Fish farm or aquaponics: keep aeration running in a power cut

Keep the blowers, air pumps, main circulation pump and alarm on a source that takes over by itself, within seconds. That can be a battery system running as an island, a standby generator, an emergency oxygen tank, or a mix. Our guide to emergency power supply for your business compares UPS, generator and battery in general. For ponds and sites far from the grid, an off-grid system with solar panels and a battery runs the aeration every day. It keeps going when the grid fails.

You run a trout farm, a recirculation hall with perch or tilapia, or an aquaponics greenhouse. The aeration runs day and night, and the pumps never stop. Then one night the grid drops, and the only question that matters is how many minutes you have.

The risk is not the cut itself, but the time between the cut and the moment someone restores oxygen. An alarm that runs on the same power as the blowers. A generator that nobody test-started this month. A cut on a warm summer night, when the water holds the least oxygen. A fish farm can lose a whole stock in one night, and a fish is not an animal you can lead outside for fresh air.

Ponds are not new to us. In Beernem, a research and education site runs fully off-grid, with a private home, greenhouses and ponds on the same system. Solar panels (17 kWp) and a battery carry the site. An emergency generator only helps out when use is high or the sun is weak.

How long can fish survive when the aeration stops?

Often only hours, and sometimes less. The FAO manual on small-scale aquaponics says fish may die within hours at low dissolved oxygen (DO, the oxygen dissolved in the water). It calls DO the water quality factor with the most immediate and drastic effect.

How much time you have depends on the fish per cubic metre, the species, the water temperature and whether the water keeps moving. Three facts make the clock run faster:

• Warm water holds less oxygen than cold water. The FAO shows that the oxygen water can hold drops as the temperature rises. A summer night is the worst moment for a cut.
• Ponds lose oxygen at night. By day, algae and plants add oxygen through photosynthesis. At night they stop and use oxygen themselves, the FAO pond manual explains.
• Trout need more than carp. The FAO names 2.5 to 3 mg/L as the level where trout die in large numbers. Carp and tilapia can tolerate 2 to 3 mg/L, but 5 to 8 mg/L is the healthy range for fish, plants and bacteria in aquaponics.

Fish at the surface gasping for air are a late sign. The FAO calls it an emergency that needs attention at once.

Why is a recirculation system or aquaponics more at risk than a pond?

Because the stocking density is higher and the water only stays clean while it flows. A recirculation system (RAS) is a closed system that cleans and reuses the same water. There, a power cut stops two things at once: the oxygen supply and the cleaning.

The FAO guide to recirculation aquaculture spells it out. Get the main pumps running again as fast as possible. The ammonia the fish excrete builds up to toxic levels when the water no longer flows over the biofilter. The guide asks for the water flow to be back "within an hour or so".

In aquaponics, the design matters too. The FAO calls nutrient film channels (NFT, plants in a thin film of water) "vulnerable to power outages". Deep water culture, with plants floating on rafts, can withstand short interruptions because the large volume of water buffers the changes.

Growing vegetables above the fish in a greenhouse? Then the fish side decides the back-up, not the plants. Our article on a battery in a greenhouse or nursery covers the lighting and heating side.

Does the law require backup power or an alarm for a fish farm in Belgium?

Not in the way it does for pigs and poultry. The EU directive on farm animals counts fish as farm animals. Its detailed rules, including the one on a back-up and an alarm for ventilation, do not apply to fish.

Article 4 of Directive 98/58/EC applies the annex with those rules to animals "other than fish, reptiles or amphibians". What remains is the general duty in Article 3. The keeper must take all reasonable steps to ensure the welfare of the animals in his care.

For a barn with mechanical ventilation, the rules are much stricter. Our article on ventilation in a pig or poultry barn during a power cut explains the duty to have a back-up and an alarm there.

Which alarm warns you when the power or the oxygen fails?

An alarm that calls a person, not only a siren in the hall. The FAO guide to recirculation says the control system must have an alarm that calls the staff before a major failure happens. It recommends a reaction time under 20 minutes, even where automatic back-up is installed.

A good alarm on a fish farm watches at least these points:

• Power supply: grid lost, or one phase missing on a three-phase site.
• Oxygen: an oxygen probe in the tanks, with a low limit.
• Water: the level in the tanks or sump, and the flow of the main pump.
• The back-up itself: battery low, generator failed to start, oxygen tank nearly empty.

Give the alarm its own battery, so it keeps calling when the grid is gone. The FAO guide lists three weekly or monthly routines: an alarm test, a test start of the generator and a check of the emergency oxygen. Write the date down each time.

Pure oxygen, generator or battery: which backup keeps the fish alive?

The FAO guide to recirculation names pure oxygen as the number one safety precaution, with a fuel-driven generator behind it. A battery system adds what those two lack: a takeover without any gap, and use on normal days.

The emergency oxygen system in the FAO guide is simple: a holding tank of pure oxygen and diffusers in all tanks. When the power fails, a magnetic valve opens and oxygen flows to each tank. Set the flow in advance so the tank lasts long enough to fix the failure.

A battery system with an island function (it runs on its own, without the grid) keeps blowers and pumps turning without a gap. According to Victron, its MultiPlus-II takes over in less than 20 milliseconds. The battery also bridges the seconds a generator needs to start. Our article on a manual or automatic transfer switch for your generator explains that handover.

How big should the backup power for aeration and pumps be?

Big enough for the loads that keep the fish alive, for as long as a cut can last on your line. List those loads first; the arithmetic is the easy part.

A worked example with round figures. A blower of 1.5 kW, a circulation pump of 2.2 kW and 0.3 kW of controls and alarm draw 4 kW together. For eight hours that is 32 kWh. Add a margin, because a battery loses some capacity as it ages. Measure your real consumption before anyone sizes a system.

Can solar panels and a battery run the aeration of a fish farm every day?

Yes, and that is what makes the battery pay. Aeration runs 24 hours a day. Solar panels cover it by day and charge the battery; the battery carries the night. When the grid fails, nothing changes for the fish.

That is how the site in Beernem works: fully off-grid, with ponds and greenhouses on the system. The generator is kept for periods of high use or little sun, and the goal is to use it as little as possible. On the farm in St-Georges-sur-Meuse, the street had no connection and the farm ran fully on a generator. With solar panels and a battery, the generator now only serves as back-up.

On a site with a grid connection, a semi off-grid system does the same work with the grid as charger. When the public grid fails, the site keeps working on the off-grid installation. Our article on a generator with a battery explains how the two share the work. Pumping water from a well or a ditch on solar? Read whether an irrigation pump can run on solar, with or without a battery.

When is a battery not the right backup for a fish farm?

When the cut can last for days and the loads are large. A pond farm with heavy aerators spread over hectares needs fuel logistics or pure oxygen, not a battery sized for days.

Two other cases. A single hobby pond or one tank is better served by a small battery air pump. And a large recirculation farm with its own medium-voltage cabin has its own engineers; there, a battery is one part of a larger design.

What happens when you ask us to secure your fish farm?

We start with what must keep the fish alive, and for how long.

1. Step 1: Analysis and sizing. We list the blowers, pumps, controls and alarm, and look at what they draw. If data is missing, we measure your consumption first.
2. Step 2: Technical design. We choose the battery, the inverter power, the solar panels where they fit, and the handover to a generator.
3. Step 3: Installation and commissioning.
4. Step 4: Monitoring and support. You follow the system in the app and the VRM portal, with an alert when the battery runs low.