Disinfection for decentralized wastewater reuse in rural areas through wetlands and solar driven onsite chlorination

Abstract

Increasing water scarcity is of growing concern in Europe, especially in Mediterranean countries along coastlines. Wastewater reuse reduces water stress, but often requires the absence of pathogen indicators and the application of chlorine to assure residual disinfection. However, the effluent qualities of typical Wastewater Treatment Plants (WWTP) show immense chlorine demands. This makes the supply, handling and dosing of typical WWTP effluent challenging, especially in rural regions. In the work presented here, a vertical flow constructed wetland (VFCW) was combined with a small-scale solar-driven Onsite Chlorine Generation system (OCG) to further improve effluent qualities for different WWTPs and to produce chlorine stock solution directly at the site. To test different operational conditions the VFCW received WWTP effluent from a) an Activated Sludge and b) a High-Rate Algae Pond system. The VFCW reduced TSS by 92%, COD by 80%, and NH₄ by over 99% and the chlorine demand by 85%. The log-unit reduction of the VFCW/OCG system reached ≥5.1 for total coliforms and ≥4.6 for E. Coli. During VFCW passage the already high electrical conductivity further increased to beyond permissible reuse limits due to high evapotranspiration (ET) rates of the planted vegetation Arundo donax. Unique aspects of this setup were that neither chemicals nor external electricity were required to run the system. The elevated chloride concentration of the treated WW (819 ± 132 mg/L) proved sufficient for the production of chlorine stock solution. However, the solar-driven OCG system tested here consumed considerably more electricity compared to other OCGs available on the market. Nevertheless, the system presented here may be considered an efficient disinfection alternative for decentralized WW reuse applications at remote sites with both limited access to grid electricity and strict requirements for pathogen indicators.

Keywords: Disinfection; Disinfection-by-products; Electrochlorination; Onsite chlorine generation; Vertical flow constructed wetlands; Wastewater reuse; Water scarcity.