Phosphorus Re-dissolution and Recycling from Activated Sludge in Full-Scale Wastewater Treatment Plants

Abstract

Wastewater treatment is essential to protect the environment and human health. While traditionally the focus has been on pollutant and nutrient removal, wastewater treatment plants (WWTPs) are now evolving into facilities for the additional recovery of resources. A key target for recovery is phosphorus (P), an essential element for agriculture and critical to ensure global food security. Given the finite and geopolitically vulnerable nature of global phosphate rock reserves as well as the regulatory pressure for a circular resource use, the development of methods for a sustainable nutrient management and P recovery is becoming increasingly important. Adapting the enhanced biological P removal (EBPR) process for a biological P recovery could be a promising alternative to cost- and resource intensive chemical methods. By utilizing the metabolic capability of polyphosphate accumulating organisms (PAOs) enriched in the microbial community of activated sludge (AS), P can be released from their intracellular polyP pool under anaerobic conditions. So far, most studies are based on laboratory enriched PAO cultures, limiting the real-world applicability in full-scale systems. This thesis aimed at exploring a targeted P re-dissolution approach for non-acclimated AS of full-scale WWTPs by leveraging the P cycling ability of PAOs. Key objectives included process optimization for a rapid P re-dissolution without sludge disintegration (cell lysis), identifying an efficient carbon source, and evaluating the recovery process at pilot-scale in a full-scale WWTP. Results showed that P is effectively released from non-acclimated AS within a short period of 1–4 h by adding acetate. Laboratory batch experiments revealed that AS from pure full-scale EBPR systems exhibits the highest P re-dissolution efficiency, with up to 56% of total sludge P being released. WWTPs combining EBPR and chemical P removal (CPR) show reduced P yields (19–24%), likely due to precipitant use limiting PAO activity and polyP availability. Systematic investigation of volatile fatty acid (VFA) supplementation revealed acetate at a 200 mg/L dose as the most efficient substrate for a high, fast and consistent P re-dissolution from mixed EBPR/CPR sludge. The molar P yield/C consumed ratio was 0.45. Other substrates, such as formate, propionate and butyrate were less effective. After full acetate consumption, ongoing P release was observed, which gave new insights into metabolic limitations within PAOs possibly due to polyP/glycogen depletion and disruption of the membrane potential. At pilot-scale, acetate-induced P re-dissolution in a mixed EBPR/CPR WWTP produced a P-rich stream suitable for fluidized bed precipitation using milk of lime. A precipitation efficiency of 99% was achieved. A P-enriched dolomite pellet with slow-release fertilizer characteristics was produced, potentially suitable for pH regulation in acidic soils and provision of the plant nutrients Mg, Ca and P. With a yield of 1.9% P, the recovery remained lower than in laboratory-scale, which was attributed to technical challenges in the extraction of the P-rich stream and biological P re-dissolution variability of the EBPR/CPR sludge. Overall results show that, particularly in pure EBPR systems, an acetate-mediated approach enables a rapid and targeted re-dissolution of P. Accordingly, the P-depleted AS can subsequently be returned to the aeration stage of the WWTP or disposed by co-incineration. Further research in pure EBPR systems and on long-term stability is recommended to enhance the recovery efficiency. Retrofitting WWTPs for an on-site EBPR based P recovery could contribute to a more circular P use, by providing a valuable recovery product. Concomitantly it may reduce sludge disposal costs and dependance on chemical precipitant usage.