The variations in case of Biochemical Oxygen Demand, total solid and total suspended solid are especially significant such variations in prescribed standards are also responsible for their being fully accepted by those who are responsible for designing suitable abatement processes.
There are well established downward trends in the case of some pollutants, especially of BOD. Some industrially advanced countries, e.g., the USA, are planning to eliminate the total organic load in a reasonable period of time there are a couple of other important reasons for the lack of keen interest in the development of suitable abatement techniques.
The implementation of abatement technique generally demands investment with practically no return. Secondly, sugar factories are well spread out and the total pollution load vis-a-vis assimilation capacity does not make it absolutely essential for reduction of different pollutants to the prescribed limit.
The effluents, from the sugar industry, occasionally cause serious problems due to their heavy organic load, which normally requires a lot of oxygen resulting in the exhaustation of oxygen’s, required foilisli to survive However, considering the great difference between the actual and prescribed values of pollutants, the implementation of a suitable abatement technique is considered essential.
The main raw material for this industry is sugarcane. A typical flow diagram for sugar production is given in Fig. 15.1. The sugarcane is cut into pieces and crushed in a series of rollers to extract the juice in the ‘mill house’. The fibrous residue of the mill house is discarded as bagasse.
The cane juice from the mill house is slightly acidic. It is treated with sufficient milk of lime to adjust the pH to approximately 7.0-8.4. It is, then, heated to a temperature of 212 -215°C and allowed to settle. Lime combines with pectin and other impurities and also coagulates the albuminous material present in the juice.
After settling, the juice is filtered and pumped into conical tanks, through the bottom of which carbon dioxide is bubbled to precipitate the excess of lime as calcium carbonate. Any soluble calcium sucrate decomposes into black sugar.
This juice, known as carbonated juice, is, then, treated with sulphur dioxide. This operation, known as sulphination, completes the neutralization of lime and the decomposition of calcium sucrate. In addition, the colour of the juice is blackened.
The clarified juice is filtered to remove precipitated calcium sulphide. It is concentrated by boiling under reduced pressure in multiple effect evaporators. There, the steam produced in the first evaporator is used to boil the juice in the second.
Kept at a lower pressure, the steam produced in the second is again employed to boil the juice in the third at a still lower pressure, and so on until the concentrated juice is finally passed to the vacuum pans where further evaporation reduces the water content to 60%.
The 60% crystallized syrup from the vacuum pans is then are to crystallized, where compete crystallization occurs. The vacuum pans are, then, centrifuged to separate the sugar crystal from the mother liquor. The spent liquor is discarded as black strap molasses. The sugar is dried and bagged for transport.
Source and Characteristics of Wastes Waste Water from Mill House:
Waste water from the mill house includes water used to extract the maximum amount of juice and that used to cool the roller bearings. Such as the mill house waste
These wastes contain high BOD due to sugar and oil from machineries. Waste Water from Filter Clothes Washing
These clothes, used for filtering the juice, need occasional cleaning. The wash water, thus, produced, though small in volume, contains high BOD and suspended solids.
A large amount of water is required in the barometric condenser of the multiple effect evaporators and vacuum pans. The water is usually, partially or fully recalculated, after cooling through a spray pond.
This cooling water gets polluted as it picks up some organic substances from the vapours of the boiling syrup in the evaporator and vacuum pans.
The water from the spray pond, when it overflows, becomes a part of the waste water, usually low in BOD. But due to poor maintenance and bad operating conditions, a substantial amount of sugar may entrain in the condenser water.
This polluted water, instead of being recalculated, is discarded as excess condenser water. These discharges contribute substantially to waste volume and moderately to BOD in many sugar mills.
The leakage and spillages of juice syrup and molasses in different sections and periodical washing of floors also contributes to waste water. Though these wastes are small in volume and are discharged intermittently, they have a very high BOD.
Effect of Waste on the Receiving Stream:
Organic pollutants present in the effluent, viz., sugar and other carbohydrates, when it stagnates, by the rapid consumption of oxygen, followed by anaerobic stabilization, cause bad odor, black colour and fish mortality due to formation of hydrogen sulphide.
Treatment of Sugar Industry Effluent:
The various treatment processes are as follows:
The numbers of processes that have been adopted to treat sugar industry effluent are:
1. Prevention at source
2. Disposal on land for irrigation
3. Trickling filter and activated sludge process
4. Anaerobic treatment
5. Disposal in lagoons or stabilization ponds.
Prevention at Source:
The quantity of pollution material, being discharged into waste waters, can be reduced considerably by taking the following measures:
1. Good housekeeping, through proper operations and maintenance improves the process efficiency, thereby, reducing the total amount of pollution material in the waste waters.
2. Efficient separation of evaporators to reduce entrainment to the minimum.
3. Routine inspection of different units, particularly pumps, conveyors, pipes and other vessels.
4. By keeping a check on the extent of entrainment of sugar syrup through the barometric condenser of the evaporators. Loading of the evaporators and vacuum pumps, when boiling at incorrect liquid level, and variation in vacuum pans, at excessive rate operation, leads to loss of sugar through condenser water.
Improper design of these units, particularly the entrainment in separators, also results in boiling and splashing in the units. By giving proper attention to reduce this entrainment, we can reduce the effluents as well as increase efficiency.
5. Judicious use of water can reduce the waste problem, arising from floor sweeping and washing.
6. By giving a short detention time in the holding tank to the filter, cloth washing before mixing with other effluents.
7. Molasses has an extremely high BOD of the order of 90, 000 mg/l, hence, proper handling and storage to avoid spillage becomes essential.
8. The material leaking from pumps and vessels in sugar factories can be collected and returned to the respective processes by making enclosures and collection pits around the pumps and vessels.
Disposal on Land for Irrigation:
The effluent from cane sugar factories can be disposed off on the land as irrigation water. The process has been practiced by many factories in Uttar Pradesh and Bihar but has not gained much appreciation due to accumulation of sludge on the field, odor nuisance and large land requirement. It was observed to reduce suspended solids and BOD by 99 percent and 65 percent, respectively.
However, there are difficulties due to accumulation of sludge on field, used for disposal of unfiltered waste water from sugar factories. This difficulty can only by avoided by using only 1/3 of the available space, each year, for waste disposal, and using that section for the next two years for growing crops, grain or vegetables.
Jenson used a system for irrigation in which he used a shallow pond for removal of suspended solid resulting in very lush grasslands.
Trickling Filter and Activated Sludge Process:
The treatment of sugar mill effluent, in a high rate trickling filter, reduced pollution by 70%, which could be further reduced to 95% after stabilization in a tank for 3 days. However, the process has the following limitations:
(a) The treatment requires large and expensive plants
(b) It needs skilled supervisors for handling
(c) It is difficult to operate in the winter season
(d) It requires a large quantity of water for dilution.
Anaerobic Treatment of Sugar Industry Effluent:
Anaerobic treatment of sugar industry effluent is a biochemical process, caused by the facultative and anaerobic group of bacteria which, converts the organic matter into volatile acid, and methanogenic bacteria, then, converts the volatile acid to carbon-dioxide and methane.
Methanogenic bacteria are strictly anaerobic. They thrive best when pH is close to neutral but to fail to thrive at pH 6.0. The bacteria are heterogenous in cellular morphology- some are rods, others cocci known as sarcinae.
The process of anaerobic digestion consists of a closed digester size, depending upon the quantity of effluent, having an inlet for the addition of waste, two outlets for methane gas and for disposal of slurry.
For starting the process of digestion, the digester is seeded with 20% solution of cowdung in water for one week at 37 + 10°C. Then, the diluted effluent is added, the concentration is gradually increased. At each loading, digester is run for sufficient time at several total displacements.
The anaerobic digestion of cane sugar industry in a closed digester for methane recovery is a promising pollution control measure as the process results in BOD up to 100 mg II. The effluent from the anaerobic digester can be used for irrigations purposes.
It is interesting to observe that most sugar industries are concentrated in rural areas. Here, the pollution load viz-a-viz assimilation capacity is much less because, normally, there is no concentration of other industries.
These parts of the country still depend upon tube-well water for irrigation. Normally, due to lack of electricity, tube-well water is not available during the peak period of winter crops. It is a matter of coincidence that sugar industries are in full swing at this time and their effluent can be used for irrigation purposes.
The only drawback, to be considered in this technique, is the formation of sludge on the soil. The sludge should be removed from the effluent before it is used for irrigation purposes or using 1/3 of the available space each year of waste disposal.
The energy consumption in rural India is very low. The chul has used in rural areas are not designed on a scientific basis and, naturally, are not only inefficient but create a lot of pollution, especially of air.
This is normally a localized problem and the persons associated with food preparation are badly affected. With some capital investment, it is possible to arrange for gas production from sugar industry effluents.
The effluent, after gas production, could be used for irrigation purposes. Should there be much difficulty in getting land for this purpose, trickling filter and activated sludge may be considered as a suitable alternative for effluent treatment.