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.
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