Microwave-assisted digestion and NaOH treatment of waste-activated sludge to recover phosphorus by crystallizing struvite

A number of studies of waste-activated sludge (WAS) pretreatments, aimed at releasing phosphorus (P) from WAS and increasing the amount of P that can be recovered, have been performed. Here, a microwave-assisted digestion and NaOH treatment (MWs & NaOH) coupled crystallizing struvite, to promote the solubilization, transformation, and recovery of P from WAS, is proposed. Microwaves (MWs) can cause cavities to form in WAS, weakening the bonds between extracellular polymeric substances and the solid phase. Irradiating with MWs significantly increased the efficiency at which P was dissolved (i.e. transferred from the solid to the liquid phase) and the efficiency at which organic P was hydrolyzed and transformed into inorganic P when the NaOH treatment was performed. The P solubilization and transformation characteristic achieved in different treatments was examined by scanning electron microscopy and three-dimensional excitation emission matrix analysis. The MWs & NaOH method released 34.20–43.73% of total P from WAS, and 23.48–32.07% of the total P was recovered by crystallizing struvite at pH 9.5 and Mg:P ratio of 1.5:1. It would cost about USD 85–103 per ton of dry WAS to treat WAS using the MWs & NaOH method.