Phosphorus removal and recovery from sludge centrate by membrane and steel-making slag
- Publication Type:
- Thesis
- Issue Date:
- 2022
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Phosphorus is an essential element for all lives on Earth. It is also a finite resource, derived primarily from phosphorus rock. Given the imminent depletion of minable phosphate rock, phosphorus recovery from nutrient-rich streams is essential for future generations. Sludge centrate as a by-product from anaerobic digestion of sewage sludge is a phosphorus-rich stream that can be considered as an alternative to renewable phosphorus supply. This study initiated a systematic approach to develop an innovative integrated framework using steel-making slag and membrane-based processes to maximise phosphorus removal and recovery from sludge centrate. The proposed complete framework involved pre-treatment process (i.e. biogas sparging and sand filtration), an enrichment process by forward osmosis followed by a recovery process using steel-making slag and post-treatment of the recovery process effluent by steel-making slag and a membrane photo bioreactor. The obtained results demonstrated the proof-of-concept of biogas sparging to control membrane fouling and enhance nutrient enrichment during sludge centrate preconcentration by forward osmosis. Biogas sparging also resulted in a significant improvement in the enrichment of phosphate ions to close to the theoretical value based on mass balance calculation. In other words, phosphate ions were retained in the concentrated sludge centrate for subsequent recovery. Results in this study highlighted for the first time the potential of nutrient recovery from sludge centrate using calcium and other alkali metals from steel-making slag. Up to 96% phosphate and 71% ammonia could be recovered from sludge centrate at the optimal conditions. The results also showed that pre-treatment by sand filtration and forward osmosis enrichment was essential to achieve high nutrient recovery. Sand filtration pre-treatment decreased the total suspended solid of sludge centrate by eightfold, leading to mitigated membrane fouling and reduced nutrient loss during forward osmosis preconcentration. In addition, the study demonstrated the feasibility of using steel-making slag to polish the aqueous solution followed by the application of steel-making slag in quenching residual P from the recovery process effluent. At the optimal conditions (i.e. pH 8.5 and steel-making slag dosage of 5 g/L), approximately 98% phosphorus removal could be achieved with the output level of less than 0.1 mg/L. Furthermore, the study successfully demonstrated the feasibility of using a novel sequencing batch membrane photobioreactor for simultaneous nutrient removal and algal biomass production from sludge centrate. In comparison to the batch mode reactor, the membrane photobioreactor allowed for continuous cultivation of microalgae with 40% higher biomass content.
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