Valorization of sludge materials after chemical and electrochemical treatment

Abstract

Wastewater sludge requires effective dewatering, lower metal content, retention of nutrients, and biological stability prior to use for other applications. This thesis presents a comprehensive sludge treatment study by chemical and electrochemical processes to develop alternative scope for sludge usage. Firstly, Fenton-ultrasonication was applied to treat anaerobically digested sludge (ADS) by optimizing three process variables (Fe2+, H2O2, and sonication time). 36 mM of Fe2+, 320 mM H2O2 with 30 min of sonication showed the best performance for total organic carbon (TOC), extracellular polymeric substances (EPS), and heavy metals. Secondly, pressure-driven electro-dewatering (EDW) was assessed by a lab-scale device 5, 15, and 25V. After EDW, dry solid content increased up to 56%, and EDW did not show remarkable effects on heavy metals and E.coli quantification. Thirdly, the effect of inorganic coagulant PTS combined with organic Ce-CTA was investigated on EPS, heavy metals, and nutrient contents in ADS at different levels of pH. pH 3 and 9 showed promising results after coagulation-flocculation and suggested the production of various sludge-based materials (adsorbents, catalysts, building materials, etc.). Finally, sludge beads were prepared for recovery of rare earth elements (REEs) from aqueous effluents. Among the REEs studied, Fenton and hydrochloric acid-treated sludge beads demonstrated a higher affinity towards Sc3+ and Sm3+ ions, displaying the maximum adsorption capacities of 2.80 to2.83 mg/g and 4.03 to 4.16 mg/g. To perform an economic evaluation of Fenton and electro-dewatering systems, chemical costs and required energy were calculated and the electrochemical system found them to be more appropriate for sludge treatment than Fenton.