A Comparative Study On The Mesophilic And Thermophilic Anaerobic Digestion Characteristics Of Typical BNR Process Sludge

Sludge anaerobic digestion technology attracts more attentions gradually in recent years due to its effective recycling energy and significantly reducing the characteristics of the sludge volume.Mesophilic anaerobic digestion（MAD）is widely used in sludge treatment due to its good stability in treating activated sludge from the traditional process,but its application to treat sludge of long sludge retention time from the biological nutrient treatment（BNR）process is not successful in our country.However,MAD has been changed to thermophilic anaerobic digestion（TAD）in some European wastewater treatment plants（WWTPs）in recent years.Based on the investigation of BNR process sludge of different scales,this paper compared different characteristics of MAD and TAD to treat BNR process sludge of different scales firstly.Then the differences of MAD and TAD to treat BNR process sludge with high solid content were explored.This study would provide references for the application selection of anaerobic digestion to treat BNR process sludge.The main results and conclusions are as follows:（1）The anaerobic digestion methane production potential of BNR process sludge of large WWTPs was 20%higher averagely than the corresponding value of BNR process sludge of small WWTPs.TAD significantly increased R_maxand P_oof BNR sludge and shortened its digestion time,especially for the anaerobic digestion of oxidation ditch process sludge.Only TAD of BNR sludge can meet the requirements of"Guideline on Best Available Technologies of Pollution Prevention and Control for Treatment and Disposal of Sludge from Municipal Wastewater Treatment Plant"that requires more than 40%VS degradation percentage.（2）Under the condition of the same total solid（TS）content,TAD had greater methane production potential and faster digestion rate than that of MAD.In the MAD groups,methane production potential increased from 146.42 L/kg VS to 205.01 L/kg VS with the content of TS increased from 4%to 8%.However,when the content of TS reached 8%,the methane production in anaerobic digestion may be inhibited due to the accumulation of propionate（more than 1000 mg/L）.（3）The specific resistance of the digested BNR process sludge of TAD was greater than 1010s2/g,and the dewaterability was more difficult than that of MAD.Moreover,high temperature and high solid content will bring higher pollution level of digested mixture,but the calorific value of the digested residue produced by MAD and TAD was lower than 6700 J/g dry sludger.In addition,the moisture content was higher than 87%,and the total nutrient element content,including nitrogen,phosphorus and potassium,were less than 5%.（4）The continuous experiment of high solid content BNR process sludge showed that the average daily gas production of per unit reactor volume in the TAD was 254.7m L/L when the organic load rate（OLR）was 1.79 g VS/L/d,which was 3.68 times of the corresponding value in the MAD.However,when OLR increased to 2.87 g VS/L/d,the daily gas production of per unit volume of TAD and MAD increased to425.4 and 146.9 m L/L,and the methane content was also close to 60%.（5）Under the same OLR of 1.79 and 2.87 g VS/L/d,VS degradation rate of TAD with high solid content BNR sludge were 43%and 30%,which were 1.4 and 1.5times of the corresponding value of MAD.It showed that only TAD could meet the requirement of VS degradation ratio of above 40%when the OLR was 1.79 g VS/L/d.In addition,TAD of high solid content BNR process sludge has better energy balance and the higher net energy of per unit reactor volume TAD,which were+0.33 and+1.55 k J/L/d at OLR of 1.79 and 2.87 g VS/L/d.However,the corresponding values of MAD were only-2.53 and-2.89 k J/L/d under the corresponding OLR.