Parameters Affecting Microorganisms and the Process Performance in Biological Wastewater Treatment

Abstract: The aim of the thesis was to investigate how different abiotic and biotic parameters may affect microorganisms and the process performance in various biological wastewater treatment systems. The type of carbon source in the influent wastewater may have a profound effect on the composition and the activity of denitrifying activated sludge bacteria in nutrient removal. Of four investigated carbon sources (acetate, methanol, crude syrup, hydrolyzed starch) in long-term continuous cultivations, acetate and methanol were found to be more advantageous as carbon sources for denitrification in wastewater treatment than the carbohydrate products. Not only did they induce a higher denitrification rate and yield and a lower sludge production, they also enriched for a higher amount of true respiratory denitrifying bacteria. The length of the sludge age, general process conditions and seasonal variations have a significant impact on the microbial composition and treatment performance. In a long-term investigation on two activated sludge plants, designed for enhanced biological phosphorus removal, but at different sludge loadings (low and high, with and without nitrogen removal, respectively) it was shown that different types of separation problems were encountered due to different sludge characteristics. The worst sludge characteristics were observed in the high loaded system, especially during the winter period. Another significant difference between the two systems was that the sludge production was different, being lower in the low loaded system. Apart from abiotic process parameters, biological interactions within the microbial community may also have a great effect on the bacterial degradation activity. This was exemplified by studying the effect of grazing of microscopic predators (protozoa and metazoa) on nitrifying bacteria in an aerobic biofilm model system. It was demonstrated in a long-term experiment, by means of addition of specific inhibitors to the predators, that they may have a negative effect on nitrifying bacteria. Based on a development of a mathematical model of the experimental results, the negative effects were further postulated to be due to not only grazing, but also due to the oxygen consumption of the predators in the biofilm. Based on the knowledge on how different parameters affect the microbial composition and the process performance, a new process was tested in order to decrease the sludge production in biological wastewater treatment. The process was based on optimizing the roles of bacteria and predators in separate stages. In the first stage, bacteria were stimulated for decomposal of the waste, whereas in the second stage predators were stimulated in order to feed on the bacteria produced in the first stage. This process was tested on laboratory scale with both synthetic as well as authentic, industrial wastewaters from the pulp and paper industry. The results turned out to be promising, indicating that the sludge production may be decreased by as much as 40-95%, depending on the type of wastewater and design of the process.