Method and Apparatus to Detoxify Aqueous Based Hazardous Waste

The present invention relates to a process and apparatus for hazardous waste processing, and in particular to a Process and apparatus for detoxifying aqueous based hazardous wastes.

Typically, organic hazardous waste streams are incinerated at approved Hazardous Waste Facilities. When the waste streams comprise primarily water and the BTU content falls to less than 5,000 BTU per pound of waste, alternative treatment technologies become economically viable.

Systems are known for processing hazardous wastes to remove organic pollutants therefrom. Examples of such systems are described, respectively, in U.S. Pat. No. 4,349,415 and U.S. Pat. No. 4,375,387. Organic extraction such as described in these patents uses carbon dioxide flowing upwardly in a column countercurrent to a stream of liquid hazardous waste, as a solvent to strip organics from the waste matrix. At or near its critical point the carbon dioxide behaves like a liquid organic solvent in that it dissolves organic substances in the waste stream, and it behaves like a gas in that its extraction rate is higher than the separation that might be obtained by other means. The organics laden carbon dioxide or extract is removed from the top of the column, while "clean" water is drawn off at the bottom. The organics laden extract is transported to a separator wherein the temperature and pressure are lowered. The organics separate from the carbon dioxide solvent and are recovered in a concentrated form while the carbon dioxide solvent is recycled to the extraction column.

A hazardous waste matrix typically contains heavy metals, along with a variety of organics such as ketones, alcohols, acetates, and aromatic and aliphatic hydrocarbons. Critical fluid extraction as described in the aforesaid patents efficaciously removes organics in aqueous based hazardous wastes, however, the "clean" water drawn off after extraction, is likely to contain toxic heavy metal components.

Processes are known for removing heavy metals from waste water streams. U.S. Pat. No. 4,166,032 describes a system in which heavy metals are separated from the waste water stream by contacting the waste water with a fibrous metal base cellulose xanthate substrate. After the heavy metals are chemisorbed onto the adsorbent cellulose xanthate, the substrate must be separated from the waste water to permit stripping of the heavy metals therefrom. The cellulose xanthate may be removed from the waste water by sedimentation, centrifugation or filtration. Alternatively, a porous bed of cellulose xanthate may be established in a flowthrough contacting reactor.

U.S. Pat. No. 4,342,650 describes a method for converting substantially untreated sludge into useful substances. The treatment involves a chelating step in which heavy metals are removed from suspension and recovered as a recyclable concentrate. The '650 patent describes a chelation process resulting from preliminary steps of disintegration and enzyme hydrolysis which produces a low molecular weight slurry wherein the metal ion content is readily accessible and upon which chelation is effected.

The processes and apparatus known in the art do not concurrently process organics and heavy metals. A process such as described in the '415 and '387 patents only removes organics from the hazardous waste stream. The '032 Patent requires the introduction, removal and further processing of cellulose xanthate adsorbent in the contact removal of only heavy metals in a waste stream. The '650 patent requires significant and costly disintegration and enzyme hydrolysis preprocessing in the chelation process which removes only heavy metals from waste slurry.

The present invention comprises a method and apparatus for detoxifying aqueous based hazardous wastes which incorporates a combination of process technologies that effect the concurrent removal of a variety of organics and the removal of heavy metals.

According to the invention a process and apparatus is provided in which aqueous based wastes are processed by removing solids, decanting phase separated product, optimizing chemically for carbon dioxide extraction, removing toxic metals and organics via liquified carbon dioxide extraction, and removing residual metals and organics via flocculation/filtration and activated carbon adsorption. The process of chemically optimizing the aqueous waste stream includes the addition of chemicals to optimize subsequent treatment and chelating agents to inhibit the precipitation of heavy metals so that they are maintained in the waste stream as an organometallic complex which can be extracted via carbon dioxide extraction and concentrated for final disposal or recycling.

Features of the invention include the use of relatively inexpensive carbon dioxide as a solvent for disengaging both organics and heavy metals from the waste stream. The use of carbon dioxide as a solvent also allows for highly efficient and economic recycling of the solvent for reuse in the system.

These and other features and advantages of the present invention will become more apparent in light of the following detailed description of an illustrative embodiment thereof, as illustrated in the accompanying drawings, of which:

FIG. 1 is a block diagram of a process and apparatus for aqueous based waste processing according to the invention;

FIG. 2 is a block diagram representation of a pretreatment sub-system in the process and apparatus of FIG. 1;

FIG. 3 is a block diagram of a liquified carbon dioxide extraction system in the process and apparatus of FIG. 1; and

FIG. 4 is a diagrammatic representation of a post-treatment sub-system in the process and apparatus of FIG. 1.

A process and apparatus for processing aqueous based hazardous wastes according to the invention, as illustrated in FIG. 1 comprises waste pretreatment 100, liquified carbon dioxide extraction 200, and post-treatment or polishing 300.

Waste received in the process is subject to pretreatment 100 which comprises the removal of solids 102 from the waste, as solids are not compatible with the process and apparatus according to the invention. Subsequent to removal the solids may be dried and readied for off-site disposal 104. An aqueous waste stream is transported to storage facilities 105 for storage, decanting, chemical treating and for establishing a feed for downstream extraction 200 and post-treatment polishing functions 300.

The aqueous waste stream is fed from storage facilities to the extraction facility 200, which comprises liquified carbon dioxide extraction apparatus 202. The waste stream, having been chemically treated with chelating agents to be discussed hereinafter, has heavy metals suspended therein as chelates, in addition to organic components. The liquified carbon dioxide extraction 202 produces a "dirty" liquid carbon dioxide from which a concentrated organic/organometallic chelate mixture 204 is produced for further processing, recycling or off site disposal 206. A refined aqueous stream or raffinate also exits the liquid carbon dioxide extraction process 202. However, the "clean" aqueous stream may contain residual heavy metals that were not removed via carbon dioxide extraction and which are further processed in post-treatment 300.

Post-treatment/polishing 300 involves the removal of residual heavy metals 302 in the aqueous stream by adding chemicals to the stream to facilitate flocculation and/or precipitation of the residual metals for removal by filtration, centrifugation or other processes known in the art. The aqueous stream is then subjected to an organics polishing step 304 wherein any residual toxic organics are removed from the stream prior to discharging 306 the effluent from the process.

Referring now to FIG. 2, pretreatment 100 is effected in a series of tanks which receive the hazardous waste and in which the waste is prepared for processing. A first set of three tanks 106, 107, and 108 serve as receiving tanks for initially storing wastes received through a strainer 110 which eliminates large solids from the waste stream. Physical phase separation and feed pretreatment are initiated in tanks 106, 107 and 108 as described hereinafter. Downline from the receiving tanks 106, 107, 108 is a mechanical solids separation system 112 comprising a centrifugal separation system such as a bowl type centrifuge, which centrifugally separates solids from the waste stream.

A second set of two tanks 114, 116 receives the waste stream from the centrifuge 112 and serves the dual function of storing waste for physical phase separation and solids settling and of providing a feed for further processing. Depending upon the composition of the waste stream, solids settling and pretreatment is further facilitated by the addition of chemicals, acids, bases, oxidizing and reducing agents, and polymers to enhance solids settling and separation or break emulsion layers to effect a relatively solids free aqueous waste stream compatible for processing according to the invention.

Also in the second tank set 114, 116 a feed is established for further processing. Chemicals are added to optimize the waste stream. By optimizing the pH of the process feed, downstream extraction may be enhanced. Chelating agents are added to complex the heavy metals in the waste stream so that they can be removed by the downstream carbon dioxide extraction. The chemicals added for process feed optimization will depend on the particular composition of the hazardous waste matrix.

According to the invention, specific waste streams may contain variable concentrations of heavy metals, such as chromium. Treating such a waste stream resident in feed tank set 114, 116 by chemical chelation, as with ammonium pyrrolidine dithiocarbamate (APDC) at an appropriate pH, results in a suspended organometallic chelate which is extracted with the liquified carbon dioxide, removing it from the waste stream. Such chelation chemistry is substantially in accordance with the example hereinafter.

The following examples are set forth solely for the purpose of further demonstrating the process and apparatus according to the invention and not for the purpose of limiting same.

Other Heavy Metals Extracted with APDC and Liquified CO.sub.2

The chemically treated waste stream, having had solids removed therefrom, is filtered through strainers, sieves, screens or fabric filters 118 to provide final protection to downstream pumping equipment. The waste is passed out of pretreatment 100 to be processed in the liquified carbon dioxide extraction function 200. The extraction function comprises an extraction unit 208 in which liquid carbon dioxide is used as a solvent to strip organics and heavy metals, in the illustrative example chromium, from the waste stream. Aqueous waste from the pretreatment 100 is pumped to a feed drum 210 of the extraction system 208. A feed pump 212 delivers the waste to the top of an extraction column 214. The extraction column 214 is a pressurized vessel which has a series of sieve tray downspouts through which the organics and heavy metals laden aqueous stream flows downwardly. Liquid carbon dioxide is fed into the bottom of the extraction column 214 and flows countercurrent to the waste stream. The liquid carbon dioxide, which has a lower density than water, is dispersed by the perforations in the sieve trays as it jets upwardly. During the countercurrent contact of the carbon dioxide with the waste stream, the liquified carbon dioxide acts as a high solubility solvent dissolving the organic/organometallic chelate from the waste stream. Refined waste or raffinate, having had organics and heavy metals removed exits the extraction column near the bottom thereof and is depressurized through medium pressure vessel 217 and low pressure vessel 219 for delivery to post-treatment 300, discussed hereinafter. A carbon dioxide/organic/organometallic chelate extract exits the top of the extraction column 214 and flows to a solvent recovery column or vessel 216.

The liquified carbon dioxide/organic/organometallic extract flows from the extraction column 214 into the solvent recovery column 216 and to the bottom thereof. A heat exchanger 222 utilizes recoverable heat from carbon dioxide recompression to heat and vaporize the liquified carbon dioxide in the recovery column 216, resulting in separation of carbon dioxide from an organic/organometallic chelate mixture. The organic/organometallic chelate mixture exits the bottom 226 of the solvent recovery column 216, whereas vaporized carbon dioxide exits the top of the column 216 for recompression in a main compressor 220. Hot recompressed carbon dioxide is passed through the heat exchanger 222 to provide the necessary heat for the solvent recovery system. A condenser 224 further cools the recycled liquified carbon dioxide prior to reuse in the extraction column 214, to act as a solvent, as discussed hereinbefore.

The concentrated organic/organometallic chelate mixture flows from the recovery column 216 to a medium pressure vessel 228 and a low pressure vessel 230 for depressurization. Residual carbon dioxide in the stream may be vaporized and recycled back to the solvent recovery column 216 after being recompressed through a low pressure compressor 232 and a medium pressure compressor 234 connected to medium pressure vessel 228. Organic/organometallic chelate mixture exits the bottom of the low pressure vessel 230 to storage tanks (not shown) for further processing, off-site disposal or recycling.

Referring now to FIG. 4, the aqueous effluent (raffinate) from the extraction function 200 enters post treatment and is treated as required to remove residual heavy metals and organics. The post treatment required is significantly downsized due to the effects of the chelation/extraction process. The raffinate is typically treated with caustic, to adjust the pH to approximately 10. This promotes the formation of hydroxides with the metallic cations to form metal hydroxide precipitates that can be removed with a filtration system. Sodium sulfide can also be added to form a sulfide precipitate, as well as a variety of polymers which which can be added to aid in the formation of particles for subsequent treatment. Further, high molecular weight chelating agents may be added to form macromolecular complexes that can be removed by filtration.

Metal hydroxide and floc laden raffinate is processed in an ultrafiltration or microfiltration system 308, depending on membrane pore size requirements. The ultra/micro filtration system 308 circulates precipitated and flocculated raffinate to an ultra/micro filtration membrane 310. The membrane 310 passes water and unprecipitated ions, such as sodium, calcium, nitrate etc., through the membrane 310 as permeate, concentrating the heavy metal hydroxides and other suspended particle on a retentate side 312 of the membrane 310. The permeate, passing through the membrane to a permeate side 314 of the filtration system 308, is passed to a granular activated carbon system 316. The granular activated carbon adsorbs any residual toxic organics that may be present in the permeate. Subsequently, fully processed effluent or detoxified water is discharged.

The hydroxide precipitates and other suspended particles or retentate that remains on the retentate side 312 of the filtration system 308 is monitored. When the retentate reaches a certain suspended solids concentration level, the slurry is passed to a solids dewatering tank 318. The dewatering tank 318 may also receive solids which settle out in the receiving tank set 106, 107, 108 and the feed tank set 114, 116. The hydroxide sludge and solids settle further and thicken in the dewatering tank 318. Further dewatering of the slurry is accomplished by processing with the centrifuge 112 (FIG. 2) or with a plate and frame filter press 324. Aqueous filtrates are recycled through the system as treatment requirements dictate.

One of ordinary skill in the art can appreciate that it is desirable to have various recycle loops incorporated in the aqueous waste processing apparatus according to the invention, such as lines 320 and 322 illustrated in FIG. 4, so that the waste stream can be rerouted and reprocessed at certain stages if treatment goals are not obtained.

Although six tanks are disclosed herein for functions including receiving, separating, feeding and thickening waste, it should be appreciated that more or fewer tanks can be employed and that the tanks and interconnections can be reconfigured to accommodate the various functions.

While strainers, sieves, bag filters, screens and centrifuges are described for physical solids separation herein, one of ordinary skill in the art will appreciate that various other means of physical separation, such as fabric or yarn wound filters and/or gravity sedimentation or floatation techniques known in the art can be used to effect the solids free aqueous based waste stream for processing according to the invention.

Further, although solids separation is discussed in the context of pretreatment, it will be appreciated that additional solids removal can be incorporated within the process, such as by the presence of filtration and screening devices located throughout the interconnecting piping.

It should be appreciated by one of ordinary skill in the art that the particular reagent to be added in the process feed tanks will depend on the metal or metals to be chelated. Further, while the reagents and optimizing chemicals are added herein in the feed tanks, chemical addition and feed optimization can take place in various process locations prior to extraction.

While residual metals and organics removal are effected by liquified carbon dioxide extraction and activated carbon adsorption, it will be appreciated that other solvents could be used to effect removal.

Although the invention has been shown and described with respect to an illustrative embodiment thereof, it should be understood by those skilled in the art that the foregoing and various other changes and additions in the form and detail thereof may be made without departing from the spirit and scope of the invention as delineated in the claims.