The present invention relates to the treatment of waste waters containing suspended solids and/or emulsified oil. More specifically, the invention relates to a method for increasing the cross-flow microfiltration fluxes of waste waters containing suspended solids and/or emulsified oil.
In treating both domestic and industrial waste waters, removal of suspended solids, including solids of colloidal dimensions, is generally a required unit operation. Through-flow filtration, utilizing, for example, multimedia filters, and cross-flow filtration utilizing, for example, ultrafiltration and reverse osmosis, have been widely used in the art to remove such suspended solids.
Conventional through-flow filters, however, disadvantageously require back washing to eliminate filtered particles which continuously accumulate on and enter into the filtration barrier. Thus by its very nature, through-flow filtration is a batch process wherein the filtration flux (the rate of transfer of the permeate across the filter surface) declines relatively rapidly if the driving pressure differential across the filtration barrier is held constant. A decline in filtration flux is thus a well-recognized problem.
In cross-flow or ultrafiltration, however, because the direction of the feed flow is tangential to the filter surface, accumulation of the filtered solids on the filtered medium can be minimized by the shearing action of the flow. Cross-flow filtration thus affords, at least in principle, the possibility of a quasi-steady operation with a nearly-constant flux when the driving pressure differential is held constant. Unfortunately, this theoretical possibility has not been achieved in practice. Thus, the problem of declining filtration fluxes has continued to plague cross-flow filtration of domestic and industrial waste waters.
A method of cross-flow microfiltration utilizing thick-walled, porous thermoplastic tubes has recently been developed by Hydronautics, Incorporated of Laurel, Md. The filtration characteristics of these tubes, sold by Hydronautics, Incorporated under its trademark HYDROPERM.TM., combine both the "indepth" filtration aspects of multi-media filters and the "thin-skinned" aspects of membrane ultrafilters. The HYDROPERM" tubes differ from conventional membrane ultrafilters by having pore sizes of the order of several microns wherein the length of a pore is many times that of its diameter. These tubes are described in greater detail in report No. 77-ENAS-51 of the American Society of Mechanical Engineers entitled "Removal of Suspended and Colloidal Solids from Waste Streams by the Use of Cross-Flow Microfiltration," which is incorporated herein by reference.
In general, any effluent from which suspended solid removal is desired will contain a wide range of particulate sizes, ranging in diameter from several microns down to colloidal dimensions. Because of the "in-depth" filtration characteristics of the thick-walled, thermoplastic tubes, such as HYDROPERM.TM. tubes, particles smaller than the largest pore size of a tube enter the wall matrix, whereas particles larger than the largest pore size are retained at the walls of the tube.
Those particles entering into the wall matrix ultimately become entrapped within it because of the irregular and tortuous nature of the pores. As microfiltration proceeds, the pore structure of a tube is gradually clogged by entrapped particles, resulting in a gradual filtration flux decline, approximately exponentially related to filtration time. Thus, the art has sought a method for preventing filtration flux declines in thick-walled, microporous thermoplastic tubes.
Further, in addition to seeking to overcome the problem of flux decline, the art has continually sought a method for increasing the filtration flux of cross-flow microfiltration tubes above those levels presently obtainable. Increases in the flux allow for more economical processing of waste waters. The art has also sought a method for reducing the suspended solids content of permeate.
The present invention increases the solids content of waste waters by utilizing certain solid additives in specific amounts. Suprisingly, substantial increases in cross-flow microfiltration fluxes are achieved and the permeate contains less suspended solids. Although tests, to be described in detail infra, show that notwithstanding the additive, the cross-flow microfiltration fluxes decline over a twenty-four hour period, the fluxes after twenty-four hours are nonetheless significantly higher than the starting fluxes achieved in identical cross-flow filtration systems lacking the additives used in the amounts required by the present invention. Thus, the significant increases in cross-flow microfiltration fluxes obtained by the present invention, both initially and over a twenty-four hour period, are a significant contribution to the cross-flow microfiltration art.