After reading this article you will learn about:- 1. Introduction to Waste Discharge from Distilleries 2. Characteristics of Wastewater Discharged from Distilleries 3. Problems Associated with Waste Treatment 4. Possible Solutions and Remedial Strategy for Treatment of Waste from Distilleries.
Introduction to Waste Discharge from Distilleries:
Distillery is one of the polluting industries in India. Spent wash generated by distilleries is dark brown in colour and therefore immediately noticed by the nearby population if let out in the river even after necessary biological treatment.
If the ground water table is very shallow in a particular area, like Indo Gangetic Plains, ground water contamination is even quicker. Many such cases have been reported in this area about contamination. Spent wash stored in unlined lagoons in distillery percolate into ground water table, and bore wells discharge light tea coloured sharbat in place of colourless drinking water.
Hardness of such well water is more than 2000 mg/L at times. Due to frequent occurrences of such incidents and more due to depleting reserves of clean drinking water, people have become more conscious about distillery waste disposal. One can see very frequent news items appearing in newspapers about health hazard and environmental degradation due to untreated or partially treated distillery effluents.
Every distillery owner is now giving equal importance as that of manufacturing unit to effluent treatment plant (ETP). Unfortunately, howsoever, sophisticated may be the ETP, distillery wastes are always associated with lots of difficult problems encountered while handling them. Based on our own experience in distillery ETPs, an attempt is made in this paper to high light such problems and remedial solutions available, if any for them.
Alcohol Manufacturing Process:
Molasses is the mother-liquor left-over after crystallization of sugar from concentrated cane juice. It is used as raw material in the distilleries for producing ethyl alcohol from the fermentable sugars contained in molasses.
In traditional batch type, fermentation process, number of fermenters are in operation and they are decanted in batches one after the other. In this process, molasses with solids-content of about 76-90% is diluted up to 20-25% solids and fed to the fermentation tank where it is inoculated with propagated yeast culture in about 10 : 1 proportion. Fermentation in the fermenter continues for about 20 to 24 hours after the final inoculation is added to it.
The basic reaction in the fermentation process is:
C6H12O6 → 2C2H5OH + 2CO2 + 26 Calories
Fermented wash which is the main product of this step is decanted and the remaining sludge known as yeast sludge is discharged from the bottom of the fermenters to the pollution load from distilleries. The fermented wash containing 6-9% alcohol is preheated to about 90° C by exchanging heat with the spent wash flowing out of the analyzer column after alcohol distillation, and is then fed to the top of the analyser column for steam stripping of alcohol, which condenses at the top of the column as 40% alcohol.
The down-coming discharge from the column is known as spent wash and it contributes maximum pollution load from the distillery. The 40% alcohol stream from the top of the analyzer column is next fed to the bottom of the rectifier column where the temperature is maintained at about 95° to 100°C. Water and alcohol vapour get condensed at different levels in this column and rectified spirit (95%) is withdrawn leaving behind the spent lees.
New distilleries are now mostly based on continuous fermentation process which is the latest. The basic difference in the continuous fermentation process, as compared to batch type fermentation, is that there is one or fixed number of fermenters irrespective of distillery capacity. Continuous fermentation process started with use of reboiler where the effluent volume would be one-third that of the conventional batch type distillery and the COD/BOD concentration would be almost 2.5-3 times higher.
Due to various operating difficulties, most of the continuous fermentation distilleries are now operating without using reboiler. Where reboiler is not adopted the characteristics of spent wash are slightly concentrated and the volume is reduced by about 20% as compared to batch type fermentation. Alcohol production from starchy material is also practised in India but on a very limited scale. Hence, the problems associated with its spent wash are not covered in this paper.
Characteristics of Wastewater Discharged from Distilleries:
The major sources of process wastewater from molasses-based distillieries are:
(a) Yeast sludge from fermenters or molasses sludge from clarifiers meant for settling of fermented wash;
(b) Spent wash from the analyser column;
(c) And Spent lees from the rectifier column.
The volume of fermenter sludge commonly called yeast sludge is about 0.3 L/L of rectified spirit produced. This sludge has solid content of about 30% by weight which comprises mostly of the spent yeast and mineral matter. The spent yeast is highly biodegradable and has BOD of up to 25000 mg/L. The composition of the yeast sludge generated from a typical batch type fermentation-based distillery is given Table 1.
Spent wash is the principal polluted stream generated from distilleries. Spent wash stream is a continuous process effluent and its volume is about 12-15 L/L of rectified spirit produced. A typical analysis of the spent wash is presented in Table 2. Spent less generation from the rectifier column is continuous and is usually recycled.
If not recycled, generation is usually at the rate of 1.7-2.0 L/L of rectified spirit product. BOD/COD values of the spent less are usually low and depend on the alcohol and/or fuel oil contamination in it. This waste stream is hot and colourless with very less suspended solids in it.
Problems Associated with Waste Treatment:
For the sake of presentation, three problem zones have been identified viz. Yeast sludge handling, spent wash treatment and disposal of treated spent wash. Each problem zone is further subdivided into problem area and then specific problems are discussed in detail.
Necessity of further research work is also highlighted if techno-economical solutions are not available as on date. In batch fermentation process, yeast sludge is collected from bottom of fermentors after settlement of fermented wash while in continuous fermentation, it is collected from Lamella clarifiers.
The characterisitics of the sludge in both cases are more or less similar so also the problems associated with it. The spent less generated is usually mixed with the spent wash or let out separately. No specific treatment plant is generally given for spent lees treatment.
Recently, many industries are discharging spent lees in the secondary treatment plant. Three major options are available for treatment of spent wash, i.e., composting, concentration and incineration, and biological treatment.
Concentration and incineration is not practised in India currently because of operational problems and high O & M cost. Composting has got limited applications since land and press mud should be available in a plenty in nearby area. Biological treatment is the most preferred and widely accepted practice of spent wash treatment in India and hence only this option is considered in this paper.
The generalised treatment sequence under this option is as given below:
Yeast Sludge Handling:
In batch type fermentation distillery, yeast culture is transferred to fermenters where diluted molasses undergoes fermentation for a stipulated period of 24-30 hours. Once the fermentation is over agitation in the fermenter due to CO2 generation also subsides and sedimentation of sludge takes place. The supernatant fermented wash is then pumped to distillation column.
The settled sludge in the fermenter is discarded and discharged separately which contains mostly yeast sludge and molasses sludge to a great extent. Unfortunately, withdrawal of supernatant from the fermenter is not done efficiently i.e., without creating turbulence in sludge zone. Lot of solids find their way into distillation column and ultimately come along with spent wash. Moreover, dissolved solids in the spent wash precipitates at higher temperature in the distillation column which further increases the inorganic sludge content in the spent wash.
In continuous fermention, there is no separate yeast sludge stream as such. However, fermented wash is generally taken to Lamella clarifiers where yeast sludge and molasses sludge is removed from fermented wash prior to its feeding to distillation column. Many times the settling efficiency of these clarifiers is very low due to various reasons and solids are carried over along with supernatant liquid ultimately reaching spent wash.
The underflow from carry clarifiers, which is the sludge stream, many times, carry clarified fermented wash also while draining the sludge and gets diluted due to carry over of liquid along with sludge and created problem of handling due to increased volume. The nature and settling characteristics of yeast sludge are not fully understood/explored so the best treatment option is yet to be evolved. (Table 3).
Yeast sludge stream is allowed to pass through a series of pits and sludge is allowed to settle in the pits. The settled sludge is then periodically removed manually and disposed of as it is. The sludge so collected still contains appreciable quantity of moisture and possess odour nuisance and also handling problems.
Centrifuge filters press etc. If used for yeast sludge dewatering involve huge O&M cost and may not be cost-effective. No commercially proven technologies are available so that the sludge stream is concentrated, thickened and dewatered before it is put into usable applications.
Today, possibly the only use of semi-dried sludge, being practised in India is to use it for cattle feeding or land conditioning. Similarly, as on date, no reliable technologies are available in Indian market which can cost effectively produce by-products of a good commercial value from yeast sludge making it revenue generating system.
Spent Wash Treatment:
The problem zone is sub-divided into 2 problem areas viz., anaerobic treatment and aerobic treatment.
Each of the problem area is discussed in detail as follows:
This is the most popular and widely accepted form of treatment adopted in distilleries. Almost 80-90% of distillery ETPs in India has anaerobic reactors. Variety of anaerobic systems are in operation on commercial scale. Biogas generated through the anaerobic reactors made the system pay back for its cost within 2-3 years.
The anaerobic treatment is associated with various problems commonly observed as given below:
The quality of molasses varies depending on the sugar recovery in sugar mill. Lower the recovery in sugar mills, better is the grade of molasses produced. The molasses are graded based on its TRS content. More the TRS, better is the yield in distillery and less is the pollutional load in spent wash.
If the TRS content is low, more molasses (that to O with higher non-fermentable matter) is required to produce one litre of alcohol resulting in higher effluent volumes and higher COD values. Obviously as molasses source changes the FS content and characteristics of spent wash change affecting the treatment system. Unfortunately procurement of molasses from different sources is inevitable in distillery industry and so also this problem.
It is also observed that if the fresh molasses is used for distillation, the spent wash generated creates many process problems such as excess solids loading and foaming in anaerobic reactor. It is, therefore, advisable to store the molasses for some-time, allow some ageing and stratification to take place in molasses tanks and then use it for fermentation.
Since, the waste is highly complex in nature and contains very high COD and TDS concentration, it offers a limited removal of BOD and COD through anaerobic route. Only up to 85-90% of the BOD and 60-70% of COD can be removed in anaerobic reactors even with a detention time of up to 8-10 days. The residual 10-15% BOD is very significant as it would mean input BOD concentration in the range of 5000-7500 mg/L for subsequent secondary treatment. In such cases BOD values may be less than the above given values.
Yeast is highly proteinous in nature and undergoes biodegradation in anaerobic phase producing various products which are foam-conducive. Continuous yeast ingress into anaerobic reactor is found to generate foam. In addition to this, yeast sludge also exerts a high organic load on anaerobic reactors resulting overloading of the treatment system.
Yeast sludge, molasses sludge and inorganic solids post-precipitated after distillation and entered in spent wash settle at the bottom of the anaerobic reactor and create problems like choking outlet weirs in case of down flow reactors and disturbing sludge blanket in case of UASB reactors also leading to rise in pH, and foaming in reactors.
Since lot of sulphur compounds are used in the manufacturing process of sugar; and sulphuric acid is used for pH adjustment of diluted molasses in distillery, sulphate concentration in spent wash is very high and may range between 4000-8000 mg/L. Due to reduction of sulphates to sulphides in anaerobic reactor, free H2S content in the digester liquid increases and if it crosses tolerable limits, it exerts free H2S toxicity to methanogens. Consequently H2S content in the bio-gas also increases leading to various problems.
H2S content in biogas beyond 2000 mg/Nm3 is not acceptable to biogas engines and also create corrosion problem or deposition on boiler tubes. Biogas becomes highly corrosive due to H2S content in presence of air and warrants use of special materials of construction (MOC) at such places. During start up periods, biogas is usually vented through flare stack and H2S content aggravates the odour problem to such an extent that the local residents around the factory start feeling insecure and go for agitation.
Presently, the biogas is utilised in boilers substituting for coal or furnace oil etc. to produce steam. However, that is not the most efficient utilisation of the biogas. Few distilleries have gone in for better utilisation of biogas where it is burnt in high pressure boilers and the steam produced is used to drive turbine to produce captive power. The exhaust steam from the turbine is then used for distillation.
It is found that the efficiency of biogas utilisation is maximum using biogas engines, more in high pressure boiler-turbine-distillation route and lowest in direct burning in low pressure boiler to produce steam. Unfortunately, biogas engines are not in practice in distillery ETPs due to limitation on allowable H2S content in biogas. Boiler-turbine-route is economical only if the distillery capacity is 40 KL and above.
During shutdowns, though there is no fresh feed of spent wash given to the anaerobic reactor, appreciable quantity of biogas is produced in initial few days as plenty of food is still available in the reactor. Unfortunately, it cannot be burnt in boiler as that is also under shutdown. The biogas is burnt at flare stack during such periods without its proper utilisation.
At present no gas flow meter is available which gives the biogas flow accurately over a long period of use. Among the available options. Orifice type, Thermal type and Vortex type are more popular. All these flow meters suffer from one or the other problem.
In case of Orifice type flow meter serious errors are induced whenever the physical properties of the biogas, like viscosity or specific gravity, change due to variations in CH4 content. Thermal type flow meter performance is badly affected by the moisture content of biogas and due to deposition of muck on its sensor.
Vortex type flow meter requires a certain minimum flow so as to create vortices for flow measurement. It would not work if the deposition of much take place between the pipe wall and bluff body. Incidentally, a little quantity of much is always present in the biogas.
The post-anaerobic effluent will has BOD in the range of 5000-7500 mg/L which is required to be brought below 100 mg/L for disposal on land or below 30 mg/L for disposal in water bodies through secondary treatment. It is impossible to get BOD up to 30 mg/L through biological treatment whatsoever may be the treatment system. Probably it is due to the fact that the most difficult biodegradable portion is left out at the end of the treatment.
Further, anaerobic treatment of anaerobic effluent, with 5000-7500 mg/L BOD is not economically viable as even additional 10% BOD removal would need 4-6 days HRT. Fixed film aerobic system such as bio filter may not work efficiently for such high BOD effluents as oxygen transfer through fixed film becomes limited. The only option, therefore, left out is activated sludge process. In this process the power requirement offset the savings earned in anaerobic phase.
In the aerobic treatment process, BOD of the influent more than 5000 mg/L may not be tolerated since it affects oxygen transfer efficiency. Further, BOD of the anaerobic effluent always fluctuates depending on the influent characteristics and process efficiency subjecting the aerobic unit to shock loading and thus decreasing the efficiency.
The design of the secondary treatment system also needs to be on conservative side due to low biodegradability of the anaerobic effluent. The reason for this is very high ratio of refractory matter to biodegradable matter in the waste. COD/BOD ratio of 2.5-2.8 of spent wash prior to anaerobic treatment becomes 5—6 after anaerobic treatment. Very high TDS mostly inorganic in nature makes oxygen transfer limited in aeration tanks.
Hardly any research work is reported in literature on aerobic treatment of post anaerobic distillery effluent. Hence, the experience is also limited in this field. Another severe problem faced by secondary treatment systems is foaming in aeration tanks specially when surface aeration is employed. Exact reasons of foaming are not known but major contribution is due to the characteristics of waste itself.
Treated Spent Wash Disposal-Pollution Control Boards (PCBs) nowadays are insisting for acquiring land by the distilleries if land disposal is the selected option for distillery waste disposal. It is necessary, therefore, to have a storage lagoon followed by acres of land for irrigation using treated spent wash having BOD less than 100 mg/L. Though 2-staged aerobic treatment brings BOD to acceptable limits.
Total dissolved solids (TDS) and Sodium Absorption Ration (SAR) may not be within the acceptable limits or irrigation on land. Till date no definite guidelines are available for recommending application rates of treated spent wash over land for irrigation. Long term effects of such irrigation on soil characteristics are still not evaluated and made available to public. Possible contamination of ground water table due to treated spent wash especially in terms of colour is not ruled out in case of land application.
Other simple option for disposal would be discharge into river, but it calls for BOD less than 30 mg/L and COD less than 250 mg/L. It is very difficult or near impossible to achieve BOD below 30 mg/L by aerobic biological treatment. Moreover, achieving COD below 250 mg/L is impossible even after tertiary treatment.
Even if the BOD is brought down below 100 mg/L colour of the effluent is still objectionable. To bring the colour to acceptable limit, tertiary treatment is to be opted for. Tertiary treatment in the form of either membrane filtration, physico-chemical treatment of oxidation using chlorine/ H2O2 very costly and may range from Rs. 1.5 to Rs. 2.0/L of alcohol produced. Handling of by-products from the treatment is another problem in itself and unless utility for such by-products or end products is found out, its handling would be newly emerged problem.
Possible Solutions and Remedial Strategy for Treatment of Waste from Distilleries:
While there are many problems involved in distillery waste treatment as outlined in Table- Ill, solutions are available for most of the problems as on date. In case of few problems, solutions could be worked out if concentrated efforts are put in for developmental work. This section discusses such solutions and identifies problem areas where further R&D work is required to arrive at proper solutions.
Yeast Sludge Handling:
Separation of yeast sludge or molasses sludge can be carried out in properly designed settling tanks. We have observed that yeast settling tanks (YST) with 12 hrs. detention and with proper inlet/outlet brought down total suspended solids (TSS) to acceptable levels for anaerobic treatment process. At the same time it was observed that due to removal of settleable solids, sulphates in spent wash were also reduced to some extent further improving the performance of anaerobic processes.
Well-designed conventional sludge drying beds under operation are successfully handling yeast sludge in a distillery in Western India. With a 7 days drying cycle, well drained semisolid sludge cakes are obtained which offer a great ease of handling and disposal.
If yeast sludge separated in YST is dried, it can be used for many suitable applications such as cattle feed supplement, land conditioner etc. It can act as a good coagulant or coagulant aid for physicochemical treatment of industrial waste waters. By-products such as activated carbon, amino acids, binder material or filler materials could be produced using dried yeast sludge. However, it would need more study to evaluate its cost-effectiveness.
Spent Wash Treatment:
Molasses procured from different sugar factories have different characteristics. Special care, therefore, is to be taken, otherwise it will show negative effect on both fermentation and effluent treatment units. Molasses before procurement could be analysed for the parameters such as non-fermentable sugars, sulphates etc. which affect not only the fermentation process but also the effluent treatment process.
Spent wash with high suspended solids tend to cause foaming in the anaerobic reactors. This can be avoided by ageing the molasses i.e., after a storage period of at least 2-3 months prior to fermentation in distillery. It was also found that the performance of anaerobic reactors improve greatly if aged molasses is used for alcohol production. Sulphate concentration in spent wash also found to be less if aged molasses is used.
If high sulphate concentration of spent wash is encountered even after using old molasses and allowing yeast sludge settling in YST, FeCl3 can be added to Raw Spent Wash (RSW) to precipitate sulphates but its excess usage is to be avoided. In case of sulphide toxicity reactor content can be diluted to some extent by addition of plain water.
Excess sulphate concentration in spent wash also leads to high H2S content in the biogas and other allied problems. Scrubbing the biogas with water or alkali as a H2S absorbent is a common solution for H2S removal. However, with this system, moisture content of the biogas could go up. Such system involves higher recurring costs. Regional Research Laboratory (RRL) Thiruvananthapuram has also developed a system which scrubes anaerobic reactor liquid (and no biogas) for H2S removal.
If distillery goes under shutdown, biogas generated can be used in canteen or can be utilised for domestic purpose. This can avoid smell nuisance due to unburnt H2S escaping through flare stack. Foaming in aeration tanks of secondary treatment plants of distilleries is quite common.
Though the reason for this is not known we may attribute this to the nature of the waste. Installation of slow speed aerators with proper submergence can reduce foaming to some extent. Further, reduction in foaming can be attained by using deformers which could be Turkey Red Oil (TRO) or silicon based defoamers. Addition of defoamers also enhances oxygen transfer further improving the process.
If the TDS concentration is high, oxygen transfer is hindered. To achieve proper oxygen transfer, TDS concentration should be reduced by adding plain water so as to maintain BOD of inlet to 1st stage aeration tank at 5000 mg/L. Based on our experience, we found that at such BOD concentrations, oxygen transfer is satisfactory.
Treated Spent Wash Disposal:
Land disposal of treated effluent from secondary treatment plant after reducing BOD to 100 mg/L is widely practised now a days, the loading rate is to be governed by the nature of soil or strata characteristics. If the percolation is minimum, it may form run off, and if the soil is highly porous, ground water table would be contaminated.
To achieve optimum loading rate leachate studies are to be conducted simulating the field conditions. Since the soild characteristics vary from place to place, leachate studies are to be conducted wherever soil nature is observed to have changed. Optimum loading rates can be determined along with the expected leachate characteristics in continuous study. This would help in estimating the pollution hazard to ground water table.