Membrane Technology

Although there is no effluent limitation set for color in the regulations, the plant management intends to treat its wastewater further, to reduce public opposition to their effluents. The underlying motivation behind studying baker’s yeast wastewater as a feed for membrane processes is that the removal of color from this wastewater by other conventional processes has been proven difficult.

Chemical treatment using alum and some polyelectrolytes was found to be quite effective; but the associated operational costs were rather high. Physicochemical treatment alternatives, such as adsorption using activated carbon and oxidation using hypochlorite and hydrogen peroxide were also tested and found to be ineffective. Pressure driven membrane processes such as ultrafiltration (UF) and nanofiltraton (NF) found many applications in the field of water treatment in recent years. To date, there have been many applications of membrane processes for the treatment of colored effluents. Decolorizing wastewaters using various membrane-based pressure driven processes has been investigated by several researchers. MF and UF are low pressure membrane processes for separating suspended solids, macromolecules and colloids from a feed stream. MF as a pretreatment step can significantly enhance water quality with reasonable cost. Decolorizing textile desizing wastewater, municipal wastewater and groundwater supplies are some examples of the applications of membranes in this area. However, MF/UF membranes were not sufficiently effective to retain all the contaminants to ensure that the permeate composition complies with the strict COD discharge standards. Therefore, recently, NF has become a popular alternative process. It is claimed that, the performance of the membrane is partly controlled by the ion exchange capacity of the skin layer in NF. High rejections by NF open up the possibilities for process efficiency improvements particularly in the food industries as in the recovery of soybean oligosaccharides, olive oil vegetation water and biotechnology industries. The ultimate goal in the study is thus to further treat the plant’s wastewater towards the removal of color and COD. NF and UF type of membrane processes are adopted for the removal of these pollutants from the baker’s yeast wastewater. More than 83 % color removal achieved when five membranes with differing molecular weight “cut-off” were used.

Pre-treatment of spentwash with ceramic membranes prior to anaerobic digestion is reported to halve the COD from 36,000 to 18,000 mg/l. The total membrane area was 0.2m2 and the system was operated at a fluid velocity of 6.08 m/s and 0.5 bar Transmembrane pressure. In addition to COD reduction, the pre-treatment also improved the efficiency of the anaerobic process possibly due to the removal of inhibiting substances. Kumaresan et al. employed emulsion liquid membrane (ELM) technique in a batch process for spentwash treatment. Water–oil–water type of emulsion was used to separate and concentrate the solutes resulting in 86% and 97% decrease in COD and BOD, respectively. Electrodialysis has been explored for desalting spentwash using cation and anion exchange membranes resulting in 50–60% reduction in potassium content In another study, Vlyssides et al. reported the treatment of vinasse from beet molasses by electrodialysis using a stainless steel cathode, titanium alloy anode and 4% w/v NaCl as electrolytic agent. Up to 88% COD reduction at pH 9.5 was obtained; however, the COD removal percentage decreased at higher wastewater feeding rates.

In addition, reverse osmosis (RO) has also been employed for fermentation  wastewater  treatment. A unit in western India is currently processing effluent obtained after anaerobic digestion, followed by hold-up in a tank maintained under aerobic conditions, in a RO system. 290m3/d of RO treated effluent is mixed with 300m3/d of fresh water and used in the process for various operations like molasses dilution, make-up water for cooling tower, fermenter washing, etc. Yet another unit in southern India is employing disc and tube RO modules for direct treatment of the anaerobically digested spentwash. The permeate is discharged while the concentrate is used for biocomposting, with sugarcane pressmud.  Reported pilot trials on fermentation spentwash using a hybrid nanofiltration (NF) and RO process. Both the NF and RO stages employed thin film composite (TFC) membranes in spiral wound configuration with module dimensions of 2.5 inches diameter and 21 inches length. NF was primarily effective in removing the color and colloidal particles accompanied by 80%, 95% and 45% reduction in total dissolved solids (TDS), conductivity and chloride concentration, respectively, at an optimum feed pressure of 30–50 bar. The subsequent RO operation at a feed pressure of 50 bar resulted in 99% reduction each in COD, potassium and residual TDS.