Environmental aspects of the anaerobic digestion of the organic fraction of municipal solid wastes and of solid agricultural wastes.

In order to obtain more detailed information for better decision making in future biogenic waste treatment, different processes to treat biogenic wastes in plants with a treatment capacity of 10,000 tons of organic household wastes per year as well as agricultural codigestion plants were compared by life cycle assessments (LCA). With the tool EcoIndicator, anaerobic digestion is shown to be advantageous as compared to composting, incineration or a combination of digestion and composting, mainly because of a better energy balance. The management of the liquid manure in agricultural codigestion of organic solid wastes causes increased gaseous emissions, which have negative effects on the LCA, however. It is recommended to cover the slurry pit and to use an improved manure management in order to compensate for the additional gaseous emissions. In the LCAs, the quality of the digester output could only be taken into account to a small extent; the reasons are discussed.

Co-digestion of organic solid waste and sludge from sewage treatment.

Solid organic wastes were codigested together with sludge of a sewage treatment plant (STP). In the practical part of the study, a plant to pretreat the organic solid wastes provided by local super markets was constructed at the STP of Frutigen, Switzerland. Up to more than 1 cubic metre of wastes was added to the fermenter of the STP every day. Data collected during 14 months of practical works, showed that for raw fruit and vegetable wastes a two step pretreatment is necessary: First the wastes were chopped and afterwards reduced to a size of 1-2 millimetres, in order to get a homogeneous suspension together with the primary sludge. The vegetable wastes showed excellent digestibility: They seemed to accelerate the digestion process as well as to increase the degree of the anaerobic degradation of the sludge. The energy demand for both, pretreatment and digestion, was 85 kWh/ton of fresh wastes. 20% of the energy was used for the hygienization, a step which does not seem to be necessary for this kind of waste in most of the cases, however. After using the gas for energy conversion, a net yield of 65 kWh/ton of electricity and 166 kWh/ton of heat was measured. Treating cooked kitchen wastes, the net energy production will be higher, because in this case a one step pretreatment will be sufficient. The pretreatment and treatment costs for codigestion on STP's were calculated to be in the range of 55 US$/ton treating half a ton per day and 39 US$/ton treating one ton, respectively. A theoretical feasibility study showed that in Switzerland there is a short term potential on STP's for the codigestion of about 120,000 tons of biogenic wastes per year without big investments. Economic studies about codigestion on agricultural biogas plants showed that the codigestion is a must at the current energy prices, which are far too low for agricultural AD without an additional income by treating solid wastes for third parties.