4.3. Trade-off between filter volume and surface
Removal of pollutants is related to filter
medium volume, whereas nutrient uptake and
assimilation into plant biomass is based on filter
surface exposed to sunlight. The surface-to-volume
ratio defines the maximum nutrient recycling
capacity of the system. An engineer
designing a filter system is forced to choose
between high specific removal rates on little
space and nutrient recycling potential requiring
a larger space. Is nutrient recycling a luxury
only achievable in rural areas, where space limitation
is a lesser problem than in urban environments?
At the point where the additionally
required surface generates income by producing
marketable goods, higher infrastructure spending
to improve nutrient recycling capacity is
no luxury any longer. Our experiments showed
that this can be achieved using aquaponic systems,
with tomatoes providing almost the same
fruit yield when irrigated with fish water than
with mineral fertilizer. And surprisingly, the
fruits do not have any fishy taste.
Studies with constructed wetlands treating
municipal wastewater gave hints that there is no
benefit in increasing the depth of the treatment
beds beyond 0.5m and that shallower beds may
perform better [29]. Beds with a water depth of
0.27m removed more COD, BOD5, ammonia
and dissolved reactive phosphorus than beds
with a depth of 0.5m [30]. These findings may
encourage opting for shallower treatment beds,
where the nutrient recycling potential of plants
can be better utilized.