How Does Commercial Composting System Work?
The agitated bed system combines controlled aeration with periodic turning. The composting takes place between walls that form long, narrow channels referred to as beds. A rail or channel on top of each wall supports and guides a compost-turning machine.
A loader places raw materials at the front end of the bed. As the turning machine moves forward on the rails, it mixes the compost and discharges the compost behind itself. With each turning, the machine moves the compost a set distance toward the end of the bed. The turning machines work in a similar way to wind-row turners, using rotating paddles or flails to agitate the materials, break up clumps of particles, and maintain porosity. Some machines include a conveyor to move the compost. The machines work automatically without an operator and are controlled with limit switches.
Most commercial composting systems include a set of aeration pipes or an aeration plenum recessed in the floor of the bed and covered with a screen and/or gravel. Between turnings, aeration is supplied by blowers to aerate and cool the composting materials. As the materials along the length of the bed are at different stages of composting, the bed is divided into different aeration zones along its length. Several blowers are used per bed. Each blower supplies air to one zone of a bed and is controlled individually by a temperature sensor or time clock.
The capacity of the system is dependent on the number and size of the beds. The width of the beds in commercially available systems ranges from about 180 to 600 cm, and bed depths are between about 90 and 300 cm. The beds must conform to the size of the turning machine, and the walls must be especially straight. To protect equipment and control composting conditions, the beds are housed in a building or a greenhouse or, in warm climates, covered by a roof.
The length of a bed and frequency of turning determine the composting period. Where the machine moves the materials 300 cm at each turning and the bed is 30 m long, the composting period is ten days with daily turning. It increases to 20 days where turning occurs every other day. Suggested composting periods for commercial agitated bed systems range from two to four weeks, though a long curing period may be necessary.
Silos
Another in-vessel technique resembles a bottom-unloading silo. Each day an auger removes composted material from the bottom of the silo, and a mixture of raw materials is loaded at the top. The aeration system blows air up from the base of the silo through the composting materials. The exhaust air can be collected at the top of the silo for odour treatment. A typical composting time for this method might be 14 days, so one-fourteenth of the silo volume must be removed and replaced daily. After leaving the silo, the compost is cured, often in a second aerated silo. This system minimizes the area needed for composting because the materials are stacked vertically. However, the stacking also presents compaction, temperature control and air flow challenges. Because materials receive little mixing in the vessel, raw materials must be well mixed when loaded into the silo.
Rotating drums
This system uses a horizontal rotary drum to mix, aerate and move the material through the system. The drum is mounted on large bearings and turned through a bull gear. A drum about 3.35 m in diameter and 36.58 m long has a daily capacity of approximately 50 tonnes with a residence time of three days. In the drum, the composting process starts quickly; and the highly degradable, O-demanding materials are decomposed. Further decomposition of the material is necessary and is accomplished through a second stage of composting, usually in wind-rows or aerated static piles. In some commercial systems, the composting materials spend less than one day in the drum. In this case, the drum serves primarily as a mixing device.
Air is supplied through the discharge end and is incorporated into the material as it tumbles. The air moves in the opposite direction to the material. The compost near the discharge is cooled by the fresh air. In the middle, it receives the warmed air, which encourages the process; and the newly loaded material receives the warmest air to initiate the process.
The drum can be open or partitioned. An open drum moves all the material through continuously in the same sequence as it enters. The speed of rotation of the drum and the inclination of the axis of rotation determine the residence time. A partitioned drum can be used to manage the composting process more closely than the open drum. The drum is divided into two or three chambers by partitions. Each partition contains a transfer box equipped with an operable transfer door. At the end of each day's operation, the transfer door at the discharge end of the drum is opened and the compartment emptied. The other compartments are then opened and transferred in sequence, and finally a new batch is introduced into the first compartment. A sill in place at each of the transfer doors retains 15 percent of the previous charge to act as an inoculum for the succeeding batch. Upon discharge, the compost can go directly into a screen to remove oversized particles, which can be returned to the drum for further composting.
On a smaller scale, composting drums can be adapted from equipment such as concrete mixers, feed mixers, and old cement kilns. Although less sophisticated than commercial models, the functions are the same: mix, aerate, and ensure that the composting process starts rapidly.
Transportable containers
A different type of in-vessel system, relies on a transportable vessel and a central composting facility. A number of local farms participate and provide manure as a raw material. Each farm receives a transportable vessel, which resembles a solid waste roll-off container. In its base, the container has aeration pipes that are connected to a blower. At the farm, the manure and dry amendments are loaded daily into the container and aerated for several days until the container is picked up and delivered to the central facility to finish composting. When the composting container is picked up, the farm is provided with another empty container to continue the cycle. The farm supplies the manure and receives bulking agent, compost and/or revenue in return.
A loader places raw materials at the front end of the bed. As the turning machine moves forward on the rails, it mixes the compost and discharges the compost behind itself. With each turning, the machine moves the compost a set distance toward the end of the bed. The turning machines work in a similar way to wind-row turners, using rotating paddles or flails to agitate the materials, break up clumps of particles, and maintain porosity. Some machines include a conveyor to move the compost. The machines work automatically without an operator and are controlled with limit switches.
Most commercial composting systems include a set of aeration pipes or an aeration plenum recessed in the floor of the bed and covered with a screen and/or gravel. Between turnings, aeration is supplied by blowers to aerate and cool the composting materials. As the materials along the length of the bed are at different stages of composting, the bed is divided into different aeration zones along its length. Several blowers are used per bed. Each blower supplies air to one zone of a bed and is controlled individually by a temperature sensor or time clock.
The capacity of the system is dependent on the number and size of the beds. The width of the beds in commercially available systems ranges from about 180 to 600 cm, and bed depths are between about 90 and 300 cm. The beds must conform to the size of the turning machine, and the walls must be especially straight. To protect equipment and control composting conditions, the beds are housed in a building or a greenhouse or, in warm climates, covered by a roof.
The length of a bed and frequency of turning determine the composting period. Where the machine moves the materials 300 cm at each turning and the bed is 30 m long, the composting period is ten days with daily turning. It increases to 20 days where turning occurs every other day. Suggested composting periods for commercial agitated bed systems range from two to four weeks, though a long curing period may be necessary.
Silos
Another in-vessel technique resembles a bottom-unloading silo. Each day an auger removes composted material from the bottom of the silo, and a mixture of raw materials is loaded at the top. The aeration system blows air up from the base of the silo through the composting materials. The exhaust air can be collected at the top of the silo for odour treatment. A typical composting time for this method might be 14 days, so one-fourteenth of the silo volume must be removed and replaced daily. After leaving the silo, the compost is cured, often in a second aerated silo. This system minimizes the area needed for composting because the materials are stacked vertically. However, the stacking also presents compaction, temperature control and air flow challenges. Because materials receive little mixing in the vessel, raw materials must be well mixed when loaded into the silo.
Rotating drums
This system uses a horizontal rotary drum to mix, aerate and move the material through the system. The drum is mounted on large bearings and turned through a bull gear. A drum about 3.35 m in diameter and 36.58 m long has a daily capacity of approximately 50 tonnes with a residence time of three days. In the drum, the composting process starts quickly; and the highly degradable, O-demanding materials are decomposed. Further decomposition of the material is necessary and is accomplished through a second stage of composting, usually in wind-rows or aerated static piles. In some commercial systems, the composting materials spend less than one day in the drum. In this case, the drum serves primarily as a mixing device.
Air is supplied through the discharge end and is incorporated into the material as it tumbles. The air moves in the opposite direction to the material. The compost near the discharge is cooled by the fresh air. In the middle, it receives the warmed air, which encourages the process; and the newly loaded material receives the warmest air to initiate the process.
The drum can be open or partitioned. An open drum moves all the material through continuously in the same sequence as it enters. The speed of rotation of the drum and the inclination of the axis of rotation determine the residence time. A partitioned drum can be used to manage the composting process more closely than the open drum. The drum is divided into two or three chambers by partitions. Each partition contains a transfer box equipped with an operable transfer door. At the end of each day's operation, the transfer door at the discharge end of the drum is opened and the compartment emptied. The other compartments are then opened and transferred in sequence, and finally a new batch is introduced into the first compartment. A sill in place at each of the transfer doors retains 15 percent of the previous charge to act as an inoculum for the succeeding batch. Upon discharge, the compost can go directly into a screen to remove oversized particles, which can be returned to the drum for further composting.
On a smaller scale, composting drums can be adapted from equipment such as concrete mixers, feed mixers, and old cement kilns. Although less sophisticated than commercial models, the functions are the same: mix, aerate, and ensure that the composting process starts rapidly.
Transportable containers
A different type of in-vessel system, relies on a transportable vessel and a central composting facility. A number of local farms participate and provide manure as a raw material. Each farm receives a transportable vessel, which resembles a solid waste roll-off container. In its base, the container has aeration pipes that are connected to a blower. At the farm, the manure and dry amendments are loaded daily into the container and aerated for several days until the container is picked up and delivered to the central facility to finish composting. When the composting container is picked up, the farm is provided with another empty container to continue the cycle. The farm supplies the manure and receives bulking agent, compost and/or revenue in return.
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