| In addition to degradation of organic matter,anaerobic digestion（AD）of organic waste（water）s can produce recyclable volatile fatty acids（VFAs）which can serve as an external carbon source for the nitrogen and phosphorus removal process in wastewater treatment,and can also generate methane（CH4）as high value-added bioenergy.AD is one of the effective ways to realize sludge stabilization,resource recovery,treatment and disposal.However,AD foaming is a widespread problem in the development and application of AD.Biofoam can occupy the effective volume of the reactor and reduce its efficiency of biogas production.In the long run,it can severely affect the reactor’s stable operation and increase maintenance costs.Organic load（OL）reportedly is a common factor leading to foaming.The previous studies on foaming have mainly focused on traditional anaerobic digestion process,and thus far,there are still few reports on whether foaming could occur and how the foam was formed in AD of thermal-alkali pretreated sewage sludge under elevated organic loading rates.Therefore,in this context,this paper first explored the possibility of organic loading-induced foam formation and investigated the key surface active substances that promoted foam induction using an anaerobic sequencing batch reactor for VFAs production from sludge hydrolysates.Then the effects of these substances on the foaming of methanogenic systems were investigated by means of semi-continuous sludge anaerobic digestion experiments,and furthermore,from a microbial perspective,the microorganisms that might influence foam formation were analysed in-depth using molecular biology techniques.Finally,the key surface active substances were extracted from the digestate to better understand the differences in their effects on foam performance and further clarify the mechanisms involved in the foam formation and stabilization during AD,which would provide key knowledge and technical support for the establishment of early warning and regulation mechanisms of foam in anaerobic processes.Based on the above experimental trials,the following conclusions could be drawn:（1）In the fermentative VFAs productive process frin the hydrolysate,increasing OL substantially enhanced the foam potential of the sludge hydrolysate,but did not evidently induce foam formation in the process.The foaming potential of the hydrolysate at 20 g COD/L was strikingly higher,and however,seemingly the low gas production could not further sustain foam formation in the process.The soluble proteins and humic acids in the process were proven to have prominent surface activity,and both were significantly negatively correlated with surface tension and positively correlated with viscosity.The elevated OL led to a significant increase of protein and humic acid contents in the reactors,resulting in an evident ascent of viscosity and a descent of surface tension,thus substantially promoting the foaming potential of the fermentation broth.（2）By increasing the organic loading rate（OLR）,the methanogenic process of sludge AD also foamed.The results showed that 5.0 g COD/（L·d）was the critical threshold for the occurrence of foam,and when the OLR of the system was increased to 7.0 g COD/（L·d）,the foam phenomenon increased significantly.And the occurrence and enhancement of the foam phenomenon showed a good agreement with the changes of the foam potential test,viscosity,surface tension and other indicators（such as macromolecular contents）.High OLR can significantly lead to the accumulation of substrates（proteins）and intermediates（extracellular polymeric substances,VFAs）.Correlation analysis showed that the proteins in the soluble extracellular polymeric substances（S-EPS）and humic acid together contributed to the formation of foam,and a significant accumulation of propionic acid in the reactor was observed during periods of severe foam.In addition,analyses of the extracellular polymers using three-dimensional fluorescence spectroscopy,Fourier Transform Infrared Spectrometer（FTIR）and o-phthalaldehyde（OPA）pre-column derivatization reversed-phase high performance liquid chromatography indicated that the proportion of irregularly curled structures in the secondary structure of the protein had doubled compared with that of the stable phase.The proportion of hydrophobic amino acids increased by about 6%in the profile of amino acids,which might be the underlying cause for the proteins that promote foam formation.（3）Analyses of the microbial community structure showed that Firmicutes was th e predominant hydrolytic fermenting microorganism during the anaerobic digestion proc ess.The relative abundance of acidifying microbes such as Proteobacteria and Synerg istota evidently varied before and after foam formation.The relative abundance of Pro teobacteria decreased significantly while the relative abundance of Synergistota showed an increasing trend.This change may led to an accumulation of VFAs in the reactor,which raises the surface tension of the sludge and in turn promotes foam formation.With the archaeal commu-nity,the abundance of Candidatus＿Methanofastidiosum strikin gly differed between the sludge layer and the foam layer,which was usually detected in viscous anaerobic conditions,which was presumably related to foam stabilisation.（4）The results of the foam potential tests showed that humic acid solutions led to the highest amount and strongest stability of foam,followed by protein solutions and propionic acid solutions.By evaluating the surface properties and the foam evolution,the mechanisms via which these three surface active substances affected the foam properties are deciphered.The viscosity and surface tension of the three solutions were first measured and the adsorption properties of the three substances at the gas-liquid interface were investigated by fitting the adsorption isotherm curves of the three substances.The results showed that the viscosity of the humic acid solution was the highest,followed by the protein solution and the propionic acid solution at the same concentration conditions.Compared with sludge proteins and propionic acid,HA had the lowest critical micelle concentration（CMC）and could be adsorbed more quickly to saturate the gas-liquid interface,substantially reducing surface tension.In addition,the trends in bubble diameter distribution and drainage rate over time for the foams produced by the three substances were in the following order:HA<protein<propionic acid.The results showed the mechanisms how surface active substances affected the foam formation were could be summarized as:the surface active substances such as proteins and humic acids increased foam volume by reducing surface tension of bubbles,meanwhile increased the viscosity and thus consequently reduced the flow rate of the liquid in the bubble film,thus slowing down the drainaging process process of the bubbles and improving foam stability. |
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