A reduction in digestion plant size can be obtained by increasing the solids content. The amount of work in transporting and spreading the digestate is also reduced.

Ong et al. (2000) obtained an increase in gas yield compared to a continuously fed and stirred reactor in a continuously fed nonstirred reactor with the outlet in the middle. It seems that some solids removal from the bottom has to take place, as accumulation of inert solids will reduce the effective volume of the reactor.

Shyam (2001) demonstrated that cow manure can be digested at 18% total solids. The method increases the gas yield with 40% using practically the same equip­ment as before.

LOADING AND UNLOADING
OF DIGESTERS

Manure-based (wet) digesters use pumps for loading. Straw is mixed with digestate and pumped in the digester. An alternative is using an auger and pushing the straw to below the liquid level in the digester. Heavy solids are removed from the bottom of the digester using a rotating rake and pump. Straw disintegrates during digestion and is pumped off together with the digested manure.

In the scum layer plant straw can be loaded with front loaders using an inlet shaft with an outlet to below the liquid level. Shredded straw can also be blown into the inlet shaft. Solids are removed in the same way as in wet digesters. The remaining digestate is pumped out. Plug flow dry digesters are loaded and unloaded with augers.

Leachate recirculation digesters are operated in batch mode. The digesters are opened and the digestate is unloaded with front loaders. Capacity of these digesters is relatively low due to the loading procedure.