38 Effluent Treatment Methods

K Kalaiarasi

1. Introduction

Textile processing industry consumes large volume of water required for various operations and releases a wide variety of pollutants during the processing of fibres, fabrics and garment construction. About 200 L of water are used to produce 1 kg of textile material. About 30% of water is used for bleaching, 16% for dyeing, 8% for printing, 14% for boiling and remaining for other uses. The water released by the textile and dyestuff industries are well beyond the standard limits and have to be treated due to their impact on water bodies and growing public concern over the toxicity and carcinogenicity of the pollutants.

2. Objective

To enable the students to gain knowledge about different effluent treatment methods
To know about different effluent parameters.

The textile effluents are treated in three stages. They are

1. Primary treatment

2. Secondary treatment

3. Tertiary treatment

3. PRIMARY TREATMENT

The main objective of primary treatment is to remove suspended solids and bring the pH to neutral range.

Primary treatment method includes

3.1. Screening

3.2. Sedimentation

3.3. Equalisation

3.4. Neutralisation and

3.5. Coagulation

3.1 Screening

Screening is the first step of primary treatment which removes larger suspended particles such as fibres, yarns, pieces of fabric, lint etc. Removal of these substances is important as these may clog pipelines, generators, pumps and machineries. Screens of various sizes and shapes are used, depending on the nature of solid (fine or coarse) to be removed.

Short fibres and smaller suspended particles are removed by fine screen. Larger suspended particles are removed by coarse screen which consists of metal bars or heavy wires spaced 25 to 50 mm apart. They are placed at an angle of about 60 degree to the horizontal axis of the flow or curved to the radius of a circle. Fine screens consists of woven wire or perforated plates and its poresize [The size of the opening ranges from 0.8mm to 6mm] ranges from 0.18mm to 0.4mm. The screens must be cleaned at regular intervals. Nowadays modern mechanical screens are used .These screens are of two types, rotary self-cleaning gravity type units and circular overhead fed vibratory units.

3.2 Sedimentation

Sedimentation is the process of settlement of larger particles by gravity. It is an essential pre-requisite for secondary treatment. It removes fibre and scum. Most of the solids settle along with heavy metals. The sludge formed at the bottom of the tank is removed as underflow by vacuum suction.

3.3Equalisation

Different kinds of wastes are produced by various textile processes and discharged at different intervals. Hence treatment cannot be done as the effluent exhibit different characteristics from time to time. Treatment will be effective only when the parameters such as BOD,pH etc., are constant. Effluents from various processes are collected in a holding tank to make the characteristics of the effluent uniform. This process of stabilisation of pH and BOD is called as equalisation. The size and shape of the equalisation tank varies with the amount of waste generated. It should have the capacity to hold the entire effluent generated. Generally they are rectangular and square in shape. Proper mixing of waste water is brought about by appropriate distribution and baffling, mechanical agitation and aeration.

Several inlet pipes were spaced at regular intervals across the width of the tank for thorough mixing. Horizontal and vertical distribution is brought about by providing over and under baffles. Mechanical agitators are placed along the centre line of the length of the tank spaced equidistantly with 10 rpm to 20 rpm. Bleaching and dyeing effluents containing reducing substances and sulphur compounds are aerated.

Advantages

Reduce the cost of treatment by mixing acidic and alkaline effluents thereby bringing the pH to neutral.
Prevent the shock loading on secondary treatment.
Equalisation will bring down the temperature of the effluent which will otherwise hinder the secondary biological treatment systems.

3.4. Neutralization

The effluents from textile processing industries have extreme pH value. Hence it has to be treated before letting out in to the environment or for secondary treatment. The effluent of acidic nature will corrode the cement, concrete and metals of the pipeline, pumps and other structures. It also de-emulsify the fats, grease , soaps etc., and produce sticky gums which may block the pipelines. Alkaline effluents are also dangerous as it is corrosive in nature. Hence, textile effluents have to be neutralized before discharged in to the environment.

3.4.1 Neutralization of acid waste

Mixing of acidic and alkaline effluentsOne of the effective and cheapest method of neutralization is mixing the acidic effluents with alkaline effluents in a large holding tanks. Mixing of the effluents may be controlled using valves.
Lime slurry method Lime slurry is the cheapest source for neutralising large volume of acidic effluent,. In this method, the sludge produced is voluminous and take more time than caustic-soda method.
Caustic-soda method The most effective neutralizing agent is caustic soda. The product formed are soluble in nature.

Advantages

Hardness of the waste water is not increased Smaller amounts of caustic-soda are required Very smaller volume of sludge is formed

Disadvantages

Products formed with caustic soda are highly soluble in water
Caustic soda is expensive and hence cannot be used for treating large volumes.

3.4.2 Neutralization of alkaline waste

Alkaline effluents are neutralized by two methods

1. Sulphuric acid treatment

This method is the most efficient method of neutralization of alkaline wastes. Predetermined amount of sulphuric acid placed in a storage tank is added to the alkaline waste by drip feed method or sulphuric acid is first added to the tank followed by alkaline effluent.

Advantages

Smaller volume is required
Effective method of neutralizing alkaline wastes

Disadvantages

Sulphuric acid is expensive Sulphuric acid is highly corrosive and difficult to handle Requires special materials for storage and pumping

2. Boiler-flue gas treatment

Carbon dioxide present in boiler flue gas act as a neutralizing agent. It reacts with alkaline wastes and form carbonates. The equipment consists of a blower, pipeline to carry the gas, filter to remove sulphur and unburnt carbon particles and a gas diffuser for diffusing carbon dioxide in to waste water. Hydrogen sulphidereleased during the process should be vented to the upper atmosphere. This method is economical but pH can be reduced only upto 9.0.

3.5. Coagulation

Coagulation means treatment of water with coagulants to remove colloidal and fine suspended impurities. The particles are destabilized and aggregated to form floc by the addition of some chemicals. The formed flocs adsorp and entrap the suspended particles and settles rapidly.

Coagulants

Most commonly used coagulants for the treatment of textile effluents are alum, ferric sulphate, ferrous sulphate, chlorinated copperas, ferric chloride and calcium chloride. The amount of coagulant depends on the composition and pH of the waste. Each coagulant has specific iso-electric point at which maximum coagulation occurs. The optimum pH for aluminium coagulant is 5.5 to 6.8. Hence lime is added. Aeration of the effluent increases coagulation.

Coagulant aids

To enhance the process of coagulation, some chemicals are added in smaller quantities such as activated silica and poly electrolytes that promote the rapid settling of flocs. These chemicals are called as coagulant aids. They act by reducing the charge on the colloid.

Steps involved in coagulation

1.Flash mixing: The coagulants, coagulant aids and chemicals for pH adjustments are rapidly mixed for 4-6 minutes in a smaller tank for greater dispersion.

2.Flocculation: The process of floc formation is called flocculation. The flash mixing is followed by gentle stirring of waste water and coagulants in a larger tank for 15-45 minutes for formation of flocs without being broken down by the turbulence.

3.Settling: The flocs formed entangle other suspended particles and settle quickly leaving a clear supernatant liquid. A period of 1-6 hours is required for settling.

Advantages

Some coagulants such as alum, chlorinated copperas are effective in colour removal Reduction of BOD by 63%
For small textile mills that cannot afford for secondary treatment, primary treatment with a final chemical coagulation is effective and attractive

Disadvantages

Formation of large quantities of sludge

4. SECONDARY TREATMENT

The effluent from the primary treatment contains dissolved organic materials and small amount of suspended solids. In secondary treatment, biological action is used to remove the organic materials. Complex organic molecules are broken down into simpler substances by microorganisms during secondary treatment hence secondary treatment is also called as secondary biological treatment.

Objectives of secondary treatment

To remove dissolved and residual organic matter To reduce COD, BOD and toxicity
There are two types of secondary treatments. They are aerobic and anaerobic.

Aerobic treatment involves oxygen for the degradation of organic molecules by aerobic microorganisms and the end product is CO2 and water. Anaerobic treatments are reductive and take place in the absence of oxygen. The end product is methane. Aerobic treatment is most widely used as it is more rapid.

4.1. Activated sludge process

The most versatile biological oxidation method for treating textile waste is activated sludge process. In this method, the textile effluent after primary treatment is fed in to an aeration tank where it is subjected to continuous biological degradation by microbial flocs suspended in the reaction tank. Oxygen is supplied by diffusers or by surface aerators. The effluent from the aeration tank is passed to sedimentation tank for settling. A part of the sludge is returned to the aeration tank for effective microbial population.

Aerobic
(C, H, O, F, S) + O2	CO2 +H2O + H2SO4 + NH4+ + NO3-
Organic wastes	microorganisms

The time required for activated sludge process ranges from 6-12 hours depending on the pollution load. The active centres of biological oxidation are microbial mass which include bacteria, fungi, protozoa and nematodes. Among these, bacteria are the most important as they are the work horses that degrade the organic substances.Oxygen is an important material for the activated sludge process. A minimum of 0.5 mg/L of oxygen should be present. If oxygen supplied is less than the amount required for the microbial metabolism, then anaerobic conditions prevail resulting in the growth of unwanted filamentous fungi and protozoa which retard floc compaction and settling.

Essential nutrients such as nitrogen and phosphorus should be supplied to the microorganisms in the form of urea, monoammonium and diammonium phosphate. If the nutrients are deficient, then growth of filamentous fungi will be stimulated which prevent settling of suspended particles. Other nutrients such as potassium, calcium and magnesium if not present in adequate quantities, should be supplied. Iron, molybdenum and cobalt are required in trace amounts. pH is an important parameter that determines the efficiency of the treatment. Hence an activated sludge system should be maintained at optimum pH and supplied with buffers to prevent change in pH during the process. The optimum pH range is 6.5 to 9.0. Below 6.5, fungi will compete with the bacteria. pH measurement is made at regular intervals.

Modifications of activated sludge process include

1. tapered aeration

2. stepped aeration

3. extended aeration

4. contact stabilization and

5. high rate aerobic treatment

Advantages

BOD reduction upto 95%
Removal of colour upto 80% Lower capital and maintenance

Disadvantages

Sensitive to shock loads of toxic and organic substances Production of larger quantities of sludge
Volume and concentration of sludge recycled should be controlled carefully

4.2 Trickling filtration

Trickling filtration is an effective aerobic biological oxidation process. In this method, the textile effluent after primary treatment is sprinkled over a bed of broken stones. The microbial film formed on the stones oxidizes the organic waste present in the effluent during its passage. The treated effluent is allowed to settle and then discharged. Trickling filters are also termed as “bio-filters and percolating filters”. The most common trickling filters are circular. Plastic and PVC material are used in place of stones. The gaps between the stonesallow air to penetrate the bed and this is improved by having ventilation at the base. The depth of bed must be more than 3 meter. The waste water is sprinkled by moving or fixed sprinklers. Trickling filter does not require inoculation but develops its own population, which is a complex mixture of bacteria, fungi, protozoa, algae and larger organisms like insects and worms. Once the biofilm reaches a certain thickness, layers will be detached and these aggregates are collected in a settling tank.

The aerobic microorganisms form a gelatinous film on the surface of the stone and oxidize both dissolved and colloidal organic matter in to simpler substances. The emergent liquid is free from degradable organic matter. The removal of organic matter depends on the pollution load, temperature, size of stone, microbial surface area, depth of the filter, retention time and oxygen transfer.

The growth and metabolic activities of microbes are affected by temperature of the waste water. Lower temperature decreased the efficiency of degradation whereas higher temperature kills microbes. Hence optimum temperature should be maintained.

The shape of the stone should be spherical ranging from 2-10 cm in size. Smaller stones are preferred as they have maximum surface area. For maximum efficiency, thin layer of the microbial film should be formed. Usage of plastic and PVC material instead of stones are efficient. The depth of the filter bed ranges from 1m-3m. The base is constructed with vitrified clay blocks as it can withstand the corrosive nature of the wastes. The filter is connected with a sedimentation tank where final settlement occurs.

Advantages

Remove BOD by 40-85%, suspended solids by 80-90% and dissolved solids upto 30%
Treated effluents are of better quality
Recover quickly from shock loads of toxic substances
An important modification of trickling filters is high rate bio-filters where high density plastic sheets or tubes in circular form are used. It allows the effluent without any blockage and air for the microbial growth. It can accommodate high hydraulic loadings. High rate bio-filters are used to treat scouring and dye house effluents.

4.3 Aerated lagoons

Aerated lagoons are activated sludge units operated without sludge return. In this method, large holding tank or ponds are lined with cement or butyl rubber or polythene. The depth of the tank varies from 3-5m. The effluent after primary treatment is passed on to these tanks and aerated mechanically. Required oxygen is supplied by floating aerators. The contents of the lagoon are mixed which enhances the oxidation of dissolved organics present in the effluent. Retention time varies from 2 to 6 days, during which time a healthy flocculent sludge is formed which carry out oxidation of dissolved organics.

Advantages

BOD is removed by 90%
Operation and maintenance are simpler
Able to resist shock loads of toxic substances Efficient in treating variable wastes

Disadvantages

Reduced performance during winter
Treated effluent from lagoons contain bacteria and hence need further purification

4.4 Oxidation ponds

The oxidation pond is a simple and economical method for textile effluent treatment. An oxidation pond is a large shallow pond in which effluent is added at one end and treated effluent is removed at other end. Oxidation of organic wastes is brought about by bacteria and flagellated protozoa. Oxygen is supplied by the algae present in the pond. The more the algae in the pond, more will be the amount of oxygen released which inturn increases the rate of purification. The growth of algae is enhanced by the nutrients present in the waste. If the waste lacks essential nutrients, it has to be supplied. The algae inturn takes up CO2 released by bacteria. For sunlight penetration, mixing of the waste by wind and natural aeration, thepond is built shallow (0.5 to 1.5 m). Depth of pond more than 1.5 m makes it anaerobic. The bottom of the pond is covered with polythene sheeting or linings of bitumen and asphalt to prevent seepage. Concrete slabs are placed at the sides to prevent vegetation and erosion. The retention time varies from 10 to 30 days. Reduction in BOD is achieved upto 80%.

4.5 Anaerobic digestion

Anaerobic digestion is carried out mainly for the digestion of sludge. In this method, digestion is carried out in the absence of air in a closed chamber of depth 6m. The organic wastes are converted into methane, ammonia, hydrogen sulphide and organic acids by microorganisms. The retention period ranges from 10-12 days. For effective treatment, the contents are made uniform by stirring. The temperature is maintained at 35°C. This treatment is useful as a preliminary treatment for strong waste. BOD reduction is upto 80-90%.

The advantages of anaerobic digestion are that the processes produce much less biomass or sludge, forms methane (biogas), no aeration required and accompanying smell is less as the process is enclosed. The disadvantages of anaerobic digestion are that the process requires good mixing, a temperature of 37°C, long retention time and a substrate with high BOD.

5. TERTIARY TREATMENT

Tertiary treatment is carried out to remove dissolved inorganic solids, colorants, non-biodegradable organics and metals. It is also called as advanced waste water treatment. It involves physical, chemical or biological processes or their combinations depending on the impurities to be removed.

5.1 Reverse osmosis

Reverse osmosis can remove particles ranging in size from 0.0001 to 0.01 µm and can operate at 90% efficiency. The process of osmosis can be reversed by applying sufficient pressure to the concentrated solution to overcome the osmotic pressure and force the net flow of water through the membrane towards the dilute phase. The solute concentration (impurity) builds upon one side of the membrane while relatively pure water passes through the membrane.

The waste water containing dissolved solids is passed through a semi permeable membrane or a pressure higher than the osmotic pressure of the effluent. The waste water passes through the membrane leaving a concentrated higher on the surface. Reverse osmosis creates a diffusive transfer barrier or microscopic openings so that only water molecules pass through. When textile dye effluent is passed through the semi permeable membrane, a concentrated dye solution is left over. The most commonly used membranes are cellulose acetate, cellulose acetate butyrate and cellulose triacetate.

Advantages

Excellent for color removal.
Substantial reduction in BOD and COD. Recovery of dyes and heavy metals.
Recovery of sizing material such as poly vinyl alcohol (PVA)andcarboxy methyl cellulose (CMC).

Disadvantages

Membranes are expensive and it has to be replaced frequently as the dyes clog the membranes

5.2 Dialysis

Solutes are separated from solution based on the difference in the rates of diffusion.
Cellulose nitrate, cellophone and patchment membranes are commonly used membranes.

5.2.1 Electro-dialysis

The electro-dialysis process uses a series of high capacity, a highly cross-linked ion-exchange resin which selectively transfers ions but not water. In contrast to the reverse osmosis, electro-dialysis employs the removal of solute from the solution rather than the removal of the solvent. This process uses selective permeable membrane and an electrical potential difference to extract ions from solution. Two types of membranes (anionic and cationic) are arranged alternatively to form many compartments between the electrodes placed at the two ends. When voltage is applied across the cell containing waste water, the anions migrate to the positive electrode and the cations migrate to the negative electrode. This causes solution in alternate compartments to become more concentrated while that in the remaining becomes more dilute. The process does not require any chemical additives and has low energy requirement. A major disadvantage in this process is that the organic molecules are not removed and they clog the membranes. This process also leaves concentrated waste water which requires a high degree of treatment.

5.3 Activated carbon adsorption

Compounds such as tannins, lignins, surfactants, color and odour producing substances cannot be removed by secondary biological treatment. Adsorption on activated carbon is one of the major effluent treatment technique for the removal of organic and inorganic substances. Adsorption is a surface phenomenon and larger the surface area, greater is the adsorption. Activated carbon is prepared by subjecting charcoal of carbon material (wood or coal) to oxidizing steam at higher temperatures of about 1700°F.

C + H2O ——- H2 + CO (water gas)

The water gas is released from the charcoal to develop a very porous structure in the charcoal thus the adsorption process increases. The effluent is passed over the bed of activated carbon. The organic and inorganic waste present in the waste water is adsorbed on to the adsorbent by weak forces such as vanderwals forces. The effluent emerges out free of contaminants. Lint and suspended solids affect the efficiency of the adsorption process as it clogs pumps and valves. Hence they are removed prior to passing through the activated carbon.

pH is an important factor that affects adsorption process. pH should be adjusted to optimum to achieve maximum efficiency. Granular activated carbon is employed for adsorption process. The pore size and high surface area are the important features of activated carbon.

There are four types of adsorption system configuration.

Moving beds
Fixed beds in series
Fixed beds in parallel
Expanded beds.

Moving bed adsorbers operate on a counter current basis. Effluent enters from the bottom through the bed and leaves at the top. The exhausted carbon is removed from the bottom and fresh carbon added at the top. Fixed bed in series system have the capacity of handling suspended solids unlike moving bed adsorbers. Fixed beds in parallel are capable of removing both suspended solids and dissolved organics. The choice of adsorber design depends on the concentration of contaminants, economy and the quality of effluent.

Advantages

Lower cost
Simplicity in operation

Conclusion

The significance of water is universally known and the value of life depends on the ability to manage available water in the greater use of people. The production processes of textile materials involve larger utilization of water which results in the release of polluted water. Hence it is important to explore various treatment processes for the reduction of pollution load and reuse the treated water.

you can view video on Effluent Treatment Methods

Web links

vikaspedia.in/health/sanitation-and-hygiene/effluent-treatment-plant
nptel.ac.in/courses/116104045/lecture5.pdf
nptel.ac.in/courses/116104045/lecture5.pdf
https://en.wikipedia.org/wiki/Sewage_treatment
https://www.britannica.com/technology/wastewater-treatment
https://www.organicawater.com/primary-secondary-tertiary-wastewater-treatment-work/
http://textilelearner.blogspot.com/2013/06/effluent-treatment-plant-process.html
https://www.omicsonline.org/open-access/production-characterization-and-treatment-of-textile- effluents-a-critical-review-2157-7048.1000182.php?aid=22482
http://eprints.ugd.edu.mk/11670/1/METHODS%20FOR%20WASTE%20WATERS%20TREATMENT%20I N%20TEXTILE%20INDUSTRY.pdf
http://www.indiantextilejournal.com/articles/FAdetails.asp?id=141
https://www.ncbi.nlm.nih.gov/pubmed/22585363