The world demand for sugar is the primary driver for Sugarcane Industry. In many developing countries, especially in Asia and South America, Sugarcane industry is one of the most prevalent agricultural industries. As a consequence, sugar cane industry has significant wastewater production. Rapid urbanization and industrialization in the developing countries like India are facing severe problems in collection, treatment and disposal of effluents. Un managed sugar mill effluent is found to have high contents of COD, BOD, TSS, TDS, and low contents of DO resulting in large scale contamination of land, water and air. This leads to serious public health problems and environmental degradation. Unfortunately, due to lack of knowledge, financial support and sometimes unwillingness to spend on treatment of wastewater, most of sugar industries in developing countries discharge wastewater without adequate treatment. For treating the biodegradable impurities, the natural choice is biological treatment, which could either be aerobic or anaerobic. But anaerobic digestion is an attractive waste treatment practice in which both pollution control and energy recovery can be achieved. Sugar industry wastewater was treated in a UASB ( Up flow Anaerobic Sludge Blanket). In this review, the sugar industry wastewater generation sources, characteristics, recent advancements in the aerobic, anaerobic, and physico-chemical treatment technologies, Up flow Anaerobic Sludge Blanket Reactor technology and the areas needing further research have been explored.
India is the world's largest sugar-consuming country and the second largest in terms of sugar production. The growth of sugar factories along with the sugar industries segment depicts the sugar industry scenario in India. Cane cultivation areas increased to 5,354,000 hectares in 2012–2013, which were 5,055,000 hectares in 2007–2008. Moreover, there were 516 industries in operation in 2007–2008. Currently, in 2012–2013, this range increased to 526, producing 25.14 million tons of sugar. Consequently, the amount of wastewater generated from those industries has also increased.
It is one of the major commercial crops grown in Karnataka and is one of the major sources of revenue for majority of the farmers. The production of sugar involves enormous amounts of water and energy. The main energy source is combustion, usually bagasse and other low quality fossil fuels with high sulphur content are used. In sugar production, the water used for processes such as cane washing, clarification of juice, cleaning of evaporators, heaters and purging boilers, cooling systems and sanitary services are discarded.
Sugar industries waste waters are characterized by high Biological Oxygen Demand (BOD), Chemical Oxygen Demand (COD), TSS (Total Suspended Solids) and Total Dissolve Solids(TDS). They generate 0.2-1.8 m3 /tonne waste water with COD 1800 to 3200 mg/L, BOD 720 – 1500 mg/L. Wastewater from sugar industry generally contains carbohydrates, nutrients, oil and grease, chlorides, sulphates, and heavy metals. Rapid industrialization has tremendously increased the volume of wastewater to be disposed off, while the capacity of receiving water to accept the increasing inorganic and organic loads remains the same. The BOD/COD causes rapid depletion of oxygen content of the waters, creates foul smell, renders the stream unfit for propagating aquatic life, drinking and for other purposes.
This has resulted in a rapid deterioration of quality of surface water and at the same time stimulated concerned government agencies to introduce and enforce more stringent legislation. Because of the forced legislation, industries are looking for low cost solutions for the required reduction of pollution load.
Primary treatment of sugar industry wastewater includes filtration, sedimentation, and load equalization. Whereas, secondary treatments are biological methods like lagoons, aerated ponds, Up-flow Anaerobic Sludge Blanket (UASB) [2], Expanded Granular Sludge Blanket, Fluidized Bed Reactor (FBR). Sometimes combined anaerobic and aerobic treatments are also used for sugar industry wastewater treatment.
The processing steps involved in production of sugar are milling, clarification, evaporation, crystallization, and centrifugation. In milling process, sugar cane is crushed to extract juice [1]. During milling process, some amount of water is added to crushing cane which is known as imbibitions water, to increase the efficiency of juice extraction. After extraction of juice, fibrous residue, which is known as bagasse, is generally utilized as fuel for boiler after drying. The extracted juice is very turbid and greenish in colour, which is then clarified and bleached with Ca(OH)2 and SO2 dosing followed with clarification by continuous clarifier.
The clear juice is decanted, and thickened sludge is send to the rotary drum vacuum filter for the recovery of remaining juice contained in the sludge. During the filtration process, water is added to enhance the efficiency of process, and the de-watered sludge known as ‘press mud’ is discarded and utilized as fertilizer. The clear juice is then sent to vacuum multiple-effect evaporators, where juice is concentrated. Afterwards, sucrose crystallization is carried out using pans where the remaining water is evaporated under vacuum. Product leaving the vacuum pans is called ‘massecuite’, which is then centrifuged, washed, dried, screened, and packaged.
In view of generated wastewater volume and characteristics, sugar industries are one of the most polluting industries. Volume of effluent generated depends on the cane crushing capacity of industry and management of water. Sugar industries in India generate about 1,000 L of wastewater for one ton of sugar cane processed. Therefore, the sugar industry having the capacity of 2,500 Tons Crushed Per Day (TCD) will generate about 106 L of wastewater in a running session of six months. It has also been reported that Mexican sugar industries generates 45.9 m3/s waste water (713.83_106m3) for six months running session.
Sugar industry wastewaters are produced mainly by cleaning operations such as. Washing of milling house floor, various division of boiling house like evaporators, clarifiers, vacuum pans, centrifugation, etc. which generate huge volume of wastewater. Also, wash water used to filter cloth of rotary vacuum filter and periodical cleaning of lime water and SO2 producing house becomes a part of wastewater. Periodical cleaning of heat exchangers and evaporators with NaoH and Hcl to remove the scales on the tube surface which contributes organic and inorganic pollutant load to wastewater. Leakages from pumps, pipelines, centrifuging house also contribute to wastewater produced. Except this, wastewater is also produced from boiler blow down, spray pond overflow, and from condenser cooling water which is discharged as wastewater when it gets contaminated with cane juice.
The waste water generated from different sub streams can be classified as follows[4],
The effluent consists of water used from cleaning the mill house floor which is liable to be converted by spills and pleased sugar juice (This clearing up operation will prevent growth of bacteria on the juice-covered floor).Water used for cooling of mills also forms part of the waste water from this source. Basically this water contains organic matter like sucrose, bagacillo, oil and grease from the bearings fitted with the mill equipments.
The waste water from boiling house results from leakages through pumps, pipelines and the washes of various sections such as evaporators, juice heaters, clarification, pans crystal is action, and centrifugation etc. The cooling water from various pumps also form part of waste water.
The water used in boiler contains suspended solids, dissolved solids like calcium salts, magnesium salts, sodium salts, fatty salts etc. These salts get concentrated after generation of stream from the original water volume. These solids have to be expelled time to time to save the boiler being covered up by scales.
The excess condensate does not normally contain any pollutant and is used as boiler feed water for the washing operations. Sometimes it gets contaminated with juice due to entertainment of carryover of solids with the vapours being condensed. The treatment requirement in this case is almost negligible and can be replaced by fresh water or let out directly as irrigation water after cooling it to ambient temperature.
Condenser cooling water is recirculated again unless it gets contaminated with juice, which is possible due to defective entrainment separators, faulty operation beyond the design rate of evaporation etc. If it gets contaminated, the water should go into the drain invisibly. This volume of water is also increased by additional condensing of vapour which is drained from the boiling juice in the pan.
The heat exchangers and evaporator are cleaned with caustic soda and hydrochloric acid in order to remove the formation of the deposits of scales on the surface of the tubing. In India, most of the sugar factories let this valuable chemical go into drains. The soda and acid wash contribute considerable amounts of organic and inorganic pollutions and may cause shock loads to waste water treatment.
The sugar industry is an important consumer of both drinking and industrial water used in the refining process. Waste water produced has a high organic load and, initially in the refining process, also has high particulate load. Thus, treatment of these wastewaters requires a process that combines mechanical, chemical, and biological treatment measures. Screening, grit removal, flow equalization, sedimentation, or dissolved air flotation are used to reduce Suspended Solids (SS) load from sugar industry wastewater. Biological treatment methods are applied for the reduction of soluble organic matter and disinfections. Biological treatment includes aerobic and anaerobic process. Except biological methods, physicochemical methods are also used for sugar industry wastewater treatment.
Physico-chemical parameters such as pH, electrical conductivity, COD, chloride, calcium, magnesium, sulphate and TDS were relatively high in the sugar factory effluent and severely affected the seed germination [7]. There was a gradual decrease in the percentage seed germination and germination value with sugar industry effluent concentration. The untreated sugar industry effluent could possibly lead to soil deterioration and low productivity. Terrestrial and aquatic environmental pollution could be averted by proper treatment of the effluents using suitable conventional methods like Electro Coagulation Technique.
Sugar industry effluent is conventionally treated by adopting various physico-chemical and biological methods. These conventional processes suffer the disadvantage that the reagent costs are high and the soluble COD removal is low. Moreover, chemical treatments could induce a secondary pollution due to the fact that chemical additives may contaminate the treated water. Coagulants in addition to increasing the amount of sludge production increase the total solids in the effluents; adsorption process necessitates back-washing and use of membranes has the problem of scaling and frequent membrane fouling. The EC technique has been successfully used for the treatment of various waste waters such as domestic wastewater, cyanide containing wastewater, tannery wastewater, textile wastewater, slaughter-house wastewater etc. Electro Coagulation process is used for the treatment of sugar industry wastewater using iron electrodes.
Electro coagulation (EC) [8] is a process in which the anode material undergoes oxidation with formation of various monomeric and polymeric metal hydrolyzed species. These metal hydroxides remove organics from wastewater by sweep coagulation and/or by aggregating with the colloidal particles present in the wastewater to form bigger size flocs which ultimately are removed by the settling. During EC, coagulants are obtained in by the dissolution of the anode.
Electro-chemical treatment process is an emerging wastewater treatment technology. Electro Chemical treatment method involves Electro-Oxidation, Electro- Coagulation, and Electro-Floatation. In Electro-Oxidation (EO) treatment, organic materials are oxidized to carbon dioxide and water or other oxides by electrochemically generated reactive oxygen and/or oxidizing agent. Whereas, Electro Coagulation process involves generation of anode material hydroxides and/or poly hydroxides which remove the organics by coagulation. Electro-flotation process removes pollutants with the help of buoyant gases bubbles generated during electrolysis, which take with them the pollutant materials to the surface of liquid body.
Since, sugar industry wastewater contains mostly sugars and volatile fatty acids, which are easily biodegradable; therefore all the biological (anaerobic and aerobic) treatment processes are suitable.
Aerobic biological treatment generally involves degradation of organic in the occurrence of oxygen. Conventional aerobic treatment includes activated sludge, trickling filters, aerated lagoons, or a combination of all [9]. Sugar industry waste waters are biodegradable except oil and grease which are not easily degraded by anaerobic processes [10], because oils produce longchain fatty acids during the hydrolysis step which causes retardation in methane production [11]. Long-chain fatty acids were reported to be inhibitory to methanogenic bacteria [12].
Ahmad and Mahmud [13] conducted experiments in batch reactor to show whether the aerobic treatment for sugar industry wastewater is acceptable. It was reported that the aerobic biodegradation of wastewater is agreeable. It was also reported that the COD reduction can be predicted at given parameters with the help of relationship suggested by Tucek et al. [14]. Earlier, lagoons were used for sugar industry wastewater treatment [15, 16] because of being an economic process. But, larger area requirements and emission of unpleasant and annoying odour during the treatment process [16] are some of the disadvantages of lagoons. Aerated lagoons were also used in past and showed lesser residence time and area required compared to lagoons, to treat sugar industry wastewater, but oxygen consumption and HRT were found to be high, and still large area requirement is a disadvantage[17]. Effluents from Mumias Sugar factory is treated using ponds before discharging into Nzoia River. To explore the pollution of river due to this activity, Moses' et al. [17] examined the samples for temperature, pH, BOD, COD, TDS, and TSS and concluded that the values were well and with in discharge limits defined by NEMA (National Electrical Manufacturers Association) and the World Health Organisation (WHO).
Hamoda and Al-Sharekh[18], examined the performance of a system, Aerated J.P. Kushwaha / desalination and Water Treatment, submerged fixed-film (ASFF), in which bio-film was attached on submerged ceramic tiles with diffused aeration condition. It was concluded that the ASFF process is capable of handling severe organic loadings of 5–120 g BOD/m2 with minute decrease from 97.9to 88.5% in BOD removal efficiency and from 73.6 to67.8% in COD removal efficiency. Nitrification rate was also decreased but at higher rates. None of the studies showed completely/nearly complete organic removal. Therefore, an additional biological treatment stage is needed. Hybrid systems of comprising anaerobic and aerobic treatments have been approved capable of giving high COD removal efficiency with smaller amount of required energy. Yang et al. reported a combined anaerobic(UASB) and aerobic treatment system for effluent from primary treatment of sugarcane mill waste water for its application for drip irrigation, and P99% organic and solids removal were reported at HRT(Harmone Replacement Therapy) of 2 d. This treated wastewater hold better water quality for drip irrigation.
Anaerobic digestion is an established technology, used for the treatment of wide variety of organic wastes throughout the decades. It is one of the several biological processing strategies which produce bio energy and/or bio chemicals while treating industrial and agricultural wastes. Anaerobic biodegradation can be separated into two phases in order to enhance treatment efficiencies and/or produce bio-products. Two-phase anaerobic systems have been extensively studied and numerous advantages of phase separation over conventional anaerobic digestion have been described and demonstrated in numerous studies. Some of the advantages include, increased process stability and control, smaller reactor volumes and high tolerance to toxicity and shock loads. These advantages enable the two-phase anaerobic systems to treat many kinds of solid, industrial and agro industrial wastes such as distillery, landfill leach ate, coffee, cheese whey, dairy, starch, fruit, vegetable solid, food, pulp and paper, olive mill, abattoir, dye wastewaters, primary and activated sludge.
Along with its applications as the first step of a phase separated anaerobic waste treatment configuration, anaerobic acidification can be exploited separately for bio-product formation. For example anaerobic acidification could be useful for the production of organic acids (e.g. VFA) which have variety of industrial uses. VFAs (Volatile Fatty Acids) are utilized for the manufacture of various organic compounds and some plastics. Moreover, those organic acids play an instrumental role as a carbon source in the removal of nutrients from waste waters. Recently, acid phase anaerobic digestion received considerable attention and relevant studies covered a wide range of operational parameters including; pH, waste type, HRT, temperature, mixing of wastes and OLR (Outgoing Longwave Radiation).
Up-flow Anaerobic Sludge Blanket (UASB) [5,6] reactor is used for the anaerobic process. In this anaerobic treatment complex organic matter gets converted into methane gas through the stages like hydrolysis, acidogenesis and methanogenesis [3]. UASB is widely applicable for treating various types of wastewater. UASB has advantages over aerobic treatment. UASB is used for treating wastewater in sugar industry, distillery, dairy industry, slaughter house and high strength municipal wastewater.
Mainly the following are the four key biological and chemical stage in UASB process:
HydrolysisMostly organic concentration of wastewater is complex in nature. For the bacteria in anaerobic digesters to access the energy potential of the material, these complex organic constituents should break down into their smaller constituent parts. These constituent parts, or monomers, such as sugars, are readily available to other bacteria. The process of breaking these chains and dissolving the smaller molecules into solution is called ‘hydrolysis’. Through hydrolysis the complex organic molecules are broken down into simple sugars, amino acids, and fatty acids. Acetate and hydrogen produced in the first stages can be used directly by methanogens. Other molecules, such as volatile fatty acids with a chain length greater than that of acetate must be first be catabolised into compounds that can be directly used by methanogens.
AcidogenesisThe biological process of acidogenesis results in further breakdown of the remaining components by acidogenic (fermentative) bacteria. Here, vase is created, along with ammonia, carbon dioxide, and hydrogen sulphide, as well as other by-products.
AcetogenesisThe third stage is acetogenesis. Here, simple molecules created through the acidogenesis phase are further digested by acetogens to produce largely acetic acid, as well as carbon dioxide and hydrogen.
The terminal stage of anaerobic digestion is the biological process of methanogenesis. Here, methanogens use the intermediate products of the preceding stages and convert them into methane, carbon dioxide, and water. These components make up the majority of the biogas emitted from the system. Methanogenesis is sensitive to both high and low pH and occurs between pH 6.5 and pH 8. The remaining, in digestible material the microbes cannot use and any dead bacteria remains constitute the digest.A simplified generic chemical equation for the overall processes outlined above is as follows,
Although, generally the anaerobic process is used for the treatment of sugar industry wastewater, this method is limited due to the production of long-chain fatty acids during hydrolysis of oil and grease. Also, anaerobic processes do not completely remove nutrients/organics. Therefore, anaerobically treated effluents need further treatment. Aerobic SBR may be the promising treatment technology for the sugar industry wastewater, because aerobic SBR (Sequencing Batch Reactor) has been reported to give good removal efficiency in terms of both nutrients and other organics.
UASB reactor is feasible for treating variety of wastewater. Performance of UASB reactor gets affected by ph, HRT, OLR, temperature and VFA to alkalinity ratio. Proper HRT should be provided to give sufficient contact time between wastewater and bacteria. For avoiding VFA accumulation in UASB reactor and for getting effective biogas production of sodium bicarbonate, alkalinity should be provided. VFA to alkalinity ratio should be maintained between 0.5 - 0.8 for good performance of UASB reactor.