Impact of Orthophosphate on Achiveable Dry Cake Solids Content and Polymer Consumption in the Dewatering Process – A Summary From Us-Phosphory Recovery Tests and Two Full Sclae Installations in Europe

Phosphates have been used for decades in the food industry to increase the water holding capacity of proteins. While the increase in water holding capacity is desired for the food industry, it certainly is not for Wastewater Treatment Plants (WWTPs). At WWTPs with Biological phosphorus removal (Bio-P) processes, high orthophosphate concentrations occur due to the stored polyphosphates, which get released from bacterial cells during anaerobic digestion. The presence of high orthophosphate levels leads to negative consequences in plant operation and maintenance. But also the presence of high orthophosphate makes sludge more difficult to dewater. Anaerobic digestion plants switching from chemical P-removal to Bio-P can expect a decrease of dry cake solids by 3 to 5 percentage points and an increase of polymer consumption by up to 30 %. Data gathered from tests and full scale installations in Europe suggests that this negative impact can be mostly attributed to the orthophosphates being released in an anaerobic environment such as anaerobic digesters and to a smaller degree the imbalance of monovalent vs. divalent cations. In this article, operational data from utilities that have been using Bio-P, lab data from tests and US-pilot scale test results will be presented.