Enhanced Hydrolysis And Acidification Of Waste Activated Sludge By Biosurfactant Rhamnolipid

Although the activated sludge process is an economic and effective wastewater treatmentwidely used throughout the world, this process generates a large amount of waste activatedsludge (WAS), which requires considerable costs to handle and dispose.It generates a newinternational tendency of sludge minimization and resource utilization. Hydrolysis is the ratelimiting step of sludge digestion. Therefore, enhancing sludge hydrolysis is important baseand premise to reduce sludge. Compared with other sludge solubilization technology such asthermal, chemical (oxidants, odour inhibitors, alkaline, etc.), mechanical, ultrasonic andenzymatic methods, hydrolysis by additional biosurfactant could effectively accelerate thesolubilization of extracellular polymeric substances (EPS) from WAS and subsequentlypromote the accumulation of SCFAs. In addition, biosurfactants can be biodegradable,thereby not posing a threat to the environment.The article mainly aims at explaining theenhancing hydrolysis and acidification of waste activated sludge by biosurfactant rhamnolipid.Furthermore, the effect of pH on the hydrolysis and acidification of WAS in thepresence of biosurfactant RL was also investigated.Effect of biosurfactant rhamnolipid (RL) on hydrolysis and acidification of wasteactivated sludge (WAS) was investigated. The results indicated that rhamnolipid could greatlyreduced surface tension of sludge liquid, resulting in stimulating the hydrolysis rate andenhancing the production of short-chain fatty acids (SCFAs). With the increase of RL dosagefrom0.2to0.5g/g DS, the maximum SCOD, protein and carbohydrate concentrationincreased. After6h of hydrolysis, SCOD, protein and carbohydrate concentration raised from371.9,93.3and9.0mg/L to3994.5,800.0and401.4mg/L at RL0.3g/g DS, respectively.Furthermore, the release of NH4+-N, PO43--P and accumulation of SCFAs also improved in thepresence of RL. The maximum SCFAs was1829.9mg COD/L at RL0.3g/g DS, while it wasonly377.7mg COD/L in the blank test. The SCFAs mainly consisted of propionic acid andacetic acid, accounting for50%-60%of total SCFAs.In this investigation, the effect of pH (4.0-11.0) on WAS hydrolysis and acidificationenhanced by RL was studied. The results demonstrated that WAS hydrolysis increasedmarkedly with the addition of RL compared with the blank (without RL addition). Thehydrolysis and acidification of WAS in the presence of RL at alkaline pHs was more efficientthan that at acidic pHs and near neutral pHs. After6h of hydrolysis, the soluble protein andcarbohydrate was1034.2and482.9mg/L (pH=11.0), whereas it was only196.9and60.0 mg/L (pH=4.0),71.5and22.9mg/L (the blank), respectively. After2or3days offermentation, the accumulated SCFAs came to the highest, and then decreased with furtherincreasing time at all investigated pH values. The analysis of SCFAs composition showed thatacetic, propionic and iso-valeric acids were the three main products at any pH valueinvestigated. Higher pH contributed to greater proportion of acetic acid and less proportion ofiso-valeric acid, lower pH resulted in greater proportion of iso-valeric and less proportion ofacetic acid in the initial fermentation. The proportion of acetic acid for the system withbiosurfactant RL addition was16.65,36.33and62.94%respectively at pH4.0,7.0and11.0after1day. Meanwhile, it was correspondingly40.34,12.60and11.01%for iso-valeric acid.The variation of the surface tension of sludge liquid and the microstructure of sludgeparticle during sludge hydrolysis were also investigated. In addition, the effect of pH(4.0~11.0)on the morphological of sludge floe at RL0.3g/gDS was studied. It was found that RL couldgreatly reduced surface tension of sludge liquid, resulting in dissolution and dispersion ofsludge. The efficiency of sludge hydrolysis was improved with the increase of pH value,higher pH contributed to more dispersion of sludge flocs.