Introduction:
India is one of the world’s largest producers of sugar from sugarcane. Crystal sugar is produced in organised sugar mills located in different parts of the country, the highest concentration being in the state of U.P. in the North and Maharashtra in the West. There is also a widespread small scale industry (Khandsari) producing raw sugar.
Harvested sugarcane is shredded and washed to remove dirt using water sprays. This water mostly helps in extracting maximum sugarcane juice. The remaining solid fibrous material is known as bagasse.
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After removing solid residues from sugarcane juice by using filter cloth technique, the juice is purified by either double sulphitation or double carbonation, the former being used to larger extent in India.
The sulphitation process involves heating of juice to 70°C and milk of lime (2.0-1.5%v/v) is added therein. About 8-11% gaseous sulphur dioxide is then passed through to bring down alkaline juice pH to neutral.
The treated juice/is boiled by indirect steam heating. It is then sent to clarifiers where some insoluble compounds settle as a sludge and others rise to form scum. Coagulants are also added.
In double carbonation process, milk of lime (6-10%v/v) of juice is added and then carbon dioxide gas is passed to produce alkalinity of 350-450 mg/l as CaCO3. Second carbonation is carried out at pH 8.5-9.2 after which gaseous sulphur dioxide is passed to have a pH of 6.8-7.0 and the treated juice is clarified as described above.
For both the process, chemicals used for purification of juice are listed below:
Triple superphosphate (45-48% P2O5) 0.12-0.016 kg per 1000/kg of sugarcane is used to improve clarification. Clarified juice is concentrated in multiple effect evaporations in which 75% of water from the sugarcane juice gets removed producing a syrup containing 60-70% solids.
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The syrup is further concentrated in vacuum pans (pan-boiling), yielding a dense mixture of crystals and syrup called ‘massicuit’. It is centrifuged for separation of crystals as raw sugar. The remaining syrup, known as ‘molasses’ is returned to the vacuum pans for further crystallization and finally the spent molasses is removed from the process as the by-product.
Molasses is the basic raw material for alcohol industry (distillery). Other than molasses sources of alcohol production are pineapple, grapes, sugarbets, barley, etc.
Distillery wastewater, generally known as stillage, slops, vinnasse or dunder, constitutes a high value, high strength acidic waste that presents significant disposal or treatment problems.
Intensive treatment of distillery wastewaters has become imperative because of strict water quality legislation and decrease in land availability. Production of ethanol from agricultural materials for liquid fuel supplement poses a serious threat to water quality in several regions of the world.
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Ethanol fermentation can be affected by using any naturally occurring sugar, starch or celluloses material coupled with appropriate pretreatment. In the case of starch and cellulose, acid or enzymatic hydrolysis to glucose or fructose is necessary prior to fermentation. Cane or beet molasses is used in the manufacture of rum where side fermentations produce aldehydes and ketones which characterises the flavour of particular rums.
On the hand, whisky and brandy are produced from grain starch. Industrial ethanol is rarely made by fermentation—the main route via oxidation of ethylene and has only been economical where there exists a cheap supply of molasses increased oil prices have, however, forced many countries to re-evaluate the production of ethanol as a liquid fuel particularly from sugarcane or cassava.
In case of grain distilleries, it is always economical to recover the spent grain by evaporation for use as an animal feed—the evaporate condensate thus obtained is quite amenable to conventional aerobic treatment. This, unfortunately, is not the case in molasses fermentation.
Typically, the molasses is diluted to roughly 12-20% sugars, acidified with sulphuric acid and supplemented with nitrogen and phosphorous. In a typical fermentation, the batch is inoculated with yeast and allowed to ferment for a time ranging from 12-60 hours at around 30°C-the ethanol concentration in the final product (‘beer’) is typically from 7-10%.
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The ‘beer’ then enters a two-stage or three-stage distillation train to give a final concentration of upto 95% ethanol. Azeotropic distillation is required to obtain absolute alcohol.
The first column of the train, essentially a stripping column, removes the bulk of water and other constituents from the ethanol (which then undergoes concentration and rectification). It is this stream that is principal component of stillage. Unless the yeast has been separated for recycle or its food value, the stream will also contain the spent yeast cells.
It should be noted at this point that one means of reducing the volume of waste is to ferment higher strength molasses. In the process used presently, the fermentable concentration in the initial medium is limited by alcohol inhibition of yeast growth and alcohol production.
There would be conspicuous savings in distillation energy and volume of wastewater generated if alcohol tolerant strains of yeast or bacteria can be developed. The quantity of inorganic to be removed will not be affected by such improvement in the fermentation.
The molasses on dilution will have a specific gravity of 1.09 to 1.10. The fermenter is charged with mash containing 5% yeast culture in suspension. During fermentation process considerable amount of heat and carbon dioxide are produced.
Temperature is controlled by using cooling coil heat exchangers. Carbon dioxide is collected, washed and compressed into cylinders for supply to aerated water industry.
Today India has 254 distilleries with a total installed capacity of 2440 million litres of ethanol per annum. Some distillation units are down stream of larger sugar factories but there are many that carry molasses from sugar factories to produce alcohol elsewhere in the country. The availability of by-product molasses from organised sugar mills as well as the small-scale industry led to the establishment of fermentation plants for manufacture of ethanol.
Alcohol distilleries have been classed as the most polluting amongst the chemical industries. Almost once a year a Seminar/Workshop/Panel Discussion is held in some part of the India to discuss the Molasses Alcohol Effluent Treatment. The main impediment in the way of implementation of effluent treatment schemes has been the large capital investment involved in setting up the treatment plants.
The capital cost of the treatment plant is as much as the cost of the distillery itself and in the case of the old distilleries, this cost is almost prohibitive. The standards which are set for achievement by the Pollution Control Authorities is unrealistic have not been achieved so far in India and unlikely to be feasible within the norms of the industry.
During the early history of the industry the distilleries have taken recourse to releasing the waste on open land causing pollution and foul odour in the surrounding area and contamination of groundwater.
The pollution control laws now required this effluent to be treated and rendered harmless. A concerted effort is, therefore, now being made by the industry to set up treatment plants.
Effluents and Wastes from Sugar Industry and Distilleries:
I. Sugar Industry:
1. Solid Wastes:
There are two types of solid wastes generated in a sugar mill.
They are:
(i) Press Mud:
The solid waste generated from filtre cloth and scum, is known as press mud or filtre cake, can be used for filling low-lying area with care not to pollute groundwater and considering its nutrient value as manure for the agriculture fields. Table 15.1 shows analysis of press mud or filtre cake.
Wax from the press mud can be extracted and may be recovered as a by-product. The recovered wax can be used in manufacturing of shoe polish and carbon paper.
(ii) Bagasse:
Bagasse is another form of solid waste produced after crushing shredded sugarcane and removing sugarcane juice. Bagasse contributes about 0.33 tonne per tonne of sugarcane crushed.
It has a calorific value of 1917 kcal/kg and, therefore, it is presently used as a fuel for steam generation in sugar mills. However, bagasse can produce 0.3 tonnes paper per tonne bagasse.
Therefore, sugar mill can either manufacture paper or sell bagasse to paper manufacturing units and earn some revenue. This is also a profit oriented by-product from sugar mill.
2. Wastewater:
During various process operations, wastewater generation takes place.
In sugar mill normally wastewater generation sources are as follows:
i. Mill House:
Mill house wastewater is derived from continuous gland cooling and intermittent floor washings. The volume reported to be large and contains high amount of oils, grease and sugar from spills and leaks and thus exerts high BOD (Table 15.2).
ii. Boiler Blow-Down:
Boiler blow-down is fairly clean water except that it contains high dissolved solids and phosphates.
iii. Filtre Cloth:
Filtre cloth is periodically washed and these washings constitute a source of wastewater discharged intermittently. It contains high suspended solids and BOD due to sugar juice retained, (Table 15.3).
iv. Condensates:
The vapour from the last effect evaporator and pan boiling are separately cooled in barometric condensers and the condensate goes to the pond. Part of the cooled water from the pond is recycled into the sugar mill, but a sizable amount of water from the pond is discharged as the wastewater.
If the mill operates without overloading, the evaporator and vacuum pan condensate is very clean and the entire quantity can be reused into the mill. But many a times overloading and poor operations can lead to significant sugar losses in the condensates through entrainment and get polluted. Even though, BOD of the pond overflow is not very high and therefore, does not significantly add more pollution load.
v. Condensers Washings:
Evaporators, juice heaters, pans, etc. are cleaned once in 20 days’ time for removal of scale. Caustic soda, sodium carbonate and hydrochloric acid are used for scale removals.
After chemicals cleaning the spent caustic is stored for reuse in a few mills, but in majority of mills, it ends into drains along with other cleaning agents and equipment wash water.
vi. Occasional Leaks:
Leaks from pumps and pipes in evaporators boiling, centrifuge house along with periodical floor washings, constitute another source of wastewater. Although the volume is not large, it represents the most polluting fraction of sugar mill waste as it exerts high BOD due to large concentration of sugar contained in it.
vii. Molasses Mixed Water:
Molasses contains significant concentration of uncrystallised sugar and other organic compounds. On an average the quantity of molasses production is about 4.45% of the sugarcane crushed. Leaks in pipes and pumps occasionally add to the volume of the wash water and increase pollution load considerably due to its high BOD (around 900000 mg/l)
II. From Distillery:
1. Solid Effluents:
Every litre of alcohol produced gives 1.30-1.40 kg of total solids. Considering that 1000 million litres of alcohol are produced, the total solids to be handled in the effluent from all the distilleries in a year will be 1.3-1.4 million tonnes, which is a colossal amount. The investment in effluent is anywhere between Rs.500-1000 per kl. of alcohol produced.
Yeast sludge containing about 30% solids settles down in the fermentation vats and constitutes another major source of waste. Yeast sludge and other wash waters are discharged along with the dealcoholised spent wash.
2. Wastewater:
Fermentation of molasses and the subsequent distillation gives rise to 12-14 litres of effluent per litre of ethanol manufactured mainly in the form of spent wash from the analyser column and the fermenter sludge.
The effluent has a very high BOD and COD, is acidic in nature (pH 3.5-4.5), and is discharged at high temperature. Wastewaters discharged by distillery producing industrial alcohol from sugarcane molasses poses problems of disposal to acceptable standards due to their high BOD, COD, colour, odour, etc.
The wastes resulting from a distillery are spent wash, condenser water, CO2 gas plant wastewater and yeast sludge. Condenser water and carbon dioxide gas plant wastewaters are relatively pure and do not contain appreciable amount of pollution and as such pose no treatment problems.
Yeast sludge drained from fermentation vats are dried and used as a potential source of manure or used for the preparation of poultry feed. However, sometimes the yeast sludge is also drained along with spent wash resulting in a tremendous increase in the organic strength of the wastewater.
Generally the quantity of process water in distilleries varies with an average value of 130 litre 1/1 per litre of alcohol produced. The generation of spent wash also varies with the production of alcohol. The volume of spent wash generated ranges from 14.1 to 15.6 (average 15.1) for each litre of alcohol produced.
The waste is hot, coloured and acidic, containing very high concentration of organic matter and exhibits high BOD. The distilleries usually discharge wastewaters as spent wash and condensate water which together form the final effluent. The general characteristics of the distillery effluent are given in Table 15.4.
In general, the distillery industries are among the major polluters of the aquatic environment. The waste generated from distilleries is highly organic in nature. The basic question is no longer whether this waste can be anaerobically biodegraded to methane, since most organics are amenable to anaerobic treatment, but rather at what rate it is degradable, to what degree it is degradable, how the maximum yield of methane can be obtained, and also how the overall process can be made cost effective.
Treatment of Wastes from Sugar Industry:
Methods of Treatment:
Distillery wastes in the form of ‘spent wash’ are amongst the worst pollutants produced by industries, both in magnitude and in strength. For every litre of alcohol manufacture, 10-15 times of spent wastes are discharged as spent wash with high BOD.
These cause heavy pollution in receiving waters as well as in land of application. The heavy capital outlay involved in treatment has put the distilleries in dilemma about the course of action.
The spent wash creates a lot of problems and some are listed here:
Effects of Pollution:
Effects of pollution in the natural streams –
(i) Lowering of pH value of the stream.
(ii) Increase in organic load.
(iii) Depletion of oxygen content.
(iv) Discolouration.
(v) Destruction of aquatic life.
(vi) Bad smell.
Effects of pollution on land –
(i) Pollution of groundwater.
(ii) Charring of vegetation and crops.
(iii) Accumulation of salts.
(iv) Increase of salts.
(v) Increase in cropping period.
(vi) Increase in electrical conductivity of soil.
The general practice followed for the disposal of effluent from sugar and distillery industries is to let the wastewater into rivers or sea either without treatment or with partial treatment. In some places it is being treated and disposed off along with domestic wastewater.
In majority of the cases industries are located in places where there is no facility for the collection and disposal of domestic waste, following processes are used:
(i) Direct use after neutralisation and dilution for irrigation purposes.
(ii) Evaporation and incineration of spent wash for the recovery of potassium salt.
(iii) Ammonification process.
(iv) Manufacture of torula yeast from spent wash.
(v) Anaerobic treatment such as anaerobic lagoons and anaerobic digestion.
A comparative study of the aforementioned processes revealed that anaerobic treatment is the only promising method of treatment in present-day context. The treatment with anaerobic lagoon and conventional digesters are applied to some extent in the field.
With the development of processes with longer biological solids retention time approach, the present-day research has been directed in the application of these processes.
A brief review of different processes is presented here:
(i) Physico-chemical methods.
(ii) Biological treatment methods.
(iii) Other methods/systems.
(i) Physico-Chemical Treatment Methods:
In general, physico-chemical treatment of stillage has met with little success. Sedimentation is found to be unsatisfactory even with the addition of coagulants and other additives such as alum, lime, ferric chloride, other aluminium salts, iron, bentonite, activated charcoal and soap. Settling is also found to lead to anaerobic conditions and odour.
The stillage is flocculated by centrifugation aided by the auto-flocculating of the micro-organisms. The flocculated sludge was easily settled and found suitable for animal feed. AMINODAN – a commercial process removed minimum 70% BOD by using chemical coagulation and flotation technique.
Reverse osmosis on distillery effluent has been used by recycling water back to the corn mashing stage. Stillage of 12,000 mg/l total organic carbon is treated using a DRS-90 membrane and gave fermentation alcohol yields 5.8% higher than when fresh water is used.
By using an Osmetik RO unit with various membranes, stillage from a beet molasses plant with BOD of 10,000 mg/l can be treated by reverse osmosis. Of the feed volume 80% is recovered as filtrate with a BOD5 of 600 mg/l while the remaining 20% comprises the retentate containing 15% DS and after evaporation is suitable as a fodder additive. The filtrate can be used successfully as a molasses diluent.
Stilage could be treated by electro-flocculating at temperatures ranging from 49-82°C and pH values from 3.8-7.0 with-the addition of 0.5% NaCl can be preferred over electro dialysis and electro-osmosis which are too expensive and too limited in their degree of purification to be viable treatment alternatives.
(ii) Biological Treatment Methods:
The characteristics of distillery wastewater have COD/BOD ratio of 1.8-1.9 indicating thereby that wastewaters are amenable to biological treatment. Further, BOD/N/P ratio of 100/2.4/0.3 to 100/2.75/0.8 and high BOD of the distillery effluent indicates that anaerobic treatment methods are more suitable to any aerobic biological treatment.
Should the avenues of stillage utilisation and by-product recovery be closed? Biological treatment of stillage offers the only means of disposal. Additionally, the liquor remaining after by-product recovery is not generally suitable for discharge to a receiving water body and requires further treatment – biological treatment is often the most effective. Biological treatment methods comprise anaerobic and aerobic systems.
Anaerobic Reactors and Processes:
It is important to distinguish between reactor types and treatment processes. In the latter case, one or several reactors are included in a total treatment and scheme. Various reactor types are used for anaerobic processes.
Many of these have different names, which might result in the impression that there are numerous anaerobic reactor types. However, when looked upon the detail, the numbers of different reactor types are listed in Table 15.5. Anaerobic reactors can be combined and equipped with various sludge/water separators to a multitude of processes.
There are three basic processes, Fig. 15.1:
(a) Parallel processes are two or more anaerobic processes, each single process comprising both the acid and the methane step.
(b) Staged process means two (or more) anaerobic processes in series, each single process including both the acid and the methane step.
(c) Phased process mean and anaerobic process where the acid and the methane step is separated.
An important factor in process layout consideration is the solubility index of wastewater. Wastewaters with a high solubility index (0.8-1.0) or low solubility index (0.0-0.2) are basically treated in single pass processes whereas wastes with medium solubility index (0.2-0.8) in many cases could be treated advantageously in parallel process.
Phasing of the anaerobic process can be used in every case and might result in reduced total reactor volume and better process control possibilities. Sulphate rich wastes might result in reduced total reactor volume and better process control possibilities advantageously be phased or staged in order to confine a possible inhibition caused by hydrogen sulphide.
In a combined process, the inhibition will be effective on slow growing methane bacteria and thus result in a relatively higher increase in reactor volume. This comment also holds for other toxic materials, e.g., those present in pulp and paper industry washes.
Reactor types like fixed and moving beds are, in general, believed to withstand the effect of toxic compounds and other environmental factors better than expanded, fluidised and recycled beds plus sludge blanket reactors.
i. Fixed Bed Reactor:
The micro-organisms are attached to an inert media which can be any one of the media known for aerobic trickling filtres. The wastewater passes the bed with vertical flow (up or down flow). In all configurations, a substantial percentage of the biomass is present as suspended flocs entrapped in the voids of the inert media.
In general, this reactor type is operated without recycle. This results in a plug flow pattern in the reactor, although the gas production tends to stir up the flow pattern by rising bubbles. Recycle may be used to control biofilm thickness to a certain degree, or to overcome toxicity/pH problems.
Excess sludge is removed from the reactor together with treated wastewater. Fixed bed reactors are best suited to warm, strong wastewaters, mainly soluble. Production of microbial biomass is small (hence need for back washing is reduced) and surplus sludge and power requirements are lower than for competitive aerobic systems.
Although anaerobic up-flow filtres give higher BOD removals than anaerobic disasters at the same loadings, they require recalculation and as such have higher capital operating cost.
Or alternatively, use two-stage fixed-film reactor. A waste similar to raw stillage, with and artificial land leachate of pH 5.4 and COD of 54,000 mg/l containing mainly fatty acids with some protein, tanneries and high molecular weight carbohydrates. It is possible to effect COD removal from 53% to 98%.
These removals occurred at retention times from 75 to 74 days corresponding to recalculation ratios (recycle: fresh feed) of 1.4 to 34 in an anaerobic up-flow filter (height/diameter = 10.4). It would appear that anaerobic up-flow filters offer one unexplored avenue for stillage treatment.
Long term experiments have been conducted on two staged fixed film reactor for the treatment of spent wash from some distilleries in India. Some of the basic results are given in Table 15.6.
Various sizes of stones from 0.5″ to 1.5″ were tried as filter media. From experimental studies, it is revealed that 1″ stone media works reasonable well. Decreasing stone media below 1″ choked the filter bed, although removal rate constant was quite large.
A mathematical model is used to determine this rate constant for both the reactors and is of the order of 40 kg COD/m3.d for Reactor I and 20 kg COD/m3.d for Reactor II.
Tracer studies conducted by using:
(i) Impulse dose from which level of mixing was determined. Dispersion coefficient D/UL varied from 0.0257 to 0.0464 and
(ii) Step decrease dose determined the behaviour of the system, viz.-
(a) Stagnant zone – 25 to 28%
(b) Mixed zone – 40 to 52%
(c) Plug flow zone – 30 to 35%
ii. Moving Bed Reactor:
The micro-organisms are attached to an inert (plastic) media which is moved through the, wastewater. An example of this type of reactor is the RBC. The media can be partially or fully submerged.
The velocity between the media and the wastewater gives some sort of biofilm thickness control. Excess sludge leaves the reactor together with treated wastewater. Aerobic RBC appears to offer little practical advantage over trickling filter.
iii. Fluidised Bed Reactor:
The micro-organisms are attached to an inert media which might be sand, activated carbon or garnet. The biofilm covered media is fluidised by a high vertical velocity, demanding a very high degree of recycle.
The single particles almost have a fixed position in the bed, but are gently moved around. Still, each particle tends to stay located within a rather small bed volume. The bed expansion is controlled by vertical velocity and the recycle outlet level.
Biofilm thickness is controlled by the led regeneration strategy and by the size and density of the inert media in combination with vertical velocity. The gas production may create foaming and flotation at top of the reactor. This must be controlled by hydraulic or by mechanical means in order to prevent particle escape together with the treated wastewater.
Excess sludge can be removed from the bed regeneration stream taken out from the top of the fluidised bed where biofilm tends to have maximum thickness. No investigations have been carried out for this system. It holds a future promise with high organic concentration wastewater because of high methane yield.
iv. Expanded Bed:
The micro-organisms are attached to an inert support media, which might be sand, gravel, anthracite or plastic. The diameter of the media is comparable to that used in fluidised bed but often slightly bigger.
The biofilm covered media is expanded by sufficiently high vertical velocity, obtained by a high degree of recycle. The bed expansion is kept at a level where all particles still keep their place within the bed. This system has not been tried for distillery waste.
v. Recycled Bed:
In recycled bed reactor, the micro-organisms are attached to an inert media which could be sand, anthracite, iron particles, etc. A substantial part of the biomass is found in suspended flocs.
The bed is kept suspended by mechanical stirring and gas stirring. Separation of the bed and waste water is performed in a separator from which the bed with the attached biofilm and suspended settled flocs are recycled to the reactor.
Excess sludge can be drawn from the bed recycle pipe. The separation of solids and liquid is the key point in this reactor type as in several others because separation of gas producing particles is difficult. Gas stripping or cooling of the influent to the separator are two methods which can be used to control this problem.
vi. Recycled Flocs (Contact Reactor):
The reactor is basically identical to the recycled bed reactor except for the inert media. This means that in micro-organisms have to form flocs in order to stay in the reactor. Inert particles in wastewater may act as carrier material and turn the reactor into a recycled bed reactor.
The separation of wastewater and flocs is the critical part of the reactor. Again, gas stripping or cooling helps to control the separation. This reactor offers immense possibilities of treating wastewater from distillery and sugar industry.
vii. Sludge Blanket Reactor (Clarigester Type Reactor):
The flocs are kept in suspension by the effect of gas bubbles. In order to avoid mechanical mixing, the wastewater must be evenly distributed over the bottom of the reactor. An often used technique is to distribute the incoming water through bottom inlet pipes.
These are by no means mandatory for sludge blanket generation. The flocs have a biofilm structure and are formed in granules of 1-5 mm diameter.
In the lower part of the reactor this dense layer of sludge granules is present. This result in sludge concentration profiles with considerable concentration, variation within the reactor.
Some sort of the reactor separation, either internal (in the reactor), or external is needed. This system needs further investigation for the treatment of high strength distillery waste effluents.
viii. Conventional Digesters:
The high dissolved BOD5 of stillage generally precludes aerobic treatment on economic grounds. Anaerobic treatment, however, offers the capability of heating raw, undiluted stillage producing only low quantities of sludge and giving an effluent that is readily polished in an aerobic system. In some situations, particularly, when the waste stream has a large inorganic salt, dilution may be advantageous.
ix. Open Lagoons:
Anaerobic lagooning is a cheap and effective method of treatment of waste water and offers an alternative in terms of capital and operating costs, provided the land is available and inexpensive.
Many distilleries, however, are in sugar or grain growing areas and the cost of land near the distillery may be high. Other distilleries are in urban areas where in addition to the high land costs, odour problems are likely to be encountered.
x. Anaerobic Activated Sludge Process:
This process has been developed with the objective of conceiving active methane formers and achieving high MCRT with low HRT for maximising substrate utilisation. The sludge formed during anaerobic digestion is light in weight due to mineralisation and bulky due to entrained biogas and accordingly tends to float and escape with supernatant causing substantial loss of methanogens. Recalculation of this sludge maintains a sound population dynamics of methane gas and achieves active digestion.
xi. Acid Methane Segregation Process:
Methane and acid formation from laboratory scale units yielded spent wash with 95% BOD5 removal. The maximum yield coefficient, maximum substrate utilisation/mass of micro-organisms, half velocity constant and decay coefficient have been found for both methane formers and acid formers.
Types of Aerobic Treatment Process:
Aerobic biological treatment can be achieved by any of the following processes:
(a) Trickling filters
(b) Activated sludge systems
Oxidation pond cannot be used as the wastewater is dark brown in colour due to presence of humus and would retard photosynthetic action of algae.
(a) Trickling Filters – Trickling filters are generally used in two roles, either prior to an activated sludge or other aerobic process or to polish the effluent from some other treatment process.
(b) Activated Sludge Process – Direct treatment of stillage using activated sludge process is rarely cost effective. In most cases, activated sludge treatment follows dilution of the raw stillage or is used subsequent to fodder yeast growth.
(iii) Other Methods/Systems:
i. Ammonification and Nitrification:
This process is dependent on ammonifying and nitrifying bacteria which are sensitive, slow growing, temperature and pH dependent. This process suffers when BOD5 exceeds 15,000 mg/l and needs dilution of wastewater around the year. This method also does not offer any possibility of energy recovery.
ii. Drying:
Spent wash is dried in open shallow pits and needs large surface area. Sediments can be removed and used as a fertiliser. Odour nuisance could be expected besides causing groundwater pollution. There is no scope for energy recovery.
Spent Wash Treatment with Municipal Waste:
The treatment of combined stillage and municipal wastewaters has received scant attention due to the fact that such treatment requires a large sewage to stillage flow ratio to give the required dilution for ensured success. This excludes the use of this disposal procedure in low population density regions.
A laboratory scale activated sludge plant can be used for this purpose. About 25% stillage in municipal sewage with 21,440 mg/l COD gave only 1.3% reduction in COD in 9 hours whilst a 10% stillage in sewage solution averaged 13.9% COD reduction. Best result of 22.8% COD removal in 4.5 hours is obtained with combined 10% stillage feed pre-neutralised to pH 7.2-7.4.
The combined waste of 355 mg/c BOD5 can be treated in an activated sludge unit for 2 hours resulting in an average BOD5 removal of 80%. There may be no bulking/foaming problems through excess gas production in the yeast waste water pipeline, which required lowering the temperature of this waste to at least 24°C and release of gas at high points in the lime once a day.
This also meant a considerable decrease in the associated odour problems. Pilot plant experiments in waters from rivers and tributaries used as waste outlets prior to treatment give encouraging results. Removal of 76% BOD5 is effected by an activated sludge plant in 45 minutes at a loading rate of 1.17 kg BOD5, /m3.d and a MLSS of 1.29 g/l.
The addition of yeast waste at 7% of the total BOD5 give better overall BOD5, removal than in the absence of this waste. Problems with bulking and foaming are encountered only in separate treatment of yeast wastes and no sulphur bacteria are detected. ‘Trickling filters’ can be used to treat 1% rum stillage in domestic sewage.
Disposal of Distillery Wastes:
Because of the geographical location of distilleries and the non-existence of proper collection and treatment system for domestic wastewater in the neighbouring towns, these distilleries in India are forced to dispose off their effluents either into inland waters or on to land.
The Indian Standards (IS: 2490, 1974; IS: 7968, 1976) require that the BOD and COD of the treated effluent should be brought down to 30 and 250 mg/l for disposal into inland waters.
The respective values for disposal on land should be less than 100 and 250 mg/l. None of the treatment alternatives achieve this target. Hence, further treatment or dilution before final disposal is essential.
Because of the large quantity of spent wash water and relatively high values of BOD and COD in anaerobically treated effluent, it is almost impossible to meet the dilution water requirements.
There are very few studies reported which attempt further reduction in BOD/COD of anaerobically treated spent wash water. Flow diagram 15.2 shows distillery waste treatment method.
Present Status of Treatability of Distillery Effluents:
Recent advances in solving the problem of disposal of liquid effluent generated by distilleries have led to the application of anaerobic retained biomass reactors. Several reactor configurations which promote biomass retention have been suggested. These include stationary fixed film, expanded or fluidised bed and up-flow sludge blanket reactors.
From operational point of view, particularly under the present day Indian conditions, stationary fixed film reactors appear to be more suitable for the treatment of distillery wastes. These reactors can be operated in either up-flow mode (Up-flow Stationary Fixed Film, USFF) or in down flow mode (Down Flow Stationary Fixed Film, DSFF).
A review of the literature indicate that most of the investigations were carried out employing USFF reactor while it is suggested that DSFF reactor may prove to be better for the treatment of distillery wastes. However, number of questions such as follows; remain to be answered before employing DSFF reactor for reducing the pollutional strength of the distillery effluents.
The questions are as follows:
(i) Is it viable to treat raw distillery waste (without any dilution) using DSFF anaerobic reactors?
(ii) Is it advantageous to dilute the waste before it is fed to the DSFF anaerobic reactors?
(iii) If the answer to above question is yes, then what should be the extent of dilution?
(iv) To what extent can the reduction is organic strength (BOD/COD) of wastes can be achieved by application of DSFF anaerobic reactors?
(v) How much energy (biogas) can be recovered in the process of reducing pollution due to these effluents?
In general, the literature on anaerobic treatment, particularly on the application of fixed film reactors emphasise the importance of several kinetic and process constants for rational design of the system.
In this regard following questions need to be answered:
(i) Are the reliable estimates of the various kinetic and process constants available for the anaerobic treatment of wastes?
(ii) Are these constants influenced by the substrate level (BOD, COD or VFA)?
Further, the main problem regarding safe disposal of effluents is to treat large quantities of highly organic wastes to very low organic concentrations to satisfy the standards. The anaerobic treatment alone cannot solve this problem. Hence, it is essential that further treatment of anaerobically treated effluents be carried out.
In this regard answers to the following questions are required:
(i) Is it possible to significantly reduce the organic strength of anaerobically treated effluents?
(ii) How is the process efficiency influenced by varying biological solids retention, time?
(iii) What are the values of kinetic constants which are required for rational process design?
(iv) How the settling characteristics of the mixed liquor change with change in biological solids retention time?
It is not feasible to treat undiluted waste using stationary fixed film anaerobic reactors. The waste can be treated to bring down the COD to 9000-10000 mg/l by feeding diluted waste in a stationary fixed film anaerobic reactor.
The optimum influent COD and hydraulic retention time appear to be close to 50000 mg/l and 10 days respectively. The dilution can be achieved with either raw water or by circulating the effluent of the stationary fixed film anaerobic reactors.
The separation of acid and methanogenic phase does not give any additional advantage at higher overall (including both phases) HRT < 10 days but may yield better results at lower HRT’s. The kinetic constants µm and K appear to be of less significance and the process kinetics may be defined by overall rate constant and should be determined for a specified influent COD. The reactor type (semi-continuous completely mixed anaerobic reactor or continuous flow completely mixed anaerobic reactor) seem to affect the evaluation of process kinetics.
The aerobic degradation of anaerobically treated distillery effluent can significantly reduce COD (∼ 67%) but it appears to be difficult to achieve the effluent COD less than 500 mg/l without dilution. The settling characteristics of the activated sludge developed from anaerobically treated distillery effluent depend on specific growth rate at which the reactor is maintained and improve with increase in BSRT.
Thus this examines the extent of pollution created by distilleries and the different methods available for the treatment and disposal of distillery wastes. In the present day context, fixed film anaerobic reactors appear to be the only choice for reduction in pollutional strength of distilleries.
However, process design and control criteria for full exploitation of these reactors are yet to be developed. This is because of the lack of information on various aspects such as desirable influent COD, optimum level of volatile fatty acids concentration in reactor, the reliable estimates of the kinetic constants and their dependence on the substrate levels (COD or VFA) etc.
Hence, investigations on these lines are warranted. It is also indicated that it is not possible to bring down the BOD/COD levels to acceptable standards by anaerobic treatment alone and not many studies have been conducted on further treatment of anaerobically treated distillery effluents. Thus this area is also open for research.