Treatment of sugarcane molasses distillery wastewater is challenging due to the presence of complex phenolic compounds (melanoidins and polyphenols) possessing antioxidant properties. This work examines a combination of forward osmosis (FO) and adsorption to recover water and antioxidant compounds from distillery wastewater prior to conventional anaerobic digestion (AD) for biogas production.
Distillery wastewater was concentrated using commercial FO membranes (two thin film composite (TFC) and one cellulose triacetate (CTA) membrane) and magnesium chloride hexahydrate (MgCl2.6H2O) as draw solution. Initial experiments were done with 10% (v/v) melanoidins model feed. For distillery wastewater, maximum rejection of organics (in terms of chemical oxygen demand, COD), melanoidins and antioxidant capacity was obtained with 3 M MgCl2.6H2O. Performance of all three membranes was evaluated over 24 h FO experiments and over five cycles with membrane cleaning between cycles. The virgin and used membranes were characterized using zeta potential, scanning electron microscopy and Fourier transform infrared spectroscopy. The HTI-TFC membrane with average flux of 2.8 L/m2h, water recovery of 81% and antioxidant capacity rejection of 86% was rated the best in terms of water recovery, rejection and ease of cleaning.
The FO concentrated distillery wastewater was subjected to adsorption on polymeric XAD16 resin followed by sequential desorption with water-acidified ethanol-water. Batch adsorption-desorption tests were conducted with different concentrations of distillery wastewater (water recovery of 80%, 40%, 60% and 20% by FO) followed by column tests. The total recovery of melanoidins was lower (17%-33%) compared to polyphenols (40%-53%) across the FO concentrated distillery wastewater.
Based on the FO and adsorption results, four treatment schemes were evaluated to recover (i) water and biogas (ADS1: FO-AD); (ii) antioxidants and biogas (ADS2: adsorption-AD); (iii) water, antioxidants and biogas (ADS3: FOadsorption- AD; (iv) ADS4: adsorption-FO-AD). ADS4 was most promising with recovery of 91% polyphenols, 72% melanoidins and 72% water. Treatment schemes ADS2, ADS3 and ADS4 also resulted in higher methane production compared to anaerobic digestion of raw distillery wastewater. The adsorption- FO process has the potential to be integrated in the overall zero liquid discharge (ZLD) systems in practice in Indian distilleries.
Key Words of the Research: Distillery wastewater, Forward Osmosis, Adsorption, Antioxidants, Anaerobic digestion, Resource recovery.