working in parallel, followed by two parallel horizontal-subsurface flow constructed wetlands (HFCWs), with hydraulic loading rates of 0.5 and 0.75 m³/m².d, respectively. The two units were planted with Phragmites australis at a density of 4 plants/m². Wastewater samples were collected at the inlet of the storage tank and at the outlet of the whole system (VFCWs, HFCWs) stages. The main removal percentages of total suspended solids (TSS), biochemical oxygen demand measured in a 5-day test (BOD₅), chemical oxygen demand (COD), total nitrogen, and total phosphorus were respectively 95%, 93%, 91%, 67%, and 62%. The system showed a very high capacity to remove total coliforms, fecal coliforms, and fecal streptococci (4.46, 4.31, and 4.10 Log units, respectively). Artificial neural networks (ANNs) were used to model the quality parameters (TSS, BOD₅, COD) and total coliforms and fecal streptococci. Based on the obtained results, the ANN model could be considered as an efficient tool to predict the studied phytoremediation performances using HCWs.

Bacteriophage removal efficiency as a validation and operational monitoring tool for virus reduction in wastewater reclamation: Review

The multiple-barrier concept is widely employed in international and domestic guidelines for wastewater reclamation and reuse for microbiological risk management, in which a wastewater reclamation system is designed to achieve guideline values of the performance target of microbe reduction. Enteric viruses are one of the pathogens for which the target reduction values are stipulated in guidelines, but frequent monitoring to validate human virus removal efficacy is challenging in a daily operation due to the cumbersome procedures for virus quantification in wastewater. Bacteriophages have been the first choice surrogate for this task, because of the well-characterized nature of strains and the presence of established protocols for quantification. Here, we performed a meta-analysis to calculate the average log₁₀ reduction values (LRVs) of somatic coliphages, F-specific phages, MS2 coliphage and T4 phage by membrane bioreactor, activated sludge, constructed wetlands, pond systems, microfiltration and ultrafiltration. The calculated LRVs of bacteriophages were then compared with reported human enteric virus LRVs. MS2 coliphage LRVs in MBR processes were shown to be lower than those of norovirus GII and enterovirus, suggesting it as a possible validation and operational monitoring tool. The other bacteriophages provided higher LRVs compared to human viruses. The data sets on LRVs of human viruses and bacteriophages are scarce except for MBR and conventional activated sludge processes, which highlights the necessity of investigating LRVs of human viruses and bacteriophages in multiple treatment unit processes.

Setback distances between small biological wastewater treatment systems and drinking water wells against virus contamination in alluvial aquifers

Contamination of groundwater by pathogenic viruses from small biological wastewater treatment system discharges in remote areas is a major concern. To protect drinking water wells against virus contamination, safe setback distances are required between wastewater disposal fields and water supply wells. In this study, setback distances are calculated for alluvial sand and gravel aquifers for different vadose zone and aquifer thicknesses and horizontal groundwater gradients. This study applies to individual households and small settlements (1-20 persons) in decentralized locations without access to receiving surface waters but with the legal obligation of biological wastewater treatment. The calculations are based on Monte Carlo simulations using an analytical model that couples vertical unsaturated and horizontal saturated flow with virus transport. Hydraulic conductivities and water retention curves were selected from reported distribution functions depending on the type of subsurface media. The enteric virus concentration in effluent discharge was calculated based on reported ranges of enteric virus concentration in faeces, virus infectivity, suspension factor, and virus reduction by mechanical-biological wastewater treatment. To meet the risk target of <10^-4infections/person/year, a 12 log₁₀ reduction was required, using a linear dose-response relationship for the total amount of enteric viruses, at very low exposure concentrations. The results of this study suggest that the horizontal setback distances vary widely ranging 39 to 144m in sand aquifers, 66-289m in gravel aquifers and 1-2.5km in coarse gravel aquifers. It also varies for the same aquifers, depending on the thickness of the vadose zones and the groundwater gradient. For vulnerable fast-flow alluvial aquifers like coarse gravels, the calculated setback distances were too large to achieve practically. Therefore, for this category of aquifer, a high level of treatment is recommended before the effluent is discharged to the ground surface.

Fecal Bacteria, Bacteriophage, and Nutrient Reductions in a Full-Scale Denitrifying Woodchip Bioreactor

Denitrifying bioreactors using woodchips or other slow-release carbon sources can be an effective method for removing nitrate (NO) from wastewater and tile drainage. However, the ability of these systems to remove fecal microbes from wastewater has been largely uninvestigated. In this study, reductions in fecal indicator bacteria () and viruses (F-specific RNA bacteriophage [FRNA phage]) were analyzed by monthly sampling along a longitudinal transect within a full-scale denitrifying woodchip bioreactor receiving secondary-treated septic tank effluent. Nitrogen, phosphorus, 5-d carbonaceous biochemical oxygen demand (CBOD), and total suspended solids (TSS) reduction were also assessed. The bioreactor demonstrated consistent and substantial reduction of (2.9 log reduction) and FRNA phage (3.9 log reduction) despite receiving highly fluctuating inflow concentrations [up to 3.5 × 10 MPN (100 mL) and 1.1 × 10 plaque-forming units (100 mL) , respectively]. Most of the removal of fecal microbial contaminants occurred within the first meter of the system (1.4 log reduction for ; 1.8 log reduction for FRNA phage). The system was also efficient at removing NO (>99.9% reduction) and TSS (89% reduction). There was no evidence of consistent removal of ammonium, organic nitrogen, or phosphorus. Leaching of CBOD occurred during initial operation but decreased and stabilized at lower values (14 g O m) after 9 mo. We present strong evidence for reliable microbial contaminant removal in denitrifying bioreactors, demonstrating their broader versatility for wastewater treatment. Research on the removal mechanisms of microbial contaminants in these systems, together with the assessment of longevity of removal, is warranted.

Seasonal determinations of algal virus decay rates reveal overwintering in a temperate freshwater pond

To address questions about algal virus persistence (i.e., continued existence) in the environment, rates of decay of infectivity for two viruses that infect Chlorella-like algae, ATCV-1 and CVM-1, and a virus that infects the prymnesiophyte Chrysochromulina parva, CpV-BQ1, were estimated from in situ incubations in a temperate, seasonally frozen pond. A series of experiments were conducted to estimate rates of decay of infectivity in all four seasons with incubations lasting 21 days in spring, summer and autumn, and 126 days in winter. Decay rates observed across this study were relatively low compared with previous estimates obtained for other algal viruses, and ranged from 0.012 to 11% h(-1). Overall, the virus CpV-BQ1 decayed most rapidly whereas ATCV-1 decayed most slowly, but for all viruses the highest decay rates were observed during the summer and the lowest were observed during the winter. Furthermore, the winter incubations revealed the ability of each virus to overwinter under ice as ATCV-1, CVM-1 and CpV-BQ1 retained up to 48%, 19% and 9% of their infectivity after 126 days, respectively. The observed resilience of algal viruses in a seasonally frozen freshwater pond provides a mechanism that can support the maintenance of viral seed banks in nature. However, the high rates of decay observed in the summer demonstrate that virus survival and therefore environmental persistence can be subject to seasonal bottlenecks.

Key design factors affecting microbial community composition and pathogenic organism removal in horizontal subsurface flow constructed wetlands

Constructed wetlands constitute an interesting option for wastewater reuse since high concentrations of contaminants and pathogenic microorganisms can be removed with these natural treatment systems. In this work, the role of key design factors which could affect microbial removal and wetland performance, such as granular media, water depth and season effect was evaluated in a pilot system consisting of eight parallel horizontal subsurface flow (HSSF) constructed wetlands treating urban wastewater from Les Franqueses del Vallès (Barcelona, Spain). Gravel biofilm as well as influent and effluent water samples of these systems were taken in order to detect the presence of bacterial indicators such as total coliforms (TC), Escherichia coli, fecal enterococci (FE), Clostridium perfringens, and other microbial groups such as Pseudomonas and Aeromonas. The overall microbial inactivation ratio ranged between 1.4 and 2.9 log-units for heterotrophic plate counts (HPC), from 1.2 to 2.2 log units for total coliforms (TC) and from 1.4 to 2.3 log units for E. coli. The presence of fine granulometry strongly influenced the removal of all the bacterial groups analyzed. This effect was significant for TC (p=0.009), E. coli (p=0.004), and FE (p=0.012). Shallow HSSF constructed wetlands were more effective for removing Clostridium spores (p=0.039), and were also more efficient for removing TC (p=0.011) and E. coli (p=0.013) when fine granulometry was used. On the other hand, changes in the total bacterial community from gravel biofilm were examined by using denaturing gradient gel electrophoresis (DGGE) and sequencing of polymerase chain reaction (PCR)-amplified fragments of the 16S rRNA gene recovered from DGGE bands. Cluster analysis of the DGGE banding pattern from the different wetlands showed that microbial assemblages separated according to water depth, and sequences of different phylogenetic groups, such as Alpha, Beta and Delta-Proteobacteria, Nitrospirae, Bacteroidetes, Acidobacteria, Firmicutes, Synergistetes and Deferribacteres could be retrieved from DGGE bands.

Unit Process Wetlands for Removal of Trace Organic Contaminants and Pathogens from Municipal Wastewater Effluents

Treatment wetlands have become an attractive option for the removal of nutrients from municipal wastewater effluents due to their low energy requirements and operational costs, as well as the ancillary benefits they provide, including creating aesthetically appealing spaces and wildlife habitats. Treatment wetlands also hold promise as a means of removing other wastewater-derived contaminants, such as trace organic contaminants and pathogens. However, concerns about variations in treatment efficacy of these pollutants, coupled with an incomplete mechanistic understanding of their removal in wetlands, hinder the widespread adoption of constructed wetlands for these two classes of contaminants. A better understanding is needed so that wetlands as a unit process can be designed for their removal, with individual wetland cells optimized for the removal of specific contaminants, and connected in series or integrated with other engineered or natural treatment processes. In this article, removal mechanisms of trace organic contaminants and pathogens are reviewed, including sorption and sedimentation, biotransformation and predation, photolysis and photoinactivation, and remaining knowledge gaps are identified. In addition, suggestions are provided for how these treatment mechanisms can be enhanced in commonly employed unit process wetland cells or how they might be harnessed in novel unit process cells. It is hoped that application of the unit process concept to a wider range of contaminants will lead to more widespread application of wetland treatment trains as components of urban water infrastructure in the United States and around the globe.

Transport and removal of coliphage PRD1 in constructed wetlands

The Convection Dispersion Equation (CDE) was used to calculate PRD1 and Br(-) transport parameters in a subsurface flow constructed wetland. Transport parameters from Br(-) displacement were applied into the CDE to estimate a 0.96 day(-1) first order decay coefficient (k). The PRD1 breakthrough curves were also simulated to obtain effective cross-sectional area (Ac), longitudinal dispersion coefficient (D), convective velocity (v), and k. There was practically no difference between Ac and D for both tracers. However, the estimated convective velocity was higher for PRD1 than for Br(-). Further simulations were conducted by taking experimental concentrations from prior research on surface and subsurface flow constructed wetlands. Dispersion number (d) was estimated to be between 0.17 and 0.029 by using PRD1 and Br(-) transport parameters. These parameters were also used to calculate wetland dimensionless removal (K). An analytical solution for the zero moment of the observed breakthrough curves was applied to estimate PRD1 fraction recoveries in the wetland by using d and K. The results of the present study suggest that this analytical solution may be an alternative design tool for pathogen removal estimation in subsurface flow constructed wetlands.

Removal of human enteric viruses and indicator microorganisms from domestic wastewater by aerated lagoons

Aerated lagoons offer a low-cost and simple approach to treating domestic wastewater in small municipalities. The objective of the current study was to evaluate, for each cell in the lagoons, the removal of indicator microorganisms and human enteric viruses under warm (summer) and cold (early spring) conditions. The two sites are located in southwest Quebec, Canada. Samples were assayed for thermotolerant coliforms, enterococci, Clostridium perfringens , somatic and male-specific coliphages, and culturable human enteric viruses (HEV). The results show higher removal under warm ambient conditions for all microorganisms. Thermotolerant coliforms and enterococci were removed to a greater extent than C. perfringens and HEV. HEV removal was only observed in warm ambient conditions. The removal of coliphages was different from the observed removal of HEV. The use of coliphages as surrogates for HEV has been proposed, but this does not seem appropriate for aerated lagoons, as the removal of coliphages overestimates the removal of HEV. Given the low observed removal of HEV during this study, the effluents remain a significant source of pathogens that can affect drinking water treatment plants drawing their raw water from receiving streams. Ultraviolet disinfection of treated wastewater effluent is a possible solution.

Occurrence and persistence of enteroviruses, noroviruses and F-specific RNA phages in natural wastewater biofilms

Enteroviruses and noroviruses are pathogenic viruses excreted by infected individuals. Discharged in wastewaters, some of these viruses can be captured by biofilms. In the present study, we assessed the occurrence and persistence of these viruses in wastewaters and in corresponding biofilms. Natural wastewaters and biofilms were analyzed monthly from January to July using real-time RT-PCR. Enterovirus RNA was detected in wastewater in June while norovirus RNA was detected from January to March. In contrast, biofilm analysis revealed the presence of both enterovirus and norovirus genomes throughout the study period. For instance, enterovirus and norovirus genogroups (GG) I and II were detected in 50, 46 and 37% of the biofilm samples, respectively (n=24). In a laboratory experiment, persistence of norovirus GGI RNA (quantified using molecular techniques) and F-specific bacteriophages (quantified using both culture and molecular techniques) was assessed in wastewater and corresponding naturally-contaminated biofilms at both 4 and 20 degrees C. The concentrations of viral genomes (norovirus GGI and F-specific RNA phage) were very stable in biofilms. Indeed, no significant decrease was observed during the persistence experiment that lasted 49 days. Furthermore, regardless of our experimental conditions, viral genome and infectious F-specific bacteriophages persisted longer in biofilm than in wastewater. According to our results, wastewater biofilms may contribute to the persistence and dispersal of pathogenic viruses outside of epidemic periods.

Transport of coliphage PRD1 in a surface flow constructed wetland

A tracer study was conducted in a 3-ha surface flow constructed wetland to analyze transport performance of PRD1, an enteric virus model. The convection-dispersion equation (CDE), including a first-order reaction model, adequately simulated transport performance of PRD1 in the wetland under an average hydraulic loading rate of 82 mm/d. Convective velocity (v) and longitudinal dispersion coefficient (D) were estimated by modeling a conservative tracer (bromide) pulse through the wetland. Both PRD1 and bromide were simultaneously added to the entering secondary treated wastewater effluent. The mass of bromide and PRD1 recovered was 76 and 16%, respectively. The PRD1 decay rate was calculated to be 0.3/day. The findings of this study suggest that the CDE model and analytical moment equations represent a suitable option to characterize virus transport performance in surface flow constructed wetlands.

Effects of pH and temperature on the survival of coliphages MS2 and Qbeta

The RNA F-specific coliphages, MS2 and Qbeta, have been used as virus indicators in water and wastewater studies. It is therefore useful to have a good understanding concerning the effects of environmental factors on their survival in order to choose an appropriate candidate for assessing microbial safety in relation to water quality management. The effects of pH and temperature on the survival of these two coliphages were investigated. MS2 survived better in acidic conditions than in an alkaline environment. In contrast, Qbeta had a better survival rate in alkaline conditions than in an acidic environment. The inactivation rates of both coliphages were lowest within the pH range 6-8 and the temperature range 5-35 degrees C. The inactivation rates of both coliphages increased when the pH was decreased to below 6 or increased to above 8. The inactivation rates of both coliphages increased with increasing temperature. Qbeta behaved peculiarly in extreme pH buffers, i.e. it was inactivated very rapidly initially when subjected to an extreme pH environment, although the inactivation rate subsequently decreased. In general, MS2 was a better indicator than Qbeta. However, within the pH range 6-9 and at temperatures not above 25 degrees C, either MS2 or Qbeta could be used as a viral indicator.

Microbial water quality improvement by small scale on-site subsurface wetland treatment

It has been demonstrated that large constructed wetlands used for domestic wastewater treatment are useful in the reduction of enteric microorganisms. This study evaluated the ability of three small-scale, on-site subsurface wetlands with different vegetation densities to remove total coliforms, fecal coliforms, coliphage, Giardia and Cryptosporidium. These wetlands were found to be equally efficient in the removal of enteric bacteria and coliphage as larger constructed wetlands. Giardia and Cryptosporidium were usually undetectable after passage of the wastewater through the subsurface wetlands. Coliphage removal increased with increasing vegetation density.

Evaluation of biotracers to monitor effluent retention time in constructed wetlands

AIMS

With concern surrounding the environmental impact of chemical tracers on the aquatic environment, this paper presents the initial evaluation of biotracers used to determine the effluent retention time, an important performance indicator, in a Free Water Surface Constructed Wetland.

METHODS AND RESULTS

Production of the biotracers, coliphage MS2, and the bacteriophage of Enterobacter cloacae and antibiotic resistant endospores of Bacillus globigii is described in detail. Their subsequent use in three separate tracer experiments - January, March and June (2000) - revealed the variability of retention time with respect to effluent flow. The biotracer MS2 showed the constructed wetland had a retention time of 8-9 h at a mean discharge of 0.9 l s-1, increasing to 10-12 h at a mean discharge 0.3 l s-1. A similar retention of 9-10 h at a mean discharge of 0.3 l s-1 was calculated for the Ent. cloacae phage. In contrast, use of endospores revealed considerably longer retention times at these mean discharge rates; 12-24 h and 36-48 h, respectively.

CONCLUSION

Biotracers could provide a useful and environmentally friendly technique to monitor effluent retention in constructed wetlands. At this stage the phage tracers appear particularly promising due to ease of isolation and recovery.

SIGNIFICANCE AND IMPACT OF THE STUDY

Initial results are encouraging and have highlighted the potential of biotracers as alternatives to chemical tracers, even in microbially-rich waters.

Fate of physical, chemical, and microbial contaminants in domestic wastewater following treatment by small constructed wetlands

In order to evaluate the efficacy of constructed wetlands for treatment of domestic wastewater for small communities located in rural areas, small-scale wetland mesocosms (400 L each) containing two treatment designs (a mixture of Typha, Scirpus, and Juncus species; control without vegetation) were planted into two depths (45 or 60 cm) with pea gravel. Each mesocosm received 19 L/day of primary-treated domestic sewage. Mesocosms were monitored (inflow and outflow samples) on a monthly basis over a 2-year period for pH, total suspended solids (TSS), 5-day biochemical oxygen demand (BOD(5)), total Kjeldahl nitrogen (TKN), dissolved oxygen (DO), and conductivity. Microbiological analyses included enumeration of fecal coliforms, enterococci, Salmonella, Shigella, Yersinia, and coliphage. Significant differences between influent and effluent water quality for the vegetated wetlands (p<0.05) were observed in TSS, BOD(5), and TKN. Increased DO and reduction in fecal coliform, enterococcus, Salmonella, Shigella, Yersinia, and coliphage populations also were observed in vegetated wetlands. Greatest microbial reductions were observed in the planted mesocosms compared to those lacking vegetation. Despite marked reduction of several contaminants, wetland-treated effluents did not consistently meet final discharge limits for receiving bodies of water. Removal efficiencies for bacteria and several chemical parameters were more apparent during the initial year compared to the second year of operation, suggesting concern for long-term efficiency and stability of such wetlands.

Giardia and Cryptosporidium removal from waste-water by a duckweed (Lemna gibba L.) covered pond

AIMS

To determine the ability of duckweed ponds used to treat domestic waste-water to remove Giardia and Cryptosporidium.

METHODS AND RESULTS

The influent and effluent of a pond covered with duckweed with a 6 day retention time was tested for Giardia cysts, Cryptosporidium oocysts, faecal coliforms and coliphage. Giardia cysts and Cryptosporidium oocysts were reduced by 98 and 89%, respectively, total coliforms by 61%, faecal coliforms by 62% and coliphage by 40%. There was a significant correlation between the removal of Giardia cysts and Cryptospordium oocysts by the pond (P < 0.001). Influent turbidity and parasite removal were also significantly correlated (Cryptosporidium and turbidity, P=0.05; Giardia and turbidity, P=0.01).

CONCLUSIONS

The larger organisms (parasites) probably settled to the bottom of the pond, while removal of smaller bacteria and coliphages in the pond was not as effective.

SIGNIFICANCE AND IMPACT OF THE STUDY

Duckweed ponds may play an important role in wetland systems for reduction of Giardia and Cryptosporidium.

Fate of indicator microorganisms, Giardia and Cryptosporidium in subsurface flow constructed wetlands

Limited information is available on the ability of subsurface flow wetlands to remove enteric pathogens. Two multi-species wetlands, one receiving secondary sewage effluent and the other potable (disinfected) groundwater were studied from February 1995 to August 1996, at the Pima County Constructed Ecosystems Research Facility in Tucson, Arizona. Each wetland had a retention time of approximately 4 days. The objectives of this study were (1) to evaluate the ability of multi-species subsurface wetlands to physically remove Giardia cysts; Cryptosporidium oocysts, total and fecal coliforms, and coliphages; and (2) to determine the likely impact of local wildlife on the occurrence of these indicators and pathogens. In the wetland receiving secondary sewage effluent, total coliforms were reduced by an average of 98.8% and fecal coliforms by 98.2%. Coliphage were reduced by an average of 95.2%. Both Giardia cysts and Cryptosporidium oocysts were reduced by an average of 87.8 and 64.2%, respectively. In the wetland receiving disinfected groundwater, an average of 1.3 x 10(2) total coliforms/100 mL and 22.3 fecal coliforms/100 mL were most likely contributed by both flora and fauna. No parasites or coliphages were detected.

Removal of pathogenic and indicator microorganisms by a constructed wetland receiving untreated domestic wastewater

Wetlands containing floating, emergent and submergent aquatic plants, and other water-tolerant species have been found to economically provide a mechanism of enhancing the quality of domestic wastewater. The use of constructed wetlands for the removal of indicator bacteria (total and fecal coliforms), coliphages, protozoan parasites (Giardia and Cryptosporidium) and enteric viruses was investigated. A pilot scale constructed wetland consisting of two cells, one planted with bulrush and the other unplanted bare sand, were used to compare their efficiency in removing pathogens from raw sewage. Overall more than 90 percent of all microorganisms studied were removed by either of the two systems with a 1 to 2 day retention time. Removal of all mentioned microorganisms was greater from the surface flow in the unplanted cell than in the planted cell, except for Giardia and Cryptosporidium, although the differences were not statistically significant. Enteric viruses, coliphages and indicator bacteria were found to penetrate 2 m below the surface, although concentrations were reduced by greater than 99 percent in both cells. Less virus penetration into the sand occurred in the planted wetland versus the unplanted wetland. Water temperature was found to be the most important factor in the removal of enteric bacteria and viruses, while turbidity reduction was related to Giardia removal. These results demonstrate that significant reductions of pathogenic microorganisms can occur in constructed wetlands receiving untreated domestic wastewater with only a 1-2 day retention time.

Virioplankton: viruses in aquatic ecosystems

The discovery that viruses may be the most abundant organisms in natural waters, surpassing the number of bacteria by an order of magnitude, has inspired a resurgence of interest in viruses in the aquatic environment. Surprisingly little was known of the interaction of viruses and their hosts in nature. In the decade since the reports of extraordinarily large virus populations were published, enumeration of viruses in aquatic environments has demonstrated that the virioplankton are dynamic components of the plankton, changing dramatically in number with geographical location and season. The evidence to date suggests that virioplankton communities are composed principally of bacteriophages and, to a lesser extent, eukaryotic algal viruses. The influence of viral infection and lysis on bacterial and phytoplankton host communities was measurable after new methods were developed and prior knowledge of bacteriophage biology was incorporated into concepts of parasite and host community interactions. The new methods have yielded data showing that viral infection can have a significant impact on bacteria and unicellular algae populations and supporting the hypothesis that viruses play a significant role in microbial food webs. Besides predation limiting bacteria and phytoplankton populations, the specific nature of virus-host interaction raises the intriguing possibility that viral infection influences the structure and diversity of aquatic microbial communities. Novel applications of molecular genetic techniques have provided good evidence that viral infection can significantly influence the composition and diversity of aquatic microbial communities.

Les virus des eaux usées et leur élimination au cours des traitements des effluents pollués

Les virus représentent une forme de pollution biologique véhiculée par les eaux usées. Dans une station d'épuration naturelle ou un pilote de laboratoire, le traitement biologique d'un effluent contaminé entraîne une réduction de la charge virale.

Celle-ci est considérable à l'étape du traitement secondaire. Les bactériophages sont considérées comme indicateurs de contamination fécale. Ce sont également des modèles de virus entériques. Dans une étude expérimentale, deux pilotes ont été utilisés (l'un à lagunage naturel, l'autre à boues activées) pour suivre la cinétique et évaluer le taux d'élimination du coliphage somatique X-174 et celui à ARN F-spécifique MS2. L'élimination des deux phages, qui atteint 99 % dans les deux systèmes, est comparable à celle rapportée dans les études antérieures sur les virus entériques. L'influence de plusieurs facteurs sur le retrait et l'inactivation virale (adsorption à la matière solide, action des microorganismes, effet des rayonnements solaires et des composés dissous) a été abordée. Révélée dans des travaux antérieurs, l'importance de 1'adsorption a été montrée dans le cas des pilotes et aussi en mettant en contact le X-174 et le MS2 avec des minéraux argileux (la montmorillonite et le kaolin). L'effet des rayons solaires est connu dans le milieu marin. Dans une étude in vitro, une exposition de 12 h au soleil a causé l'inactivation quasi-totale des suspensions phagiques. Enfin, une baisse de la concentration phagique est enregistrée après un contact prolongé dans une eau de lagunage filtrée. En revanche, la phagocytose des phages par Tetrahymena pyriformis n'a pas été évidente.