Regulated disinfection byproduct formation over long residence times.

Design and operation of drinking water treatment plants and associated distribution systems with long residence times are complicated by the formation of regulated disinfection byproducts (DBPs), comprised of total trihalomethanes (TTHM) and five haloacetic acids (HAA5). Treated water dissolved organic carbon (DOC) concentrations, the unit processes required to meet those DOC concentrations, and disinfection strategies (e.g., booster chlorination) are the primary design and operational considerations that can require extensive testing or modeling to determine. In this study, twelve different treated drinking waters were generated at the bench-scale using ferric chloride coagulation and granular activated carbon adsorption from four parent raw waters collected from the San Juan River representing spring runoff, monsoon, and low flow events. Treated drinking waters with DOC concentrations of 0.9, 1.4, and 1.9 mg/L were tested for regulated DBP formation under simulated distribution system (SDS) conditions over residence times as long as 56 days and compared to 7-day formation potential (FP) testing. SDS free chlorine concentrations were maintained between 0.2 and 1.0 mg/L as Cl2 through periodic booster chlorination. Maximum SDS TTHM and HAA5 concentrations were 0.208 and 0.074 mg/L, respectively, with formation consistently varying by approximately ±20% across the four different parent raw waters despite having been treated to the same three DOC concentrations. An average of four existing TTHM models consistently underpredicted TTHM formation by approximatively 20%. Long considered a conservative measure of DBP formation, FP testing also underpredicted SDS DBP formation at 56 days by approximately 40% on average. The DBP testing approach presented in this study allowed for the development of several significant linear relationships for predicting DBP concentrations based on treated water ultraviolet light absorbance at 254 nm, water temperature, and cumulative free chlorine demand.

Regeneration of pilot-scale ion exchange columns for hexavalent chromium removal.

Due to stricter regulations, some drinking water utilities must implement additional treatment processes to meet potable water standards for hexavalent chromium (Cr(VI)), such as the California limit of 10 µg/L. Strong base anion exchange is effective for Cr(VI) removal, but efficient resin regeneration and waste minimization are important for operational, economic and environmental considerations. This study compared multiple regeneration methods on pilot-scale columns on the basis of regeneration efficiency, waste production and salt usage. A conventional 1-Stage regeneration using 2 N sodium chloride (NaCl) was compared to 1) a 2-Stage process with 0.2 N NaCl followed by 2 N NaCl and 2) a mixed regenerant solution with 2 N NaCl and 0.2 N sodium bicarbonate. All methods eluted similar cumulative amounts of chromium with 2 N NaCl. The 2-Stage process eluted an additional 20-30% of chromium in the 0.2 N fraction, but total resin capacity is unaffected if this fraction is recycled to the ion exchange headworks. The 2-Stage approach selectively eluted bicarbonate and sulfate with 0.2 N NaCl before regeneration using 2 N NaCl. Regeneration approach impacted the elution efficiency of both uranium and vanadium. Regeneration without co-eluting sulfate and bicarbonate led to incomplete uranium elution and potential formation of insoluble uranium hydroxides that could lead to long-term resin fouling, decreased capacity and render the resin a low-level radioactive solid waste. Partial vanadium elution occurred during regeneration due to co-eluting sulfate suppressing vanadium release. Waste production and salt usage were comparable for the 1- and 2-Stage regeneration processes with similar operational setpoints with respect to chromium or nitrate elution.

Enhanced DOC removal using anion and cation ion exchange resins.

Hardness and DOC removal in a single ion exchange unit operation allows for less infrastructure, is advantageous for process operation and depending on the water source, could enhance anion exchange resin removal of dissolved organic carbon (DOC). Simultaneous application of cationic (Plus) and anionic (MIEX) ion exchange resin in a single contact vessel was tested at pilot and bench scales, under multiple regeneration cycles. Hardness removal correlated with theoretical predictions; where measured hardness was between 88 and 98% of the predicted value. Comparing bench scale DOC removal of solely treating water with MIEX compared to Plus and MIEX treated water showed an enhanced DOC removal, where removal was increased from 0.5 to 1.25 mg/L for the simultaneous resin application compared to solely applying MIEX resin. A full scale MIEX treatment plant (14.5 MGD) reduced raw water DOC from 13.7 mg/L to 4.90 mg/L in the treated effluent at a bed volume (BV) treatment rate of 800, where a parallel operation of a simultaneous MIEX and Plus resin pilot (10 gpm) measured effluent DOC concentrations of no greater than 3.4 mg/L, even at bed volumes of treatment 37.5% greater than the full scale plant. MIEX effluent compared to simultaneous Plus and MIEX effluent resulted in differences in fluorescence intensity that correlated to decreases in DOC concentration. The simultaneous treatment of Plus and MIEX resin produced water with predominantly microbial character, indicating the enhanced DOC removal was principally due to increased removal of terrestrially derived organic matter. The addition of Plus resin to a process train with MIEX resin allows for one treatment process to remove both DOC and hardness, where a single brine waste stream can be sent to sewer at a full-scale plant, completely removing lime chemical addition and sludge waste disposal for precipitative softening processes.