When a tough phosphorus permit leaves you no room to wait.
That was the reality for Larchmont Water Pollution Control Plant — a 1 MGD extended aeration facility outside Savannah, GA. When their 2018 permit renewal came back with a 0.5 mg/L total phosphorus limit, they'd never even been required to monitor for phosphorus before. Building a new plant wasn't an option. Neither was waiting.
Start with what you know — and what you don't
Before recommending anything, our team arrived on site to map phosphorus levels, assess biological activity, and establish a baseline. That work matters more than most people expect. You can't optimize a process you haven't measured.
Bench-scale jar testing drove product selection from there. After screening aluminum sulfate, ferric options, and multiple polyaluminum chlorides, Larchmont's program landed on a dual-product approach: DelPAC at the splitter box ahead of the secondary clarifiers, paired with sodium aluminate fed at the front of the plant.
Why two products outperform one
DelPAC drove the removal — fast. Within 12 hours, effluent phosphorus dropped from roughly 3.5–4.0 mg/L to below the 0.5 permit limit.
Sodium aluminate handled what DelPAC alone couldn't: maintaining basin pH and alkalinity. Fed at the head of the plant, it stabilizes the biological environment and trims phosphorus along the way — allowing a reduced DelPAC dose on the back end.
The plant went from an estimated 400–500 gallons per day of aluminum sulfate to roughly 30 gallons per day of sodium aluminate and 21 gallons per day of DelPAC.
The results
Effluent phosphorus held at an average of 0.35 mg/L — well below permit
97% reduction in TSS
60% reduction in BOD
A stabilized, consistent process despite highly variable influent loads
Our tank telemetry system proved its value years later when effluent phosphorus crept up and the plant assumed feed rates were unchanged. Historical data told a different story: feed rates had quietly dropped while flow and load had climbed. No jar testing required — the answer was already in the data.
What this means for your plant
Phosphorus limits are tightening. Whether you're facing a new permit or trying to optimize an existing program, the fundamentals hold: know your baseline, test before you commit, think beyond single-product approaches, and partner with people who've done this before.
Questions about your phosphorus program? Reach out at webinar@usalco.com or visit usalco.com.
