Nitrogen removal from waste treatment pond or activated sludge plant effluents with free-surface wetlands

Abstract

Due to their dynamic mode of operation, waste treatment pond and activated sludge plant effluents always contain quite large amounts of nitrogen relative to those found in natural lakes, streams, and oceans. Typical activated sludge plant effluents contain 25 mgl-1 N, mostly as ammonia. In most aquatic milieux, concentrations of ammonia > 1 mgl-1 N are potentially toxic to fish and other wildlife. Nitrification and denitrification of activated sludge plant effluent alleviates the ammonia problem at a considerable cost but the discharges still usually contain over 10 mgl-1 NO3-N which exceeds the WHO standard for drinking water. A great advantage of waste treatment pond effluents is that nitrogen is normally already present as nitrate or particulate-N (algae and bacteria), and nitrate concentrations are < 5 mgl-1 NO3-N. However, even 1 mgl-1 of nitrate-N is sufficient to cause eutrophication in unpolluted lakes, streams, and oceans and some of the particulate-N discharged will be recycled to give eutrophication downstream. Where sufficient diluting water is available, these higher effluent concentrations are not a problem. Unfortunately, clean diluting water is becoming a scarce commodity in many areas, particularly the 17 semi-arid states in the USA and in most developing countries where rainfall is needed for drinking water and wildlife support. One solution for nitrogen removal is new design for free surface constructed wetlands which have considerable potential for nitrogen polishing of waste treatment pond effluents. Particulate-N can be removed by using wetlands as large filters but the nitrogen often recycles and is released as ammonia in winter and spring. Denitrification of nitrate to N2 gas removes the problem permanently. In particular, the relatively low BOD, high nitrate and low ammonia effluent from some stabilization ponds is ideal for nitrate removal (denitrification) by free surface wetlands. Rates of nitrate removal of 200 to over 5,000 mg N m-2 d-1 can be achieved with initial nitrate values of 2-14 mgl-1 NO3-N. These rates are 1-2 orders of magnitude greater than occur in most natural lake, estuarine or wetlands sediments and can be mostly attributed to denitrification rather than growth of rooted plants. In two weeks 20 mgl-1 NO3-N can be reduced to less than 1 mgl-1. After wetlands treatment the water is suitable for release into water-depleted live streams or lakes where a low eutrophication potential is vital for native biota. It is recommended that pond effluent be routed thorough constructed wetlands whenever possible since both better water quality and wildlife benefits occur.