WO1992018611A1

WO1992018611A1 - Air recovery system

Info

Publication number: WO1992018611A1
Application number: PCT/US1992/003130
Authority: WO
Inventors: Larry J. Finn
Original assignee: Bedminster Bioconversion Corporation
Priority date: 1991-04-15
Filing date: 1992-04-13
Publication date: 1992-10-29

Abstract

An air recovery and odor control system including a tipping area (10), rotating digesters (20), processing area (12), and aeration area (14) for use in a composting process which system recovers exhaust air produced by the composting process operations for reuse in the composting stages which occur in the digesters (20) and aeration area (14) of the system prior to release of the air to the atmosphere whereby to deodorize the air, improve operating efficiency and enhance pathogen kill.

Description

AIR RECOVERY SYSTEM

Field of the Invention

The present invention relates to an air recovery control system for use with apparatus for making organic fertilizer, sometimes referred to as compost, or other fermentation products, from organic waste material such as municipal solid waste, sewage sludge, and the like. Background of the Invention Prior art systems for achieving composting of solid waste and sewage sludge typically employ one or more multi-stage digesters in which material being treated undergoes staged microbial decomposition. The conventional digester is divided into two or more compartments or stages and during material processing is rotated while air is circulated through the digester at controlled rates under predetermined conditions in a flow direction counter to the material flow. The climate in each stage is maintained to achieve the optimum development of the type and species of microorganism predominant in that stage. Spent air is vented from each stage to the atmosphere as needed to maintain an optimum climate in each of the operating stages. Temperatures are kept below 150 degrees F (65.56 C) to insure the maximum rate of composting. Moisture usually does not require adjustment during this phase of the operation, but if the material becomes too dry for composting, the digester vessel is equipped with a water manifold which allows the addition of moisture to the mass undergoing treatment. Since this phase of the composting

SUBSTITUTE SHEET is conducted entirely within an enclosed vessel, the only source of odor production is the exhaust air. Typical of such prior art systems and methodology of operation are those set out and described in D.S. Patents 3,245,759 and 3,138,447, the teachings of which are hereby incorporated by reference.

The method and apparatus for manufacture of compost described in those patents is designed to produce aerobic decomposition of organic waste materials by maintaining within the apparatus in which the method is carried out, conditions suitable for optimum propagation of the different types of aerobic bacteria on which such decomposition depends. The apparatus comprises a digester in the form of a cylindrical drum mounted for rotation on an axis which is slightly declined towards the discharge end relative to the horizontal. The interior of the digester is divided into a series of compartments or chambers by a plurality of transverse partition is provided with transfer buckets which are selectively opened and which, when opened, transfer material from compartment to compartment from the higher to the lower end of the drum, the raw waste organic material being fed into the digester at the higher end and the finished being withdrawn at the lower end. While the apparatus described in those patents is suitable for carrying out a composting process, environmental, regulatory and recycling forces have led to a refocusing of disposal options. Landfilling, which

SUBSTITUTE SHEET currently accounts for approximately 78 percent of municipal solid waste disposal, is in severe jeopardy. The number of landfills has decreased from 20,000 in 1978 to 6,000 in 1988 and is estimated to decrease to 2,100 by the year 2000. In addition, incineration, which accounts for approximately 11 percent of municipal solid waste disposal has come under intense public scrutiny. Public opposition has been responsible for the curtailment of a significant number of proposed incineration projects. The co-composting technology to which the present invention has application embodies a fermentation reactor which is employed to accelerate the microbial conversion of solid waste and sewage sludge into a high quality compost. The process has the ability to compost municipal solid waste and sewage together hence the term co-composting, thus addressing the two principle waste management problems communities will face in the next few decades. unlike incineration and landfilling, which both generate a number of toxic by-products including harmful emissions of gases, heavy-metal laden ash and leachate, the co-composting process of this invention produces no toxic by-products. Additionally, it is a non-grind, non-shred, in-vessel aerobic process that produces a high quality compost while simultaneously sanitizing non-σompostable residual tailings. The process results in municipal solid waste volume reductions as high as 85 percent while simultaneously providing a solution to the sewage sludge problem. This is extremely important in light

SUBSTITUTE SHEEl^" of the fact that, based upon current projections, municipal solid waste generation will exceed landfill capacity by approximately 25 percent by the year 2000. Summary of the Invention It has been discovered that a surprising number of advantages result if the air heretofore exhausted to the atmosphere is retained within the system and reused in critical phases of the fermentation process. By recovering the exhaust air produced by the process operations and reusing it in critical phases of the operation system efficiency of the overall system is improved, pathogen kill is enhanced and odor control is improved.

As noted above, the air recovery and odor control system comprising this invention is meant to serve the purpose of taking advantage of the exhaust air produced by the process operations and reusing it in the further operations of the system. The reuse of the process produced air is a means of limiting the amount of air exhausted into the outside atmosphere, which is of major concern to environmentalists and concerned citizens. The reuse of the preconditioned air is also a benefit in that external, ambient temperature air is not required for the most important steps in the process, namely, the curing of the compost during the process to further reduce pathogens. During this critical stage, it is important that the temperature be maintained at a fairly precise level for the entire period designated by the United States Environmental Protection Agency guidelines in order to achieve the

SUBSTITUTE SHEET •

5 prescribed pathogen destruction. Should external ambient air be used in a cold climate for example, without prior heating and the concomitant expenditure of energy, the time to achieve the correct temperatures for the process would be extended and more of the microbes which are essential to carrying out the process, could be damaged prior to reaching the proper temperature level.

I have discovered that by taking advantage of the exhaust air produced by the process operations and reusing it in the further operations of the system the air exhausted into the atmosphere is substantially deodorized and as noted above use of the preconditioned air is also a benefit in that external, ambient temperature air is not required for the most important process step of pathogen destruction.

In the typical system utilizing a multi-stage digester for the co-composting of municipal solid waste and sewage sludge, the residence time in the digester is normally three days. Upon removal from the digester, the material, now compost plus nondegradables, is rough screened by a one-inch (2.54 cm) trommel. Two fractions emerge from the trommel. One is the residue which fails to pass through the screen. This material is discharged back onto the tipping floor of a typical composting facility and sent to landfill. The rough compost is then conveyed to an area in which further composting or curing is carried out. For purposes of this invention this phase of further processing of the rough compost will be referred to

SUBSTITUTE SHEET generically as curing. The term curing area as used herein shall mean an area where organic material that has undergone the rapid initial stage of composting is further stabilized into a humus-like material. It is in this stage of the process in which the pathogens primarily undergo destruction. The curing stage of the process is an important phase of the dynamic aeration system which comprises the subject invention in that it is during the approximately two-week stay of the compost in the curing area that the pathogens are destroyed. To accomplish this step of the process the temperature of the compost must be maintained within certain parameters. The compost must be kept at a point in excess of 55 degrees C, yet under approximately 65 degrees C for a period of not less than 72 hours. These are the requirements and guidelines for pathogen destruction and for the production of compost to be considered safe for handling and reuse.

By maintaining an essentially closed system in which effluent air from the curing beds and digesters is captured for reintroduction and reuse in the system the release of harmful pathogens is avoided while at the same time a more efficient and environmentally acceptable process is achieved. The process is further enhanced by totally enclosing the three major areas of this operating facility. These areas typically are the tipping building, the aeration building, and the process building details of which are set out below.

Brief Description of the Drawings

SUBSTITU i E SI For the purpose of illustrating the invention, there is shown in the drawings a form which is presently preferred; it being understood, however, that this invention is not limited to the precise arrangements and instrumentalities shown.

FIGURE 1 is an isometric overview of a composting facility embodying the present invention;

FIGURE 2 is an aerial view of the facility shown in Figure 1 with portions of the enclosure removed to show the internal construction of the plant;

FIGURE 3 is a flow diagram of the overall composting facility detailing the aeration and odor control system;

FIGURE 4 is floor plan showing constructional details of the facility and its operational features;

FIGURE 5 is a detailed showing of one form of curing channel utilized in the composting process; and

FIGURE 6 is a transverse sectional view taken along line 6-6 of Figure 5. Detailed Description of the Invention

Referring now to the drawings and more particularly to Figures 1 and 2 thereof there is shown a completely enclosed composting facility comprised of three major areas, tϊie tipping building 10, a processing building 12 and an aeration building 14. The tipping building floor is where solid waste is dumped and sorted. Unacceptable waste, for example, white goods, car batteries, tires, large pieces of wood, etc., is rejected and sent to a

- SUBSTITUTE SHEET landfill. As diagrammatically shown in Figure 3, the acceptable waste is then moved as by means of an end loader 16 from the tipping floor into ram pits 18 positioned at the entry of the digesters 20 from where the waste is loaded directly into the digester drum by means of a screw driven or hydraulic loading ram. Sewage sludge 22 delivered to the plant is stored in a liquid sludge tank 24 from where it is pumped by liquid sludge pumps 26 directly into the digesters 20 as needed to maintain the proper carbon/nitrogen ratio essential to efficient composting.

The sewage sludge is added to the solid waste to bring the moisture content of the overall digester charge up to about 50 percent and to bring the carbon/nitrogen ratio to 35:1 or less. The material is then processed through the digester for a period of three days. As previously noted, the digester is typically divided into three fermentation chambers or stages by means of internal partitions. Material is discharged from the digester after approximately three days of residence time. Upon removal from the digester, the material, now compost plus nondegradables is transported by belt conveyors 28 and 30 to a trommel screen 32 where it is rough screened into two fractions. One is the residue a nondegradable material, sometimes hereinafter referred to as inorganic material, which fails to pass through the screen and which is discharged back onto the tipping floor by belt conveyor 33 where it is pushed into a transfer trailer for landfill disposal. The nondegradables which have passed through the

SUBSTITUTE

digester are sanitary because of the high temperatures encountered in the digester and will not decompose for produce leachate in the landfill. The second fraction is rough compost, sometimes hereinafter referred to as organic or degradable material, which is conveyed to the curing area 34 by belts 35 and 81.

The first area treated in the air recovery system is the floor area in the tipping building 10. Ceiling mounted vents or hoods 36 overlie the floor area and serve as an intake for effluvium emanating from this area and recover it for use in the channel area 34 via ducting 46.

The air circulation and recovery in the tipping building^* is accomplished by means of steel duct work with intake vents 36 running from a 15,000 cfm (424.75 com) electric vent fan 42. This results in between 3 and 4 air changes per hour for the building. Even though the air recovery system is quite sufficient to handle the odors produced, the tipping floor is washed down daily and raw garbage is not allowed to remain on the floor for more than a few hours.

Another area of the tipping building where odors can occur is in the area of the ram pits 18. As the ram is withdrawn from the digester to allow for more waste to be inserted, internal air from the digester is released. While this air is not as malodorous as that from the tipping floor itself, it is nonetheless far from pure. This air is recovered by air intake hoods 44 for recirculation by fan 48 to the aeration channel area 34 via

SHEET ducting 46.

Unlike the tipping building, within the aeration building 14, the detectable odors are not the offensive, malodorous type. The prevalent odors encountered at this location are the musty, earthy type variety characteristic of aerobic composting. As already noted, the process functions carried out in the curing area 34 are an important feature of the dynamic aeration system comprising this invention. It is during the two-week stay of the compost in the curing channels 50, as best seen in Figures 2, 4 and 5, that pathogen destruction is achieved. To accomplish this step of the process, the temperature of the compost must be maintained within prescribed parameters. The compost must be kept at a point in excess of 55 degrees C yet under approximately 65 degrees C for a period of not less than 72 hours. These are the requirements and guidelines of the United States Environmental Protection Agency for pathogen destruction and for the production of compost to be considered safe for handling and reuse. As noted above, make up air for the aeration channels comes via ducting from the air intake system in the ceiling of the tipping building which air is that which is exhausted from the fermentation reactors or digesters 20.

Air for the curing channels 50 is furnished by a series of blowers which force air into the compost deposited within the curing channels through a network of perforated pipe 51. A 15,000 cfm to 18,000 cfm blower fan moves the air to the intake point of the small, 1 hp 400

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11 cfm electric blowers 52 which served to aerate the compost stored in the concrete curing channels 50. These blowers are controlled by computer 54, Figure 4, and are activated/deactivated based on parameters pre-programmed into the computer that.operate the motor control center 56. When the temperatures reach a point outside the designated range, the computer is programmed to activate the blowers to deliver cooler air to and through the compost housed within the channels, to lower the rising temperatures. Once the temperatures have been returned to the optimal zone, the blowers are deactivated by the computer. This is a continuously occurring cycle, as the temperatures within the compost naturally increase when left unregulated. Monitoring of the temperatures within the individual aeration zones within each curing channel is achieved through use of thermocouples placed in the channel walls. Their output is fed to the computer 54 which, in turn, controls operation of the blowers 52.

A prime benefit of the air recovery system is that external, ambient air is not required to be used for the compost curing step. In many geographic areas, the ambient air has a great temperature differential, the introduction of outside air into the compost pile could damage many of the beneficial microbes, while causing the blowers to have to be activated/deactivated at a much quicker and much less regular rate. Also, the ambient air may not have the correct humidity or moisture levels, resulting in further variation of the warming/cooling cycle as well as the

SUBSTITUTE SHEET blower activation schedule.

As the air migrates thorough the six to eight feet of compost in the curing channels 50, it becomes more humid. Being warmer, as this air exits the compost pile, it naturally rises towards the building's ceiling. At this point, the air is collected by air intakes 60 and 72 (See Figure 3} to any one or all of three locations, the final compost curing area 66, to the soil filter 74, or for reintroduction into the digesters 20 via duct means 62 as individual conditions warrant. Air intake vents 72 located in the ceiling are adapted to collect effluent air and vent it to the atmosphere through the soil filter 74. The soil filter is a mixed intermediate consisting of two parts compost and one part gravel. The filter itself has a surface area of 0.5 square feet (.0465 square meters) per cfm (.0283 cubic meters per minute). The collected air is then blown through the filter and, in transit, is filtered of the odor producing elements. At this point, the air is dispersed into the surrounding environment with no ill or disturbing effects. As indicated above, located within the aeration building is a ceiling mounted air intake fixture 60 that, if desired, by means of valving 77, can direct exhaust air flow through the duct system 62 to combine with the makeup air for the digesters 20 fed by rotary blowers 79. A floor plan of a composting facility depicting operational features of the system is shown in Figure 4. Constructional details of a preferred channel construction used for retention of rough screened compost is shown in

SUBSTITUTE SHEET Figures 5 and 6. Each channel or trough is constructed of a 8 inch (20.32 cm) thick x 84 inches (213.36 cm) high x 195 feet (59.44 meters) long reinforced concrete walls 80, each having a maximum compost storage area of 6 feet (1.83 meters) deep x 9 feet (2.77 meters, 10.16 cm) 4 inches wide x 180 feet (54.86 meters) long or 10,244 cubic feet (290.30 cubic meters) of capacity. Auto-loading of these channels is accomplished by using a tripslinger conveyor belt 81. When the compost deposited within a trough reaches the desired height, a censor located within a side wall activates the tripper which is then moved forward to the next available channel The channels can also be loaded manually with a wheel loader. Each trough is divided into individual treatment zones 82, as best seen in Figures 4 and 6, along its length. Each treatment zone occupies a length of approximately 25 feet. The floor of each trough, Figure 5, is comprised of a composite of 9 inches (22.86 cm) of washed rock 84 overlaid by a geotextile fabric 86 which, in turn, is overlaid by a 3 inch (7.62 cm) washed gravel bed 88. Imbedded in this materiel is an air plenum 89 which feeds a figure eight air dispersion loop 90 as seen in Figure 6. The dynamic channels can conveniently be equipped with a mechanical compost turner or robot not shown. The robot can be programmed to turn over the compost in each channel or trough once per processing day. The passing of the turner or robot causes the compost to move towards the discharge end of the channel approximately 12 1/2 feet (3.81 meters) per turning. This will move the

SUBSTITUTE SHEET compost completely through this curing process in twelve working days or two work weeks.

As previously discussed, each dynamic channel is divided into five individually controlled aerated zones 82. Each of the blowers 52 which feet the individual aeration zones is rated at 370 cfm (10.49 cubic meters per minute) at 6 inches (15.24 cm) of static pressure. An underfloor pipe interconnects each blower to the end orifice of the in-place figure eight perforated plumbing 90. Each of the blowers in turn is preconnect to its own electrical starter which is controlled by the programmable logic controller or motor control circuit .56. This PLC is then slaved to the computer 54. In addition, each aerated segment of the dynamic channel has a temperature censor located in the channel wall at a height of three feet which is connected to the programmable logic controller for temperature monitoring of the compost in that segment of the channel. The operator can then manipulate the blowers and the temperature via the computer 54. Each of the segments will have a high temperature set point and a low temperature set point. When the temperature reaches the high set point, a blower will come on and will stay on until the temperature fails to the lower limit. This cycle will continue indefinitely as outlined above for each segment of the channel. The computer 54 is then used to record the temperatures in each segment on regular intervals and to store this data for subsequent report writing and proof of meeting the process to further reduce pathogen

SUBSTITUTE SHEET requirements. During the course of a two-week period, the robot will automatically move the compost to the discharge end of the channels. At this point, the compost can be easily accessed with a front end loader 96 (Figure 2) . When the compost is removed from the dynamic curing channels, it is stacked in a pile in the compost curing area 98. The piles should not exceed 10 feet (3.05 meters) in height. The static curing pile is turned no less than once each week and restack closer to the final screening station 100 each time it is turned. Air is directed to the compost piles through channels formed in the concrete slab floor of the process building. The air is covered from the aeration^*building ceiling by means of a 10,000 cfm electric fan 102 to the make up area for the small (lhp, 400 cfm) (1 hp, 11.34 cubic meters per minute) blowers which supply air through the in-slab channels. The channels are covered with a perforated grating upon which the compost is piled for further storage and curing. Air is blown through the channels and into the compost to insure that no anaerobic conditions appear. Compost is then stored here for a period of time as prescribed by local regulations. Satisfying these regulations, the compost is ready for a final screening and preparations for shipment. This final aeration step in the process creates much the same effect as the external soil filter in that it filters air prior to its being released into the atmosphere. Air used in the process building for the further curing of compost as just described is derived from an intake fixture and duct that

SUBSTITUTE SHEET draws off a portion of the building's preconditioned air for use in this process step. [A proprietary screening process of the inventor assigned to the present assignee is that set out in Application Serial Number 07/667,849, the teaching of which is hereby incorporated by reference.] The final screening [as taught in the referenced application] utilizes a dual motion screen 104 (Figure 2) . The screen preferably comprises one or more concentric decks. The first or inner deck includes a screen having relatively large holes for screening out larger granular materials. The second or outer deck includes a screen having relatively smaller holes for screening out fine materials. The two-stage deck screening system provides efficient material separation so as to produce a superior homogeneous product. After final screening, the compost is stored in a finished compost storage area 106. There is adequate space in this area to store the compost for an additional two weeks.

To recap, a prime benefit of the air recovery system that external, ambient air is not required to be used in the composting process. In many geographic areas, the ambient air has a great temperature differential, the introduction of the air into the compost pile could serve to damage many of the beneficial microbes, while causing the blowers to have to be activated/deactivated at a much quicker and much less regular rate. Also, the ambient air may not have the correct humidity or moisture levels, resulting in further variation of the warming/cooling cycle ^SUBSTITUTE SHEET as well as the blower activation schedule.

The entire air recovery/odor control system plays an important part in the overall process and produces benefits that are not realized in the use of nonpreconditioned air drawn from outside the plant. Its integral role in the process results in a highly energy efficient system, a superior end product and a process having little if any malodorous air effluents and having reduced pathogens.

SUBSTITUTE SHEET

Claims

I claim:

1. A composting facility comprising: a horizontally disposed, multi-stage digester drum mounted for rotation about its longitudinal axis and having one or more partitions in a plane transverse the longitudinal axis of the drum dividing it into a series of treatment compartments, an entry at one end of said drum for introducing material to be treated and a discharge port at the opposite end of said drum for discharging treated material; a tipping area for deposit of waste materials; means for storing sewage sludge; means for feeding a predetermined charge of waste and sewage sludge into said digester; means for rotating said digester drum thereby to effect turning and mixing of the waste and sewage being processed within said drum; screening means for segregating the output of said digester into organic and inorganic fractions; a plurality of curing channels for storage of the organic fraction; material handling means for conveying the output of said digester to said screening means and for conveying the organic fraction segregated by said screening means to said curing channels; means for continued periodic turning of the organic fraction deposited in said curing channels; an aeration system for circulating air through the digester

SUBSTITUTE SHEET drum at controlled rates under predetermined conditions of temperature and humidity in a flow direction counter to the material flow through said digester, said aeration system including blowers and associated ducting for conveying air to said curing channels for dispersion through the organic fraction deposited therein; means for venting process gases from said digester as required to maintain optimum climatic conditions within the digester for effective microbial decomposition of material undergoing treatment; enclosure means preventing release to the atmosphere of emanations from the tipping area and process gases emanating from the digester and curing channels; and means for recovering such emanations within said enclosure and for recycling same through said digester and curing channels thereby to deodorize the air, improve operating efficiency, and enhance pathogen destruction.

2. A system for converting solid waste and sewage sludge into compost, comprising: one or more compartmentalized horizontally mounted rotatable cylindrical digester drums means for feeding a predetermined charge of waste and sewage into said digester drums; means for rotating said drums thereby to effect turning and mixing of the waste and sewage; means for screening the output of the digester into biodegradable and non-biodegradable fractions: means for storing the biodegradable fraction;

SUBSTITUTE SHEET means for effecting continued periodic turning of the stored fraction; material handling means for receiving the output of said digester drums and for conveying the output to said screening means and for receiving the screened biodegradable fraction and conveying same to said storage means; means for supplying air to said digester and the biodegradable fraction contained within said storage means; means for venting process gases from said digester drums and storage means; enclosure means preventing release of process gases to the atmosphere emanating from said digester drums and stored fraction; means for recovering process gases within said enclosure; and means for recycling the recovered process gases for reuse in the system.

3. The system set forth in Claim 8 wherein said means for recovering process gases comprises a series of fans, ducts, and air intake hoods disposed in overlying relating to both said stored biodegradable fractions and said digester-gas venting means such that emanations therefrom are recovered for reuse in the composting process.

4. The process of converting municipal solid waste and sewage sludge into compost which comprises: providing an enclosed facility; biodegrading a composite charge of waste and sludge in a compartmentalized horizontally mounted rotating cylindrical

SUBSTITUTE SHEET digester drum; screening the output of said digester into organic and inorganic fractions; depositing the organic fraction in a curing area; supplying preconditioned air to said digester and curing area for use therein; and recovering process gases emanating from said digester and curing area within said facility for reuse in the digesting process.

SUBSTITUTE SHi=E