Abstract
Phosphorus, a crucial component of life, may cause eutrophication if it is discharged untreated into the aquatic ecosystem. Phosphate (PO43-) may exist at an elevated level in anaerobic digestion (AD) effluents and can lead to the clogging of pipes by forming struvite crystals. This study was conducted to assess the responses of coagulant type, dosage and process conditions to phosphate removal efficiency from anaerobic sludge. The experiments were performed in two steps. First, a sensitivity test was conducted to compare five coagulant types (alum, poly-aluminum chloride (PAC), FeCl2, FeCl3 and PAC + FeCl3) at standard coagulation conditions. The results showed that PAC would be the best coagulant among the tested, while a combination of PAC and FeCl3 may be beneficial under circumstances. Second, an optimization study was performed for PAC using response surface methodology employing central composite design. Among the three independent variables (coagulant dosage, slow mixing duration and agitation speed), the dosage was the sole significant variable for phosphate removal efficiency, while the other two had limited effects. A future study to optimize the rapid mixing conditions would give additional insights into the process. The results of this study may be useful to design a process to counteract phosphate discharges from AD plants, as well as to reduce the risks of pipe clogging and maintenance problems due to crystalline struvite formation in the later stage of AD.
Highlights
Received: 10 January 2022As a component of nucleic acids and many other biomolecules, phosphorus (P) is an essential element for living organisms [1]
Phosphorus is regarded as one of the macro-nutrients that are required for the growth of microorganisms in biological waste and wastewater treatment processes, such as anaerobic digestion (AD) [2]
The phosphate removal efficiency elevated as the coagulant dosage increased
Summary
As a component of nucleic acids and many other biomolecules, phosphorus (P) is an essential element for living organisms [1]. Phosphorus is regarded as one of the macro-nutrients that are required for the growth of microorganisms in biological waste and wastewater treatment processes, such as anaerobic digestion (AD) [2]. Removing one of the precursors (i.e., ammonium, magnesium and phosphate) from anaerobic sludge can limit struvite precipitation in the plants [8]. Coagulation of phosphate in the anaerobic sludge can be accomplished by establishing a coagulation process after the digestion stage. The dosage of coagulant, the agitation strength and the reaction time should affect the phosphate removal efficiency. Selection of optimal coagulant and process conditions can lead to an efficient phosphate removal from anaerobic effluents and control of the formation of struvite [11]. This study was designed to collect information to establish the coagulation process for phosphate removal from AD sludge. Response surface methodology (RSM) using a central composite design (CCD) was employed for the optimization [12]
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