Phosphate Recovery from Sewage Sludge by Density Concentration

Abstract

Phosphate is removed from wastewater to prevent eutrophication. Chemical phosphorus removal by iron dosing is commonly used for this purpose, where phosphorus is among others bound in the mineral vivianite (Fe(II)3[PO4]2 · 8 H2O). A molar Fe:P ratio of 2.5 in sludge will capture 70-90% of total phosphorus in vivianite after anaerobic digestion, a digestion method which offers ideal conditions for vivianite formation. Vivianite is paramagnetic, enabling it to be recovered by magnetic separation for recycling purposes. Phosphorus recycling from wastewater is of great importance, as in worst-case scenarios, 40-60% of all phosphate rock resources will be extracted by 2100. It is furthermore estimated that in Europe 20-30% of the fertilizer demand can be met by using phosphorus that ends up in sludge in European sewage treatment plants. To reduce processing costs of magnetic vivianite recovery, a pre-concentration step based on density concentration can be introduced, as vivianite particles are denser than organic material in sludge. This research investigated the feasibility of vivianite concentration from digested sewage sludge with a Multotec MX7 spiral concentrator. Spirals are applied in the mining industry for coal washing and pre-concentration of ores which are suspended in water. Particle separation on spirals is based on centrifugal and gravitational forces. Digested sewage sludge is more viscous than water, which affects the separation process on spiral concentrators. To investigate the influence of this high sludge viscosity on the spiral separation efficiency, laboratory experiments were performed. Diluting the sludge furthermore improved the concentration of solids towards
the product flow. However, spiral efficiencies were only 9%, while efficiencies over 80% were found for synthetic sludge, meaning that particle separation was limited. The iron concentration was highest in the product outlet and increased with dilution. The phosphorus content, however, was similar for all outlets and dilutions, suggesting that only Fe minerals other than vivianite, such as siderite and pyrite, concentrated towards the product flow. Additionally, a concentrated bed of sand grains was observed near the column after a dilution of 20%. While diluting the sludge improved the concentration of sand and heavy Fe minerals, it did not induce vivianite concentration.