Using Of Internal Carbon Source In Excess Sludge To Strengthen Denitrification Of Wastewater

The influent water of most municipal wastewater treatment plants in our country had the problem of low carbon and nitrogen.In order to achieve deep denitrification,it is usually necessary to add external carbon source to make up the deficiency of carbon source.Activated sludge process was the most widely used wastewater treatment method,which had the advantages of mature technology,simple operation and good treatment effect,but it would produce a large amount of excess sludge.At present,the treatment accounts of excess sludge had accounted for 25～65%of the total operating cost of the municipal wastewater treatment plants.How to improve the nitrogen removal effect of wastewater economically and efficiently,reduce the cost of excess sludge disposal had become two major challenges for municipal wastewater treatment plants.Therefore,this paper studied the effect of pretreatment conditions on the release of internal carbon source from excess sludge firstly,and further analyzed denitrification effects by using internal carbon source of pretreatment excess sludge.In addition,the effects of sludge reduction and dewatering performance were also analyzed,as well as the technical application and economic applicability on this basis were also analyzed.The main findings were as follows:（1）During the pretreatment process,the pretreatment temperature（50,60,70,80,90°C）,p H（9,10,11,12,13）,and time（15,30,45,60,90,120 min）had important effects on the release of internal carbon sources in excess sludge.To investigate the effect of heating temperature on the pretreatment effect of excess sludge.When the treatment time was 120min,the release of carbon source increased rapidly with the temperature within the range of 50～80℃.When the treatment temperature increased from 80°C to 90°C,the solubility chemical oxygen demand（SCOD）concentration only increased by 0.36%,while the ammonium nitrogen（NH₄⁺-N）and total phosphorus（TP）concentrations increased by16.32%and 29.48%,respectively.To investigate the effect of the initial p H on the pretreatment effect of excess sludge.Under the pretreatment temperature of 80℃and the treatment time of 120min,the release of carbon source increased gradually with the initial p H range of 9～11.p H>11,the release increased rapidly.When the p H value increased from12 to 13,SCOD increased by 24.52%,while the ammonia（NH₄⁺-N）and total phosphorus（TP）concentrations increased by 70%and 71.70%,respectively.To investigate the effect of treatment time on pretreatment effect.The release of carbon source gradually increased with the pretreatment time range of 15～60 min.When the pretreatment time increased from 60 min to 120 min,sludge cracking rate only increased 2.42%,NH₄⁺-N and TP concentration increased 6.70%and 20.67%.Therefore,from the perspective of internal carbon source release,nutrient dissolution and energy saving,the optimum pretreatment conditions were p H of 12,the treatment temperature and time were 80°C and 60 min,respectively.（2）When thermal pretreatment,alkali pretreatment,and hot–alkali pretreatment sludge were used to enhance denitrification.The denitrification process was divided into two stages:the first stage（0～9 d）was the adaptive stage,and the effluent concentration of nitrate（NO₃^--N）gradually increased with time;the second stage（9～23 d）was the stable stage,the effluent concentration of NO₃^--N remained fluctuation in a certain range.When the internal carbon source of thermal pretreatment sludge strengthened denitrification,the removal rate of NO₃^--N in the range of 54.45～75.78%;When the internal carbon source of thermal pretreatment sludge strengthened denitrification,the removal rate of NO₃^--N in the range of 40.07～47.73%;When the internal carbon source of hot–alkali pretreatment sludge strengthened denitrification,the removal rate of NO₃^--N in the range of 50.91～59.40%.The effluent concentration of nitrite（NO₂^--N）showed a trend of rising first and then decreasing,the effluent concentration of NH₄⁺-N gradually decreased with time.（3）When the denitrification process run 23 days,the（MLVSS）reduction efficiency of sludge in the thermal pretreatment group was 25.64%,and the specific resistance to filtration（SRF）value was 1.95×10¹² m/kg;the MLVSS reduction efficiency of sludge in the alkali pretreatment group was 24.72%,and the SRF value was 3.16×10¹² m/kg;the MLVSS reduction efficiency of sludge in the hot–alkali pretreatment group was 27.77%,and the SRF value was 2.50×10¹² m/kg.The Extracellular Polymeric Substances（EPS）results showed,the EPS concentration of thermal pretreatment,alkali pretreatment,and hot–alkali pretreatment sludge decreased from 54.30 mg/VSS to 32.45,36.44 and 32.58after denitrification reaction.Among them,the loosely bound extracellular polymer（LB-EPS）,which was closely related to the sludge dewatering performance,decreased from 4.88 mg/VSS to 3.08,3.09,and 3.29 mg/VSS,respectively.（4）Based on the simultaneous fermentation and denitrification in the same reactor,a new concept of"nutrient removal-sludge reduction-phosphorus resource recovery"was proposed,from the perspective of resource recovery and utilization to solve the two problems in municipal wastewater treatment plants:insufficient carbon source and difficult treatment of excess sludge.Through economic analysis,it was found that could save 52.39RMB,when used 1 t thermal pretreatment excess sludge with 95%moisture content as carbon source.