Zero Discharge, a possible future for Recirculating Aquaculture Systems?
Johan Verreth and Ep Eding
Fish Culture and Fisheries Group, Wageningen University, The Netherlands.

Abstract

Intensification of fish farming inevitably leads to higher waste production and an increased ecological footprint in terms of water use and fossil energy demands. A solution to these ecological concerns is the rearing of fish in closed recirculating aquaculture systems (RAS). RAS are especially suited to cut water requirements. Average water needs per kg production may decrease from several m3 in common flow-through systems to less than 100 litres in RAS. Similarly, COD discharge may be reduced to 10%, comparing RAS with a common stagnant fish pond.

In Europe, environmental regulations have encouraged the development of RAS, especially where taxes have to be paid on the discharged amounts of COD, Nitrogen and/or Phosphate. Commercial farming of fish in RAS has become standing practice in the Netherlands with a commercial expertise of 15 years, farming mainly eel, African Clariid catfish, tilapia, turbot and recently, also sole. The applied water treatment configuration usually consists of a suspended solids removal unit, UV treatment, a trickling filter and oxygenation. The farm effluent is often collected in a septic tank to concentrate waste and to further reduce waste discharge costs. The best environmental performance nowadays is realised by systems which also incorporate a denitrification unit, a flocculation unit, a belt filter and a sludge storage facility. These systems meet the strictest environmental criteria for effluent discharge. Yet, feed nutrients which are not retained in the fish biomass are still discharged, either as less harmful volatile products (N2, CO2), or concentrated in a sludge which can be used for composting.

A more challenging way to improve the ecological sustainability of recirculation systems is the incorporation of water treatment processes, which convert waste nutrients into harvestable products. Innovations for such sustainable recirculation systems are:
  1. fixed bacterial biofilms are replaced by harvestable biofilms in suspension (to be used as single cell protein);
  2. the incorporation of phototrophic organisms such as micro-algae or macro-phyta, which can either be harvested directly or by herbivorous organisms;
  3. the incorporation of detrivorous organisms to convert remnants of solid waste.