Anaerobic Digestion Technology

Anaerobic Digestion (AD) is a biological process that happens naturally when bacteria breaks down organic matter in environments with little or no oxygen. It is effectively a controlled and enclosed version of the anaerobic breakdown of organic waste in landfill which releases methane.

Almost any organic material can be processed with AD, including food scraps, fats, oils & grease (FOG), tissue paper, garden waste, sewage and animal waste.

The two main products of anaerobic digestion are biogas and a solid residual.Noble Exchange products and solutions for Bio-degradable waste management systems are based exclusively on Anaerobic Digestion technologies.

Anaerobic digestion process

Biogas is produced through anaerobic digestion of organic waste (or biomass) and can be burned to produce electricity or heat. Biogas is a natural source of energy and its production relies on natural processes. The food waste is placed in digestion tanks where it is broken down by microorganisms in an oxygen-free environment. During the decomposition process, the microorganisms release methane gas or Biogas.

Look at the figure of Anaerobic digestion process to understand the process better.

Anaerobic digestion process

NEX Anaerobic Digestion and Compressed biogas generation Plant

Digestion

The biological degradation of the substrates takes place in the first and second stage digesters. Under anaerobic conditions, a temperature of 35-40°C and a continuous mixing, biogas is being produced by the conversion of the dissolved organic matters. The process is a bacterial decomposition of wet organic matter, comprising the hydrolysis of insoluble biodegradable organic food matters, the acidogenesis forming acids from smaller soluble organic molecules and the methanogenesis, where biogas is produced. The emitted biogas is collected in the roof of the digester tanks, which also functions as a gas storage room. The digesters are insulated with expanded polystyrene and cladded with coated sheets from outside; This is to avoid heat losses and to overheat due to external conditions.

Digestion

Upgradation of biogas and conversion to CBG (Compressed Biogas): The generated biogas consists of various gases such as Methane, Carbon Dioxide, Hydrogen Sulphide (in minor quantities) and moisture in vapour form. The generated gas is scrubbed with water to remove the CO2 and contain around 95 – 97% pure methane and the rest CO2.

The Upgradation Technology system is a four-step system consisting of the following process:

  • The designed system uses the technique of Vacuum Pressure Swing Absorption (VPSA)

    Step-1

  • Removal of H2S and moisture using a special bed molecular sieves and a combination of desiccant filter and activated carbon

    Step-2

  • Removal of CO2 from Biogas by absorption process involving the use of scrubbing medium (i.e. molecular sieves in this case)

    Step-3

  • The system works in swapping mode where pressurising and depressurising works in alternate continuous mode. When one column is under pressurising for scrubbing the required gases out, the second column works under decompression mode

    Step-4

  • Drying of clean gas for further application using special sieves again

    Step-5

Advantages:
  • High methane content in Clean Gas (up to 95% methane and above)
  • Complete heat recovery
  • High energy efficiency with minimum methane loss to the scrubbing medium
  • High level of automation resulting in easier operation of the system
  • No expensive chemicals required
  • Low pressure resulting in low maintenance of the system parts
  • Consistent results (even for part load operation)
High-Pressure Compressor

It is the most critical stage of the compression, and all safety features are included as per the PESO requirements, including Zone Classification. Cleaned/upgraded biogas will be filled in the cylinders. Each Bio CNG cylinder will have the water capacity of 75 litres which will be equivalent to 10.55 Kg of compressed gas holding. The cylinders will be supplied in groups called “The Cascades''. Each cascade will house 40 such cylinders which will have the capacity to store 462 Kgs of compressed gas. The cylinders will be of high-pressure seamless steel CNG cascade cylinders. Three Bank systems and copper vent piping as per IS: 7285 having Working Pressure at 15° C – 255 bar and with Hydro Test Pressure – 425 KGF/CM2.

Digested slurry

The digested slurry can be upgraded to an organic bio-fertilizer. The separated water from the slurry can be re-used for the processing of the next day’s operations. There is also an Effluent treatment plant installed for the separated water to bring the CoD down to the pollution control board limits. The treated water can be used for flushing or gardening as well as the daily addition for the biogas plant usage.

Unique Features of Processing Plant:
  • Odorless & Noiseless. The central processing plant is based on an anaerobic digestion system, gas up-gradation system and compression system that converts the organic waste slurry into compressed biogas (CBG).
  • Equipped with a submersible agitator which helps in better digestion and circulation of the slurry inside the digester and avoid settling of the solids inside the digester.
  • Designed to handle all types of biodegradable waste including fruit & vegetable peels, process & plate waste, used kitchen oil/grease.
  • Designed for continuous feeding of waste.
  • Ensures maximum volatile solids degradation & avoidance of heat losses.
  • Waste water from the system is re-circulated to ensure minimal fresh water requirement or discharge.
  • Conforms to European standards of safety and environment. Several technical agencies have analyzed the proposed NEX technology and have found it to be an environmentally efficient way of disposing of the wet and organic/ food waste.
  • By-product generated from the process will be nitrogen rich, organic fertilizer that replaces conventional chemical fertilizers for farming and gardening purposes.
  • Compressed Biogas (CBG) produced at the processing plant can be marketed to institutional clients for replacement of conventional energy sources such as LPG, diesel, kerosene, furnace oil, etc.