Integrated treatment systems
Nature-based wastewater treatment technology covers several methods that can be used alone or in combination. Three mains systems can be used for treatment of effluent water from land-based aquaculture farms: ground filtration, constructed wetland, and pond systems. Ground filtration systems can further be divided into open ponds or subsurface trenches (Fig. 5.8). In a filtration system the soil is used as a filter medium, and is actually the same as the depth filter described earlier. The water is distributed above the filter bed and trickles through the soil in which filtration, adsorp-tion/precipitation and biological degeneration will be major processes. Nutrients, organic matter and micro-organisms will be removed. The local soil, if suitable, is normally used as a filtration medium and therefore it is also called on-site treatment. However, soil (e.g. sand) or other suitable porous media such as Leca may also be trucked to the site. The main problem with ground filtration for treatment of effluent water from aquaculture is the large amount of water to be treated and the low hydraulic capacity of soil-based systems. Even if some improvement can be achieved by using suit-able porous media the area needed is still large. Normal values for domestic wastewater when using subsurface trenches are about 10000 l/m2/day, with some higher values in open ponds. Effluent water from aquaculture facilities has been less well studied and there is a lack of available values. Because the pollutant concentrations are lower, the load may be somewhat higher, but if the system is overloaded it will function sub-optimally and purification will be reduced. Results from treat-ment of domestic wastewater in a cold climate lat-itude (69°N) with an annual mean temperature of 1.2°C show the following results: 70% nitrogen removal, 99% phosphorus removal, 70% chemical oxygen demand (COD) removal and close to 100% removal of faecal coliforms. The normal flow was 750 m3/day in a 2000 m2 open basin, but during snow melt it can be up to 3500 m3/day.
If the system becomes totally clogged a tractor with a shovel can remove the upper 5–10 cm of sand, after which the system can be used again. A typical value for ground filtration systems for wastewater is 100 l/m2/day, but of course this varies with the soil conditions. If using such values for outlet water from fish farming, a tremendous area is necessary for water purification. When using depth filtration in fish farming there is a need for high hydraulic capacity.
Constructed wetlands are commonly used for various types of polluted water such as domestic and industrial wastewater, and have also been uti-lized to treat wastewater from aquaculture.4 The outlet water is sent into the wetland that contains wetland-adapted plants and a porous earth medium. The plants take up and utilize the nutri-ents in the outlet water; in addition, the roots of the plants create an environment that increases the purification processes. By sending outlet water from a salmon hatchery to an abatement pond coupled to a constructed wetland, the following removal rates were attained: solids above 98%, ammonia above 84%, biodegradable organics and phosphorus above 90%.34 Both ground filtration systems and constructed wetlands will, in addition to removal of nutrients and organic matter, inactivate micro-organisms. The great disadvantage with the systems is the low hydraulic capacity in relation to the amounts of water in aquaculture.