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During the last two decades, wastewater treatment in systems combining conventional biological processes and hydroponics has been tried at several locations. In this article, six different systems are described and their treatment results compared. Five systems were found in literature, and the sixth system was constructed and operated by the microbiology group at KTH. These systems can be divided into three subgroups: i) demonstration systems with small inflow and long hydraulic retention time (HRT), ii) pilot systems with small to moderate inflow and moderate HRT, iii) full scale systems with large inflow and short HRT. In general, removal of organic matter seems to be most efficient in systems resembling an active sludge process. Systems with long HRT appears over-dimensioned as long as the volume is not simultaneously used for treatment and production. Nitrogen was efficiently removed through conventional biological processes, whereas phosphorus removal through mainly sedimentation and adsorption in the systems was not very efficient. Nutrient removal by means of up-take through plant growth has not contributed significantly in any of the described treatment systems. None of the treatment plants have had the primary objective of biomass production. Hence, potential
removal and recycling of nutrients through a productive system still remains to be answered.
wastewater treatment, hydroponics, BOD, nitrogen, phosphorus, greenhouse.
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