ZERO LIQUID DISCHARGE ( ZLD ) in TIRUPUR DYEING FACTORIES
A Techno-Commercial and Environmental Challenge
R. Manivasagam, B.Tech., MBA, MIE., FIV.,
Chartered Engineer and Approved Valuer - Technical, Management & Project Consultant
Tirupur, Exports, Dyeing and Pollution
Tirupur is an important centre in India for Knitted garments (like T-Shirts) manufacturing and exporting. For manufacturing knitted garments we have to impart colour to the fabrics (bleaching and dyeing). Majority of the bleaching and dyeing is done on cotton knitted fabrics. Many types of dyes are used in cotton dyeing. But Reactive Dyes are the most widely used dyes in cotton dyeing because of its ease of application, fastness & brilliancy. Lot of dyes, chemicals & chemical formulations are used in reactive dyeing. They are wetting oil, detergents, soda ash, caustic soda, hydrogen peroxide, stabilizer, peroxide killer, reactive dyes, sequestering agents, defoamers, lubricants, salt, soaping agent, acetic acid, hydrochloric acid, fixing agent, softening agents, etc. Salt is the main chemical which exhausts (pushes) the reactive dyes into the cotton fibre. Without salt reactive dyes can not be applied on the cotton fibres. The salt can be either Common salt (sodium chloride) or anhydrous Sodium sulphate or Glauber’s salt (sodium sulphate with 10 water molecules). On an average every Kg of cotton fabric dyed uses about 0.3 to 0.8 Kg of salt and about 30 to 90 litres of water for dyeing.
All these chemicals and dyes are used in dyeing at various stages at various temperatures with water as medium of application. About 75-85% of the reactive dyes are absorbed and fixed in the cotton fabric and the remaining hydrolised dyes go in the waste water. About 60-80% of softener fixes with the fabric and the remaining go into the waste water. All other chemicals are used just to help the dyeing and washing process but do not get fixed with the fabric. These unused dyes, chemicals & formulations are mixed and carried away in the waste water in its original form or in modified form. The waste water or effluent has got the pollution load mainly in the form of pH, BOD (Biochemical Oxygen Demand). COD (Chemical Oxygen Demand), TSS (Total Suspended Solids), TDS (Total Dissolved Solids), etc. BOD is a measure of the amount of oxygen that is consumed by bacteria during the decomposition of organic matter in the effluent. COD measures the amount of oxygen that is consumed during the decomposition and oxidation processes, specifically the decomposition of organic matter and oxidation of inorganic matter, or chemicals in the effluent. In simple terms BOD & COD refers the amount pollution due to the chemicals in the effluent. More the chemicals in the effluent more is the oxygen required to remove them and hence higher the BOD & COD. Total Suspended Solids (TSS) refers to the un-dissolved chemicals and particles in the effluent in a suspended state. TDS refers mainly the salts dissolved in the effluent. The salt used in dyeing is the main reason for the high level of TDS in effluent. Soda ash & caustic soda also contribute to TDS. There are some more pollution parameters also, but not discussed here.
Why Zero Liquid Effluent Discharge?
The intensity of pollution parameters present in the liquid effluent is described by the term ppm. The term ppm means parts per million and is equal to mg / litre. If you dissolve one gram of salt in one litre of water then the TDS will be 1,000 ppm. From the soft-flow dyeing machines, the final mixed effluent before treatment has TDS: 4,000 to 6000 ppm, BOD : 300 to 800 ppm, COD : 800 to 1,500 ppm, TSS : 300 to 600 ppm. The Pollution Control Board (PCB) norms for effluent discharge into inland surface water are, TDS : 2,100, BOD : 30, COD : 250, TSS : 100. By a normal Primary & Secondary treatment we can easily achieve BOD, COD & TSS values within norms. But this treatment can not reduce TDS. Therefore we can not achieve a TDS of 2100 by normal means and practically we can never discharge treated effluent into inland surface water confirming to PCB Norms. Hence the concept Zero Liquid Effluent Discharge (ZLED) or Zero Liquid Discharge (ZLD) has arisen automatically. Note that this is not Zero Effluent Discharge (ZED). ZED means nothing is discharged but ZLED or ZLD means no liquid effluent is discharged. This is because there is always a gas emission & solid waste generation in the factory & and also during the effluent treatment process.
The route to achieving Zero Liquid Effluent Discharge (ZLD)
In order to achieve Zero Liquid Effluent Discharge, we have to recover the water and salt separately from the effluent and reuse it in the dyeing process. This is a very complicated, tedious and expensive process. This involves a lot of money in investment and also huge running costs. Apart form this we need to apply lot of chemistry, engineering & technology know-how. There are many steps involved in achieving Zero Liquid Effluent Discharge (ZLED). Effluent is treated in Effluent Treatment Plant (ETP) comprising of Primary Treatment, Secondary Treatment and Tertiary Treatment. Water is recovered in Reverse Osmosis Plant (RO). Salt is recovered by using Multiple Effect Evaporation (MEE) with Crystallisation.
Effluent Treatment Plant – Primary Treatment
In primary treatment some amount of TSS, BOD, COD and majority of colour are removed. Here we have several methods. The conventional method is treatment with ferrous sulphate & lime. In this method sludge production is high. But this method is very effective in removing most of the colour & TSSOzonation method the effluent is treated with Ozone and the colour and chemical molecules are broken down. Here the sludge volume is reduced. Ozonation breaks down the colour and chemicals but does not remove them completely. Hence COD reduction is less. Ozonation plant is very expensive, power-hungry and a complicated machine. With the Electro-Coagulation method the effluent is passed through electric current between iron plates. Coagulation takes place using dissolved iron. Sludge generation is moderate. This is an efficient system but requires lot of current & iron electrodes and the machine is difficult to be maintained. Another method is the Electro-Chemical oxidation of the effluent at the oxidation cells using electric current. In gas Chlorination method, Chlorine gas is passed through the effluent and reacted. Sludge production is also less. But chlorine gas handling is very dangerous and operation is batch-wise and complicated. Fenton Oxidation is another process in which effluent is oxidised with hydrogen peroxide. This process has got high running cost.
Effluent Treatment Plant – Secondary Treatment
The secondary treatment is a biological process. This involves Anaerobic or aerobic methods. Anaerobic method involves use of bacteria to treat effluent without air to eliminate BOD & COD loads. Aerobic method involves bacteria with air to eliminate BOD & COD loads. There are many types of treatments in both aerobic & anaerobic methods. Activated Sludge Process (Aerobic) is the most widely used and the cheapest method. This method is very effective in removing about 80-90% of the inlet BOD & COD load. In this method air is continuously passed through effluent and bacteria are cultivated. The retention time is about 16-30 hours. The bacteria eat the pollutant chemicals and digest them. There are other types of Bio-reactors like Fluidised Bed Bio Reactors also very effective. Without primary treatment, we can also directly go to the biological method Activated Sludge Process (Aerobic) to remove BOD & COD load. Here the retention time is about 48-72 hours. But some colours still remain in the effluent and we will have to use resin filters to remove the colour. But then the resin has to be regenerated. This involves cost and back wash has to be managed. Membrane Bio Reactors (MBR) are the latest one involving activated sludge process & membrane separation of sludge and suspended particles and also acts as ultra filtration. This involves huge investment cost.
Effluent Treatment Plant – Tertiary Treatment
The tertiary treatment prepares the effluent for passing through the RO Plant. Sand Filters & Carbon Filters are regularly used. Ultra Filtration and Conventional tertiary clarifier with lime and FeCl3 treatment are very effective methods. Ultra Filtration is very important to protect RO Membranes from early fouling. Many RO Membranes very easily foul and fail because of poor tertiary treatment.
Water Recovery by Reverse Osmosis Membranes
Reverse Osmosis Membrane separation is the use of membranes like very fine filters to separate water and TDS. Here the treated effluent from the tertiary treatment is passed through membranes at high pressures. Here only the pure water passes through the membrane and recovered for reuse. All the TDS causing salt is removed as reject. This reject is sent to another set of RO membranes to recover more water and reduces the reject volume. The RO process can recover 85-90% of the feed water for reuse. Life of the RO membranes depends upon the primary, secondary & tertiary treatment effectiveness. To prevent RO membrane fouling, we should have the strict control over the RO inlet parameters like the BOD, COD, TSS, Silica, Iron etc.. If everything is good, then the membranes will work for about 3 years. Even a small mistake will immediately spoil the membranes. The membrane replacement cost is very high. The final rejects from these processes are sent to Nano Filtration and or Multiple Effect Evaporators. After RO II or RO III, Multi Effect Evaporators & Crystallisers are used to recover Common Salt or Glauber’s salt for reuse in dyeing.
Salt Recovery
There are two routes possible for recovering the salt for reuse. One is the Common Salt route and the other is the Glauber’s salt route (for discussion purposes this refers to anhydrous sodium salt or Glauber’s salt). We have to choose any one of the methods.
Common Salt Recovery
First let us see the common salt route. The combined effluent is treated by primary treatment, secondary treatment, and tertiary treatment and through reverse osmosis plant in two or three stages. Here we recover about 85% to 90% of the water for reuse. Here we start the with 4,000-6,000 ppm TDS effluent and finally get recovered water with less than 100 TDS for reuse and the final concentrated reject with about 25,000-35,000 ppm TDS that is mostly due to common salt. The entire RO Reject is sent to Multiple Effect Evaporator for evaporation and concentration, Salt Settler and Pusher Centrifuge. Here we get Mixed Salt as output. Since this has got lot of contamination we can not use this in dyeing. This has to be disposed off as Solid Waste.
Glauber’s salt Recovery
Now let us see the Glauber’s salt route. The combined effluent is treated by primary treatment, secondary treatment, and tertiary treatment and through reverse osmosis plant in two or three stages. Here we recover about 80% to 90% of the water for reuse. Here we start the with 4,000-6,000 ppm TDS effluent and finally get recovered water with less than 100 TDS for reuse and the final concentrated reject. Then this is sent to Multiple Effect Evaporator for evaporation and concentration. The condensate water is recovered and reused. Evaporation is done till we get a strength of 1.20 specific gravity equal to about 230 to 250 grams/litre (2,30,000 to 2,50,000 TDS). This solution is passed through an Adiabatic Cooling Crystalliser. Here the solution is cooled to 8-10 ‘C and the Glauber’s salt crystallises. This solution is passed through a pusher centrifuge and the Glauber’s salt crystals are separated. This Glauber’s salt has a mild colour tint. This Glauber’s salt can be used in the dyeing process for about 85% of the colour. From the pusher centrifuge the reject is re-circulated to Multi effect Evaporator and or Crystalliser feed to get the maximum salt recovery. However about 5 to 10% of the solution has to be periodically sent to solar evaporation ponds to remove contaminants. The Glauber’s salt recovery will be in the range of 75-80%. For the balance 20-25% quantity, fresh Glauber’s salt can be used for very light colours. Glauber’s salt has got about 55% moisture & 45% sodium sulphate. Therefore we get 2.25 times by weight more of Glauber’s salt than sodium sulphate. In dyeing process we will have to use 2.25 times Glauber’s salt more than the anhydrous sodium sulphate that we normally use. The recovered Glauber’s Salt can also be converted into pure anhydrous dry Sodium Sulphate by installing additional Evaporative Crystallisers & Salt Driers. This will be exactly similar to the fresh purchased Sodium Sulphate.
Common Salt versus Glauber’s salt
Common salt route has a recovery of about 75 - 80 %. In the common salt route, the recovered Mixed Salt will have lot of contaminants which may affect dyeing process and hence can not be reused.
In the Glauber’s salt route, the Glauber’s salt recovery is better than the common salt. Glauber’s salt recovery is about 75 to 80 %. Glauber’s salt recovery with Multiple Evaporator and Crystallizer is a very proven and a common technology in the chemical industry. During viscose rayon fibre manufacturing for every Kg of fibre produced, one Kg of Glauber’s salt is produced as by product. This Glauber’s salt is recovered using Multiple Evaporator and crystalliser. The crystallised Glauber’s salt is converted into pure anhydrous dried Sodium Sulphate by melting, washing & colour removal, again evaporation and drying. Several thousand tons of anhydrous Sodium Sulphate is produced in this way and sold. This Sodium Sulphate is already bought and used by the dyers regularly. In many chemical industries where Glauber’s salt is a by product, the same method is used for Galuber’s salt recovery. Hence Glauber’s salt route is a better option than common salt route.
Final Reject Management
The final reject from Multi Effect Evaporator or Crystalliser are sent to Solar Evaporation Ponds for final drying. We need lot of surface area for evaporated evaporation. Therefore this needs lot of space within the factory and involves lot of construction costs. Also during rainy season, there will be no evaporation and hence practically we will have to stop production.
If we want to reduce or avoid Solar Evaporation ponds, then we can install Forced Circulation Multiple Evaporators for converting all the concentrated salt reject into Mixed waste Salt that has to be disposed off as Solid Waste. In this method practically almost all the reject is evaporated and hence there is no need for Solar Evaporation Ponds.
ZLD Difficulties and Precautions
The Zero Liquid Effluent Discharge is a very tough task. Every treatment method has got its own merits and demerits. Various combinations of methods have to be selected to reach a realistic practical treatment method. Some times conventional proven methods are better than new unproven methods. We should never be carried away by the plant suppliers claims. We have to see the practical methods and get realistic data. We have to spend a lot of time and energy to select a method. The plant must be designed very lavishly. We should always design the plant only for 20 hours of operation per day. All the tanks, pipe-lines, pumps, equipments, machinery, RO membranes, Multiple Evaporators should be designed for at least 20 – 30 % more than the requirement. During back washing, sand filters, carbon filters & Ultra Filtration generates contaminated wash water of about 20% of effluent feed. This is sent back to effluent collection tank and hence the plant needs higher design capacity by 20 – 30 %. Automation & Instrumentation is a must to safeguard the plant. A good laboratory is a must. All the critical equipments must have stand by ones. We should never buy a technology or equipment or plant just going by assumptions.
Biological plant needs continuous monitoring. Any mistake may stop the entire plant for several days. RO Plant needs maximum protection. Any small mistake will easily damage all the Membranes and it will cost several Lakhs of Rupees. Multiple Evaporators will easily get scaling & choking. For this we have to select a combination with costly Forced Circulation Multiple Evaporators instead of simple Falling Film Multiple Evaporators. Sludge handling will be a very tough task.
Any treatment method or plant or equipment proposed must satisfy the following two fundamental science laws. Any scheme violating these two laws will not work properly and will never give desired results. The Law of Conservation of Mass states that the total mass of all of the reactants in the chemical reaction is equal to the total mass of all the products in the chemical reaction. In other words, what goes in must come out. Mass, or matter, can neither be created nor be destroyed. Nothing can vanish. The Law of Conservation of Energy states that energy cannot be created or destroyed, but can change its form. Every change needs some material, action or energy. Nothing is free. Everything has got a cost.
Water Loss
Lot of water is lost in the treatment process. Water evaporates at the surface in storage tanks by a natural evaporation process. There is a water evaporation loss is collection tanks, clarifiers, biological aeration tanks, storage tanks, etc. Due to huge amount of air blowing in the biological aerators, water is lost as vapours. Some amount of water is lost in sludge. There is a loss in boiler blow-down evaporation & steam condensate evaporation. Water is lost during cloth drying equal to cloth weight. Water is lost in concentrated reject sent to solar evaporation ponds. A huge quantity of water is lost in evaporator cooling tower by cooling water evaporation loss and drift loss. A total of about 20-25% of the effluent treated will be the water loss. This loss account is very important in making Water Balance calculations.
Recovery & Loss in Zero Liquid Effluent Discharge
Item % Recovery for Reuse % Loss
Water 75 – 80 % 20 – 25 %
Common Salt 35 – 50 % 50 – 65 %
Glauber’s Salt 75 – 80 % 20 – 25 %
Solid Waste Problem
While we are busy with Zero Liquid Effluent Discharge, we have forgotten one thing. We are generating thousands of tons of sludge as solid waste. This sludge has to be disposed off in a Secured Land Fill (SLF). The SLF has to be maintained for at least 30 years from the closure date. The un-recovered Common Salt or Glauber’s salt and the contaminants are sent to solar evaporation ponds for natural evaporation or sent to final Forced Circulation Multiple Evaporators for converting into Solid Waste. If we assume that all the dyeing factories totally use about 600 tons of salt daily, then the rejected salt will be about 100-150 tons every day. Imagine this for a whole year and for several years. We do not know what we are going to do with this future. It is a sleeping monster. In future this solid waste management will be another tough task. May be we will have to use incineration to burn them with modern machinery fitted air pollution control equipments. Still we may end up with salt.
Is Zero Discharge Possible ?
If you choose the right technology, good plant and equipments, make the necessary investment, capacity to bear the treatment expenses, proper maintenance, ready to stop the production if there is a breakdown in the Zero Discharge Plant and the commitment to achieve Zero Discharge then it is possible to achieve Zero Discharge. This possibility is subject to the above mentioned Water Loss %, & Salt Loss % values and sending contaminants to solar evaporation ponds. Technically it throws lot of challenges. Mostly cost of recovery is always higher than the cost of the product. But one should never consider Waste Water Treatment as a cost centre. It is an ecological, social and statutory requirement.
Alternatives to Zero Liquid Effluent Discharge:
Sea Discharge
Sea discharge is one alternative way. But again there are stringent laws for the quality of effluent that is to be discharged into the sea. We will have to ensure the limits of colour, BOD, COD, TSS and any other toxic substances. Only high levels of TDS in the form of common salt will be allowed. In sea discharge we have two methods. One is to treat the mixed effluent with primary, secondary and tertiary treatments and dispose into the sea. Here we loose all the water and salt in to the sea. Another method is to treat with the above methods and recover water for reuse by RO Treatment. Then send the final reject to sea discharge. Here we recover about 85% of the water for reuse and avoid only the Multiple Evaporator loosing the salt. But the reject from RO Plant will have more contaminants more than the sea Discharge Norms. Hence the RO reject can not be easily discharged into the sea. Therefore only the fully treated water (Primary, Secondary & Tertiary ) can be discharged into the sea. But for this we have to carry-out a large scale Environment Impact Assessment study. This may involve several Ministries, Departments & Institutions like, Environment Ministry in State & Centre, NEERI, Central & State PCB, Coastal Regulations, Highways, etc., From concept to commissioning this may take several years. We may also face lot of resistance from the concerned public, NGOs, sea related parties etc..
Inland Surface Water Discharge
We have already seen that since the mixed effluent has a TDS of about 4,000-6,000 ppm, we can not do discharge into inland surface water. There is one possibility of mixing 1 – 2 times normal raw water with the treated effluent till the TDS reaches less than 2,100 ppm and then discharge. This is to be done with special permission from the Government. Even this is possible only after doing primary, secondary and tertiary effluent treatment. If you assume a total effluent generation from Tirupur Dyeing factories to be around 10 crore litres per day, then we need another 10 – 20 crore litres of normal water per day to be mixed with it. This will be against the principle of worldwide philosophy of “Water Conservation”.
Things to Think Over
We use lot of plant, machinery, equipments, civil structures, chemicals, fuel and power to achieve Zero Liquid Effluent Discharge. But some where some other factory may be making pollution in producing those items. Effluent treatment is never holistic but it shifts pollution from one form to another form and from one place to another place.
“Sustainable Development is development that meets the needs of the present without compromising the ability of the future generations to meet their own needs”.
NOTE : All the values given in the above article depends upon the type of dyes, chemicals and processes used in the factory. It varies from one factory to another factory. The % recovery and ppm levels also depend upon the inlet values and the efficiency of the treatment systems. These values should be taken only as a guide and the real values can be arrived at the site only. There are many more treatment technologies available which are not discussed here.
