Removal of Organic Contaminants From Waste Water

This invention relates to a process for reducing the organics content of an aqueous alkaline waste stream containing organic esters having pesticidal properties by acidification and extraction with a C.sub.5 -C.sub.10 hydrocarbon.

Aqueous waste streams containing contaminating amounts of organic compounds are generated in a variety of industrial processes, for example in the manufacture of insecticides. Before such aqueous streams can be discharged into a receiving stream, the organics content must be substantially reduced in order to meet regulatory standards. For example, the production of pesticidally active cyano-substituted carboxylic acid esters e.g. by Francis coupling, may result in an alkaline aqueous waste stream containing not only small amounts of the ester together with dissolved starting materials, but in addition alkali metal salts. These esters have prominent insecticidal and acaricidal activities, but low toxicity to mammals. Disposal of such waste streams e.g. by incineration is both difficult and expensive owing to the presence of the alkali metal salts.

A process for reducing the organics content of aqueous alkali waste water containing at least one organic ester having insecticidal properties which comprises

adjusting the pH of said water in the range from about 2.5 to about 5.5

extracting said water with a C.sub.5 -C.sub.10 hydrocarbon, and separating an aqueous product having lower content of said at least one organic ester.

In accordance with the invention alkaline waste water containing at least one organic ester having insecticidal properties is treated to substantially reduce the organics content by acidification, extraction of the organics into certain hydrophobic hydrocarbons and separation of the extracted water.

The manufacture of pesticidally active organic esters, particularly cyano-substituted carboxylic acid esters are described, e.g. in U.S. Pat. Nos. 3,835,176; 4,110,360, 4,110,361; 4,110,362; 4,110,363 and 4,123,451 incorporated herein by reference. For example, in the manufacture of esters such as .alpha.-cyano-3-phenoxybenzyl-2,2,3,3-tetramethyl cyclopropane carboxylate or .alpha.-cyano-3-phenoxybenzyl 2-(4-chlorophenyl)-3-methyl butanoate often results in an alkaline waste stream containing organics including the esters and starting materials for their manufacture, together with alkali metal and/or ammonium salts. Typical alkali metal salts include chlorides, sulfites, phosphates and cyanides of e.g. sodium, potassium and lithium. In the event the water contains cyanide ions, they should be removed by any conventional method such as chlorination, treatment with a peroxide such as hydrogen peroxide, or hydrolysis with an aldehyde such as formaldehyde prior to practice of the present invention. The products of such treatment are either non-toxic or readily biodegradable in contrast to the refractory nature of the organics removed by the instant process to conventional biodegradation.

In the typical practice of the invention alkaline waste water containing e.g. from 10-2,000 ppmw of the pesticidal esters and up to about 18,000 ppmw of organics is charged to an acidification zone wherein acid is added to the water to lower the pH to below about 5.5 and preferably in the range from about 2.5 and 5.4, particularly in the range from 2.6-3.6. Although still lower pH's may be employed, generally these will be too corrosive and adversely impact materials of construction or require more expensive equipment. The acidification zone which may comprise one or more vessels comprised of acid resistant materials is preferably equipped with a mixing device to rapidly lower the pH to the desired range with the minimum amount of acid. Although in theory any acid may be added preference is given to strong mineral acids such as hydrochloric acid, sulfuric acid phosphoric acid and the like. Of these hydrochloric acid is most preferred since it has lower tendency to form surface active reaction products with the organics present in the waste water.

The water having a pH below 5.5 is extracted with a C.sub.5 -C.sub.10 hydrocarbon. The hydrocarbon may be aliphatic, cycloaliphatic, aromatic or mixtures of these. Exemplary hydrocarbons include n-pentane, n-hexane, cyclohexane, benzene, toluene, n-heptane, methyl cyclohexane, ortho, meta- and para xylenes, n-octane, ethyl benzene, n-nonane and the like. Particularly preferred are C.sub.5 -C.sub.8 aliphatic hydrocarbons and mixtures thereof containing less than 10% vol aromatics, especially n-heptane.

The liquid-liquid extraction may utilize co-current flow but preferably employs counter-current flow. The extraction may be conducted separately but preferably is conducted simultaneously with the acidification.

In a preferred embodiment, the extraction is conducted in two stages with intermediate separation of the organic rich hydrocarbon. In a particularly preferred embodiment the water is both acidified and counter-currently extracted with hydrocarbon followed by intermediate settling to substantially separate the organics-rich (fat) hydrocarbon, which is then removed, followed by a second counter-current extraction of the water product from the first extraction, with fresh hydrocarbon. Although it is possible to employ a different hydrocarbon for each extraction stage, for simplicity and ease of separation, use of a single hydrocarbon composition e.g. n-heptane is preferred.

The ratio at which the hydrocarbon and water are employed in the extraction step(s) have an effect on the overall efficiency of the system. The preferred ratio will depend upon the particular organics, particular hydrocarbon, and mixing efficiency, but may be easily determined by experimentation. Generally volume ratios of hydrocarbon to water in the range from about 0.2:1 to about 5:1 have been suitable, with volume ratios in the range from about 0.5:1 to 2:1 being preferred. When two-stage extraction is employed substantially equal amounts of the hydrocarbon will be employed in each stage.

Since the alkaline waste water that is to be treated by the process of the invention may contain solid or semi-solid impurities such as silica, and/or organics at concentrations in excess of their solubilities, it is advisable to remove these materials by known techniques, e.g. filtration, centrifugation, decantation and the like, prior to treatment by the process of the invention in order to improve the efficiency of the extraction step.

The process according to the invention may suitably reduce the content of pesticidal ester to below about 100 parts per billion by weight (ppbw). As a rule the process will not be applied to water containing less than about 10 or more than about 20,000 parts per million by weight (ppmw) of total organics.

After separation from the water having lowered organics content, the organics rich hydrocarbon may be regenerated, by e.g. fractional distillation, to separate an overhead stream substantially free of pesticidal ester, and which, if desired, may be recycled to the extracting step(s) to extract additional ester-containing feed water. The residual organics from the fractional distillation may be incinerated to recover the heat value thereof, or subjected to further purification for reuse.

The invention is further illustrated but is not intended to be limited by the following example.

A synthetic ester-containing alkaline water representative of the waste water from the manufacture of a pesticidal ester is prepared having the composition shown in Table 1.

The ester content too is very significantly reduced according to the invention.