| Citric acid?2-hydroxy-1,2,3-propanetricarboxylic acid? is one of the most important organic acids and has been widely used in food, beverage, chemical and medicine industries. Currently, the production of each ton of citric acid will produce 8 m3 of fermentation wastewater which contains high concentration of chemical oxygen demand(COD, 15000-20000 mg·L-1) with low p H value?4.5-4.8?. The discharge of this wastewater will cause serious environmental pollution. Majority of wastewater from citric acid production is commonly treated through anaerobic-aerobic digestion, however, the equipment needs large land and capital investment and operation costs are high. Moreover, the treated wastewater still cannot meet national discharge standard and has brought great economic and environmental burden for citric acid manufacturers. To solve the problem mentioned above, this paper established an integrated citric acid-methane fermentation system based on the theory of the “fermentation ecological engineering”, and its technical feasibility was evaluated in terms of anaerobic digestion and citric acid fermentation performances. The major inhibitors which affected the stability of the proposed system were confirmed and their influencing mechanisms were studied. Based on the results, corresponding optimization methods were proposed. The main research contents and results were shown as follows:?1? An integrated citric acid-methane fermentation system was established based on the theory of the “fermentation ecological engineering”. In this system, cassava and corn starch were used as raw materials for citric acid production after milling, liquefaction and sterilization operation, citric acid fermentation wastewater was treated with anaerobic digestion and then recycled as process water for the next batch of citric acid fermentation, thereby avoiding wastewater discharge and reducing water resource consumption. When anaerobic digestion effluent?ADE? without treatment was reused for citric acid fermentation, citric acid production decreased by 34.1% compared with the control and high concentration of ammonia, sodium and potassium ions were confirmed to be the major inhibitors. 001×7 strong acidic cation exchange resin was used to pretreat of ADE prior to reuse and the recycling experiment was performed successfully for 10 batches. The average citric acid production was comparable to the fermentation with tap water, while the average COD removal rate of anaerobic digestion approached to 94.7 ± 0.8%. These results indicated that both citric acid fermentation and anaerobic digestion were efficient and stable in operation, and the integrated citric acid-methane fermentation system was feasible in technique.?2? Preliminary strategy was proposed to remove inhibitors from the integrated citric acid-methane fermentation system. A mutant strain A. niger MZ-11 which exhibited resistance to 200 mg·L-1 Na+ and 300 mg·L-1 K+ in ADE, was screened through ultraviolet mutagenesis. Meanwhile, air stripping was used to remove excessive ammonia and part of precipitable metal ions in ADE. The treated ADE was recycled to citric acid fermentation and the recycling experiment was performed for 10 batches. The anaerobic digestion was efficient and stable in operation and the average methane yield was 292.3 ± 25.1 m L·g-1 TCODremoved which approached to the normal biogas fermentation, while the average citric acid production was 145.9 ± 3.4 g·L-1 and 2.5% lower than that with tap water(149.6 g·L-1). Excessive Na+ contained in ADE was found to be the major inhibitor for citric acid fermentation and its concentration could accumulate to 1000 mg·L-1 in recycling.?3? Effect of high sodium concentration in ADE on the integrated citric acid-methane fermentation system was investigated. 1000 mg·L-1 Na+ contained in citric acid fermentation could influence the normal metabolism of A. niger and decline the medium p H sharply which decrease the activity of glucoamylase and isomaltase secreted by A. niger, and influence the breakdown of isomaltose and part of other dextrins, causing the decrease of available total sugar concentration and citric acid production. When sodium hydroxide was fed into fermentation medium at initial stage?4-24 h?, the decline rate of the medium p H slowed down and the inhibition caused by high concentration of Na+ was effectively eliminated. As a result, citric acid production was greatly improved and was comparable to the fermentation with tap water.?4? Effect of volatile fatty acids?VFAs? in ADE on the integrated citric acid-methane fermentation system was studied. In recycling, acetic and propionic acid were the two most abundant acids in ADE and were potentially harmful to citric acid fermentation. Fermentation with tap water in initial p H 4.5, the critical inhibitory concentration of acetic acid and propionic acid for citric acid fermentation was 480 and 296 mg·L-1, respectively. High concentration of acetic acid or propionic acid in ADE could prolong the lag phase of the citric acid fermentation, while the A. niger vitality declined and even complete autolyzed, resulting in the significant decrease of citric acid production. The influence of acetic acid and propionic acid on citric acid fermentation was associated with the medium p H and their inhibitions became stronger as the initial p H of medium decreased. Therefore, anaerobic digestion system should be operated in high-efficiency and steady state to maintain the VFAs concentration at a reasonable level. When acetic acid or propionic acid was accumulated in recycling, increasing the initial p H of citric acid fermentation medium properly could relieve their inhibition for the integrated citric acid-methane fermentation system.?5? For the inhibitors in ADE which affected the stability of the proposed system, including ammonia and sodium, corresponding optimization methods were proposed and their technical feasibility and stability were also tested. ADE was pretreated with air stripping to remove Ca2+ and Mg2+ which could cause electrodialysis membrane pollution, and then treated with electrodialysis. The removal rates of Na+ and NH4+ in ADE were above 90% and the water recovery reached to 95% at the applied voltage of 15 V and the flow rate of 50 m L·min-1. The dilute water was recycled for citric acid production and 130 U·g-1 glucoamylase was added at the start of the fermentation, the inhibition caused by ADE was completely eliminated and citric acid production was comparable to the fermentation with tap water. The technical feasibility and stability of air stripping and electrodialysis treatement were tested and the recycling experiment was performed successfully for 10 batches. The average citric acid production was 142.4 ± 2.1 g·L-1 which was comparable to the fermentation with tap water(141.6 g·L-1), while the anaerobic digestion was efficient and stable in operation and the average methane yield was 297.7 ± 19.8 m L·g-1 TCODremoved which approached to the normal biogas fermentation.?6? To ensure the long-term stable operation of the integrated citric acid-methane fermentation system, the recycling indicators of the ADE was established. The anaerobic digestion effluent used as process water for citric acid fermentation should meet the following requirements:?1? conductivity ? 750 ?S·cm-1;?2? Na+ ? 200 mg·L-1;?3? K+ ? 300 mg·L-1;?4? ammonia ? 50 mg·L-1;?5? acetic acid ? 480 mg·L-1;?6? propionic acid ? 296 mg·L-1?... |
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