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Langata Rd, Nairobi, Kenya.
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Biogas Plant Main Facilities

Digester

Digester is a tank of cylindrical form (for better mixing during the fermentation) with internal diameter – 24m and height – 6m. It is built of cast-in-situ reinforced concrete based on sulphate-resistant cement with thickness of walls and bottom – 0,25m. Digester has a volume 3040 m³.

Digester ‘s ceiling is a wood cover which is laid over the wooden beams. In the center of digester there is a column with chapiter. Column serves as support for the concrete roof. On the tank’s wall and in the bottom, there is to be installed pipelines for heating, intended for assurance and maintenance of the optimal fermentation process temperature at mesophilic conditions. Full unloading of the fermented substrate from the digester goes by pipe with D225.

Digester bottom has a slope 1% to the wall, to the side where pipeline of the bottom overflow enters digester. For heat conservation and reduction of heat energy consumption, digester walls and bottom are insulated outside with 50 mm slabs of extruded polystyrene foam. Over the heater, the substructure walls and bottom are insulated with roll damp proofing.

Superstructure and substructure heat insulation is protected by shaped sheet from outside mechanical damages and rodents. Six mechanical mixers are installed in digester. They ensure homogenization and uniform substrate temperature distribution, and also prevent encrustation on the surface of substrate. Mixers are serviced from outside metal platform.
Thereby, normal conditions are created for microorganism’s lifecycle and for organic substance decomposition in digester.

Digester with concrete roof

Specifications
The reservoir of the concrete Height (in light): approx. 8.0 m Diameter (inside): approx. 22.0 m
The total volume of approx. 3.040 m³

Post-Digester

Digested mass of each reactor goes to Post-Digester where the residual organic matter decomposition. Organic compounds are carbohydrates and protein in a torn condition after fermentation in digesters – this allows more fully digested organic substrate and obtain additional biogas. The process of fermentation in post-digester is faster than in the digesters. Post-digesters are equipped gasholders for storage the biogas, as well as the alignment of its pressure.

Gasholder is a half-spheric tank of membrane PELD. Biogas is collected to it during the process of anaerobic fermentation of raw materials. Gasholder takes shape under the surplus internal pressure (0 – 5 mbar) of biogas in the tank. Annular slot in the wooden ceiling of the digester gives easy entering of biogas to gasholder. Outer cover is a protective cover of polyester material covered with PVC.

It protects gasholder from ultraviolet radiation. Leakproof in cover and gasholder attachment point to the wall of digester is ensured by pressure lock. For overpressure release (> 5mbar) and exclusion of under pressure in the tank (<0mbar), it is provided an ultrahigh and ultralow pressure safety valve.

Post-digester with protection dome belt support

Specifications

  • The tank made of reinforced concrete Height of wall: ok.8,0 m
  • Inner Diameter: Approx. 29.0 m Gross Amount: ok.5200 m³
  • Permissible excess gas pressure to 10 mbar Permissible ambient temperature: 40 ° C

Filtrate tank

Reinforced concrete tank designed to collect filtrate after separator. Tank is equipped with level sensors and a submersible pump. Tank functions as buffer tank for the transport of filtrate to the technology or lagoon biogas plant.

Filtrate Tank Specifications

  • Tank made of reinforced concrete Height: approx.3,0 m
  • Volume: approx. 300 m³
  • Permissible excess gas pressure to 10 mbar Permissible ambient temperature: 55 ° C

Technical building / operator

Technical room is designed to accommodate operator, heat substations, compressor, power panels and automation systems and other auxiliary systems, sanitary facilities. Along the perimeter of exterior walls is 70mm thick concrete blind area, 1.0 m wide with a slope from the building of 3% on gravel compacted soil. Walls are made lighter, filling insulation, sheathing, exterior steel corrugated sheet in the middle – drywall

Specifications

  • Building is one-story rectangular shape, degree of fire resistance 3 dimensions: width 6m
  • Length 12m Height 3m
  • Foundations columnar, solid brand of concrete B-20

Napier Grass Storage (silo)

Silos – are intended for conservation (silage) and storage grass. Preservation of grass in silos occurs in an oxygen-free environment which is conducive to their lactic fermentation. Contained in the silage carbohydrate (sugar and starch) under the action of lactic acid bacteria transformed into lactic acid, which is founded.

Preservative in silage. For this green mass, shall be placed in facilities with low permeability for air walling. When laying silage sealed rolling tractors, cars and so on. Is sometimes used self-compacting under its own weight, which is less effective. Silage fodder produced in the trenches, pits, towers and clamps.

Specifications

  • Storage made of reinforced concrete Height: 4m
  • Width: 75m Length 118m
  • Volume: approx. 35500 m³

Biological ponds

Anaerobic fermentation substrate, bacteria absorbed about 90% of organic matter. The first stage of treatment – separation. PRESS screw separator separates suspended particles on thin screens. The resulting filtrate from separation contains 1 to 0.5% of dry matter. The filtrate is sent to the site pre-treatment.

The second stage – aeration and sedimentation in the first biological pond. After pre-treatment, biological wastewater falls into the pond, this is an earthen lagoon, built from natural materials with waterproofing properties (clay, stone). Volume 1600m3 pond, holding time 8 days. After the first pond overflow drains by gravity into the biological landscaped pond.

The third stage – post-treatment in a green Biological pond. Algae and plants in the pond are fed organic matter contained in the effluent. Large masses of green plants trimmed water from the remaining organic matter. The pond has a capacity of 1600m3, the holding time of 8 days. Passing landscaped pond overflow drains by gravity into the third – stocking the pond.

Fourth stage – final cleaning in the stocking and green Biological pond. At this stage, water flows into the pond is almost pure. In the design will be worked out in detail the scheme, taking into account local climatic conditions, soils, etc. The proposed scheme can be simplified or updated measures for wastewater treatment based on the results obtained by empirical evidence.