The potential of constructed wetlands to cleanse water of nutrients is immense. This fact is noted in the extensive constructed wetland portion of this website. However, small nurseries may be unable to dedicate large land areas to wetland construction. In light of these specialized needs, systems have been developed that utilize less space while providing natural water treatment.
Small scale nutrient removal systems collect and clean water near each area of production rather than collecting runoff water from the entire property. Additionally, small scale systems may be constructed underground to allow for nutrient removal within the same land area as nutrient release occurs. Small-scale nutrient removal systems require careful management to maintain adequate drainage. Drainage is critical to system function. Before construction can take place, nitrogen loading rates and runoff flow rates must be estimated. Each nursery will need to determine if the labor expenses incurred in installation, management, and maintenance are offset by the need to remove nitrogen from runoff waters.
Placement of systems under roads and production areas is only possible if there is good drainage. Kaldness media has been used for ammonia removal in aquaculture. This media provides good drainage and enormous surface area that can be covered with organisms capable of removing nitrogen from water. As long as waters do not contain chemicals toxic to denitrifying organisms, these systems are effective.
Microbes remove nitrogen by converting nitrate into nitrogen gas. This removes the nitrogen from the water and places in the atmosphere (which is already 70% nitrogen). The process of nitrogen conversion requires a carbon source such as sugar or grass clippings.
Carbon requirements are dependent upon the amount of nitrate to be removed. Nitrate loading (as it is termed) depends upon several factors:
Typically the kinds of fertilizers, the amounts applied and the frequency of application will vary significantly by season.
Irrigation can have large effects on the amounts of nutrients in runoff waters. Every irrigation event will result in nutrient loss, but overwatering results in nutrient waste. It is important to know the flow rates and amounts of water released from production areas during irrigation events to predict the size of the nutrient removal system required.
Microorganisms that remove nitrogen should be collected from the nursery drainage system if possible. Nearly every aquatic system will be capable of nitrate removal to some degree. Microbes can be harvested by placing Kaldness media into retention ponds for several months. The substrate can then be used as part of the denitrifying system. All the microbes need is a low oxygen environment that is frequently exposed to nitrogen and carbon.
Small lab and field systems have been capable of 90% nitrate removal in only 30 minutes with a flow rate of 2.5 gallons per minute. As a result, these systems have great potential to help small operations that wish to remove nitrogen from their runoff waters.