Effect Of Sludge Return Site On System Effectiveness Based On Side-Stream Chemical Phosphorus Recovery In A~2/O Process

A~2/O process is the most widely used biological nitrogen and phosphorus removal process in practice,which is expected high phosphorus removal and maximum recovery can be achieved by introducing side-stream chemical phosphorus precipitation in this process.In order to ensure the concentration of sludge in the system,the sludge in the intermediate sedimentation tank(sludge after phosphorus release)needs to return to A~2/O main-steam after side-stream chemical phosphorus recovery.However,at present,there is no clear understanding on where the sludge should be returned to the main-stream after phosphorus release.Different return site may have different effect on the system.It is not clear how the return site affect operation of the system.This paper is based on the A~2/O side-stream chemical phosphorus removal system,the activated sludge after phosphorus release was returned to different sites(aerobic tank?anoxic tank and anaerobic tank)of main-stream system.The effects of three different side-stream on the nitrogen and phosphorus removal performance of the whole system?the activity changes of phosphorus accumulating bacteria and the transformation of metabolites were studied,which provided a reference for phosphorus recovery and application in different side-stream processes.The main conclusions of study are shown as following:(1)The side-stream sludge returned to the aerobic tank,the nitrogen and phosphorus removal performance of the system were improved,and the concentration of soluble phosphate in effluent was(0.07±0.04)mg/L;The ratio of phosphorus recovery was 27.45%,which could maintain the advantage of side-stream phosphorus recovery within short-term.The activity of phosphorus accumulating bacteria was restrained,and the total phosphorus release amount and the glycogen content of the system decreased,but the phosphorus content of sludge did not decrease with a long period operation of the side-stream.It was considered the decrease of phosphorus release was caused by the lack of glycogen content.On the other hand,the sludge became loose,settling performance deteriorated,so that the intermediate sedimentation tank was clogged up,resulting the system operation was blocked.(2)The side-stream sludge returned to the anoxic tank,the phosphorus removal of the system could be improved at the beginning of the operation,the concentration of soluble phosphate in effluent was(0.07±0.04)mg/L;After 20 days,the sludge settling performance became worse,the phosphorus removal performance deteriorated,but the nitrogen and the organic matter removal performance were not significantly affected.The rate of anaerobic phosphorus release and aerobic phosphorus uptake decreased,but the rate of anoxic phosphorus uptake increased.The phosphorus removal ratio between anoxic denitrifying phosphorus-accumulating and aerobic polyphosphate increased from 43.20%to 53.38%,the denitrifying phosphorus removal ratio was strengthened;The metabolic patterns of PHA and glycogen in sludge microorganism were not changed,but the amount of PHA synthesized in anaerobic stage decreased gradually;The maximum recovery amount of side-stream phosphorus accounts for 24.75%of the influent phosphorus content,which could achieve considerable phosphorus recovery effect;When the system collapsed,the side-stream chemical phosphorus recovery was stopped,the functions of the system were gradually restored,and the continuous operation of the system could be realized.(3)The side-stream sludge returned to the anoxic tank,the concentration of TN in effluent was higher,system denitrification performance declined;The phosphorus removal performance was improved in the short term,but the phosphorus release and the phosphorus content of sludge decreased gradually;The activity of phosphorus accumulating bacteria was inhibited,both anaerobic phosphorus release rate and anoxic/aerobic phosphorus uptake rate decreased;There were no effect on the transformation of PHA and glycogen,but the maximum amount of PHA synthesis in anaerobic stage was decreased;The outside of microorganism celles were surrounded by a large number of filamentous bacteria and the sedimentation performance of activated sludge becomed worse;The phosphorus recovery rate accounts for 54.09%,which can achieved certain phosphorus recovery.