Recovery of phosphorus and volatile fatty acids from sewage sludge

Abstract

Sewage sludge is one of the largest organic waste with a great potential for recovery of renewable energy and nutrients through biological processes such as anaerobic fermentation. In anaerobic fermentation, organic materials are converted into valuable products such as volatile fatty acids (VFAs) and at the same time, nitrogen and phosphorus are released, suggesting a potential of co-recovery of both VFAs and nutrients. However, low sludge solubilization hinders VFAs production and nutrient release. Therefore, this research aims to develop a fermentation based-process by applying sludge pretreatment and/or optimizing operating conditions to improve sludge solubilization and maximize VFAs production and nutrient release.
In this thesis, the effects of sludge pretreatment with thermal, alkaline, and combined alkaline-thermal methods, effects of operating temperature and pH of continuous fermentation, and effects of sludge types and upstream wastewater treatment conditions on sludge solubilization, VFAs production, and nutrient release were investigated. Microbial community analysis and techno-economic assessment were also performed. It was found that VFAs production, and nutrient release could be improved by both application of sludge pretreatments and manipulating fermentation temperature and pH. Alkaline types, pH, and temperature were the important factors influencing VFA production and phosphorus release in fermentation processes. The harsher conditions led to the higher sludge solubilization but it did not always lead to higher yields of VFAs and soluble phosphate. On the other hands, VFAs production relied on microbial activities, which could be inhibited by strong alkaline at pH 10. Phosphorus release was also inhibited under alkaline pH due to precipitation of phosphate, while acidic pH was more favorable. Microbial community could be shaped by altering pH and temperature to facilitate the production of VFAs and release of nutrients. Key hydrolytic/fermentative bacteria were identified. Altering pH and temperature also facilitate selectivity of individual VFA production, which could be beneficial for downstream VFA separation process.
Moreover, it was found that sludge types and upstream wastewater treatment condition, i.e., SRT, had effects on sludge characteristics and hence influenced VFA production and nutrient release. Combined thermal and alkaline treatment only improved VFAs yield in sludge from long SRT process. Phosphate release also varied depending on sludge types and contents of total phosphorus and metals. From techno-economic assessment, product yields and solid reduction should be considered to determine the feasibility of each method. Furthermore, recovery of VFAs into PHAs could provide higher benefits than the use as a carbon source in biological nutrient removal process, meanwhile phosphorus recovery could be restricted by high operation cost.

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Identifiers

ORCID for Tararag Pincam: orcid.org/0009-0009-8025-6068

ORCID for Yongqiang Liu: orcid.org/0000-0001-9688-1786

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