In some areas of Denmark nutrient removal is required even for very small wastewater plants, that is down to 500 pe (pe = person equivalents). The goal for the removal is 80% removal of nitrogen and 90% removal of phosphorus, or in terms of concentrations: 8 mg nitrogen/l and 1.2 mg phosphorus/l. The inlet concentrations are typically 40 mg N/l and 10 mg P/l. The paper presents the results from two such plants with a capacity of 800 pe. Phosphorus removal is made by simultaneous precipitation with ferrous sulphate. Nitrogen removal is carried out using the recirculation method. Both plants were originally rotor aerated oxidation ditches. They have been extended with a denitrification reactor and a recirculation pumping station. At present both plants have been in activity for about 3 years and with satisfactory results. Average concentrations of nitrogen (summer) and phosphorus is 7 mg/l and 0.9 mg/l respectively.
Nitrogen removal seems to be a practical solution on these small plants. It is,though, sensitive to temperature and highly oxidized rain water. Phosphorus removal seems to be an easily run and relatively non-sensitive technique at least when using simultaneous precipitation.
