Excavation and Layout of a Fixed-Dome Biogas System

An Overview: Biogas construction is the most critical part of the biogas-making process. This is because it entails the actualization of the idea to be a tangible project. A certain order must be followed in construction, from laying out of the first item to the last in a well-organized sequence. This article aims at providing a clear and step-by-step guideline in the excavation process and the foundation of a fixed-dome biogas system. The following steps are covered: Excavation and methods to be used in pit digging, identification of a central point, base construction, laying of the bricks, designing of hooks, inlet, and outlet piping and plastering, supporting, and curing the wall.

Excavation of the base
A concave base may be favored for two reasons. First, the liquid matter of the bio-slurry will always be on top. Second, the solid bio-slurry matter entering the biodigester will sink quickly.
This means that the solid matter will take longer to digest, while the liquid substance on top will be pushed out of the biodigester and into the expansion chamber quickly. When new materials enter the biodigester, the base causes spontaneous mixing to occur without the use of an agitator.

Pit Excavation: Methods of Digging the Pit
The size of the digester determines how deep the pit should be dug. Any digester’s radius is enlarged by 40-45 cm to provide for more operating space around it. When the groundwater table is high, a small hole is dug away from the digester pit to collect water, which can then be manually excavated or pumped out as the digging progresses.

Soil should be scooped from the hole and placed at least one meter away from the pit’s edge. It is recommended that digging be avoided during wet seasons. Tractors and excavators, as well as common digging tools like a shovel, can be employed.

Location of the Centre of the Biogas Cylinder

  1. The center of the base of the dugout pit is important for digging out the concave base of the biodigester and laying the bricks. To find the center of the dugout pit, use the top of the pit as a reference point,
  2. To coincide with the pegging, a wooden board needs to be laid over the top of the dugout pit. Security needs to be enforced for it is impenetrable,
  3. A rope is connected to the pegging perpendicular to the plank, forming a cross with the plank,
  4. At the spot where the rope crossed the plank, a nail is hammered in place. This is the exact location of the dugout pit’s center,
  5. If a plumb-up is not available, a rope connected to the nail can be used to determine the vertical height from the center.

Leveling of the Base
The concave base demands extra attention in base leveling. A clear water hose filled with water and laid across the base with one end immediately opposite the other is the ideal way to do this.
More soil is removed until the water level at both ends is the same. This is done to keep the concave base level all the way around. A spirit level can be used to ensure that the base is leveled in the case of a flat base.

Casting of Base Concrete

  1. The base is usually cast with concrete. A concrete base is necessary to prevent the seeping of slurry into underground aquifers,
  2. For good concrete strength, the cement, sand, and gravel should be mixed in the ratio of 1:2:4. The sand used to prepare the concrete should be coarse and contain very little if any, clay or topsoil. Materials such as tree roots, leaves, and other dead organic matter should also be sieved out of the sand,
  3. Bricks can be used as a guide to ensure regular thickness. To ensure good strength, care should be taken to ensure that the concrete is only mixed on demand,
  4. Using a radius stick, establish the slab ring,
  5. Level the base and in the case of a cone, dig the depth according to measurements,
  6. Establish the slab height by the use of pegs leveled by a spirit level,
  7. Pour concrete on the slab to a depth of 75mm together with the floor of the digester,
  8. Immediately lay one or more courses of bricks/blocks with the radius stick.

The first course of bricks is half bricked, with the other courses being in quarter-brick fashion. The mortar ratio of cement to land to size is 1: ¼:4. It is important to use the technical drawing from the beginning in order to have all the dimensions in place.

Note: The tools to use when casting base concrete include the following: Radius stick, brick trowel, spade, hoe, wheelbarrow, measuring boxes.

Brick Work
Note that before starting to install bricks, the concrete base must be allowed to cure for at least 24 hours. During the construction period, regularly spraying water on the concrete will help.

Construction of the brick pillar

  1. A central pillar should be built before the bricks are laid. To build the pillar, a little weight is tied to the pole that serves as the concave base’s center and radius,
  2. The weight is left to settle on its own. The bottom of the cylinder is marked with the center of the cylinder where it settles on the concrete,
  3. The weight is surrounded by four bricks, with the weight in the center of the area between the bricks,
  4. The bricks are laid on top of the base such that a hollow pillar is formed,
  5. The height of the pillar must be equal to the height of the concave base,
  6. The hollow pillar is filled with sand to about four-fifths of the height,
  7. In the hollow space, a rod is placed in the center and filled with sand such that it coincides with the cylinder’s center. Normal mortar is placed around the rod to make it sturdy,
  8. The rod is allowed to firm up till the following day,
  9. All the stones/blocks/bricks are dipped in water to clean and improve binding before they are used for construction,
  10. A 60 cm slurry discharge manhole is being built at the same time. To assure vertical walls, use a spirit level or plumb bob,
  11. Reserve space to position inlet pipes while constructing.

The tools in the construction of a brick pillar are: Radius stick, spade, brick trowel, bucket, hoe, wheelbarrow, measuring boxes

The bricks should be of good quality, with a size of 24: 12: 9 cm. Bricks with a thickness of less than 8 cm should not be used. Before being set into a 1 cm mortar bed, the bricks are soaked in water for at least 20 minutes (Ullrich, 2008). Depending on the size of the biodigester and the required wall thickness, the bricks can be put flat or on their edges. To be chosen, the bricks must also be clean. The wall thickness of a biodigester increases with its size.

To maintain regularity and, more crucially, the strength of the walls, all mortar joints must be kept to a maximum of 3 mm. A piece of foam soaked in water must be used to remove mortar debris. Spread a thin layer of cement powder on the concrete base before placing the first set of bricks.

The aim of the thin coating layer of cement is to improve the binding between the initial layer of mortar joints and the concrete. Throughout the building, only wet bricks must be used. Initially, normal masonry mortar (1 part of cement to 4 parts of mortar) is used in laying the bricks.

It will be more difficult to keep the bricks in place as the dome grows larger. Because of the dome’s curvature, the bricks are liable to slip. The bricks must be kept in place with a hook as soon as they are laid to guarantee that they stay in place. There must be no shaking, shifting, or movement once the bricks have been put. Bricks that are later found to misalign are removed and laid again. The mortar formulation should be changed as the dome becomes more prominent from 1:4 to 1:3 and later 1:2 to ensure quick bonding (Sangha, 2013).

A freshly prepared mortar must be used at all times. Mortar that has been prepared for more than 30 minutes must be thrown away. The top of the dome grows rounded as it becomes more prominent. After putting the bricks in place, more weights can be added to the hooks to keep them in place.

Additional weights can be added by extending the ropes that hold the weights. Scaffolds will be required in the dome to assist the masons as the wall grows taller. Backfill the dome with excavated dirt while the brickwork is being done. This is required to allow the masons to freely move around the dome.

To avoid the growth of plants in the brick joints on the outside of the dome, the mortar joints must be sealed with freshly made mortar. To increase stiffness and flexibility, all masonry work is often maintained moist.

Design of Hooks


Hooks are used to put the bricks in place from not slipping. To ensure that the bricks stay in place, they must be held in place with a hook as soon as it is laid. After the bricks have been laid, no shaking, shifting or movement must be allowed (Bensah, et al., May 2021).

The hooks are made of ½ ‘’ smooth iron rods. The hooks are designed such that it could hold the brick firmly but also be removed easily without shifting the already laid brick. The size of the hook depends on how the bricks are laid.

Inlet and Outlet Pipe Installation

Inlet and outlet pipes are integral parts of the digester acting as passageways for organic materials to enter and exit. PVC pipes or covered brick gutters can serve as the inlet or outlet pipes. The pipes must be properly sized to avoid choking leading to digester failure.

Long pipes must be avoided as much as possible otherwise inspection chambers should be incorporated in the design to allow for easy cleaning in case of choking or clogging. Inlet and outlet pipes must be incorporated into the brick layering.

Never should the brickwork be tampered with by way of breaking or chiseling to insert a pipe. This action will obviously weaken the structure and it may lead to either gas leakage or system collapse. The pipe should rest below a brick projecting about 2 cm to the inside of the dome. Keep the pipe in position by tying the pipe to ground pegs.

To avoid choking or clogging, a pipe with a minimum of 10 cm internal diameter should be used for the inlet pipe and 15cm for pipe diameter for the outlet pipe that connects the digester to the expansion chamber. The outlet pipe usually starts at the 4th layer of bricks and continues above the dome of the expansion chamber. This will enable poking when there is a blockage. Seal the pipe joint between the outlet pipe and the brickwork with a cement mortar in the ratio of 1: 1/4: 4. At the level of the expansion chamber, cut out the pipe to allow for slurry flowing in and out.

Alternatively, the inlet and outlet pipes from and into the main digester can be constructed with bricks instead of pipes. Excavation of the earth for the inlet and outlet pipes must be done together with the dome and the expansion chamber. Pipes with plumbing traps are highly recommended for separating water with the gaseous mixture. Some examples are P-traps, S-traps, Q-TR traps, gully traps, bottle traps, intercepting traps.

In the case of a toilet, the connection of the pipe can be installed lower than the inlet pipe from the mixing chamber but it should not go above the lower slurry level to avoid gas escape through the toilet. The toilet attachment to the plant needs to be constructed with a pan without a siphon or trap to avoid excessive water inside the digester affecting the hydraulic retention time and total solids in the slurry.

Note that this pipe connects the digester and the mixing/dung chamber at the cowshed, it is inclined at an angle directly to the center of the digester and reinforced with concrete/mortar on the outside.

Plastering the Digester Wall
Plastering should be done at the inside of the digester with a ratio of 1:1/4:4 and a depth of 2cm. The wall should be washed with cement water before plastering.

Support of the Lower Part of the Digester

Soil backfilling and compaction around the lower part of the digester should be done up to 3 courses high. The soil compaction is important, and it must be properly done because once the digester is in use, the block/brick wall is subjected to high pressure from the inside. Therefore, it must be supported from the outside.

Curing of the wall
To avoid cracking and to encourage binding, a routine sprinkling of water needs to be done for 5 to 7 days.


The construction of a biogas plant is very critical and highly detailed. As such, the steps in the construction of the biogas plant must be properly studied and followed to the letter. Artisans must be qualified and take time in revising the manual for proper implementation of the biogas plant plan. The quality of the construction materials must be ascertained before implementation. To sum up, if the correct procedures are followed, this manual would serve as an educator in biogas plant construction parameters.

Bensah, E., Antwi, E. and  Ahiekpor, J. (2021). Guide for the Design and Construction of Fixed-Dome Biodigester. Retrieved from https://www.researchgate.net/publication/351818542

Ullrich, G. (2008). Second supervisor training on biogas plant construction. Promotion of Private Sector Development in Agriculture, Nairobi, Kenya.

SKG Sangha (2013). Biogas Plant Construction Manual. Retrieved from https://www.build-a-biogas-plant.com/PDF/SKGConstrution%20manual2013.pdf

This article is an excerpt from the book “The Blue Flame: Biogas Training Blueprint”

Suggested Citation in APA
Wanjohi, A.M. (2022). The Blue Flame: Biogas Training Blueprint. Nairobi: Kenya Projects Organization

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