Treatment and resource recovery options for first and second generation bioethanol spentwash - A review

A decline in the availability of fossil fuel resources coupled with deleterious environmental concerns has prompted further research into biofuels. Conventional bioethanol production via a first-generation approach may soon become superseded through integration with lignocellulosic feedstocks. However, the underlying concerns pertaining to the disposal of high-strength liquid waste (i.e. spentwash) remain both unchanged and constitute a substantial cost to bioethanol manufacturers. Therefore this review details current efforts in the literature to elucidate various approaches for spentwash treatment and investigate the potential for resource recovery. Insight into the composition of distillery wastewater is given in the lead-up to a thorough discussion encompassing the origin, transformation and characterisation of the highly problematic melanoidin compounds entrained within this effluent. Close examination of advanced organic characterisation methods used by researches yields further insight into the nature of spentwash dissolved organic matter (DOM). Employment of both biological and physio-chemical treatment schemes to alleviate the environmental footprint of such high-strength wastewater are also reviewed. Opportunities to dramatically improve the economic viability of biofuel production by exploiting the potential for resource recovery in the form of energy, organic/inorganic constituents and effluent reuse are discussed. Overall, the review culminates by highlighting recommendations for future work to accelerate the onset of an environmentally benign bio-refinery.

Effect of distillery spentwash fertigation on crop growth, yield, and accumulation of potentially toxic elements in rice

The safe disposal of industrial effluents always remained a challenging process because of their high level of nutrients, toxic elements, and salts. A pot experiment was conducted to investigate the effects of various concentrations (5%, 10%, 15%, and 20%) of sugar industry effluent spentwash (SW) fertigated with tab water (TW), on soil properties, crop growth, physiological parameters, yield components, and accumulation of potentially toxic elements (PTEs) in rice (Oryza sativa L.) grains and straw. The results showed that soil physico-chemical properties were modified with rise in SW concentration. Application of 5% SW significantly enhanced the plant growth, and yield components. Photosynthesis rate, transpiration rate, and stomatal conductance were significantly higher under 5% SW concentration in comparison with control. However, SW concentrations of > 5% showed inhibitory effects for all growth, physiological, and yield components. Accumulation of PTEs showed increasing trend with rise in SW concentration. However, under 5% SW concentration, all the PTEs in rice grain and straw were within the permissible limits (PLs) recommended by FAO/WHO and no health hazards were detected by health risk assessment. Based on the study results, 5% SW fertigation with TW can be applied as fertilizer for enhancing the growth and productivity of rice. Graphical abstract.

Development of high-performance supercapacitor electrode derived from sugar industry spent wash waste

This study aims at developing supercapacitor materials from sugar and distillery industry wastes, thereby mediating waste disposal problem through reuse. In a two-step process, biomethanated spent wash (BMSW) was acid treated to produce solid waste sludge and waste water with significantly reduced total organic carbon (TOC) and biological oxygen demand (BOD) content. Further, waste sludge was directly calcined in presence of activating agent ZnCl₂ in inert atmosphere resulting in high surface area (730-900m²g^-1) carbon of unique hexagonal morphology. Present technique resulted in achieving two-faceted target of liquid-solid waste remediation and production of high-performance carbon material. The resulted high surface area carbon was tested in both three and two electrode systems. Electrochemical tests viz. cyclic voltammetry, galvanostatic charge-discharge and impedance measurement were carried out in aqueous KOH electrolyte yielding specific capacitance as high as 120Fg^-1, whereas all solid supercapacitor devised using PVA/H₃PO₄ polyelectrolyte showed stable capacitance of 105Fg^-1 at 0.2Ag^-1. The presence of transition metal particles and hetero-atoms on carbon surface were confirmed by XPS, EDX and TEM analysis which enhanced the conductivity and imparted pseudocapacitance to some extent into the working electrode. The present study successfully demonstrated production of high-performance electrode material from dirtiest wastewater making process green, sustainable and economically viable.

Effect of sugar industry spentwash (diluted) on the characteristics of soil and sugarcane (Saccharum officinarum L.) growth in the subtropical environment of Sindh, Pakistan

Spentwash is a rich source of organic matter and essential plant nutrients in addition to excess salts. Sugar mills in Pakistan discharge about 3.48 million tons of spentwash annually, with no proper procedures for its disposal or utilization. To test the effect of diluted spentwash on soil and the soil's ability to support plant growth, sugarcane (Saccharum officinarum L.) variety CPF-237 was planted. The experiment was conducted in a randomized complete block design involving factorial combination of four concentrations of each spentwash (0, 10, 20, and 30%) and mineral fertilizers (0, 1/3, 2/3, and the full recommended rate of NP). The 10% spentwash plus 2/3 mineral fertilizer treatment substituted 33% each of N and P and 100% of K, saving mineral fertilizer cost (Rs. 48600 ∼ US$458) in addition to 29.54% increase in yield over full NP fertilizer. The same treatment also improved the soil organic matter (65%), N (20%), P (25%), and K (230%) over full NP treatment alone. An increase in the salt content of the soil was detected within the prescribed limits, with the exception of HCO₃.

Microbial decolorization of spentwash: a review

Spentwash is one of the most complex and cumbersome wastewater with very high BOD, COD and other organic and inorganic toxic constituents. It is dark brown colored and difficult to treat by normal biological process such as activated sludge or anaerobic lagooning. The color is due to the presence of melanoidins, caramels and other polymers. These compounds have anti oxidant properties which render them toxic to microorganisms. Spentwash disposal into the environment is hazardous and has a considerable pollution potential. It affects the aesthetic merit. Its decolorization by physical or chemical methods have been investigated and were found unsuitable. In the recent past, increasing attention has been directed towards utilizing microbial activity for decolorization of spentwash. This review reveals various groups of microorganisms which have potential in spentwash decolorization. The role of enzymes in decolorization and the microbial degradation of individual compounds imparting color to spentwash are also discussed.