Biogas use in Reconstruction
Practical Action
Plug Flow Digester
This is a type of Earth-pit plant, therefore appropriate in stable soil (e.g. laterite), where masonry
digesters are not necessary. Earth-pit plants can only be recommended for installation in
impermeable soil located above the groundwater table. Their construction is particularly
inexpensive in connection with plastic sheet gas-holders.
For earth-pit plants, it is sufficient to line the pit with a thin layer of cement (wire-mesh fixed to
the pit wall and plastered) in order to prevent seepage. The edge of the pit is reinforced with a
ring of masonry that also serves as anchorage for the gas-holder. The gas-holder can be made of
metal or plastic sheeting. If plastic sheeting is used, it must be attached to a quadratic wooden
frame that extends down into the slurry and is anchored in place to counter its buoyancy. The
requisite gas pressure is achieved by placing weights on the gas-holder. An overflow point in the
peripheral wall serves as the slurry outlet.
Advantages: Low cost of installation (as little as 20% of a floating-drum plant); high potential for
self-help approaches.
Disadvantages: Short useful life; serviceable only in suitable, impermeable types of soil.
Plug flow digesters (Figure 3) constitute a type of earth-pit plants that have long, narrow concrete
tanks with either a rigid or flexible cover. The vessel is a long narrow (typically a 5:1 ratio; 5 times
as long as the width) insulated and heated tank made of reinforced concrete, steel or fiberglass
with a gas tight cover to capture the biogas.
Figure 3: cross-section diagram of a plug flow digester. Source: http://www.plugflowdigester.com/
The plug flow digester (tank) is built partially or fully below grade to limit the demand for
requiring additional heating. Plug flow digesters are used only at dairy operations that collect
manure by scraping. In theory, manure in a plug flow digester does not mix longitudinally on its
trip through the digester but can be imagined to flow as a plug, advancing towards the outlet
whenever new manure is added.
When the manure reaches the outlet it discharges over an outlet weir arranged to maintain a gas
tight atmosphere but still allow the effluent to flow out. In actuality the manure does not remain
as a plug and portions of the manure flow through the digester faster than others and some settles
or floats and remains in the digester. Biogas produced by the digester is used to heat the digester
to the desired temperature.
Excess biogas can be used to run an engine generator. Heat can also be recovered from the
engine generator and used for space or floor heating, water heating or steam production to offset
the cost of purchased electricity, propane, natural gas or fuel oil used on the farm for daily
operations.
5