Environmental assessment of pig slurry biological treatment plants. 1. Mass balance and prediction of nutrient fluxes

In Britanny, land application of livestock wastes from confined animal operations is seen as a major source of nitrogen in agricultural drainage channels. In this context, treatment aiming at removal of nitrogen from livestock wastes, particularly pig slurry, is becoming increasingly common in France. The aim of this study was to evaluate the fate of nitrogen, phosphorus, copper and zinc during biological treatment of pig slurry, including the separation of the raw and/or treated pig slurry. Based on the experimental data obtained, a predictive model was developed to calculate the nutrient fluxes according to the plant characteristics. According to the results obtained in this study, the biological aerobic treatment technique is efficient in Brittany for pig slurry treatment, particularly for nitrogen removal (60-70%). The treatment produces final products that need to be managed by the farmer. The solid separated fractions could be exported out of the farm. Supernatant could be used for irrigation on the farm with a reduced environmental risk compared with the use of raw slurry. The biological sludge, could be spread on the farm as organic fertilizer. However, the use of the sludge on a nitrogen basis as fertilizer could overload the fields with phosphorus, copper and zinc. Moreover, exportation of the solid separated fractions could be more and more difficult due to their heavy metals content. A reduction of the copper and zinc levels in pig diets is then necessary.