Chemical oxygen demand (COD) is widely used for wastewater monitoring, design, modeling and plant operational analysis. However this method results in the production of hazardous wastes including mercury and hexavalent chromium. The study examined the replacement of COD with total organic carbon (TOC) for general performance monitoring by comparing their relationship with influent and effluent samples from 11 wastewater treatment plants. Biochemical oxygen demand (BOD5) was also included in the comparison as a control. The results show significant linear relationships between TOC, COD and BOD5 in settled (influent) domestic and municipal wastewaters, but only between COD and TOC in treated effluents. The study concludes that TOC can be reliably used for the generic replacement of both COD (COD=49.2+3.00*TOC) and BOD5 (BOD5=23.7+1.68*TOC) in influent wastewaters but only for COD (COD=7.25+2.99*TOC) in final effluents.

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Abstract:The wastewater of one third of Ireland's population is treated on-site using domestic treatment systems (DWWTSs) that usually consist of a septic tank and soil attenuation system. Within the past four years, the legislative framework for these systems has undergone a major change with a registration and inspection regime being introduced to identify legacy sites that will require remediation work, particularly in areas of the country underlain by subsoils of very low permeability. Against this background this study aims to assess the overall sustainability of existing DWWTSs as well as alternative treatment and disposal options. The results show that main CO 2eq emissions are from the methane production in septic tanks. The reduced methane production in mechanically aerated secondary treatment systems was found to counterbalance the related emissions due to the additional energy requirements. In contrast, septic tank systems have the lowest construction and operational costs representing the most economically sustainable solution. Pressurised disposal systems are slightly more expensive but have the potential to reduce environmental impact on surface water and reduce greenhouse gas emissions. Clustered decentralised treatment solutions could be environmentally and economically sustainable but ownership, management and related financial and legal issues will need to be addressed and developed.

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