Pre-Treatment and Nanofiltration Pilot Study to Manage Increased Particulate Loading and Reduced Acid Feed Conditions

The City of Boynton Beach operates the West WTP, which is one of the first membrane softening plants in the United States. This WTP has been in operation since 1986 and treats surficial aquifer groundwater using six Nanofiltration (NF) skids each producing 1.45 mgd permeate. The WTP has experienced increased particulate loading. This paper presents results from a 6-month pilot study completed in 2021. Pilot objectives were investigating effective pre-treatment alternatives for solids removal, evaluating scale inhibitors to meet acid reduction goal and testing membrane combinations to meet finished water quality goals. Performance of a sand strainer and granular filter containing anthracite over silica sand were compared. Each unit was placed upstream of the NF pilot skid and tested at variable loading rates to assess effectiveness for particulate removal. A baseline pilot phase without pre-treatment was performed to mimic existing conditions. This resulted in solids blinding of cartridge filters and solids breakthrough that lead to particulate matter plugging lead Stage 1 elements, increasing differential pressures, and solids abrasion in membrane elements autopsied. Both pretreatment units provided adequate removal of colloidal sand to minimize particulate loading on downstream processes even during known “sand events” caused by intermittent wellfield operations and routine shutdown procedures. The acid reduction phase occurred over a 2-month period and two different scale inhibitors were selected through manufacturer coordination, verified with projection software and tested at different pH conditions. Both products were successful in limiting iron fouling and calcium carbonate scaling, confirmed through normalized performance data and membrane autopsies. Remainder of the study was conducted without acid with a consistent NF skid performance. Goals for finished water quality were strict as they need to match existing water quality from the East WTP. Based on a previous study, these goals can best be achieved by a combination of membrane selection and configuration, and raw water bypass blend. Because of this importance, 3 NF membrane combinations were selected, some configured in a double hybrid. Wet chemistry water quality analysis was performed to compare projections of vendor’s proprietary software. SDS tests were performed to establish DBP formation under varying raw water bypass blends. Best combination is using ‘medium’ rejection NF membranes with a limited bypass blend. Pilot study results provide an important guidance for treatment modifications to be included in the contract documents for the full-scale NF skid replacement project that will result in a more reliable and consistent operation.

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