Limiting Factors in Biological Nutrient Removal from Wastewater

Abstract: Some of the main factors affecting the biological removal of nutrients in wastewater treatment plants have been investigated. The influence of low concentrations of dissolved oxygen and the redox potential on the denitrification, activity of activated sludge was studied in batchwise experiments. Oxygen was found to have a negative effect on denitrification even when present at concentrations lower than could be measured with conventional oxygen probes. A linear relationship was found between the denitrification activity and the redox potential. Furthermore, denitrification with ethanol and methanol as carbon sources was studied in two parallel chemostats, as well as in batch cultivations of pure denitrifying strains. The growth rate with ethanol as carbon source was found to be 2-3 times higher than with methanol, and considerably more stable denitrification was obtained with ethanol. A simple method to determine the volatile fatty acid (VFA) potential, a measure of the readily fermentable organic fraction in domestic wastewater, was developed. This fraction is considered to be of crucial importance for the performance of enhanced biological phosphorus removal (EBPR). The VFA-potential was then measured in different municipal wastewaters, showing typical values for municipal wastewater to be between 50 and 100 mg COD/l. However, large deviations from this range were found to occur in wastewater with a large proportion of industrial effluent. The possibility of increasing the VFA-potential and improving EBPR through enhanced hydrolysis and fermentation of primary sludge in the primary clarifiers was studied in a full-scale treatment plant. An increase in the VFA-potential of 5-10 mg COD/l was observed in the clarifiers operated with enhanced hydrolysis and fermentation, and laboratory-scale fermentation of primary sludge showed that another 15-20 mg/l of VFA-COD could be extracted if the hydrolysis and fermentation processes were to be prolonged and optimized. VFA-potential determinations, in addition to measurements of poly-b-hydroxyalcanoic acids (PHA) and phosphorus, were used in studies of carbon and phosphorus transformations in the same full-scale treatment plant as mentioned above. The results indicated that approximately 20 mg VFA-COD were required to remove 1 mg of phosphorus. In the anaerobic stage of the treatment plant, 1.5 mg of PHA-COD were produced per mg of VFA-COD taken up, and a release of 0.31 mg of phosphate-P per mg of PHA-COD formed was observed.