Cutting BOD and TSS in a Facultative Lagoon: The Algae Problem Nobody Warns You About

A facultative lagoon fails TSS in summer because algae blooms carry into the effluent. These lagoons remove 75 to 95 percent of incoming BOD, yet 70 to 80 percent of the BOD leaving the lagoon can be algae, which also drives TSS. Mixing can push BOD and TSS below 10 mg/L, and discharging below the surface avoids the algae-rich top layer.

If you run a facultative lagoon, the pattern is maddeningly familiar. Your winter and shoulder-season numbers look fine. Then July arrives, the water turns pea-green, and your total suspended solids (TSS) climb over permit even though the lagoon is clearly digesting most of what comes in. Nothing about the incoming load changed. What changed is that the lagoon started growing its own solids: algae.

This is the paradox at the heart of facultative lagoon operation. The same sunlight and nutrients that help treat wastewater also feed dense summer algae blooms, and those algae cells are counted as suspended solids and as oxygen-demanding organics on your lab report. This guide explains why it happens, why chasing influent load will not fix it, and the practical levers, mixing, sub-surface discharge, and resting a cell, that bring summer BOD and TSS back down without rebuilding the plant.

For the wider system context, see the sewage lagoon and lift station management course, and for the specific question of how much mixing you need, how lagoon aeration is sized.

Why does a facultative lagoon fail TSS in summer?

A facultative lagoon works because it has two zones stacked on top of each other. Near the surface, sunlight and dissolved oxygen support aerobic bacteria and algae. Near the bottom, an oxygen-poor layer digests settled solids anaerobically. That layered design is efficient at removing incoming organic load, which is why these lagoons routinely strip out a large share of the BOD arriving at the inlet.

The trouble is that the surface algae which help oxygenate the water are themselves suspended solids. In cool, low-light months the bloom is thin and stays out of your effluent. In warm, sunny summer months the bloom explodes, and when water leaves the lagoon it carries those algae cells with it. Your incoming waste is well treated, but the lagoon has manufactured a fresh crop of solids on top of it. That is why summer, not a load spike, is when TSS violations tend to appear.

In a facultative lagoon, summer TSS is rarely an incoming-load problem. It is an algae problem the lagoon grows on its own.

The algae paradox: high BOD removal, high effluent TSS

The numbers make the paradox concrete. A well-run facultative lagoon commonly removes on the order of 75 to 95 percent of the BOD arriving at the inlet. By any reasonable measure, the biology is doing its job. Yet the effluent can still fail, because a large fraction of the BOD and solids leaving the lagoon are not leftover sewage at all.

70 to 80 percent of effluent BOD can be algae

In a facultative lagoon during a summer bloom, roughly 70 to 80 percent of the BOD leaving the lagoon can be algae rather than incoming waste. Because algae cells are both solids and oxygen-demanding organic matter, they push up TSS and BOD together on your report. This is the single most important fact to internalise: once summer blooms set in, your effluent limits are being driven by biology you grew, not by the wastewater you received.

It also explains a common frustration. Operators tighten screening, chase inflow and infiltration, and re-check the inlet, all reasonable instincts, yet the summer TSS number barely moves. The load was never the problem. The algae is.

How does mixing reduce BOD and TSS?

Mixing the water column is the most direct lever you have against algae-driven solids. Two things happen when you introduce gentle, continuous mixing with aeration. First, algae cells that depend on floating in the sunlit surface layer get circulated down into darker water, which limits how dense the bloom can grow. Second, the added dissolved oxygen keeps the water column aerobic, so the biology stays healthy rather than tipping toward the anaerobic conditions that create odor and release stored organics.

Done well, mixing can bring both BOD and TSS down substantially, in some cases below 10 mg/L, by disrupting the blooms that dominate summer effluent solids. The mechanism is prevention through circulation, not a chemical kill. That matters, because a chemical kill leaves the dead algae behind to sink and rot.

Bottom-up diffused aeration is the form of mixing best suited to this job, because it lifts and turns the entire water column from the floor up rather than only disturbing the surface. If you are weighing diffused against surface systems, our lagoon aeration sizing guide walks through the trade-offs and the dissolved-oxygen targets for BOD reduction versus nitrification. You can see the diffused hardware itself in our diffused lagoon aeration systems.

Sub-surface discharge and resting a cell

Where you draw effluent from matters as much as how you treat it. Algae concentrate in the sunlit top of the water column. If your outlet skims from that top layer, you are pulling out the algae-rich water by design. Moving the draw point below the surface, a sub-surface or mid-depth discharge, lets you release the clearer water underneath the bloom while leaving the densest algae behind. On many lagoons this single change meaningfully lowers effluent TSS in summer with no new mechanical equipment.

Resting a cell is a second low-capital tactic. In a multi-cell system, temporarily taking a polishing cell offline from active loading gives its solids time to settle before discharge resumes. The quiescent period lets algae and other suspended matter drop out of the water column so the water you finally release is clearer.

• Discharge below the algae layer. A sub-surface or mid-depth outlet avoids the algae-dense surface water that fails TSS.
• Rest a polishing cell. A quiescent settling period lets suspended algae drop out before discharge.
• Mix to prevent the bloom. Continuous circulation limits how dense summer algae can get in the first place.
• Protect retention time. Solids that settle only stay settled if flow is not racing across the cell (see short-circuiting below).

Retention time is the quiet variable

None of these tactics work if wastewater is racing from inlet to outlet faster than the lagoon was designed for. When flow short-circuits, effective treatment volume shrinks, solids do not have time to settle, and any bloom you were counting on to settle out simply leaves with the flow. If your TSS is high and your retention time feels short, read how lagoon short-circuiting steals retention time before you invest in anything else. Baffling and mixing frequently recover treatment you already paid to build.

Ammonia is the companion problem here. The same warm, well-oxygenated conditions that help control summer algae are also what nitrifying bacteria need, so operators fighting TSS are often fighting an ammonia limit at the same time. The five conditions for nitrification explains why dissolved oxygen and reducing BOD first are the levers that move ammonia, and why they overlap with your TSS strategy.

Why not just kill the algae with a chemical?

It is tempting to reach for a copper-based algaecide and knock the bloom down fast. The problem is what happens next. Copper sulfate kills the algae, and the dead cells then sink to the bottom, rot, and release the nutrients that fuelled them straight back into the water, which feeds the next bloom. You have not removed the solids or the nutrients. You have relocated them to the sediment and set up the next cycle, while copper accumulates in the bottom over time.

That is why circulation, not a chemical kill, is the durable control. Mixing prevents the bloom from getting dense in the first place and keeps the water column aerobic, so you are managing the cause rather than paying to fight the same symptom every summer. Over several seasons, a mixing and biological approach is usually the lower lifetime cost as well as the more reliable one.

Off-grid mixing for remote and prairie lagoons

Not every lagoon has a convenient power drop. Many rural, prairie, and remote community lagoons sit far from a service line, which is exactly the context where operators assume mixing is off the table. It is not. Wind-driven aeration runs off-grid with no power line, delivering the same bottom-up circulation from the floor of the cell. If your site has no reliable electricity, off-grid windmill aeration can provide the mixing that controls summer solids without a utility connection.

Sizing an off-grid system follows the same logic as a wired one: it is driven by lagoon surface area, depth, and your treatment goal, then adjusted for how windy the site is. Rather than guess, talk to our wastewater team for a free sizing recommendation. You can also see the full range of municipal options on our wastewater lagoon and lift station page.

Can a facultative lagoon meet secondary limits?

It can, but you have to respect where the constraint actually lives. A facultative lagoon typically has no trouble with BOD; its weak point is summer TSS driven by algae. If your permit targets secondary treatment levels, the realistic path is a combination: mix to suppress the bloom, discharge below the surface to avoid the algae-rich layer, rest a polishing cell where the system allows, and protect retention time so solids settle and stay settled.

Where those tactics are not enough on their own, additional polishing or filtration may be part of the answer, but most operators find they can move the summer number meaningfully with mixing and discharge changes before they consider a capital rebuild. The starting point is understanding that you are managing an algae problem, then sizing the mixing to your specific lagoon. We are happy to review your cell layout, depth, and effluent numbers and give you a recommendation at no cost.

A summer TSS action plan

1. Confirm the diagnosis: if TSS and BOD climb together in warm, sunny months while incoming load is steady, you are looking at an algae problem, not a load problem.
2. Check retention time and short-circuiting first, because settled solids only stay put if flow is not racing across the cell.
3. Move the effluent draw below the surface so you discharge clearer water from beneath the algae-rich top layer.
4. Add continuous bottom-up mixing to limit bloom density and keep the water column aerobic.
5. Where the system allows, rest a polishing cell to let suspended algae settle before discharge.
6. Avoid copper algaecides that kill algae which then sink, rot, and refuel the next bloom.
7. Size the aeration to your lagoon volume, depth, and goal, and ask for a free recommendation if you want a second set of eyes.

The bottom line

A facultative lagoon that meets BOD but fails TSS in summer is not broken. It is doing exactly what it was designed to do, and the algae it grows to help treat the water is the very thing pushing your solids over the limit. The fix is not a bigger chemical budget or a plant rebuild. It is managing the bloom: mix the water column, discharge below the surface, protect retention time, and let biology work in aerobic conditions. If you want help matching that to your site, reach out for a free sizing and cost review.