Running a lagoon system: the checks that actually matter

You inherit three ponds, a diffuser that may or may not be running, and no manual telling you what normal looks like. That is the ordinary starting point for a lagoon operator. Most lagoon literature is written for the engineer who designed the pond, not the person walking the berm every week.

About a quarter of the country's community wastewater systems run lagoons instead of a mechanical plant, most of them small. One pond handles both settling and secondary treatment at once, which is a big part of why a lagoon costs less to build and run than the alternative.

What you need day to day is simpler than an engineering manual: what healthy looks like, what the walkdown covers, what a color or smell is telling you, how the pond turns over each spring and fall, when sludge is worth digging out, and the handful of upsets behind most compliance letters.

The design math and exam-style formulas live in a separate guide, linked at the end.

What does a healthy lagoon look like?

A healthy lagoon reads clean at a glance. Clear water with no odor and almost no suspended material is normal. So is brown water with an earthy smell and small flocs, and neither needs a fix.

If you run a facultative system, the primary cell should hold a deep, sparkling green, and the later cells should carry good dissolved oxygen. Wind should raise a visible ripple across the surface. No wave action can mean the surface has gone anaerobic, or that an oily film is sitting on top.

Color and smell are the fastest read you have on a lagoon, faster than any lab result. Learn what your own ponds look like on an ordinary day, because that baseline is what tells you something changed.

Walking the berm: what the routine check covers

EPA's pond operations manual puts the daily and weekly walkdown at about one to two hours for a conscientious operator, leaving the rest of the shift for lab work and everything else on the list. Walk or drive the perimeter of every cell and look for:

• Scum building up on the surface or at the discharge boxes, checked daily
• Anaerobic signs, odor, black color, floating sludge, or gas bubbles breaking the surface, checked daily
• Burrowing-animal damage in the dikes, checked weekly
• Weeds growing in the water, checked weekly
• Dike erosion or dike leakage, checked weekly
• Fence damage, checked weekly

Clean the inlet screens and clear trash daily. Check the inlet flow meter and float well on whatever schedule your permit or equipment calls for.

Keep the dikes seeded with grass above the waterline and mow them regularly. Vegetation control does double duty: it prevents erosion and insect problems, and it keeps the surface open to the wind. That wind is part of what keeps a pond from going anaerobic.

None of this is recreational water. Ponds have drowned operators, and a liner gets slick and hard to climb out of once it is wet. Work in pairs near open water, fence the site, and post warning signs.

Why do lagoons flip color and smell each spring and fall?

A facultative lagoon is layered, an oxygen-rich top over an anaerobic bottom, and twice a year those layers invert. Fall cooling triggers the first turnover, and spring overturn follows directly after the ice goes out.

That turnover stirs the bottom, and the pond runs turbid, unstable, and smelly for a stretch. It settles back into its normal layers once temperatures stabilize.

Winter stratification sets in once the water cools below about 4 degrees Celsius and its density drops. In many northern states, a pond returns to normal three to four weeks after ice breakup, which is when operators plan seasonal discharge.

Nitrification also slows or stops through winter on a cold-climate lagoon and typically comes back once the water warms.

Aerated ponds usually keep discharging right through winter. An ice-covered facultative pond often cannot, and must store what it holds until permit conditions return.

How much sludge is normal, and when do you dig it out?

Most lagoons only need solids removed every 10 to 20 years, so a little sludge buildup is not an emergency. Watch it, do not chase it.

The field trigger you act on is depth, not the calendar. EPA's lagoon troubleshooting guidance puts that trigger at about 25 percent of the operating depth, measured at 24 to 36 points across the cell. A 60-inch lagoon, for example, hits that mark at 15 inches of sludge.

When you do plan a cleanout, contact your regulatory agency before you start. Most states want a written plan covering the removal method, liner protection, sludge testing for volatile solids and metals, and final disposal. Land application of the sludge may need its own separate permit.

Pumping is the removal method most operators use. Take care with it, since liner damage during a cleanout can mean repair, replacement, or years of extra monitoring.

Reading upsets by color and smell

Most lagoon upsets show up in the pond's color and odor before they show up in a lab result:

• Grey-black floating sludge, gas bubbles, and a septic smell point to organic overload, low oxygen, or raw wastewater short-circuiting through untreated. Add aeration, add baffles or another cell, fix the inlet-outlet design, recirculate, or remove accumulated sludge.
• Green water with a grassy or earthy smell is an algal bloom, usually paired with a pH above 9 and a long detention time. Recirculate, add a settling pond, or move to land discharge.
• Floating mats of blue-green scum with a fishy smell are a blue-green bacterial bloom. Treat it the same way as an algal bloom, but be careful about pulling a cell out of service to fix it. That move can make the bloom worse instead of better.
• Red streaks with heavy Daphnia usually follow an algal bloom. Run the aerators longer and recirculate.
• Solid red or pink water with a rotten-egg smell means the pond has gone anaerobic under serious organic overload or under-aeration. Increase aeration and recirculate.

Two numbers back up what your nose already tells you. Effluent pH climbing above 9.0 confirms an algal bloom, and it matters because ammonia gets more toxic to aquatic life as pH rises. Weed or duckweed cover crossing 40 percent of the surface signals organic overload worth acting on before it spreads.

Short-circuiting is worth checking for directly rather than guessing at it. Float oranges or tennis balls at the inlet, or run a dye test, and time how long they take to reach the outlet. That shows whether water is skipping straight through instead of taking its full detention time.

Single-cell lagoons are the most exposed to this problem.

When aeration is part of your system

If your lagoon has mechanical or diffused aeration, dissolved oxygen is the number to watch. Aerated lagoons should hold at least 2 mg/L of dissolved oxygen in every cell as a normal target. Even at the heaviest loading, an aerated pond should not fall below 1 mg/L throughout.

If your lagoon is partial-mix, favor nighttime for aerator runtime. Algae make their own oxygen off sunlight during the day, so aerating on top of that daytime supply mostly off-gasses it and wastes the power.

For diffused air systems, check the blower daily and watch the aeration pattern for dead spots or a ruptured line. Measure dissolved oxygen at a few points in the pond each week to confirm it is distributing evenly. Keep large debris away from any aerator so nothing fouls it.

When to bring in a second set of eyes

Some fixes are yours to make on the spot: more aeration, a cleared inlet, a mowed berm. Others are not.

If a bloom, a short-circuiting cell, or a compliance problem does not clear up with the moves above, it is time for help. Bring in your state's lagoon program or an outside engineer before it becomes a violation.

The same goes for anything that touches the liner: sludge removal, dike repair near the seal, or a suspected leak. A damaged liner is expensive to fix, and worse to leave for later.

Multi-cell systems also give you tools most operators forget they have. You can move water from cell to cell to fix a low-oxygen spot, isolate a cell that has gone anaerobic, or rest a cell to recover.

You can also recirculate water from the last cell back to the first to raise its oxygen and reseed it with algae. Reach for those tools before assuming a problem needs an outside fix.

For the detention-time math, loading-rate formulas, and design depths behind these operating numbers, see the fixed-film and lagoons guide. If a pond upset turns into an unauthorized discharge, the SSO response and reporting guide covers what happens next.