The effect of environmental conditions on faecal coliform decay in post-treatment of UASB reactor effluent

The removal of faecal coliforms (FC) in waste stabilization ponds is partly caused by natural decay processes. This work distinguishes between light m ediated and light independent processes, since only the upper layer of a st abilization pond receives solar radiation. Light attenuation by algae matte r or other particles causes darkness in the rest of the pond. The purpose o f this work was to investigate the decay processes in stabilization ponds i n order to yield improved design of pond systems. Experiments were carried out with buffered effluents from an Upflow Anaerob ic Sludge Blanket (UASB) reactor, treating domestic wastewater. The FC deca y rate was determined and compared for light and dark conditions, in aerate d bottles and beakers, respectively. The effective environmental factors we re also investigated and mathematical expressions were derived for their ef f ect on the FC decay rate. Special attention was given to the effect of li ght attenuation by algae matter. It was found that light independent FC decay in aerated UASB effluent is ca used by a shortage in carbon sources, since glucose addition prevented deca y for over ten days. The nutrient content of UASB effluent was satisfactory for a long-term survival. The FC decay in the dark parts of stabilization ponds is therefore under conditions of carbon and nutrient sufficiency expe cted to be negligible. Under conditions of carbon shortage, the light indep endent FC decay was found to be temperature dependent, but not pH dependent tin the range 7.2-9.1). The FC decay in beakers exposed to solar radiation was much faster then und er dark conditions. The light mediated decay was affected by the pH, DO con centration and the solar radiation intensity, but not by the temperature. T he addition of autoclaved algae matter strongly reduced the FC decay in the beakers, due to light attenuation. It is therefore expected that the light attenuation by algae matter in stabilization ponds also reduces the FC dec ay. This could partly offset the stimulating effects of algae photosynthesi s (i.e. increased pH and DO concentrations) on FC decay. It seems therefore that there is an optimum algae concentration for maxim um FC decay. This c an lead to a new design approach, based on regulation of algae growth in st abilization pond systems.