Biological phosphorus removal from wastewater with a moving bed biofilm reactor process

The human population is increasing and more wastewater, rich in nutrients, is produced. Increased levels of nutrients such as phosphorus, nitrogen and organic matter in water bodies present a risk for eutrophication, toxicity to the water-living fauna and oxygen depletion. Therefore, it is important to operate the wastewater treatment plants efficiently to limit the release of nutrients to the receiving waters and decrease the energy demand and environmental impact. Today, biological phosphorus removal (bio-P) from wastewater is achieved with activated-sludge processes. Bio-P processes with biofilm, such as moving bed biofilm reactors (MBBR) or fixed film reactors, are not yet in use in full-scale but research is ongoing. It is of interest to develop an MBBR phosphorus removal process since the process can have advantages over existing processes. MBBR processes often take less space than activated sludge processes, and less chemicals are needed compared to chemical precipitation. However, the biological phosphorus removal process requires altering of anaerobic and aerobic conditions causing challenges in the process design. In this study, a semi-continuous moving bed biofilm reactor process for denitrifying phosphorus removal from wastewater was designed and operated. The function of the process along with the capacity of the process was evaluated in terms of aerobic phosphorus uptake rates, anaerobic phosphorus release rates, and anaerobic soluble carbon uptake rates. No denitrifying phosphorus removal was achieved in the intended experimental setup with the existing conditions. However, after several changes in the experimental setup and in the feed composition the desired bio-P activity was achieved. The changes that supported the development of desired bio-P activity were the following: 1) The soluble carbon was prevented from being present in the aerobic and deoxidising reactors. 2) The temperature in the reactors was decreased from 15°C to 10°C. 3) More counter ions in form of magnesium and calcium were added to the main influent. Due to the experimental setup used when the achieved bio-P activity was obtained, no total nutrient removal could be measured. The highest achieved aerobic phosphate uptake rate was 0.13 g/(m2∙d) and it was measured after 76 days. The simultaneous anaerobic phosphate release rate was 0.2 g/(m2∙d) while the highest anaerobic phosphate release rate was 0.35 g/(m2∙d) which was achieved after 91 days. After 76 days, the anaerobic soluble carbon uptake rate was 6.4 g/(m2∙d). The microscopic study showed that some of the biomass contained polyphosphate granules at this time. The achieved rates, except the soluble carbon uptake rate, were 6-11 times lower compared to the literature. However, the rates were still increasing at the last measurement and the process had not reached its full potential before the study was terminated. The intended process for denitrifying phosphorus removal was not tested after the temperature decrease and addition of counter ions. Therefore, more studies have to be conducted on the intended denitrifying phosphorus removal process to enable evaluation of the potential of the process, and to determine if the treatment requirements can be met.