Phosphorus is an essential element for plant growth so that it has been used for agriculture. However, according to recent studies, mineral phosphate which is the main source of phosphorus might be drained within 50 years (Takaoka, 2010). To cope with this problem, the phosphorus recovery processes in wastewater treatment plants (WWTPs) have been developed and applied in pilot or full scale. Among them, the struvite crystallization process is the most popular, because of fast crystallization in the process and slow N and P release in agricultural application.
In this study, a feasibility test of the struvite crystallization process has been done for a typical WWTP in Korea. We firstly investigated characteristics of process water or sludge from the anaerobic digester, the centrifuge and the entire sludge treatment system in the WWTP, so as to evaluate the feasibility of struvite crystallization for the process water/sludge. A chemical equilibrium model (Visual MINTEQ; vminteq.lwr.kth.se) was used to evaluate the P-removal potential of the struvite crystallization process in the given chemical conditions of the process waters/sludge (Celen et al., 2007). Also, batch aeration tests were performed to evaluate the kinetics (i.e. speed) of struvite crystallization. The PO4-P concentrations of (1) the return flow from the entire sludge treatment system, (2) the sludge from the anaerobic digester and (3) the centrate from the sludge centrifuge were measured at 12.2 ± 1.36, 72.4 ± 2.28 and 37.4 ± 0.43 mgPO4-P/L, respectively. These PO4-P concentrations were 2~5 times lower than those of WWTPs in developed countries. Applying the struvite crystallization process to the return flow might be nonsense with such a low PO4-P concentration. However, the PO4-P concentrations of the digested sludge and the centrate were found to be high enough for applying the struvite crystallization process.
The chemical equilibrium model was used to estimate the P-removal potential of the struvite crystallization process, in the given chemical conditions of the digested sludge and the centrate, and to find the optimal Mg dose and pH. When the ratio of the Mg dose to the PO4 equivalent amount (one Mg mole reacts with one PO4 mole; see the equation above) becomes higher than 1.5, the equilibrium PO4-P concentration became stable, approaching the lowest concentration. Also, above pH 8.5, the equilibrium PO4-P concentration approached the lowest concentration.
Batch aeration tests indicated that the PO4 removal kinetics by struvite crystallization was found to fit well to the 1st order kinetics (Figure 2) (Nelson et al., 2003). pH was found to determine not only the equilibrium (i.e. final) PO4-P concentration but also the kinetics (i.e. speed) of the reaction. pH 9 was shown to speed up PO4-P removal, compared to pH 7 or 8. Also, note that aeration increased pH without adding NaOH, from 7.3 to pH 8.8 for the sludge and from 8.4 to 9.1 for the centrate, because it purged out CO2 and other acidic gases in the sludge and the concentrate.
This study proved the feasibility of the struvite crystallization process and provided the optimal conditions for application to a WWTP. Findings from the chemical equilibrium modelling and the batch aeration tests may be further used for design and operation of pilot- or full-scale struvite crystallization processes.