Evaluation of post-treatment of permeate from an anaerobic membrane bioreactor treating dairy wastewater with focus on water reuse and nutrient recovery

Literature shows that anaerobic membrane bioreactors (AnMBRs) have high treatment efficiencies when treating different types of wastewater including dairy effluent (Dereli, et al., 2012, Liao, et al., 2006, Saddoud, et al., 2007). However, they have very low nitrogen and phosphorus removal and as such they end up discharging nutrient-rich permeate, albeit free from suspended solids and microbes (Grundestam and Hellström, 2007, Szabo-Corbacho, et al., 2019). This provides an opportunity for not only reusing the permeate but also recovering nitrogen and phosphorus. Therefore, the main objective of this research was to study the post-treatment of permeate from an anaerobic membrane bioreactor (AnMBR) treating real dairy wastewater, with regard to nutrient recovery and water reuse potential. A 10 L AnMBR fitted with a cross-flow, ultrafiltration membrane was used to treat real dairy wastewater from Conaprole CIM plant in Montevideo, Uruguay. The resulting AnMBR permeate was kept in both aerobic and anaerobic conditions, and filtered at 5 and 10 bar using a 200 Da nanofiltration (NF) membrane and a reverse osmosis (RO) membrane. Binary salt solutions were formulated using (NH4)2SO4 or PO4-P standard solution as the base with either NaCl, KCl, NaHCO3, (Na)2SO4 or MgSO4 as secondary salts to evaluate the effect of ions on the rejection of NH4-N and PO4-P when filtering using NF and RO. Nutrient retention rates were determined and attempts were made to recover struvite and hydroxyapatite from NF and RO concentrates. NF and RO permeates were characterized and evaluated for possible reuse applications. Membrane performance indicators for both NF and RO were derived from the equipment settings. Salt solutions trials showed that the presence of monovalent and divalent ions mainly affected the rejection capacity of NF membrane compared the RO membrane, implying the steric effect was probably the most dominant separation mechanism as opposed to the Donnan or dielectric effect. The presence of divalent anions increased NH4-N rejection, while the presence of monovalent cations had little or no effect. The presence of both monovalent and divalent cations and anions had very little or no effect on PO4-P rejection. There was no apparent difference in the operability of NF and RO units when using either aerobic or anaerobic AnMBR permeate. However, the results were insufficient to conclude whether or not short-term fouling occurred. Anaerobic AnMBR permeate had a higher NH4-N and PO4-P concentrations compared to aerobic AnMBR permeate with concentration factors for between 0.6 – 3 for NH4-N, and 0.5 – 1.2 for PO4-P. Struvite and hydroxyapatite recovery was unsuccessful, possibly due to low phosphate concentration in the samples. RO permeates were found to be suitable for reuse applications at the Conaprole plants that currently utilize well-water such as cleaning, feedwater for cooling and steam generation as well as for external applications e.g., irrigation water, restricted urban use, and environmental use. As a follow-up to the study, it is recommended: (i) to conduct longer duration filtration trials using aerobic and anaerobic AnMBR permeate to investigate short-term fouling, (ii) to run multistage NF and RO filtration and evaluate the retention capacity for both NH4-N and PO4-P and the respective concentration factors; (iii) to conduct the trials for chemical precipitation of both HAP and MAP using 1 L samples with higher PO4-P concentration ( > 100 mg/L) with the MgCl2 and CaOH2 concentrations (iv) to repeat microbiological analysis using fresh samples to ascertain whether the permeates meet the fecal and total fecal criteria for reuse application for indirect potable use such as groundwater recharge and impoundments