Treatment of textile wastewater by electrochemical method

In textile process industries, dyeing and finishing processes are two important steps. These steps involve the dyeing of man-made or natural fibers to the desired permanent colours. Hence dyeing and finishing processing have become an integral part of the textile manufacturing process. However, in the dyeing and finishing processes, a considerable amount of wastewater is generated. Textile wastewater contains strong colour, a large amount of suspended solids, a highly fluctuating pH and a high temperature and COD concentration. Because of these characteristics, treatment of this textile wastewater has been rather difficult.

Traditional methods for dealing with textile wastewater consist of various combinations of biological, physical and chemical methods. Because of the large variability of the composition of textile wastewaters, most of these traditional methods are becoming inadequate and treatment cost of textile waste effluents has been increasing rapidly. Ozonation method has been shown to be quite effective in decolorizing textile wastewater.

The another alternative method for treating textile effluent is electrochemical method. This method is already tested to treat various industrial wastewater, but it has been rarely used for treatment of textile wastewater.

CHARACTERISTICS OF TEXTILE WASTEWATERS:

The most notorious part of textile wastewater is its strong colour. Depending on the type of dyestuff used, the colour of the textile wastewater varies from red, brown, blue, purple and black due to their intensified and dark varieties. Textile wastewater can change colour from day to day, or even several times a day because the dyestuff used in the dyeing process changes frequently due to customers’ requirements. The variation of colour also causes frequent fluctuation in the COD content of the textile wastewater.

The large pH swing in the textile wastewater is another strong negative point. The pH variation is primarily caused by different kinds of dyestuff used in the dyeing process. The pH of the wastewater can change from 2 to over 12. Such a big variation is a big problem for ASP or chemical treatment process. Hence proper pH adjustment has become a necessary part of the textile wastewater treatment process.

The another problem is high temperature. In comparison to most industrial wastewaters, the temperature of textile wastewater is unusually high. During the dyeing process, rinse waters of up to 90°C are used in various steps. The near 40°C temperature of the textile wastewater is attributable to these hot rinse waters. The high temperature renders immediate treatment of the textile wastewater released directly from the dyeing process impractical and usually entails proper prior heat dissipation to lower the temperature to 30°C or below.

Other important pollutants of textile wastewater may consist of small amounts of polyvinyl alcohol (PVA) and starch used for sizing of the chemical or man-made fibers. The sizing agents have a very high COD content of over 10,000 mg/1, depending on their concentrations. They enter the wastewater after the desizing process and contribute significantly to the COD content of the textile wastewater.

Along with above pollutants; pH, BOD, total solids and the amount of wastewater produced vary widely from one operation to another, mixing of these wastewaters in the equalization tank forms a final waste effluent which possesses more stable physical and chemical properties. In general, the final textile waste effluent can be broadly categorized into three types of wastewater according to their COD content and the colour intensity, these are the high, medium and low strength wastewaters. The high strength wastewater has a COD concentration exceeding 1600 mg/l and a strong dark colour with very low transparency. The medium strength wastewater has a COD content between 800 and 1600 mg/l while the low strength wastewater has the lowest COD concentration of less than 800 mg/l. The colour in the wastewater usually varies in intensity according to the above strength classification. But in many circumstances, the low or medium strength wastewater may also have a very strong colour.

Electrochemical Method:

In this method, normally uses cast iron cathode and anode enclosed in electrolytic reactor. For this method pH of the textile wastewaters in the electrolytic cell was adjusted to around 7, The mechanism of the electrochemical process in aqueous systems is quite complex. There are three possible mechanisms involved in the process: electrocoagulation, electro flotation and electro oxidation. Oxidation and reduction of the electrochemical process occur, respectively, at the anode and cathode of the iron electrodes according to

Anode:

4Fe ⇌ 4Fe²⁺ + 8e^–

4Fe²⁺ + 10H₂O + O₂ ⇌ 4Fe(OH)₃ + 8H⁺

Cathode:

8H⁺ + 8e^– ⇌ 4H₂

With sufficient power supply, the dye molecules are reduced at the cathode to small organic molecules. Some of the small organic molecules and the suspended solids are captured by Fe(OH)₃ which is removed by sedimentation. This method is significantly reducing COD and decolourization of textile wastewater.

The study shows that, the pH effect of the textile wastewater on the treatment efficiency is found not to be significant when it is within the range between 5 and 10 and hence in the majority of cases, adjustment of the pH prior to electrochemical treatment is not necessary. Addition of a small amount of polyaluminum chloride (PAC) up to 40 mg/l has been observed to significantly enhance the treatment efficiency. A current density of about 92.5 amp/m² is also found to yield a maximum COD removal.