University of Tropical Agriculture Foundation -UTA-
Royal University of Agriculture
of Cambodia

Biodigesters

A legitimate criticism of the plastic biodigester (using tubular polethylene film) in a village situation is that it is easily broken by scavenging animals, which are not always easy to control.  Using stronger materials, such as bricks and cement,  for the digestion chamber is a feature of the Chinese and Indian models.  However, these models are expensive (at least USD250.00 for a system with 6 m³ of liquid volume). In Cambodia and Vietnam, and in countries with similar low per capita income, the cost must be kept  low if the technology is to have an impact at the level of the average household. 

  The “hybrid” biodigester, developed first in Vietnam by VACVINA (Claude Potvin 2000, personal communication), and now being adapted to conditions in Cambodia, appears to offer the most interesting line of improvement. 

The “hybrid” (combination of Indian / Chinese and plastic plug-flow designs) biodigester system uses a chamber made of bricks and cement (or “shuttered” concrete) to replace the plastic bag; the rest of the system is the same as for the “plastic’ model.  The use of the polyethylene reservoir (Photo 1) enables the system to operate at low pressure, thus avoiding the more complicated (and expensive) construction used in the “dual-compartment” “Chinese” model or the "Indian" floating dome system,  which in turn leads to reduced costs.  For the farm with more than 6 - 8 pigs there is the opportunity to use the roof of the biodigester as the floor of the pig pen (Photos 2-14). 

 Preliminary estimates of the costs of materials for a biodigester 6m long and 1m² section (liquid volume about 5 m³) are of the order of USD60.00, excluding the labour. 

 The simple construction of the “hybrid”  biodigester facilitates construction and the skills to do this are available at household level in almost all villages, thus the labour can in most cases be provided by the family members.  

Photo 1. Biogas reservoir made from tubular polyethylene film

The principle of the "hybrid"  biodigester is that "biogas" is lighter (has lower density) than air, thus it will  migrate (rise) to the highest points in the biodigester system. Thus the digestion chamber does not have to made to the same "exacting" specification as in the "Chinese" dual compartment model where pressures may be as high as 1m of water gauge.  The pressure in the "plastic" and "hybrid" biodigester is no  more 1-2 cm water gauge.  

The digestion chamber is based on the same "trench" system as the plastic biodigester but is lined with bricks and coated with cement, with a roof that is "cast"  in situ  (Photo 3) or formed from slabs that are cast separately (Photo 4) and then laid on top of the chamber (Photo 4).

Photo 6. The channel around the pens which conveys the manure and washings into the biodigester

Photo 7. The entrance to the syphon which will convey the manure and washings into the biodigester.  Additional manure (eg: from cattle) can be added at this point

Photo 8. The pipe from the syphon that exits into the biodigester

Photo 9. The "man-hole" that provides an entrance into the biodigester, together with the cover

Photo 10.  The cover in place over the "man-hole".  A seal with clay and water will be put around the  edges.

Photo 11.  The passage giving entrance to the pens and which is also the roof of the biodigester.  The"man-hole" is in the foreground.

The diagrams show the general layout of the 8 pens for the sows with a creep area for the piglets (Figures 1 ands 2).  The biodigester chamber is 1.2 m wide and 1m deep. At the two points (*) where four pens coincide,  immediately above the centre line of the biodigester, a siphon is fitted through which the washings enter the biodigester chamber (Figure 3). A square hole ([ ]), fitted with a gas-tight cover, is situated in the roof of the digestion chamber, in the part that is outside the pig pen (Figure 1). This provides access into the digestion chamber for cleaning or other maintenance tasks. The cover sits inside a water seal to prevent leakage of the biogas (Figure 4).

Figure 1: Floor plan of the 8 pens (concrete surface with 7% slope for the sow pen; and earth floor in the creep area for the piglets) and the biodigester chamber

Figure 3: Vertical section of the biodigester chamber showing the siphon through which the washings from the pig pen (faeces, urine and water) enter the digestion chamber. The siphon serves as water trap preventing the escape of the gas

Figure 4: Details of the entrance to the digestion chamber, showing the cover (in red), retaining shoulder (green) and water seal (blue), the latter made from a mixture of clay and water

Photo 19: The arms of the siphon are cut so that the exit of the siphon is 15cm below the roof of the digestion chamber

Photo 20: The siphon ready to be installed

Photo 21: The siphon installed in the roof of the digestion chamber

Photo 22: The siphon located in the point where 4 pens coincide.  The floors of these  4 pens will be made with 7%  slope towards the siphon

Photo 22: The part of the digestion chamber outside the pig pen where the entrance hatch will be placed,

Photo 23: The outlet for the biogas

12 February 2001

We thought the problem had been solved and that the "plastering" and "brushing" (with a cement-water suspension) of the walls and roof of the gas space of the biodigester had been successful. The system began to produce gas (Photos 30 and 31).  

Photo 32: The PVC pipe attached to the syphons under the pig pen

1 March  2001
The improvement was short-lived. The gas again began to escape. Reluctantly we had to modify the system, leading the manure / water suspension from the syphons through PVC pipe to the "dependable" polyethylene tube system (Photos 32 and 33).