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Keys to water quality management in RAS

RAS water quality issues and how to address them


April 14, 2020
By From the experts at The Freshwater Institute

Topics
Clogged media due to calcium carbonate precipitate in a CO2 stripper. Remove media and replace with clean media.

Intensive RAS offer the advantage of providing an optimized environment for the fish. However, water quality conditions in RAS will deteriorate due to failing equipment, deferred maintenance or cleaning schedules, or improper management. Basic troubleshooting knowledge can help correct water quality issues in a timely manner.

Low dissolved oxygen
Measure and monitor water flow rates and adjust. Reduced flow rates through oxygenation unit processes will result in lower than expected dissolved oxygen delivery to fish culture tanks. Valves and/or pump speeds should be adjusted to achieve design flows or replace failing pumps.

Check liquid levels in liquid oxygen tanks or percent gas phase from oxygen generators product gas daily. If liquid oxygen supply tanks are emptying or under-delivering, oxygen cannot be transferred to the water. Have the supply tanks refilled immediately or service generator equipment.

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Confirm that gas flow is unobstructed. A closed valve or clogged supply line, especially at the entry of oxygenation unit processes, can prevent or impede oxygen gas flow. Confirm solenoid valves are functioning properly and clean supply lines as needed.

Check for clogged unit process distribution plates. Water overflowing an oxygenation unit process will dilute dissolved oxygen concentrations downstream of the unit process. Clean entries to these unit processes and clear obstructions from any distribution plate holes to ensure the full flow or entire design flow is being treated.

High carbon dioxide
Measure and monitor water flow rates and adjust. Reduced flow rates will result in higher than expected carbon dioxide return to the fish culture tanks. Valves and/or pump speeds should be adjusted to achieve design flows or replace failing pumps.

Check fan or blower performance. A failed or underperforming fan reduces the gas to liquid ratio in the CO2 stripper and reduces removal. Replace the underperforming fan or blower.

Confirm stripper media is not clogged. Clogged media can cause water to pool in a stripping tower and prevent blown air from exiting a stripping tower. Clean or replace the media to allow unrestricted water flow through the column.

Check that outside exhaust vents are unrestricted. Blocked exhaust vents can impede air flow through a stripping tower similarly to pooling water. Open any valves or clear obstructions to achieve full air flow.

Check and reduce ambient air CO2 concentrations. Ambient inside air with high gas-phase CO2 concentrations results in a lower concentration gradient than outside air and could reduce removal efficiency when inside air levels rise. Measure CO2 concentrations in the air and increase building ventilation, if necessary.

High ammonia or nitrite
Monitor and adjust alkalinity. Nitrification within the RAS biofilter consumes alkalinity. Alkalinity needs to be replenished in tight systems by the addition of sodium bicarbonate or another source to maintain alkalinity greater than 100 mg/L as CaCO3.

If alkalinity is low, add approximately 0.25 kgs of sodium bicarbonate per kilogram of feed to maintain alkalinity and adjust future dosing to meet the system’s specific needs.

Confirm biofilter media levels. Insufficient amounts of media in the biofilter will not provide enough surface area to “house” a bacterial population needed for adequate nitrification. Measure the static media bed height. Add additional media, if necessary.

Correct “dead spots” or hardened sand in the biofilter. Unagitated or unfluidized media in a biofilter can become anoxic, killing established nitrifying bacteria and reducing the total surface area available. Adjust blower plate position and output to agitate all areas of the filter or break-up hard media with a long-handled brush.

Excessive solids
Monitor and correct culture tank hydraulics. Water in a circular culture tank should have enough rotational velocity to concentrate solids to the middle bottom drain of the tank for quick and easy removal from the tank.

Measure inlet and bottom drain flow rates and make corrections as needed. The orientation and velocity of the inlet water may need to be adjusted to create the rotation desired. Increase velocity by reducing the size and/or number of holes in the inlet.

Check for proper solids filter operation. Backwash cycles should be running properly and directing solids to the correct drains or lift stations. Automatic drains should be activating on schedule and removing solids. Correct abnormal filter operations and confirm solids are exiting the system.

Look for holes or tears on filter seals or media screens. Holes in filter media screens will allow solids to pass mechanical filters and can reduce or prevent proper backwash cycles from removing solids from the RAS. Replace any worn screens or plug any holes. Replace any leaky or worn seals.

Flush drain lines to clear collected solids. Flushing collected solids away from the system ensures that they cannot reenter the system water column. Regularly flushing drain lines also helps maintain appropriate flow rates by eliminating potential restrictions. Open sump bottom drain valves or pull standpipes until the water runs clear.

For more information about depuration visit FreshwaterInstitute.org.


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