WO1988004282A1

WO1988004282A1 - Sludge restructuring and conversion method

Info

Publication number: WO1988004282A1
Application number: PCT/US1987/001477
Authority: WO
Inventors: Lennert G. Erickson; Howard E. Worne
Original assignee: Waste=energy Corp
Current assignee: Waste=energy Corp
Priority date: 1986-12-08
Filing date: 1987-06-18
Publication date: 1988-06-16
Anticipated expiration: 1989-06-08

Abstract

A method for the restructuring of wastewater treatment sludges and conditioned solid wastes (1) to remove non-biodegradable solids and to convert biodegradable matter into methane fuel gas (17). The restructuring process includes particulate size reduction (2), enzyme hydrolysis (3), alkaline chemical hydrolysis (5), and removal of non-biodegradable particulate matter (10), including metallic hydroxides. The resultant BioFeedstock (12) includes biodegradable matter in solubilized form suitable for rapid metabolization in biological reactions, including anaerobic digestion (15). Intermediate chemical reactions are employed to refine the digester biogas (14) into higher grade methane fuel gas. The intermediate biological treatment sludges are reconstructed and recycled as additional BioFeedstock. The separated non-biodegradable particulate matter (11) is discharged as a biologically inert concentrate.

Description

SLUDGE RESTRUCTURING AND CONVERSION METHOD

BACKGROUND OF THE INVENTION

This is a continuation-in-part patent appli¬ cation related to our prior patent application filed on December 8, 1986, U.S. Patent Office Serial Number 06/939,231, which in turn is a continuation of U.S. patent application Serial No. 06/791,186, filed October 25, 1985, now abandoned.

This invention relates to improvements to known methods fo biological purification of wastewaters by removal of solids, part of which are converted into methane gas with the remainder being a residue sludge in which potentially biodegradable matter¹ comprises typically more than 50% of the original waste solids. Our invention provides methods for restructuring, recy¬ cling, and converting substantially increased propor- tions of biodegradable matter into methane fuel gas with a corresponding reduction of biodegradable matter discharged as residue sludge.

For example, the methods of our invention are adaptable to the improvement of Municipal Wastewater Treatment Plant processes to increase on site conversion of biodegradable matter into fuel gas and to reduce substantially the volume, biodegradability, moisture content, and pathogen content of the residue sludge.

For example, the methods of our invention are adaptable to converting the biodegradable fraction of Municipal Solid Wastes into methane fuel gas with a corresponding reduction in the volume and biodegradabil¬ ity of the waste residue.

Municipal Wastewater Treatment Sludges, are the Conditioned Sludge residues removed in purification of municipal wastewaters in modern Publicly Owned Treat¬ ment Works (POTW). Wastewater Influent is usually processed through screens and degritting equipment to remove rags and other debris, including rocks and heavy granular material. The wastewater is then treated to separate solids, including Primary Treatment Sludge (usually gravity precipitated) and Secondary Treatment Sludge or Waste Activated Sludge, usually precipitated following aerobic treatment.

Usually such intermediate process sludges are stabilized in an anaerobic Digester in which about 50% of the biodegradable matter is converted into Digester Gas, or Biogas, containing about 60% Methane and about 40% carbon dioxide and other non-fuel gases. The resi¬ due Digester Sludge (usually 2-3% solids) is dewatered and discharged as Wastewater Treatment Sludge, usually containing substantial amounts of chemicals which had been added to facilitate dewatering prior to disposal. Usually over 50% of digester sludge is undi¬ gested organic matter including biologically fresh an- aerobic microbial biomass. According to U.S.A. Envi¬ ronmental Protection Agency (EPA) publications, Coliform bacteria is present at typical concentrations of 30,000 to 6,000,000 per 100 miHiliters, viral pathogens at 100-1,000 per 100 ml, and Salmonella at 3-62 per 100 ml. The parasite Ascaris Lumbricoides survives digest¬ er treatment and is found in concentrations up to 1,000 per 100 ml.

U.S. government investigators report heavy metals in digester sludge from cities in the United States at the following median (dryweight) concentrations: Cadmium 31 mg/kg

Chromium 1, 100 mg/kg Cobalt 100 mg/kg

Copper 1,230 mg/kg Lead 830 mg/kg

Mercury 6.6 mg/kg

Nickel 410 mg/kg

Silver 100 mg/kg

Zinc 2,780 mg/kg The costs of dewatering and disposing of this volatile organic material in an ecologically acceptable way are more than $150.00 per dryweight ton in many areas of the United States. Agricultural usage and incineration are restricted by the content of toxic heavy metals and other constraints. Sludge production and the costs of sludge processing-to-disposal are ex¬ pected to increase substantially during the next decade.

POTW sludges are notoriously difficult to dewater because of their content of gelatinous material and microbial cell structures which remain intact, both of which tenaciously retain water and immobilize micro- particulates by adsorption and absorption.

Municipal Solid Wastes (MSW) are the major source of biodegradable organic materials from urban areas, being produced at an annual rate of about 40,000 dryweight tons per 100,000 population. Also produced annually is about 20,000 tons of MSW inorganic and non- biodegradable material. Most of this material is cur¬ rently disposed of in landfills; however, disposal costs are increasing because of a developing shortage of con¬ venient disposal sites and the need to control seepage of effluents containing heavy metals and other toxins. MSW incineration is an alternative method of stabiliza¬ tion and energy recovery; however, in some areas it is unacceptable to discharge into public airspace the re¬ sidual products of combustion which may include vapor¬ ized heavy metals and toxic organic compounds. The energy potential of the 50-70% biodegrad¬ able fraction of MSW is illustrated by the fact that this material, over a period of years, converts to biogas by anaerobic bacterial decomposition within the landfills. This may result in the potentially hazardous seepage of gases from such sites and seepage of leachate containing heavy metals and other toxins into the ground water. At some of these MSW disposal sites, accumulat¬ ing biogas is removed through wells and refined to utility grade methane fuel gas.

The potential biochemical energy content of MSW cellulose is described by Paul F. Bente, Jr., Exec¬ utive Director of the Bio-Energy Council, Arlington, Virginia, on page 178 of the Bio-Energy Directory and Handbook - 1984: _. "The single largest source of par¬ tially pretreated cellulosic feedstocks is MSW, It is collected and available year round. From 50-75% of MSW has materials with 50 percent or more cellulose, whereas naturally occurring lignocellulosic materials have only about 30-45%. The high cellulose content occurs because the MSW contains much waste paper in which the cellulose portion has been both pretreated and enriched by the paper pulping process."

Resource Recovery technologies are well known for classifying MSW to remove non-biodegradable objects, including metals, glass, ceramics, rocks, plastic, and rubber. The remaining, mostly biodegradable, material may be further processed into a Conditioned Sludge con¬ taining 5-10% solids suitable for restructuring and conversion into methane fuel gas according to the method of our invention.

SUMMARY OF THE INVENTION This invention relates to methods for improv- ing the operation, effectiveness, and capacity of con¬ ventional municipal and industrial wastewater treatment plants employed for purification of water containing organic wastes. A major feature of the method of our invention involves the restructuring of conditioned conglomerate sludges by disintegrating the biodegradable fraction into soluble form. Such restructuring facili- tates removal of non-biodegradable particulates, includ¬ ing metals, grit, fibrous material, and other inert waste. A specific process sequence is described:

(i) Particulate Size Reduction: This pro¬ cess step includes the use of grinding, comminuting, and homogenizing machines to accomplish progressive particulate size reduction and dispersal of the sludge solids.

(ii) Enzyme Hydrolysis: The in-process sludge flows into a reactor into which hydrolytic enzymes are added. In the resultant reaction, contact between the enzymes and the dispersed particulate matter results in the hydrolysis of the polysaccharide gel fraction of the sludge and also serves to hydrolyze or degrade cel¬ lular and particulate organic matter by disintegrating large molecular structures into smaller components.

This hydrolytic preconditioning is an important inter¬ mediate step in the method of our invention since bio¬ degradable material is conditioned into dispersed forms that will react readily in our subsequent chemical hy- drolysis step.

(iii) Chemical Hydrolysis and Conversion of Metals into Hydroxides: The final step in hydrolyzing the organic matter is an alkaline hydrolysis reaction. Microbial life-forms are destroyed, and soluble metals are converted into metallic hydroxides. Practically all the biodegradable matter is converted into soluble form. Because of the prior preconditioning by enzyme hydrolysis, the desired solubilization of biodegradable matter into solubilized peptides and amino acids is completed rapidly at relatively low temperatures and with relatively low concentrations of caustic chemicals, thus conserving nutrients for subsequent conversion reactions. (iv) Residual Solids Removal: A centrifuge may be used to remove suspended solids which are biologi¬ cally inert or which cannot be converted economically into digester gas. Such solids include metals, metallic hydroxides, grit, ash, biologically resistant fibers and biodetritus. Since all cellular and gelatinous matter has been disintegrated, the residual particulate solids are readily separable and have a relatively low capacity for retention of water. Centrifuge cake is produced at 20-25% solids. The residual sludge can be dewatered to 40% solids or more in a filter press oper¬ ating at relatively moderate pressure. The residue is inert, practically free of biological activity, and may contain a substantial proportion of resins and resin- coated cellulose fibers. This residue may be economi¬ cally reduced to a low-volume ash concentrate by incineration.

(v) BioFeedstock: The intermediate BioFeed¬ stock thus produced includes practically all the treated biodegradable material in soluble form as defined by

Standard Methods for the Examination of Water and Waste¬ water. 16th Edition, (1985), American Public Health Association, et al, Washington. This intermediate pro¬ duct is biologically inactive since the pH is about 10.0 and practically all the microbial cell structures have been disintegrated.

The method of this invention includes the means for purification of digester biogas produced on site. The alkaline characteristic of the intermediate BioFeedstock is neutralized by infusion and absorption of the acidic carbon dioxide content of the digester biogas. Hydrogen sulfide and other impurities are re¬ moved in this gas scrubber step. The desirable results obtained are neutralization of the BioFeedstock to about H 7.5 and enhancement of the energy content of the biogas by removal of the non-fuel constituents from the gas produced. The method of this invention includes the means to increase substantially the inflow feed pro¬ cessing capacity and biogas production capacity of an¬ aerobic digesters. Tests show that the solubilized nutrient BioFeedstock of this invention is metabolized at a rate approximately eight times faster than for metabolization of typical feedstocks comprising primary or secondary treatment sludges.

The method of this invention includes the means to increase substantially the recovery of energy in the form of methane gas by restructuring and convert¬ ing the biodegradable fraction of wastewater treatment sludge and municipal solid wastes usually disposed of as wastes. The method of this invention is adaptable to improving the functioning of wastewater treatment fa¬ cilities by restructuring recycled, intermediate, bio¬ logically-reactive sludges into biologically compatible nutrient BioFeedstocks. The method of this invention includes the means to reduce substantially the volume and biodegrad- ability of wastewater treatment sludge and municipal solid wastes usually disposed of as wastes.

A major environmental objective is to reduce pollution inherent in the land disposal or incineration of municipal wastewater sludges and solid wastes.

A major economic objective is to save capital and operating costs otherwise required for treatment-to- disposal of municipal wastewater sludges and solid wastes. Other features and innovations will be de¬ scribed in the following discussion of the drawings illustrating the preferred embodiments of our invention.

DESCRIPTION OF THE DRAWINGS Figure 1 is a flow diagram which illustrates the method of our invention for restructuring conditioned sludges into high quality methane fuel gas, a low moisture residue of non-biodegradable solids, and a liquid effluent with a low solids content.

Figure 2 is a flow diagram which illustrates the method of our invention adapted to the functioning of a municipal wastewater treatment facility to decrease the production of disposal sludge and to increase the production of methane fuel gas.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to Figure 1, there is indicated a source of conditioned biodegradable sludge inflow 1. Suitable urban biomass sources of biodegradable materi¬ als include sewage sludges from wastewater treatment plants and the biodegradable fraction of municipal solid wastes (garbage). Other sources may include residues from food and beverage processing and manures from ag¬ ricultural animals and fowl. Such biomass materials may be conditioned by methods well known in the art into aqueous slurries containing 5-10% (dryweight) sol¬ ids with particulates not exceeding about 0.25 inch in diameter and from which relatively high specific gravity particulates such as glass, ceramics, rubber, rocks, and sand have been removed. Particulate Size Reduction processing 2 may include the use of grinding, comminuting, and homoge¬ nizing machines, well known in the art, to accomplish progressive particulate size reduction and dispersal of the sludge solids. Typically, the resultant size range of granular particulates in the sludge will be less than 20 microns in diameter, and the length of smaller diameter fibrous particulates will range up to about 50 microns.

The in-process sludge flows into an Enzyme Hydrolysis reactor 3 into which a solution of hydrolytic enzymes is added at 4. In the resultant reaction, con¬ tact between the enzymes and the dispersed particulate matter results in the hydrolysis of the polysaccharide gel fraction of typical waste sludges and also serves to hydrolyze or degrade particulate organic matter. The sludge should be recirculated or mixed as necessary to facilitate contact between the enzymes and the par- ticulate solids. The enzyme hydrolysis reaction may be accomplished most effectively within the pH range 6.5 to 7.2 and the temperature range 20°C to 55°C maintained for four to ten hours in a reactor including a recircu- lation mixer. The effectiveness of the hydrolytic re- action increases with temperature and time, and these factors should be adjusted to the characteristics of the raw sludge to be processed while taking economics into consideration.

The final step in hydrolyzing the organic matter is a Chemical Hydrolysis reaction 5. Caustic chemicals are added at 6, usually sodium hydroxide, sodium carbonate, potassium hydroxide, or potassium carbonate, in quantity sufficient to raise the alkalin¬ ity to pH 10-11, preferably to about pH 10.5. The al- kaline hydrolysis reaction may be accomplished most effectively within the temperature range 50°C to 90°C maintained for two to four hours, preferably under pressure at about twenty pounds per square inch in a reactor including a recirculation mixer. The effec- tiveness of the hydrolytic reaction increases with tem¬ perature, time, and pressure, and these factors are also taken into account in adjusting the process to the characteristics of the raw sludge to be processed.

A unique feature of the sludge restructuring process of our invention is that the combination of homogenization, enzyme hydrolysis, and alkaline hydrol¬ ysis at elevated temperature and pH, serves to destroy practically all viral, bacterial, and parasitic micro¬ organisms. A feature of the alkaline hydrolysis step of our invention is that at pH 10.5, practically all metals which were soluble at lower pH are converted into metallic hydroxide, semi-solid forms suitable for subse¬ quent removal.

The resultant hydrolyzed intermediate product is conveyed to a mixing tank 7 to which chemical condi- tioners 8 may be added to facilitate subsequent removal of particulates and certain chemicals. For instance, a surfactant is usually added to reduce surface tension, thus to facilitate subsequent removal of particulates, including metallic hydroxides. If phosphate is present in soluble form, ferric chloride may be added to pre¬ cipitate this material in form suitable for subsequent removal.

The resultant hydrolyzed and conditioned in¬ termediate product 9 is conveyed to particulate solids removal equipment 10. Therein, centrifugal separators are preferably used to remove particulate" matter. Such matter includes biologically-resistant fibrous solids and granular non-biodegradables, including metallic hydroxides, grit, ash, and biodetritus. These residual solids may be economically dewatered by air drying or by use of a relatively low pressure filter press. This residue is practically non-biodegradable and non-patho¬ genic since the pH is about 10.0 and practically all the microbiological life forms have been inactivated in the prior hydrolytic reactions.

The thermally volatile content of the residue 11, including fibers, resin, and plastics may be reduced by autogenous combustion to a relatively low volume ash concentrate which may be refined to recover individual metals such as cadmium, chromium, nickel, and silver.

The alkaline (approximately pH 10.5) BioFeed¬ stock intermediate product 12 is neutralized in a gas scrubber reactor 13 by chemical reaction with the carbon dioxide content of the digester gas 14 produced in an- aerobic digester reactor 15. Such reactors, well known in the art, are usually vertical cylindrical tanks, approximately 35 feet high, with static mixer-diffusers, and operated at an average pressure of about 20 psi. The unique benefits of this reaction in the method of our invention are (a) the pH value of intermediate Bio¬ Feedstock 12 is economically neutralized to about pH 7.5; (b) this reaction is relatively stable at pH values in the range 7.5 to 10.5; and (σ) the quality of the relatively non-soluble methane gas 17 released at the top of reactor 13 is substantially increased by removal of the carbon dioxide. The quality of the methane fuel 17 is further enhanced by absorption into BioFeedstock 16, of hydrogen sulfide gas and other non-fuel constit¬ uents often present in digester biogas.

The BioFeedstock 16 thus produced is a unique product of our invention. It contains biologically available nutrients in soluble and microparticulate form suitable for relatively rapid microbiological metabolization. The potential for biologically incom¬ patible reactions is substantially reduced by the prior hydrolytic steps of our invention which serve to disin- tegrate the molecular and cellular structures of micro¬ organisms in the inflow sludge 1. Thus, the protoplas¬ mic constituents are made available for rapid assimila¬ tion by the anaerobic bacteria within the digester.

The digester sludge 18, including excess an- aerobic microbial biomass produced in the process of digestion, is conveyed to liquid removal equipment 19. Therein, centrifugal separators and supplemental filters are preferably used to remove a low solids liquid effluent 20, without the use of floculation chemicals. Practically all of the particulate solids thus concen¬ trated in sludge form at 21 are combined with the in¬ flow of conditioned sludge 1 for subsequent restructur¬ ing, recycling, and conversion according to the method of our invention. Alternatively, all or part of the inflow of conditioned sludge may be introduced into process at 22, directly into the digester 15. The resultant bioconversion within the digester will be less rapid and less complete; however, the excess sludge residue will be concentrated and recycled 21 into nutrient Bio¬ Feedstock 16 and returned to digester 15 to complete the bioconversion process. In this arrangement, a re¬ duced total volume of sludge solids 1 and 21 is re¬ structured into BioFeedstock 16, and the equipment siz¬ ing and operating costs related to sludge restructuring and the removal of particulate solids and liquid effluent will be correspondingly reduced. The required digester capacity will be larger; however, this is not a disadvan¬ tage in retrofit installations where digester capacity already in place will be sufficient to handle the recy¬ cle load because of the operating efficiencies of our process.

Thus, the unique "full circle" process of our invention is employed to restructure and convert the biodegradable fraction of conditioned conglomerate sludges into methane gas, a liquid effluent, and a res- idue concentrate of non-biodegradable solids. Recalci¬ trant particulates that survive such restructuring, conversion, and liquid/solid separations will automati¬ cally be recycled via the full circle process until disintegration into low molecular weight constituents results in bioconversion in the digester or separation into liquid or solid fractions.

Referring to Figure 2, there is indicated a retrofit adaptation of the method of our invention to the functioning of a typical municipal wastewater treat- ment facility. - The facilities added according to the method of our invention are indicated all inclusively at 41.

Raw wastewater influent 31 is processed through primary treatment 32 which may include removal of grit and trash and removal of settleable solids in the form of primary thickened sludge 22, typically 5-6% solids. This conditioned sludge is conveyed directly into digester 15. Such primary sludge is well suited to anaerobic digestion since it includes a wide range of un-metabolized biological nutrients with a relatively low content of active incompatible microbial life forms. The main flow of wastewater 33 containing over 50% colloidal and dissolved solids is processed through secondary aerobic treatment 34 to accomplish separation of solids 35 in the form of Waste Activated Sludge (WAS) and a clarified conditioned and disinfected low solids effluent 36.

In typical wastewater treatment plants, the WAS is thickened and conveyed directly into digester treatment. However, operational problems and ineffi¬ ciencies result because the aerobic WAS biomass is bio- logically incompatible with the anaerobic microorganisms which dominate the digester environment and because the content of non-biodegradable filamentaceous or fibrous solids cause agglomeration and foaming problems.

In typical wastewater treatment plants, the excess digester sludge 18, is dewatered to 20-30% solids and disposed of as wastewater treatment plant sludge. Dewatering costs are substantial, and operational prob¬ lems and inefficiencies result because the recycled anaerobic effluent removed in dewatering operations is biologically incompatible with the aerobic microorgan¬ isms which dominate the secondary treatment process. Waste sludge disposal costs are substantial because of the 70-80% liquid content and because of the biodegrad¬ able characteristics of most of the solids content. in the Figure 2 adaptation of the method of our invention, the total discharge of excess digester sludge 18 is restructured as described with reference to Figure 1, numbers 2 through 17, to produce a BioFeed¬ stock product 16. Part of this flow of solubilized nutrients 42 is recycled through flow-splitter valve 43 to the digester 15, combined with the input of primary thickened sludge 22. Valve 43 is adjusted to balance the average flow of liquids 22 and 42 into the digester with the average flow of liquid sludge removed at 18.

The remainder of the flow of solubilized nu¬ trients 44 is recycled through flow-splitter valve 43 to secondary treatment 34. Therein, excess water is removed for discharge as part of wastewater effluent 36, and the BioFeedstock nutrients are metabolized into additional waste activated sludge 35. WAS thickener equipment 45, preferably a centrifuge, is used to con- centrate about 95% of the particulate solids in sludge form at 46. This is combined with the inflow of di¬ gester sludge 18 for subsequent restructuring, recycling, and conversion according to the method of our invention. The aerobic liquid effluent 47 removed by WAS thickener 45 is recycled to aerobic secondary treatment 34 without

_* biological incompatibility problems.

Thus the unique "full circle" process of our invention is employed to restructure and convert the biodegradable fractions of sludges produced in metro- politan wastewater treatment plants into high quality methane gas and to correspondingly reduce the volume of solids discharged to waste. Additionally, the method of our invention is employed to improve the operations of metropolitan wastewater treatment plants by restruc- turing sludges in process to eliminate biological in¬ compatibility reactions in interactive aerobic/anaerobic treatment processes.

Alternatively, additional conditioned sludges from other sources may be combined with the inflow of conditioned sludges 18 and 46 for subsequent restructur¬ ing and conversion according to the method of our inven¬ tion. Such additional sludge inflows may include waste¬ water treatment sludge wastes from other municipalities or conditioned sludges prepared form the separated organic fraction of municipal solid wastes.

A preferred "Process for Treating Municipal Solid Waste" is described in U.S. Patent 4,540,495 issued to Clifford C. Holloway. After classification to remove oversize objects and ferrous metals, the municipal solid waste (MSW) is treated with water, steam, heat, pressure, and agitation to produce a cooked, sterilized, and ho- mogenized intermediate product. This is subsequently processed through size and specific gravity classifica¬ tion equipment to remove non-biodegradable objects. The resultant product contains substantially all of the biodegradable fraction of the MSW and is well suited for preparation of a conditioned sludge 1 for inflow to the restructuring and conversion process of our invention.

Alternative known processes have been de¬ scribed for separation of the conglomerate components of municipal solid wastes into biodegradable and non- biodegradable fractions. Such processes have employed combinations of dry and wet sorting, shredding, grinding, and other conditioning techniques. Examples of such resource classification and recovery techniques as de- scribed in the publications of Buhler-Miag, Inc., Min¬ neapolis, Minnesota, and in publications about the Re¬ source Recovery Facility owned by Metropolitan Dade County, Florida, developed by Parsons and Whittemore, New York, and operated by Resources Recovery (Dade County) , Inc.

It is well known in the art to use gas scrubber reactors to refine digester or landfill biogas to produce high quality methane gas (typically 980-1,000 Btu/Scf) by removing carbon dioxide, hydrogen sulfide, and other undesired elements. Usually ordinary water or waste- water treatment plant effluent is used as the reactor counterflow liquid. Typically the inflow fluid has a pH value of about 7.0, and fluid discharge values are lower because of the reaction with the carbon dioxide which converts into carbonic acid. Since carbon dioxide is not readily absorbed into liquids at pH levels of 7.0 and lower, it is usually necessary to operate the reactors at pressures up to 300 pounds per square inch. For reference, see BINAX system publications of Modular Products, Inc., Brea, California.

A unique feature of the method of our inven- tion is the use of intermediate BioFeedstock product 12 at pH values above 10.0 as the inflow liquid to the gas scrubber reactor 13. As is well known in the chemical arts, infused carbon dioxide gas reacts readily with caustic liquids. The advantageous result obtained in the method of our invention is that a given volume of raw digester biogas may be processed into utility grade methane fuel gas in smaller gas scrubber reactors, at lower rates of fluid flow, and at reactor pressures of only about 20 pounds per square inch. In retrofitting modern municipal wastewater treatment plants with this Sludge Restructuring and Conversion Method, the digester 15 will usually be the existing digester(s), with capacities often exceeding one million gallons and designed for hydraulic retention intervals of about 30 days. The hydraulic retention time required for processing a given quantity of biode¬ gradable sludge solids may be reduced to about 4-6 days for a 100% inflow of restructured BioFeedstock in the system configuration of Figure 1. A somewhat longer retention time may be required for the system configu¬ ration of Figure 2. In any event, the substantially reduced hydraulic retention time required for digesters operating within the methods of our invention may allow a reduction in the scale of digester operations or may provide capacity for the treatment of additional condi¬ tioned sludges.

The BioFeedstock 16 product of our invention is well suited for use with anaerobic digesters designed for highrate conversion of readily biodegradable nutri- ent feedstocks having a minimal content of non-biode¬ gradable materials. A preferred design is the Upflow Anaerobic Sludge Blanket (UASB) digester process described in published papers by Jelte Lanting, et al., of Biothane Corporation, Camden, New Jersey. This di¬ gester design features internal concentration and sepa¬ ration of digester biogas, liquids, and solids into separate discharge streams. Conversion efficiency is improved because of controllable selective retention of biodegradable solids within the digester. Laboratory tests indicate hydraulic retention times in the range 12 to 48 hours for system configurations illustrated in Figure 1. The internal separation or pre-separation of effluent and sludge residues discharged from the UASB Digester serves to reduce the cost and complexity of the liquid removal equipment 19.

EXAMPLE 1: A sample of BioFeedstock 16 de- rived from wastewater treatment sludge was tested for its biological conversion characteristics' in a labora¬ tory scale, singlestage digester. During the initial 48 hours, digester gas was produced at a rate about eight times faster than for conversion of raw sludge. EXAMPLE 2: A batch sample of wastewater treat¬ ment sludge was tested before and after the restructuring and removal of inert solids. We obtained an average 65% reduction in the concentration of the metals cadmium, chromium, copper, nickel, lead, and other metals. EXAMPLE 3: Two batch samples of BioFeedstock

16 derived from restructured digester sludge were sub¬ jected to standard procedure analytical laboratory tests. The following results were observed:

Sample A Sample B Total solids 2.3% 2.2%

Soluble solids 2.3% 2.2%

Chemical Oxygen Demand 12,500mg/l 8,310mg/l Biological Oxygen Demand 6,160mg/l 5,500mg/l

Machines suitable for use in the particulate size reduction step 2 in the process of our invention include a first stage sludge comminutor; e.g., the Gorator sludge grinding machines produced by Dorr-Oliver, Incorporated, Stamford, Connecticut, or the Maz-O-Rator machines available from Robbins and Myers, Inc., Springfield, Ohio. Subsequent processing may include the use of high-shear homogenization machines, e.g., the Dispax Ultra Turrax units produced by the Janke & Kunkel organization or the Sepratron machines produced by the Krupp organization, both in the Federal Republic of Germany. Alternative particulate size reduction technologies may be used, as are well known in the art.

Centrifuge machines well known in the art may be used for all or part of the liquid separation/solids concentration functions 10, 19, and 45 of Figures 1 and 2. Suitable high-speed, bowl-scroll machines for con¬ tinuous flow operation are available from the Sharpless- Stokes Division of Pennwalt Corporation, Warminister, Pennsylvania, and from Bird Machine Company, Inc., South Walpole, Massachusetts. Conventional filtration equip- ment may be used to supplement the liquid/solid separa¬ tion processes.

The hydrolytic enzymes employed in the process are added as an aqueous solution prepared from crude, dehydrated, nonspecific enzyme mixtures isolated from microbial fermentation broths and subsequently dehy¬ drated. Potency, depending on the enzyme involved, averages 50,000 international units of hydrolytic ac¬ tivity per gram as supplied by Worne Biotechnology, Inc., Medford, NJ. The hydrolytic enzymes may be rehy- drated by dissolving 1 lb of dried enzymes in 5 gallons of water at 25°-37°C. This solution is added into the reactors in the ratio of 1 lb of dehydrated enzymes per 1000 lbs (dry weight) of biodegradable organic material inflow in the sludge to be hydrolyzed. Hydrolytic enzymes useful for the purpose of this invention includes a wide variety of the following: proteases amylases maltase glucoamylase dextranase pentosanases polysaccharidases lipases pectinases protopectinases hemicellulase cellulases These hydrolytic enzymes are capable of hydrolyzing proteins, carbohydrates, fats, protopectins, pectins, hemicellulose, cellulose, and polysaccharides into sol¬ uble, low molecular weight fractions.

For the protection and conservation of the sensitive and valuable protoplasm content of the cell, we usually limit the enzyme hydrolysis reaction to about 12 hours.

For large volume installations, mixed cultures of viable, dehydrated, hydrolytic enzyme-producing mi- croorganisms may be employed as an inoculant. Potencies are typically about one million fungal spores per gram and about five billion viable bacteria per gram as sup¬ plied by Worne Biotechnology, Inc., Medford, New Jersey. To establish a viable growing mixed culture capable of secreting the required quantities of enzymes, one pound of the inoculant is added to a 1,000 gallon stirred fermenter containing 750 gallons of sterile media of a type meeting the growth requirements of the specific microbial strains being cultured and designed to insure maximum enzyme production. Aerobic microbial cultures are provided with sterile air at a rate which gives a dissolved oxygen of approximately 2 ppm. De¬ pending on the environmental requirements of the culture, the temperature is maintained between 20°C and 50°C, with an average of 25°C to 37°C. The media is adjusted to pH 4.5 to 7.5, the optimum being 6.5 to 7.0, depend¬ ing on the cultures being grown. Anaerobic cultures used in the process receive identical treatment without the aeration. In about 24 to 36 hours, bacteria in the finished liquid culture will have a minimum cell concen¬ tration of 3 to 5 billion microorganisms per milliliter and will have reached the maximum level of enzyme production. Due to slower growth rates, fungi will require a residence time of 3 to 5 days to reach maximum enzyme production.

The size of the vessel or vessels needed to grow the cultures and produce the enzymes averages 1,000 gallons per 250,000 gallons of liquid sludge slurry containing 10% solids.

The spent biomass should be separated from the enzyme rich media as soon as possible by filtration or centrifugal separation. After separation the biomass is added to the conditioned sludge feedstock for re¬ structuring and conversion according to the process of our invention. About 90% of each batch of the enzyme rich media may be added to the enzyme hydrolysis reactor 3 at the rate of 50 gallons per 1,000 gallons of sludge slurry having 10% solids.' ^* '

Ten percent of each enzyme batch, before sep¬ aration of the biomass, may be retained for use as an inoculant for subsequent culture batches up to a total of five such extension cultures, after which a new se¬ ries of cultures should be started with a fresh supply of dehydrated microorganisms as the inoculant.

Thus, in large installations a microbial inoc¬ ulant, plus its exocellular enzymes, may be produced economically on site from relatively small amounts of commercially produced dehydrated enzyme-producing micro¬ organisms.

Enzyme-producing microorganisms useful for the purpose of this invention include: Athelia sp. Myricoccum sp.

Alternaria sp. Thermonospora sp.

Arthrobotrys sp. Thermopolyspora sp. Arthrographis sp. Xanthomonas sp. Beauveria sp. Aspergillus sp. Botrytis sp. Acremonium sp.

Cephalosporium sp. Memnoniella sp. Chaetomium sp. Metarrhizium sp. Chrysosporium sp. Monascus sp.

Cladosporium sp. Mucor sp.

Conidiobolus sp. Mortierella sp.

Coprinus sp. Myrothecium sp.

Cytophaga sp. Paecilomyces sp.

Endothia sp. Pellicularia sp.

Entomophthora sp. Penicillium sp.

EupeniciIlium sp. Polyangium sp.

Fomes sp. Polyporus sp.

Fusarium sp. Preussia sp.

Gliocladium sp. Pythium sp. Gliomastix sp. Rhizomucor sp. Acinetobacter sp. Rhizopus sp. Bacillus sp. Sporoboromyces sp. Achromobacter sp. Serratis sp. Arthrobacter sp. Stachybotrys sp. Corynebacterium sp. Trichoderma sp. Cellulomonas sp. Verticillum sp. Clostridium sp. Candida sp. Micrococcus sp. Saccharomyces sp. Streptomyces sp. Saccharomycopsis sp. Micromonospora sp. Torula sp. Pseudomonas sp. Yarrowia sp.

Nocardia sp. Mycrobacterium sp.

Thermoactinomyces sp.

The digester fermentation and microbial growth are dependent on the optimal supply of nutrients. The bacteria have simple nutrient requirements; and the various organic materials required by some species, such as B vitamins, a small number of amino acids, or fatty acids, are supplied by other bacterial species. Inorganic minerals are usually the only other nutrients required. Most of the crude substrates utilized for methane fermentation contain sufficient minerals to satisfy the nutrient requirements for growth of the microorganisms; however, supplementation with relatively minor amounts of nitrogen, phosphorus, and other nutrients may be advantageous.

All or part of the unique BioFeedstock prod¬ uct of our invention, available at 12, may be used al- ternatively as a high performance liquified fertilizer, usually distributed in irrigation water. This biologi¬ cally available nutrient may also be useful in hydrocul- ture crop production or in the production of algae or aquatic plants. More complete information about such alternative uses has been published in our U.S. Patents No. 4,267,049 and No. 4,432,650.

For purposes of clarity and understanding, the foregoing improved Sludge Restructuring and Conver¬ sion Method has been described in some detail by way of illustration and example; however, it is to be under¬ stood that certain additions, modifications, and omis¬ sions may be practiced within the spirit of the inven¬ tion as limited only by the scope of the appended claims.

Claims

WHAT IS CLAIMED IS:

1. A method for converting biodegradable sludge to a gaseous fuel comprising: (a) hydrolyzing said sludge with hydrolytic enzymes;

(b) further hydrolyzing said sludge by alka¬ line hydrolysis to solubilize the biodegradable content of said sludge; (c) removing non-solubilized solids from said hydrolized product;

(d) neutralizing the product of said alka¬ line hydrolysis; and

(e) anaerobically digesting said neutralized product to form a gaseous fuel.

2. The method for converting biodegradable sludge in accordance with claim 1, wherein there is a non-gas residue following the anaerobic digestion of step (e), and said non-gas residue is recycled through steps (a) through (e).

3. The method for converting biodegradable sludge in accordance with claim 2, including the step of reducing the liquid content of said non-gas residue before it is recycled.

4. The method for converting biodegradable sludge in accordance with claim 1, wherein conditioned biodegradable sludge is combined with said neutralized product in the anaerobic digestion of step (e).

5. The method of converting biodegradable sludge in accordance with claim 2, wherein conditioned biodegradable sludge is combined with said non-gas res¬ idue and recycled therewith.

6. The method of converting biodegradable sludge in accordance with claim 1, wherein said neu¬ tralizing step (d) is executed to adjust the pH of the product to about 7.5.

7. The method of converting biodegradable sludge in accordance with claim 1, wherein said neu¬ tralizing step (d) is executed by contacting the solubilized portion of the alkaline hydrolysis product with the gas formed in the anaerobic digestion of step (e).

8. The method of converting biodegradable sludge in accordance with claim 7, wherein said contact is executed in a gas scrubber with the solubilized por¬ tion of the alkaline hydrolysis product at a pH of about 10-11 whereby said neutralizing step (d) occurs by re¬ action with carbon dioxide present in said gas.

9. The method of converting biodegradable sludge in accordance with claim 8, wherein contact be¬ tween the alkaline hydrolysis product of step (b) and said gas causes said alkaline hydrolysis product to remove hydrogen sulfide from said gas and thereby pro- vide a gas of increased methane content.

10. A method for converting aerobically treated biodegradable sludge to a gaseous fuel comprising: (a) hydrolyzing said aerobically treated sludge with hydrolytic enzymes;

(b) further hydrolyzing said sludge by alka¬ line hydrolysis to solubilize the biodegradable content of said sludge; (c) removing non-solubilized solids from said hydrolized product; (d) neutralizing the product of said alkaline hydrolysis; and

(e) anaerobically digesting said neutralized product to form a gaseous fuel.

11. The method for converting aerobically treated biodegradable sludge in accordance with claim

10, wherein said aerobically treated biodegradable sludge is sludge formed by secondary treatment in a waste water treatment plant.

12. The method for converting aerobically treated biodegradable sludge in accordance with claim

11, wherein said neutralizing step (d) is executed by contacting the solubilized biodegradable content of step (b) with the gas formed from the anaerobic diges¬ tion of step (e) .

13. The method for converting aerobically treated biodegradable sludge in accordance with claim

12, wherein there is a non-gas residue following step (e), and said non-gas residue is recycled through steps (a) through (e) .

14. The method for converting aerobically treated biodegradable sludge in accordance with claim

13, wherein a portion of said neutralized product of step (d), related to the amount of non-gas residue being recycled from step (e), is returned and mixed with ac- tivated sludge formed by secondary treatment in a waste water treatment plant and the mixture subjected to steps (a) through (e) .

15. A process for converting biodegradable waste to gaseous fuel in which the biodegradable waste is solubilized by alkaline hydrolysis, including the steps of neutralizing the solubilized biodegradable waste and anaerobically converting the neutralized waste to fuel gas, said neutralization being executed by con¬ tacting the alkaline hydrolyzed biodegradable waste with the fuel gas produced by said anaerobic conversion.

16. The process for converting biodegradable waste to gaseous fuel in accordance with claim 15, where¬ in said alkaline hydrolyzed biodegradable waste is at a pH of about 10-11, whereby carbon dioxide in said fuel gas neutralizes said solubilized biodegradable waste to a pH of about 7.5.