The present invention relates generally to the treatment of flowable and non-flowable organic waste, and in particular to achieving the efficient treatment of both wastes simultaneously, by anaerobic digestion using a floating bed process and apparatus.
It is known that solid (non-flowable) and fluid (flowable) organic wastes may be treated using anaerobic microbiological treatment processes. Various anaerobic systems have been developed to implement the anaerobic digestion of organic wastes. These systems typically work on fluid wastes and slurries and employ a sludge blanket containing the treating bacteria wherein the fluid waste to be treated passes upwardly through the sludge blanket. In these systems the greatest concentration of bacteria is at the bottom of the sludge blanket and least near the top of the sludge blanket. In these systems, at least a portion of the waste stream passing up through the sludge blanket is drawn off and reintroduced below the sludge blanket for another pass up through the blanket. These systems are inefficient for a number of reasons and are ineffective for the treatment of solid nonflowable materials.
The management of organic nonflowable waste materials is a particular concern in the food processing industry as well as within municipalities. In the food processing industry there are large accumulations of vegetable matter from food processing operations such as the husks and cobs remaining after corn is canned or frozen and pea shells remaining after canning or freezing peas. The organic fraction of municipal solid waste, lawn and garden wastes, animal manure, mixtures of the above, and others also occupy large volumes in landfills and are noxious odor concerns. Moreover, extreme environmental contamination may occur when rainwater leaches through such materials and flows into waterways, drinking water supplies and ground waters.
Several means of management and disposal have been attempted including land filling, incineration, use as fertilizer, soil conditioner, animal feeds, and others. However, none of these alternatives have found wide spread practice for many reasons.
Nonflowable organic wastes have one thing in common, which is consistently valuable. They are biodegradable and renewable. If the solar energy contained within these materials could be liberated and captured, a renewable energy source could be derived, while simultaneously eliminating a costly environmental problem.
Nonflowable wastes also are lighter than water and, in the presence of anaerobic microorganisms, produce tiny carbon dioxide and methane gas bubbles which further enhance the capability of these waste materials to float in the presence of water. It is conceivable to develop a floating bed of organic waste within a vessel containing a liquid seeded with anaerobic bacteria. When the liquid is passed through the bed, the bed will filter and entrap the bacteria within the float layer. The solids in the bed also will serve as the media on which these microorganisms can attach themselves. Such a floating bed would have the capability to digest the nonflowable organic material within the bed and simultaneously treat a wastewater containing biodegradable organic material passing through the floating bed. Thus, a floating organic bed reactor can simultaneously treat both flowable and non-flowable waste streams to convert the biodegradable organic material to energy as biogas, reduce the volume of non-flowable waste and remove pollutants from flowable waste streams.
Accordingly, it is an object of the present invention to provide a system for treating both liquid and solid organic wastes.
Another object of the present invention is to provide a system for the biological treatment of both liquid and solids waste streams.
A further object of the invention is to provide a system for the anaerobic treatment of organic liquid and solid wastes.
Yet another object of the present invention is to provide a system for the anaerobic treatment of organic liquid and solid wastes in a common treatment vessel.
Still another object of the present invention is to provide a method and apparatus for the anaerobic treatment of organic and solid wastes in a common treatment vessel utilizing floating bed technology.
Another object of the present patent is to produce energy from the co-digestion of the flowable and non-flowable wastes.
Still another objective of the patent is to reduce the volume of non-flowable organic waste through its conversion to energy and to treat contaminated wastewater, converting its biodegradable organic fractions to energy and leaving treated wastewater for discharge to the environment.
Briefly stated, the present invention comprises a process and apparatus for the anaerobic biological treatment of liquid (flowable) and solid (nonflowable) wastes in a closed bioreactor vessel. Both solids and liquid wastes are introduced into the vessel at an inlet end and pass longitudinally through the vessel. In transit and while in the vessel, the wastes are contacted with anaerobic microorganisms. These organisms digest the organic matter and the product of the digestion is removed from an opposite end of the vessel. The action of the microorganisms on the organic matter, which is lighter than water, generates gases, such as methane and carbon dioxide, which assist to buoy the organic solids causing them to form a floating bed of solids. The anaerobic microorganisms attach to the organic solids and become entrapped within the floating bed.
As digestion proceeds, particle size of the solid waste is reduced and the density of the material increases. Over time, and as more solids are added to the vessel, a gradient develops within the bed wherein the least digested, least dense material locates in an upper stratum of the bed and the most digested, most dense material locates towards the bottom of the bed. As the most buoyant and least digested material gathers at the upper surface of the bed, the concentration of solids in a given volume of the upper stratum increases relative to the moisture content of that volume. Should the percentage of moisture in any given volume of the bed fall below about 70% the activity of the microorganism is inhibited. Accordingly it is important to maintain the moisture content of the upper stratum of the floating bed and to seed the upper stratum with a liquid rich in anaerobic microorganisms.
Maintaining the upper stratum of the bed properly moisturized preferably is accomplished by drawing a liquid component from the bottom of the vessel and continuously sparging it over the surface of the bed. This not only maintains the upper stratum of the bed properly moisturized, but also the liquid that percolates down through the bed works to maintain a nutrient field throughout the depth of the bed. Sparging, while continuous, does not occur simultaneously over the entire surface of the bed. Instead sparging proceeds in steps across the bed front to back or across the width of the bed. In this respect liquid first is introduced so as to spray over a first portion of the bed located adjacent a front wall of the vessel for a given period of time. Prior to the termination of the spray over the first portion of the bed, a spray over a second adjacent portion of the bed farther from the front wall is started. The first spray is terminated and after a period of time the introduction of liquid begins over a third portion still farther from the front wall. The second spray then is terminated and after a period of spraying only over the third portion, the introduction of liquid begins over a fourth portion of the bed still farther from the front wall. This process continues across the vessel from front to back and repeats.
Each period of liquid introduction is followed by a quiescent period before liquid again is sprayed over a given portion. Depending upon the width of the bed the spraying of liquid over a first portion of the bed adjacent the front wall may recommence before the sequence has reached the last portion of the bed adjacent the back wall of the vessel.
Periodically, the most digested matter in the floating bed adjacent the outlet end is removed from the vessel along with a quantity of liquid. As digestion proceeds, fresh or untreated solid waste is introduced at an inlet end of the vessel opposite from the outlet end. With each successive removal of the most digested matter from the outlet end and introduction of fresh material at the opposite inlet end, the floating bed moves slightly towards the outlet end of the vessel. Over time, the entire floating bed migrates to the outlet end and is digested in transit. The gasses generated by the digestion process collect at the top of the vessel and also are removed and collected for use as an energy source.
Untreated liquids such as municipal wastewater, wastewater from a food processing operation or leachate from a vegetable storage pile or the like, also is introduced into the inlet end of the vessel to replace liquid that is removed from the outlet end. It should be appreciated that a vegetable processing operation generates a large quantity of wastewater. For example, washing vegetables prior to processing produces wastewater that contains vegetable matter and must be treated before discharge to the environment. Also, water leaching from vegetable stockpiles awaiting processing or leaching from stores of processing residue contains a high level of nutrients and must be treated prior to discharge. Wastewater and leachate often are collected in lagoons and treated by aeration prior to discharge. However, both of these liquid waste streams are treatable according to the method and apparatus of the present invention thereby reducing or eliminating the need for aeration. Further, these liquid waste streams can be used to provide a nutrient lagoon for the anaerobic microorganisms within the closed vessel at start up.
In accordance with the present invention, such liquid is introduced into and mixed with the liquid stream that is sprayed over the top of the floating bed. As the liquid mixture passes downwardly through the bed it undergoes treatment by the bacteria within the bed so that by the time the liquid is cycled to the outlet end of the vessel a substantial portion, if not all, of its organic should be digested. Accordingly, the organic material contained by the wastewater component also is treated and digested in transit through the vessel.
Thus, the present invention provides for the treatment of both solid and liquid wastes in a clean, efficient disposal operation, which reduces the volume of the biomass by anaerobic digestion so as to facilitate disposal. The invention further provides for the generation of energy in the form of methane gas that can be sold or used as the energy source for the process.
Accordingly, the present invention may be characterized in one aspect thereof by a process for the biological treatment of liquid and solid organic wastes comprising:
a) contacting organic solids in a vessel with a liquid containing anaerobic microorganisms, the solids forming particles for supporting and entrapping the bacteria and the solids being digested by the microorganisms and being buoyed in the liquid by the gases generated during such digestion so as to form a floating bed wherein an upper stratum of the bed contains the least digested and least dense material and a lower stratum of the bed contains the most digested and densest material;
b) circulating liquid from beneath the floating bed to the upper surface of the floating bed and allowing the liquid to percolate down through the bed;
c) moving the floating bed from an inlet end of the vessel to an opposite outlet end of the vessel; and
d) removing the most digested, most dense material from the vessel at the outlet end.
In yet another aspect, the present invention may be characterized by an apparatus for the biological treatment of liquid and solid organic wastes comprising:
a) a closed vessel containing an anaerobic microorganism-rich liquid for conducting the anaerobic digestion of organic matter, the vessel having an inlet at one end for introducing the organic matter to be digested into the vessel and an outlet at an opposite end for removing digested organic matter from the vessel, the organic matter forming a floating bed within the vessel having the densest most digested material in a lower stratum of the bed and the lightest least digested material in an upper stratum of the bed;
b) means for withdrawing liquid from beneath the floating bed and sparging the removed liquid over an upper surface of the floating bed;
c) means for moving the floating bed from the inlet end to the outlet end; and
d) means for removing the densest most digested material from the vessel at the outlet end.