Recycling Technologies Based On Efficient Nutrient Recovery Of Liquid Digestate

In this study,two recycling technologies based on efficient nutrient recovery of liquid digestate were investigated to treat the liquid digestate which are rich in nitrogen,phosphorus and other nutrients according to the different conditions of economical and available energy sources of large and medium-sized biogas projects.A combined process which is composed of nitrogen and phosphorus pretreatment and SBR biochemical treatment was used and tested to realize partial nutrient recovery of liquid digestate for the biogas projects which don't have economical and available energy sources.Another process was used to realize total nutrient recovery of liquid digestate for the biogas projects which have economical and available energy sources.In this process,the concentrated liquid digestate was mixed with biochar to produce soil conditioner.The main conclusions are as follows:Pretreatment technology-"Ammonia stripping plus struvite precipitation"was used in partial nutrient recovery of liquid digestate.This pretreatment technology can reduce the subsequent biological treatment load rate of SBR,while nitrogen and phosphorus in the liquid digestate was recovered.The results showed the stripping can recover 90%ammonia nitrogen of liquid digestate and pH value of the liquid digestate after stripping was about 9.2 and suitable for following-up struvite precipitation treatment.Struvite precipitation can recover nitrogen and phosphorus in liquid digestate and C/N ratio of the effluent was greatly improved,from 0.7 to 10 after struvite precipitation,which was conducive to the further treatment of the following-up SBR biochemical system.The removal rate of SS,COD,ammonia nitrogen and total phosphorus were 84.8%,85.4%,77.6%,75.9%,respectively after SBR treatment(conventional aeration)and the effluent could meet the requirements of national discharge standard of pollutants for livestock and poultry breeding(GB18596-2001).The nano aeration in SBR reactor was introduced in order to investigate treatment efficiency of novel SBR process for increasingly stringent discharge standard.The results showed that this technology effectively improved the removal rate of pollutants,and the removal rates of SS,COD and ammonia nitrogen in the SBR reactor reached 91.9%,92.0%and 85.2%respectively.The effluent could meet the requirements of national discharge standard of pollutants for livestock and poultry breeding(the second draft version)issued by the Ministry of Environmental Protection in 2014.In the process for total nutrient recovery of liquid digestate,two-stage evaporation process of liquid digestate was put forward according to the characters of ammonia nitrogen in atmospheric pressure and negative pressure evaporation process.This process can separate the condensate into two parts:one part was the condensate with rich ammonia content(11718 mg/L,atmospheric pressure evaporation),and the other part was the condensate with low ammonia content(67 mg/L,negative pressure evaporation).Three kinds of raw materials and different preparation conditions for biochar production were tested to obtain the optimal product which could adsorb and carry concentrated liquid digestate.The results showed that biochar derived from cotton straw had the best performance.The biochar produced in the optimum conditions was mixed with concentrated liquid digesate to produce soil conditioner.Compared to the blank test,the germination rate of ryegrass increased from 68%to 81%,and the yield increased by about 30%after the soil conditioner application.The results showed that this soil conditioner has obvious sustained-release effect.An ananlysis method combined SWOT and PEST model was employed to evaluate the strengths and weaknesses,as well as opportunities and challenges for resource utilization of liquid digestate from large and medium-scale biogas projects.From the viewpoint of politics,economy,society and technology,the problems and obstacles for resource utilization of liquid digestate was analysed and summarized,and possible developing strategies were proposed.