Sugarcane and Sugar -The Criticality of Water and Waste Management
By Dr. Yashpal Singh
From the Executive Summary of book entitled “Sugarcane and Sugar-Biology, Agriculture, Manufacture, Regulation, Environment and Pollution Control”, 2024 by Dr. Yashpal Singh. Published by The Wealthy Waste School India and U.P. Sugar Mills Association. Available at: https://www.amazon.in/Sugar-Cane-Agriculture-Manufacture-Regulation/dp/936039856X/ref=sr_1_1
India is sadly expected to emerge as a water scarce country by 2050. While India’s water foot print of 980 cubic meter/cap/year is lower than the global average of 1240 M3/cap/year (America 2480 M3/cap/year; China 700 M3/cap/year), our population of more than 1.2 billion people collectively contributes to a significant 12% of the total water foot print. This is not sustainable and there is an urgent need to adopt corrective action.
Sugar cane is a water intensive crop and therefore the availability of water is of critical concern. Both surface and ground water sources are now inadequate. Most of this is polluted. The Government of India has well drawn-out action plans to address to these issues. These include ensuring that wastes are properly treated before discharge, prioritizing action on the basis of pollution potential (Red, Orange, Green and White Category of Industries), setting up water quality monitoring system, stepping up regulation, introducing fines, penalties and environmental restoration through damage assessments and compensation, encouraging maximum recycle and reuse, promoting sustainable zero liquid discharge and implementation of a National Plan for conservation of aquatic ecosystems to undertake various conservation activities.
Sugar mill water balance and water conservation
Sugar cane contains 70% of water, most of which is reused but still leaves about 18 to 20% water on cane which is generated as surplus. This water is being increasingly reused by the Sugar Industries which have introduced a number of water conservation techniques leading to considerable reduction in fresh water use. Hot water is generally used in mill imbibition, milk of lime preparation, Oliver filter, pan movement, centrifugal washing, laboratory and floor washings and boilers. Installation of back pressure turbines instead of condensing turbines, air cooled condensers in condensing turbines instead of cooling towers, provision of condensate polishing units and recycling hot water would bring about more of water conservation and reduce the waste water load on E.T.P. A number of measures have been recommended to recycle, reuse and recover water. This can also reduce the waste water generation. A fresh water generation of less than 100 liters/ton of cane is achievable. Good management practices are critical to ensure optimal use of water in a sugar industry and a consequent reduction in waste water volumes and pollution loads.
Sugar Mill Effluents
Waste waters in Sugar mills are generally produced from washings on the mill house, boiling house, evaporators, clarifiers, vacuum pumps, centrifugals etc. Periodic cleaning of lime and sulphur houses and descaling of heat exchangers and evaporators also contribute to a high volume of wastes. Boiler blow downs, spray pond overflows, spillages of molasses, syrup or juice and condenser cooling waters also add to the pollution load.
The use of ion exchange resins for decolorization in refineries leads to considerable amounts of effluents being generated during the backwash. With a high TDS of 35000-40000 mg/liter this is an area of concern.
Sugar effluents are not very highly polluting in terms of concentration of pollutants. The 1:2 BOD:COD ratio suggests that they are easily amenable to biological treatment through anaerobic and aerobic pathways and may not require complex chemical treatment. The MoEF and CC, Government of India has prescribed standards for treated effluents limiting the pH to 6.5 to 8.5, BOD and T.S.S. (30 mg/L for surface waters and 100 mg/L for disposal on land); oil and grease 10 mg/L; T.D.S. 1000 Mg/L and final waste water discharge to 200 liters/ton of cane crushed. The Ministry has also prescribed a few water conservation technologies which include recycling of excess condensate to process or utilities, stabilization of E.T.P. at least one month prior to crushing season, storage of treated effluents in seepage free lagoons, installation of flow meters and installation of suitable air pollution control devices to meet the particulate matter emission standard of less than 150 mg /NM3.
The important contaminants of concern in a sugar mill could be suspended solids, biodegradable organics measured commonly as BOD/COD, pathogens, nutrients like phosphorous and nitrogen which may trigger eutrophication or organic enrichment, refractory organics, heavy metals, and dissolved solids. Sugar waste water treatment generally consists of screening, grit removal, oil and grease removal, equalization, neutralization, coagulation, flocculation, primary settling, anaerobic treatment (U.A.S.B. etc.), activated sludge process, secondary clarifier, chlorination, multigrade filters, activated carbon filters and sludge management systems and sulphate removal systems brine treatment systems and condensate polishing units are also provided.
Treated effluents, to the standards as prescribed by the Boards can be disposed into inland surface waters or on land for irrigation. The NGT has however directed that irrigation use should be allowed only after assessing the adequate availability of land and impacts of such disposal on agriculture and ground water. Guidelines have been prescribed for the utilization of effluents in irrigation. Many operational and maintenance problems may occur during the running of the Effluent treatment plant which have to be duly addressed to. Sludge bulking, black and odorous contents, excessive sedimentation of solids, excessive fouling of surface and weirs with effluent solids, intermittent surges in flow and a high concentration of grit, clay or other material are some problems encountered during the operation of the E.T.P. which can be solved by good management practices and judicious operational surveillance. The activated sludge process may also exhibit operational problems like sludge bulking and frothing in aeration basins which can be managed through appropriate systems and operation.
Rising sludge in secondary settling tanks and low pH in anaerobic digesters which may destabilize the digestor may also need to be managed properly. Techniques are available to address to the operational problems associated with running of effluent treatment plant. A regular performance evaluation is an important exercise.
Good operating practices.
A judicious handling of raw materials and products and good management and operating practices may reduce the environmental impacts of a sugar mill by 25 to 30%. This includes regulating the flow of contaminants and taking adequate measures at the cane unloading and carrier section, milling section, bagasse handling, boiler house and boiler section, clarification, evaporation and rotary filters, entrainments from pans and evaporation, washing and cleaning of juice heaters and pan evaporators, Sulphur and lime houses, molasses storage tanks etc.
Air Emissions
Cane bagasse, varying between 24 to 30% by weight of sugarcane is being extensively used for the co-generation in sugar industry. Air Pollution in sugar manufacturing depends on the kind of fuel being used in the boiling process as well as the performance of boilers and furnaces. The common pollutants from sugar mills are CO2, nitrogen oxides, Sulphur oxides, unburnt fuel and particulates. Fly ash is very light and spreads over a long distance. The MoEF and CC has prescribed that the Particulate Matter from stacks shall be restricted to 150 mg/normal M3. A number of air pollution systems are available and they include gravity settling chambers, baffle separators, louvre separators, cyclone separators, fabric filters, electrostatic precipitators, wet scrubbers, spray towers, venturi scrubbers, moving bed scrubbers, orifice scrubbers and core separator systems.
The particulate matter in exhaust gases is generally considered as 5000 mg/Nm3. For this initial particulate load for bagasse fired boilers, multi cyclones may restrict the emissions to 300 mg/NM3, spray tower scrubber can achieve 100 mg/NM3 of particulate and electrostatic precipitators can produce 50 mg/NM3.