|Technologies for the treatment of molasses based distillery effluents - Effluent Generation|
5. Effluent Generation
The wastewater (effluent) generated in a distillery is of two types viz. process wastewater and non-process wastewater. The non-process wastewater is comparatively pure and as such can be recycled. The process wastewaters of a distillery consist of fermenter sludge, spent lees and spent wash. Spent less is usually recycled. Fermenter Sludge has a higher Biochemical Oxygen Demand (BOD) and a lower volume as compared to spent wash. It is advisable to dewater fermenter sludge and dispose it off without mixing it with spent wash as it will increase the BOD of the receiving stream.
The amount of effluent (wastewater) generated in a distillery depends upon the extent of process water used and the technology adopted for the manufacture of alcohol. Most of the Indian distilleries are of the conventional batch type in which about 15 KL of spent wash is produced per KL of alcohol. In the modern continuous type distillery, spent wash generation is of the order of 10 to 12 KL/KL of alcohol (5 to 6 KL/KL of alcohol if a reboiler is used).
6. Characteristics of Indian Distillery Effluent
Alcohol can be produced from a number of substrates such as cane molasses, beet molasses, starchy substances. The characteristics of spent wash depend upon the substrate used, quality of the substrate and the manufacturing process adopted. In India, alcohol is primarily produced from cane molasses. The cane molasses spent wash has a higher BOD level as compared to beet molasses spent wash and as such more difficult to treat as compared to beet molasses spent wash. Among countries using cane molasses for the production of alcohol, the Indian cane molasses and, in turn, spent wash are the worst in quality with respect to environmental parameters. He spent wash produced by the Indian distilleries is one of the most difficult distillery effluents to treat.
Spent wash has a temperature of about 90C to 100C with a smell of burnt sugar. It is highly acidic (pH between 4.3 and 5.3) and dark brown in colour. It contains a high percentage of dissolved organic and inorganic matter with a BOD in the range of 45,000 to 60,000 mg/l.
7. The Requirements of Effluent Quality (After Treatment)
Under the The Environmental (Protection) Act, the Government has specified Minimum National Standards (MINAS) for different industries taking into account the characteristics of the effluent and the minimum acceptable quality of the treated effluent. The standards for the distillery industry stipulate that the treated effluent should have a pH in the region of 5.5-9.0, maximum BOD level of 30 mg/I for disposal into inland surface waters and 100 mg/l for disposal on land. It also states that all efforts should be made to remove colour and unpleasant odour as far as practicable.
8. Technologies For the Treatment of Distillery Effluent
Industrial effluents may be treated by a number of methods, either singly or in combination. The selection of the effluent treatment route or approach is dependent upon:-
- Effluent characteristics (before treatment)
In general, physico-chemical treatment of distillery spent wash has met with litter success. There have been some attempts to use spent wash as substrate for yeast growth or for biochemical production. It is reported that the remaining BOD content after using spent wash for yeast growth or biochemical production is still high necessitating further treatment which would be difficult as all easily degradable organics are already consumed.
Concentration of spent wash and its usage as an animal feed additive is a common practice among countries producing alcohol from beet molasses (Europe, North America). This practice has not found acceptance because Indian spent wash contains a higher percentage of inorganic substance which produce a laxative effect if the consumption of feed is not closely monitored. Besides, in Indian conditions, the cost of concentration of spent wash is prohibitive.
The following technologies have been discussed in detail:
9. World Trends
Anaerobic digestion (biomethanation/methane recovery processes), concentration of spent wash for usage as a animal feed ingredient, concentration followed by incineration, land application, composting etc. are the common methods of treating distillery spent wash in different parts of the World. Back