DYEHOUSE EFFLUENT AND ITS TREATMENT

Contaminants in dyehouse effluents For many years, the textile dyeing industry was a major source of water pollution. Increasing public awareness of this problem has resulted in much stricter legislation to protect the environment with controls on the types of contaminants and the amounts released. Governments have also become much more aggressive in updating legislation and in enforcing it. Water quality criteria are usually established for drinking water, and for surface and ground waters. The control of the discharge of effluent has become a major preoccupation for industries in Europe and North America. Legislation often sets maximum daily limits and a longer-term average limit for a whole range of contaminants. Controls in developing countries are much less stringent and, in some cases, almost non- existent. This has been a significant factor in the recent re-structuring of the dyestuffs manufacturing and textile finishing industries. There is little doubt that water quality standards will become increasingly important. Therefore, effluent treatment before discharge will be required, with increasing costs to the textile industry. The impact of textile effluent on aquatic life is an active field of research. There is considerable collaboration through ETAD (the Ecological and Toxicological Association of the Dyes and Organic Pigments Manufacturing Industry [3]) and ADMI (the American Dye Manufacturers Institute). Both have undertaken numerous studies in this area. Most textile effluent water is discharged into surface waters such as rivers and lakes, either directly or through municipal sewers. The main problem is the wide range of chemicals that it contains and the high level of dilution that usually exists. Table 8.4 lists some of the common pollutants. Of particular concern are those chemicals that are not degraded by water-borne bacteria, and which therefore persist and accumulate in the environment. Many such chemicals, such as polychlorobiphenyls, are extremely toxic and dangerous. Discharge of effluent to a municipal sewer has the advantage that the sewage treatment is able to remove many, but not all, of the contaminants. Provided the nature and approximate amounts of the contaminants are known, sewage treatment can reduce pollutant levels to the point at which discharge into surface water is then feasible. Because of the diversity of chemicals in textile effluent, it is usually characterised by a number of general criteria rather than in terms of specific contaminants. These include those described below.

(1) The volume of effluent.

(2) A measure of the amount of oxygen it will consume for oxidation of the organic chemicals it contains. This point is important because depletion of oxygen in water has a negative impact on aquatic life. The biological oxygen demand (BOD) [1] is the amount of o xygen (mg l–1 or ppm) consumed in 5 days at 20 ° C by growth of bacteria from a culture added to the water. The chemical oxygen demand (COD) [1] is based on a much faster chemical oxidation of organic compounds with hot sodium dichromate solution. The two values are often close but not equivalent. Many organic compounds are readily oxidised by hot dichromate but are resistant to microbial oxidation at ambient temperature. It is typical of textile effluent that the COD is much higher than the BOD. The COD is less affected by the usual effluent treatment processes, and is thus more persistent in the environment. Values range from 200 – 3000 mg O2 l–1 for BOD and from 500 – 5000 mg O2 l–1 for COD. The total organic carbon (TOC) in the water serves as an alternative to BOD and COD. All these can be determined by standardised analytical procedures.

(3) Floating insoluble chemicals, mainly insoluble oils and solvents.

(4) Suspended solid materials. These are quite diverse and include short fibres and insoluble dyes or compounds that have precipitated in the effluent because of a change in temperature or pH. Quantities range from 50 – 500 mg l–1. This can be estimated by filtration or by turbidity measurements.

(5) Colour. This is visible pollution. While it may not be toxic, colour does reduce light transmission into waters and limits photosynthesis. The dyeing industry discharges about 9% of the dyestuffs it consumes. This corresponds to a considerable degree of colour in a dyehouse effluent. Dyes are not easily biodegraded since, by design, they have good stability towards light and various chemical treatments. Most dyes are not of high toxicity and are eventually removed from water by oxidation or adsorption on sediment, but presence of colour in the water from a dyehouse is undesirable. It is a strong indicator of the presence of much higher quantities of dyeing assistants, almost all of which are present in the effluent. Even a simple chemical such as acetic acid can significantly increase the BOD.

(6) Acidity. The pH may vary from about 4 up to near 12. The acidity of water affects aquatic life and effluent must be neither too acidic nor too alkaline on discharge. This also applies for discharge into a municipal sewer because the micro-organisms used in sewage treatment are equally susceptible.

(7) Toxic chemicals. For the textile industry, the major offenders here are heavy metals such as chromium and copper, organochlorine compounds from insecticides or moth-proofing agents, and sulphides from dyeing with sulphur dyes. We also study about- Effluent treatment, Dye-house water treatment, Water treatment in dyeing, importance of water treatment in the dyeing, Water quality in dyeing, benefits of treated water in dyeing, different types of water treatment plant, Methods of water treatment.