United States _ Water Engineering Environmental Protection Research Laboratory Agency Cincinnati OH 45268 Research and Development EPA/600/S2-86/075 Jan 1 987 «>EPA Project Summary Dewatering Wastewater Treatment Sludge by Clathrate Freezing: A Bench-Scale Feasibility Study Benjamin Molayem and Tevfik Bardakci Preliminary investigations were con- ducted on the technical feasibility of clathrate freezing applied to the dewater- ing of municipal wastewater sludge. This project has successfully demonstrated the fundamental feasibility of the process, although contact times of 15 to 20 min- utes were necessary to achieve high dewatering. Also, other c lath rating agents should be examined. Considerable work would still be required, therefore, for bring- ing the process to full-scale commer- cialization. The work was done under a cooperative agreement with the Metro- politan Sewer District of Greater Cincin- nati, Ohio, subcontracting to Benmol Cor- poration, who initially suggested the process. This Project Summary was prepared by EPA's Water Engineering Research Labor- atory, Cincinnati, OH, to announce the major findings of a project that is documented in a separate report of the same title (see Project Report ordering information at back). The Process The clathrate dewatering process would use a liquid refrigerant to form a complex with the water in sludge. The complex, or clathrate as it is termed, freezes to form a solid crystal. In the clathrate crystal, molecules of water surround a molecule of clathrating agent. The water and clath- rating agent are likely held together by a combination of irrtermolecular attractions and crystal geometry that permits the trapping of one molecule within the voids of another's crystal lattica The clathrate freezes at temperatures above the freez- ing point of water. Therefore, if sludge is immersed in an excess of clathrating agent at a temperature between the freezing points of the clathrating agent and water, clathrate crystals could form without the formation of ice crystals. It is proposed that water inside sludge particles and cellular material will diffuse outward and form clathrate crystals in the bulk liquid phase, producing a suspension of totally dehydrated sludge particles, clathrate crystals, and the liquid clathrating agent. It should then be possible to separate the clathrate crystals from the sludge solids by virtue of their density difference, and thus effect a water removal and dewater- ing of the sludge. Upon melting of the clathrate, water can be separated and clathrating agent recycled to the process. Easily liquified compounds that form clath- rates with water include propane, many Freons, other hydrocarbons and gases, and halogens. Project Objectives and Methods The objectives of this initial study were to concentrate on the three following con- cerns relating to technical feasibility: (1) Extent of clathrate formation; (2) Ease of achieving phase separations required for the process; and (3) Extent to which water is removed from the bulk of the sludge solids. In the preliminary study of technical feasibility, Freon-11 was used as the clath- ------- rating agent. Although liquid propane would be preferred for a commercial pro- cess due to its lower cost, working with Freors-11 was much easier. Propane studies would require higher pressures, and propane is flammable. With Freon-11 experiments can be conducted nearer to normal laboratory conditions with con- siderably less complicated techniques. The feasibility was investigated in a simple apparatus where sludge and liquid Freon-11 were placed in a 1 liter test-tube shaped vessel, stirred with a laboratory mixer, and allowed to settle. The test-tube shaped vessel was immersed in a 1.5 °C water bath. Primary, secondary, and mixed sludges were tested, The mixed sludge had a 60/40 volume ratio of primary-to- secondary sludge, typical of that processed at the Mill Creek Treatment Plant in Cincinnati. Results Average water removals by clathrating varied between 48 and 97.9 weight percent (9.25-52.6% dry solids in the product), depending upon contact time and sludge type. Sludge was mixed with liquid Freon-11 for both 10 minutes at a time and also for as much as 20 minutes with the phases separating but remaining in contact for as long as 2 hours. Single contacts and two-stage contacts (two repeated contacts of the sludge with pure Freon-11) were used. To obtain the higher degrees of dewatering, the longer and/or two-stage contacts were necessary. Oath- rate crystals obtained were of two types, a rather soft grayish crystal with a molar ratio (water to Freon-11) of about 17 to 1 and a hard white crystal with a molar ratio of about 8 to 1. Procedures could be adjusted to obtain the latter kind of crystal which separated well under gravity. In summary, high water removals with good phase separations appeared to be possible. Although basic technical feasibility was shown, considerable work would still be required to develop the process to the point of commercialization. First, the conditions favorable for high water removals need to be more accurately defined. Next, studies would be desirable with less expensive propane rather than Freon-11, and phase separation devices used in full-scale processes, such as hydroclones, should be evaluated. A careful economic analysis would also b< desirable. Even though the process does show possibilities, it is fairly complex with considerable development work still re- quired. For the present, additional EPA funding is not planned, but continued development by Benmol Corporation may take place. The full report was submitted in fulfill- ment of Cooperative Agreement No. CR 810549 by the Metropolitan Sewer District of Greater Cincinnati, under the sponsor- ship of the U.S. Environmental Protection Agency. Benjamin Molayem and Tevfik Bardakci are with BENMOL Corporation, Alexandria, VA 22314, Harry E. Bostian is the EPA Project Officer (see below). The complete report, entitled "Dewatering Wastewater Treatment Sludge by Clathrate Freezing: A Bench-Scale Feasibility Study," (Order No. PB 86-239 779/AS; Cost; $11,95, subject to change) will be available only from: National Technical Information Service 5285 Port Royal Road Springfield, VA 22161 Telephone: 703-487-4650 The EPA Project Officer can be contacted at: Water Engineering Research Laboratory U.S. Environmental Protection Agency Cincinnati, OH 45268 United States Environmental Protection Agency Center for Environmental Research Information Cincinnati OH 45268 BULK RATE POSTAGE & FEES PAID EPA PERMIT No G 35 Official Business Penalty for Private Use $300 EPA/600/S2- 86/07 5 ------- |