Continuous Separation Process

1. Technical Field

This invention relates to a continuous process for separating organic compounds from water.

2. Description of the Prior Art

The chemical process industry for some time now has been researching and developing chemical engineering processes for the removal of synthetic organic compounds from the environment. Typically, these artificially produced organic compounds have been identified as potentially toxic to man either directly or indirectly through the food chain. Examples include polychlorinated biphenols (PCBs), chlorodiphenols, Aroclors.RTM., Kanechlors.RTM., and their cogeners.

PCBs were first synthesized over 100 years ago. Under standard conditions, they exist as heavy liquids weighing from about 1.2 to 1.4 kilograms per liter in the form of a slightly viscous oil to a sticky resin. PCBs are derivatives of the compound biphenol in which 1 to 10 hydrogen atoms have been replaced with chlorine atoms. PCBs are fire resistant and have high boiling points. PCBs have extremely high chemical and thermal stability and are quite useful in commercial applications such as dielectric fluids for capacitors and transformers, as heat transfer and hydraulic fluids, pigments, plasticizers, fluids for carbonless copying paper and electromagnets, and as components in cutting oils.

PCBs were widely used in the last 50 years. A lack of recognition of the hazards led to their wide distribution in the environment. Although the United States Environmental Protection Agency has labeled PCBs as having fairly low acute toxicities, some adverse effects have been found in humans, laboratory animals, and other organisms. There is some evidence that PCBs bioaccumulate and may be carcinogenic. Concern over the PCB contamination problem has led to toxic substances control legislation that will require the eventual elimination of the use of PCBs in the United States.

Separation and removal from water of PCBs in very low levels, i.e., part per billion levels, is a complex and expensive task. PCBs are colorless, odorless, and relatively insoluble in water. The Industrial Environmental Research Laboratory of the U.S. Environmental Protection Agency has specified the solubility of PCBs in water to be in the range of 0.007-5.9 mg/l or 7-5900 parts per billion (ppb). This solubility level indicates that physical separation techniques most likely would be unsuccessful in separating PCB from waste water to below 7-5900 ppb, and that a chemical separation process would be the technique of choice for achieving lower PCB levels.

Conventional chemical engineering practice employs a carbon adsorption process for removing chlorinated hydrocarbons, including PCBs, from an aqueous waste stream. The waste water is passed through a vessel filled with activated carbon in the form of carbon slurry or granules. Impurities from the aqueous stream are removed by adsorption onto the carbon. Activated carbon has an affinity for organics and is commonly used on a commercial scale for organic contaminant removal from waste water. However, activated carbon adsorption of PCBs has a major drawback in that it requires the disposal or regeneration of the carbon adsorbor.

Other conventional physical/chemical separation processes are used to remove PCBs from waste water. Sedimentation, coagulation, and flocculation are examples of conventional physical or chemical separation process which can be used for PCB removal from waste water. According to O'Brien and Gere Engineers, Inc., conventional techniques can remove 50 to 80% of the PCBs at influent levels in the range of 1 to 20 micrograms per liter. The New York State Department of Environmental Conservation has also concluded that these conventional techniques can remove 50 to 80% of the PCBs at influent levels in the range of 1 to 10 micrograms per liter and that 90% removal via a coagulation/filtration process represents the highest reduction consistently achievable in existing plants. While higher removals may be theoretically possible, the degree of control required for these removals and the normal variations in the quality of surface waters combine to set 90% removal as a practical limit.

Governmental regulatory agencies at the federal and state levels have mandated a reduction in PCB content in waste waters to levels less than 1 ppb. Reductions in other synthetic chemicals, such as dioxin, similarly have been proscribed to levels below that detectable by conventional analytical techniques.

Even though PCBs, dioxins, or other regulated organic substances have not been used for many years in a particular industrial facility, these substances have been observed in the discharges even after substantial cleanup programs have been carried out. A process is needed to clean up low level amounts of these contaminants in waste water discharged from most industrial facilities.

It is an object of the present invention to provide a continuous process for removing slightly soluble synthetic organic compounds from water.

It is another object to provide a continuous process for removing organic compounds from waste water while eliminating the disposal or regeneration problems associated with contaminated filter media.

It is a further object to provide a continuous process for removing organic compounds in low level amounts from waste water in a cost-efficient manner.

In accordance with the present invention, a process is provided for removing an organic compound from water including the steps of passing an influent of water containing an organic compound through a filter bed of hard, substantially non-porous, inorganic particles to separate the organic compound and form a water effluent of reduced organic concentration, washing the particles to remove the organic compound and form a concentrated organic effluent, and recycling the washed particles to the filter bed.

The process of the present invention includes processing an influent of water and organic compound through a filter bed of particles composed of a hard, substantially non-porous, substance. The process is performed continuously by including the steps of washing the particles to remove the organic compound and recycling the washed particles to the filter bed. As opposed to carbon absorption which requires regeneration or disposal of the filter media, the process of the present invention provides a continuous washing and recycling of the filter media used in the filter bed.

The filter media particles employed in the process of the present invention are characterized as hard and substantially non-porous. Such a material for the filter particles is required to perform the continuous process with high efficiency. The continuous process of the present invention further is unique in using inorganic particles for the filter bed. It has been found that organic particles such as petroleum derivatives, e.g., plastics or resins, do not work in this continuous process. Such organic particles act as an adsorbent or adherent material, to which adheres the organic compound contaminant to be removed from the waste water. The washing step to remove organic compounds from the organic filter particles does not work, thus necessitating the disposal of the organic filter media.

The present process is intended for low level amounts of synthetic organic compounds such as PCBs. By "low level" amounts, amounts in water in the range of about 1 to 20 ppb are intended. The process includes pre-treating steps to bring the waste water influent to within the prescribed range. One or more physical or chemical pre-treatment steps can be incorporated into the process to precipitate the organic compound, such as PCB. These physical/chemical pre-treating steps include emulsion breaking, sedimentation, coagulation, flocculation, and clarification to remove oil, sediments, and other foreign materials. These pre-treating steps can be implemented in conventional chemical engineering designs to bring the influent to the filter bed to within the range of about 1 to 20 ppb PCB.

The organic compounds removed by the present process include synthetically produced organic compounds, typically characterized by high molecular weight and low solubility in water. Such synthetic organic compounds include polychlorinated biphenols (PCBs), chlorodiphenols, Aroclors.RTM., Kanechlors.RTM., and their cogeners. The process can be implemented as a tertiary cleanup for the removal of such organic materials to parts per billion levels.

In one aspect, the process implements an equalization tank upstream from the filter bed. The equalization tank overcomes varying influent flow rates and provides a steady flow rate through the filter bed. A steady flow rate to the filter bed is important to avoid hydraulic shear of organic contaminants from the filter media prior to the particle washing step.

Chlorine can be added to the equalization tank to inhibit microbiological growth. Influent from the equalization tank to the filter bed can be pumped or gravity fed to overcome the pressure drop through the filter bed.

The filter media of hard, substantially non-porous, inorganic particles preferably has a specific particle size. It has been found that a particle size in the range of about 0.9-0.95 mm at a uniform coefficient of less than about 1.5 in a 40-inch deep filter bed effectively removes low level organic compounds from the waste water. The specified particle size provides high hydraulic loading rates, e.g., above 4 gallons per minute per square foot, without high pressure drop and while avoiding local fluidization of the filter media and consequent breakthrough of the suspended solid. The particle size of 0.9-0.95 mm allows for the proper transport of the media in the filter bed itself. In this way, the media will not compact and stop moving as other smaller-sized media will do in such a filter.

It has been found that silica is particularly effective as the filter media in the process of the present invention. Silica provides a relatively hard material which resists degradation when the process is performed in a highly turbulent or highly agitated filter bed. Silica is substantially inert, thereby resisting chemical degradation over long-term filter media contact.

It has been found that a round grain-shape assists in the transport of sand in the filter bed and performs well. Angular media lock together and inhibit effective filter media transport.

The step of passing influent water containing organic contaminants through the filter bed preferably is carried out in countercurrent flow. The influent water enters one end of the filter and flows toward the opposite end, which opposite end serves as the entrance for washed filter particles. In this way, the most contaminated water contacts the most contaminated filter particles and the cleanest water contacts the cleanest washed filter particles.

It has been found that the washing step must employ sufficiently agitated or turbulent flow rates to scour and hydraulically shear the particles and separate the organic compound. In another aspect, the washing step includes shocking the filter media particles to shear the organic compound from the solid particles. Shocking can be achieved by an ultrasonic wave generator. The washing fluid, e.g., water, then becomes contaminated with particulate matter and organic compound.

The particle washing step can be performed by passing a washing media in countercurrent flow past the spent filter media particles. In one aspect, the process of the present invention includes washing the particles with a portion of the water effluent formed in and exiting the filter bed.

The concentrated organic effluent formed when washing the filter media particles to remove the organic compounds can be recycled to the pre-treating steps where the organic compounds will be effectively removed in a concentrated form as part of the primary waste water. The concentrated form permits effective removal in the primary and secondary physical/chemical separation steps of the same PCBs which previously passed through in low level amounts. This also results in lower quantities of PCB contaminated materials for disposal than would have resulted if carbon adsorption were used.

ln one embodiment, a filter bed suitable for use in the process of the present invention is shown and described in U.S. Pat. No. 4,126,546 which is hereby incorporated by reference into this detailed description of the present invention. It has been found that such a filter combined with the specific filter media particles, described supra, surprisingly removes organic compounds such as PCBs with high efficiency. However, the present process is not limited to the apparatus configurations and process limitations as disclosed and claimed in U.S. Pat. No. 4,126,546. Rather, the particle washing step of the present invention, for example, can take place outside of the filter bed apparatus.

A process in accordance with the present invention has been found to reduce PCBs in waste water to an unexpectedly low level. This continuous process incorporated physical/chemical pre-treating to remove approximately 85 to 95% of the PCBs in the incoming waste water. The PCB concentration in the incoming waste water ranged from 360 to 12,000 ppb. The remaining PCBs in the pre-treated water passed into an equalization tank which provided a steady flow rate to the filter. Water samples were analyzed for PCBs using EPA Test Method 608 for Organochlorine Pesticides and PCBs. The process continuously and consistently reduced PCB levels to below 1.0 parts per billion (ppb) and in many cases to below 0.5 ppb with an influent PCB concentration ranging from about 1.0 to 8.0 ppb. Since the solubility of PCBs ranges from 7 to 5900 ppb, this present invention, removing PCBs to below 1 ppb, is more efficient than could have been predicted. Further, this present invention's ability to achieve better than the commonly accepted theoretical limit of a 90% PCB removal could not have been predicted. Additionally, the invention operates continuously to remove PCBs to these low levels without media replacement. A reject line from the filter bed carried separated PCBs from the filter bed back to the clarifier where the PCBs collected as sludge.