6 Waste water characteristics of municipal wastewater and textile industry

J.S. Laura

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1 Introduction

 

Municipal waste water (MWW) is also called sewage, include domestic wastewater derived from households, business buildings and institutions etc. It includes various waters with diverse wastes composition. Municipal waste water includes waters from different activities such as :

 

  • -Washing water also known as greywater or sullage, it comes from cleaning and washing of clothes, floors, cars, dishes, kitchen washings, and bathing, etc.
  • –Human excreta, includes faeces and urine, often mixed with used toilet paper or wipes; this is also known as blackwater
  • -Urban runoff from roads, car parks, roofs, sidewalks/pavements, contains animal faeces, litter, waste or leaked oils, gasoline/petrol, diesel or rubber residues from tires, soap scum, metals from vehicle exhausts, etc.
  • –Highway drainage containing oil, de-icing agents (in colder places), rubber residues particularly from tires etc.
  • -Surplus manufactured liquids from domestic sources (drinks, cooking oil, pesticides, lubricating oil, paint, cleaning liquids, illegal disposal of pesticides, used oils, etc.)
  • –Storm drains (may include trash)
  • -Industrial waste water after treatment in effluent treatment plant

Characteristics and sources of major pollutants

 

Municipal wastewater contains more than 99 % of water and the remaining 1% includes suspended and dissolved organic and inorganic matter as well as microorganisms. Because of these impurities, physical, chemical and biological characteristics of MWW are impaired. The pollutants which affect the water quality come from different sources. The solids (settle able, suspended or dissolved) come from cleaning and washing, dirt and dust, drainage of soil and sand by natural run off, weathering and erosion etc. High BOD is caused by organic waste contributed by human and animal excreta (cowdung, dog’s faeces), and waters from kitchen produced in process of cooking, and washing food remains. The major organic components in domestic waste water are carbohydrates, proteins and fats, oils and grease. Carbohydrates and proteins are easily biodegradable. Fats, oils and grease are more stable which decomposes slowly by microorganisms. Chlorides come from the softener used to treat hard water and from added chlorine. Human excreta also add chlorides, nitrogen, phosphorus and pathogenic  microbes- viruses, protozoa, bacteria, helminths, to MWW. Total nitrogen is comprised of organic nitrogen, ammonia, nitrite and nitrate. Organic nitrogen is found as amino acids, amino sugars and proteins. Later along with urea readily converted into ammonium by microbial action. Phosphates in MWW is mainly contributed by detergents. Sulphate is naturally present in water depending on the source geology of water; it is also a major dissolved component of rain water. Major constituents and their concentration in typical domestic wastewater are given below in the table.

 

Urban areas in time are growing in space with increase in population. Along with that sewage network is also increasing. In 2001 share of class 1 cities in total Indian urban population was 64 %. Since urban population is increasing and domestic water consumption is more, and thus wastewater generation is also increasing in such areas. National average for per capita water supply is 179 MLD (million litres per day). Water Supply in Class-I Cities is 44769 MLD in 2008. Total water supply of Class-I Cities and Class-II Towns in combination is 48093 MLD

 

The sewage generated in class-I cities is estimated to be 35558.12 MLD. 93 % of total wastewater is generated in Class-I cities only. The total wastewater generated in class-II towns is 2696.70 MLD. So less is the gap between water supply and waste water generation more efficient is the use of water. Total sewage treatment capacity in Class-II towns is 233.7 which are 8% of the total sewage generated. Taking class-I and class-II cities and town together, total of about 38000 MLD of sewage is generated but the treatment capacity exists for only about 12000 MLD. Thus, there is a large gap between generation and treatment of wastewater in India. Even the treatment capacity existing is also not effectively utilized due to operation and maintenance problem. Operation and maintenance of existing plants and sewage pumping stations is not satisfactory, as nearly 39% plants are not conforming to the general standards prescribed under the Environmental (Protection) Rules for discharge into streams as per the CPCB’s survey report. Status of 35 metropolitan cities with more than 10 Lac Population is being presented here.

 

 

 

Effect of discharge of untreated MWW

 

The municipal wastewater generated needs to be collected, treated and disposed properly. If it is not collected it causes contamination of the locally available freshwater supplies. Such untreated wastewater can have broad degenerative effects on both public health and our ecosystem. Discharge of untreated sewage in water courses both surface and ground waters is the most important water polluting source in India.

 

Nutrient enrichment causes eutrophication, which is one of the major stresses on coastal waters and severely deteriorate the coastal water quality. Theses excess nutrients particularly nitrogen and phosphorus reach the coastal waters through wastewater generated from coastal urban centres. The average nitrogen, phosphorus and potassium contents in municipal wastewater from Indian Cities are computed as 30 mg/l, 7.50 mg/l and 25 mg/l respectively, which are in agreement within the range given in the Central Public Health and Environmental Engineering Organisation (CPHEEO manual, 1993). These values are being used for calculating the nutrient load reaching the coastal waters. While the average BOD load from unit wastewater was estimated by Central Pollution Control Board as 200 mg/l is taken for calculation of BOD load.

 

Since sewage is single major source for deterioration of water resources and the quality of treated sewage is generally of lower standard which further add to the problem, so Central Pollution Control Board (CPCB, New Delhi) has laid down the standards for discharge of treated water in water resource as well as for land disposal.

 

 

 

Textile Industry

 

7.2 Introduction

 

Textile and clothing is one of the largest and oldest industries present in the world. It is among the major employment generating industries and highly contribute to the economies in many nations. In India, textile units are scattered all over the country and constitutes one of the country’s major export sectors. India makes a major contribution to world trade in cotton yarn, accounting for some 25% of the total. Tamil Nadu, Gujarat, Punjab and Maharashtra are among the states which have large number of textile industries amounting to 1895 i.e. about 81 per cent of total industries. This sector consumes large volume of water and chemicals for manufacturing and processing of textiles. There is large variation in demand of type, texture and color combination of fabrics, which results in significant fluctuation in effluent characteristics of manufacturing mills. The pollution of water from the textile industry is a major cause of concern. The textile manufacturing process and chemicals used are being discussed in the subsequent sections.

7.2.1 Textile Industry- manufacturing process

The transformation of raw material to the final usable form involves different stages. The integrated textile industry produces finished goods from raw fibers. Initially, raw fibers are

 

transformed into yarn, thread or webbing. Then the yarn is converted into fabric in looms. Fabric is then dyed or printed to convert into finished product. In short, process flow is- Raw Material à Fiber manufacturing à Yarn manufacturing à Fabric production à Dyingà Fished goods

 

The textile industries can be categorized on the basis of the types of fiber they use. Three main categories of fibers used by textile industries are –

 

  1. Cellulose fibers (e.g. cotton, rayon, linen, ramie, hemp and lyocell)
  2. Protein fibers (e.g. wool, angora, mohair, cashmere and silk) and
  3. Synthetic fibers (e.g. polyester, nylon, spandex, acetate, acrylic and polypropylene)

 

Various manufacturing processes are carried out for different types of textiles. The general production process of textiles can be broadly divided into two categories: the spinning process (the dry process) and the wet process (involves the usage of dyes). The various important wet processes involved in the textile industry are as follows:

 

  1. Sizing / Slashing: This process involves sizing of yarn using starch, polyvinyl alcohol (PVA) or carboxy methyl cellulose (CMC) to give necessary tensile strength and smoothness required for weaving. The water required for sizing varies from 0.5 to 8.2 liter/kg of yarn.
  1. Desizing: The desizing process is used to remove the sizing components from the cloth to make it suitable for dyeing and further processing. This can be done either through acid (sulphuric acid) or with enzymes. The required water at this stage varies from 2.5 to 21 liter/kg of yarn.
  1. Scouring / Kiering: The desized cloth is subjected to scouring. This process involves removal of natural impurities such as greases, waxes, fats and other impurities. This can be done either through conventional method (kier boiling) or through modern techniques (continuous scour). Kiering liquor is an alkaline solution containing caustic soda, soda ash, sodium silicate and sodium peroxide with small amount of detergent. The water required for this process varies from 20 – 45 liter/kg.
  1. Bleaching: Bleaching removes the natural coloring materials from the cloths. This process generally involves some oxidizing agent. More often the bleaching agent used is alkaline hydrochloride or chlorine. For bleaching the good quality fiber, normally peroxide is used. The chemicals used in peroxide bleaching are sodium peroxide, caustic soda, sulphuric acid and certain soluble oils. For cotton, bleaching agents used are hypochlorite or hydrogen peroxide, while for wool hydrogen peroxide and acids are used. For bleaching of synthetic cloths different agents used are hydrogen peroxide, sodium hypochlorite, peracetic acid, sodium chlorite etc.
    1. Mercerizing: The process of mercerization provides luster, strength, dye affinity and abrasion resistance to fabrics. It is generally carried out for cotton fabrics only for easy dyeing. Mercerization can be carried out through cold caustic soda solution followed by washing with water several times. The water required for this process varies from 17 to 32 liter/kg.

     

    1. Dyeing and printing: Dyeing is the most important and complex step in wet processing which provides attractive color to the product. Dyeing is carried out either at the fiber stage, or as yarn or as fabrics. For dyeing process, hundreds of dyes and auxiliary chemicals are used. Some of the dyes used are given in table (6.1).

     

    Table 6.1: Types of dyes used in textile industry and their example

 

Printing is usually done by rollers in large mills, or by stamping using dye mould in cottage industries, using various dye pastes. Chemical treatment is followed by printing to fix the color.

  1. Finishing: Finishing is the final step in manufacturing and involves water-proofing, pre-shrinking, fire-proofing, brightening etc. to produce a better quality cloth. Finishing involves chemical treatment and residues are removed by washing and rinsing.

Water use and textile wastes effluents

The textile industry is considered to be one of the biggest threats to the environment. The various processes carried out in the textile industries produce large amounts of gaseous, liquid  and solid wastes. As stated earlier, this industry uses a large amount of water for all of its manufacturing steps, which is needed for:

 

  • (a) The application of chemicals onto textiles and
  • (b) Rinsing the manufactured textiles

 

The amount of water consumed by various types of fabrics is listed in table (6.2).

 

The water let out after the production of textiles contains a large amount of dyes and other chemicals which are harmful to the environment. Besides these dyes, the other chemicals discharged are oxidizing agents, reducing agents, acids, bases, salts, enzymes, oils, wetting agents, detergents, retardants, developers, stripping agents and finishes etc. Most of these are not to be retained by final product, so after serving their purpose they are discarded into the waste water. Some major categories of water pollutants found in textile wastewater are as given below.

 

  1. Colour: In the textile dyeing process, there is always a portion of unfixed dye which gets washed away along with water. The textile wastewater is found to have high concentration of these unfixed dyes. Proportions of these are shown in table (6.3).
  1. Dissolved solids: Dissolved solids this industrial effluents include dirt, impurities, soluble excretion or secretions, residual matter and process chemicals etc. TDS are difficult to treat with conventional treatment systems. Discharge of high level of dissolved solids in the environment can contaminate ground water and surface water, which could be harmful to vegetation.
  1. Residual chlorine: Chlorine compounds are added to the textile effluents from bleaching process. These compounds can react with the other compounds in wastewater and form other toxic substances. These are very harmful to the aquatic life.
  1. Others: Textile effluents usually contain some other contaminants such as non-biodegradable organics (refractory materials), organic pollutants originating from organic compounds of dye stuffs, acids, sizing materials, enzymes, tallow etc. These impurities are responsible for high BOD and COD of effluent.

The level of toxicity or harmfulness of the textile effluents varies among industries. The brief account of various pollutants produced at different steps of textile mill is given in the table (6.4).

The organic substances (dyes, starches, detergents etc.) in textile waste undergo chemical and biological changes and deplete dissolved oxygen consuming it from receiving water. Additional heat is also added by the cooling water from various heat treatment processes in the textile plants. Table (7.5) summarizes the range of the pollutants found in textile wastewater

 

Standards of wastewater discharge

 

If effluent of such a high level of pollutants is discharged into the natural water, it will affect the aquatic life, and will render the water unusable for other purposes as well. So the levels of parameters of water quality need to be brought down to the specified level, so as not to affect the receiving waters. The Indian standards given by Central pollution control board (CPCB) for discharge of waste water from textile industry are given in the table (6.6).

 

 

Effluent treatment in textile industry

 

Wastewater produced from textile industry has to be cleaned from oil, color, chemical, organic matter, which are introduced in various steps of fabric manufacturing. The cleaning process depends on the kind of wastewater and amount of wastewater generated. The basic processes used for textile wastewater treatment are given below:

 

Primary and Secondary Treatment: The first step in the waste water treatment is to mix and equalize the waste water streams that are discharged at different time, and different intervals from different stages in the processes. Some industries also prefer screening, oil trap prior to equalization for removal of solids and oil and grease. Equalization ensures that the effluent have uniform characteristics in terms of pollution load, pH and temperature. The effluent is then subject to flash mixing for the addition of coagulants such as lime, alum, ferrous sulphate, ferric chloride, polyelectrolyte and processed through clariflocculator or flocculator and settling tank. Selection of appropriate coagulants and doses of chemicals are determined on the basis of treatability study of effluent samples. The chemical treatment helps in reduction of colour and suspended solids. A significant reduction in BOD and COD values is also observed. This physico-chemical treatment is followed by biological treatment process, with settling which further reduces BOD and COD values.

Tertiary Treatment: Textile effluents may require tertiary or advance treatment methods to remove particular contaminant or to prepare the treated effluent for reuse. Some common tertiary operations are removal of residual organic color compounds by adsorption and removal of dissolved solids by membrane filtration. The waste water is also treated with ozone or other oxidizing agent to destroy many contaminants. Evaporation and crystallization are other methods to minimize effluent disposal problems.

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References

 

  1. Sengupta, B., 2007. Advance Methods for treatment of textile industry. Delhi State Industrial Development Corporation, CPCB, MoEF. http://cpcb.nic.in/newitems/27.pdf
  2. Textile industry and water treatment. https://www.lenntech.com/textile_industry_and_water_treatment.htm
  3. Moustafa S. (2008) . Process analysis & environmental impacts of textile manufacturing. Dyes and chemicals.
  4. Ntuli F, Omoregbe I, Kuipa P, Muzenda E, Belaid M (2009). Characterization of effluent from textile wet finishing operations. WCECS.
  5. Mostafa, M., 2015. Waste water treatment in textile industries- the concept and current removal technologies. Journal of Biodiversity and Environmental Sciences, 7(1):501-525