Snail Away: Snail Mitigation of Nitrification-Only Tertiary MBR

ABSTRACT
Physa genus snails were observed during nitrification-only tertiary membrane bioreactor (N-only tMBR) operation in support of Pure Water Southern California, a joint program between the Los Angeles County Sanitation Districts (LACSD) and the Metropolitan Water District of Southern California (MWD) to provide potable reuse water to Southern California. Development and testing of bench-scale protocol and pilot-scale testing evaluated several mitigation strategies including influent ultra-fine screening, free ammonia exposure, acute anoxia, chlorination, sodium chloride and iron. Results indicated that ultra-fine screening and free ammonia exposure were the most effective strategies to implement. These strategies will be tested at demonstration-scale at a 0.6 mgd facility to validate snail suppression effectiveness.

INTRODUCTION
Pure Water Southern California is a joint program being pursued by the Los Angeles County Sanitation Districts (LACSD) and the Metropolitan Water District of Southern California (MWD) to provide a new source of water for Southern California. Effluent from LACSD's largest facility, the A.K. Warren Water Resource Facility (Warren) will be treated for nutrient removal and advanced water treatment to meet potable reuse standards prior to distribution in the region (Figure 1). In advance of implementation, the Grace F. Napolitano Innovation Center's Demonstration Plant (Demonstration Plant) was built to evaluate nitrogen removal alternatives. While operating in nitrification-only tertiary membrane bioreactor (N-Only tMBR) mode, snails identified as Physa genus were observed in the membrane tanks, likely sourced from Warren secondary effluent (SE) (Figure 2). While initially addressed with more frequent membrane cleaning, snail proliferation could cause significant increases to both capital and operational costs at full-scale. Planning began in 2023 to provide viable snail mitigation options for future retesting at demonstration-scale and planning for full-scale operation. Potential mitigation measures based on literature review and previous experience were compiled and ranked with the most promising selected for evaluation (Table 1).

OBJECTIVES
The objective was to evaluate the most viable snail prevention, mitigation, and recovery measures identified from literature review and determine the most effective approaches for implementation at the Demonstration Plant and consideration for full-scale implementation.

METHODOLOGY
A bench-scale protocol using snails harvested and grown at the Warren Facility was developed that included the verification of snails of the genus Physa. The snails were placed in an acclimation tank filled with nitrified Warren SE, aerated, and fed with activated sludge from nitrification-denitrification water reclamation plants (Figure 3). The snails were acclimated and tested after reaching maturity. Ten snails were tested per batch in aerated Warren SE (unaerated for anoxic testing) in each test beaker. Ten snails were added to a second beaker to serve as the control. After each test, snails were placed in aerated SE for recovery and observed at 24 and 48 hours. The 48-hour mortality was reported. A separate protocol was developed for iron due to suppression from bioaccumulation. Mitigation measures tested at bench-scale are shown in Table 2.

Pilot-scale evaluation consisted of operating an existing pilot-scale tMBR (Figure 4) in N-only mode from April to September 2023 to determine if snails would proliferate under these conditions (Table 3). Monthly MBR snail observations were performed. An additional pilot was constructed to evaluate ultra-fine screening. A 75-micron and a 150-micron screen were evaluated alongside a 9.5 mm screen as control (Figure 5). Snails were seeded inside each mesh screen, and the holding tank was monitored to measure screen-size effectiveness. Further pilot modification was performed to optimize successful bench-scale strategies and mitigation strategies that were inconclusive at bench-scale due to the acute testing protocol.

<B>RESULTS AND DISCUSSION
Bench-scale testing results (Table 4) indicated that the 4000 mg/L NH4Cl at pH 9 showed the highest mortality with an average 48-hour mortality > 97.5%. Anoxic conditions, short-term chlorination, sodium chloride and iron exposure showed minimal to no mortality when compared to control. However, anoxic conditions will be further evaluated at the demonstration scale due to observed suppression in previous nitrification-denitrification modes at the Demonstration Plant. Anoxic conditions and chlorination may be more effective on suppression of younger life stages during continuous operation. Therefore, chlorination will also be re-evaluated at pilot-scale in flow-through mode and at demonstration scale.

Operation of N-only tMBR was successfully replicated at pilot-scale (Figure 6). However, monthly MBR observations did not show snail proliferation (Figure 7), possibly due to weekly hypochlorite and citric acid cleaning and will be further evaluated individually. Results showed that 75-micron screening was effective at snail suppression, but 150-micron screens might allow eggs to pass through the screen after one month (Figure 8). Additional work of ultra-fine screening is planned at pilot-scale and will also be incorporated in future testing at the Demonstration plant.