CHAPTER FOUR
4. ENVIRONMENTAL IMPACT ASSESMENT
4.1 Pollution control and safety
Sugar industry is basically seasonal in nature and operates only for 120 to 200 days in a year
(early November to April). A significantly large volume of waste is generated during the
manufacture of sugar and contains a high amount of pollution load particularly in terms of
suspended solids, organic matter, and press mud, bagasse and air pollutants. Therefore an
attempt has been made to present an overview of waste management in sugar industry in
Ethiopian.
4.1.1 Waste generation
A. Wastewater
Mill house: Mill house wastewater is derived from continuous gland cooling and intermittent
floor washing and contains high amounts of oils and grease and sugar from pills and leaks.
Boiler Blow-down: Boiler blow-down is fairly clean water except that it contains high dissolved
solids and phosphates.
Rotary filter: Filter cloth is periodically washed and constituents a source of waste water.
Condensates: The vapors from the last effect evaporator and pan boiling are separately cooled in
multi jet condensers and the condensate goes to the pond. A part of the cooled water from the
pond is recycled into the sugar mill, but a large portion is discharged as wastewater. If the mill
operates without overloading, the evaporator and vacuum pan condensate is quite clean and the
entire quantity can be reused. But many a times, overloading and poor operating conditions can
lead to significant sugar losses in the condensates through entrainment and thus polluting the
water.
Occasional Spills and Leaks: Leaks from pumps and pipes in the evaporators and centrifuge
house, along with periodical floor washings, constitute another source of waste water. Although
the flow is intermittent and volume discharged is not large, it represents the most polluting
fraction of sugar mill wastewater.
Condensate Washings: Evaporators, juice heaters, pans, etc. are cleaned once in 20 Days for
removal of deposited scales. Caustic soda, sodium bicarbonate and hydrochloric acid are used for
scale removal. Normally the caustic soda washings are stored and reused for cleaning operations.
Most of the sugar mills discharge these chemicals into the drains. After the equipment is boiled
with caustic soda and rinsed with fresh water, it is cleaned with dilute hydrochloric acid using an
inhibitor. The wastewater is discharged into the drains, as the recovery of the chemicals may not
prove to be economical. It is seen that the wastewater has small organic load but inorganic
content may be high to pose a shock-load to wastewater treatment facility (occasional discharge,
once in fortnight). It is suggested to have a holding tank and mix this wastewater gradually to the
final effluent to avoid shock loading on the treatment plant.
Lime Houses: The washings of lime house would contain a considerable amount of inorganic
solids, which include carbonates. The effluents from these two units when combined would give
neutral pH value of waste. This wastewater does not contribute to organic pollution but can be
characterized as inorganic wastewater.
4.1.2 Wastewater parameters
BOD: - This is the measure of the oxygen consuming capabilities of organic matter .During
decomposition, organic effluents exert a BOD that can deplete oxygen supply BOD is generally
measured and expressed in parts per million or milligrams per liter. The effluents from a raw
sugar factory can vary between hundred to several thousand mg/l.
Dissolved Oxygen: It is measured and expressed as parts per million or mg/l
Total Suspended Solids (TSS): - Suspended solids when they settle form sludge on the stream,
lakebed and they are most damaging to the life in water. The different modes of disposal of
wastes are:
1. Disposal into water bodies
2. Evaporation in open pits
3. Disposal into ocean
4. Press mud for fertilizer
5. Bagasse for paper and pulp and fiber
B. Solid wastes
Bagasse: It is estimated that bagasse contributes to 33.3% residue of the total cane crushed. It
has a calorific value of about 1920 kcal/kg and is mainly used as fuel in boilers for steam
generation.
Press Mud: It contains all non-sucrose impurities along with CaCO3 precipitate. Press mud from
defecation process contains valuable nutrients like nitrogen, phosphorous, potassium, etc, and
therefore used as fertilizer.
C. Air pollutants
The bagasse, on burning, produces particulates, viz., un-burnt fibers, carbon particles and
gaseous pollutants like oxides of nitrogen, water vapor and other organic compounds. Of the
particulate waste, the heavier particles slowly settle down in the surrounding area. Such dust fall
leads to the problems of cleaning, reduction in property value, effect on vegetation, etc. The
main gaseous pollutants are CO, which is altogether not measured by any unit, and CO2 is
reported to be in the range of 12 14%.
Wastewater reduction and by-product recovery:
The following areas are important to economize the usage of water.
A. Cooling water
1. Mainly used for condenser, bearing cooling, lime houses and crystallizer for formation of
crystal
2. In condenser, water gets mixed with vapor. However, adjusting pH along with make-up
water to keep dissolved solids in check can recycle it.
3. Evaporator cooling water contains entrained sugar and base because of excess of lime and
can be recycled. Improvement in the designs of evaporator/pan boiler can reduce the loss.
Losses will also be due to evaporation, splashing and percolations.
4. Keeping the temperature of incoming water between 30o and 35oC can reduce losses due to
evaporation. Splashing and percolation can be checked by proper maintenance.
5. Cooling water for bearings, power generation, etc., can be reused safely.
B. Process water
Sugar mill employs both hot and cold water for its various processes such as Filter cake washing,
lime preparation, dilution for lowering Brix, Dilution in evaporators and pans, Massecuite,
Magma making and Crystal washing in centrifugal.
1. Water requirement before evaporator storage is about 1/5 to of steam used while that used
after evaporator requires approximately equal amounts, as for steam. Careful attention is
required after evaporator stage to control water usage.
2. Hot water can be used in place of cold water to reduce the quantity of steam required.
3. It is preferable to use 18 20% maceration by equally adjusting it from the top and the
bottom of bagasse bed feeding to the last mill at a pressure of 7 14 Kg/cm2 rather than
merely pouring the same at 25 to 30% of cane (about 5 7% water can be saved).
4. If maceration is high enough, there will not be any need of dilution water for juice.
5. To reduce water quantity, light molasses can be used for magma making.
Washing water: Therefore require treatment and should not be recycled. Periodic cleaning
results in high BOD and it also contain caustic soda and weak acids. Returning it to the service
water tank can reuse water.
Testing Water: This water is safe for returning it to the service water tank. Oil and Grease
providing suitable oil and grease traps can eliminate this.
Chemical Reuse: The stored and settled supernatant can be reused with a little addition of fresh
caustic soda for next cleaning operation.
Molasses Handling: It is necessary to store molasses in RCC tanks or steel tanks above ground
level. Otherwise, there is a possibility of ground water contamination.
The high BOD of molasses may cause pollution problems due to mishandling.
C. Product recovery
The by-products available from sugar mills are bagasse, furnace ash, and molasses and filter
mud. The uses of these by products are given below. If all the by products are used for
transformation into value added products, (resource recovery), it will minimize the pollution to
large extent.
Bagasse: These are used for steam, power, charcoal, briquettes and methane & producer gas.
Molasses: These are used for fertilizer and cattle feed.
Filter mud: For fertilizer.
Boiler ash: For foundry material.
4.2 Safety
Sugar in boiler feed water causes water to foam, which will lead accidents. If not present in large
quantity, it is decomposed by heat into products that are detrimental to the tubes and shells of
boilers causing pitting and overheating. If sugar is present in small amounts their traces will be
eventually accumulated on the boiler tubes as a harmful and dangerous carbonaceous deposit.
The breakdown of sugar also forms harmful organic acids. To prevent this lime is added to feed
water to maintain pH = 8.0. A pronounced odor develops in the steam if boiler water contains
sugar. Under such conditions the contaminated feed water is turned to sewer and the boilers are
blown off. To prevent these hazards tests are conducted to determine amount of sugar traces in
water. The most commonly used tests are Naphtha test and Aresenomolybdate test.
