Management of Phosphorus from Enhanced Biological Phosphorus Removal Recycle Streams Using Lanthanum Chloride

Abstract

Enhanced biological phosphorus removal (EBPR) in wastewater treatment processes is gaining popularity as a substitute for chemical precipitation because of its cost-effectiveness and decreased production of sludge. Nevertheless, downstream solids processing like sludge digestion, storage, and dewatering can trigger a secondary release of polyphosphate stored within EBPR sludge. This research investigates the use of lanthanum chloride as a coagulant for removing phosphate from liquors produced during the dewatering of EBPR sludge. The hypothesis is that lanthanum can achieve higher orthophosphate removal compared with traditional coagulants like ferric chloride or aluminum sulfate. Results showed that on a molar basis, lanthanum could remove over 99% of orthophosphate from both digested and nondigested EBPR sludge dewatering liquors at metal to phosphate ratios of 1∶1 and 1.1∶1, respectively. Comparable removal using alum and ferric chloride required higher molar ratios, typically 1.6∶1 or more. Furthermore, in liquors with high initial orthophosphate concentrations (>100 mg/L), lanthanum chloride attained over 85% orthophosphate removal even at pH as low as 2. Conversely, as the dose of ferric chloride increased, the removal rates declined because of significant pH depression. Alum was comparatively the least effective coagulant for removing phosphate for the tested conditions. For low initial orthophosphate concentration solutions (15 mg/L), both ferric chloride and lanthanum chloride attained about 90% removal of orthophosphate at a molar dose ratio of 2∶1. Thus, using ferric chloride as a coagulant is practical in solutions with low initial orthophosphate concentrations. However, for solutions with high initial orthophosphate concentrations (such as return streams from EBPR systems), the potential of lanthanum chloride as a coagulant shows considerable promise.