Variation of dissolved organic matter during excess sludge reduction in microbubble ozonation system

Progress of improving waste activated sludge dewaterability: Influence factors, conditioning technologies and implications and perspectives

Large amounts of waste activated sludge (WAS) as a by-product generated from the biological treatment in wastewater treatment plants (WWTPs) is of high moisture content (MC), organic pollutants, heavy metals and pathogenic bacteria, it may cause serious environmental ecological risk without appropriate disposal. More than one half of the total operation cost is accounted for sludge disposal in a WWTP. Dewatering is an essential and important step during the sludge treatment and disposal process for it could efficiently reduce its volume, and be beneficial to the subsequent treatment and disposal of sludge. However, sludge should be conditioned before mechanical dewatering because of its high hydrophilicity. In this work, it presented a comprehensive review on sludge dewatering including summarizing the dewaterability measurement indexes, affecting factors, conditioning technologies, the improvement mechanisms. Finally, based on the eventual disposal and low carbon emission target, the implications and perspectives development of sludge conditioning were discussed. Based on the above discussion, there is no unified theoretical insight of the improvement mechanism of sludge dewaterability. In addition, the relationship between the microstructure of organic matters in sludge floc and the dewaterability should be deepened. Especially, how to choose the optimal conditioning technology for sludge dewatering lies in the physical and chemical properties of sludge, however, the carbon emission of the conditioning and dewatering process also needs to be considered. Accordingly, green, low-cost and organic conditioning agents are the direction of future research, and the establishment of automatic operating system and real-time evaluation index system is the key challenge.

Effects of microbubble pretreatment on physiochemical and microbial properties of excess activated sludge

Fast increased amount of excess activated sludge (EAS) from wastewater treatment plants has aroused universal concerns on its environmental risks and demands for appropriate treatments, while effective treatment is dependent upon proper pretreatment. In this study, air-supplied microbubbles (air-MBs) with generated size of 25.18 to 28.25 μm were used for EAS pretreatment. Different durations (30, 60, 90, and 120 s) yielded sludge with varied physiochemical conditions, and 60 s decreased sludge oxidation status and significantly increased adenosine triphosphate (ATP) content. Soluble, loosely-bound, and tightly-bound extracellular polymeric substances (SEPS, LB-EPS, and TB-EPS) were extracted from the sludge through a stepwise approach and examined through three-dimensional excitation-emission matrix (3D-EEM) and quantitative analysis. The results showed that 60- and 120-s treatments generated stronger fluorescence intensities on dissolved organic matters (DOMs) of protein-like and fulvic acid in LB-EPS and TB-EPS, which indicated the decrease of counterparts in EAS, and therefore facilitated sludge dewaterability and reduction. The dominant microbial communities in EAS, including Proteobacteria, Bacteroidota, Chloroflexi, and Actinobacteriota, were not significantly affected by MB pretreatment. The results collectively revealed the effects of MB pretreatment on EAS and indicated that MBs could be an effective pretreatment technique for EAS treatment process.

Effect of dissolved organic matter on selective oxidation of toluene by ozone micro-nano bubble water

The oxidation capacity of ozone micro-nano bubble water (OMBW) was always higher than ozonated water due to enhanced contact by bubble interface, while the effect of coexisted dissolved organic matter (DOM) on the oxidation efficiency was still unclear. In this paper, batch experiments were carried out to investigate the selective oxidation of toluene by both OMBW and ozonated water (OW) with coexisted DOM in water. Five types of background solutions were applied in this study, including humic acid solution, fulvic acid solution and three types of diluted landfill leachates at the same content of total organic carbon. Results showed that coexisted DOM had a greater inhibition effect on toluene oxidation rate by OMBW, and the oxidation rate of toluene by OMBW and OW became close. It was mainly caused by the decreased reaction rate between toluene and hydroxyl radical (k_T-OH·) in OMBW after the introduction of DOM, which competed for the adsorption sites on micro-nano bubble interface. The fraction of ozone to oxidize toluene as well as k_T-OH· was in positive correlations with SUVA₂₅₄ and the content of humic acid-like substances, but negatively correlated with E₂/E₃, content of tryptophan-like proteins and content of fulvic acid-like substances. In addition, increasing the ozone dose was not effective in increasing the utilization rate of ozone in OMBW due to limited adsorption sites on micro-nano bubble interface. The paper was conductive to the application of ozone micro-nano bubble water in groundwater remediation with complex water matrices.

Interaction Mechanisms and Application of Ozone Micro/Nanobubbles and Nanoparticles: A Review and Perspective

Ozone micro/nanobubbles with catalytic processes are widely used in the treatment of refractory organic wastewater. Micro/nanobubble technology overcomes the limitations of ozone mass transfer and ozone utilization in the application of ozone oxidation, and effectively improves the oxidation efficiency of ozone. The presence of micro/nanobubbles keeps the catalyst particles in a dynamic discrete state, which effectively increases the contact frequency between the catalyst and refractory organic matter and greatly improves the mineralization efficiency of refractory organic matter. This paper expounds on the characteristics and advantages of micro/nanobubble technology and summarizes the synergistic mechanism of microbubble nanoparticles and the mechanism of catalyst ozone micro/nanobubble systems in the treatment of refractory organics. An interaction mechanism of nanoparticles and ozone microbubbles is suggested, and the proposed theories on ozone microbubble systems are discussed with suggestions for future studies on systems of nanoparticles and ozone microbubbles.

Partial ozonation of returned sludge via high-concentration ozone to reduce excess sludge production: A pilot study

Partial ozonation of returned sludge via high and low concentration of ozone were compared to evaluate their efficiency in excess sludge production reduction. A pilot-scale system of anaerobic/anoxic/oxic (A/A/O) + ozonated sludge recycle (OSR) process was operated for 97 days, to investigate the effects of different ozone concentration (380 mg/L and 150 mg/L) on the nutrient removal capacity, sludge reduction rate, the excess sludge properties including settling, dewatering and anaerobic digestion (AD) performance. It was found that at the same total ozone dosage (13 mg/g MLSS, 25 mg/g MLVSS), the ozone of 380 mg/L achieved much higher organic matters and total excess sludge reduction (41.6% and 25.9%) than 150 mg/L applied (31.0% and 18.2%). It also laid less deterioration effect on the effluent quality and had better nutrient (COD, NH₄⁺-N, TN) removal capacity than 150 mg/L applied. Meanwhile, little difference was found in the settling, dewatering and AD properties of excess sludge from the two A/A/O + OSR processes. Meanwhile, sludge solubilization rate, BIOLOG ECO microplate, 16S rRNA sequencing were applied comprehensively to illustrate the reasons for above advantages of the elevated ozone dosage applied. It was clarified that compared to 150 mg/L, A/A/O + OSR with ozone of 380 mg/L had higher sludge solubilization rate, less impact on bacterial community distribution and utilization capacity of carbon sources in bioreactors.

Performance of microbubble ozonation on treated tropical peat water: Effects on THM4 and HAA5 precursor formation based on DOM hydrophobicity fractions

The hydrophobicity properties of dissolved organic matter (DOM) found in tropical peat water has an impact on the formation of carcinogenic DBPs such as trihalomethanes-4 (THM4) and haloacetic acids-5 (HAA5). This study was conducted to determine the effect of microbubble ozonation on changes in DOM fraction and its effect on the formation of THM4 and HAA5. Alum coagulation and activated carbon adsorption were carried out to reduce the DOM concentration before microbubble ozonation. Microbubble ozonation was carried out at acidic (pH 5.5), neutral (pH 7) and alkaline (pH 8.5) conditions to determine the effect of pH. Coagulation and adsorption of activated carbon were successful in reducing the presence of the hydrophobic acid fraction (HPOA) in peat water completely, but the transphilic (TPH), charged hydrophilic (HPIC) and neutral hydrophilic (HPIN) fractions remained in the water. Microbubble ozonation succeeded in decreasing the presence of TPH fraction but increased the formation of HPIC and HPIN. The degradation of the TPH fraction resulted in reduced formation of chlorinated THM4 and HAA5 (C-THM4 and C-HAA5). On the other hand, the formation of HPIC and HPIN fractions increased the formation of brominated THM4 and HAA5 (B-THM4 and B-HAA5) after the final chlorination process.