Effect Of THSF As External Carbon Source On The Biological Nitrogen Removal Performance Of Low C/N Wastewater

Lack of organic carbon sources in the influent of major wastewater treatment plants（WWTPs）in China limits the biological nitrogen removal efficiency.Thus,WWTPs need to add additional organic carbon sources to enhance the biological nitrogen removal performance,which will increase the operation and maintenance costs.Therefore,the search for economical and inexpensive organic carbon sources has become an important part of energy saving and consumption reduction for WWTPs.In this study,the thermal hydrolyzed sludge filtrate（THSF）of wasted activated sludge was used as an external carbon source to investigate its effect on the denitrification performance of A²/O system treating low C/N domestic sewage.Firstly,the influent water quality,operation efficiency and carbon emission characteristics of a low C/N and operating load WWTP were analyzed,the denitrification characteristics of low C/N domestic sewage and the carbon emission characteristics of such WWTPs were discussed.A lab-scale A²/O system was used to carry out by continuous experiments to compare the denitrification performance of low C/N domestic sewage,when THSF and sodium acetate were used as external carbon sources,respectively.The effects of THSF at different temperatures on nitrification rate and denitrification rate was analyzed by batch activity test.The main conclusions are as follows:（1）Taking a low C/N and operating load WWTP as the object.The concentrations of BOD₅,COD,NH₄⁺-N,TN and TP in the influent were 16～260,104～536,17.80～78.10,20.00～89.20 and 0.74～7.66 mg/L,respectively.The average value of BOD₅/TN was 2.70.The ammonia oxidation rate,nitrite oxidation rate and maximum denitrification rate of activated sludge in the secondary treatment unit were 3.56,4.29 and 13.73 mg/（g VSS?h）,respectively.The anaerobic phosphorus release rate and aerobic phosphorus uptake rate of PAOs were 4.03 and 3.19 mg/（g VSS?h）,respectively.The analysis of carbon emission characteristics shows that the total carbon emission is 1539.18 t CO_2eq/a,and the direct carbon emission and indirect carbon emission are the same.The main sources of carbon emission are nitrogen monoxide（N₂O）emission and power consumption,which account for 37.85%and 42.43%of the total carbon emission,respectively.There is a significant positive correlation between total carbon emissions and power consumption,indicating that power consumption is still the key to determining total carbon emissions.Therefore,when ensuring the efficient of nitrogen and phosphorus removal,low C/N and operating load WWTPs should also pay attention to the carbon emissions caused by power consumption.（2）The continuous operation test was carried out with THSF as the external carbon source of A²/O system.When the volume addition ratio of THSF increased to 0.6%（C/N was 8.27）,the effluent TN concentration decreased to 13.90±4.25 mg/L,and the TN removal efficiency reached 79.26%,which was approximately the same as that of sodium acetate（80.43%）.The mass balance analysis of this system showed that the COD provided by THSF accounted for approximately 45.77%of the total influent COD,of which approximately 20.49%COD（THSF）was used for denitrification,which was equivalent to the proportion of sodium acetate（20.72%）.Approximately 21.82%of COD in THSF was used for microbial proliferation,which was lower than that of sodium acetate（33.69%）,indicating that THSF as an external carbon source contributed to wasted activated sludge reduction.Therefore,THSF as an external carbon source can obtain good denitrification performance.The variations of microbial activity and community structure showed that THSF had no significant effect on nitrification rate,and the maximum nitrification rate was maintained at 3.14 and 2.24 mg/（g VSS?h）.The denitrification rate increased from 14.73 to 17.55 mg/（g VSS?h）,approximately 1.19 times.The results of16S r RNA gene sequencing showed that the relative abundance of Rhodobacter increased from 0.14%to 3.50%,and the relative abundance of Azospira and Terrimonas increased（accounting for 3.56%and 0.88%,respectively）.Therefore,the microbial community in activated sludge was changed after THSF addition.（3）The batch test results of the effect of THSF on nitrification rate showed that the ammonia oxidation rate and nitrite oxidation rate reached 3.68±0.20 and 3.51±0.17 mg/（g VSS?h）,respectively,in the absence of THSF.By changing the temperature of thermal hydrolysis pretreatment,when the volume addition ratio of THSF was 4%,the ammonia oxidation rate of 145℃,165℃and 185℃test groups decreased to the lowest,which were1.21±0.59,0.43±0.19 and 0.44±0.41 mg/（g VSS·h）,respectively.For the nitrite oxidation rate,the volume addition ratios were 3%,2%and 1%at 145℃,165℃and 185℃,respectively.The nitrite oxidation rates were 1.23±0.20,1.85±0.24 and 2.06±0.31 mg/（g VSS·h）,respectively.Therefore,the influence of THSF on nitrification rate may be due to the limited oxygen mass transfer efficiency by high concentration of COD and refractory organics produced in thermal hydrolysis pretreatment process.（4）The batch test results of the effect of THSF on denitrification rate showed that when the THSF with thermal hydrolysis temperature of 125℃,145℃,165℃and 185℃were used as external carbon sources,the TN removal rates were 97.35%,97.45%,96.79%and 86.65%respectively under the same COD supply ratio,indicating that thermal hydrolysis temperature was an important factor affecting denitrification efficiency.The production of refractory organic matter（organic nitrogen）increases at high thermal hydrolysis temperature,which will reduce the biodegradability of organic carbon sources in THSF.At the same time,in order to solve the problem of high energy consumption and high effluent COD concentration caused by refractory organics in the production of THSF,the thermal hydrolysis temperature can be appropriately reduced on the premise of ensuring high nitrogen removal efficiency.