Biogas plant for sorted municipal solid waste

Main income sources for waste sorting plant :
- waste recycling;
- recycled materials sales (paper, metal, plastic etc).

While municipal solid waste sorting among all the wastes big volume of food wastes can be found that makes 30-35% of total waste volume. If Anaerobically digested food wastes can produce biogas.

As a result waste sorting plant can have profit in the form of biogas or electricity by burning biogas.

Main by product of organic material decomposition is biogas. Biogas is similar to natural gas by its composition. Synonyms for biogas are such words as methane gas, sewage gas and marsh gas.

Biogas can be:
- used as natural gas;
- stored, pumped;
- burned for generation of heat and electric power;
- used as fuel for vehicles.

Information

Biogas - is gas consisting of approximately 50-70% of methane (CH₄) and 50-30% of carbon dioxide (CO₂). Biogas forms in the process of anaerobic decomposition of organic substrates and actually is the product of bacteria metabolism.

Anaerobic digestion - the process organics decomposition in an oxygen free conditions.

With the help of anaerobic digestion biogas technology allows in short period of time (30 days) produce organic fertilizer containing biologically active substances and micro elements. Here are the main advantages of organic fertilizer produced in biogas plant:

Maximum ammonia preservation and accumulation. Within continuous process of manure storage (composting) 50% of ammonia is lost. Thanks to anaerobic digestion in biogas plant total ammonia N completely preserved in bio-fertilizer, besides dissolved ammonia NH4-N content increased by 10-15%.

Presence of active microflora. High level of organic matter humification serves as a powerful impact for soil microorganisms activation. Ammonia fixing and other microbiological processes are also considerably intensified.

Biogas plant is the most efficient system for recycling of food wastes into organic fertilizer. Organic fraction conversion technology is performed by means of biochemical decomposition (hydrolysis) of high-molecular compounds into simple compounds. As far as anaerobic digestion makes 50-60% organics decomposition digested biomass is free of smell and is ready for use organic fertilizer.

Food wastes after sorting at MSW sorting plant have some distinctive features in comparison with other organic wastes:

1. Presence of inorganic compounds in 15% quantity from total volume of waste.
2. Morphology and fraction size.

That is why food waste are recycled using special two stage technology. The peculiarity is in preparation stage of food waste. After sorting food waste of different fraction size transported to mechanical disintegration unit. Produced homogenous mass containing inorganic and mineral compounds directed to inorganic detachment stage.

Inorganic materials detachment (with density lower then water) is performed with the help of hydraulic sedimentation in special tanks. Inorganic parts with density less then 800 kg/m³come to upper layer and removed be special device. Neutralization of water solution from bacteria and viruses is made in tube sterilization unit during 1 hour at 70 oC temperature.

Using pipe water solution is transported to closed tank which serves as accumulation vessel for sterilization. From that tank water is fed into the tube sterilization device equipped with heat exchanger at inlet stage. Water heating is made by heat exchanger and kept in tubes for a required period of time. In that way water solution is circulated for in hour in a closed circle consisting of accumulation vessel – pump – sterilization unit. During the heating in tank water solution density decreases to 850 kg/m³that makes inorganic particles detachment with higher density more effective. In order to perform this process water flow is changed at an angle 900 degrees hence accumulating small heavy at the tank bottom. These particles are discharged with the help of screw discharger that is installed at the bottom of the tank.

Prepared water solution of organic substances acquires quality of nutrient substrate for bacteria. After sterilization stage substrate pumped to hydrolysis reactor for 8-10 days retention time. In hydrolysis reactor special temperature conditions secured, the humidity is increased and pH level is carefully controlled. From the hydrolysis reactor substrate is supplied in doses to digester, that is very significant factor for bacterial balance preservation. Biogas accumulated in outer bag gas holders made of strong and tensile material. The main condition for digestion is substrate temperature 25-28 С^o and complete mixing. That is why tank reactor is equipped with wall heating system and mechanical mixing system.

Substrate contains simple alcohols and acetate, which are soluble in water. Produced substrate loaded under the pressure to digester where final 30 days stage of waste recycling into biogas and organic fertilizer takes place.

Required conditions for digestion are constant temperature of substrate in the range of 36-38 Сo, even and proportional substrate loading within whole digester square. Substrate supply to digester is made 8-12 times per day in a program time mode with the help of pump.

Digester is a gas proof and hermetically sealed tank made of reinforced concrete. In order to maintain even temperature its internal side equipped with bottom and walls heating system. Such system passes through compulsory hydraulic testing. In order to prevent heat losses the outer side of digesters walls insulated. Mechanical mixer is inside the digester and ensures complete and careful mixing. Fermented biomass discharge takes place with the same periodic as loading. Biogas plant control is made by central program module in a time-programmed mode in accordance with limit sensors values.

Produced biogas collected to the gasholder. Gasholder is can be used as gas proof cover of the digester and execute the function of gas storage orto be performed as a outer bag gasholder. Gasholder membrane is ultraviolet and ozone resistant and has low throughput of biogas. The gasholder material is resistant to fire and is very tensile. Gas extraction is made by pipeline, which equipped with automatic condensate discharge unit and safety devices which secure gasholder from excess pressure. All devices work on limit sensors data. From the gasholder biogas is continuously fed to the co-generation unit or biogas treatment system. Digested biomass directed to separation unit. Mechanical separation unit works 8-12 times per day in a time-programmed mode and detaches digested biomass to solid and liquid fertilizer.

Solid fertilizer transported by loader and liquid fraction is pumped by pipeline to lagoon for storage Solid fertilizer can be applied to the own fields or dried, packed and sold.