A feasibility study of novel and economic seawater-based urine phosphorus recovery (SUPR®)

Phosphorous (P) is a valuable resource that will be depleted by the end of the century, while it is excessively discharged via sewage without recovery, causing water eutrophication. Recovery of P from human urine via phosphate precipitation may not only make up nearly 5-10% of mined phosphorous extraction but also mitigate risk of water pollution significantly. However, current urine P recovery practice is limited by urine collection efficiency and high cost mainly arising from addition of precipitants such as magnesium salts. As a freshwater-stressed and densely populated urban city, Hong Kong may offer a cost-effective solution for these constraints. First, vast majority of inhabitants live in high-rise buildings, enabling a cost-effective collection of human urine; second, Hong Kong has practiced seawater toilet flushing for 80% of the residents since 1958s, providing an alternative free source of precipitants. This study tested the technical feasibility of a seawater-based system for recovering phosphorus from urine. The calculation of saturation index (SI) and a series of lab-scale preliminary experiments were adopted to investigate the suitable condition for full P recovery from urine. A lab-scale SUPR reactor was set up and run for more than half year based on the preliminary experiment results to explore the optimum operation condition for more than 90% of P recovery from urine. The characteristics of precipitation product were analyzed and its utility as phosphate fertilizer was assessed. Urea hydrolysis plays an essential role in efficient seawater-based P recovery from urine. Nearly complete P recovery was achieved within 10 min without any pH adjustment when hydrolyzed urine accounted for 10-80% of the seawater and urine mixture. Moreover, the bacteria from seawater could speed up urea hydrolysis that was also benefited from a higher temperature. For SUPR reactor, more than 90% of urine P recovery could be achieved with continuous operation when HRT was 1.5 hours and mixing ratios of urine to seawater was 1:3. Suspended sludge from reactor could greatly enhance urea hydrolysis in the reactor due to the growth of bacteria, which was proved by SEM images and Live/Dead double staining. Although the composition of products varied along with volumetric ratios of urine to seawater, struvite dominated in final phosphate precipitates, which contained some beneficial elements for crops, ensuring its utility as a phosphorous fertilizer. Keywords: human urine, seawater, phosphorus recovery, urea hydrolysis, phosphorous fertilize