Effects of ultrasonic pretreatment on sludge dewaterability and extracellular polymeric substances distribution in mesophilic anaerobic digestion

Effect of ultrasonic pretreatment on sludge dewaterability was determined and the fate of extracellular polymeric substances （EPS） matrix in mesophilic anaerobic digestion after ultrasonic pretreatment was studied. Characteristics of proteins （PN）, polysaccharides （PS）, excitation-emission matrix （EEM） fluorescence spectroscopy and molecular weight （MW） distribution of dissolved organic matters （DOM） in different EPS fractions were evaluated. The results showed that after ultrasonic pretreatment, the normalized capillary suction time （CST） decreased from 44.4 to 11.1 （sec·L）/g total suspended solids （TSS） during anaerobic digestion, indicating that sludge dewaterability was greatly improved. The normalized CST was significantly correlated with PN concentration （R2 = 0.92, p 〈 0.01） and the PN/PS ratio （R2 = 0.84, p 〈 0.01） in the loosely bound EPS （LB-EPS） fraction. Meanwhile, the average MW of DOM in the LB- EPS and tightly bound EPS （TB-EPS） fractions also had a good correlation with the normalized CST （R2 〉 0.66, p 〈 0.01）. According to EEM fluorescence spectroscopy, tryptophan-like substances intensities in the slime, LB-EPS and TB-EPS fractions were correlated with the normalized CST. The organic matters in the EPS matrix played an important role in influencing sludge dewaterability.

Recovery of phosphorus as struvite from sewage sludge ash

Phosphorus （P） is an element vital for all living organisms, yet the world＇s reserves of phosphate rock are becoming depleted. This study investigated an effective P recovery method from sludge ash via struvite precipitation. Results showed that more than 95% of the total P content was extracted from sludge ash by applying 0.5 mol/L HC1 at a liquid/solid ratio of 50 mL/g. Although heavy metal leaching also occurred during P extraction, cation exchange resin efticiently removed the heavy metals from the P-rich solution. Orthogonal tests showed that the optimal parameters for P precipitation as struvite would be a Mg：N：P molar ratio of 1.6：1.6：1 at pH 10.0. X-ray diffraction analysis validated the formation of struvite. Further investigations revealed that the harvested precipitate had a high struvite content （97%）, high P bioavailability （94%）, and low heavy metal content, which could be considered a high quality fertilizer.

Enhanced anaerobic digestion and sludge dewaterability by alkaline pretreatment and its mechanism

To investigate the influences of alkaline pretreatment on anaerobic digestion （AD） and sludge dewaterability after AD, waste activated sludge was adjusted to different pH values （8, 9, 10, 1 1, 12） and placed at ambient temperature for 24 hr. The samples were then adjusted to the initial pH and subjected to 25 days of AD. The results showed that, when compared with the control （pH 6.8）, total suspended solids （TSS） and volatile suspended solids （VSS） reduction following pretreatment at pH 9-11 increased by 10.7%-13.1% and 6.5%- 12.8%, respectively, while biogas production improved by 7.2%-15.4%. Additionally, significant enhancement of sludge dewaterability after AD occurred when pretreatment at pH 8-9 was conducted. The proteins and carbohydrates transferred from the pellet and tightly bound extracellular polymeric substances （TB-EPS） fractions to the slime and loosely bound EPS （LB-EPS） fractions after pretreatment and during the AD process, and the concentrations of proteins and carbohydrates in the slime fraction had a good linear relationship with the normalized capillary suction time （CST）. During the AD process, the normalized CST was positively correlated with the organic materials in the loosely bound fraction of the sludge matrix （R2/〉 0.700, p 〈 0.01）, while it was negatively correlated with the organic materials in the tightly bound fraction （R2≥ 0.702, p 〈 0.01）. These results suggest that alkaline pretreatment could break the EPS matrix and release inner organic materials, thus influencing the efficiency of the AD process and dewaterability after AD.