Effect Of Humus On Anaerobic Digestion Of Sludge And Its Shielding Method

The signing of the “Paris Climate Agreement” means the advent of the carbon neutral operation of wastewater treatment plants?WWTPs?.Excess sludge is the primary source of carbon and energy self-sufficient operation,which generates energy through anaerobic digestion?CH₄?.In the case of excess sludge amount,the energy self-sufficiency rate of sewage treatment plant depends on anaerobic digestion efficiency.Many factors can affect sludge anaerobic digestion efficiency.Among them,the humus composition is extremely special,not only can hardly be degraded in the anaerobic digestion system,but also inhibits the anaerobic digestion process.Therefore,it is necessary to investigate the inhibition of anaerobic digestion by humus and take technical /method to shield the inhibition of anaerobic digestion by humus.Based on the artificial cultivation of excess sludge with few humus and metal ions,the commercial humic acid,which is similar to the humus in the sludge,was selected as the research object to carry out experimental research.Firstly,the effects of different humic acid dosage on anaerobic digestion of sludge were investigated.The results showed that 15% of humic acid?VSS?inhibited anaerobic digestion most seriously and the CH₄ yield was 91.4 ml/g VSS,which decreased by 35.1% compared without humic acid dosage.Dosage of humic acid did not have any significant impact on the CH₄ content in biogas.Humic acid could inhibit the hydrolysis process of organic matter during anaerobic digestion.The concentrations of soluble polysaccharide and protein with 20% humic acid dosage were 2.20 and 1.58 times higher than control group respectively.Humic acid as an exogenous electron acceptor,can promote the anaerobic digestion process of acidification.On the 5th day,VFAs concentrations were the highest in every group.VFAs concentrations were 1176 mg/L with 20% humic acid dosage,which increased by 11.3% compared with control group.The inhibition of humic acid on anaerobic digestion of sludge can be effectively shielded by metal ions.In this study,three ways of adding Ca²⁺,Al³⁺ and different concentrations of Ca²⁺ and Al³⁺ combinations was chosen to shield inhibition effects caused by humic acid.Results of adding Ca²⁺ showed that the yield of CH₄ was the highest,reaching up to 96 m L/g VSS,when 100 mg/L Ca²⁺ was added.The inhibition rate of CH₄ was mitigated by 65.4%.In the form of cation exchange,Ca²⁺ can mitigate hydrolysis inhibition of polysaccharide and protein by humic acid during anaerobic digestion.The more Ca²⁺ was added,the better the shielding effect was achieved.However,when the dosage of Ca²⁺ was higher than 200 mg/L,the effect of shielding hydrolysis inhibitory did not increase.Ca²⁺ did not significantly affect the pH during anaerobic digestion,the pH value in each group was within 7.18⁷.45.Also,Ca²⁺ did not have any significant influence on the proportion of VFAs component in supernatant.With the increase of Ca²⁺ dosage,the removal rate of VSS showed the rule of rising first and then decreasing.Results of adding Al³⁺ showed that CH₄ yield was the highest with the addition of 70 mg/L Al³⁺,reaching up to 88.2 m L/g VSS.The inhibition rate of CH₄ was mitigated by 65.7%.The inhibition effect of humic acid on polysaccharide and protein hydrolysis was mitigated by Al³⁺ mainly through electrostatic action and catching-sweeping.The residual concentration of protein by adding 70 and 100 mg/L Al³⁺ was 11.9 and 16.4 mg/L respectively,92.5% and 89.7% lower than control group.The effect of Al³⁺ on pH was not obvious in anaerobic digestion system,pH value ranging from 7.07 to 7.37 in each group.Al³⁺ could promote the formation of acetic acid,inhibit the formation of propionic acid,and had no remarkable effects on pentanoic acid during anaerobic digestion.Higher dosage of Al³⁺ may lead to more organic matter entrapped,which hampered the degradation of organic matter by anaerobic digestion bacteria.Results of adding Ca²⁺ and Al³⁺ showed that CH₄ production,up to 108.5 m L/g VSS,recovered best when 50 mg/L Ca²⁺ and 10 mg/L Al³⁺ were added.The inhibition rate of CH₄ was mitigated by 80.1%.Al³⁺ shielding hydrolysis inhibition of polysaccharide and protein by humic acid was better than Ca²⁺.When reaching the best shielding effect,the concentration of Ca²⁺ was 2.5⁵.00 mmol/L,which is 2 times as many as Al³⁺.The pH value of combining Ca²⁺ with Al³⁺ groups was stable in the range of 6.94⁷.54.The lowest concentrations of VFAs were 50.64 mg/L in combination with low concentrations of Ca²⁺ and Al³⁺.Al³⁺ may inhibit the generation of propionic acid,but not have any effect on n-valerate when increased gradually.At the end of anaerobic digestion,22.8% of the highest VSS removal rate was achieved by adding low concentrations of Ca²⁺ and Al³⁺.The combination of low concentrations Ca²⁺ and Al³⁺ had an "1+1>2" effect.While under the condition of medium or high concentration of Al³⁺,Al³⁺ can form aluminum hydroxy compounds through hydrolysis effect,which may combined preferentially with humic acid.At this time,dosing Ca²⁺ may play the weak role of shielding,so mitigating inhibition by humic acids had little difference.