Technical-economic Analysis Of Digestate Treatment And Environmental Impact Assessment Of Biogas Fertilizer Application

With the continuous advancement of Chinese rural circular economy development and ecological civilization construction,the treatment technology of biogas slurry and residue has gradually become the focus of development.The relatively backward biogas slurry treatment technology limits the development of the industrialization of biogas projects.The sustainability of the project mainly depends on the ability to treat biogas slurry,and improper management can lead to serious environmental problems.Based on the biogas slurry treatment process,this paper focuses on the technical and economic issues of biogas slurry treatment process and the utilization of biogas fertilizer and value-added biogas fertilizer.（1）In order to have a deeper understanding of different biogas slurry treatment processes,and accurately predict and analyze the actual operation in a shorter time,the Aspen Plus^?was used to simulate the process flow,and the total capital investment and total operating cost were calculated according to economic parameters.The results of technical economic evaluation showed that the scale of the biogas project was 10 t/d,total capital investment and total operating cost of ammonia stripping process were lower than sewage treatment process 34.5%and 2.5%,respectively,and sewage treatment process can reduce fertilizer cost RMB 7,200 and RMB 900.The sales revenue of ammonium sulfate from ammonia stripping process was RMB 10,000.Total capital investment and total operating cost of the ammonia stripping process were 37.3%and 19.2%lower than sewage treatment process at 1,000 t/d scale.By sewage treatment process and ammonia stripping process,chemical fertilizers cost can be reduced by RMB 710,000 and RMB 90,000,and ammonia stripping can obtain RMB 1.13 million in sales of ammonium sulfate.The annual cost of treating biogas slurry of 10 t/d biogas project was about 300 RMB/t,and by using sewage treatment process and ammonia stripping for 1,000 t/d scale was about 10 RMB/t and 7 RMB/t,respectively.It is recommended to simplify the biogas slurry treatment process for small-scale biogas projects.Using biogas slurry treatment technology in large-scale biogas projects can reduce the cost per ton of biogas slurry;the ammonia stripping can produce ammonium sulfate for sale,and ammonia stripping has the greater economic benefits.（2）After the application of biogas fertilizer,it will affect the environment due to the influence of the migration and transformation of nitrogen in the soil.At present,there is no research on the nitrogen balance in the soil-crop system by the application of biogas fertilizer.Based on field experiments,Vensim^?was used to establish a soil-crop system’s nitrogen dynamics model,and to explore the dynamic changes of nitrogen in soil by biogas fertilizers with different nitrogen levels.When the nitrogen application amount was 85kg/hm²and 170 kg/hm²,the final NH₄⁺contents of the soil were 77.6%and 59.6%lower than the original,the NO₃^-contents were 37.9%and 32.2%lower.The ammonia volatilization and denitrification amount of 170 kg N/hm² were 64.5%and 137.7%higher than 85 kg/hm²,respectively,but the nitrogen application amount of 85 kg/hm² has the highest ammonia volatilization rate（2.5%）.When the nitrogen application amount was less than 170 kg/hm²,the nitrogen of biogas fertilizer cannot be maintained for the growth of crops due to the large amount of loss.When the nitrogen application amount was 340kg/hm² and 680 kg/hm²,the soil will accumulate ammonium nitrogen and nitrate nitrogen.The loss increases with the increase of nitrogen application,but the loss rate was equivalent,indicating that the nitrogen application was more than 170 kg/hm²,the nitrogen loss rate had nothing to do with fertilizer applied amount.In order to ensure the higher nitrogen utilization rate of fertilizers and smaller loss,the nitrogen application amount was recommended to be in the range of 170 kg/hm²-340 kg/hm².（3）The substitution of biogas fertilizers for bio-based fertilizers can help promote the sustainable development of agriculture,but there was still a lack of comparative analysis on the environmental impacts of biogas fertilizers and bio-based fertilizers.This study used Life Cycle Assessment,combined with economic analysis to comprehensively evaluate the bio-based fertilizers（ammonium sulfate liquid and ammonium sulfate liquid powder）,biogas fertilizers and chemical fertilizers.The environmental impact value of biogas fertilizer increased in proportion to the amount of nitrogen applied.The greenhouse effect values of the nitrogen application amount with 170,340 and 680 kg/hm² had increased significantly,which were 23.3%,133.7%and 370.1%higher than those with 85 kg/hm²,respectively.Under equal nitrogen conditions（340 kg N/hm²）compared with biogas fertilizer,total environmental impact of ammonium sulfate liquid and ammonium sulfate liquid powder were 53.0%and 53.3%lower and can reduce 92.8%heavy metal emissions and reduce ecological toxicity.The carbon equivalent emissions of biogas manure and biogas-based ammonium sulfate liquid fertilizer are equivalent.After reducing the amount of nitrogen application,the ecological efficiency of biogas fertilizer was obviously improved.Although a certain amount of production cost has been increased,the eggplant yield of ammonium sulfate liquid was 63.42 t/hm²,which was 27.9%higher than the equal nitrogen biogas fertilizer;the economic benefit was 687,200 RMB/hm²,which was 130.4%higher than biogas fertilizer.The replacement of biogas fertilizer with bio-based fertilizers were feasible in terms of economic benefits.The ecological efficiency of biogas fertilizer,ammonium sulfate liquid and powder with equal nitrogen levels were 1.01,3.74 and 1.83,respectively.After value-added treatment,biogas fertilizer can improve ecological efficiency,bio-based liquid fertilizer can obtain higher economic value with less environmental impact and improve ecological efficiency.