| Aimed at improving settleability and dewaterability of activated sludge, this paper evaluated effect of chitosan and lysozyme pretreatment on sludge settlement, dewatering velocity and level, meanwhile, established the optimum dosage of chitosan and lysozyme, the best simultaneous pretreatment as well as the best experimental condition.The research discussed the mechanism of sludge dewatering preliminarily through measurement of Zeta potential, analysis and investigation of flocculation morphology of activated sludge and EEM spectra of SMP in sludge supernatant.As weak cationic polymeric flocculant, chitosan pretreatment combined with lysozyme could significantly improve sludge settleability which was beneficial to overcome the shortcomings in the worse performance of sludge sedimentation caused by lysozyme pretreatment. The optimum dosage of chitosan and lysozyme were 0.01g/gDS and 0.10g/gDS respectively, and the best simultaneous pretreatment was that sludge pretreated by chitosan first following by lysozyme which were determined through the single condition experiments and the orthogonal experiments. Under that conditions, the initial settling speed of sludge floc reached to about 7.13mm/min which increased nearly 14 times compared with that of the raw sludge floc,the moisture content of vacuum filtration was 71.3% and the specific resistance of filtration reduced to 0.25×1012m/kg, which decreased by 23.3% and 95.8% respectively less than the untreated sludge.Furthermore, the experimental results also ascertained that the optimum range of concentration were 20000-25000mg/L, the optimum temperature was 30? and the optimum pH value was 7, which were perceived as the best experimental condition of sludge pretreated by chitosan and lysozyme simultaneously.The research showed that Zeta potential, flocculation morphology, fractal dimension and EPS had important influence on settleability and dewaterability of activated sludge. On account of cationic amino group on molecules of chitosan and lysozyme, pretreatment by appropriate amount of chitosan and lysozyme could compress twin electrical layer and reduce the |?| potential of sludge.The |?| potential of sludge pretreated by the combition of 0.01g/gDS chitosan and 0.10g/gDS lysozyme decreased from 4.23mV of the untreated sludge to 0.28mV, which promoted destabilization and flocculation of sludge particles.Flocs of untreated sludge were tiny and dispersed, and the value of two-dimensional fractal dimensions (D2) was only 1.48; sludge particles were agglomerated to form large flocs by flocculation of chitosan, D2 value of sludge flocs increased to 1.69, nonetheless, the floc structure was loose and many pores existed inside the floc; sludge floc structure was further compressed and the internal pores were reduced by lysozyme pretreatment,thus sludge floc became larger and compacter when pretreated by chitosan and lysozyme together,and the maximum value of D2 increased to 1.74,which resulted in the rapid settling and dewatering velocity. Analysis of EEM spectra of SMP in sludge supernatant indicated that protein-like, humic-like and fulvic-acid-like were essential components of SMP with characteristic fluorescence peaks at (Ex/Em=280/350)? (Ex/Em=290/370) and (Ex/Em=230/385?360/450). The fluorescence intensity of protein-like barely changed when sludge pretreated by chitosan alone, whereas it increased by 25.8% with sludge pretreatment by lysozyme alone or combination of chisotan and lysozyme.The results indicated that chitosan pretreatment played a major part in destabilization and flocculation of sludge particles, which would barely change the organic matter composition in sludge; on one hand, the lysis of lysozyme resulted in dissolution of tryptophan concerned with emiocytosis and intracellular water, on the other hand, lysozyme pretreatment changed the composition and structure of EPS caused that protein-like substance dissolved out and diffused into sludge supernatant as the composition of SMP, simultaneously bound water were discharged from EPS. Therefore, colloidal particle agglomerated together to form large flocs by chitosan pretreatment first,the composition and structure of EPS were impoved effectively and the bound water in EPS were released through lysozyme pretreatment followed, which further enhanced significantly sludge dewaterability. |
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