Total emission analysis of sewerage systems and wastewater treatment plants

• Main Author: Jack, Andrew G.
• Other Authors: Akunna, Joseph
• Published: Abertay University 1999
• Subjects: 628.168; Water pollution & oil pollution
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.297649

The proposed methodology to most effectively manage intermittent combined sewage discharges into urban watercourses in the UK is given in the Urban Pollution Management (UPM) manual. The method is based on the use of detailed computer models of the sewerage system, wastewater treatment plant and receiving watercourse. Solving intermittent discharge problems using UPM, often requires the installation of in-sewer storage tanks. However, recent research from Germany and elsewhere (e.g. Austria and Denmark) has shown that this type of solution may be of little benefit with respect to the total emissions discharged from the entire system, where emissions from both the Combined Sewer Overflows (CSOs) and the Wastewater Treatment Plant (WTP) are considered together. This is because, in certain situations, WTP efficiency can be compromised by the prolonged periods of dilute (low nutrients and substrate) inflows which can result from the draining down of in-sewer storage tanks. The earlier research in Germany and elsewhere has been concerned with long term total emissions (annual loads) and not the problems specific to individual sites, or the benefits and/or limitations of storage with respect to acute pollution. Thus the principal objective of the research described here has been to substantiate and quantify the total emission problem by means of detailed modelling, via an evaluation of the likely storage volumes which could give rise to total emissions problems for the Perth wastewater system. Following this, a general method has been developed to investigate and resolve total emission problems related to acute pollution effects. As WTP disruption due to flow dilution can last for a prolonged period after even a single rainfall event, computational simulation times need to be long enough to represent the delay in WTP performance returning to normal operating conditions. As long term continuous simulation is usually impractical due to protracted computational times, a method referred to as the Total Emission Analysis Period (TEAP) has been developed. This will define the minimum required computational time and rainfall inputs to be used to ensure that the effect of in-sewer storage on total emissions could be modelled. Utilising the TEAP method to analyse total emissions it has been concluded that increasing volumes of storage would not be expected to create a total emission XXVI problem with respect to the Biochemical Oxygen Demand (BOD). Consequently, it was concluded that the best storage volume with respect to BOD was the minimum volume which would allow compliance with receiving water quality standards. No direct comparison could be made with the conclusion derived from the German research due to the long term nature of their analysis, however, it would appear from an interpretation of their results, that similar findings were obtained. With respect to ammonia, it was found that increases in total emissions can occur as, ammonia concentrations, unlike BOD, do not increase at the start of a storm due to first foul flush effects. Consequently, any increased emissions from the WTP would not be offset via a reduced CSO spill load. It was also found, however, that increasing volumes of storage would not be expected to exacerbate acute pollution problems within a receiving watercourse and that both large and small storage volumes had the potential to give rise to very similar degrees of WTP disruption. This was due to the way in which different hydraulic loading conditions (caused by the different volumes of storage) affected the bacterial concentrations in the reactor. The conclusion that storage would not provide a significant benefit for ammonia total emissions was supported by the Austrian and Danish research.