Mechanisms And Resource Utilization Of Sludge Disintegrated By Ionizing Irradiation And Thermal Hydrolysis

Treatment and disposal of large quantities of waste activated sludge?WAS?produced from conventional processes have become technological and economic problems for many wastewater treatment plants?WWPTs?worldwide.On the other hand,WAS possesses recovery value due to abundant organic compounds?carbohydrates,proteins and fats?and nutrients?nitrogen and phosphorus?in sludge flocs and cells.In this study,⁶⁰Co gamma-ray irradiation was utilized to disintegrate WAS and enhance its physicochemical characteristics and dewaterability.Meanwhile,the involved disintegration mechanism was analyzed.Anaerobically digested sludge?ADS?was also treated through gamma-ray irradiation,and the effects on dewaterability and involved mechanisms were investigated.Struvite precipitation method was applied to recover P in the post-irradiation liquor.The effects of low-temperature thermal hydrolysis process?THP?on solubilization and dewaterability of WAS and the underlying reaction mechanisms were systematically studied in order to recycle superior C-source.A pilot-scale THP combining a two-stage anoxic/oxic?A/O?process for biological denitrification was operated to determine the feasibility of the solubilized C-source from the sludge.A nitrogen balance model of the two-stage A/O process was established based on the pilot-scale experiment.This study provides theoretical and practical bases for sludge reduction and recycling.The main conclusions are shown as follows:?1?The WAS disintegration process employed gamma-ray irradiation at doses of1–15 kGy and involved two stages,namely,dewaterability enhancement and high-efficiency disintegration stages.In the first stage??4 kGy?,irradiation slightly disintegrated the sludge flocs and cell structure and released a small quantity of extracellular polymeric substances?EPS?and particulate COD into the soluble phase.The solid matter content and particle size slightly decreased.The dewaterability of WAS and thickened WAS?TWAS?were enhanced remarkably due to the proper level of particle size,soluble EPS content and disintegration rate of sludge.In the second stage,remarkable disintegration and solubilization of sludge flocs and cells occurred at doses above 4 kGy.In this stage,the soluble COD,soluble EPS content and turbidity of the sludge supernatant increased obviously,whereas the sludge particle size and organic content in the solid phase decreased.Further enhancement of settliability was observed,but dewaterability of WAS and TWAS were deteriorate.Thus,gamma-ray irradiation at doses of 1-4 kGy could enhance the dewaterability of WAS and TWAS remarkably;while irradiation at doses higher than 4 kGy was beneficial to WAS disintegration.?2?ADS was treated by gamma-ray irradiation at doses of 2–15 kGy.The dewaterability of ADS was improved,but the nutrient content in the supernatant increased.The dewaterability of ADS was gradually improved with increasing dose from 5 to 10 kGy.No further improvement of dewaterability was observed at dose above 10 kGy,and the irradiation product may remain radioactive.Thus,the absorbed dose of 5–10 kGy would be potentially successful for ADS dewaterability improvement.The optimal dewaterability was achieved at 10 kGy,which could reduce the water content of sludge cake and the filtration time(TTF₅₀)from initial80.08%to 73.46%and from initial 660 s to 195 s,respectively.The release of soluble EPS and the reduction of particle surface charges were found to be the major reasons for dewaterability enhancement.Irradiation and cationic polyacrylamide?CPAM?co-conditioning achieved minimal synergetic effects.Furthermore,PO₄^3--P in the post-irradiation liquor could be recycled through struvite precipitation method,and the recovery rate was 86%.?3?Single-factor bench-scale experiment was conducted at 60–120°C for 60min and sludge water content of 97.4%.Organic compounds?SCOD,proteins,carbohydrates and lipids?and nutrients?N and P?were efficiently released into the sludge supernatant after thermal treatment,while sludge dewaterability gradually deteriorated.The undesirable dewaterability may be due to the release of abundant soluble EPS.60°C was proved to be the start point of sludge cell disintegration and lysis by conducting the DNA test.After thermal hydrolysis under different conditions,the BOD₅/COD of the sludge supernatant was 0.51–0.62,and the highest BOD₅/COD was observed at 80°C,showing desirable biodegradability.The SCOD/TN of the sludge supernatant was 16.7–26.2 under various conditions,indicating satisfied C/N.The results of BOD₅/COD and SCOD/TN demonstrated that the supernatant of the solubilized sludge can act as superior C-source for biological denitrification.?4?Integrating the lab-scale experiment results and pilot situation showed that the optimal treatment conditions comprising of sludge water content of 98%,treatment time of 90 min,treatment temperature of 80°C led to superior C-source under the pilot condition and achieved favorable SCOD/TN?15.8?from the solubilized sludge.During 40 days of stable operation,the effluent from the continuous two-stage A/O bioreactor was satisfactory with the addition of initial and secondary C-source.?5?A nitrogen balance model of two-stage A/O process was established based on the pilot-scale experiment.The pilot results were utilized to verify the model,which indicated that the effluent could comply with the preset discharge standards by adjusting the secondary C-source dosage,mixed-liquid suspended solid?MLSS?and internal recycle rate under different influent conditions.The model showed high simulation accuracy,which could be applied to optimize the two-stage A/O process.