After reading this article you will learn about:- 1. Profile of the Sugar Industry 2. Process Description of Sugar Industry 3. Water Requirement 4. Sources of Wastewater 5. Wastewater Characterization 6. Wastewater Treatment 7. Solid Waste Problem 8. Conclusion.
Profile of the Sugar Industry:
Sugar is one of the significant agricultural products and industries processing sugar are vital for Indian economy. Sugar industry is one of the important agro-based industries in India. The industry has created significant socio-economic impact on rural agro-based economy in particular, and Indian economy in general. The residues from Sugar are part of the natural products.
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These being highly putrescible create environmental hazards if permitted to be thrown away without proper treatment and subsequent careful disposal. The cane Sugar manufacturing industry is essential in the production of many varieties of foods.
Sugar is used in cakes, ice cream, candy, and soft drinks as well as in other foods and beverages. In India, most of the Sugar mills are situated in the countryside and operate for about 4 to 8 months just after the harvesting of the Sugarcanes.
Sugarcane is normally harvested manually in India, which eliminates the carrying of soil and trash to the factory along with the Sugarcanes. A large volume of waste of organic nature is produced during the period of production, and normally they are discharged onto land or into nearby watercourses, usually small streams, practically without pre-treatment.
Conditions become worse as the stream flow reaches a very low level and when enough dilution water is not available during the period of operation of the Sugar mills (early November to end of May or June).
Putrefaction of the polluted stream water caused by the heavy discharge of organic waste, resulting in odour nuisance near Sugar mills has been a very common phenomenon. When the untreated Effluents are discharged into the environment, they disrupt the ecological area of living organisms.
Effluent discharges from Sugar mills constitute a number of chemical pollutants, such as oil and grease, carbonate, bicarbonate, nitrite, phosphate, in addition to total suspended solids, dissolved solids, volatile solids and scopes of other toxicants.
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These pollutants could bring about changes in temperature, humidity, oxygen supply, pesticide, stress, etc., amounting to a partial or complete alteration in the physical, chemical and physiological spheres of the biota.
Basically Sugar is sucrose, a disaccharide extracted from Sugarcane.
Sugar manufacturing can be categorized as follows:
1. Gur and Raw Sugar (sakar) manufacture.
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2. Khandsari-manufacture of unrefined low grade Sugar.
3. Double Carbonation, Double Sulphitation Process
4. Double Sulphitation Process.
As per COINDS/8/1980-81 of Central Pollution Control Board (CPCB), Sugar factories are classified on the basis of crushing capacity as under:
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Small unit –up to 1500 TCD
Medium -1501 to 3000 TCD
Large – Above 3000 TCD
Process Description of Sugar Industry:
The various steps involved for the production of Sugar are as follows:
1. Procurement of Sugarcane.
2. Milling of Sugarcane.
3. Juice Preparation.
4 Juice Concentration.
5. Syrup Processing and Crystallization.
6 Sugar Crystal Separation, Drying, Packaging and Molasses Handling.
7. Bagasse Utilization.
The Sugarcanes are cut into pieces and crushed in a series of rollers to extract the juice, in the ‘mill house’. The milk of lime is then added to the juice and heated, when all the colloidal and suspended impurities are coagulated. During this treatment, much of the colour is also removed.
The coagulated juice is then clarified to remove the sludge. The clarifier sludge is further filtered through filter presses and then disposed off as solid waste. The filtrate is recycled to the process and the entire quantity of clarified juice is treated by passing sulphur dioxide gas through it. This process is known as “Sulphitation” (Fig.1); colour of the juice is completely bleached out in this process.
1. Procurement of Sugarcane:
An early variety of Sugarcane matures by second week of November. The late variety of Sugarcane matures towards the middle of the season. Depending upon the state of maturity, the Sugarcane is harvested in the field, where the roots and green leaves are removed manually and the Sugarcane is prepared for transportation to the factory. Depending on the location of the field, the cane is transported to the factory by bullock carts, tractor trolleys and trucks to the factory.
2. Milling of Sugarcane:
The Sugarcane is unloaded at the mill house by winch and crab on to the cane carrier, which feeds the fiberizing system. Here the Sugarcane is shredded into small pieces. The shredded Sugar cane is then squeezed through a series of pressure milling rolls containing grooved surface. Weak juice or water is added to last but one roller so that recovery of juice is of the order 95-97%.
3. Juice Preparation:
Double Sulphitation process is the latest process adopted for juice clarification. In this process, juice is heated at 70°C and is treated with lime and sulphur dioxide. The juice is adjusted to neutral pH and passed to the heat exchanger to raise its temperature to the boiling point. It is then sent to clarifier where the juice is clarified and then sent to multiple effect evaporators. The sediment from the clarifier is sent to vacuum filters. The juice mud is taken out as solid waste and the extracted juice is mixed with raw juice.
4. Juice Concentration:
The clarified juice is concentrated to about 65% solids from about 15% solids before entering the first multiple effect evaporator sending steam in the first evaporator. Vapours from the first evaporation are fed to the second evaporator and so on.
Spent steam from the first evaporator is returned to the boiler for reuse as feed water for steam generation. Spent steam from the second and third evaporator is used for process, and vapours from the last evaporator are condensed through condensers.
5. Syrup Processing and Crystallization:
The concentrated juice or syrup from the evaporator is again bleached by passing sulphur dioxide through it and the pH of the syrup drops down to about 5.4. It is then sent to the vacuum pan where the thickened syrup is boiled for three to four times as per purity in order to extract the sucrose content. It is then sent to crystallizers to deposit any additional sucrose content on the crystals. Fine Sugar is used as seed crystals.
6. Sugar Crystal Separation, Drying, Packing and Molasses Handling:
The Mixture of crystals and liquor, called ‘massecuite’, is sent to high-speed centrifuges. The liquor is re-concentrated and cooled successively to obtain more than one crops of crystals. The final mother liquor, called ‘Molasses’, which is still very rich in Sugar content is sent to steel storage tanks. Molasses is sold to various distilleries and other users against permit issued by excise department.
The separated crystals are passed on to hopper conveyors where hot and cold air is passed through the crystals. The appropriate size is dried and sent to elevators. The elevators feed the grading system bins. The fine crystals are reused for seeding. Finally the finished product is bagged and stored in godowns.
7. Bagasse Utilization:
The pulp expelled after extraction of juice is called ‘Bagasse’. As it comes out of the mill house, it contains about 50% moisture. A number of drying processes have been tried in the industry but unfortunately none of these were found industrially viable. Therefore, the wet Bagasse with 50% moisture is carried to boiler house by Bagasse carrier. It is able to generate about 2 kg of steam per kg on wet basis itself. With efficient boilers coming in the market, the factories are able to save about 10-20% Bagasse.
The excess Bagasse is carried to Bagasse yard from where it is sold to paper mills and other users. During the general cleaning or shut down, Bagasse return carrier brings the Bagasse back to the boiler. A small fraction is sent to bailing plant where Bagasse is compressed and tied by G.I. wire to form small bails. The size of these bails is about 40 to 50 centimetre cube. The Bagasse can be stacked in the form of bails. This is done very carefully as Bagasse is known to be susceptible to auto ignition.
Water Requirement of a Sugar Industry:
Major units that consume water in a Sugar plant are:
(i) Boiler feed water.
(ii) Cooling water for condenser.
(iii) Process water for maceration, lime preparation, dilution for control of brix, dilution in evaporators and massecuite dilution, filter mud, fly ash handling, and cane wastewater.
The Sugar cane received from the field contains about 70% moisture on an average. Majority of this water has to be discharged as factory wastewater. Material balance of a typical plant with respect to water requirement and wastewater generation is show in in Fig 2.
Sources of Wastewater in a Sugar Industry:
There are various sources of wastewater generating in a sugar industry. The quantity of the effluent depends on the size of the factory as depicted in Table 1. The break-up of the effluent generated in the various units of a typical sugar plant is shown in Table 2.
Centrifugation produces another type of wastewater called ‘Molasses’. Molasses is an important raw material for distilleries.
Wastewater Characterization of a Sugar Industry:
The effluent characteristics from a typical sugar plant are presented in Table 3.
Wastewater Treatment of Sugar Industry:
Like any other industry, the pollution load from Sugar mills can also be reduced with a better water and material economy practiced in the plant. Judicious use of water in various plant practices, and its recycle, wherever practicable, will reduce the volume of waste to a great extent. Volume of mill house waste can be reduced by recycling the water used for splashing.
Dry cleaning of floors or floor washings using controlled quantity of water will also reduce the volume of waste to a certain extent. The organic load of the waste can only be reduced by a proper control of the operations. Overloading of the evaporators and the vacuum pans and the extensive boiling of the syrup lead to a loss of sugar through condenser water, this in turn increases both volume and strength of the waste effluent.
Conventional Treatment Method:
The conventional system of treating wastewater is by Activated Sludge Process (ASP) from various units of a sugar plant is shown in Fig. 3. The various units include Bar Screen, Skimming Tank, Equalization Basin, Aeration Unit, Clarifier and Sludge Drying Beds.
Other Treatment Options:
1. Anaerobic Lagoon Technique.
2. Anaerobic Digestion.
3. Up flow Anaerobic Sludge Blanket (UASB).
4. Up flow Blanket Filter (UBF).
5. Aerobic Bioconversion.
6. Fixed Film Fixed Bed Reactor.
Anaerobic treatment of the effluent, using both lagoons and digesters have been found to be more effective and economical. Anaerobic lagoon with a detention time of 15 days and 0.38 kg BOD/m3d loading can achieve 89.6% BOD removal.
A BOD reduction of about 70% can be expected from an Anaerobic digester, with a BOD loading of 0.65 kg/m3/day and a detention time of 2.4 days under a controlled temperature of 37°C. The effluents of the anaerobic treatment units are found to contain sufficient nutrients (nitrogen and phosphorus). Further reduction of BOD can be accomplished in aerobic waste stabilization ponds.
Where sufficient land is available, a two stage biological treatment, with Anaerobic lagoons followed by Aerobic waste stabilization ponds, is recommended for Indian conditions. Now-a-days use of fixed film technique is gaining much momentum.
Anaerobic high rate treatment technology by UASB is reported to be mostly suitable as it handles high organic load at low hydraulic retention time (HRT); at less than 12 hours at thermophilic temperature range of 45°C, more than 90% COD removal can be achieved. UASB technique is the best among other available treatment options, as claimed by researchers.
UASB is now popular because of certain advantages of the system. Sugar mills operate on a seasonal basis viz., November-April (about 150 to 250 days in a year). Active sludge can be stored well in the reactor without any harm to the biological anaerobic sludge; the reactor can start operations as and when required.
In UASB process, wastewater enters the reactor from the bottom at low velocities and flows upward through a bed of relatively dense (granular / flocculent) sludge and a blanket of sludge particles.
The substrate comes in contact with a sludge suspension (biomass) and eventually gets converted to biogas. The gas thus produced bubbles through the bed and brings about an excellent mixing so that adequate contact is made between the biomass and the substrate.
At the top of the UASB reactor, a simple device called gas-solid separator allows the rising gas bubbles to get separated from the liquid phase. Liquid flowing beyond the gas solid separator is free from gas bubbles and a quiescent zone is created.
This zone acts as a setting compartment where sludge particles carried up by the gas settle and slide over the inclined walls of the separator and back into the reactor. A fine quality treated effluent leaves the setting compartment. An important feature of the UASB process is the quality of the sludge with respect to setting characteristics.
After a few weeks start-up of the reactor, the sludge gets converted into granular form, thus attaining very good settle-ability. This facilitates large hydraulic and organic loading potential of the reactor at a reasonably low HRT with a quite large Solids Retention Time (SRT). This results in more substrate stabilization.
An Up flow Blanket Filter is a new hybrid reactor and is a combination of UASB and packed bed reactor system. Plastic rings submerged in the liquid act as solid support for biomass. The reactor can operate at 27°C and 5 to 51 kg COD / m3.d organic loading rates.
Aerobic Bioconversion of sugar mill effluents is practiced in Mosul (Iraq). A study reported that batch experiments of Aerobic bio-oxidation was inexpensive and were generally in the design range. Kinetic coefficients and mathematical methods for Aerobic bio-converters were also determined.
The Anaerobic filter is an up flow digester provided with inert packing material such as rock, plastic, sand, and refractory bricks. Wastewater is passed upwards at very low velocities through the packing material providing contact of the wastewater with attached and entrapped (interstitial) anaerobic biomass (fixed film) within the reactor, thus allowing biodegradation of the influent organic matter. Anaerobic microorganisms appear to accumulate in the media void and media surface.
The substrate comes into intimate contact with a large active biomass as it passes through the reactor. Based on these principles, wastewater from sugar manufacturing plants can be treated in a Fixed Film Fixed Bed Reactor at 4-8 days HRT and 5-16 kg COD/m3.d organic loading. More than 80% COD stabilization can be achieved at an average loading of 12.5 kg COD/m3.d at 5 days HRT.
The biogas production of 0.284 m3 methane / kg COD may be expected with 69.4% methane content of biogas. The reactor has content drawbacks as substantial volume of the reactor gets occupied by the packing material. Further, these filters are easily clogged by accumulated biomass and suspended solid contents in the influent. Clogging of filter bed leads to channeling resulting in poor performance of reactor.
Solid Waste Problem Arising in Sugar Industry:
Two forms of solid wastes are normally generated in the manufacture of cane Sugar viz., Bagasse and Press mud. Every 1,000 tons of processed Sugarcane generates about 270 tons of Bagasse. The Sugar industry is faced with the problem of proper and economical disposal of large quantities of Bagasse and Pressmud.
The most common method of disposing Bagasse is to burn it in boilers operated at sugar mills. Burning bagasse presents problems like polluted emissions with high degree of moisture and bulkiness. Utilization of Bagasse as a boiler fuel is impaired by the high degree of moisture (45-60%). In addition, its bulkiness requires the construction of special furnaces to operate efficiently.
Considering the high cellulose content of bagasse and the organic matter, it offers a potential renewable source of biomass for biochemical conversion to methane by anaerobic fermentation. Bagasse and Molasses contain significant concentration of un-crystallized sugar and other organic compounds; the sugar content can be extracted. In addition, the residual digested sludge can have beneficial uses as fertilizer or as a soil conditioner.
Bagasse Reuse:
Bagasse is used as a raw material in paper and pulp industry after drying in the manufacture of paper, paperboard, paper bag, carton, etc. Bagasse can produce 0.3 tons paper per ton Bagasse. It finds use in the manufacture of briquettes. Recent technology is to utilize the Bagasse as adsorbent and it is found to be economical. The other solid waste form the sugar industry is ‘Presumed’ which comes out from filter press.
It is used for manure. Also called filter cake, the Presumed can be used for filling low-lying area where ground water is not too shallow. Wax from the Press mud can be extracted and may be recovered as a by- product. The recovered wax can be used in manufacturing shoe polish and carbon paper. Table 4 presents the characteristics of Press mud. Bagasse has a calorific value of 1917 kcal kg and therefore is used as a fuel for steam generation in sugar mills.
Conclusion to the Sugar Industry:
Since the sugar industry operates on a seasonal basis, wastewater production is also obviously seasonal. A large variation is observed in the quality and quantity of wastewater generated in various sugar mills. The wastewater has a high COD, BOD, Suspended Solids and also most acidic pH. Equalization and lime treatment is needed prior to further treatment.
The wastewater has very low nutrient content such as nitrogen and phosphorus; therefore, they need to be supplemented during biological treatment of this wastewater. But the UASB reactor is versatile and it can be put into operation within a couple of days after feeding wastewater to the reactor, even after a long rest. Therefore, UASB is a viable treatment option for the sugar industry effluent.
Sufficient flexibility becomes essential in framing up a methodology for proper treatment and disposal of sugar industry Effluent as there is considerable variation in the size and scale of the industry. Furthermore, the treatment methodology selected should be comprehensive so as to safeguard the environment which contains intricate system of living and non-living components bound together by a highly complex interrelationship.
Appropriate use of the state-of the-art technology must be made either through design of a proper methodology or through the rules governing its use.
It takes a long time for the activated sludge to get stabilized after the start of the season. It is therefore recommended that either Effluent Treatment Plant (ETP) is kept in operation by using colony waste or the process shall have to be modified.
By using Bagasse in the manufacture of paper, some of the pollutional problems, especially colour and high amount of recalcitrant COD from paper industries may be solved.
By-products like Molasses, Bagasse and Press mud earn revenue to the sugar industry.
None of the process streams involved in sugar manufacture generate toxic effluents, and henceforth they can be used for irrigation purpose at lower concentrations.
As discussed, most of the solid waste generated is utilized by other industries; hence, solid waste disposal is not a big problem for sugar industry.