Effects Of Calcium Addition On Different Forms Of Biogas Fertilizer And Application Modes In ‘Ammonia Stripping’ System

Anaerobic digestion is an important means of manure treatment in livestock and poultry farms.However,during the digestion process,a large amount of heavy metals in livestock and poultry manure are concentrated in biogas residue,resulting in an increased risk of heavy metals in biogas residue applied to farmland.The’ammonia stripping’system will add Ca（OH）₂to adjust the p H and assist flocculation.Ca²⁺will be introduced when the biogas slurry is returned to the anaerobic digestion system,which may affect the distribution and morphological changes of gas production and heavy metals.In this study,different types and concentrations of Ca agents were added to the anaerobic digestion process to improve the anaerobic digestion biogas production while exploring the effect of adding Ca agents on the migration and transformation of heavy metals.The effect of calcium-containing biogas residue on the migration and transformation of heavy metals in soil and the enrichment of heavy metals in ryegrass was further explored.The biogas slurry in the’ammonia stripping’system was flocculated by enriching local microorganisms,and the obtained bacterial fertilizer was combined with organic/inorganic fertilizer for ryegrass cultivation to explore the effects of combined application of bacterial fertilizer on ryegrass yield and migration and transformation of heavy metals in soil and ryegrass.（1）Three different types（Ca Cl₂,Ca（OH）₂,Ca CO₃）and two gradients（100 mg/kg,2500 mg/kg）of Ca were used as additives for anaerobic digestion.The total biogas production of the group with high concentration of Ca（OH）₂was the highest,reaching422.8 m L/g·VS,which was 83.7%higher than that of CK.The addition of Ca Cl₂and Ca（OH）₂can reduce the methane content,while the addition of Ca CO₃will increase the methane content.The contents of Cu,Zn,Cd and Pb in biogas residue were higher in low concentration group（A1,B1,C1）than in high concentration group（A2,B2,C2）,indicating that with the increase of adding concentration,Cu,Zn,Cd and Pb in biogas residue were transferred to biogas slurry.The migration rate of heavy metals in biogas residue was low and the stability was high.In order to reduce the contents of Cu,Zn,Cd and Pb in biogas slurry,it is not suitable to add high concentration.The content of Cu,Zn,Cd and Pb in the biogas residue of the low concentration Ca（OH）₂treatment group was higher than that of the other groups,and the transfer amount of Cu,Zn,Cd and Pb increased by 9.9%,4.2%,40.1%and 4.6%,respectively,compared with CK.The addition of low concentration Ca（OH）₂treatment was beneficial to strengthen the transfer of Cu,Zn,Cd and Pb from biogas slurry to biogas residue.（2）Ryegrass with strong enrichment of heavy metals was selected as the test crop.Ca-containing biogas residue as bio-organic fertilizer can effectively alleviate the toxic effect of heavy metals on ryegrass and promote the increase of biomass.Compared with CK₁（no fertilizer blank group）,the yield of biogas residue was significantly increased after application,and the growth rate was between 49.4%and 64.4%.Compared with CK₂（no Ca biogas residue group）,the yield of Ca-containing biogas residue group was higher,among which the high concentration Ca（OH）₂and low concentration Ca CO₃group increased the most,increasing by 19.7%and 19.4%respectively,which was consistent with the highest biogas yield of high concentration Ca（OH）₂and low concentration Ca CO₃group.The application of Ca（OH）₂-containing biogas residue and Ca CO₃-containing biogas residue significantly inhibited the activity of Cu,Zn,Cd and Pb in soil,and inhibited the enrichment of Cu,Zn,Cd and Pb in ryegrass.Ca Cl₂-containing biogas residue also inhibited the accumulation of Cu and Cd in ryegrass,but increased the accumulation of Zn and Pb in ryegrass.（3）Ca-containing biogas residue has the same effect on soil improvement as soil improvement by direct application of Ca agent.The high concentration of Ca（OH）₂-containing biogas residue not only increased the soil p H but also had the best fixation effect on Cu and Zn in the soil.The high concentration of Ca CO₃-containing biogas residue had the best fixation effect on Pb.The application of Ca Cl₂-containing biogas residue activated Zn,Cd and Pb in soil.Compared with CK₂,the contents of Zn,Cd and Pb in soil decreased after the application of Ca Cl₂-containing biogas residue.The contents of Zn,Cd and Pb in the low concentration of Ca Cl₂-containing biogas residue decreased by 4.0%,3.4%and 4.4%,respectively,and those in the high concentration of Ca Cl₂-containing biogas residue decreased by 6.8%,2.2%and 7.1%,respectively,indicating that the activation of Zn,Cd and Pb was absorbed by ryegrass,and salinity increased the mobility of Zn,Cd and Pb in soil.（4）In the’ammonia stripping’system,the biogas slurry was first cultured and then flocculated to improve the stripping efficiency.The addition of carbon source to the biogas slurry significantly promoted the growth of microorganisms.From the economic point of view,it was found that when glucose was added for the enrichment of indigenous microorganisms in biogas slurry,the growth of microorganisms and proteins was the highest when 6 g/L glucose was added for 3 days.Flocculation was carried out with 0.3g/L bioflocculant and 5 g/L Ca O,respectively.The concentration of bioflocculant was 6.2×10⁷CFU/m L,and the concentration of Ca-containing flocculant was 4.1×10⁷CFU/m L.In order to achieve the purpose of value-added,the microbial fertilizer obtained by the flocculation of the local microorganisms in the biogas slurry was combined with organic/inorganic fertilizer.On the whole,the yield of organic fertilizer-bacterial fertilizer combined application was significantly higher than that of inorganic fertilizer-bacterial fertilizer combined application.The best way and proportion of organic fertilizer-bacterial fertilizer combined application is 25%T1（25%Ca-containing bacterial fertilizer instead of organic fertilizer）,the yield is up to 2.8 kg/m²,and its leaves are the most dark green and the growth trend is the best.Under the treatment of organic fertilizer/inorganic fertilizer-bacterial fertilizer,the content of Cu,Zn,Cd and Pb in soil is closely related to the proportion of combined application and the mode of combined application.The content of Cu,Zn,Cd and Pb in soil of organic fertilizer-bacterial fertilizer is significantly higher than that of inorganic fertilizer-bacterial fertilizer.The contents of Cu,Zn,Pb and Cd in ryegrass under different combined application treatments have different trends.The contents of Cu and Zn in ryegrass under high proportion combined application treatment（50%T1/F1）and Pb and Cd in ryegrass under low proportion combined application treatment（25%T2/F2）were significantly lower than those in pure organic fertilizer/inorganic fertilizer（0%T/F）.