Phosphorus Release and Recovery from Digested Anaerobic Sludge in an Electrochemical System

The mining of non-renewable phosphate rock is currently the primary phosphorus source for food production. Phosphorus recovery from secondary wastes is a key component for circular economy. Municipal digested anaerobic sludge, or digestate, is a liquid-solid mixed liquor produced from anaerobic digestion that is enriched with phosphorus (P). Around 40% of the phosphorus discharged by anthropogenic activity will end up in the wastewater sludge, making digestate a promising phosphorus source. Conventional method for phosphorus recovery from the digestate like struvite precipitation is practically challenging. One of the main reasons is the heavy chemical consumption and the high risk of handling corrosive acid/base. Electrochemical methods are of strong research interest lately due to the elimination of chemical use, flexible process control, and low environmental impact. In this study, we designed an electrochemical nutrient recovery cell (ENRC, Fig. 1) to achieve simultaneous phosphorus release and nutrient recovery from the digestate. The digestate was sampled from Missouri River Treatment Plant in late September of 2021. Most phosphorus exists in the solid fraction of the digestate and need to be firstly released into the aqueous phase, usually by lowering the pH, before the recovery. As shown in Fig.1, water electrolysis reaction will induce the acidification of the digestate and increase aqueous PO43- concentration in the anode. Meanwhile, electric field will drive NH4+ to migrate across the cation exchange membrane (CEM) into the recovery chamber. Results show the digestate was acidified from pH 8.0 to 2.0 in the anode of the ENRC after operating for 5 h under the current density of 25 A m-2, similar to the pH adjusted by the conventional HCl adjustment method. Both methods released 67.6~74.1% of total phosphorus in the digestate and achieved the comparable PO43--P concentrations of 253.47 ± 3.05 mg L-1 and 277.60 ± 15.2 mg L-1 (p-value > 0.05), rising from 27.72 ± 0.95 mg L-1 in the untreated digestate (Fig. 2). In contrast to the high NH4+-N concentration of 995.07 ± 53.11 mg L-1 in the HCl treated digestate, electrochemical leaching removed >99% of NH4+-N from the digestate (Fig. 2). The HCl adjustment increased Ca2+ concentration from ~89 mg L-1 to 479 mg L-1, much higher than 31 mg L-1 Ca2+ in the ENRC anode effluent, likely due to Ca2+ migration through CEM into the recovery chamber. The HCl treatment introduced a high concentration of Cl- (~4700 mg L-1), which was avoided in the ENRC anode effluent that had ~120 mg L-1 Cl- (Fig. 2). The nutrients were successfully recovered in the ENRC recovery chamber after continuous operation for 5 cycles. As shown in Fig. 3A, the ENRC released 147.59~215.24 mg L-1 PO43--P from the digestate at the end of each cycle, accounting for 40~58% of total phosphorus in the digestate. The catholyte removed 96-98% of the PO43--P and produced an effluent with < 5 mg PO43--P L-1. The PO43--P concentration in the recovery solution reached 142.62 ± 7.78 mg L-1 after the first cycle and gradually accumulated to 404.56 ± 20.44 mg L-1 after 5 cycles, which accounted for 72% of the total PO43--P released in the anode. Mass balance revealed that after removing the recovery solution, 6.9% of the released PO43--P remained or adsorbed in the recovery chamber (Fig. 3B). In addition, 9.8 % of the released PO43--P precipitated in the cathode chamber due to the high pH environment. The NH4+-N concentration in the recovery solution linearly increased to 3493.56 ± 194.13 mg L-1 h-1 after five cycles of operation (Fig. 3A), accounting for ~90% of the total NH4+-N in the digestate. After adjusting the pH to 8.5 in the final recovery solution, more than 99% of the PO43--P was precipitated to produce a solid with light brownish color. The inorganic contents in Fig. 3C shows that PO43--P contributed to ~13.62% of dry mass in the product, which was equivalent to 31.19% P2O5 content and would meet the 25%~37% P2O5 requirement for economic grade phosphate rock. Calcium was the major metal and took up ~16.54% of the dry weight in the product, indicating a good soil phosphorus availability. After the nutrient extraction, around 42% of the dissolved COD was removed and the digestate settleability was significantly improved. This work has demonstrated an effective electrochemical approach to extract nutrients from the waste digestate and can be extended to treat phosphorus-rich biosolid wastes generated from different sectors. This work has been published: Wang, Z. and He, Z.* (2022) Electrochemical phosphorus leaching from digested anaerobic sludge and subsequent nutrient recovery. Water Research. Vol 223, Article 118996.