Reinventing Wastewater Treatment Plants for Energy Neutral Treatment and Enhanced Fertilizer Production

[b]Abstract[/b]: This project aims to transform conventional WWTPs into energy neutral, resource recovering facilities by integrating three key technologies: cloth media filtration (CMF) for enhanced primary treatment and carbon diversion, anaerobic digestion (AD) for increased biogas and ammonia production, and membrane evaporation (ME) for ammonia recovery and fertilizer generation. Pilot scale studies demonstrated that CMF can reduce aeration requirements by up to 21% while producing a more concentrated primary solids stream. The ME system, by increasing the pH of centrate from ~8 to 10, allowing for sufficient settling of solids, and removing smaller particles with a UF membrane, can achieve more than 80% ammonia recovery without membrane fouling. TEA modeling indicates CMF+ME can lower costs and improve nitrogen recovery economics, which depend strongly on influent TN concentration. [b]Project Approach and Methodology[/b] Three processes CMF, AD, ME were evaluated (Figure 1) but this abstract focuses on only the demonstration CMF and ME pilot-scale testing. [i]Demonstration Scale CMF[/i]: A new dewatering pile cloth filtration system (DCMF) system (Figure 2) was developed at Colorado School of Mines to enhance solids capture. Wastewater flows through cloth media under hydrostatic pressure, forming a solids mat. When headloss triggers cleaning, the drum rotates, exposing cloth to air for gravity dewatering and vacuum solids removal. Settled solids beneath the drum are periodically pumped out. The DCMF system was tested with an upstream sequencing batch membrane bioreactor (SBMBR) and compared to a standard 2 mm fine screen. Four cases: FS H, DCMF H, FS L, DCMF L were tested (Table 1). [i]Data Acquisition[/i]: DCMF monitored TSS, water levels, and flows; SBRs tracked TSS, DO, and pH. Weekly composites were analyzed for COD, nutrients, and solids. Solids streams (CPS SS, DCMF FC, DCMF SW) were sampled frequently. Aeration time and blower energy were quantified via SCADA and amperage data. [i]Membrane Evaporation (ME)[/i]: ME pilot work focused on ammonia recovery from UF pretreated centrate (Figure 3). Pilot Site and Pretreatment: South Platte Renew (SPR) provided centrate from digesters. Pretreatment combined a 300 µm screen and a 60 m² UF membrane operated under controlled pressure and regular backwashing. Pretreated centrate was transported to the lab; the nitrogen concentration was adjusted to 1,323±129 mg/L-N by adding ammonium chloride to the pretreated centrate. [i]Membrane Contactor Operation[/i]: ME batches (~238 L) were run at 37 °C. Feed pH was raised to 8.5 to enhance ammonia volatilization; small UF modules removed pH induced solids. A 40 m² membrane transferred ammonia to an acidified distillate loop forming ammonium sulfate. After each experiment, systems were rinsed and backwashed. [i]Technoeconomic Analysis (TEA)[/i]: TEA models for CMF and ME were developed independently. This abstract discusses only the TEA of the ME system. The centrate pretreatment and membrane contactor process modelled for a 20 MGD plant, using SPR operational data (Figure 4). Table 2 summarizes all the operating conditions and key parameters for our model. The operating conditions and design parameters for the pretreatment process and membrane contactor system are summarized in Table 3. [b]Results and Discussion[/b] [i]Demonstration Scale CMF Performance[/i]: Across all phases (FS H, DCMF H, FS L, DCMF L), DCMF consistently performed better than fine screening (Figure 5). DCMF achieved substantially higher COD and TSS removal in both high and low strength conditions. Fine screening provided minimal solids removal and limited COD reduction. [i]SBMBR Permeate Quality and Aeration Demand (Figure 6)[/i]: In high strength phases (FS H, DCMF H), SBMBR permeate quality met treatment goals for sCOD (<30 mg/L) and ammonia (<2 mg N/L) under both upstream pretreatments. At low strength (FS L), effluent ammonia exceeded 20 mg N/L and sCOD surpassed 50 mg/L, indicating insufficient aeration time. In contrast, DCMF L maintained target effluent quality, showing that improved solids removal upstream reduces oxygen demand downstream. [i]Aeration Energy (Table 4)[/i]: Aeration energy analysis showed notable savings with DCMF pretreatment. Compared to fine screening there was 21% reduction in aeration energy under high strength conditions and 16% reduction under low strength conditions (actual savings would be higher if FS L had met treatment goals). [i]Pilot Scale Membrane Evaporation (ME) Performance[/i]: Pretreated centrate from SPR was processed through the ME pilot system to evaluate nitrogen recovery (Figure 7). pH elevation to 8.5 increased ammonia availability but also caused solids formation. During pilot runs, ME achieved:83.4% nitrogen recovery at 37°C and pH 8.5; Stable distillate pH (<4), enabling ammonium sulfate formation. [i]ME TEA Results[/i]: ME system viability depends heavily on membrane cost (Figure 8) and influent TN concentration (Figure 9). Increasing membrane price raises CAPEX and OPEX proportionally. Lower TN concentrations result in lower flux through the membrane and more membrane area (high CAPEX) and total revenue declined linearly with decreasing concentration. Higher TN concentrations seem to favor economics.