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Field Demonstration of the Performance of a Geotube® Dewatering System to Reduce Phosphorus and Other Substances from Dairy Lagoon Effluent

S. Mukhtar, K. Wagner, L. Gregory

Two upper North Bosque River segments were designated as impaired in 1998 due to point source and nonpoint source (NPS) pollution of phosphorus (P) to these segments in the watershed. As a result, two Total Maximum Daily Loads (TMDLs) were applied which called for the reduction of annual loading and annual average soluble reactive P (SRP) concentrations by about 50%. This demonstration was conducted to evaluate the efficacy of a prospective new technology, the Geotube® dewatering system that may aid dairy farmers in reducing P from lagoon effluent to be applied to waste application fields and thus reducing NPS pollution.

In this Geotube® dewatering system, effluent is pumped from the dairy lagoon using a PTO-driven chopper pump into a PVC pipe with a series of elbows that facilitate thorough mixing of the chemical pretreatment. Alum and a polymer are added to the effluent agglomerate solids and precipitate P as it flows through the elbows to the Geotubes®. Two 14’ x 50’ geotextile fabric tubes were installed on a 6 millimeter impermeable polyethylene sheet next to a primarily dairy lagoon that received flushed manure. After the tubes were filled, they were allowed to dewater for a period of 6 months. Rainwater typically sheds off of the tubes and does not soak into the tubes. At the first two sampling events in March and April 2005, samples of the dairy lagoon effluent, the lagoon effluent after the addition of the chemical pre-treatment, and the effluent dewatering from the tubes were taken and flow rates into the tube were measured. At the last sampling event in October 2005, samples of residuals and depth of the dewatered residuals were taken from both tubes. Samples from the three events were analyzed for concentration of solids, nutrients, metals and pH.

Results showed that the Geotube® dewatering system performed very well in filtering solids from the dairy lagoon effluent, removing an average of 93.5% of the total solids between the two pumping and dewatering events of March and April. It was effective in removing nutrients and metals as well. The average percent reduction of SRP for the two events was very high at 85%. It should be noted that these findings were limited to the sampling of the tubes in March and April and the tubes continued to dewater for several months. Therefore, any changes in the concentration of the dewatering effluent, volatilizing solids and precipitating substances after the sampling events could not be accounted for.

A brief economic analysis of this dewatering system was furnished by the technology provider. Cost estimates for a long-term dewatering system were $90,000 to treat 1.9 million gallons of dairy lagoon effluent containing 15+ years worth of nutrients and solids that settled to the bottom of the lagoon at a 2000 head lactating cow open-lot dairy. This estimate includes all capital and operating costs except removal of residual solids. Costs will vary depending on the size of the dairy and the length of time between lagoon treatments using Geotubes®.

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