Characteristics of textile wastewater

Characteristics of textile wastewater

Textile Printing and dyeing processes include pre-treatment, dyeing and printing, finishing.

The main pollutants are organic matters which come from the pre-treatment process of pulp, cotton gum, cellulose, hemicellulose and alkali, as well as additives and dyes using in dyeing and printing processes. Pre-treatment wastewater accounts for about 45% of the

total, and dyeing/printing process wastewater accounts for about 50%~55%, while finishing process produces little.

In Vietnam, chemical fiber accounts for about 69% of total in which polyester fibers accounts for more than 80%. Cotton accounts for 80% of the natural fiber production. Therefore, the dyeing wastewater analysis of production and pollution is based on these two fibers.

Pre-treatment of cotton includes desizing and scouring. The main pollutants are the impurities in the cotton, cotton gum, hemicellulose and the slurry, alkali in weaving process.

The current average COD concentration in the pre-treatment is 3000 mg/L. The main pollutants in dyeing/printing are auxiliaries and the residual dyes. The average concentration of COD is 1000 mg/L and the total average concentration is 2000 mg/L after mixing.

Pre-treatment of polyester fibers mainly involves in the reduction with alkali. The so-called reduction is treating the polyester fabric with 8% of sodium hydroxide at 90oC for about 45 minutes. Some polyester fabrics will peel off and decompose into terephthalic acid and ethylene glycol so that a thin polyester fabric will have the feel of silk. This process can be divided into continuous and batch type. Taking the batch type as an example, the concentration of COD is up to 20000 mg/L-60000 mg/L. The wastewater from reduction process may account for only 5% of the volume of wastewater, while COD accounts for 60% or more in the conventional dyeing and finishing business.

The chroma is one pollutant of the wastewater which causes a lot of concerns. In the dyeingprocess, the average dyeing rate is more than 90%. It means that the residual dyeing rate in finishing wastewater is about 10%, which is the main reason of contamination. According to the different dyes and process, the chroma is 200 to 500 times higher than before. pH is another factor of the dyeing wastewater. Before the printing and the dyeing process, pH is another factor ,the pH of dyeing wastewater remains between 10 to 11 when treated by alkali at high temperature around 90oC in the process of desizing, scouring and mercerization. Polyester base reduction process mainly uses sodium hydroxide, and the total pH is also 10 to 11. Therefore, most dyeing water is alkaline and the first process is to adjust the pH value of the textile dyeing wastewater.

The total nitrogen and ammonia nitrogen come from dyes and raw materials, which is not

very high, about 10 mg/L. But the urea is needed while using batik techniques. Its total

nitrogen is 300 mg/L, which is hard to treat. The phosphorus in the wastewater comes from the phosphor detergents. Considering the serious eutrophication of surface water, it needs to be controlled. Some enterprises use trisodium phosphate so that the concentration of phosphorus will reach 10 mg/L. So, this phosphorus must be removed in the pre-treatment.

In the production process, suspended substance comes from fiber scrap and undissolved

raw materials. It will be removed through the grille, grid, etc. The suspended solids (SS) in the outflow mainly come from the secondary sedimentation tank, whose sludge has not

been separated completely which will reach 10-100 mg/L as usual.

The potential specific pollutants from textile Printing and dyeing is shown in Table 1

Table 1. Specific pollutants from textile and dyeing processing operations

No.

Process

Compounds

1

Desizing

Sizes, enzymes, starch, waxes, ammonia.

2

Scouring

Disinfectants and insecticides residues, NaOH, surfactants,soaps, fats, waxes, pectin, oils, sizes, anti-static agents, spent solvents, enzymes.

3

Bleaching

H2O2, AOX, sodium silicate or organic stabiliser, high pH.

4

Mercerizing

High pH, NaOH

5

Dyeing

Colour, metals, salts, surfactants, organic processing assistants,sulphide, acidity/alkalinity, formaldehyde.

6

Printing

Urea, solvents, colour, metals.

7

Finishing

Resins, waxes, chlorinated compounds, acetate, stearate, spentsolvents, softeners.

Discharged wastewater by some industries under uncontrolled and unsuitable conditions

is causing significant environmental problems. The importance of the pollution controland treatment is undoubtedly the key factor in the human future. If a textile mill discharges the wastewater into the local environment without any treatment, it will has a

serious impact on natural water bodies and land in the surrounding area. High values of COD and BOD5, presence of particulate matter and sediments, and oil and grease in the

effluent causes depletion of dissolved oxygen, which has an adverse effect on the aquatic

ecological system.

Effluent from textile mills also contains chromium, which has a cumulative effect, and higher possibilities for entering into the food chain. Due to usage of dyes and chemicals,

effluents are dark in color, which increases the turbidity of water body. This in turn hampers the photosynthesis process, causing alteration in the habitat (Joseph Egli, 2007).

Technology of sewage treatment textile dyeing

Physicochemical wastewater treatment

Wastewater treatment is a mixture of unit processes, some physical, others chemical or biological in their action. A conventional treatment process is comprised of a series of individual unit processes, with the output (or effluent) of one process becoming the input

(influent) of the next process. The first stage will usually be made up of physical processes. Physicochemical wastewater treatment has been widely used in the sewage treatment plant which has a high removal of chroma and suspended substances, while it has a low removal of COD. The common physicochemical methods are shown as followed.

Biological wastewater treatment method

The biological process removes dissolved matter in a way similar to the self depuration but in a further and more efficient way than clariflocculation. The removal efficiency dependsupon the ratio between organic load and the bio mass present in the oxidation tank, its temperature, and oxygen concentration.

The bio mass concentration can increase, by aeration the suspension effect but it is important not to reach a mixing energy that can destroy the flocks, because it can inhibit the following settling. Normally, the biomass concentration ranges between 2500- 4500 mg/l, oxygen about 2 mg/l. With aeration time till 24 hours the oxygen demand can be reduced till 99%. According to the different oxygen demand, biological treatment methods can be divided into aerobic and anaerobic treatment. Because of high efficiency and wide application of the aerobic biological treatment, it naturally becomes the mainstream of biological treatment.

The following is a process of textile wastewater treatment technology combined between Physicochemical wastewater treatment and Biological wastewater treatment method, for the wastewater properties of the textile industry as above, this process can be said. It is more efficient, saves construction area, construction and operation costs compared to traditional technologies, high efficiency of BOD, COD, SS treatment.

 Wastewater from the sewer system of the plant in the collection systems flowing into the treatment tank, firstly the waste water is passed through the trash can for the purpose of eliminating rubbish and other forms of fiber. Wastewater after passage is collected into the collecting pit and pumped through the air conditioning tank, where the wastewater is moderated and blended as well as supplied with gas for stable disturbance of the flow as well as the concentration of Substances present in wastewater.

After passing through the regulating tank, the wastewater is pumped through the agglomerated tank, where chemicals such as alum, polymer and some environmental modifiers such as H2SO4 are added at a certain dose. Increase the efficiency of the coagulation process. From the leachate tank the effluent flows through the cotton tank, in the cotton tank the polymer chemicals are added at a certain dosage, under the effect of both the stirring system of the dirt in the suspended liquid suspended in form. Sediments are deposited in the sedimentation tank and adhered to the granular surface of the filter media at the most rapid and economical filtration.

Wastewater from the cotton tank before self-flowing through the settling tank 1 in order to separate the deposits formed in the cotton tank, where, under the effect of gravity and the density difference between water and sludge, come down here. The wastewater after removal of the SS is led to an intermediate tank in order to stabilize the flow before pumping into the gas tank.

Water after physical treatment and sediment separation ensures that most of the heavy metals are removed into the aerosol effluent which is blended with the air supplied by the air blowers via the gas distribution system. The tank will help microorganisms in the aerobic form (activated sludge) to break down the organic residues in waste water into simple inorganic substances. Here, BOD is processed, COD is about 90-95% efficiency From the basin, the wastewater is led to the second settling tank.

Here the process of separation between water and activated sludge. Activated sludge settles to the bottom and is conveyed to the sludge reservoir through the bottom sludge collection system, while effluent above the surface flows into the disinfection tank. In the disinfection tank, Ca(OCl)2 solution Pump tank to thoroughly treat the disease germs such as E.Coli, Coliform, ... Water after treatment in the sedimentation tank 2 continues to lead to the tank level filter used to store water for pressure filter tank to remove the color and the The remaining residue will then be discharged into the disinfection tank.

Here the water is mixed with Chlorine solution simultaneously with the appropriate time to carry out the process of sterilization. Waste water after passing the disinfection tank meets QCVN 13: 2008/BTNMT standard, column B.

In some cases, the effluent after disinfection will not discharge directly and the receiving source will be pressurized to the filter tank. After passing through the filter tank, the remaining residue in the effluent will be The treatment is thorough, and the wastewater is fed through a static mixer to disinfect it again before it reaches the receiving source. Sludge sludge at the bottom of the settling tank 1 The sludge at the bottom of the reservoir 2 is pumped into the silt reservoir. In the sludge tank, excess sludge is pumped into the sludge compacted tank to reduce the volume before it is put to disposal in the next steps. This technology is the optimum technology to ensure the wastewater treatment meets the required standards, applying the chemical treatment method to best reduce temperature and color, and high COD removal efficiency.

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