Technoeconomic Evaluation of Alternatives for TTHM Compliance in Class A Reuse Effluent

INTRODUCTION Total trihalomethanes (TTHMs) are disinfection byproducts formed through chlorine-based disinfection processes and are an increasing threat to wastewater, reuse, and drinking water systems. Recent regulations for direct and indirect reuse systems require reduction of TTHM levels. However, little guidance is available on appropriate technologies and methods for effective TTHM mitigation. Both removal technologies and alternative disinfection approaches offer possible solutions. This paper describes the comprehensive evaluation of TTHM mitigation alternatives that was conducted for a 15 megaliter per day (MLD) water reclamation facility (WRF) producing Class A reuse-quality effluent. Historical WRF operational data showed TTHM concentrations regularly exceed permitted limits. The permitting requirements for disinfection and TTHMs provide the basis of compliance for all alternatives considered. The facility's Aquifer Protection Permit (APP) specifies a TTHM alert level (AL) and discharge limit (DL) for TTHMs and fecal coliform. Alternatives were developed based on the treatment goal of reducing TTHM levels below the AL of 80 micrograms per liter (ug) and include peracetic acid (PAA), chloramines, air stripping, and source control. Figure 1: THM Speciation with Alert Level and Discharge Limit PAA DISINFECTION PAA can be a cost-effective alternative to chlorine-based disinfection systems. Existing disinfection doses and performance can indicate the efficacy of PAA disinfection, but there is little industry experience with achieving the disinfection performance required at this WRF. Bench testing was recommended to identify an appropriate dose range (Figure 2). While the bench testing showed good recovery of PAA through the contact period (Table 1), the maximum dose tested of 4 mg/L and 40 minutes contact time did not provide the effluent pathogen reduction required to comply with regulatory discharge limits. Figure 2: PAA Bench Test Results for 2 ppm Dose Table 1: PAA Recovery in Bench Tests Bench testing was inconclusive with respect to the design dose. However, recovery rates suggest higher doses would not reliably achieve disinfection performance targets as available residual PAA provided little disinfection between 20 and 40 minutes of contact. Additional bench or full-scale demonstration testing would be required to reduce uncertainty in design dose and refine cost estimates. CHLORAMINE DISINFECTION Under the proper ratio, chloramines bind up chlorine in a combined form and eliminate the pathway of TTHM formation. Chloramine disinfection requires a longer reaction time and CT target (total chlorine residual multiplied by reaction time). Disinfection kinetics can be complex and site specific; however, California's Title 22 regulations establish requirements for tertiary filtered disinfected water irrespective of free or combined chlorine. These requirements include a minimum CT of 450 mg-min/L in addition to total coliform limit of <2.2 MPN per 100 mL, corresponding to a minimum modal contact time of 90 minutes based on peak dry weather design flow. The limits from California's regulations were used as a conservative planning level basis and the expanded contact chamber footprint is conceptually shown in Figure 3. Figure 3: Conceptual Expansion Required for Chloramine Disinfection AIR STRIPPING Air stripping is a treatment method that effectively removes volatile organic compounds (VOCs) from a contaminated water source. The process forces air through contaminated water through the water via air stripping towers or aeration tanks. Several process alternatives for air stripping were considered including air stripping towers (3 or 6 tower configuration), surface aerators within a tank, and floating spray aerators within a tank. Vendors offer performance guarantees at the stated percentages to ensure that effluent water meets the permit's requirement, 80 ug/L, under a set of defined influent conditions (e.g., flow rate, TTHM speciation, temperature), as shown in Table 2. Table 2: Air Stripping Alternatives Performance Comparison SOURCE CONTROL ALTERNATIVES An evaluation of DBP precursors is also being performed to inform the identification and evaluation of potential source control alternatives. The results of this evaluation are pending and will be incorporated into a final presentation. RESULTS PAA was considered infeasible due to excessive risk and was eliminated for consideration. Variations of chloramine disinfection and air stripping were progressed to conceptual design for capital and annual operating cost estimates. The introduction of ammonia nitrogen and formation of NDMA from chloramines are considerable issues for the WRF. The net present values (NPV) for each alternative were estimated and air stripping towers were determined to be the most cost-effective alternative. This solution requires the smallest footprint (even in the two-pass system), has the lowest life cycle cost, and provides a confident performance guarantee given the plant sees its projected flows. This work serves as a valuable reference for utilities with TTHM concentrations exceeding current or anticipated effluent limits in drinking water, reuse, and wastewater applications. Table 3: Alternatives Cost Comparison Summary