Large volume of sewage is produced during manufacture of cane sugar and it contains a high pollution load. The quantity of sewage produced from a cane sugar industry and its characteristics vary widely depending on local conditions and methods of plant operations.
Process Involved in Manufacture of Cane Sugar:
The various processes involved in cane sugar manufacturing can be categorized into:
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(a) Physical processes — (1) crushing, and (2) evaporation
(b) Chemical processes — (3) clarification
Crushing:
The sugarcane is harvested manually or mechanically. The manual harvesting eliminates much of the trash and soil from sugarcane, which reaches the factory in a condition in which it can be straightaway put through the extraction process. However, in recent times mechanical harvesting is becoming popular due to shortage of labour. Where mechanical harvesting is employed cane washing is necessary to remove extraneous material.
The cane is cut into short lengths in a shredder and then crushed in a series of roller mills to squeeze out the juice. The partially crushed cane is wetted with water in the later stages of crushing operation to aid maximum extraction. This operation is known as maceration. The extracted juice is screened to remove coarse particles of cane or bagasse and then sent to process.
Clarification of Juice:
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The screened juice is treated with lime to raise pH to 7.6 to 7.8 and then heated to 102°C, when coagulation of colloidal and other suspended impurities takes place. Bulk of the colour of the juice is also removed by treatment with lime, carbon dioxide is often passed through the juice for controlling its pH value.
The flocs formed in this process are settled and removed by passing the juice through a clarifier. The clarifier sludge is filtered in a vacuum filter or filter press; press mud is disposed of as solid waste or in the form of slurry and the filtrate is recycled to the process. In the sulphitation process, juice is further heated by passing sulphur dioxide gas through it when the colour of juice is bleached.
Evaporation:
The clarified juice is preheated by passing it through heat exchangers and then evaporated in multiple effect evaporators and fed to a single effect evaporator for further concentration. The later process is known as ‘pan boiling’ in which syrup is thickened to a point where numerous small crystals of sugar are formed.
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The syrup known as massecuite is then taken to crystallizers where complete crystallization of sugar takes place. The massecuite is then fed to centrifugal baskets where the sugar crystals are separated from mother liquor which drains away as molasses. During the process of centrifugation water is sprayed on the surface of sugar crystals to remove the adhering molasses.
The weak molasses thus obtained are recycled to the process. From the centrifugal baskets sugar is taken out and dried in a drier and bagged. A flow diagram of the process of manufacturing cane sugar indicating the sources of sewage is shown in Fig. 19.3.
Fig. 19.3 Flow Diagram of Cane Sugar Manufacture indicating Various Sections Contributing Sewage.
Sources, Quantity and Characteristics of Sewage:
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Sources:
Sewage is contributed from different sections of cane sugar industry as indicated below:
(i) Cane Wash Water:
Cane washing is necessary where mechanical harvesting is employed. These waters contain high concentration of suspended solids which are stabilized in colloidal from by organic compounds acting as protective colloids. Cane wash waters may also contain substantial concentrations of sugar, because of damage to cane during mechanical harvesting.
(ii) Effluent from Mill House:
Large amount of water is used for extraction of juice and cooling the bearings of mill tandems. This cooling water picks up oils and grease as well as some juice from spillover and leakage.
(iii) Effluent from Filter Cloth Washing:
The sludge from clarifier after clarification is filtered in filter presses. The filter clothes are washed periodically, which enhances the suspended solids concentration and biochemical oxygen demand of the sewage.
(iv) Effluent from Boiling House and Floor Washing:
The effluents from boiling house results from leakage from pumps, pipes, evaporators, crystallizers and especially from centrifuges together with periodic washing of floor. Although the discharge is intermittent and not large in volume, it represents the most polluting fraction of effluents because of its high biochemical oxygen demand.
The juice heaters usually foul very quickly, so it is necessary to clean heating surfaces throughout the plant to remove accumulation of scales. This is accomplished by boiling with dilute caustic soda solution and inhibited dilute hydrochloric acid followed by rinsing with water.
The sewage from boiler blow-off is of intermittent nature, but contains relatively high solids and alkalinity and low biochemical oxygen demand.
(v) Condenser Water:
In evaporators large quantity of condenser water is produced which constitute a large proportion of sewage from cane sugar industry. In the evaporation and crystallization process sugar particles gain access to cooling and condenser waters by entrainment in evaporators and vacuum pumps. This is major source of organic pollution in these waters.
Quantity:
The average quantity of sewage produced from a cane sugar industry is of the order of about 3000 litres per tonne of cane crushed. Table 19.2 shows the average quantity of sewage produced from the different sections of the cane sugar industry.
Characteristics:
The characteristics of major fractions and of combined sewage excluding the condenser water are shown in Table 19.3 and Table 19.4 respectively. Figures given in Table 19.3 represent the average of several cane sugar industries in the country and those in Table 19.4 give the total range of variation. It may be observed that there is wide variation in the values of the various parameters. As such actual survey of the conditions and analysis of sewage are necessary to determine the quantity and quality of the sewage to be handled in a particular factory.
Pollutional Effects of Sewage from Cane Sugar Industry:
The various pollutional effects of sewage from cane sugar industry are as indicated below:
(i) The effluents from some sections of the cane sugar industry contain considerable concentration of suspended solids, which deposit and cause blockage in drainage and ditches and also delayed pollutional effects because of slow decomposition of the settled matter.
(ii) Effluent from cleaning operation and heat exchangers may be acidic, alkaline or neutral. These are damaging due to high salt concentration and toxic to aquatic life.
(iii) The effluents have a high concentration of sugar and other carbohydrates. During anaerobic conditions, obnoxious odour develops in the contaminated stream. Such a septic condition results in production of hydrogen sulphide gas in contaminated water. The effluent is imparted black colour because of precipitation of iron by hydrogen sulphide as the stabilization proceeds.
(iv) These effluents have a high biochemical oxygen demand. Discharge of these untreated effluents in water courses results in depletion of oxygen content, making environment unfit for fish and other aquatic life.
Methods of Treatment and Disposal of Sewage from Cane Sugar Industry:
The various methods of treatment and disposal of sewage from cane sugar industry can be grouped under four main heads:
(a) Waste prevention at source
(b) Disposal on land
(c) Conventional biological treatment
(d) Treatment in lagoons or stabilization ponds
(a) Waste Prevention at Source:
The quantity of polluting material getting into sewage can be reduced by taking certain measures for proper operation and maintenance of the manufacturing plant. Prevention and treatment of sewage should begin within the industry itself. Good housekeeping, efficient operation of evaporators to reduce entrainment to the minimum and proper handling and storage of molasses to eliminate spillovers are important in this connection.
Routine inspection of different units, particularly pumps, conveyors, pipes and other vessels and judicious use of water in the mill reduces the problem arising from floor sweeping and washings. The filter cloth washings may be given short detention in a holding tank before being allowed to mix with effluents from other sections.
(b) Disposal on Land:
Use of sewage from cane sugar industry for irrigation is considered to be the best method of its disposal, after preliminary treatment to remove oil and suspended matter and correct pH value. However, the use of large fraction of this sewage for irrigation purposes may result in the development of serious odour problem.
Further unfiltered sewage may cause accumulation of sludge in the filed. This difficulty may be avoided by using only one third of available space each year for sewage disposal and using that portion for next two years for cultivation of grain or vegetable crops. In some cases depending on the soil texture and pH value, occasional furrowing of land may help to dissipate the odour problem to considerable extent.
(c) Conventional Biological Treatment:
The treatment of sewage from cane sugar industry by activated sludge process and trickling filter has been studied by many investigators, with varying degree of success. It may be necessary to add nutrients like nitrogen and phosphorus to bring it to the level of BOD : N : P to 100 : 5 : 1 in aerobic biological system.
However, it has been the general observation that biological treatment would require large and expensive plant as well as skilled supervision and would be difficult to operate since the sugar season lasts for a short period of only 4 to 5 months, and that also particularly during the winter.
(d) Treatment in Lagoons or Oxidation Ponds:
Treatment of sewage from cane sugar industry by lagooning or in oxidation pond has been found most feasible and economical. It has been found that in regions where winter conditions are not severe, the period of retention of sewage in lagoons or in oxidation pond is not too high and degree of purification is reasonable.
In this method sewage is treated in two stages, the first stage being primarily anaerobic digestion in open deep pond and the second stage being aerobic oxidation in open shallow pond. The first stage is usually preceded by equalization of flows and characteristics of the sewage in an equalizing cum digestion pond having a detention period of one day.
The anaerobic digestion pond may have a liquid depth of not less than 2.5 m and a detention period of 6 days. At an organic loading of BOD in the range of 0.24 to 0.32 kg/m3/day in the anaerobic digestion pond, the reduction in BOD may reach the order of 60 per cent. The effluent from this pond is further purified in an aerobic oxidation pond having a liquid depth less than 1.2 m and a detention period varying from 10 to 12 days.
A BOD loading of 4750 kg/m3/day may be applied in the oxidation pond. The BOD removal in the oxidation pond may exceed 70 per cent. The overall reduction in BOD in this treatment plant may be of the order of 90 per cent or more. The final effluent will have a BOD between 60 to 100 mg/l. However, even with this BOD the effluent cannot be discharged in inland surface waters and it may be necessary to dilute it to get the required value (30 mg/l). In case of land disposal for irrigation, the effluent need not be diluted.
Incorporation of a suitable algal culture tank for acclimatizing algae to the effluent and dosing algae continuously to the oxidation pond is useful in successful operation of the oxidation pond.
Fig. 19.4 shows a flow diagram for the treatment of sewage from cane sugar industry.
Fig. 19.4 Flow Diagram for Complete Treatment of Sewage from Cane Sugar Industry.
Byproducts of Cane Sugar Industry and their Disposal:
The major byproducts of cane sugar industry are as follows:
(i) Bagasse
(ii) Press mud or Press cake
(iii) Molasses
(i) Bagasse:
The production of bagasse depends upon the fibre content of the sugarcane, which varies from 13 to 15% in western and southern zone and from 15 to 17% in eastern and northern zone. Bagasse production is about 30% of the cane crushed.
The industrial use to which bagasse can be put are:
(a) Fuel; and
(b) Raw material for manufacture of paper, pulp, newsprint and insulation board.
Bagasse can also be used after treatment for production of plastics moulding powder, cattle feed, biogas, manure, etc.
(ii) Press Mud or Press Cake:
The production of press mud is about 3% of cane in sulphitation factories and 7% in carbonation factories. The sulphitation press mud also contains sugarcane wax varying from 8 to 10% of the mud.
The sulphitation press mud is almost entirely used as manure in the field, while carbonation press mud is generally used to fill up pits and in some cases for production of lime for building purposes. Attempts have been made to extract wax from sulphitation press mud, which can be used in place of imported carnauba required in manufacture of carbon paper, shoe and other polishes, wax paper, emulsion for protective coatings on fruits, etc.
(iii) Molasses:
Molasses is the main byproduct of cane sugar industry. Production of molasses depends largely on the quantity and to some extent on the quality of the cane crushed which varies from region to region. Its average production is 4.4% of cane processed.
Molasses have an extremely high biochemical oxygen demand of the order of 900 000 mg/l, hence proper care should be taken so that there are no chances in the mill for the molasses to spill over into the sewage. Further considering the hazards of severe pollution of streams, special attention has to be paid to the disposal of molasses.
Molasses are mainly utilized by the distilleries. However, the production of molasses exceeds the amount required by all the distilleries of the country. About 70% of the molasses produced in the country is utilized by the distilleries.
Besides this, molasses is also used as:
(a) Cattle feed, and
(b) Tobacco curing.
There is a good demand of molasses abroad and some quantity is exported. ;