Obtaining three cleaner products under an integrated municipal sludge resources scheme: Struvite, short-chain fatty acids and biological activated carbon

Phosphorus recovery and carbon reuse from municipal sludge have been regarded as an effective method of recycling wastes. In this study, an innovative proportional acidification method was developed to release phosphate from municipal sludge and acquire struvite, short-chain fatty acids (SCFAs), and acidulated biological activated carbon (ABAC) as three cleaner products. Acidulated treatment could adequately release phosphate from municipal sludge. Over 95% of total phosphorus was extracted at pH 2.0, and 98.9% of soluble phosphate was recovered by activated brucite ultra-fine powder (ABP) as struvite crystal within 30 min at pH 7.0-7.5. The optimal volumetric ratio of acidulated sludge mixture and unacidified supernatant was 4:6 after fermentation process, while 45.4% of strong acid was saved compared to sludge complete acidification. The field emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD) revealed that struvite crystallization was a process of magnesium ions dissolved and reconstruction. 381.1 mg of phosphate and 159.3 mg of ammonium could crystallize with adding 1 g ABP. Acetate and propionate occupied 74.2%-82.3% of total SCFAs at neutral fermentation condition, and one kilogram dry sludge could recover 0.079 kg acetate and 0.023 kg propionate. Moreover, the FE-SEM micrographs and Brunauer-Emmett-Teller (BET) analysis indicated that ABAC had better physical properties than conventional biological activated carbon (BAC). The recovery process of all three products avoided creating new pollutants, and scheme economic assessment showed roughly balance of chemical cost and products return. This integrated scheme can promote sustainable recovery nutrients from municipal sludge in the form of large-scale applications.