With China becoming the largest producer of MSG, scientific evaluations of the environmental effects of MSG production are crucial for energy conservation, emission reductions and the green development of the MSG industry. Additionally, life cycle environmental impact factors are more comprehensive and convincing than some traditional energy saving and emission reduction factors (such as material reduction and pollution reduction) in evaluating the environmental effects of CP technologies.
5.1 Environmental impact of the applied CP technologies
The potential environmental impacts of producing 1 t of MSG without applying the abovementioned three CP methods (Scenario 2) are assessed in this article. The results in Fig. 3 show that compared with those in Scenario 2, all of the characteristic impact factors are lower under the current production technology conditions (Scenario 1). Among these factors, the value of POFP decreases the most by 92.40%, followed by those of WU, AP, RI, PED, GWP, and EP, which decrease by 66.00%, 38.12%, 27.13%, 25.36%, 22.42%, and 21.31%, respectively.
In terms of the production process and specific indicators, the POFP of the tail liquid treatment process decreased the most, from 6.45E+02 kg NMVOC to 4.29E+01 kg NMVOC. The main reason for this change was the implementation of the CP technology mentioned in Section 2.3. This technology increased the removal efficiency of the VOCs in the exhaust gases from 45% to 95%, which prevented the MSG companies from being shut down by environmental authorities.
There is also a significant decline in the WU in the ER process by 81.41% because of the application of the CP technology discussed in Section 2.2. The process of extracting GA via the isoelectric crystallization and ion exchange method requires a large amount of water to wash the resins, but this consumption issue is avoided in the concentrated continuous isoelectric method. Moreover, the concentrated continuous isoelectric method can reduce the heavy discharge of waste water; as a result, the EP value of the ER process decreased by 71.70%. In addition, the AP and PED values in the ER process declined by 43.81% and 40.84%, respectively, because of the reductions in concentrated sulfuric acid and liquid ammonia in the concentrated continuous isoelectric method. Although the extraction rate of glutamate in the concentrated continuous isoelectric method (90%) is lower than that in the isoelectric crystallization and ion exchange method (95%), the overall environmental effects are much lower.
The largest rate of decrease in the characteristic impact factor for the fermentation process is for the PED (23.01%) because the temperature-sensitive fermentation bacteria increased the acid yield of glucose with the CP technology mentioned in Section 2.1.
5.2 Opportunities for improvement
The potential environmental impact of 1 t of MSG is assessed in this article. The results show that maize and tail liquor are the two most important contributors to the related environmental effects. Maize planting and harvesting contributed to 90.37% of EP, 34.04% of RI, 26.60% of PED, and 19.19% of WU. Tail liquor utilization contributed to 96.72% of POFP and 20.12% of GWP.
A total of 2.12 tons of maize is used to produce 1 t of GA through maize-starch conversion (conversion rate is 70%), starch-glucose conversion (conversion rate is 98%), glucose-glutamate conversion (conversion rate is 65%) and glutamate extraction and refining (extraction rate is 90%). All the conversion and extraction rates except the GA extraction rate have reached maximums with the currently used technologies. Although the extraction rate of GA based on the isoelectric crystallization and ion exchange method can reach 95%, the process has been gradually eliminated due to the production of a large amount of washing wastewater and separation wastewater. Therefore, the only way to reduce the potential environmental impact of maize is to focus on the maize planting and harvesting process in the next few years. The environmental effects of this process mainly come from the emissions associated with irrigation, transportation energy consumption and the application of pesticides and fertilizers. Hence, the main measures used to reduce the environmental effects of maize planting should focus on targeted seed selection, water-saving irrigation, scientific fertilization and pest control in the future.
The environmental contribution of the fermentation tail liquor is mainly due to VOC emissions and energy consumption by the spraying granulation process. Although the CP technology used for VOC removal with electrostatic separation equipment has increased the removal rate to 95%, the remaining 5% of small VOCs still cannot be removed. Even if some new technologies can remove this 5% of VOCs, the high economic cost would increase the burden on MSG enterprises. Therefore, low-temperature tail liquid utilization technology needs to be promoted to avoid VOC generation and high energy consumption.
In addition to maize and tail liquor, the contributions of concentrated sulfuric acid and liquid ammonia to the environment are also significant. The contributions of concentrated sulfuric acid to AP and RI are 58.51% and 24.91%, respectively, and the contribution of liquid ammonia to PED is 26.25%. Concentrated sulfuric acid mainly provides H+ and regulates pH for glutamic acid extraction. Because sulfate will not corrode containers (compared with hydrochloric acid) and is affordable, it is the best choice at present. Therefore, greener sulfuric acid production is the optimum choice for reducing the environmental impact of the full life cycle. For example, the application of a high-efficiency vitriol catalyst and improvements in desulfurization technology are effective measures that could reduce sulfur dioxide emissions in the sulfuric acid production process (Oni et al. 2018). Liquid ammonia mainly provides a nitrogen source and regulates pH in the glutamate fermentation process. Because of its high nitrogen content, liquid ammonia is currently the most ideal nitrogen source. Hence, reducing the environmental contributions of liquid ammonia production, especially those related to energy consumption, is the best way to improve the environmental effects of the MSG life cycle. The effective methods for reducing energy consumption during ammonia production include continuous pressure coal gasification, trace carbon removal and the use of evaporative cooling technologies (Chao et al. 2012).
The above analysis shows that the implementation of CP technologies has effectively reduced the environmental effects of the MSG industry over the past ten years, but there is little room to improve the environmental impacts of MSG enterprises through CP technologies (alternatively, new CP technologies are still in the research stage), except by addressing the tail liquid utilization stage. Therefore, in addition to improvements within MSG enterprises, life cycle theory should be applied to realize the sustainable green development of the MSG industry, as witnessed for the green manufacturing system integration projects organized by the Ministry of Industry and Information Technology (MIIT) of the People's Republic of China. The projects aim to meet the goals of "Made in China 2025" by establishing green design platforms, improving key green technologies and constructing green supply chains. Specific to the MSG industry, green raw materials (such as maize, sulfuric acid and liquid ammonia) and green supply chain construction can significantly reduce the environmental impact on the whole life cycle. Moreover, an industrial symbiosis network of "maize planting-fermentation-biological byproducts-agricultural planting" can be constructed to extend the industrial chain and improve the utilization rate of resources, and a typical circular economy pattern is successfully built focused on the MSG manufacturers in East China.