Digested slurry is recycled through a continuously stirred reactor in order to maintain a high level of
bacterial concentration and, hence high performance. The contact process is a suitable approach
for both mobile substrates and substrates with a high concentration of solids.
An upflow-type fermenter with a special hydraulic configuration serves simultaneously as a
suspended-solids filter with a high bacterial density and correspondingly high biodegradation
A vehicle (balls of plastic or clay) is kept "floating" in the fermenter to serve as a colonizing area for
A vehicle (plastic pellets or lumps of clay, rock or glass) provides a large, stationary colonization
area within the fermenter. Fixed-bed fermenters are suitable for wastewater containing only
dissolved solids. If the wastewater also contains suspended solids, the fermenter is liable to plug
The acidic and methanogenic phases of fermentation are conducted separately, each under its own
optimum conditions, in order to maximize the fermentation rates and achieve good gas quality.
The treatment of wastewater marked by heavy organic pollution must always be looked upon as an
individual problem that may require different processes from one case to the next, even though the
initial products are identical. Consequently, trials must always be conducted for the entire chain:
production process - purification - wastewater utilization - and energy supply/ use.
Thanks to their uncomplicated, robust equipment, the contact process and fixed-bed fermentation
stand the best chance of success in developing countries.
The fact that practically identical production processes often yield residues that hardly resemble one
another also applies to industrial waste materials. Here, too, pretrials and individual, problem-
specific testing are called for in any case.
The potential range of organic waste materials is practically unlimited. Of particular interest for the
purposes of this manual, however, are waste materials from factory farms and slaughterhouses.
Large-scale stock farming
The characteristics of dung from cattle, pigs and chickens were described in chapter 3.2. In factory
farming, the dung yield is heavily dependent on the given type of fodder and how the stables are
cleaned. Thus, pinpoint inquiries are always necessary.
The large quantities of substrate, often exceeding 50 m³/d, lead to qualitative differences in the
planning and implementation of large-scale plants, as opposed to small-scale plants. This has
consequences with regard to substrate handling and size of plant:
- Daily substrate-input volumes of more than 1 m³ cannot be managed by hand. Pumps for filing
the plant and machines for chopping up the substrate are expensive to buy and run, in addition
to being susceptible to wear & tear. In many cases, careful planning can make it possible to use
gravity-flow channels for filling the plant.