Electric resistivity during electrodialytic recovery of phosphorous from sewage sludge ash

This paper focuses on changes in potential drops over different parts of a two-compartment electrodialtyic cell during a process developed for P recovery from sewage sludge ash and simultaneous removal of heavy metals. The cell has two compartments separated by a cation exchange membrane. The ash suspension is in the anode compartment. The major aim of this work is to obtain knowledge needed to optimize the energy consumption for the process. Initially, a low conductivity of the suspension caused a relatively high resistance in the suspension itself. The adjacent catholyte contrast with a high conductivity and a low pH. A high membrane resistivity was seen because of the concentration and pH gradients. During the treatment, the gradients leveled out and, the overall voltage reached a low level. However, at this point, where the suspension is acidic, protons are major current carriers, which is a waste of energy to the overall recovery process, since current should be spent to transport metal cations across the membrane. Thus, the investigation points at two major issues for future optimization; the gradients over the cation exchange membrane (pH, conductivity, potential) and improved utilization of the produced protons to be for P extraction rather than carrying current.