柠檬酸-沼气双发酵耦联系统中钠离子影响机制的探究及其去除

Exploration of effect of sodium ions on citric acid fermentation with integrated citric acid-methane fermentation systemand its removal

为解决柠檬酸发酵过程中产生的废水问题,提出了柠檬酸-沼气双发酵耦联系统。在该系统中,柠檬酸发酵废水经中温厌氧消化和水资源化处理后作为配料水回用于下一批次柠檬酸发酵,从而实现废水零排放。厌氧消化出水中的主要抑制物质是Na^+,在回用过程中其质量浓度最高达到1 000 mg/L,会严重抑制柠檬酸发酵。采用电渗析法对厌氧消化出水中的高浓度Na^+进行去除,循环回用10批后,平均柠檬酸产量为142.4±2.1 g/L,达到去离子水发酵水平(141.5 g/L)。1 000 mg/L Na^+会造成柠檬酸发酵前期培养基pH快速下降,黑曲霉细胞分泌的糖化酶和异麦芽糖酶活力降低,部分糊精和异麦芽糖不能有效水解,柠檬酸产量显著下降。在柠檬酸发酵开始时添加18 g/L CaCO_3可以延缓发酵初期培养基pH的下降速率,促进部分糊精和异麦芽糖的水解,从而使可利用糖浓度增加,减轻高浓度Na^+对柠檬酸发酵的抑制作用。但是,CaCO_3会造成发酵前期培养基pH较高,副产物草酸大量积累,柠檬酸产量为139.2 g/L,仍低于去离子水发酵水平。

To solve the potential extraction wastewater pollution in citric acid production, an integrated citric acid-methane fermentation system was proposed. In this system, the extraction wastewater was treated through meso- philic anaerobic digestion （AD） and then the anaerobic digestion effluent was further treated and recycled to make mash for the next batch of citric acid fermentation, thus the wastewater discharge was eliminated. Excessive Na^＋ con- tained in AD effluent was confirmed to be the major inhibitor for citric acid fermentation and its concentration could accumulate to 1 000 mg/L in recycling. AD effluent was treated with electrodialysis system to remove sodium and then recycled to make mash for the next batch of the citric acid fermentation. The recycling was performed for 10 batches andthe average citric acid production was 142.4 ± 2.1 g/L,which was comparable to that of the fermentation with tap water （ 141.5 g/L）. Effect of high concentration of sodium on the citric acid fermentation was investigated and the re- sults indicated that 1 000 mg/L Na^＋ contained in citric acid fermentation could decline the medium pH sharply at ini- tial stage, which decreased the activity of isomaltase and glucoamylase secreted by Aspergillus niger, and influenced the breakdown of isomaltose and part of other dextrins, thus caused the decrease of citric acid production. 18 g/L cal- cium carbonate was added to medium at the start of the citric acid fermentation, the decline rate of the medium pH slowed down, which could promote the breakdown of isomahose and part of other dextrins, and thus could increase a- vailable sugar concentration and relieve the inhibition caused by 1 000 mg/L Na^＋. However,as the medium pH main- tained at the high level because of the calcium carbonate addition, part of available sugar was converted to by-product oxalic acid and the citric acid production was 139.2 g/L, which was still lower than that of the fermentation with tap water.