164 Organic waste recycling: technology and management
Semi-continuous operation
This involves feeding of the digester on a more regular basis. Feeding is done
usually once or twice a day. The digested organic matter is also removed on the
same time interval basis. This type of operation is suitable when there is a
steady flow of organic matter. The digester volume will have to be large enough
to serve both as a reactor and a gas storage tank. Total gas production per unit
organic weight of organic matter loaded is usually high. Most of the operational
biogas plants in the field are of this type.
Continuous operation
In this mode of operation the feeding and removal of organic matter take place
at all times. The amount of material to be digested is kept constant in the
digester by overflow or pumping. It has been used in treatment of liquid wastes
or organic wastes with low solid concentrations. Continuous operation relies
heavily on external energy inputs for pumping and mixing, and therefore has
limited application in the areas where energy resources are of limited supply.
It should be noted, however, that seed inoculum is added at the beginning of
the process of anaerobic digestion (start-up) and actual operation starts as soon
as the microbial population establishes itself, and gas production plus
percentage of methane gas in total gas production stabilize. In field
implementation of anaerobic digestion, the animal manure itself acts as seed
inoculum and the process may reach its stability within 20-30 days of operation
(depending upon temperature, digester size and type of substrate).
4.4.2 Types of digesters
Various designs of biogas digesters for actual field operation range from a
simple design to a sophisticated one. It is generally known that, the increased
level of sophistication in design would result in a higher demand on manpower
with appropriate skills, which is normally in short supply. Furthermore, more
advanced designs increase the cost associated with construction and operation,
with small apparent increase in the level of gas production.
Double-stage digesters (i.e. first-stage for acid formation and second-stage
for CH 4 formation) are mostly designed for experimental purposes to provide
more insight into, and an understanding of, the complex nature of the anaerobic
process. Single-stage digesters, on the other hand, are of more practical nature.
In general, types of digesters can be divided into two main groups, i.e. those
utilizing the dispersed-growth bacteria and those utilizing the attached-growth
bacteria. Because most digesters are operated as flow-through without sludge
recycle, the dispersed bacteria overflow in the digester slurry, making the HRT