Improving Dewaterability of Digested Sludge from EBPR Facilities

Enhanced biological phosphorus removal (EBPR) is generally viewed as the sustainable compliance approach for phosphorus removal and it offers and economical way for phosphorus recovery. Recently the negative impact of EBPR on the dewaterability of digested sludge has garnered the attention of several large utilities. These impacts include a substantial loss of dewaterability, as much as 4 to 6 percentage points and require significantly higher polymer doses. This impacts the economics of EBPR as well as phosphorus recovery.Review of existing literature and data from several reference facilities yielded different theories as to the specific cause and effect relationship between EBPR and dewaterability. One links the poor dewaterability to the increase in mono to divalent ion ration K+/Mg2+, which is a direct result of EBPR. Another theory points to the water binding properties of phosphate in the presence of extracellular polymeric substrates (EPS).Possible solutions may include adjusting the mono to divalent ion ratio by chemical addition or elutriation, cell lysis, metal salt addition, aeration, pH adjustment, or any combination thereof. The challenge in testing solutions is the inherent scale of typical digestion and dewatering facilities, which makes pilot testing costly while common bench tests methods are not well suited.To research this subject and develop solutions or optimize systems, a bench scale dewatering press was developed, which produces cake between 10% and 25%. It functions in principal similar to a belt or plate filter press. This press was used to evaluate the impact of various chemicals on sludges from different EBPR plants.