Inorganic profiles of chemical phosphorus removal sludge

Iron dosing is commonly used to remove phosphorus from wastewater but little is known about how this changes the distribution of iron, phosphorus, calcium, magnesium and trace metals in activated and digested sludge. This research compared the inorganic profiles of sludge from full-scale processes (activated sludge and anaerobic digestion) with and without iron dosing, with the aim of identifying changes in inorganic distribution resulting from iron dosing. Sludge phosphorus and metals were fractionated using sequential chemical extraction. Bioavailable iron was lower in iron-dosed activated sludge, as was bioavailable phosphorus (6·5 g/kg compared with 1·8 g/kg), with most of the iron and phosphorus bound as iron-hydroxy-phosphates. Similar results were found for anaerobically digested sludge after iron dosing; iron and phosphorus in the sludge increased by 4 and 1·35 times, respectively, but bioavailability was decreased. The ratio of chemical oxygen demand to bioavailable phosphorus in the digester was 840 : 1 after iron dosing. By contrast, calcium, magnesium, copper and zinc were increasingly bioavailable in the digester after iron dosing. The reported changes were linked to the iron content of the sludge; hence the level of iron dosing is key to minimising changes in sludge inorganic profiles.