What are the quality requirements for building biogas digesters?

There are various materials for building biogas digesters, such as cement, sand, stones, bricks, steel, lime and so on. At present, most rural areas use concrete and brick biogas digesters. With the development of rural economy and the deepening of commercialization of biogas digesters, commercialized biogas digesters are gradually accepted by many farmers.

(1) cement

Cement is the main raw material for building biogas digesters. At present, there are more than 30 kinds of cement produced in China, among which ordinary portland cement (strength and safety index meet GB 175 standard), slag portland cement and pozzolanic portland cement (strength and safety index meet GB 1344 standard) can be used to build biogas digesters. Because the concrete grade used in the walls and ring beams of small and medium-sized biogas digesters such as rural household biogas digesters is usually below 200, ordinary portland cement No.325 and No.425 are generally used, and high-grade cement is not needed. The content of sulfate, carbonate and other harmful substances in groundwater exceeds the specified value, which will corrode ordinary cement, so slag cement or pozzolanic cement should be selected.

(2) Stone

Stone is a coarse aggregate for preparing concrete. Because the wall thickness of biogas digester is 40 ~ 50 mm, the particle size of stones should not exceed 1/2 of the wall thickness, so stones with particle size less than 20 mm should be used. Stones include gravel and pebbles. Gravel particles have rough and angular surfaces, and have strong adhesion to cement, but they have large porosity and need to be filled with more mortar. Because of the low workability of concrete, it is more difficult to water and tamp than using pebbles in construction. In practical operation, it is better to use gravel close to cube, and the strength of gravel should be greater than 1.5 times that of concrete. Pebble, also known as gravel, is mainly used to build ponds. Fine pebbles with particle size of 10 ~ 20mm, needle-like particle content less than 15% and weak particle content less than 10% are used. After classification, the gap is small and the capacity is large. The stones used to build the pond should be clean, and the impurities such as soil should be less than 2% after being washed with water, and there should be no organic matter such as weeds and plastics. Weathered gravel is not suitable for use. For the choice of stone, we can consider the local actual situation and use local materials.

(3) sand

Sand is the fine aggregate of concrete. In concrete mixture, cement slurry is wrapped on the surface of sand particles and fills the gaps between sand particles. Because the smaller the sand particles, the more cement is needed to fill the gaps between the sand particles and wrap them, so medium sand with a particle size of 0.35 ~ 0.5 mm is generally used. In addition, there are large and small particles in the medium sand, such as the proper combination of large and small particles, the total surface area and porosity of sand are small, the amount of cement is small, the middle layer of cement slurry formed is thin, the free water is relatively small, and the configured concrete has small voids, strong bonding force, high concrete strength and good durability. Natural sand, such as river sand, mountain sand or sea sand, can be used to build biogas digesters, but it is required that the sand has pure composition, hard texture, no organic matter, no more than 3% soil content, less than 0.5% mica content and no organic impurities such as firewood and plastics. When the content of organic impurities in sand is high, it can be washed with clear water until it meets the requirements before use.

(4) steel bar

Usually, when building a household biogas digester, the roof of the skylight and the roof of the water pressure room need some reinforcement, and other components may not need reinforcement. However, where the soil is uneven or the foundation bearing capacity is poor, an appropriate number of steel bars should be configured when building the pool. The first-class steel bar (No.3 steel) with a diameter of 4 ~ 40mm is commonly used in biogas digesters. When in use, oil stain and rust should be removed and straightened. The bending and hook at the end shall be made into an arc of 180 according to the clearance diameter greater than 2.5 times the diameter of steel bar.

(5) Brick

There are three kinds of ordinary clay bricks, No.50, No.75 andNo. 100, which require regular appearance, uniform size, smooth all sides, no deformation, generally no cracks, uniform cross-section structure and brittle knocking, and avoid using underfired bricks, crisp bricks and threaded bricks. The standard size of brick is 240 mm× 1 15 mm× 53 mm, and the geometric size of brick used to build the pool is not limited by the standard size. Bricks used to make pool covers should have complete edges and corners, otherwise the masonry quality will be affected.

(6) Lime

Lime is a kind of gas-hardened inorganic cementing material, which is made of limestone by high temperature calcination. It is mainly used as a modified material for masonry mortar and sealing mortar. Adding lime to cement slurry can increase toughness, water retention and workability. Before using lime, it is usually necessary to pour a certain amount of water to make lime mature and become hydrated lime (hydrated lime). In the process of lime curing, powdered hydrated lime is generated when the amount of water added is small, and becomes lime paste or lime slurry with the increase of water added. The speed of lime curing is related to the quality of lime. There is a glassy hard shell on the surface of quicklime, which not only has a slow curing speed, but also has many uncured particles. If partially solidified lime is used in concrete or mortar, the volume will expand because the lime is still solidified, which will lead to concrete cracking or partial shedding, which will seriously affect the construction quality of the pool. There are limestone blocks in immature lime, and there are often more slag after ripening. Therefore, in the process of use, lime should be sieved and fully matured to eliminate uncured particles in lime. In addition, lime can not be used as a cementing material alone to build biogas digesters, and the dosage should be controlled as a modified material. Generally speaking, it is required that the impurities and powder in lime for pond construction should not exceed 3%, and the cinder and stone chips should not exceed 8%.

(7) Water

Clean, clean and neutral water is selected for pool construction, but acidic water or alkaline water cannot be used, and drinking water is usually selected.

(8) sealing material

For biogas digesters made of concrete, brick, stone and other materials, because there are a large number of capillary channels in these building materials, in order to meet the requirements of biogas fermentation without water leakage and air leakage, it is necessary to brush sealing materials on the cement mortar foundation layer to close the capillary channels.

The sealing material must have good sealing performance, corrosion resistance, toughness, adhesion and wear resistance. It requires small shrinkage, convenient construction and low price.

(9) Pipeline

The pipes needed to build biogas digesters require good air tightness, aging resistance, corrosion resistance and low price. Commonly used pipes are steel pipes, cast iron pipes and plastic pipes.