Introduction
Urban development and population growth can place pressure on groundwater-based potable water supplies and wastewater treatment systems, which are common in various parts of the world. Managed aquifer recharge (MAR) using highly treated effluent from municipal wastewater treatment plants (WWTPs) is recognized as a promising strategy for indirect potable reuse, helping to meet increased potable water demands and associated increases in wastewater discharges (Figure 1).
Figure 1. MAR with reclaimed water schematic. (Reprinted from Yuan et al. (2016) Water Quality Research Journal, 51(4):357-376, with permission from the copyright holders, IWA Publishing.)
Treated wastewater effluent can contain residual contaminants that may affect human health, making it important to identify contaminants requiring additional pre-treatment prior to injection or filtration as part of MAR systems. A list of representative (critical) contaminants can be used to provide an initial assessment of which treatment technologies might be required. The selection process needs to be efficient since there are a large number of potential regulated contaminants, and studying the removal efficiency for each of them for different treatment alternatives would be costly and difficult to implement in practice. We developed an approach that can be applied in any jurisdiction considering the implementation of MAR for potable water use, and applied the approach to a municipal WWTP located in Ontario, Canada that receives wastewater containing mainly household waste and little or no discharge from industry. The WWTP system utilizes an activated sludge biological treatment with tertiary filtration and ultraviolet disinfection.
Methodology
We developed water quality targets for microbial and chemical contaminants by considering regulated and guideline limits from five sources:
- Three water reuse regulatory documents (Florida, Idaho, California) – limits specific to MAR applications from jurisdictions with regulations or guidelines.
- Guidelines for Canadian Drinking Water Quality – limits related to potable water.
- Ontario Provincial Water Quality Objectives – limits related to environmental protection.
Regulated contaminants were classified into five main groups, considered the best indicators for water quality based on exposure and health effects: microbial parameters, disinfection by-products, inorganic chemicals, volatile organics, and non-volatile organics. We typically selected the strictest available regulatory limits as MAR water quality limits, with consideration of known reduction values related to WWTP technology used at the case study site. Drinking water guideline values were mainly used since the ultimate goal was for potable applications, although in some cases additional or more stringent values were set based on water reuse or environmental water quality guidelines.
Outcomes
Predominant contaminants were selected through statistical analysis of monitoring data for the WWTP’s effluent. These contaminants are based on regulated substances that are routinely monitored in wastewater effluent, and therefore this approach would be generally applicable since similar data would be available for any type of treated municipal wastewater. The following parameters had higher concentrations than the defined water quality targets:
- Micro-organisms: E. coli.
- Nutrients: nitrate, nitrite, total ammonia, and total phosphorus.
- Metals: aluminum, manganese.
- Salts: chloride.
We grouped regulated potential additional contaminants according to their physicochemical type or application. This group of contaminants includes compounds that are not routinely monitored in WWTP effluents, but are included in regulated water quality limits and also have a high probability to exist in wastewater effluents. Since the list of regulated contaminants is large, we selected representative contaminants from each group based on several factors, including potential for widespread and/or elevated occurrence in wastewater, and expected removal during wastewater treatment processes. Information from the literature was included in the selection process, including typical concentrations in WWTP effluents. Two micro-organisms (Giardia and Cryptosporidium) were selected, as well as multiple non-volatile organics, including herbicides, pesticides, plasticizers, and detergents. Industrial (by-) products were excluded since the WWTP in the study does not receive industrial waste.
An example municipal WWTP (stock photo)
Emerging contaminants include four main categories: perfluorochemicals, pharmaceuticals, antibiotics, and personal care products. These contaminants are not regulated in drinking water or wastewater, but may produce adverse ecological and human health effects. Their main source into the environment is usually through municipal sewage and therefore they are an important consideration for MAR with reclaimed water. A literature review identified compounds that have relatively high usage volumes, high concentrations in wastewater effluents, and increased potential for ecological and/or health effects. To better manage monitoring requirements, representative compounds were selected from each of the emerging contaminant groups using the same approach as for regulated contaminants:
- Perfluorochemicals: representatives are commonly detected in aquatic environments, can bioaccumulate in mammals, have poor biodegradability, are persistent in wastewater effluents, and exceed provisional health advisory values developed by the United States Environmental Protection Agency.
- Pharmaceuticals: representatives have high consumption volumes, are present in wastewater effluents, and have shown toxic effects on fish, crustaceans, algae and bacteria, even in low concentrations.
- Antibiotics: representatives are commonly used and frequently detected in wastewater effluents.
- Personal care products: representatives have toxicity effects on aquatic life, and are found in WWTP effluents in concentrations exceeding regulated environmental limits.
As research in this area continues to develop, it is expected that the treatment and monitoring requirements of trace and emerging compounds in water will change over time.
Conclusions
The identification of critical contaminants for MAR with reclaimed water is important for implementation in jurisdictions without existing regulations or guidelines. According to the results of this study, the following conclusions can be drawn:
- Recharge water quality targets for MAR can be defined based on current water reuse, drinking water, and environmental water quality regulations or guidelines.
- Statistical analysis of WWTP effluent monitoring data can facilitate the selection of critical contaminants.
- Potential contaminants can be addressed by selecting representative contaminants which have a high probability to exist in wastewater effluents.
- Literature data can be useful in identifying contaminant concentrations for which wastewater effluent data are not available at a particular location.
Our approach selects representative compounds for each group of contaminants that have the greatest possibility to exist in municipal wastewater effluents and to cause potential adverse human and environmental health effects. The approach used in this study would be useful for feasibility assessments to assess treatment requirements for future MAR projects using WWTP effluents, not only in Canada but also worldwide.
Yuan, J., M. I. Van Dyke, and P. M. Huck (2017). Identification of critical contaminants in wastewater effluent for managed aquifer recharge. Chemosphere 172, 294-301.
Contact: Peter Huck, Department of Civil and Environmental Engineering; Michele Van Dyke, Department of Civil and Environmental Engineering
For more information about the Water Institute, contact Amy Geddes.
