Economic evaluation of ion-exchange processes for nutrient removal and recovery from municipal wastewater

Excessive nutrient discharge from wastewater treatment plants (WWTPs) causes eutrophication, necessitating stringent effluent limits (e.g., <1 mg NH₄-N/L, <0.5 mg PO₄-P/L). Nutrient recovery aligns with circular economy principles, but traditional methods like biological nutrient removal (BNR) face challenges such as instability and high energy consumption. Ion exchange (IEX) processes, while previously limited by selectivity, clogging, and regeneration costs, show promise due to recent advances in media development. Synthetic zeolites like mesolite exhibit high ammonia adsorption capacity (4.6-4.9 meq/g), and hybrid ion exchange resins (HAIX) with ferric oxide nanoparticles enhance phosphorus removal, offering extended operating cycles. However, the economic feasibility of IEX hinges on efficient regenerant cleanup and nutrient recovery. This study evaluates the performance and economics of IEX processes as tertiary treatment for a 10,000 population equivalent WWTP, comparing it with traditional BNR.

Existing literature highlights the environmental impact of nutrient-rich wastewater discharges and the need for stricter regulations. The depletion of natural phosphorus reserves and the potential for economic benefits from nutrient recovery are also emphasized. Research on advanced IEX materials shows improved ammonia removal efficiency using synthetic zeolites like mesolite, owing to their consistent Si:Al ratios. Studies on HAIX resins demonstrate superior phosphorus removal capabilities compared to traditional methods. While promising, IEX's widespread adoption has been hindered by high regenerant disposal costs. Previous work demonstrated the potential for regenerant reuse and nutrient recovery, which is crucial for economic viability. This study builds upon these advancements by conducting a full-scale economic evaluation of IEX, incorporating nutrient recovery into the cost analysis.

Three flowsheets were designed for a 10,000 population equivalent WWTP: 1) BNR with iron dosing, 2) activated sludge process (ASP) + IEX, and 3) anaerobic membrane bioreactor (AnMBR) + IEX. The BNR process (A2O) was designed using the SRT method, considering UK climate conditions (14°C average wastewater temperature). The ASP+IEX flowsheet involved a conventional ASP for BOD removal, followed by a drum filter and two IEX columns (one for nitrogen, one for phosphorus). The AnMBR+IEX flowsheet utilized an optimized intermittent dead-end biogas sparging regime to minimize energy consumption. IEX column design was based on pilot-scale data, specifying media type, empty bed contact time (EBCT), and regeneration parameters. Regenerant cleanup and nutrient recovery were integrated, utilizing hollow fiber membrane contactors (HFMC) and chemical precipitation to recover ammonium sulfate and hydroxyapatite, respectively. Capital expenditure (CAPEX) and operational expenditure (OPEX) were estimated using the factorial method and unit cost data, respectively. A 40-year whole life cost (WLC) was calculated, considering a 7% discount rate. Greenhouse gas emissions were also considered using established conversion factors.

All three flowsheets met effluent quality requirements (COD < 20 mg/L, NH₄-N < 1 mg/L, PO₄-P < 0.5 mg/L), but process stability varied. BNR proved sensitive to temperature fluctuations and shock loads, requiring ferric dosing and tertiary filtration to meet stringent limits, resulting in higher costs. ASP+IEX and AnMBR+IEX effectively removed nutrients, with additional COD removal by HAIX. IEX was less sensitive to temperature and shock loads than BNR. Regenerant reuse and nutrient recovery were successfully integrated, achieving >85% nutrient recovery. CAPEX was highest for BNR (£3.94M), followed by AnMBR+IEX (£3.6M) and ASP+IEX (£3.48M). OPEX was highest for BNR (£316k/year), followed by ASP+IEX (£282k/year) and AnMBR+IEX (£177k/year). WLC for BNR was £8.4M, while ASP+IEX and AnMBR+IEX were £7.4M and £6.1M, respectively. AnMBR+IEX showed significant energy recovery through biogas production, further reducing costs and emissions. Recovered nutrients could potentially generate revenue, although the market value depends on product purity and quality.

The results demonstrate the economic and environmental advantages of IEX for nutrient removal and recovery. IEX offers superior reliability and lower costs compared to BNR, especially when stringent effluent limits are required. The integration of nutrient recovery significantly reduces OPEX. The AnMBR+IEX configuration is particularly attractive due to energy recovery, aligning with circular economy principles. While nitrogen recovery from wastewater might not currently compete with industrial production, the non-renewable nature of phosphorus makes recovery increasingly crucial. The marketability of recovered products depends on purity, quality standards, and market demand. The study highlights the potential for higher revenue if recovered products are sold as high-value reagents rather than fertilizers.

Ion exchange processes, particularly when coupled with AnMBR, offer a technically reliable and economically viable alternative to traditional BNR for nutrient removal and recovery from municipal wastewater. The benefits include lower whole-life costs, reduced greenhouse gas emissions, and the potential for revenue generation from recovered nutrients. Future research should focus on optimizing IEX processes for large-scale applications and exploring wider market opportunities for recovered nutrient products.

The economic evaluation is based on specific design parameters and cost data, which may vary depending on location and specific conditions. The market analysis for recovered nutrients is preliminary and further investigation is required to fully assess the economic potential. The long-term performance and durability of IEX media under full-scale operating conditions need further verification.