Wastewater treatment in molasses-based alcohol distilleries for COD and color removal: A review

Abstract

Molasses-based distilleries are one of the most polluting industries generating large volumes of high strength wastewater. Different processes covering anaerobic, aerobic as well as physico-chemical methods have been employed to treat this effluent. Anaerobic treatment is the most attractive primary treatment due to over 80% BOD removal combined with energy recovery in the form of biogas. Further treatment to reduce residual organic load and color includes various: (i) biological methods employing different fungi, bacteria and algae, and (ii) physico-chemical methods such as adsorption, coagulation/precipitation, oxidation and membrane filtration.

This work presents a review of the existing status and advances in biological and physico-chemical methods applied to the treatment of molasses-based distillery wastewater. Both laboratory and pilot/industrial studies have been considered. Furthermore, limitations in the existing processes have been summarized and potential areas for further investigations have been discussed.

Introduction

Ethanol manufacture from molasses generates large volumes of high strength wastewater that is of serious environmental concern. The effluent is characterized by extremely high chemical oxygen demand (COD) (80,000–100,000 mg/l) and biochemical oxygen demand (BOD) (40,000–50,000 mg/l), apart from low pH, strong odor and dark brown color (Central Pollution Control Board (CPCB), 1994, Central Pollution Control Board (CPCB), 2003). In India, which is the second-largest producer of ethanol in Asia with a projected annual production of about 2300 million liters in 2006–07 (Subramanian et al., 2005), alcohol distilleries are rated as one of the 17 most polluting industries. Apart from high organic content, distillery wastewater also contains nutrients in the form of nitrogen (1660–4200 mg/l), phosphorus (225–3038 mg/l) and potassium (9600–17,475 mg/l) (Mahimairaja and Bolan, 2004) that can lead to eutrophication of water bodies. Further, its dark color hinders photosynthesis by blocking sunlight and is therefore deleterious to aquatic life (FitzGibbon et al., 1998). Studies on water quality of a river contaminated with distillery effluent displayed high BOD values of 1600–21,000 mg/l within a 8 km radius (Baruah et al., 1993). Adequate treatment is therefore imperative before the effluent is discharged. In addition to pollution, increasingly stringent environmental regulations are forcing distilleries to improve existing treatment and also explore alternative methods of effluent management. For instance, Indian distilleries were stipulated to achieve zero discharge of spentwash to inland surface water by December 2005 (Uppal, 2004).

In an earlier review on this subject, Sheehan and Greenfield (1980) discussed treatment options practiced in the 1970s. More recently, Wilkie et al. (2000) have examined characteristics and anaerobic treatment of effluent obtained from different feedstock used for ethanol manufacture. This review focuses on the advances in molasses-based distillery wastewater treatment in the last two decades and the emerging technologies in this field.