Potential for CSO treatment with horizontal flow constructed wetlands: influence of hydraulic load, plant presence and loading frequency

Current and future approaches to wet weather flow management: A review

Sewers can become hydraulically overburdened during high-intensity precipitation resulting in untreated water entering receiving streams. Combined (CSOs) and sanitary sewer overflows (SSOs) cause adverse public health and environmental impacts as well as management challenges for many wastewater utilities. This novel review presents information regarding wet weather flow regulation, impacts, and current management methods, and offers ideas for future approaches in the United States. Currently, storage followed by conventional municipal water reclamation facility treatment after precipitation events is often employed. Stand-alone alternative technologies include high-rate solids removal, rapid disinfection, filtration, and green infrastructure. However, most current stand-alone approaches do not address soluble BOD₅ or emerging contaminants in stormwater and wastewater. As the needs for wet weather flow management change, future approaches should include a goal of zero overflows and achieve effluent quality as good as or better than conventional treatment. To help achieve zero overflows and complete treatment, the "peaker facility" concept is proposed. The peaker facility often remains idle but treats excess flow when needed. Considering the challenges of remaining idle for long periods, starting up quickly, and handling high flows, chemical oxidation may be an applicable peaker facility component. However, more research and development are needed to determine best practices. PRACTITIONER POINTS: Combined (CSO) and sanitary sewer overflows (SSOs) pose both environmental and public health risks as untreated water is discharged into lakes and rivers during high-intensity rain events. Current stand-alone approaches for managing or treating CSOs focus on particulate BOD/COD and solids removal, and do not typically address soluble BOD or emerging contaminants in stormwater and wastewater (including pathogens). New wet weather policies and regulations encourage more holistic approaches by wastewater utilities, and future approaches should include a zero-overflow goal for all CSOs and SSOs. To help achieve zero overflows, the concept of the "peaker facility" is proposed. Chemical oxidation may be an applicable component of peaker facilities for its short detention time and ability to remove, oxidize, or inactive water impairment-causing contaminants.

Constructed wetlands for combined sewer overflow treatment: A state-of-the-art review

Combined sewer overflows (CSOs) are a major source of surface water pollution and degradation. This is particularly visible where sewage collection with combined sewer and centralized treatment are well established, such as in Europe and North America: an overwhelming number of surface water bodies are in insufficient status of ecology, hydrology and physico-chemical parameters. Therefore, several countries have started implementing constructed wetlands (CWs) as mainstream on-spot treatment. This paper summarizes the main design approaches that can be adopted. We identified eight different schemes for the implementation of CSO-CWs, based on our international experience and documented by a literature analysis. The performance review includes conventional water quality parameters, as well as pathogen and emergent contaminant removal. Furthermore, modelling tools for advanced design and for understanding a wide applicability of these green infrastructures are presented. This paper also provides a review on other side benefits offered by the adoption of Nature-Based Solutions for CSO treatment, such as ecosystem services, and the most common issues related to their operation and maintenance. Our analysis has produced a list of key factors for design and operation, all derived from full-scale installations in operation up to more than ten years.

Assessing local materials for the treatment of wastewater in open drains

In the present study, three low-cost filter aggregate materials were tested and compared for organic matter and fecal coliform (FC) removal at the laboratory scale. Setups were subjected to synthetic wastewater at two hydraulic loading rates (HLR), i.e. 4 cm/day and 40 cm/day. The hydraulic retention time (HRT) at the two HLRs varied from 4 days to 12 h, respectively. The result obtained shows that the biochemical oxygen demand (BOD₅) removal efficiency of aggregate materials decreased with the increase in HLR. Both at high and low HLR, the terracotta aggregate material exhibited maximum BOD₅ loading removal and without significant difference for the case of FC removal efficiency for all the three aggregate materials. At higher HLR, cell debris and biofilm loss from the aggregate material contributed to the chemical oxygen demand (COD) levels in the treated water. The terracotta aggregate material provided best organic matter removal at both HLRs. The study demonstrates the potential of incorporating inexpensive and readily available local materials into decentralized, frugal green infrastructure interventions capable of lowering the quantum of harmful biological contaminants in open storm water channels in rapidly urbanizing cities of developing countries, and that the terracotta aggregate material provided best organic removal at both HLRs.

Reduction of micropollutants and bacteria in a constructed wetland for combined sewer overflow treatment after 7 and 10 years of operation

Repeated investigations on constructed wetlands for the treatment of combined sewer overflows, also named bioretention filters or retention soil filters, are necessary to provide information on their long-term performance. In this study, a sampling campaign was conducted on micropollutants, indicator microorganisms and standard parameters ten years after such filters were in operation and three years after the first investigation; it revealed that the filters lost capacity to remove chemical substances with no or only slow biological degradability. This was the case e.g. for phosphate (decrease from 29 to 11%), diclofenac (67 to 34%) and TCPP (34% to negative reduction). They continued to remove easily degradable parameters such as COD (stable around 75%) stably. The indicator microorganisms Escherichia coli (1.1/0.8 log₁₀), intestinal enterococci (1.3/0.8 log₁₀) and somatic coliphages (0.6/1.0 log₁₀) showed comparably low process variations given the difficulties in sampling and analysing microbial parameters representatively as well as given natural variations in microbial behaviour and growth. Additionally, for bisphenol A, we found a temperature-related difference of removal efficiencies: while in the cold months (winter), the removal was only 53% on average, it increased to 90% in the warm months (summer). As for the long-term prospective of retention soil filters, decision-makers need to identify the most important pollutants in a specific catchment area and adapt the filter design accordingly. If pollutants are targeted that lead to an exhausted filtration capacity, post treatment or the exchange of charged filter material is necessary. However, for easily biologically degradable substances, so far, there is no limit in their use.

Contributions of combined sewer overflows and treated effluents to the bacterial load released into a coastal area

The impact of combined sewer overflow (CSO) on the receiving water body is an issue of increasing concern, as it may lead to restrictions in the use and destination of the receiving body, such as bathing or recreational area closures, fish and shellfish consumption restrictions, and contamination of drinking water resources. Recent investigations have mainly referred to the occurrence and loads of suspended solids, organic compounds and, in some cases, micropollutants. Attempts have been made to find correlations between the discharged load and the size and characteristics of the catchment area, climate conditions, rainfall duration and intensity. This study refers to a touristic coastal area in the north-east of Italy, which is characterized by a combined sewer network including 5 CSO outfalls which, in the case of heavy rain events, directly discharge the exceeding water flow rate into channels which, after a short distance, reach the Adriatic Sea. The study analyzed: i) rainfall events during the summer period in 2014 which led to overflow in the different outfalls, ii) the inter- and intra-event variability with regard to E. coli, Enterococci and conductivity, and iii) the hydraulic and pollutant (E. coli and Enterococci) loads discharged by the local wastewater treatment plant and by all the CSO outfalls. Finally, it estimated the contribution of each source to the released hydraulic and pollutant loads into the receiving water body. Moreover, it was also found that the modest water volume discharged by all CSO outfalls (only 8% of the total volume discharged by the area) contains >90% of the microbial load.