Effluent treatment

Any effluent treatment programme must consider whether the water contaminants are biodegradable, over what time scale, and – if persistent – how they will influence conditions down stream. Even the influence of subsequent water chlorination for drinking purposes on the potential formation of toxic organochlorine compounds must be considered. This a complex subject requiring detailed on-site analysis before arriving at specific recommendations and their implementation. One very important strategy in reducing water pollution from a textile finishing plant involves minimising waste and optimising process methods so that fewer contaminants are discharged in the effluent. The following list gives some examples of this.
 (1) re-use of excess dye solution remaining at the end of a process – this can be incorporated into other dyeing recipes;
(2) mixing acidic and alkaline effluents before discharge to avoid excessive acidity or alkalinity;
(3) replacing starch-based sizing materials with polyvinyl alcohol, which can be recovered by membrane separation techniques and recycled;
(4) process changes and optimisation of the use of problematic chemicals – for example, for the oxidation of leuco vat dyes, hydrogen peroxide or sodium perborate can be substituted for sodium dichromate. Replacement of dyes that require aftertreatment with chromium or copper salts will have a beneficial impact on the immediate water environment. Glucose can replace sodium sulphide as a reducing agent for sulphur dyes. The dye manufacturers now offer ‘greener’ products and processes such as low salt/high fixation reactive dyes. Many companies are examining the possibilities of recycling water and chemicals, and of recovering heat from hot effluent. Various types of processes are used in textile effluent treatment. Dyehouse effluent has a composition that is highly time-dependent in terms of both the types and quantities of contaminants. A first stage in treatment is often an equalising lagoon. Equalisation involves holding the combined process effluents for a given period to allow stabilisation of pH and BOD, and time for sedimentation of some solids. This considerably reduces fluctuations in the composition of the water leaving the dyehouse, which can upset down stream processes such as activated sludge treatment. Biological treatment with organisms may be aerobic (with oxygen) or anaerobic (without oxygen). Many textile mills in urban areas discharge their effluent into municipal sewers. The sewage may be treated in an activated sludge plant. The effluent is mixed with micro-organisms, aerated and then the sludge allowed to settle. The phosphate and nitrogen nutrients needed for bacterial growth are not a problem if the industrial effluent is mixed with regular sewage. Activated sludge treatment considerably reduces the BOD by aerobic oxidation, and adsorption or coagulation of the contaminants. The effects are improved if the sludge is combined with activated carbon. This also protects the micro-organisms from heavy metals. They are sensitive to sudden changes in conditions and industrial effluents in the sewer system should be of relatively constant composition and concentration. The main problem is that of sludge disposal. After suitable treatment, it can be used as fertiliser, or for landfill. In a trickle filter plant, the effluent percolates through a filter bed with the bacteria growing on the surface of the filter medium. These aerobic microbial oxidation processes reduce the BOD, COD and TOC. The effect on colour from dyes, however, is often only marginal. Some dyes are adsorbed to some extent by the biological treatment. This has only limited effects in removing hydrolysed reactive dyes from the effluent. Because of the high levels of waste, textile auxiliary products should ideally biodegrade rapidly in water, although this is often not the case. Alkyl benzene sulphonate detergents, with a branched alkyl chain, such as that derived from propene tetramer, caused mountains of foam on rivers throughout the industrialised world in the 1960s because of their low rate of biodegradation. One of the most common effluent treatment methods is that of precipitation. This often involves a combination of precipitation of insoluble salts, coagulation of colloidal material and flocculation. This is similar in principle to the method used for clarifying water described in Section 8.1. Addition of lime (CaO) to the effluent is quite common. This neutralises any excess acidity and precipitates many types of anionic compounds. Treatments with alum or ferric chloride are also popular. The aluminium or ferric hydroxide, along with precipitated aluminium or ferric salts, removes colloidal matter and a number of anionic dyes. This reduces the COD, colour and suspended solids. Sedimentation is assisted by adding a flocculant such as a polyacrylic acid derivative, or a cationic polymer, the latter being able to bind hydrolysed reactive dyes. Again, the disposal of the sediment sludge needs to be considered. There are a variety of chemical technologies for effluent treatment. Few of these are used in the textile industry because of their cost. This situation may change as environmental protection becomes even more socially and politically acceptable. Chlorination with sodium hypochlorite and acid eliminates much organic material in waste water but may generate even more toxic organochlorine compounds. Oxidation by ozone is much safer but the cost of generating ozone by electrical discharge through oxygen gas is still prohibitive. Other technologies involve reverse osmosis and membrane filtration, adsorption on active carbon, or generation of coagulants by electrochemical techniques. The highest standards of effluent treatment require combinations of different types of treatment. These will probably become more significant, despite the expense, as regulatory controls are increasingly enforced.