How to Start a RAS Biofilter

Biofiltration to oxidize ammonia excreted by fish is critical for an effective RAS. Water treated in a biofilter is contacted by Nitrosomonas and Nitrobacter bacteria colonized on  media with high surface area to oxidize ammonia to nitrite and nitrite to relatively nontoxic nitrate. 

Extended shutdowns or disinfection will kill the bacteria in a biofilter, requiring recolonization before fish can be safely stocked into a RAS. Restarting a biofilter can be accomplished with simple preparation and close water chemistry monitoring. 

Starting up a RAS from a shutdown or disinfection can cause solids to re-suspend and shock load a system. Brush settled solids and allow solids filtration to clean the water prior to biofilter preparation. The RAS can be "short-cycled" by bypassing the culture tank or nonessential unit processes once relatively clear of solids. This will reduce the volume of the RAS and the amount of chemical needed to start the biofilter. If the water in the RAS is usually chilled, this is a good time to consider a modest temperature increase to reduce the total time necessary to recolonize the biofilter. After RAS configuration is set, close down the water leaving and entering the system to operate on as close to 100% recirculation as possible.  

The water quality can now be managed to create a healthy environment for the bacteria. During the process, maintain pH of 7–8, alkalinity above 150 mg/L, and dissolved oxygen near saturation entering the biofilter. When water quality is conducive to biofilter start-up, an ammonia source can be added to the water. Begin to monitor the water quality daily for conversion of ammonia to nitrite. Once this conversion begins, add ammonia to feed the Nitrosomonas bacteria and monitor pH, dissolved oxygen, and alkalinity. The conversion of nitrogenous wastes consumes alkalinity, requiring continued addition of an alkalinity source to maintain concentrations above 150 mg/L.  

Once ammonia and nitrite are completely converted to nitrate, return the RAS to normal configuration and slowly acclimate the water to the target temperature set point. At this time, challenge the biofilter by adding an amount of ammonia equivalent to what will be produced by the expected feed load upon stocking fish. If the biofilter can treat this load and maintain safe concentrations of ammonia and nitrite, the RAS should be ready for fish. This step ensures that the newly stocked fish are not exposed to stressful water quality and can be grown to their full potential once stocked. Continue feeding ammonia and managing water quality until the RAS is stocked.

Find out more at freshwaterinstitute.org.