| Recycling of drinking water treatment sludge, as an effective approach to enhance coagulation process, is a hot aspect of research and application as well as an important form of sludge reutilization in recent years. Based on the accumulation of disinfection by-products(DBPs) precursor, heavy metal and pathogenic microorganism during sludge recycling, using ultrasound to enhance coagulation of sludge recycling was proposed in this paper. We mainly studied the effect of the operation condition of recycled sludge and its characteristics on coagulation performance and flocs properties in coagulation process, as well as the feasibility of sludge recycling process. Additionally, we systematically investigated the conditions of ultrasound on the change of sludge properties, subsequently the coagulation characteristics and involved mechanism of recycling sonicated sludge was revealed.As indicated by the performance of coagulation performance of sludge recycling and its factors, the pollutants removal efficiency depended on the operation condition, the source water and sludge property. In the case that the recycled sludge with a proper total suspended solid concentration was dosed at the first slow mixing period at a lower recycling ratio, the better pollutants removal could be obtained. When the recycled sludge containing powder activated carbon, organics removal was enhanced regardless of the increase of powder activated carbon dose, but the recycling process was more effective for a specified raw water quality. Recycling filter backwash water containing low total suspended solid concentration could improve the removal performance of humic-like substance and accumulate the protein-like substance. Furthermore, the molecular weight distribution of organic fractionation in settled water depended on that of source water and corresponding filter backwash water.Different ultrasound conditions exerted different effects on sludge property. A low energy density of 0.03W/m L could disintegrate sludge and solubilize protein, polysaccharide and humic-like substance into sludge supernatant. In bath sonoreactor, the sludge disintegration was greater at low frequency(25/40 k Hz) than that at medium or high frequency(68/160 k Hz), while in probe reactor the sludge disintegrating was greater at a lower frequency(25k Hz). The more vigorous sludge disintegration and more released protein and polysaccharide were achieved at the highest energy density of 5W/m L. Meanwhile, p H and charge of sonicated sludge surface was kept at a certain level, however, the size of sonicated sludge flocs decreased with the increase of BET specific surface area. In addition, the sludge dewaterability became worse and the sludge settling performance improved better at relatively lower frequencies(25/40 k Hz) and higher energy densities(3/5W/m L), which were significantly related to the solubilized organics.The results of recycling sonicated sludge(with/without solubilized matters) at low energy density of 0.03W/m L within bath sonoreactor indicated that the better coagulation efficiency was gained without solubilized matters. There was no improvement on coagulation efficiency when recycling sludge with solubilized matters, the quality of the settled water even got worse, which was ascribed to the VSS reduction rate, the solubilization of protein and the reduction rate of flocs size. The results of recycling concentrated pre-sonicated sludge without solubilized matters indicated that lower frequencies(25/40 k Hz) within shorter sonication time less than 5min was more favorable for turbidity removal, while higher frequencies(125/160 k Hz) with longer sonication time(10~15min) for organics removal. Additionally, as compared to conventional coagulation process without sludge, the flocs generated by sludge recycling process under different ultrasound conditions achieved a significantly improvement of flocs size and better strength, while the recovery ability bacame worse. Moreover, the flocs formed preliminarily exposed to lower frequencies(25/40 k Hz) were more resistant to breakage, the recoverability was more irreversible, and the flocs were larger and more irregular.When recycling sonicated sludge at low energy density of 0.03W/m L within 10~20min in probe sonoreactor, the quality of settled water was enhanced, in addition, the lower frequency was slightly better for pollutions removal compared with the higher frequency. The optimized energy density, sonication time and sludge recycling ratio using response surface methodology displayed high similarity with the parameters obtained in single-factor experiments of recycling sonicated sludge without solubilized matters when the organics removal in settled water was the highest. Thus, the established model could well predict and simulate the organics removal and the optimal operation conditions of sludge recycling process.The results of molecular weight distribution, hydrophobicity and fluorescence of the organic fractionations in settled water indicated that recycled sludge increased the accumulation risk of weak hydrophobic acid, hydrophilic matter and <3k Da fractionations, as well as protein-like substances. Further studies on the removal of heavy metals(Cd2+ and Cu2+) and microorganisms in recycling process with sonicatd sludge(with/without solubilized matters) indicated that the accumulation risk of Cd2+ and Cu2+ was not effectively solved by ultrasound pretreatment, while the safety of microorganism could be improved. Based on the removal properties of DBPs precursors, heavy metals and microorganisms in recycling process, the paper presented an innovative method to ensure the chemical and microbiological safety of sludge recycling process, that is, using ultrasound to solubilize and degrade the organics and to inactivate microorganism in maximum, then the solubilized matters were separated to decrease the burden of recycling process.The research of this paper was of theoretic innovation, the change law of the sludge under ultrasound treatment was clarified, and the operation strategy to ensure chemical and microbial safety of sludge recycling process was proposed. The obtained conclusions could provide theoretical and technical support for the enhanced coagulation of low temperature and low turbidity water, and sludge reduction and resource reutilization. |
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