Leading the School: Demonstrating 50 µg/L Effluent TP Limits in Hatchery Pilot Testing

Summary
New Hampton Fish Hatchery is required to meet an ultra-low total phosphorus (TP) limit (<50 µg/L) while maintaining simple operation and reliable performance. Challenges included limited technology options, budget constraints, the facility's remote location, and inexperienced staff, ruling out complex solutions. The selected technology, DynaSand D2-dual-stage continuous backwash filtration-was piloted in 2024, marking its first U.S. application for hatchery effluent treatment. Results showed a single-stage filter with upstream metal salt addition consistently met the 50 µg/L TP target. Full-scale design, informed by the pilot study, is set for completion in Spring 2025.

Relevance
With populations encroaching into natural watersheds, instream nutrient standards aim to maintain water quality, driving ultra-low TP limits like those at this facility. This work presents treatment technology selection and evaluation, pilot results and a compliance roadmap.

Background
Modernization at New Hampton Fish Hatchery aims to reduce source water use while doubling fish production, increasing effluent phosphorus loading and necessitating advanced treatment. Four phosphorus removal technologies were evaluated and DynaSand D2, identified as the most efficient option, was piloted in Milford, NH, in Fall 2024. Pilot objectives included assessing TP removal efficiency, optimizing operational strategies, and determining critical design parameters. A dissolved air flotation (DAF) system was also identified for efficient management of reject solids. Bench-scale DAF tests evaluated thickening efficiency and polymer optimization.

Methodology
Technology Evaluation: Four technologies were evaluated using a triple bottom line criteria scoring technique.
Pilot and Bench Testing: The pilot used dual-stage continuous backwash. Stage 1 used coarser sand for bulk removal; Stage 2 employed finer sand for polishing. Alum was dosed upstream to sequester soluble reactive phosphorus (sRP), with filters targeting particulate phosphorus (PP) (Figure 1).
DAF bench testing simulated flotation assessing various polymer types including linear cationic, structured cationic, and non and anionic emulsion.

Results
Treatment Technology Evaluation
DynaSand D2 scored the highest against economic and non-economic criteria compared to Blue Pro (Nexom), cloth media filtration (Aqua-Aerobics), and Actiflo (Veolia) (Figure 2).

Pilot Testing
- Average hatchery effluent TP was 80 ± 16 µg/L with a sRP/PP/sNRP split of 60/22/14. The sNRP fraction remained consistently below 20 µg/L (Figure 3).
- October 21—27: Pilot effluent TP from both stages remained below 50 µg/L, except for one testing day when manual alum dosage variations caused Stage 1 to exceed the limit (Figure 4).
- October 28—30: Feed TP was supplemented by 35 µg/L sRP to simulate the modern hatchery effluent. Stage 1 achieved the 50 µg/L TP target at 11 mg/L alum dosage (100 lb-Alum/lb-sRP) (Figure 4).
- October 31: Stress testing was performed during fishpond brushing which spiked the pilot feed TP to 2,500 µg/L with sRP/PP/sNRP split of 32/76/1. Despite limited alum dosing (10 lb-Alum/lb-sRP), filters reduced sRP from 800 to 55 µg/L and TP to 265 µg/L (Figure 5).
- November 1—3: Filters were operated with no alum upstream to assess the extent of removal. TP removal of 60 µg/L reduced to 15 µg/L on Day 3 (Figure 6).
- November 3—4: The filter was operated with intermittent backwash (Ecowash) running 20% of the time. No performance improvement was observed and even with at 11 mg/L alum (100 lb-Alum/lb-sRP), Stage 1 effluent did not meet the target (Figure 4).

Bench Testing
Polydyne Clarifloc C-9545 optimized with alum achieved highest capture rates during floatation (Figure 7).

Discussion
The hatchery effluent sNRP fraction, consistently below 20 µg/L, requires no specific treatment. Meeting the 50 µg/L TP limit requires combined sRP and PP concentrations below 30 µg/L. Pilot tests confirmed that Stage 1 effluent reliably met the target when sufficient alum was dosed for sRP sequestration (Figure 8). Stress testing and no alum operation revealed some sRP removal capacity, likely attributed to residual adsorption by a metal hydroxide coating on the sand formed from prior alum overdosing, which adsorbed sRP not sequestered by alum upstream. However, this residual capacity diminished after several days of operation. Initially optimized alum dosage of 11 mg/L was not effective to reduce the Stage 1 effluent TP to less than 50 µg/L, after three days of operation without alum.

DAF bench tests showed linear cationic polymers were effective for coagulation, suggesting negatively charged particles in the effluent. Combining alum with optimized polymer dosages produced larger flocs which effectively removed via flotation.

Key Take Aways
- Instream nutrient standards increasingly drive ultra-low TP limits for wastewater and hatchery facilities, as populations encroach on natural watersheds.
- Dual-stage continuous backwash filtration ranked highest among technologies for ultra-low TP, excelling in performance, reliability, O&M ease, and flexibility.
- For wastewater with TSS below 20 mg/L and negligible sNRP, the pilot showed a single-stage filter achieving <50 µg/L TP could be an ideal alternative for wastewater tertiary treatment and hatcheries.