Identification and modelling of dry weather processes in gully pots

This paper presents the results of a monitoring and modelling programme, carried out to study the processes occurring in gully pots during dry weather. The monitoring programme involved estimation of the change in gully pot liquor quality, under field and laboratory conditions. The contents (i.e. liquor and sludge) of pots draining five different types of roads were monitored over dry periods in the winter and summer. A bench scale study was carried out to study the influence of temperature variations and sludge digestion by-products on gully liquor quality. The change in quality was measured in terms of chemical oxygen demand (COD), dissolved oxygen (DO) and ammonium concentration. The trends for change in COD and DO, were found to be broadly similar for all road types. However, ammonium transformation was found to follow different patterns at different locations. Several dry weather processes, such as COD decay, ammonium transformation, oxygen depletion and uptake by sludge, oxygen transfer from the atmosphere and benthic release of COD, have been identified. Other processes, found to take place during dry weather, include development of a scum layer over the surface of the gully liquor and sludge bulking. A model is proposed which can predict the change in gully liquor quality in terms of COD, DO and ammonium concentrations. Data collected during the study has been used to successfully calibrate and verify the model.