Phytoremediation of Anaerobic Digester Effluent for Water Purification and Production of Animal Feed

Abstract

The application of phytoremediation for purification of an anaerobically treated dairy manure and production of forage crops was investigated. Four crops (two cereals and two grasses) were examined for their ability to grow hydroponically and to remove pollutants (nutrients) from dairy wastewater. The preliminary experiments showed that timothygrass and orchardgrass did not perform well as aquatic plants. Only 24 and 29% of the seeds germinated after 19-21 days giving a crop yield of 21 and 19 t ha^-1 for timotygrass and orchardgrass, respectively. Wheat and barley grow very well as aquatic plants with a seed germination of 83 and 73 (in 7 days) and a crop yield of 106 and 86 t ha^-1 for wheat and barley, respectively. The effect of light duration, seeding rate, wastewater application rate and fungicidal treatment on the wheat crop yield and pollution potential reduction were studied. The results indicated that with this system, a wheat forage crop could be produced in 21 days from germination to harvest. A treatment combination of wastewater application rate of 900 mL day^-1, a seeding rate of 400 g and a light duration of 12 hrs gave the best results for crop yield (3.81 kg of wheat tray^-1). Based on thirteen harvests per year, a total possible yield of 3300 t ha^-1 per year can be achieved with the system. This is more than 102 times grater than the yield obtainable from a filed grown conventional forage of 245 t ha^-1 per year. Wheat had a superior nutritional value (higher digestible energy, higher carbohydrates, fat, protein and mineral contents and less crude fiber) compared to the other field forage crops. It also contained higher macro and micro nutrients (Sodium, Magnesium, Manganese, Iron, Copper, Boron, Selenium, Iodine and Cobalt) than field forage crops. Removal efficiencies of 72.4, 88.6 and 60.8 % can be achieved for the total solids, Chemical Oxygen Demand (COD) and ammonium nitrogen, respectively. A nitrate nitrogen concentration of 7.1 mg L^-1 was also found in the effluent from the hydroponic system. This is below the Canadian Environmental and Health Guidelines of 10 mg L^-1.