Calculating the Feed Oxygen Demand (FOD) of Aquafeeds

Abstract

The biological oxygen demand of aquafeeds will be called the feed oxygen demand (FOD). It is the amount of dissolved oxygen needed to oxidize organic carbon and ammoniacal nitrogen applied to culture systems in feeds and not recovered in biomass of the culture species at harvest.   The following equation can be used to estimate FOD:

FOD =[C_f- (FCE  C_a)] 2.67 + [N_f-(FCE N_a)] 4.57

where, C_f, N_f, C_a, N_a = decimal fractions(%÷100) of carbon and nitrogen in feed and culture animals, respectively; FCE = feed conversion efficiency (1÷FCR). The FOD value will vary with species, feed quality, feeding practices, and feed conversion efficiency, but calculations suggest that FOD usually will be 1.2 to 1.3 kg O₂/kg feed.

The biological oxygen demand or BOD is widely recognized as an important water quality variable. It is used mainly as an index of the pollution strength of municipal, industrial, and agricultural waste waters (Eaton et al. 2005). The BOD of an effluent is a measure of the amount of oxygen needed by microorganisms in oxidizing organic carbon to carbon dioxide and ammoniacal nitrogen to nitrate (Sawyer and McCarty 1967). Oxygen required to oxidize organic carbon is referred to as the carbonaceous biological oxygen demand (CBOD), and oxygen needed to oxidize ammoniacal nitrogen is called the nitrogenous oxygen demand (NOD). The two fractions usually are combined and reported as the BOD. The 5-day BOD (BOD₅) is most commonly determined, and for typical wastewater and aquaculture pond water, the BOD₅ concentration is about two-thirds of the quantity of oxygen needed over a longer period of time to completely oxidize the organic carbon and ammoniacal nitrogen (Boyd and Gross 1999). The amount of oxygen needed to satisfy the BOD of an effluent completely is known as the ultimate BOD(BOD_u). For example, suppose that the BOD_u concentration in an effluent averages 200 mg/L (200 g/m³) and 250 m³ of this effluent are discharged into a lake each day. The effluent will impose an oxygen demand of 50 kg/day (200 g BOD/m³ x 250 m³/day) on the lake.

Dissolved oxygen concentration is likely the single most important water quality variable in aquaculture (Boyd and Tucker 1998). An adequate concentration of dissolved oxygen is needed to avoid stress and to assure good growth of the culture  species. Moreover, sufficient dissolved oxygen is needed in aquaculture systems to oxidize wastes and prevent them from accumulating to levels harmful to fish, shrimp, and other culture species. The primary wastes in aquaculture systems are organic carbon and ammoniacal nitrogen resulting from the fraction of aquafeeds not converted to biomass of the culture species and harvested. The purpose of this report is to present a method for calculating the oxygen demand of aquafeeds.