Enhancing The Performance Of Main-stream Nitrogen And Phosphorus Removal By Side-stream Techniques

Biological nutrient removal（BNR）process in wastewater treatment plants（WWTPs）doesn’t perform well due to the low carbon influent.In order to meet the strict discharge standards,WWTPs usually add carbon sources or chemicals.However,these effective measures would increase the operational costs and carbon emissions.Under this circumstance,the ratios of C/N and C/P should be improved directly or indirectly rather than adding carbon source or chemicals.Side-stream techniques can be considered to enhance the main-stream nitrogen and phosphorus removal,such as side-stream anaerobic fermentation of excess sludge or side-stream recovery of phosphorus.Firstly,the side-stream recovery of phosphorus was conducted in the pilot-scale of modified UCT for evaluating enhancement on the performance of BNR process.With sufficient organics in influent（COD≥350 mg/L,TN=50 mg N/L,TP=5 mg P/L）,the effluent can meet the first-class A level.However,low influent carbon sources（COD=250 mg COD/L,,TN=50mg N/L,TP=5 mg P/L）cause the worse effluent which cannot meet the first-class B level.With the side-stream recovery of phosphorus（the quantity ratio of side stream to influent,qs=15%）,the efficiency of P-recovery was at about 37%of total P in influent,and the effluent concentration of TP and TN sharply reduced to 0.46 mg P/L and 16.1 mg N/L.However,the continuously increased ratio（qs=30%）would not be effective for further improvement.The analysis indicated that the alkalinity with the side-stream process increase the p H value back to7.5 in main-stream units,which is a favor of nitrification for organisms and NH₄⁺-N in effluent was at low level of 0.54 mg N/L.Moreover,the higher ratio of side-stream would shift the metabolic patterns of dominated microorganisms to glycose pattern,which lead to loose structure of flocs and poor sedimentation performance.Secondly,the internal carbon source released from excess sludge in the side-stream anaerobic fermentation was also conducted to enhance BNR.The lab-scale experiments were designed for determining the optimum SRT of fermentation process.The results showed that the optimum SRT=10 d,and the total VFAs is 170 mg/L but the acidification efficiency is only36%.Therefore,the alkaline pre-treatment for excess sludge was further implemented to strengthen the fermentation and the internal carbon sources in supernatant reached to higher level at about 1486.4 mg COD/L.On the basis of lab-scale experiments,the side-stream anaerobic fermentation of excess sludge process was conducted in the lab-scale sequenced batch reactor（SBR）.The results showed that the BNR performance were worse instead of being better even if increasing the ratio of side-stream.The images by microscope showed that the flocs became loose as well as the poor sedimentation performance.It indicated that the SBR were impacted by side-stream anaerobic fermentation.Further evaluations implied that the relative abundance of functional microorganisms and their associated activities were also declined.It was supposed that the ratio of side-stream（qs=3%→6%）is too high,and thus the load of N and P was overwhelmed.As a result,the carbon source in main-stream is further limited,which deteriorated the performance of BNR.Therefore,the measurements of removing/recovering a part of N and P from fermented supernatant is further designed in the experiment,but the performance was not obvious and also the measurement was not practical.In a word,the optimized ratio of side-stream should be considered for the balance of N and P with internal carbon source released from fermentation process.Finally,the pilot-scale of modified UCT is carried out to testify the effect of side-stream anaerobic fermentation of excess sludge.The ratio was set at low level（qs=1.25%）,and the result showed that COD in fermented supernatant was only about 318 mg COD/L,which was much lower than the level of SBR.However,N and P in effluent significantly decreased to PO₄^3--P=1.6 mg P/L,NH₄⁺-N=0.59 mg N/L and NO₃^--N=8.0 mg N/L,indicating the better performance of BNR with side-stream anaerobic fermentation.The evaluation showed that PHB content andΔP/ΔPHB increased from 4.62 to 4.93 mg PHB/g MLSS and from 1.48 to9.88 mg P/mg PHB,respectively,which reflected that the capacity of phosphate accumulating organisms（PAOs）was enhanced by the fermented sludge.Moreover,P content in EPS increased from 11.63 to18.30 mg/g EPS and polysaccharide content in EPS declined from115.95 to 99.09 mg/g EPS.These results indicate that glycose metabolism pattern was inhibited but the phosphorus metabolism pattern was promoted.Above all,it concluded that the“simple”release of internal carbon sources in side-stream anaerobic fermentation of excess sludge strengthened the main-stream process.More importantly,the balance between carbon sources and N&P loads in fermented sludge should also be considered for improving the relative abundance of dominate microorganisms,associated activities and optimizing the metabolism patterns.