Urine phosphorus (P) recovery presents an opportunity not only to mitigate the P scarcity problem, but also to improve the efficiency of downstream sewage treatment works. However, the high cost of precipitant dosage makes urine P recovery financially unattractive, thus seawater has been proposed as an alternative low-cost precipitant, which is particularly of benefits for Hong Kong where seawater toilet flushing has been practiced since 1958. However, the knowledge of ureolysis – the controlling factor of urine P recovery efficiency-in this proposed Seawater-catalysed Urine Phosphorus Recovery (SUPR) system is limited.

The present study firstly evaluated the effect of ureolysis on urine P recovery when seawater was used as an alternative precipitant. For completely ureolysed u...[ Read more ]

Urine phosphorus (P) recovery presents an opportunity not only to mitigate the P scarcity problem, but also to improve the efficiency of downstream sewage treatment works. However, the high cost of precipitant dosage makes urine P recovery financially unattractive, thus seawater has been proposed as an alternative low-cost precipitant, which is particularly of benefits for Hong Kong where seawater toilet flushing has been practiced since 1958. However, the knowledge of ureolysis – the controlling factor of urine P recovery efficiency-in this proposed Seawater-catalysed Urine Phosphorus Recovery (SUPR) system is limited.

The present study firstly evaluated the effect of ureolysis on urine P recovery when seawater was used as an alternative precipitant. For completely ureolysed urine, more than 98% of urine P could be precipitated by seawater, while for fresh urine (un-ureolysed urine) the P recovery efficiency was less than 20%. The main reason of this difference was due to the pH increase during ureolysis, and only 20% of ureolysis extent could result in higher than 90% of urine P precipitation within 10 min. The results demonstrated that certain degree of ureolysis is the pre-requisite for efficient P precipitation.

However, ureolysis is generally much slower than P precipitation, thus it is the rate limiting step of urine P recovery. For optimization and application of this innovative SUPR system, the kinetics of microbial ureolysis in this system was further investigated. The results showed that the indigenous bacteria from urine and seawater exhibited relatively low ureolytic activity, but they can quickly adapt to the environment of urine-seawater mixture after short period of batch cultivation. During 4 cycles of batch cultivation, both the abundance and specific ureolytic activity of the indigenous bacteria were greatly enhanced, and the result was confirmed by a biomass-dependent Michaelis-Menten model. Consequently, only around 3 h was needed to achieve complete ureolysis after 4 cycles of cultivation, indicating that enrichment of indigenous bacteria from SUPR system by cultivation can obtain sufficient ureolytic activity for facilitating in-situ phosphate precipitation.

Based on the findings above, a lab-scale SUPR reactor was set up to verify the high-rate in-situ ureolysis and efficient P recovery in a continuous-flow condition. Nearly complete urine P recovery was achieved within a single reactor without adding any chemicals, and the quick ureolysis was possibly due to attached sludge. Terminal Restriction Fragment Length Polymorphism (TRFLP) analysis revealed the predominant groups of bacteria in the SUPR reactor were possibly originated from seawater rather than urine. Moreover, the high ureolysis rates induced by cultivated bacteria in the SUPR reactor were confirmed by the batch tests, and the results were found to be well described by a kinetic model which includes microbial ureolysis, phosphate precipitation, and weak acid/base equilibria. The influence of seawater-to-urine mixing ratios and different hydraulic retention times (HRTs) on the P recovery efficiency to cover various sanitary conditions were examined with the SUPR reactor, and a phosphorus recovery efficiency higher than 98% could always be obtained when the HRT was longer than 3 h and seawater-to-urine mixing ratio of lower than 3:1. In conclusion, this study proved that in-situ fast ureolysis can be achieved within the SUPR reactor, which can facilitate efficient P recovery, thus providing an efficient and economic method for urine P recovery.