Enhanced Hydrogen Production From Wasted Activated Sludge By Combining Ultrasonic And Free Nitrous Acid Pretreatment

Wastewater treatment plants(WWTPs)play important roles in preventing the natural environment from pollution.As the central technology of WWTPs,activated sludge process generally transforms ?35% and ?65% of the wastewater organics into carbon dioxide and water by catabolism and WAS by assimilation,respectively.Therefore,the waste activated sludge(WAS)becomes the main by-product in the sewage treatment process.On the other hand,WAS contains considerable toxic and harmful matters,which poses a severe threat to human health and ecological environment if mishandled.Apart from the toxic matters,WAS also includes a variety of organics,and most of them can be used as substrates to produce hydrogen.Therefore,hydrogen generation by using WAS has attracted more and more attention.With the development of sludge pretreatment technology in recent years,among these technologies,ultrasonic(US)technique has extensive application foreground,due to its effective and non-secondary pollution to environment.WAS was treated for hydrogen production by anaerobic fermentation,which has been confirmed by researchers.Nevertheless,hydrogen produced is also consumed substantially by hydrogen consumers.Thus,the yield of hydrogen is at a low level.It was found in this study that the addition of nitrite into acidic fermentations after ultrasonic(US)pretreatment to form free nitrous acid(FNA),to further enhance hydrogen yield.In addition,FNA is a renewable and low-cost chemical that can be in situ produced from anaerobic fermentation liquor.Therefore,the findings is economic.Experimental results showed that when 180 mg/L nitrite was added into the US(2 W/mL,15 min)pretreated waste activated sludge(WAS),the maximal hydrogen yield of 24.81 ± 1.24 mL/g VSS(volatile suspended solids)was obtained under acidic fermentation(1.0 mg/L FNA was initially formed under this condition),which was 2.21-folds(or 1.36-folds)of that from US pretreatment(or FNA treatment)alone.The mechanism investigations indicated that combination strategy(US+FNA)posed a positive synergistic effect on the disintegration of sludge and enhanced the release of biodegradable organics.The effects of US+FNA on other biological fermentation processes relevant to hydrogen accumulation were found that although US+FNA has an inhibitory effect on the biological processes of hydrogen generation and hydrogen consumption,the inhibiting effect on microbial activity of hydrogen consumption was severer.Moreover,all the pretreatment methods inhibited the activities of all the enzymes measured to some extent,while the suppression effect of different pretreatment ways to the activity of related enzymes with hydrogen production was much slighter than that to the enzymes concerned with hydrogen consumption.Finally,According to the findings achieved in this study,a concept of “closed-loop”for future operation in a WWTP with the implement of US+FNA was proposed.This model not only improves the utilization efficiency of resources to a certain extent,but also promotes the production of hydrogen.