Water Delivery and Recirculating Aquaculture Systems (RAS)

Water Delivery and Recirculating Aquaculture Systems (RAS)

A recirculating aquaculture system keeps water moving through a closed loop rather than discarding it after one pass. Water flows from fish or larval tanks, through mechanical and biological filtration, through aeration or oxygenation, and back to the tanks. Understanding every component of that loop — and being able to respond when something goes wrong — is a core hatchery skill.

How Water Moves Through a RAS

Pumps are the heart of any RAS. A centrifugal pump draws water in through the impeller eye and accelerates it outward to create pressure that pushes flow through the system. The relationship between flow rate and pressure is described by a pump curve chart: as back-pressure rises (for example, when a filter becomes loaded), flow rate drops. Always consult the pump curve and the manufacturer manual when troubleshooting low flow.

Water is distributed to tanks through header pipes and standpipes. Gate valves on each standpipe let you throttle flow to individual tanks. Turning a gate valve clockwise reduces flow; counterclockwise opens it. Always move valves slowly — sudden changes create water hammer and can stress fish.

Measuring and Adjusting Flow Rate

The simplest field method is the bucket-and-stopwatch test: hold a graduated bucket under the standpipe outlet, time how many seconds it takes to collect a known volume, then calculate flow rate. For example, if 5 gallons collects in 20 seconds, the flow rate is 5 ÷ 20 = 0.25 gal/s, or 15 gal/min. Compare your measured value to the posted target flow for that tank and adjust the gate valve as needed. Re-measure after every adjustment and record both the before and after readings in the lab log. Your target is to be within 10% of the posted value.

Biofilters: Function and Backwashing

Biological filtration converts toxic ammonia (excreted by fish) to nitrite, then to the less toxic nitrate, through colonies of nitrifying bacteria that grow on filter media. Two common media types in hatcheries are bead filters (floating plastic beads) and drum filters (rotating fine-mesh screens that handle solids mechanically).

Over time, solids accumulate and reduce flow through the biofilter. A backwash cycle flushes accumulated solids out while preserving as much bacterial biomass as possible. Always follow the written SOP exactly:

1. Notify the hatchery manager before isolating any filter.

2. Close the inlet valve to isolate the filter from the main flow.

3. Open the backwash port and run the backwash per the manufacturer cycle time — do not guess.

4. Close the backwash port and slowly reopen the inlet valve.

5. Collect a water sample immediately before and after backwash and measure turbidity. A lower turbidity reading after backwash confirms the solids load was reduced.

6. Record all readings, times, and any observations in the lab log.

Never skip isolation steps. Backwashing a filter while it is still in full service can send a turbidity pulse directly to larval tanks, causing gill damage or mortality.

Pump Failure Response

Header pump failure is the most time-critical emergency in a RAS. Fish can survive minutes to hours without filtration depending on density and temperature, but they need water movement and dissolved oxygen continuously. When a pump alarm sounds or flow stops:

1. Confirm the failure: check the alarm panel for which pump has tripped.

2. Switch to the backup pump: locate the backup pump isolation valve or switch; open the backup outlet valve, then start the backup pump motor.

3. Verify flow is restored: observe standpipe flow and check the flow meter if installed.

4. Notify the hatchery manager immediately — even if flow is restored. The underlying fault must be diagnosed.

5. Document the event: record time of alarm, time of switchover, time of notification, and any observations about the failed pump.

The entire response must be completed within five minutes. Practice the procedure until it is automatic.

Impeller Inspection and Airline Clearing

During scheduled pump maintenance, always follow lockout/tagout (LOTO) procedures before opening any pump housing. Isolate electrical power at the breaker, apply a lockout device, and verify that the pump cannot restart before reaching inside. Impeller inspection checks for wear, debris, or cavitation damage — a pitted or cracked impeller reduces efficiency and can fail suddenly.

Airlines (diffuser lines, air-stone tubing) clog with biofilm over time. Clear them by briefly back-purging with clean air or by soaking in a dilute acid or enzyme solution per the facility protocol. Never blow air lines with your mouth.

Safety Points

• PPE: wear waterproof apron, nitrile gloves, and rubber boots whenever working around pumps, drains, and biofilter ports. Eye protection is required when collecting backwash samples or handling any chemical cleaner.
• Slip hazard: wet hatchery floors are extremely slippery. Walk; never run.
• Electrical hazard: water and electricity are present in the same space. Never handle electrical panels with wet hands. Report any damaged cords immediately.
• LOTO: mandatory before any pump or mechanical component is opened for inspection. No exceptions.
• Confined-space awareness: some sump pits and filter chambers can accumulate low-oxygen air or hydrogen sulfide. Never enter without a gas monitor and a trained standby person.