NITROGEN AND PHOSPHORUS REMOVAL BY WILLOW STANDS IRRIGATED WITH MUNICIPAL WASTE-WATER - A REVIEW OF THE POLISH EXPERIENCE

Field experiments with four willow species irrigated by municipal waste water were performed in Osobowice near Wroclaw (South-West Poland) by Professor Feliks Bialkiewicz in the period 1960-1967. The experimental area, 0.5 ha of silty soil over sand, was irrigated via surface furrows 1.8 m apart, with zero, 2000 or 4000 mm of municipal waste water during the growing season (May-September), 100 or 200 mm each week. The soil was drained with parallel clay pipes at 120 cm depth, 14 m apart. Biomass production of stem wood on either a one year or two year rotation, and the quality of water draining from the experimental area, was measured. Annual yields for 2000 mm irrigation of two year rotation plants of Salix amygdalina (S. triandra) was 14.4 t DM ha-1, S. viminalis 8.7 t DM ha-1, S. americana (S. cordata) 7.1 t DM ha-1, and S. purpurea (S. sabulosa) 5.0 t DM ha-1. Yields in ''non-irrigated'' plots were lower for S. amygdalina (10 t DM ha-1) but little different for the other species, possibly due to lateral seepage of nutrients between plots. At 4000 mm irrigation, yields were little different from those at 2000 mm (14.1, 7.3, 6.4, 5.0 t DM ha-1 respectively), suggesting the plants to be nutrient saturated at 2000 mm irrigation. Quality of drainage water (biological oxygen demand BOD5, 19 mg dm-3; total N, 32 mg dm-3; P as P2O5, 6 mg dm-3) was compared with levels in the waste water supply, giving efficiencies of removal of 88% for BOD5 but only 41-43% for N and P. It is concluded that, although providing excess nutrients for willow growth, seasonal waste water loading in excess of 2000 were too high for efficient removal of nutrients and levels of N and P in the effluent remained unacceptably high. It is suggested that an irrigation load of 1000 mm (50 mm per week) would provide both an adequate supply of nutrients for plant growth and, by increasing the efficiency of nutrient removal, improve the quality of effluent leaving the plot.