Optimization of a sequential anaerobic-aerobic treatment of a saline fishing effluent

Optimization of a sequential anaerobic-aerobic treatment system to determin e ideal reactor sizes for reduction of organic matter (COD) in fishing effl uents to legal standards at minimal costs was performed. Calculations for t he sequential degradation of effluents for a base case were carried out by an objective function involving process optimization and investment-operati on cost related to treatment yield in each reactor. Process optimization co nsidering individual equipment volumes based on biological kinetics gave a minimum total volume for a conversion yield of 0.77 in the anaerobic reacto r. The objective function turned out a monotonous decreasing function withi n the 0-85% conversion yield range of the anaerobic digester. The minimum t otal cost was found at the superior limit (85%) of the conversion range. Ex clusion of the anaerobic step increases by eight times the cost to reach th e same level of COD reduction shown by sequential treatment. Experiments at bench scale showed that this system reduces the COD of a base case effluen t to 600 mg l(-1), but the final nitrogen content (810 mg l(-1)) remains fa r above the legal limit (50 mg l(-1)). Parametric sensitivity analysis showed that total annual cost is an increas ing exponential function of the COD concentration, an increasing linear fun ction of the energy cost, a decreasing linear function of the COD legal lim it and is almost insensitive to oxygen cost. However, aerobic degradation i s 27% cheaper by using oxygen instead of aeration. Implementation of the sequential system requires an annual capital investme nt of US$ 196,000 year(-1) and a discounted operation cost of US$ 138,000 y ear(-1). The cost of a plant designed to comply with the COD legal limit (1 60 mg l(-1)) is relatively low as it would only increase the price of fishm eal by 1.0% per tonne.