Wastewater plants use a lot of power during daylight hours, and many of them have acres of water that nobody else wants. That makes treatment lagoons one of the most logical places to float a solar array. It also makes them one of the easiest places to get the project wrong, because a lagoon is not a pond. It is a treatment process with a discharge permit attached.

This guide is for plant operators, public works directors and the EPCs who pitch them. It covers where the power case is strong, where the biology pushes back, and what to settle before anyone orders racking.

Why wastewater plants keep looking at their ponds

The load is large and it runs all day. According to the EPA, drinking water and wastewater plants are often the largest energy users in a municipal government, at 30 to 40 percent of total municipal energy consumption, and energy is often 25 to 30 percent of a utility's operation and maintenance costs.

Aeration drives much of that. The University of Michigan's Center for Sustainable Systems puts aeration at 25 to 60 percent of treatment plant energy use. Blowers and aerators run through the day, which lines up well with solar output and favors a behind-the-meter design that offsets on-site load rather than exporting to the grid.

The sites are also numerous. EPA's lagoon inventory identifies more than 4,500 discharging lagoon systems that are publicly or semi-publicly owned and hold NPDES permits. Many serve small and rural communities where land for a ground-mount array is limited or already spoken for.

The catch: some lagoons need the sun

This is the point a skeptical operator will raise first, and they are right to.

In a facultative lagoon, algae use sunlight to produce the oxygen that aerobic bacteria need to break down waste. EPA describes this directly: the sun provides light for algae to grow, which creates oxygen needed for aerobic bacteria. Shade a large share of that surface and you are changing the treatment process, not just adding a power plant.

Mechanically aerated lagoons are a different story. Their oxygen comes from blowers or surface aerators, so shading matters less to treatment. Field data on FPV over human-made ponds is still thin, but it points the same way. A 2025 study in Frontiers in Water monitored four floating arrays on Florida and California treatment, stormwater and irrigation ponds (none were wastewater lagoons) and found within-pond differences in algae and water quality between covered and open water were largely minimal. The exception worth noting: at one stormwater pond, dissolved oxygen was significantly lower under the panels in spring, while the two aerated sites showed little trend.

The practical reading:

Lagoon type	Oxygen source	FPV fit
Facultative (unaerated)	Algae and sunlight at the surface	Weak. Covering the surface can reduce treatment. Needs process engineering review before any design work.
Partial-mix or complete-mix aerated	Blowers or surface aerators	Stronger. Shading has less effect on treatment, but layout must clear aerators and mixers.
Polishing, holding or storage cells	Varies	Often the strongest candidates. Confirm with the plant engineer and permit writer.
Anaerobic	None required	Possible, but gas, odor and corrosion issues need their own review.

Start with the cells where sunlight is not doing treatment work.

What to settle before design

1. The permit and the process

Discharging lagoons operate under NPDES permits with effluent limits. Adding a structure to a treatment cell can change how it performs, so the plant's engineer of record should confirm the array will not affect compliance, and the state permitting authority should be consulted early. The federal wetlands question that applies to farm and stormwater ponds is a separate issue. Our permits overview covers that framing, but on a treatment plant the permit that matters most is usually the one already on the wall. Local building, electrical and interconnection permits apply either way.

2. The liner

Many lagoons are lined with compacted clay or a synthetic membrane. Anything that penetrates or abrades the liner is a problem that is expensive to fix and may itself be a compliance issue. That rules out driven anchors in the bottom of most lined cells. Bank anchoring or deadweight systems that sit clear of the liner are the usual approach. Ask for the liner as-builts before anyone draws a mooring plan.

3. Aerators, mixers and sludge

Map every aerator, mixer, inlet, outlet and baffle curtain. The array needs clearance from all of them, including room for aerators to be pulled for service. Lagoons also accumulate sludge and eventually need it removed. The layout should leave a way to dredge or pump without dismantling the array, or the array should be designed to be moved.

4. The environment on the water

Wastewater cells can hold corrosive gases and more aggressive water chemistry than a clean irrigation pond. Ask your supplier what materials the floats, fasteners and connectors use, and specify stainless or coated hardware where the plant engineer recommends it. See our components page for how the Gen3 kit is put together.

5. Access and safety

Operators already have safety rules for working near open water. The array needs a walkway plan, lockout points for the DC side and a clear rule for who is allowed on the floats. Put this in the O&M plan, not just the drawings.

How the money usually works

Many treatment plants are owned by a city, county, district or authority, which pays no federal income tax. Under elective pay, state and local governments and other tax-exempt entities can receive eligible clean energy credits as a direct payment instead of needing tax equity.

The timing is tight. Under the One Big Beautiful Bill Act, wind and solar projects claiming the technology-neutral credits must have begun construction by July 4, 2026 or be placed in service by December 31, 2027. That July date has passed, so a plant starting now is working toward the placed-in-service deadline. Our 2027 deadline timeline walks through what is still realistic, and our FEOC page explains the sourcing rules that also apply.

Public procurement adds its own calendar: engineering review, council or board approval, bidding and sometimes grant conditions. Build that into the schedule before promising anyone a credit.

A short checklist for operators and EPCs

Identify which cells are facultative, aerated or storage, and rule out the ones that depend on sunlight.
Pull the NPDES permit, liner as-builts and aerator layout.
Get the plant engineer's written view on process impact.
Talk to the state permitting authority before design, not after.
Size the array to daytime plant load using interval meter data.
Design moorings that stay clear of the liner and leave room for sludge removal.
Confirm the credit path, the placed-in-service deadline and the procurement schedule.

For a first pass at structure cost and payback, use the ROI calculator and the racking cost guide, then read how the system goes together.

Bottom line

Wastewater lagoons pair a large daytime load with open water the plant already controls, which is a strong starting point. But facultative lagoons rely on sunlight to treat waste, so the right target is usually an aerated, polishing or storage cell, not the whole plant. Settle the process question, the permit and the liner first. The racking is the easy part. If you want a second set of eyes on a specific site, contact us or find a dealer.

This article is general information, not tax or legal advice. Consult your tax advisor, engineer of record and permitting authority about your specific project.

Sources: EPA: Energy Efficiency for Water Utilities · EPA: Lagoon Wastewater Treatment Systems · University of Michigan Center for Sustainable Systems: U.S. Wastewater Treatment Factsheet · Cagle et al., Frontiers in Water (2025) · IRS: Elective Pay and Transferability · Latham & Watkins: Treasury Guidance Narrows Beginning of Construction Rules