Phosphorus Removal From EBPR Sludge Dewatering Liquors Using Lanthanum Chloride, Aluminum Sulfate and Ferric Chloride

摘要

In wastewater treatment, enhanced biological phosphorus removal (EBPR) is becoming an increasingly popular alternative to chemical precipitation (CP) because of its lower costs and reduced sludge production. However, downstream solids handling processes such as digestion, sludge storage and dewatering promote an undesirable release (i.e. secondary release) of polyphosphate that was stored within EBPR sludge. Released phosphate is recycled to the head of the plant with the liquors of sludge dewatering processes. The concentration of phosphate in recycle streams from EBPR systems can be one to two orders of magnitude higher than the influent phosphorus concentration entering the EBPR system. Plants using EBPR often have to resort to chemical phosphorus removal from the recycle streams to reduce the P loading returning to the EBPR process. The present study addresses the potential of lanthanum chloride as an alternative coagulant for phosphate removal in EBPR sludge dewatering liquors as the use of lanthanum-based coagulants has shown a strong potential for phosphorus precipitation in wastewater applications. The hypothesis is that lanthanum chloride is capable of achieving greater orthophosphate removals than ferric chloride or aluminum sulfate (alum) because lanthanum salts have shown to have a wider effective pH range and require lower doses than typical coagulants. Results indicate that on a molar basis, lanthanum is capable of removing u3e99% of orthophosphate from digested and non-digested EBPR sludge dewatering liquors at metal to phosphate ratios of 1:1 and 1.1:1, respectively. Similar removals using aluminum and ferric iron required molar ratios of 1.6:1 or greater. Additionally, in liquors with a high initial ortho-P concentration (u3e100 mg/L), lanthanum chloride achieved u3e 85% ortho-P removal at pHs as low as 2, whereas removal efficiency decreased with increasing ferric chloride dose due to pH depression. With alum, phosphate removal was essentially non-existent below pH 3. In liquors with low initial ortho-P concentrations (15 mg/L), ferric iron and lanthanum both achieved approximately 90% ortho-P removal at 2:1 molar doses. Thus, lanthanum chloride has shown to be an effective alternative coagulant in liquors with high initial ortho-P concentrations, but ferric chloride is more practical in liquors with low initial ortho-P concentrations because lanthanum and other rare earth salts are less commercially available and more costly than ferric chloride. However, for high phosphate concentrations, such as the ones present in return streams from EBPR systems, the use of lanthanum is promising. As more rare earth metal mines are open in the United States and abroad, fueled by the demand for these metals used in electronics (e.g. cell and smart phones), rare earth chlorides, which are a by-product of these mines, will become more commercially available at lower prices.