Comparison on anaerobic phosphorus release and recovery from waste activated sludge by different chemical pretreatment methods: Focus on struvite quality and benefit analysis

Abstract

Phosphorus recovery from waste activated sludge (WAS) is expected to alleviate the shortage of phosphate rock and reduce eutrophication. In this study, acid, alkali and sodium polyacrylate (PAAS) were compared to enhance phosphorus release and recovery from WAS. During anaerobic fermentation (AF) stage, the optimal pretreated conditions for ortho-phosphate release were the pH of 4 (AF 12 h), 13 (AF 12 h) and 22.4 g PAAS/L (AF 24 h) with the phosphorus release efficiencies of 40.9%, 62.6% and 31.7%, respectively. Acid, alkali and PAAS addition were beneficial for apatite phosphorus (AP), non-apatite inorganic phosphorus (NAIP) and organic phosphorus (OP) release from WAS, respectively. Strong acidic (pH = 4) and alkaline (pH = 12 and 13) conditions inhibited the release of soluble ammonia, while PAAS would not have a negative impact on the release of soluble ammonia. By means of precipitation crystallization, the ortho-phosphate could be almost recovered after acid/alkali pretreatment compared with PAAS (88.9%) at optimal Mg/P molar ratio of 1.5:1. The XRD, FT-IR and SEM-EDX analyses confirmed the main component in the product was struvite. The purity of the struvite in the product recovered from acid (named PreAC, 78.9%) and alkali (named PreAL, 89.6%) pretreated sludge were higher than that of the PAAS (named PrePA, 72.3%) by elemental analysis. The mercury and chromium content existed in PreAC were above the Control Standards of Pollutants in Sludge for Agricultural Use, whereas detected heavy metal elements level of the PreAL and PrePA were below the standard. By means of cost analysis, acid/alkali pretreatment could obtain economic benefits compared with PAAS. Thus, those discoveries would broaden the phosphorus recovery way to serve in practice.