Microbial electrolysis cell accelerates phosphate remobilisation from iron phosphate contained in sewage sludge

Abstract

Phosphate was remobilised from iron phosphate contained in digested sewage sludge using a bio-electric cell. A significant acceleration above former results was caused by strongly basic catholytes. For these experiments a dual chambered microbial electrolysis cell with a small cathode (40 mL) and an 80 times larger anode (2.5 L) was equipped with a platinum sputtered reticulated vitreous carbon cathode. Various applied voltages (0.2-6.0 V) generated moderate to strongly basic catholytes using artificial waste water with pH close to neutral. Phosphate from iron phosphate contained in digested sewage sludge was remobilised most effectively at pH ∼13 with up to 95% yield. Beside minor electrochemical reduction, hydroxyl substitution was the dominating remobilisation mechanism. Particle-fluid kinetics using the "shrinking core" model allowed us to determine the reaction controlling step. Reaction rates changed with temperature (15-40 °C) and an activation energy of Ea = 55 kJ mol(-1) was found. These analyses indicated chemical and physical reaction control, which is of interest for future scale-up work. Phosphate remobilisation rates increased significantly, yields doubled and recovered PO4(3-) concentrations increased four times using a task specific bio-electric system. The result is a sustainable process for decentralized phosphate mining and a green chemical base generator useful also for many other sustainable processing needs.

Mechanisms and Model Process Parameters in Bioelectrochemical Wet Phosphate Recovery from Iron Phosphate Sewage Sludge

“…The observed reaction rate of the remobilization was correlated to the level of an applied voltage to overcome the manifold internal resistances in the BES. The objective was to understand how far the generated chemical base in the cathode caused the observed high phosphate remobilization yields (Figure ). The needed high pH catholyte was generated in a model experiment without sewage sludge using a Shewanella oneidensis anolyte.…”

“…Therefore, the controlled examination outside of the MEC in a well-defined experimental setup was possible to monitor the remobilization process (Figure ). Further explanations for the importance of a high pH were the solubility products of the principal FePs and Fick’s first law (eq ). The excess of OH – anions led to a high concentration gradient between the reaction surface of the sludge particle and the bulk solution (∂ c i ( x )/∂ x ), involving a greater diffusive flux of OH – species ( J d , i ( x )).…”

“…This clear temperature dependence (Figure 6d) enabled to determine an accurate activation energy E a by the Arrhenius' law (eq 13), and an E a = 55 kJ/mol was calculated. 36 Although there was a clear temperature effect, it became only well detected at a pH value below 12.4. Above this pH the temperature effect was less or not visible as high chemical base concentration became more influential on the reaction rates than the temperature.…”

“…These two methods provide with CO 2 < 50% of phosphate 34 on average and for OH − excellent yields 83%, 1 92% 35 and 95%. 36 However, there are conflicting data for OH − use that show lower and variable yields of 13%−56% and this at high pH with different sewage slugdes. 37 The chemical base propelled wet phosphate extraction requires therefore further investigations to understand the kinetics of the process to exploit it as a method that by assumption should outperform currently favored extraction from ashes, what is supported by so far available data.…”

“…Additionally, carbon dioxide injection under high pressure or the use of caustic soda (OH – ) can leach phosphate from wet digested sewage sludge containing FePs. These two methods provide with CO 2 < 50% of phosphate on average and for OH – excellent yields 83%, 92% and 95% . However, there are conflicting data for OH – use that show lower and variable yields of 13%–56% and this at high pH with different sewage slugdes .…”