Introduction:
In vessel composting (IVC) is a managed process in which biodegradable waste is broken down by naturally occurring microorganism with oxygen to produce a stabilized residue called COMPOST.
Inputs:
Biodegradable Municipal waste (BMW), Agriculture waste, Food processing waste.
Process:
Waste is collected and brought to the site where it is first sorted out and shredded to a consistant size. It is than put inside a closed reactor where the composting process is speeded up through the management of water, air and heat. This process is typically takes between 7 to 21 days. The material is than subject to another screening to remove any trace of metals and goes through a further maturation period of upto 10 weeks in a shed. It can than be used as compost or soil conditioner
OUTPUT:
One Ton of BMW will produce approx. 500 KG of compost.
In-vessel composting is an industrial form of composting biodegradable waste that occurs in enclosed reactors. These generally consist of metal tanks or concrete bunkers in which air flow and temperature can be controlled. In-vessel composting can also refer to aerated static pile composting with the addition of removable covers that enclose the piles.
Offensive odours are caused by excess nitrogen or moisture. This may be controlled with a higher carbon to nitrogen ratio or increased aeration by ventilation, mixing, or by using a coarser grade of carbon material. Insects may be controlled by keeping the bin enclosed with the minimum size vents necessary for adequate air exchange. Insects outside the bin enter through the vents and tend to stay inside where the food is. A relatively dry layer of carbon material on top of the compost filters odors and, given adequate ventilation, maintains a moisture gradient that helps keep insects inside where the moisture is.
An in-vessel composting apparatus for continuous processing of food waste into bulk composting material and liquid compost tea, comprising: an insulated apparatus enclosure, a rotable horizontal drum with an input end having an axial input port and an output end having a relatively larger axial discharge port, said drum divided in length by interior partitions into at least first, second and final chambers, each said partition having an axial port by which adjacent said chambers are connected, a compost tea holding tank, means for shredding and inserting said food waste into said first chamber, means for draining and collecting excess fluid from said first chamber, means for passing remaining said food waste in incremental amounts from said first chamber to said second chamber and from said second chamber as bulk composting material into said final chamber, means for permitting low pressure outgassing from said drum, means for measuring temperature in said second chamber, means for incrementally rotating said drum about its axis, means for exchanging air in said drum for outside air, means for containing a batch volume of said excess fluid at elevated temperature for a predetermined holding period, means for percolating said batch volume of excess fluid into said final chamber and out of said final chamber as compost tea into said compost tea holding tank, and an operator station and a controller, said controller communicating with said means for shredding and inserting, said means for incrementally rotating said drum, said means for measuring temperature, said means for exchanging air, and said operator station, said means for shredding and inserting comprising an input hopper connecting to a multi-toothed rotary shredding mechanism connecting to an auger and chute assembly connecting to said axial input port of said input end of said drum, said axial ports being of sequentially larger diameter from said input port to said discharge port, said chambers further comprising interior structure for tumbling contents during rotation, said structure oriented parallel to said axis of said drum, said means for incrementally rotating said drum comprising a base frame with drum supports and drum support rollers upon which said drum rests, and a motorized drum drive system comprising a motor and gearbox attached to said base frame and coupled to said drum by at least one endless belt, said means for exchanging air in said drum for outside air comprising an exhaust fan and duct, said duct connecting to said drum in the vicinity of said input end.
Operation & Control
Before the biodegradable organic waste is loaded into the composting tunnel it must be shredded and provided with the correct level of moisture.
The control of the Composting Process is based on two parameters – the oxygen level and the lowest temperature in the composting waste.
Both these parameters are monitored by the PLC continuously. Each is kept within the operator preset ranges of oxygen level and compost temperature by increasing/decreasing the compost blower speed and admitting fresh air into the compost blower suction duct.
To assist in providing a flexible control system each compost blower speed may be varied.
The speed of each Biofilter blower may also be varied to maintain the required headspace pressure within the tunnel. This is under PLC control and completely automatic once set up.
The control of the plant must rely upon instrumentation to detect conditions throughout the plant.
In-compost temperature monitoring devices are installed at 8 positions along the length of each composting tunnel, plus in the inlet and exhaust ducts for each composting blower, the outlet of each biofilter blower, and the scrubber outlets.
Oxygen level monitoring is provided in the roof of each tunnel.
In each tunnel the pressure in the air space above the compost is monitored and compared with atmospheric air pressure. If the difference exceeds maximum process limits an actuated butterfly valve is automatically opened to atmosphere so that the pressure in the tunnel may remain within the pre-set range. An excessive pressure differential could cause damage to the tunnel doors.
Design & Build
This plant was constructed for Scottish Water to a design by Enviros Consulting of Shrewsbury. It is designed to compost 20,000 tonnes of class 3 waste per year.
The composting plant structure is made up of 4 adjacent tunnels each of which is 25 metres long, 5.3 metres wide, and 5.3 metres high. These tunnels are constructed in reinforced concrete. The roofs are also in reinforced concrete. Each composting tunnel is filled by front loader through the full width and length, and to the required batch height. Each composting tunnel may be operated independently.
The air to each tunnel is supplied through a series of ducts cast into the floor, the air emerging into the compost mass through an array of cast-in plastic nozzles.
A centrifugal blower is provided for each tunnel. This blower may provide up to 8000 m3/hour of air into the underfloor aeration system.
The suction side of the blower is connected to the airspace above the composting material so that the composting blower acts to continuously re-circulate the same mass of air through the compost. Fresh air is provided by a branch pipe on the suction side of the composting blower. Control of the fresh air inlet is effected using a modulating square damper in the branch pipe.
A second centrifugal blower (3000 m3/hr capacity) is provided for each tunnel to draw off exhaust air displaced by any fresh air introduced into the system.
These four exhaust blowers discharge into two air ducts which lead into two proprietary Scrubbers — designed, manufactured and installed by Forbes of Norfolk. The Scrubbers act to remove ammonia and particulates from the airflow and to ensure that its humidity remains high. The Scrubbers also cool the airflow. This ensures that the air outflow from the Scrubbers may be successfully deodorised by the bacteria living on the media within the Biofilter tunnels.
The whole of the Biofilter Blower airflow therefore passes through the Scrubbers and the Biofilters, and is then exhausted to atmosphere.
The Biofilter consist of 2 adjacent rooms — constructed in pre-cast concrete sections - which contain permeable media upon which de-odourising bacteria are grown.
All above-ground air ducting was fabricated in polypropylene pipe (SDR 33) using the butt fusion process, and incorporating various flanged connections to facilitate disassembly. The discharge duct from each Biofilter blower (into the Scrubber) is in round stainless steel ducting, flanged to suit. The manifold connecting each Composting Blower discharge to the four cast-in re-circulating air pipes per tunnel is also fabricated in stainless steel.
10 and 15 days at process temperatures of 35 and 25°C, respectively. The technical feasibility of the UASB-digester combination was demonstrated by continuous flow pilot-scale experiments. A pilot UASB reactor was operated for 81 days at 6 hours HRT [hydraulic retention time] and 15°C and was fed with raw domestic sewage. This period was subsequently followed by an 83 day operation period incorporating a parallel digester unit, which was operated at 35°C. The UASB-digester combination achieved removal efficiencies of total, suspended, colloidal and dissolved CODs of respectively 66, 87, 44 and 30%. Preliminary model calculations indicated that a total reactor volume of the UASB-digester system corresponding to 8.6 hours HRT might suffice for sewage treatment in Palestine. [author abstract]
