18_ Evaluation Study on Plant Design Study 2005

SNV Cambodia

Evaluation Study for Biogas Plant Designs

Final

September 2005

TABLE OF CONTENTS 1. INTRODUCTION 3

2. METHODOLOGY 3

3. EVALUATION 4

A. APPROPRIATENESS OF MODELS TO SMALL RURAL HOUSEHOLDS IN CAMBODIA AND THE USER 4

Required time investment and savings 4 Repair and Maintenance 4 Safety 5 Durability and Reliability 5

B. EVALUATION OF THE DESIGN OF ALL FIVE MODELS 6 Structural design for inlet pit and pipe 6 Structural design for gasholder and digester 7 Structural design for bottom slab (digester base) 9 Structural design for manhole 10 Structural design for slurry outlet 11

C. APPROPRIATENESS OF CONSTRUCTION MATERIAL AND TECHNOLOGY FOR MASS DISSEMINATION OF PLANT IN CAMBODIA 11

Construction Materials 11 Construction Methods 11

D. APPROPRIATENESS OF DESIGN FOR AREAS WITH HIGH WATER TABLES 13 E. CONSTRUCTION COSTS 14

4. CONCLUSION 15

APPENDIX 1. PLANT DESIGNS

APPENDIX 2. COST COMPARISON

APPENDIX 3. EVALUATION MATRIX


1. Introduction The aim of this report is to evaluate five different biogas design models and determine which model is most appropriate for mass dissemination in Cambodia. The evaluation will look at a number of factors including the appropriateness of each biogas plant to small rural households (mostly small farms), the construction techniques and materials, the design of each model and the cost of construction.

2. Methodology The five models that will be assessed are –

1. Deenbandhu (India) 2. GGC (Nepal) 3. KT2 (Vietnam) 4. Chinese Dome (built on a small scale in Cambodia), and; 5. VACVINA (built on a small scale in Cambodia)

In order to define the scope of the report and to allow a fair comparison of the models, various boundaries have been adopted. Firstly, it is assumed the plants are intended for domestic use, mostly in rural areas, where families have at least two or more cows (or the equivalent dung production from other animals like pigs). Community plants require an in depth socioeconomic analysis, they tend to be larger and will not be discussed in this report. It is assumed the gas produced will be used directly, for either cooking or lighting. The evaluation of the designs and construction techniques are based on the manuals and drawings produced for each model as detailed in the references. The construction method for the Chinese Dome has not been provided and so assumptions have been made as to how they would be constructed. The construction method for the KT2 has been interpreted from a video titled “Biogas Project Vietnam, Construction Technique, December 2003”. Fittings and accessories, like the drain pit and valves, will not be evaluated in this report, as they are common to all designs and will depend on the availability in the local markets. There are several variables, which will affect the construction cost and cost per litre output of gas. This includes the retention time of the slurry in the digester and amount of available slurry, both of which ultimately effect the size of the digester and hence the cost of it. In order to provide a fair cost comparison, models with the approximately the same digester volume (approx. 4m3 of slurry) will be used. Limitations It is important to note this evaluation is limited in its scope as it is a ‘desk study’. Experimentation and field studies may yield issues not covered in this report. For example, the physical construction of the models using local materials and established methods may reveal unforeseeable problems.


3. Evaluation

a. Appropriateness of models to small rural households in Cambodia and the user The average annual income for small rural households in Cambodia is less than US$ 1,1001 (with an average of five people per household). All five models have been designed for use in developing countries and with the fact above in mind, have been made as low-cost and appropriate to the situations for which they have been designed. Each model has different sizes, which accommodate the range of digestible material available to each household (i.e. 2 pigs or 4 cows etc)2.

Required daily person-hours for operation The biogas plant has to be readily adopted by the user for it to be a success. Models that require fewer, daily, person-hours to run it will be preferred over a model that may only be marginally more efficient. The GGC, Deenbandhu, and KT2 all include an inlet tank where mixing can take place. This feature is not essential and can be included into any model. For larger models where the daily slurry input is greater, the inlet tank feature is more essential. The VACVINA is designed for use with a stable, where material is collected on the floor, but requires a person to ‘sweep’ it into the inlet daily. However, most rural households do not have permanent enclosures for cows and pigs and so this design feature is not appropriate to the average user. All models require the dung to be mixed with water to produce a type of slurry suitable for digestion. Cow manure, for example, requires a 1:1 ratio of manure to water and this high water demand will require a person to spend a considerable amount of time collecting and transporting the water. However, this high water requirement is the case with all five designs, although a design with an inlet tank allows more flexibility (e.g. in collecting rainwater when the inlet pipe is plugged). Regardless of the design, the user will have to spend time collecting and preparing the slurry. The fixed dome plants all have a built-in gasholder, where the gas is under pressure owing to the hydraulic function of the elevated slurry in the overflow pit or pipe. Therefore, the gas flows directly to the stove where output is controlled with a valve. The VACVINA model has an external gasholder, in which the gas is not under pressure, and so time has to be spent every time gas is required to adjust rubber-tubing3, or equivalent, to ‘push’ the gas out.

Repair and Maintenance Repair and maintenance should not have to be done frequently, should have a minimal cost and should not necessarily require a skilled person to do it. Proper designing and constructing, as described below, should prevent the need for having to carry out repairs.

1 Cambodia Socio-Economic Survey (CSES) 2004 2 It is assumed the Chinese Model is available in size ranges similar to the other models although they have not been available at the time of writing this report. 3 VACVINA Manual, pg 14


The Deenbandhu manual4 recommends the plant needs to be emptied, cleaned, and repainted with sealant paint every five years. However, depending on the substrate used, the plant may require more or less frequent cleaning. Experience in the field has shown gas production in the fixed-dome plants decreases over time, typically 10 years after the plant commissioned. Digested slurry and other grit and stones tend to accumulate in “dead zones”. Even the addition of fresh material cannot dislodge the build up and subsequently the area of the digester is reduced, and fresh slurry is expelled without complete fermentation – the retention time is decreased. Access to the digester of the GGC and Deenbandhu would be through the outlet tank – from which it would be emptied. Access to the KT2, VACVINA and Chinese Dome would be through the manhole at the top of the digester – which is more awkward and time consuming than the others. Firstly, the seal at the manhole would have to be broken and emptying the digester through the manhole would require more than one person – one person to climb in and fill a bucket (or equivalent) and the second to lift out the slurry. Once the maintenance work is completed, the manhole needs to be resealed to ensure there are no gas leaks. The manhole would require regular monitoring to ensure the clay seal is still intact and there is sufficient water to ensure it does not dry and crack.

Safety The gas produced from the digester is combustible and the gasholder needs to be airtight to prevent any accidents. The gasholders for the fixed-dome models are located outdoors and underground where damage will be minimal as discussed in Section 2. The VACVINA gasholder is kept either in the kitchen or stables and the scale of damage would be greater if there were any accidents. In addition to this, if the gasholder is kept within living quarters it requires space that is not in the sun, safe and not too far from the digester. In the 1990s UNICEF supported a biogas programme under the Ministry of Rural Development of Cambodia. The programme used a model consisting of a rubber bag digester and a plastic gasholder similar to VACVINA model. According to H.E. Try Meng, Under Secretary of the Ministry of Rural Development, the programme was ceased because of technical problems - a farmhouse was reported to have burnt down owing to leaking gas.

Durability and Reliability The average recorded life expectancy of a model is important in deciding which model is best suited to rural households in Cambodia. As mentioned previously, the average income is low and investment in a biogas plant is considered costly in these terms. The user would expect to invest a model with the longest life expectancy. However, the average life expectancy of the fixed-dome plants is similar (from 20-30 years) and construction techniques play a major role in determining this. The live span of the VACVINA digester will be similar to the other models, but the plastic gasholder will need to be replaced when damaged.

4 Singh, J., Myles, R., and Dhussa, A., Manual on Deenbandhu Biogas Plant, Dehli 1987, pg 28


b. Evaluation of the design of all five models

Structural design for inlet pit and pipe The inlet pit is not an essential feature of the digester, as the design for any model can be adapted to include it. However, the KT2 provides the best design solution – the inlet tank is flat bottomed and the pipe is raised slightly above the flat base allowing grit and stones to settle just below the intake. The use of a plug at the pipe ensuring no gas or heat escapes through the inlet tank (although the amount would be minimal) and no foreign bodies enter the digester. This feature also allows slurry to be held in the mixing tank to heat in the sun where it can later be fed into the digester at an optimum temperature. The VACVINA model introduces slurry into the digester through a siphoned inlet. A siphoned inlet is more susceptible to clogging. In addition, it has no mixing tank making it harder to use when dealing with cow manure. The slurry is introduced at the top of the digester and the outlet is located near the top of the tank. Since fresh material is lighter than digested material, the probability of fresh slurry not being fully digested and leaving the digester prematurely is high. The inlet is perpendicular to the outlet and almost at the centre of the wall. This can cause the slurry to be stagnant in areas in the digester, where it is not being ‘pushed’ through, and reduce the effective volume of the digester, as shown in the figure below. Figure 15. A simplified model to show how the slurry flows from the inlet to the outlet and where it accumulates. The arrows represent the direction of flow from inlet to outlet. The Chinese Dome model has a pipe inlet for material from the latrines and all other slurry is to be fed straight into the digester. The circular inlet pit passes through the concrete dome, weakening the structure of the dome, increasing the potential for failure and cracking of the structure. The design suggests the inlet pipe from the latrines penetrates through the digester to the centre point. Although it is advantageous to introduce slurry to the bottom of the pit to ensure the material is continually ‘churned’ and therefore digests properly6, the positioning of the pipe is disadvantageous. The pipe

5 VACVINA Manual, adapted from pg 22 6 Fresh material is lighter than fully digested material. When introduced at the bottom of the tank it rises and causes the slurry to be ‘stirred’ allowing gas to escape more readily and increasing the bacteria activity. Agitation also occurs in the fixed dome plants (and not the VACVINA) as the accumulation of gas in the dome causes the slurry to be displaced and mixed.

Outlet

Inlets for latrine and animal manure

It is assumed the digester is full when new material is added daily. This causes a flow to be created between inlet and outlet, and forces old material to be pushed out. However, in this arrangement, the slurry tends to accumulate in corners and along edge as it remains largely unaffected by the flow of new material.


is subject to pressure from the slurry around it and can corrode away with time given it is situated directly in the slurry. It can also impede the movement of the slurry from inlet to outlet. New material entering the digester will be lighter and will be more likely to pass straight through to the outlet without being fully digested, given the configuration of where the material enters and leaves the digester. The University of Oldenburg has carried out research to show the flow of material within the digester, depends on the positioning of the inlet pipe. Flow from the inlet pipe that ends at the digester wall differs from the flow from an inlet pipe that protrudes into the chamber7. The experiment was based on the design for the GGC. Where the pipe ends at the wall, small circular coil patterns are formed and where the pipe protrudes, a ‘free-jet’ model is formed where the flow reaches the outlet quicker (when the protrusion equals the diameter), as shown in the figures below. Although this shows a simplified model of the slurry movement, it is representative of the general hydrodynamic flow in a digester of this shape. The designs for the Deenbandhu, KT2 and Chinese Dome models all show the pipe protruding into the digester unlike the GGC. However, they all have curved or conical bottoms and this would mean the dead zones would tend to be in that space. The fresh slurry is lighter and although the flow would be similar to the figure below, it would also tend to rise shortly after it is emitted from the pipe, leaving a ‘dead zone’ in the base. However, for the Deenbandhu, KT2 and Chinese Dome, with the protrusion and direction of the inlet pipe, the slurry can enter with enough ‘force’ to disturb the slurry at the base. Further investigation and experimentation is required to determine the flow pattern of the slurry given different digester shapes and inlet positions. Figure 28. (Left) Linear overlay of the circular coils advancement from above. (Right) Linear overlay of free-jet propagation from above.

Structural design for gasholder and digester In the fixed dome designs, the whole dome needs to be gastight, at least up to the lowest slurry point level. Cracks in the masonry or concrete arise where the external and internal 7 Kulschewski, U., Hydrodynamic flow-patterns in a “fixed-dome” Biogas Reactor, Oldenburg 2004, pg 54 8 Kulschewski, U., Hydrodynamic flow-patterns in a “fixed-dome” Biogas Reactor, Oldenburg 2004, pg 71

Inlet Inlet Pipe

Outlet


(tensile) stresses are highest. A dome shape, or curved surface can support heavier loads than a flat slab of the same material and thickness as shown in the figure below. Figure 39. Shape and load-bearing capacity, where a thicker arrow represents a larger force. Therefore, from the designs of all five models we can see the VACVINA model is more susceptible to cracks as it is constructed from flat slabs whereas the other four models have a dome or cylindrical digester. However, the KT2 and Deenbandhu fare better than the Chinese and GGC models in this case as they both have a dome structure that forms both the digester and gasholder whereas the latter only have a dome structure for the gasholder and a cylindrical structure for the digester. Components under compression are less likely to suffer from cracks. If constructed properly, when the gas space of the fixed dome is under pressure, the forces will be evenly distributed at every point and therefore, will be less likely to crack.

Figure 410. Different shapes, of the same volume, have different stress patterns under the same load (a and b). In a dome shape, the loads acting in different directions are more reliably balanced than with a vertical wall (c and d).

The gasholder for the VACVINA model is made from tubular polyethylene and therefore is not prone to cracking but tearing. Exposure to the sun can degrade the plastic causing it

9 Sasse, L., Biogas Plants, GTZ/GATE, GmbH 1988, pg 28 10 Sasse, L., Biogas Plants, GTZ/GATE, GmbH 1988, pg 29


to become brittle and eventually breakdown. Puncturing is the other main cause of leakage – through human error, rodents and possibly birds. In terms of the shape of the digester and gasholder, the Deenbandhu and KT2 offer the most structurally stable designs. The Chinese and GGC models, offer a better structural design than the VACVINA model, which flat walls and slabs have to withstand a higher bending moment. The dome shape of the digester (or the curved cylindrical walls) is less susceptible to the formation of scum, along with the varying slurry levels during gas build-up and usage. However, the VACVINA is more susceptible to having layers of scum form, as there are flat surfaces on which it can build and the through movement of the new slurry, added daily, may not be sufficient to prevent it forming. In theory, the drop from the inlet pipe should break the scum forming if the slurry enters at a considerable force – but the siphon actually slows down the slurry. The digestion process takes place in two distinct phases and keeping these as separate as possible allows efficient digestion to take place yielding more gas. The VACVINA model would provide the best solution to this problem, if the inlet and outlet were almost opposite each other on the two shorter walls. The long rectangular structure should allow sufficient time for the processes to take place without ‘mixing’. However, the inlet and outlet are not positioned correctly for this type of process. In addition to this, for the same amount of material, a curved or spherical shape gives a higher volume to surface ratio than a flat or box shape. A higher volume to surface ratio allows more slurry or gas to be stored and potentially a longer digestion period.

Structural design for bottom slab (digester base) The bottom slab has to carry the weight of the digester walls, the slurry inside it, and in the case of the fixed-dome designs the weight of the earth on the dome. The slab distributes these loads onto the ground beneath it. The larger the foundation the less settlement there is, as the load is more evenly distributed. In addition to having a well-structured base, the ground beneath the base needs to be equal, well compacted and consistent – however this is a construction technique and not a factor of the model design. A curved (shell formation) base, as used in the Deenbandhu and Chinese Dome models, provides the best load bearing capacity owing to its shape. The conical shaped base, as used in the KT2, can “carry” higher loads than a flat-bottomed base as in the GGC and VACVINA. The base supports the vertical load from the digester dome wall and only part of the horizontal load depending on the ‘protrusion’ of the base beyond the limit of the wall (as show in the figure below). Both the KT.2 and the Deenbandhu have the base extending 200mm and 150 mm respectively, from the edge of the base.


Figure 5. The base of the digester should extend beyond the limit of the walls in order to accommodate the horizontal load (also see Fig. 2 for loading patterns)

In the case of the GGC, VACVINA and Chinese Dome there is minimal horizontal load on the base from the wall because the wall is vertical. However, for the GGC and Chinese Dome, there is a similar horizontal loading pattern where the dome meets the vertical walls. The horizontal forces from the concrete dome are only countered by the backfill material. Poor compaction of backfill can result in structural damage.

Structural design for manhole The main purpose of a manhole is to allow access to the digester so that it may be cleaned or repaired and with some designs, it has a function during construction. Both the Deenbandhu and GGC are designed so the digester can be accessed through the outlet tank. However, the outlet tank needs to be emptied, for both models, in order to gain access to the digester. In terms of ease of use, both models have advantages and disadvantages - the Deenbandhu model provides easier access to the digester from the outlet tank for removing grit. It is possible to stand at the outlet tank and ‘rake out’ the grit as the base of the digester is almost at the same level as that of the outlet tank – however, the actual opening for the Deenbandhu is smaller than that of the GGC. The Deenbandhu has an arched frame opening, providing support to the rest of the dome above it – it is more structurally stable than the opening created in the GGC, which is rectangular. The Chinese Dome and KT2 have manholes at the top of the digester, which are sealed to prevent gas leaks. However, they affect the strength of the dome structure, as the openings create inherent weaknesses and a latent risk of gas leakage. The digester does not have to be completely emptied of slurry to carry out inspections and maintenance. The Chinese Dome model has two access points – through the outlet and through the manhole on the dome – both of which are similar in size and smaller than the access points for the KT2, Deenbandhu and GGC models. A single opening through the outlet pit is sufficient access to the digester and additional large openings increase the probability for gas leakages, structural failure, and complicates construction as discussed in Section 3. The VACVINA digester is accessed though a manhole in the top of the digester. A space is left when the concrete for the top is cast. If the cover for the hole is not at the same level or below the level for the bottom of the cover, a small air pocket is created where small amounts of gas will tend to accumulate – the outlet for the gas is in a different place to the manhole. The entire top for the digester is reinforced so the opening should pose little threat to the structural integrity of the digester although it does create a latent risk of gas leakage as with the models mentioned before.


Structural design for slurry outlet The VACVINA and KT2 have an outlet pipe from the digester, for the digested slurry, that leads to a storage or overflow pit, whereas the other three models have no pipe and the outlet tank is directly attached to the digester. A pipe is more advantageous when trying to conserve heat within the digester – this may not be a priority in Cambodia, where temperatures are on average high. The outlet tank of the KT2 is below ground level. During flooding, water is more likely to enter into the chamber. The tank is dome shaped to withstand higher loads and less material is required to construct it. However, owing to the shape, as mentioned previously, more expertise is required to construct it. The pressure in the digester increases when gas is created and this ‘pushes’ the displaced slurry into the outlet tank (also known as the overflow pit or compensation tank). In turn, when gas is being used, the difference in slurry level between the digester and overflow pit provides pressure to push out the gas when it is released. The VACVINA plant does not have this feature and gas does not leave the digester as readily – it is not under the same pressure. Therefore, there is no pressurised flow of gas to any appliances from the external gasholder. The manual recommends using a gas-fan to extract the gas or constricting the reservoir with ‘rubber-tubing’ to push out enough gas – increasing the probability of damaging the plastic reservoir.

c. Appropriateness of construction material and technology for mass dissemination of plant in Cambodia

Construction Materials All models are constructed from materials that are widely available in Cambodia. However, the cost of these materials varies and they will be discussed in Section 5 – Construction Costs.

Construction Methods The simplicity in construction is one of the main factors that will affect the popularity of one model over another. The construction of brick or concrete domes is not common in Cambodia and skilled or trained masons would be needed for the construction. The VACVINA requires fewer skilled masons for the construction of the actual digester. All four fixed-dome digesters require more excavation than the VACVINA and the Deenbandhu, Chinese Dome and KT2 require more complicated excavation for the base of the digester than the GGC and VACVINA, which have flat bottoms. Nevertheless, this is a small drawback as the structural load bearing benefits are greater for a concave base. The base for the Deenbandhu and Chinese Dome models, although superior in their load bearing capacity are the most difficult to construct – the Deenbandhu concave base has to be thicker at the edges than in the middle and it is made from concrete. Similarly, the cone shaped base for KT2 is also made from concrete and requires more skilled construction. The base for the VACVINA is made from crushed stones and bricks with a 5cm layer of concrete on top. It is likely be more prone to cracks under stress (e.g. uplift pressure).


The GGC and Chinese Dome models have vertical, cylindrical walls and require the same skill required to construct straight, vertical walls, as used for the VACVINA. The construction methods for the KT2 and Deenbandhu domes are similar – both employ a method of using a stick or string as centre-marker (a trammel) to create a dome shape. The curved base for the Deenbandhu and Chinese Dome models may make it harder to construct the rest of the dome above it, as there is no flat surface from which to construct. The KT2 and Chinese Dome models have a manhole on top of the dome and the curved cap of the dome does not have to be constructed. However, the Deenbandhu has a full dome, and scaffolding has to be erected within the dome to allow the cap to be finished. The scaffolding is later removed through the outlet tank, but this adds to the difficulty in constructing the dome even though it will be more structurally stable. The dome for the GGC is made from concrete. The walls for the digester are constructed and filled with soil. The soil is compacted and shaped using a template with the correct radius. The concrete is then cast over the filling and the fill material is removed via the outlet pit once the concrete is set. This method requires more labour and less skill compared to the other fixed-dome models. The dome has a concrete collar – where the concrete is thicker at the point where it meets the cylindrical walls. This requires more attention, similar to constructing the base for the Deenbandhu, and if not constructed properly, the dome could be left susceptible to cracking and leakages. Similarly, the Chinese Dome model is more complex to construct as it has more openings in the digester than the models. It is assumed the chute for the inlet is constructed from bricks and formwork would have to be constructed within the dome to support it while it set. Four sets of reinforced covers would have to be constructed - for the slurry inlet, outlet and two for the gas outlet. In addition to this, it has a reinforced concrete ring around the digester where the dome joins the walls and similar to the collar for the GGC, would require more attention. The digester cover for the VACVINA model is made from reinforced concrete and so needs to be cast in place. This involves building formwork inside the digester that can be taken out though the manhole once the concrete has set. This increases the construction cost both in terms of the duration of construction and for the use of ‘extra materials’. Similarly, the outlet cover for the Deenbandhu, GGC, KT2 and Chinese Dome are made from reinforced slabs – but these can be bought pre-made or cast using bricks. All designs refer to different wall thickness and this variation can be attributed to the fact that each country or even region manufactures bricks of different sizes. The average size for the bricks made in Cambodia and Vietnam is 8.5cm x 4cm x 18cm (b x h x l) while it is 11.5cm x 7.5cm x 25cm in Nepal and India. The Deenbandhu, GGC and VACVINA are all plastered inside the digester and outlet and inlet tanks to seal any cracks. It is assumed the Chinese Dome and KT2 are plastered similarly. The Deenbandhu model also specifies plastering the dome on the outside – with a thicker layer on the outside compared to the inside. This will increase the comparative cost minimally, but the benefits of having a ‘leak-proof’ digester outweigh this cost. The fixed-dome models require applying more gastight paint, compared to the


VACVINA where the gas space within the digester is smaller (although the VACVINA manual makes no mention of using any special gastight paints or sealants). Once the digester has been constructed, the area around it needs to be back-filled. Since the VACVINA digester is rectangular and the GGC and Chinese Dome have vertical walls, the trench dug to construct it in requires a smaller amount of backfilling. In addition to this, the VACVINA model does not hold gas in the digester and can therefore be constructed just below ground level or even partially above it – although this will negate the benefits from using sub-ground conditions to stabilise the digester temperature. In contrast, the fixed dome plants require a soil cover because of the gas pressure inside the dome. The pit, excavated to construct the digester, requires backfilling and compaction in order to counter the hydrostatic pressures from within the digester. It is also recommended the inlet tank be constructed on ‘undisturbed soil’ to prevent settlement from unconsolidated soil. This is not an issue with the Chinese Dome and VACVINA models, as they have no inlet tanks, but poses a problem with the KT2 model where the inlet tank is constructed on backfilled soil in order to gain a steep angle on the inlet pipe entering the digester. The design for the GGC and Deenbandhu models recommends placing compacted earth on top of the dome, the ceiling of which is at ground level, as seen in the picture in Appendix 1. This reduces the depth to which the pit has to be excavated, as the counter weight is added above ground level. The same principal can be applied to the other models. However, the Chinese Dome model has both inlet and outlet below the ground, which can pose a problem in flood prone areas, as discussed in Section 4. The outlet tank for the KT2 has a dome shape and is more complex to construct when compared to the other models. The fixed-dome models require more skilled and trained labour to construct the domes – some more than others as discussed above. However, there is room for error in even the simplest construction designs if it is not done properly or with the correct materials.

d. Appropriateness of design for areas with high water tables It is recommended all plants be positioned at least 30m away from any wells and downstream of surface and ground water flows - regardless of the design. Any leakages of slurry into groundwater supplies that are used for drinking can be potentially harmful as the slurry contains active bacteria. There has been no comprehensive investigation into the groundwater (water table) levels throughout Cambodia. There have been two studies carried out by the US Geological Survey – the second of which put the average groundwater level at 23m from a sample of 1100 drilled wells ranging from a depth of 2 to 209m11. However, the groundwater levels vary considerably by region and season. Where the groundwater levels are high and areas are prone to flooding, the fixed-dome design is disadvantageous due to the higher gas volume below underground. The gas creates uplift forces when below the water table. 11 Peng, P., and Pin. N., Groundwater Contamination in Cambodia, 2003, pg 2 http:// www.unescap.org /esd/water


The VACVINA model is less affected by uplift pressure as the gasholder is above ground. However, the thin bottom slab of the plant (5cm concrete) could not withstand much pressure. It is safe to assume the Deenbandhu and GGC models can withstand a certain amount of uplift forces, as some areas in India and Nepal are similarly prone to high water tables and flooding, Again, curved surfaces can handle uplift pressure better than flat surfaces. A structural analysis should be carried out to determine how much uplift pressure (height of flood level above ground) the plants could withstand without damage.

Figure 6.12 Schematic diagram of earth-pressure and water-pressure forces The outlet points (where the slurry leave the outlet tank) of the Deenbandhu, Chinese Dome, KT2, and VACVINA are all at ground level unlike the GGC. This increases the chances of water entering the digester during floods. However, not all areas in Cambodia are prone to flood or have high water tables and each site would require its own investigation before a plant is built.

e. Construction costs The cost of construction plays an important role when selecting a model for mass dissemination. The main variances in costing that will determine the value of one model over another is the cost of the materials and to an extent labour required for the construction. Therefore, fittings and accessories common to all the plants have been omitted. Similarly, inlet pits with mixers are regarded as ‘an extra’ and not included in the comparison. The unit price given for materials would be that quoted by a contractor to cover transport, overhead and labour costs. Labour costs vary considerably within Cambodia especially within the unskilled and informal sectors. The cost comparison has been made for plants that are very similar in size and have a slurry volume of approx. 4m3. For structural parts made of brick masonry, dimensions relating to the Cambodian solid brick (8.5 x 4 18cm) have been used.

12 Werner, U., Stohr, U., and Hees, N., Biogas Plants in Animal Husbandry, 1989 GTZ/GATE, GmbH 1988, pg 116


The 4m3 VACVINA plant has the lowest investment cost of the five models because of its plastic gasholder. Table 1 shows the construction cost comparison for all the respective models. A more detailed bill of quantities can be found in Appendix 2. Plant Type Cost [US$] Deenbandhu (India) 243 GGC (Nepal) 282 KT2 (Vietnam) 259 Chinese Dome (China) 293 VACVINA (Cambodia) 237

4. Conclusion The criteria discussed above were ranked and summarised in Appendix 3. The matrix shows the Deenbandhu is the most suitable plant for mass dissemination in Cambodia compared to the other four designs under evaluation. The structural design of the Deenbandhu fairs better than the other models owing to the curved base and dome-shaped digester and gasholder. The positioning of the outlet in relation to the digester allows easy access for cleaning and maintenance, without breaching the structural integrity of the dome-shape or complicating construction. The VACVINA fairs the worst of all five models in terms of structural design and durability. However, as mentioned above, the Deenbandhu requires skilled constructors and poor construction can debilitate any design. The fact the dome also serves as a gasholder, is advantageous as the gas is under pressure and so reaches the point of use easily. In areas prone to flooding, this is disadvantageous as discussed above. Amongst all the fixed-dome models, the Deenbandhu proved the most structurally sound base to counter the uplift forces. Similarly, of all the fixed dome plants the Deenbandhu appears to be the cheapest to construct. Possible Improvements for Deenbandhu The construction materials – mainly bricks – differ between Cambodia and India and the design will have to be adapted to accommodate this. In addition to this, the design for the inlet / mixing tank can be improved by incorporating the plug and positioning of the pipe features of the KT2. Similarly, the inlet pipe should not protrude into the digester and should stop at the wall to allow the slurry to flow better and maximise its digestion efficiency. Besides, it should be considered introducing a small baffle wall in the digester. More experimentation is required to verify the effects of these measures.


APPENDIX 1. Plant Designs






APPENDIX 2. Cost Comparison


APPENDIX 3. Evaluation Matrix

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f mod

els

to s

mal

l rur

al h

ouse

hold

s in

C

ambo

dia

and

the

user

(far

mer

s pe

rspe

ctiv

e)

Req

uire

d tim

e in

vest

men

t and

sav

ings

6.

0 6.

0 6.

0 5.

5 6.

0 6.

0

Rep

air a

nd M

aint

enan

ce

5.0

5.0

5.0

2.0

4.0

4.0

Dur

abilit

y an

d R

elia

bilit

y 5.

0 4.

0 4.

0 2.

0 3.

0 3.

0

Saf

ety

3.0

3.0

3.0

1.0

3.0

3.0

Tota

l a.

19.0

18

.0

18.0

10

.5

16.0

16

.0

b. E

valu

atio

n of

the

desi

gn o

f all

five

mod

els

Stru

ctur

al d

esig

n fo

r inl

et p

it an

d pi

pe

5.0

5.0

5.0

2.0

3.0

5.0

Stru

ctur

al d

esig

n fo

r gas

hold

er a

nd d

iges

ter

8.0

8.0

6.0

5.0

6.0

7.0

Stru

ctur

al d

esig

n fo

r bot

tom

sla

b (d

iges

ter b

ase)

4.

0 4.

0 3.

0 3.

0 4.

0 4.

0

Stru

ctur

al d

esig

n fo

r man

hole

5.

0 5.

0 5.

0 2.

0 3.

0 2.

0

Stru

ctur

al d

esig

n fo

r slu

rry o

utle

t 5.

0 5.

0 5.

0 5.

0 4.

0 4.

0 To

tal b

. 27

.0

27.0

24

.0

17.0

20

.0

22.0

c.

App

ropr

iate

ness

of c

onst

ruct

ion

mat

eria

l and

te

chno

logy

for m

ass

diss

emin

atio

n of

pla

nt in

Cam

bodi

a 20

.0

15.0

15

.0

15.0

15

.0

15.0

d. A

ppro

pria

tene

ss o

f des

ign

for a

reas

with

hig

h w

ater

ta

bles

10

.0

10.0

10

.0

10.0

10

.0

10.0

e. C

onst

ruct

ion

cost

s 24

.0

23.5

20

.2

24.0

21

.4

22.0

Gra

nd T

otal

10

0.0

93.5

87

.2

76.5

82

.4

85.0

Documents

18_ Evaluation Study on Plant Design Study 2005