Effect Of Oxidation And Coagulation Conditioning On Sewage Sludge Deeply Dewatering Performance Under The Ultrahigh Pressure Filter Press And Its Mechanism

With the increase of urban sewage,the amount of sewage sludge produced by municipal wastewater treatment plants(WWTPs)is quickly increasing.Large sludge production has created significant environmental problems for the government.Sludge dewatering is a crucial step that can reduce the operational costs associated with transportation and disposal of sludge and facilitate sludge minimization and recycling.Sludge with a poor compressibility,strong hydrophilic extracellular polymeric substances(EPS)and high levels of bound water caused more difficulties for deeply sludge dewatering.Recent studies widely used coagulants and advanced oxidation processes(AOPs)conditioning methods to improve sludge dewaterability.However,due to the differences in sludge characteristics or sludge sources and the lack of systematic and in-depth comparison methods,it is difficult to find out a high-efficient,economical,promising,and environment-friendly sludge treatment technique.In this study,the combined oxidation and coagulation treatment was developed and investigated to improve sludge dewaterability.The goals of this study were to(a)explore the feasibility of the oxidation and coagulation conditioning methods combined with an ultrahigh filter press for enhancement of the sludge dewaterability;(b)optimize operational conditions for sludge dewatering;(c)characterize the conditioning and dewatering mechanism associated with the above treatments and summarize the reaction mechanism;(d)compare the sludge dewatering efficiency,operational costs,and the heavy metals toxicity and phytotoxicity of dewatered sludge by the chemical conditioning methods;(e)establish a system to evaluate the dewatering efficiency under different chemical conditioning methods.Therefore,we will find out an inexpensive,high-efficient,and eco-friendly method to realize sludge deeply dewatering,which can provide some useful information for sludge deeply dewatering in large-scale applications and engineering practices.(1)The application of different aluminium and iron salt coagulants pretreatments on improving sludge dewaterability was explored.The single factor test was used to investigate the sludge dewatering efficiency of inorganic coagulants with different aluminium or iron salts.The optimal dosages of inorganic coagulants were determined.To further understand the sludge conditioning mechanism of different aluminium and iron salts,the sludge dewaterability,the sludge characteristics,and the EPS composition were traced and compared.The study generated the following conclusions:(?)Iron coagulants improved sludge dewaterability significantly more than aluminium coagulants did.With the ultrahigh pressure filtration system(Pressure of 6 MPa and compress time of 30 min),the water content of sludge cake conditioned with poly ferric sulfate(PFS)could be reduced to 56.93 wt%.(?)Compared with aluminium coagulants conditioning,iron salts had stronger charge neutralization ability,formed smaller and more compact flocs,and had a stronger acidification effect.(?)Compared with aluminium coagulants conditioning,more loosely bound EPS(LB-EPS)were removed when the sludge was treated by iron coagulants,thus improvement of sludge dewaterability.(2)Polymeric aluminium ferric chloride(PAFC)combined with anaerobic mesophilic digestion(AMD)conditioning was investigated as a novel and feasible technique to improve sludge dewatering.This study applied response surface methodology(RSM)to determine the optimal operating parameters of the combined PAFC/AMD conditioning and ultrahigh pressure filtration dewatering process that led to an optimized water content level in the dewatered cake.Under these optimal conditions,the watercontent of the dewatered cake was reduced to 54.54 wt%,thus realizing sludge deeply dewatering.The study obtained the detailed mechanisms involved with the conditioning and dewatering process,by tracing the sludge,filter cloth,and filtrate characteristics.These results indicated sludge dewaterability was closely related to sludge characteristics,the filter cloth characteristics,and the filtrate characteristics,which was helpful to understand the sludge dewatering mechanism.(3)A novel sludge conditioning process involving Ca(ClO)2,ferric coagulant,and walnut shell(WS)was applied to enhance sludge dewaterability.This study investigated the effects of the combined Ca(ClO)2/ferric coagulant/WS treatment on sludge dewaterability and EPS properties,with the goal of optimizing the operating conditions for the combination treatment.Under the optimal conditions,this combination treatment significantly improved sludge dewaterability(Specific resistance to filtration(SRF)decreased by approximately 90.84%).Using a novel high pressure filtration system,the water content of the dewatered sludge cake conditioned with Ca(ClO)2/ferric coagulant/WS was reduced to below 50 wt%,implying sludge deeply dewatering.The results presented the mechanism of the combined treatment by analyzing the morphological structure,flocs morphology,rheological behavior,and bound water.The mechanism is divided into three major processes:Ca(ClO)2 oxidation,ferric coagulant re-flocculation,and WS skeleton construction.(4)This study employs a simple zero valent iron(ZVI)pretreatment,namely a thermal and acid-washed(TA)pretreatment,combined with oxidants system to effectively improve sludge dewaterability.Single factor test was used to optimize the operating conditions of TA/ZVI/oxidants treatment.The results showed that the TA/ZVI/oxidants treatment had a significant synergistic effect to achieved deeply dewatering of sludge(Water content<60 wt%)under optimal conditions.The mechanisms for the significant enhancement of sludge dewaterability was elucidated by exploring the sludge characteristics and the ZVI surface structure characteristics.The mechanism is divided into two processes:oxidation of free radicals produced by TA/ZVI/oxidants treatment and strong flocculation by Fe3+.In the ZVI reuse experiment,the TA/ZVI/oxidants treatment remained highly effective for sludge dewatering even after four cycles.(5)AOPs based on pre-ultrasound-thermal-acid-washed zero valent scrap iron(UTA-ZVSI)was developed to improve sludge dewaterability.This study established and continuously operated a novel,recyclable,and rapid UTA-ZVSI/H2O2 system and optimize operational conditions of the UTA-ZVSI/H2O2 reactor for sludge dewaterability.The water content of the dewatered cake decreased to 44.15 wt%during optimal operational conditions.Based on the sludge characteristics,a three-step treatment mechanism involving UTA-ZVSI/H2O2 has been developed,including iron flocculation,hydroxyl radical oxidation,and skeleton building.The dewatering efficiencies of three types of representative WAS were consistently effective in the UTA-ZVSI/H2O2 reactor for up to 15 cycles.This result indicates the UTA-ZVSI/H2O2 system is a stable and efficient method to achieve the deep sludge dewatering.(6)This study thoroughly evaluated different types of sludge dewatering processes in terms of sludge dewaterability,environmental risk of dewatered sludge.Compared to coagulants,the use of the combined oxidation and coagulation as conditioners shows greater promise for sludge dewatering,as the combined oxidation and coagulation have dual roles in alleviating the environmental risk of heavy metals and the phytotoxicity of dewatered sludge.Among the tested methods of oxidation and coagulation,UTA-ZVSI/H2O2/CaO treatment achieved the highest sludge dewaterability improvement and the most effective cost.The combined oxidation and coagulation treatment significantly alleviated heavy metal toxicity and phytotoxicity of dewatered sludge compared to the coagulants.Ca(ClO)2/FeCl3/WS treatment had the lowest environment risk from heavy metals and the lowest phytotoxicity of dewatered sludge,which was highly beneficial for agricultural use.In summary,this study systematically compares the sludge dewatering efficiency,the heavy metals toxicity and phytotoxicity of dewatered sludge,which provides experimental data reference and theoretical guidance for the selection of sludge conditioning treatments.