A novel approach for biological recovery of phosphorus from wastewater

Recovering phosphorus (P) from municipal wastewater is an important concept, but can be challenging because of the low P concentration in wastewater (7−10 mg-P/L). For P recovery to be chemically and economically viable, the P concentration needs to be >50 mg-P/L. This challenge was addressed in this study by developing a novel biofilm-based post-denitrification approach termed enhanced biological phosphorus removal and recovery (EBPR-r). This process was designed to achieve nitrogen (N) removal but also facilitate a multi-fold increase in P concentration, enabling P recovery. Underpinning this process was the innovative use of a group of microorganisms termed phosphorus accumulating organisms (PAOs). However, unlike the conventional enhanced biological phosphorus removal (EBPR) processes, the EBPR-r process was designed to use PAOs as a “shuttle”, to transfer the P from the wastewater stream into a separate P recovery stream. This modified process primarily involved two steps. In the first step the PAOs biofilm were exposed to a carbon-deficient wastewater stream (e.g. secondary effluent), whereby their internal carbon storage (i.e. poly-β-hydroxy-alkanoates; PHAs) were oxidised to provide the energy for phosphate (PO43−) uptake using nitrate (NO3–) and dissolved oxygen (O2) as electron acceptors. As PO43− was taken up from the wastewater and stored internally as poly-phosphate (Poly-P) inside the biofilm, simultaneous P and N removal from wastewater was achieved. During the second step the Poly-P enriched biofilm was exposed to a smaller recovery stream, where external carbon (acetate) was added to trigger the release of cellular P. Because the volume of the recovery stream was only a small fraction of that of the wastewater stream, P was simultaneously recovered and concentrated into that stream.

The study involved a series of laboratory-scale experiments designed to achieve proof-of-concept, process understanding and optimisation of the EBPR-r process. A laboratory-scale reactor, referred to as the master reactor, was constructed and operated to enrich an EBPR-r biofilm using activated sludge as the microbial inoculum; a synthetic wastewater was used as a secondary effluent (Chapter 2). When the reactor was operated at a 4:1 volumetric ratio (wastewater:recovery stream) the PO43− was concentrated 4-fold, from 8 mg-P/L in the wastewater (7.2 L) to 28 mg-P/L in the recovery stream (1.8 L). During the P uptake phase, simultaneous NO3− removal from wastewater was achieved at a Pupt/Nden ratio of 1.31 g-P/g-N, confirming the post-denitrification ability of the process. To generate a P-enriched stream suitable for P recovery, the EBPR-r biofilm was repeatedly exposed to the same recovery stream to facilitate P accumulation (via multiple P release), and a P-enriched liquor was generated (>100 mg-P/L). In addition to P recovery, the process also enabled the recovery of other valuable metal ions including magnesium (Mg2+), potassium (K+) and calcium (Ca2+), which may facilitate some of the chemical requirements for the downstream P recovery processes. These findings suggest that EBPR-r is a post-denitrification strategy that can also facilitate P recovery during secondary wastewater treatment.

As a consequence of the absence of soluble carbon in the secondary wastewater (i.e. upstream biological treatment removes most soluble carbon in wastewater), a high level of dissolved oxygen (DO >6 mg/L) was observed during the P uptake phase. It was demonstrated that the EBPR-r biofilm could still denitrify and uptake P under such conditions. However, the effect of DO on the EBPR-r process was unclear. Therefore, to investigate the impact of DO on storage-driven denitrification and P uptake by the EBPR-r biofilm, a series of batch experiments was conducted in which a PHA-enriched biofilm (obtained following anaerobic carbon replenishment) was exposed to various DO concentrations for P uptake (DO: 0−8 mg/L; NO3−: 10 mg-N/L; PO43−: 8 mg-P/L) (Chapter 3). The results suggested that even at a saturating DO concentration (8 mg/L), the biofilm could take up P (0.043 ± 0.001 mmol-P/g-TS.h; TS: total solid) and denitrify efficiently (0.052 ± 0.007 mmol-N/g-TS.h). However, denitrification declined when the biofilm structure was physically disturbed, suggesting that this phenomenon was a result of an O2 gradient across the biofilm. Hence, for a simultaneous denitrification and P removal using EBPR-r, maintaining the biofilm structure is critical. Moreover, analysis of the data also highlighted some operational boundaries (e.g. specific DO and NO3− concentrations in the influent) necessary for the EBPR-r biofilm to achieve acceptable P and N removal. This is valuable information for developing EBPR-r as a post-denitrification strategy, where oxygen intrusion is unavoidable under carbon-deficient conditions.

The effectiveness of the EBPR-r process depends on whether the PAOs can efficiently shuttle soluble PO43− from a large volume of wastewater into a smaller recovery stream in a cyclic manner. In practice, whether or not a wastewater plant adopts a single cycle for P uptake and release will depend largely on the availability of land and infrastructure. When these factors are limiting, an alternative mode of operation for P release (i.e. carbon replenishment) may involve sequential P uptake from the wastewater. Under such condition, the biofilm could be exposed to large quantities of electron acceptors (O2 and NO3−), exceeding that required for P uptake. The impact of such a highly oxidising environment on storage polymers (and thus on the P uptake activity) of PAOs was unknown. Hence, a further objective of the study was to explore the ability of PAOs to conserve P uptake activity under P-deficient and highly oxidising conditions (Chapter 4). The results showed that even after two days of exposure to highly oxidising conditions, upon the addition of 8 mg/L of P the biofilm could facilitate a similar level of P uptake (1.20 ± 0.09 mg-P/g-TS, between 0−48 h). This suggested that the P uptake activity was conserved throughout the period when no external carbon was replenished. Nonetheless, extending this period beyond 2 days was detrimental, and only 15% of the original P uptake activity remained by day 7. This finding is significant, as it is the first evidence of the ability of PAOs to conserve P uptake activity in the context of P recovery. This unique behaviour of PAOs may enable the development of new operational strategies, such as infrequent carbon replenishment to facilitate multiple P uptake phases before anaerobic carbon replenishment. The opportunities for flexibility in operational strategies could reduce the capital and operational costs of the EBPR-r process, and thus enhance the economic viability of P recovery.

One factor that determines the cost of implementing the EBPR-r strategy is the specific use of carbon for P recovery. However, the Prel/Cupt (P-release to carbon-uptake) ratio observed in the master reactor was substantially lower than that typically observed in conventional PAOs sludge (0.08 and 0.50−0.75 mol-P/mol-C, respectively). Hence, a strategy for optimising the Prel/Cupt ratio of the EBPR-r biofilm was investigated (Chapter 5). This was achieved using a stepwise increase of P-loading (by increasing the volume) and the P uptake period, while keeping the operational settings constant for recovery (1.8 L with 350 mg/L acetate and a P release duration of 2 h). The results showed that an increase in the wastewater volume from 7.2 L (stage I) to 14.4 L (stage II) and 21.6 L (stage III) increased the Prel/Cupt ratio marginally from 0.07 to 0.08 and 0.10, respectively. This small increase was because the biofilm displayed a similar P uptake rate (0.57 ± 0.05 mg-P/g-TS.h) when exposed to the same P concentration in wastewater. To facilitate a higher Prel/Cupt ratio, the P uptake duration was extended from 4 h in stage III to 10 h in stage IV. This increased the Prel/Cupt ratio markedly, from 0.10 to 0.25, and the biofilm at stage IV (cycle length of 12 h) was able to concentrate PO43− 10-fold, from 8 mg-P/L in the wastewater (21.6 L) to >90 mg-P/L in the recovery stream (1.8 L). Corresponding to the improved P recovery capacity, canonical correspondence analysis (based on sequences obtained using 454 pyrosequencing of the 16S rRNA genes) revealed a decreasing abundance of glycogen accumulating organisms (GAOs) (family Sinobacteraceae) and an increasing abundance of PAOs (Ca. Accumulibacter Clade IIA, unable to use NO3−) during the optimisation process. Based on the chemical and microbiological data, the strategy to optimise the Prel/Cupt ratio of the EBPR-r biofilm was validated. A 3-fold increase in the Prel/Cupt ratio was achieved (from stage I to IV), implying a more efficient use of carbon for P recovery (3× carbon saving).

In summary, a novel post-denitrification strategy and process to facilitate P recovery from a low P-containing wastewater was proposed, developed and validated. The EBPR-r approach is expected to offer several advantages over conventional post-denitrification processes: (1) it facilitates P recovery in addition to N removal; (2) it enables more efficient use of external carbon (for both P recovery and N removal, rather than just for N removal); and (3) it is associated with a lower risk of carbon discharge in the effluent (as carbon is not added to the wastewater, but to the recovery stream). Importantly, this study demonstrated a pioneering approach to using PAOs to address the global issue of P scarcity. Further research in this direction should be encouraged.