Winter Lagoon Operation: Ice, Ammonia, and Keeping Bugs Alive

Winter is when a lagoon that looks fine in August starts writing violations. The biology slows down, the physics change, and the sampling gets harder to interpret all at the same time. This guide covers what actually happens under ice and what an operator can control.

What cold water does to the biology

Two things move in opposite directions. Cold water holds more dissolved oxygen, which sounds like good news. But cold water also slows every biological rate in the cell, and the organisms that suffer most are the ones you can least afford to lose.

Nitrification effectively shuts down in near-freezing water. Nitrifying bacteria are slow-growing and demanding under the best conditions, needing oxygen, warmth, and low BOD all at once, and winter removes the warmth. Ammonia that would be converted in July passes through in January. This is why winter ammonia exceedances are the classic lagoon failure mode, and why they are not, by themselves, evidence that something broke. The full mechanism and the ladder of fixes are in the ammonia removal guide.

BOD removal slows too, just less dramatically. The practical consequence is that the primary cell’s job, removing on the order of 80 percent of influent BOD5, gets harder exactly when there is the least margin to spare.

What ice does to the physics

Ice cover takes away two things a facultative lagoon depends on: wind mixing and surface reaeration. It also takes away most of the light that drives algal oxygen production.

Then it changes how water moves. Under cover, influent tends to travel across the top of the cell toward the outlet rather than distributing through the full volume. That is short-circuiting, and it means effective retention time drops below design retention right when the biology needs more contact time, not less.

Mechanical aeration is what replaces the wind. This is the strongest argument for diffused aeration in cold climates: the blower sits on shore, the diffusers sit below the ice, and the rising plume both oxygenates and maintains an open water area.

Equipment through the freeze

Diffused systems. Generally keep running. Continuous operation maintains an open water area and continuous oxygen delivery, and the equipment that can be damaged by ice is not in the ice.

Splash-style surface aerators. These are exposed machinery on a freezing waterbody. Follow the manufacturer’s winter guidance without improvising: spray that freezes on the unit builds load the mounting was not designed to carry. In hard-freeze climates, many operators pull splash units for the season or protect them in place.

Aspirating surface units. Because they push air below the surface along an angled shaft rather than throwing a spray, they behave differently in cold. Manufacturer guidance still governs.

Whatever you run, decide before freeze-up, not during. The window to work safely on a cell closes fast.

Reading winter numbers honestly

The single most useful winter discipline is to stop treating any one grab sample as the truth. Short-circuiting under cover and nitrification in the BOD test bottle can both distort results, in different directions, in the same month. Sample the same points on the same schedule and read the trend.

What you can still control in winter

• Keep oxygen moving. Continuous mechanical aeration is doing the work wind used to do.
• Protect the primary cell’s BOD removal. If cell one falls behind, downstream cells inherit work they were never designed for, and ammonia is usually the first limit to slip.
• Watch loading changes. A new industrial contributor or septage receiving hits harder in winter because there is no biological headroom.
• Do not start a bioaugmentation program expecting fast winter results. Cold slows the bugs you add just as much as the ones already there.
• Plan the spring. Ice-out turnover redistributes solids and can produce the year’s worst odor episode. See odor control for the spring turnover pattern.

Before freeze-up: a short checklist

1. Run a current loading calculation so you know what cell one is actually being asked to do.
2. Sludge judge while you can still work the cells. A blanket you did not know about is a winter retention problem you cannot fix until spring. The method is in how to measure your sludge blanket.
3. Service the blower and confirm the diffuser grid is delivering evenly.
4. Pull, protect, or winterize exposed surface units per manufacturer guidance.
5. Set the winter sampling plan, including where and when, so trends are comparable.
6. Confirm your discharge plan and permit conditions for the winter period with your state agency.