Effect of organic matter on phosphorus recovery from sewage sludge subjected to microwave hybrid pretreatment

Microwave (MW) hybrid processes are able to disrupt the flocculent structure of complex waste activated sludge, and help promote the recovery of phosphorus as struvite. In this study, to optimize struvite yield, (1) the characteristics of matter released in MW-hybrid treatments were compared, including MW, MW-acid, MW-alkali, MW-H₂O₂, and MW-H₂O₂-alkali. The results showed that selective release of carbon, nitrogen, phosphorus, Ca^2 +, and Mg^2 + achieved by sludge pretreatment using MW-hybrid processes. MW-H₂O₂ is the recommended sludge pretreatment process for phosphorus recovery in the form of struvite. The ratio of Mg^2 +:NH₄⁺-N:PO₄^3 −-P was 1.2:2.9:1 in the supernatant. (2) To clarify the effects of organic matter on struvite recovery, the composition and molecular weight distribution of organic matters were analyzed. Low molecular weight COD was found to facilitate the removal rate of NH₄⁺-N and PO₄³-P via crystallization, and the amorphous struvite crystals (< 1 kDa) from the filtered solutions had high purity. Therefore, the present study reveals the necessity of taking into consideration the interference effect of high molecular weight organic matters during struvite crystallization from sewage sludge.