Biological removal of inorganic nitrogen and organic matter in closed, intensive fish culture systems

Abstract

Over the past decade there has been rapid development of intensive fish culture systems. Especially in Northern and Western Europe (Rosenthal and Black, 1993) and North America (Colt, 1991) relatively much R&D is being conducted in the field. Intensive fish culture systems can be classified into flow-through and recirculating systems. In flow-through systems clean water is often led in a single pass through the production unit and discharged thereafter. A drawback of open systems is that, apart from their site specificity, they often pollute recipient water bodies; it is therefore expected that expansion of these systems will be limited. Recirculating systems incorporate facilities for maintaining adequate water quality. These systems are environmentally friendly as water input is minimised and, consequently, little water is discharged. Adequate water quality control in recirculating systems is essential for their successful operation. Most recirculating systems incorporate biological and mechanical filtration. Growth of nitrifying bacteria is often induced in order to convert ammonia to nitrate, while mechanical filters are used to capture organic material (fish faeces and unutilised feed). Biological digestion of organic matter and biological reduction of nitrate, essential processes in stagnant fish ponds, are little used in recirculating systems. Incorporation of these processes in recirculating systems is predicted to result not only in more stable water quality within the fish culture units but also in considerable reduction of environmental pollution by intensive fish farming. In the present study results are presented on a recirculating system in which water is purified by a combination of nitrification, denitrification and digestion of organic matter. The study describes the various stages of research towards the development of a treatment system for recirculated, intensive fish culture.