As part of a research project aiming to develop and evaluate a hydroponic system for wastewater treatment in Sweden, extended nutrient removal by microalgae was tested. The hydroponic/microalgal wastewater treatment system was built in a greenhouse in order to improve growth conditions for plants and algae. Studies on the treatment step with microalgae showed that phosphorus removal could be successfully accomplished owing to the cmbined effect of phosphorus assimilation and biologically mediated chemical precipitation of calcium phosphates. This precipitation was mainly induced by the increased pH in the algal cultures, and the pH increase was in turn a result of the inorganic carbon assimilation by the algae. The results showed that the algal growth was mainly light limited which resulted in higher algal biomass density and also lowe residual nutrients in the water at longer hydraulic retention times (HRT). In contrast the phosphorus removal rate was load limited, i.e. shorter HRT gave higher removal rates. This load dependency was due to the chemical precipitation, whereas the phosphorus assimilation was dependent on algal growth. Furthermore, results from an intensive study during summer showed that culture depths of 17 cm gave higher removal efficiencies (78% - 92%) than cultures of 33 cm (66% - 88%). On the other hand, the removal rate per area was higher in the deeper cultures, which implies that these may be preferred if area is of concern.

Nitrogen removal was achieved mainly by the assimilation of nitrate to algal biomass, and removal efficiencies of around 40% (nitrate) could be reached for most parts of the year although the nitrogen removal performance was quite uneven. Up to 60% - 80% could however be reached during summer in the shallow cultures. A net removal in total nitrogen of up to 40% was observed in the shallow cultures during summer, which was most probably a consequence of grazing zooplankton and subsequent urea excretion and ammonia volatilisation as a reslt of the high pH values.

Over the year, there were large fluctuations in algal growth and removal efficiency as a result of the seasonal variations in light and tempeature. During winter, phosphorus removal efficiencies lower than 25% were observed in the shallow tanks and lower than 10% in the deep tanks. Additional illumination during winter improved the phosphorus removal in the shallow cultures but did not have a significant efect on the deep cultures. Such additional illumination increases the total energy demand of the system, and hence alternative methods for phosphorus removal during winter would probably be more economical unless the algal biomass roduced had great commercial value.

Conventional end-of-pipe solutions for wastewater treatment have been criticized from a sustainable view-point, in particular regarding recycling of nutrients. The integration of hydroponic cultivation into a wastewater treatment system has been proposed as an ecological alternative, where nutrients can be removed from the wastewater through plant uptake; however, cultivation of plants in a temperate climate, such as Sweden, implies that additional energy is needed during the colder and darker period. Thus, treatment capacity, additional energy usage and potential value of products are important aspects considering the applicability of hydroponic wastewater treatment in Sweden.

To enable the investigation of hydroponic wastewater treatment, a pilot plant was constructed in a greenhouse located at Överjärva gård, Solna, Sweden. The pilot plant consisted of several steps, including conventional biological processes, hydroponics, algal treatment and sand filters. The system treated around 0.56-0.85 m3 domestic wastewater from the Överjärva gård area per day. The experimental protocol, performed in an average of twice per week over a period of three years, included analysis and measurements of water quality and physical parameters. In addition, two studies were performed when daily samples were analysed during a period of two-three weeks. Furthermore, the removal of pathogens in the system, and the microbial composition in the first hydroponic tank were investigated.

Inflow concentrations were in an average of around 475 mg COD/L, 100 mg Tot-N/L and 12 mg Tot-P/L. The results show that 85-90% of COD was removed in the system. Complete nitrification was achieved in the hydroponic tanks. Denitrification, by means of pre-denitrification, occurred in the first anoxic tank. With a recycle ratio of 2.26, the achieved nitrogen removal in the system was around 72%. Approximately 4% of the removed amount of nitrogen was credited to plant uptake during the active growth period. Phosphorus was removed by adsorption in the anoxic tank and sand filters, natural chemical precipitation in the algal step induced by the high pH, and assimilation in plants, bacteria and algae. The main removal occurred in the algal step. In total, 47% of the amount of phosphorus was removed. Significant recycling of nitrogen and phosphorus through harvested biomass has not been shown. The indicators analysed for pathogen removal showed an achieved effluent quality comparable to, or better than, for conventional secondary treatment. The microbial composition was comparable to other nitrifying biological systems. The most abundant phyla were Betaproteobacteria and Planctomycetes.

In Sweden, a hydroponic system is restricted to greenhouse applications, and the necessary amount of additional energy is related to geographic location. In conclusion, hydroponic systems are not recommended too far north, unless products are identified that will justify the increased energy usage. The potential for hydroponic treatment systems in Sweden lies in small decentralized systems where the greenness of the system and the possible products are considered as advantages for the users.