| Wastewater produced by wheat processing contains lots of reclaimable and valuable organics such as pentosan and protein. However, currently wheat starch processing plants rarely pay any attention to them. Most of the wastewater is directly discharged, which causes a waste of resources and gives rise to serious environmental problems. Traditional wastewater treatment methods, including evaporation and concentration, biological treatment and chemical treatment, feature high energy consumption, long process and great cost, thereby imposing a heavy financial burden on relevant enterprises. Therefore, the industry is now faced with a knotty problem as to how to treat wheat starch wastewater with low lost and reclaim the highly valuable resources.In this paper, four methods, including chromatography exchange, membrane separation, coupling of precipitated protein and membrane separation, and coupling of enzymolysis and membrane separation, were adopted to reclaim pentosan from wheat starch wastewater. Based on the experimental results, a horizontal comparison was conducted to the four methods, thereby providing reference for the industry to reclaim and recycle pentosan from wheat starch wastewater. In addition, the monosaccharide composition and polymer branching degree of pentosan were also detected in this paper.The experimental result of membrane separation showed that, with 100 m L permeate solution and 700 m L retained solution, the 10,000 Dalton(hereinafter referred to as Da) membrane could retain 97.81% of the pentosan and 97.75% of the protein. The concentration ratios of retained solution and permeate solution of pentosan and protein were respectively 15.29 and 10.48, which showed that the 10,000 Da membrane could retain most of these two substances. Whereas, in comparison, the concentration ratios of retained solution and permeate solution concerning reducing sugar and amino acid were both just a little short of 1, indicating that the 10,000 Da membrane couldn’t retain any of the reducing sugar and amino acid. Then 20,000 Da and 50,000 Da membranes were respectively used to do the experiments, the results of which showed that the concentration ratios of retained solution and permeate solution were approximately 1, indicating that neither of them could retain protein and pentosan.This experiment used protease enzymolysis, trichloroacetic acid flocculation, biofloculation, heat coagulation and isoelectric precipitation to pre-remove protein in the wheat starch wastewater, with a view to creating the precondition for reclaiming pentosan via membrane separation. As was indicated by the results, the protein removal rate of the five methods ranked this way: protease enzymolysis(87.11%) > trichloroacetic acid flocculation(68.50%) > biofloculation(20.29%) > heat coagulation(17.51%) > isoelectric precipitation(8.56%); whereas the loss rate of pentosan of the five ranked this way: trichloroacetic acid flocculation(22.89%) > protease enzymolysis(12.93%) > biofloculation(11.21%) > heat coagulation(9.16%) > isoelectric precipitation(8.24%). Taking protein removal rate and pentosan retention rate as the assessment indicators, this paper concluded that protease enzymolysis was the optimal method to remove protein. After the wastewater was treated by protease, the protein removal rate was 87.11%, and only 12.93% pentosan was lost; then the 10,000 Da membrane could be used to retain basically all the pentosan.Considering that the 10,000 Da membrane could retain both pentosan and protein, this research designed a method that coupled enzymolysis and membrane, which was to break down protein into amino acid firstly, and then 10,000 Da membrane was used to separate pentosan from amino acid and reducing sugar. When the permeate solution amounted to 1.5 times of the solution to be separated, the 10,000 Da membrane could remove 92.27% of the pentosan, retain only 24.16% of reducing sugar and 28.10% of amino acid(hydrolysates), and the total retention rate of pentosan stood at 77.36%. In addition, the 10,000 Da membrane couldn’t retain any of the amino acid. Consequently, by increasing the times of recycling and extending the ultrafiltration time, protein(amino acid) and reducing sugar in the wastewater could be removed, while the pentosan was retained.The research chose DEAE-52 cellulose column to purify wheat starch wastewater, during which water was used as the eluent and a proper amount of Na Cl was added to strengthen the effect. Freeze-thaw method and ultraviolet spectrophotometry proved that soluble pentosan made up most of the eluent, and basically no protein or amino acid was detected. Characteristic peaks of β-D-Galactopyranose and α-D-glucopyranose were respectively detected in 917.12 cm-1 and 841.12 cm-1 in the infrared spectrum, which indicated that the polysaccharide contained bothβ-glucosidic bonds andα- glucosidic bonds.By adopting acid hydrolysis method, the reclaimed pentosan was degraded into monosaccharide sugar, which was then detected by ion chromatograph.The result showed that the pentosan mainly consisted of four kinds of monosaccharide sugar--arabinose, xylose, galactose and glucose. The ratio of N(Ara)/n(Xyl) was 0.92, which indicated a very high degree of branching.Overall, the coupling of enzymolysis and membrane separation method has simpler process and a higher separation ratio compared to the other three methods. It may also bring high economic benefits and is entitled to a bright future in the wheat starch industry. |
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