Organic Waste Recycling

Biofuels production 171

Plug-flow digester (Horizontal displacement digesters)
This type of digester consists of a long trench cut into the ground and lined
either with concrete or an impermeable membrane. The digester is covered with
either a flexible cover anchored to the ground which acts as a gas holder, or with
a concrete or galvanized iron top. In the latter type a gas storage tank is
required. The main difficulty in plug-flow digestion is to ensure true plug-flow
conditions, and to achieve this, the length has to be considerably greater than the
width and depth (Figure 4.10). The digester is fed semi-continuously through
the inlet which locates at one end of the digester. This type of digester has
limited use in developing countries and only a few of them are operating in
Mexico. It can sustain relatively high loading rates of organic matter with less
frequent failure due to shock loads.

(a)

Mixing pit

Ground

Gas holder Gas pipe

Outlet tank

level

Outlet pipe

Guide pipe

Inlet pipe

Slurry

Partition wall

(a)