Two types of systems are used in digestion. The plug flow system in which no mixing takes place and the completely mixed digester. Plug flow offers the advantage that there is no need for mixers, but some of the effluents must be recirculated to innoculate the feed with organisms necessary to carry out the process. The advantages of the plug flow system have been addressed by Amon (2007). Mixing offers advantages in that the substrate is kept in contact with the microbes and the temperature is kept uniformly distributed. Mixing has three important effects on the process as it: (1) maintains uniformity in substrate concentration, temperature, and other environmental factors; 2) minimizes the formation of scum at the surface; and 3) prevents the deposition of solids on the bottom. The degree of mixing varies depending upon the feedstock and operating conditions Ahrin et al., (2001) reported that there was hardly any stratification inside a KVIC digester that was fed cattle manure and that the temperature profiles were within one degree throughout the digester. He also reported increases of 8% and 11% in gas production with continuous mixing, over mixing only two hours per day in digesters using cattle manure at 55·C and retention times of six and four days. Ahrin’s data would indicate that at longer detention times the effect of intermediate mixing would be minimal. Scum formation appears to be a primary function of the feed. With the addition of large amounts of fibrous materials and fats, the formation of a scum layer is likely. If the organic materials are in the scum layer, it is likely that they will not be available as feed to the organism degrading the materials to gas. Thus the gas production rates in a digester with scum layers are reduced. The reason for trying to avoid the accumulation of solids at the bottom of the tank is the reduction of detention time. Detention time is a primary factor in gas production.

Digestion progresses more rapidly at a higher temperature therefore, is important to get the digester feed at as high a temperature as possible and to keep the heat losses to a minimum. Insulation to reduce the heat losses from a digester is very important. In examining some of the efforts in developing countries, Prasad and Sathyanarayan (1979) reported that 54% of the total heat loss was from the cover of the Indian digesters. Thus one can see that for cold climates the Indian digester with a floating metal cover is not a viable option. The materials used to insulate the digester vary from the use of dry agricultural residues (e.g. straw, hay, and corn stalks) to commercial polyurethane materials. The example was given earlier where in Turkey the digester is built into the floor of the barn. A number of heating techniques can be used in installation.

These vary from simple solar heaters placed above the digester to heat exchangers and steam injection (bubble gun) heating. Solar heat can be one of two types; active or passive. Active systems heat a portion of the feed during the day and it is then placed in the digester. Passive systems depend upon building a solar greenhouse that captures the radiant heat energy. Chongging Biogas Office (1982) reported on a comparison of a membrane digester and the Chinese digester. Since the membrane digester absorbed the solar energy the temperature was higher, and it gave a larger gas production.

Slurry is discharged from the digester. The characteristics of the slurry depend upon the feedstock, the digester conditions, and the portion of the organic matter which is converted into gas. The effluents can all be handled by conventional liquid handling methods which would permit them to be transported to the field for use as a soil conditioner or to ponds for aquatic biomass production. In some areas the slurry has been separated using vibratory screens, settling tanks, or sludge centrifuges to separate the liquid and solid portions. The uses of the slurry as a soil conditioner have been described by Ward (1982). The important concept to keep in mind is that the slurry should contain all the initial nutrients contained in the feedstock. The solids will have been reduced in quantity leaving a more stabilized solid which will break down very slowly. During the digestion process, the organic nitrogen will have been partly decomposed into ammonia nitrogen. The ammonia nitrogen which is produced, while it is more available to plants, is also much easier to lose through either misapplication or drying.