Avoiding the problems of septic sewage

Septic sewage develops in a sewerage system when aeration of sewage in gravity sewers is inadequate or when sewage is pumped up a rising-main sewer and sulphides are formed. Many variables affect the rate at which septicity develops. Equations have been produced which describe the relation between the variables and septicity. Such equations can be applied to individual sewers or rising mains. To predict, prevent and control septicity of an entire sewerage system will be complex; preventative measures taken upstream will affect the formation of septicity downstream. This paper describes the development of a computer programme, consisting of a series of linked algorithms. The programme enables the sewerage system to be mathematically modelled in order to predict the formation of sulphides in the sewage at identified locations. It also enables the effects of applying a range of preventative measures to be assessed in order to optimise a strategy for prevention and control. The results are described and discussed for application of the model to an extensive sewerage system, serving a population of 665,000. In some circumstances, prevention of septicity may not be practical or it may be expensive. To remove odours from ventilated air, which includes hydrogen sulphide, a novel, reliable, low-cost and effective catalytic filter has been developed. This filter is described, together with performance data obtained during recent full-scale operation.