Digestion of Animal Manure

The intensivation of animal keeping in many European countries has created local manure surplusses which inevitably cause environmental pollution problems, such as mal-odour and pollution of surface and ground waters. The application of anaerobic digestion for manure stabilization combines an effective means of odour control with a recovery of energy (methane gas). As a consequence the process becomes economically a very attractive treatment method in many cases. Furthermore, if a complete manure purification is pursued, anaerobic digestion seems to be also the most proper pretreatment method. After digestion the manure should be subjected in that case successively to lime treatment (in order to obtain a concentrated sludge rich in Ca-phosphate and organic nitrogen), and an ammonia-stripping and absorption process to regain the ammonia-nitrogen in the form of a highly concentrated ammonium salt solution. Recent studies at the Agricultural University of Wageningen (The Netherlands) on laboratory and pilot plant scale indicate that wet piggery manure, consisting of both faeces and urine, is very amenable to digestion in spite of the high ammonia-nitrogen concentration (up to 4,500 mg l −1 ). The digester population turned out to be able to acclimatize to these high ammonia-nitrogen concentrations. Although stable digestion could be obtained and maintained at a temperature of 30 °C at loading rates up to 6 kg TS/m 3 day, the optimum loading rate for both the abatement of odour nuisance and energy recovery is approximately 4 kg TS/m 3 day. The methane production from piggery manure under these circumstances ranged from 0.200–0.260 m 3 methane/kg TS added. The optimum process temperature for energy recovery appeared to be about 27–30 °C when high grade fuel has to be used for heating the manure and about 35–40 °C when sufficient low grade fuel is available. A full-scale digester (300 m 3 ) for pig manure has been recently put into operation in The Netherlands. Several other full-scale digesters for pig and cattle manure are projected. The main results and experiences of laboratory and pilot plant scale experiments will be presented and evaluated.