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Water Science & Technology Vol 48 No 4 pp 189–194 © IWA Publishing 2003

Concentration of pig slurry by evaporation: anaerobic digestion as the key process

A. Bonmatí*, E. Campos** and X. Flotats***

*Laboratory of Chemical Engineering and Environment, University of Girona, Campus Montilivi s/n, 17071 Girona, Spain (E-mail: a.bonmati@lequia.udg.es)
**Laboratory of Environmental Engineering, Department of Environment and Soil Science, University of Lleida, Rovira Roure, 19, 25198 Lleida, Spain
***Laboratory of Environmental Engineering, Department of Environment and Soil Science, University of Lleida, Rovira Roure, 19, 25198 Lleida, Spain (E-mail: flotats@macs.udl.es)


ABSTRACT
Nutrient redistribution between areas with a structural pig slurry surplus and those with a shortage, is limited by the high cost of transportation and spreading, due to the high water content in slurry (more than 90%) and its relative low nutrient concentration. Water can be removed from slurry by evaporation, through the application of waste heat from a power plant or from other processes. Apart from obtaining a concentrate with an obviously higher nutrient concentration than the original slurry, another objective is to obtain clean water as condensate. The objective of this work was to study the batch vacuum evaporation of pig slurry liquid fraction, to evaluate the economic feasibility and to evaluate condensate quality as a function of both pH (4, 5 and 6) and pig slurry type (fresh slurry and anaerobically digested slurry). Results showed that condensate characteristics (ammonia nitrogen, VFA, COD) were strongly dependent on these variables. Previous anaerobic digestion presented clear advantages: it provided a fraction of the required energy and it removed organic matter, preventing its volatilisation in the evaporation process and providing higher quality condensates. These advantages make the combined treatment strategy economically more feasible than the evaporation process alone.

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