Use of sanitary sewers as wastewater pre-treatment systems

As wastewater travels through a sewer system it undergoes changes in compos ition. These changes in composition may be caused by chemical, physical and /or biological processes. At present engineers do not take into considerati on the impacts of these processes on the wastewater quality when designing wastewater treatment systems. However, the impact of these processes on the chemical oxygen demand, biochemical oxygen demand, nitrogen and phosphorus content of the wastewater can be significant. In the case of the biologica l processes, microorganisms present in the wastewater as it travels through the sewer system are similar to those found in an activated sludge process . Given that the micro-organism population and the hydraulic retention time often resemble that of an activated sludge process, it would seem only rea sonable to look further into the possibility of using sewers as wastewater treatment systems. Furthermore, the plug flow regime of a sanitary sewer is inherently beneficial in terms of wastewater treatment as it is not subjec t to short-circuiting. The first part of this paper provides a technical re view of the processes which take place in a sewer system and the resulting degradation of some of the more significant substances found in wastewater. The contribution of both the suspended biomass and the attached biomass to the degradation of substrate is also examined. The second part of this pap er examines the use of the Toxchem computer model to predict the processes which are taking place in the sewer under a variety of conditions. The goal being to determine the magnitude of the degradation of substrate and disso lved oxygen depletion in a sewer system. In obtaining a better understandin g of the processes that are taking place in sewer systems, engineers will b e able to more accurately predict the degradation of substrates in sanitary sewer systems. This will result in a reduction in the size of wastewater t reatment facilities (WWTFs). (C) 1998 Elsevier Science Ltd. All rights rese rved.