Improved microbial water quality and ozone performance following coagulation: implications for carbon based advanced treatment for potable reuse†

To facilitate broader implementation of potable reuse, it is important to fully account for pathogen log₁₀ reduction values (LRVs), including unit processes that are historically uncredited or under-credited. Despite its potential for pathogen removal, coagulation coupled with flocculation (C/F) has historically been omitted or overlooked when pursuing credits for potable reuse. However, with greater implementation of carbon-based advanced treatment (CBAT), which utilizes a combination of ozone, biofiltration, and granular activated carbon treatment as an alternative to membrane treatment (i.e., reverse osmosis), C/F may emerge as a valuable unit process for achieving improvements in water quality, operational performance, and public health protection in potable reuse systems. This study evaluated the ability of C/F with ferric chloride to improve both bulk and microbial water quality of secondary wastewater effluent and improve downstream ozone performance. This study also evaluated potential surrogates for microbial removal during C/F treatment. C/F removed 17–54% of DOC with ferric doses ranging from 10–50 mg Fe per L, with 30 mg Fe per L sufficient for meeting TOC removal requirements from the Stage 1 D/DBPR for all evaluated secondary effluents. Coagulant doses of 30 mg Fe per L obtained LRVs ranging from 2–3 for MS2 and B. subtilis spores. MS2 and B. subtilis spore removal exhibited strong (r ≥ 0.8) and significant (p < 0.05) Pearson's correlation with the removal of intact cell counts and total cell counts via flow cytometry (FCM), DOC, total adenosine triphosphate (ATP), and intracellular ATP. C/F immediately preceding ozone treatment improved inactivation of B. subtilis spores, lowered applied ozone doses, and increased ozone exposure (Ct) for similar specific ozone doses as compared to secondary effluent without C/F pre-treatment. Overall, C/F with ferric chloride was determined to be a valuable treatment step for removal of dissolved organic matter, MS2 bacteriophage, B. subtilis spores, and improvement of downstream ozone treatment. Furthermore, FCM, ATP, and DOC were determined to be strong potential candidates as surrogates for microorganism removal during C/F treatment, although further testing with pathogens is still necessary to justify LRV crediting.