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Suger Mill Wastes

 

Introduction & Manufacturing Process
Sources of Waste and Characteristics of Waste 
Effects of Waste on the receiving Steams
Treatment of Waste

 

Introduction & Manufacturing Process

Introduction

In Countries like India, Cuba and Jamaica, the sugar is produced from sugar canes, while in many other places beetroots are used as the raw materials for the sugar production. In India most of the sugar mills operate for about 4 to 8 months just after the harvesting of the sugar canes

Manufacturing Process

The sugar canes are cut into pieces and crushed in a series of rollers to extract the juice, in the mill house.Juice is extracted from the sugar cane, leaving a fibrous residue called bagasse, which can be used as a fuel for the boilers or can be disposed of as solid waste. The milk of lime is then added to the juice and heated, when all the colloidal and suspended impurities are coagulated; much of the colour is also removed during this lime treatment. Lime is added to the extracted juice to raise its pH and to prevent the inversion of the sucrose molecule to glucose and fructose. The coagulated juice is then clarified to remove the sludge. The clarifier is further filtered through filter presses, and then disposed off as solid waste.The filterate is recycled to the process, and the entire quantity of clarified juice is treated by passing sulphur dioxide gas through it. The process is known as " sulphitation process"; colour of the juice is completely bleached out due to this process.

The clarified juice is then preheated and concentrated in evaporators and vacuum pans. The partially crystallized syrup from the vacuum pan, known as "massecuite" is then transferred to the crystallizers, where complete crystallization of sugar occurs.

The massecuite is then centrifuged, to separate the sugar crystals from the mother liquor. The spent liquor is discarded as " black strap mollases". The sugar is then dried and bagged for transport. The black strap mollases may be used in the distilleries.

 

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Sources of  Waste Water and the Characteristics of the Waste

Wastes from the mill house include the water used as splashes to extract maximum amount of juice, and those used to cool the rolling bearings. As such the mill house waste contains high B.O.D due to the presence of sugar, and oil from the machineries.

The filter clothes, used for filtering the juice, need occasional cleaning. The wastewater thus produced through small in volume, contains high B.O.D and suspended solids. A large volume of water is required in the barometric condensers of the multiple effect evaporators and vacuum pans. The water is usually partially or fully recirculated, after cooling through a spray pond. This cooling water gets polluted as it picks up some organic substances from the vapour of boiling syrup in evaporators and vacuum pans.

The water from spray pond when overflows, becomes a part of the wastewater, and usually of low B.O.D in a properly operated sugar mill. Additional waste originates due to the leakages and spillages of juice, syrup and molasses in different sections, and also due to the handling of molasses.

Characteristics of Suger Mill Waste

Parameter Value
pH   4.6 - 7.1 
C.O.D mg/ 600 - 4380
B.O.D mg/l 300 - 2000
Total Solids 870 - 3500
Total Volatile Solids  400 - 2200
Total Suspended Solids  220 - 800
Total Nitrogen  10 - 40

C.O.D /B.O.D ratio

1.3 - 2.0

 

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Effects of the waste on receiving streams :

The fresh effluent from the sugar mill decomposes rapidly after few hours of stagnation. It has been found to cause considerable difficulties when their effluent gets an access to the watercourse. The rapid depletion of oxygen due to biological oxidation followed by anaerobic stabilization of the waste causes a secondary pollution of offensive odour and black colour.

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Treatment :

Disposal of the effluent on land as irrigation water is practical in many sugar mills, but it is associated with odour problem. The reasonable C.O.D/B.O.D ratio of the mill effluents indicate that the waste is amenable to biological treatment. However, generally it is found that, the aerobic treatment with conventional activated sludge process and trickling filter is not too efficient, even at a low organic loading rate. A max B.O.D reduction of 51% is observed in a pilot plant study at Kanpur, where both trickling filter and activated sludge process were tried. In view of the high cost of installation and supervision of the treatment units, and the seasonal nature of the operation of this industry, it is generally observed that the conventional aerobic treatment will not be economical in this country. Anaerobic treatment of the effluent, using both digesters and lagoons, have been found to be more effective and economical.

A B.O.D reduction of about 70% was observed in a pilot plant study with an anaerobic digester. The effluents of the anaerobic treatment units are found to contain sufficient nutrients (nitrogen and phosphorous). As such further reduction of B.O.D 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. The mill effluent however is to be pretreated primarily in bar screens and grease trap.

It is expected that the B.O.D reduction in the anaerobic process will be in the order of 60%, while overall B.O.D reduction may be in the order of 90%.

 

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