UTILIZATION AND TREATMENT OF SECONDARY WASTEWATER EFFLUENT IN SHORT-ROTATION ENERGY FORESTRY – A PILOT STUDY
Municipal secondary wastewater effluent was applied to plots, 900 m2 each, on a middle-class farming land planted with willows (primarily clones from Salix viminalis} according to the short-rotation energy forestry (SREF) concept in Sweden. Wastewater was applied at rates of 2, 4, 6, 8, 10 and 12 mm/d, respectively, throughout the growth period (May-October) during five consecutive years. Willow growth optimum for 3-year old stems on 4-year old roots amounted to 7.3-12.4 DM(dry matter)/ha/y (depending on clone) after a wastewater irrigation rate of 6 mm/d (ca 1000 mm/season) consisting of 156 kg N, 27 kg P and 162 kg K per hectare and year. Concentrations of macronutrients (N, P and K) and metals (Cu, Zn, Pb and Cd) in stem wood were independent of wastewater application rates. Removal of N, P and BOD, over the unsaturated zone (ca 1 m of silty clay) on a mass balance basis were 82-93%, 90-97% and 74-82%, respectively. Mean concentrations of N, P and BOD7 of superficial groundwater measured quarterly varied within 1.3-2.7 mg N/l, 0.10-0.27 mg P/l and 3-5 mg BOD7/1, respectively. The results indicate that application of a typical secondary effluent up to a level of maximum growth of SREF – a case characterized by “pure” wastewater recycling replacing chemical fertilizers and natural water supplies – could create free capacities in the soil-plant system concerning removal of wastewater nutrients. Hence, in a situation where nutrient removal would be the main objective, the SREF area needed could be reduced.