Design implication of carbon dioxide production within biofilters contained in recirculating salmonid culture systems

S.T. Summerfelt, M.J. Sharrer-2004 
Aquacultural Engineering, 32(1): 171-182 
Abstract:

The objectives of this research were (1) to experimentally quantify the dissolved carbon dioxide (CO₂) production within a biofilter (2) to use stoichiometry to estimate the CO₂ production that would be expected across the biofilter due to nitrification and heterotrophic oxygen (O₂) demand (3) to compare the relative amounts of CO₂ produced by the biofilter bacteria to the CO₂ produced by the fish that are cultured in the recirculating system, and (4) to discuss implications for recirculating system design that must be considered when it is recognized that a large fraction of the CO₂ produced in a recirculating system is produced by the biofilter.

During this study, the fluidized-sand biofilter in the commercial-scale recirculating salmonid culture system at the Conservation Fund Freshwater Institute (Shepherdstown, West Virginia) produced 4.1±0.2 mg/L of CO₂ while removing 0.51±0.02 mg/L total ammonia nitrogen (TAN) and removing 3.8±0.2 mg/L dissolved O₂.

Taking into account the overall stoichiometric relationship between the ammonium, bicarbonate, and oxygen consumed and the cell mass, nitrate, water, and carbonic acid produced during nitrification and nitrifier synthesis, nitrifying bacteria consume 4.6 mg/L of oxygen while producing approximately 5.9 mg/L of CO₂ for every 1 mg/L of TAN consumed. In addition, the estimated respiration of heterotrophic bacteria within the biofilter, as determined by the measured decrease in dissolved oxygen concentration across the biofilter minus the estimated DO consumption due to nitrification, accounts for another 1.38 mg/L of CO₂ production for every 1 mg/L of DO consumed in the biofilter. Accounting for both TAN and DO removal measured across the biofilter, the biofilter’s CO₂ production can be estimated as 5.0±0.3 mg/L, which is reasonably close (approximately 20% greater) to the CO₂ production that was measured experimentally. Because the fish produced 6.9±0.4 mg/L of CO₂ that was measured during the same period, then the biofilter actually accounts for 37% of the total CO₂ produced within this recirculating salmonid system. Therefore, the CO₂ stripping unit should be placed immediately after the biofilter simply as common sense to optimize water quality. If the stripping unit were placed before the biofilter, then the fish would experience a CO₂ concentration that would be at least 20% greater than if the stripping unit were placed immediately after the biofilter.

(The Conservation Fund’s Freshwater Institute, 1098 Turner Road, Shepherdstown, WV 25443, USA, e-mail: s.summerfelt@freshwaterinstitute.org)

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