A treatment plant using conventional biological treatment combined with hydroponics and microalgae is constructed in a greenhouse in the area of Stockholm, Sweden. The treatment plant is built for research purposes and presently treats 0.559 m(3) of domestic wastewater from the surrounding area per day. The system uses anoxic pre-denitrification followed by aerobic tanks for nitrification and plant growth. A microalgal step further reduces phosphorus, and a final sand filter polishes the water. During a three week period in July 2002 the treatment capacity of this system was evaluated with respect to removal of organic matter, phosphorus and nitrogen. 90% COD removal was obtained early in the system. Nitrification and denitrification was well established with total nitrogen reduction of 72%. Phosphorus was removed by 47% in the process. However, higher phosphorus removal values are expected as the microalgal step will be further developed. The results show that acceptable treatment can be achieved using this kind of system. Further optimisation of the system will lead to clean water as well as valuable plants to be harvested from the nutrient rich wastewater.

Aims: To measure the microbial removal capacity of a small-scale hydroponics wastewater treatment plant. Methods and Results: Paired samples were taken from untreated, partly-treated and treated wastewater and analysed for faecal microbial indicators, i.e. coliforms, Escherichia, coli, enterococci, Clostridium perfringens spores and somatic coliphages, by culture based methods. Escherichia coli was never detected in effluent water after >5.8-log removal. Enterococci, coliforms, spores and coliphages were removed by 4.5, 4.1, 2.3 and 2.5 log respectively. Most of the removal (60-87%) took place in the latter part of the system because of settling, normal inactivation (retention time 12.7 d) and sand filtration. Time-dependent log-linear removal was shown for spores (k = -0.17 log d^-1, r² = 0.99). Conclusions: Hydroponics wastewater treatment removed micro-organisms satisfactorily. Significance and Impact of the Study: Investigations on the microbial removal capacity of hydroponics have only been performed for bacterial indicators. In this study it has been shown that virus and (oo)cyst process indicators were removed and that hydroponics can be an alternative to conventional wastewater treatment.

Hydroponic wastewater treatment takes advantage of the nutrient removing capacity of green plants. In addition to the nutrient assimilation, the roots provide a growth substrate for microorganisms involved in the biological treatment processes. However, to maintain year-round performance by the plants, additional energy muse be provided at higher latitudes, even if the hydroponics are situated in a greenhouse. To evaluate the energy demand by hydroponics in Sweden, two theoretical operational conditions have been compared. These conditions were based on A: requirements by winter resting plants, 10°C and 400 lux 16 h day-1, and B: good growth. 20°C and 2000 lux 16 h day-1. Further, five Swedish cities at different latitudes and their respective demands to reach the two conditions were compared. These cities were Lund (55°72' N), Visbv (57°38' N) Stockholm (59°35’ N), Ostersund (63°20’ N) and Kiruna (67°83’ N). The calculations showed that under Swedish conditions, the extra heat demand always exceeds the light demand on a yearlv basis except for the high temperature and light standard in Lund. The yearly light requirements are similar for the five cities, whereas the heat energy displays strong latitude dependence, e.g. the yearly heat demand in Kiruna is almost seven times higher than in Lund to reach an average indoor temperature of 10°C.

During the last two decades, wastewater treatment in systems combining conventional biological processes andhydroponics has been tried at several locations. In this article, six different systems are described and theirtreatment results compared. Five systems were found in literature, and the sixth system was constructed andoperated by the microbiology group at KTH. These systems can be divided into three subgroups: i) demonstrationsystems with small inflow and long hydraulic retention time (HRT), ii) pilot systems with small to moderateinflow and moderate HRT, iii) full scale systems with large inflow and short HRT. In general, removalof organic matter seems to be most efficient in systems resembling an active sludge process. Systems with longHRT 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 removalthrough mainly sedimentation and adsorption in the systems was not very efficient. Nutrient removal bymeans of up-take through plant growth has not contributed significantly in any of the described treatmentsystems. None of the treatment plants have had the primary objective of biomass production. Hence, potentialremoval and recycling of nutrients through a productive system still remains to be answered.