Phosphate release of sludges from enhanced biological p-removal during digestion

Abstract

During the start-up phase of an enhanced biological phosphorus removal (EBPR) plant, the amount of eliminated phosphorus during wastewater treatment and the subsequent release during anaerobic sludge digestion was investigated. Different approaches were used to determine the mechanisms of enhanced phosphorus removal. From a comparison of the EBPR plant with a control, a strong correlation between the potassium, the magnesium and the phosphorus content of the sludge and the results gained from phosphorus fractionations we conclude that the major part of the eliminated phosphorus was stored in form of polyphosphate. During digestion of excess and a mixture of excess and primary sludge a complete release of the stored polyphosphate was found. The release of phosphorus was accompanied by a release of potassium and magnesium ions, from which only potassium remains in soluble form. Therefore, the soluble potassium concentration seems to be a good measure for the amount of phosphate released. Only a part of the released phosphate remains in soluble form. When digesting excess and mixed sludge this accounts for approximately 40% of the total phosphorus brought into the digester. The difference between the measured soluble phosphate concentration and the amount of released phosphorus was fixed, mainly due to chemical precipitation. It was found that a fixation in the form of magnesium ammonium phosphate (struvite) was likely to occur under the conditions of anaerobic sludge digestion. The amount of phosphate precipitation as struvite could be estimated using theoretical calculations at approximately 20% of the total phosphorus in the digester. Calcium dosing experiments show that calcium-phosphate precipitation plays only a minor role in phosphate fixation.

Precipitation assessment in wastewater treatment plants operated for biological nutrient removal: A case study in Murcia, Spain

“…During Poly-P hydrolysis, the release of phosphorus is followed by a release of potassium and magnesium at the same ratio as observed during the phosphorus uptake (Jardin and Pöpel, 1994). Values of 0.354 gK/gP and 0.283 gMg/gP determined in previous studies (Barat et al, 2005) were used.…”

“…The rest of the phosphorus fixed in the digester was considered to be adsorbed on the surface of solids (P ads ). Although phosphate adsorption during anaerobic digester is still not a well-understood process, it is considered as a possible mechanism for phosphorus fixation (Jardin and Pöpel, 1994;Wild et al, 1997).…”

Two strategies for phosphorus removal from reject water of municipal wastewater treatment plant using alum sludge

“…In MWWTP employing biological phosphorus removal (BPR) process, the removal of P consists of its conversion from the influent into the activated sludge followed by the removal of the P-rich excess activated sludge from the main treatment stream. However, during the ensuing sludge treatment stage, generally including thickening to reduce its volume and then anaerobic digestion to stabilize the sludge, it is believed that the P is released to the supernatant (Jardin and Popel, 1994). This waste stream is termed as reject water (other terms are return liquor, sewage sludge filtrate or centrifugate) and it is normally recycled back to the main wastewater stream due to its poor quality which makes it unacceptable for direct discharge.…”

Secondary clarifier conditions conducting to secondary phosphorus release in a BNR plant

“…Phosphorus-rich excess sludge can release phosphate back to the liquid phase during sludge handling, producing liquors with relatively high phosphate content (Pitman et al 1991). The main concern has been to minimize the phosphorus release during anaerobic digestion, thickening, dewatering and sludge storage (Pitman et al 1991;Jardin & Pö pel 1994;Mota-Filho & Batista 2003).…”