Solepur: A soil treatment process for pig slurry with subsequent denitrification of drainage water

The soil purification capacity for treating pig slurry was evaluated over five years using a hydrologically isolated held treatment plant, the so-called ''Solepur'' process. This involves three operations: (1) overdosing the managed field with surplus slurry, (2) collecting and treating the nitrate rich leachate, (3) irrigating the final treated water over other fields. The facility consists of (1) a managed held (3280 m(2)) which allows the total recovery of all the leachate water which percolates through growing ryegrass, Lolium perenne, to which the pig slurry is applied, (2) a storage-pump-reactor system for denitrification and (3) a non-managed field for completing treatment. The purpose of this study was to evaluate over a five year monitoring period, the feasibility and performances of such a process) and to determine the optimum operating conditions. From 1991 to 1995, 4931 m(3)/ha of raw pig slurry was applied to the managed field, which represents a nominal load of 986 m(3) ha(-1) yr(-1). This process decreased the COD of pig slurry by 99.9% and removed 99.9% of phosphorus and approximately 90% of nitrogen. The Solepur's final product is a limpid leachate containing a very low concentration of organic matter, but with relatively high nitrate levels resulting from oxidation of slurry nitrogen in soil. The extent of nitrification efficiency was satisfactorily demonstrated both through soil inorganic-N profiles and through continuous analysis of the leachate water leaving the overdosed field which shows nitrate concentration up to 1500 mg/l. Subsequently, the nitrogen removal from the nitrate rich leachate was achieved through batch denitrification sequences: raw pig slurry (which supplies carbon to support biological denitrification) is added to a volume of 150 to 250 m(3) of drainage water (representing a nitrogen load of 10 to 40 kg N) in the ratio 1:20 to 1:100 v/v. Parallel laboratory tests showed the influence of C/NO3--N ratio on the efficiency of nitrate nitrogen removal and demonstrated the feasibility of semi-continuous denitrification. It was confirmed that a C/NO3--N ratio of three is necessary to achieve a complete denitrification sequence. The results conclusively show that the Solepur process is a promising waste treatment system. (C) 1997 Silsoe Research Institute.