| Sewage sludge is a by-product of the traditional municipal wastewater treatment process and large quantities of sewage sludge are produced every year.The extracellular polymer substances(EPS)in sludge,mainly composed of polysaccharides and proteins,are highly stable and hydrated,thus resulting in the water content of sludge as high as 95%–99%.Therefore,sludge dewatering is a prerequisite step for sludge treatment,disposal and resource utilization.In addition,the sludge not only contains various types of organic matters and nutrient elements(nitrogen,phosphorus,potassium,etc.),but also accumulates a large amount of heavy metals and pathogenic bacteria,which makes the stabilization of harmful substances and effective utilization of nutrients in sludge become another challenge.Electrochemical technology has the advantages of strong oxidation ability,simple architecture,easy control and convenient integration with existing technologies,therefore becoming an attractive method for the degradation of organic matters in sludge.Herein,hybrid electrochemical conditioning strategies integrating the advantages of electrochemical oxidation with chemical reagents for sludge deep dewatering,heavy metal removal,phosphorus fixation,and pathogen inactivation were investigated.The main contents are as follows:1.A hybrid electrochemical conditioning strategy with Fe(II)addition was proposed,and the mechanisms of sludge deep dewatering were elucidated.The hybrid electrochemical conditioning strategy with Fe(II)addition for enhanced sewage sludge dewatering was proposed.The capillary suction time(CST)and specific resistance to filtration(SRF)were decreased by 75.57%and 90.95%after the hybrid conditioning processes,respectively.Moreover,the average median particle size of sludge decreased by 10.95%,the TB-EPS fractions dropped by 44.76%,and the zeta potential of the sludge was close to the isoelectric point(0.47 m V),and porous structure was formed on the surface of the sludge cake.The mechanism of sludge dewatering performance with the hybrid electrochemical conditioning method mainly included the oxidation process of the hydrophilic organic matter and the reconstruction process with Fe(III)flocculation.To be specific,the sludge flocs were destroyed and degraded through direct and indirect oxidation of the electrode.Meanwhile,Fenton-like reactions for hydroxyl radical generation stimulated by the addition of Fe(II)and reduction process on the cathode further accelerated the destruction of sludge flocs and the degradation of macromolecular organic matter.Furthermore,Fe(III)produced by the electrochemical processes as coagulant promote the agglomeration of small granular sludge into larger dense particles,consequently achieving appreciable dewaterability.2.The transport and transformation of heavy metals during the hybrid electrochemical conditioning processes was studied,and the mechanisms for efficient heavy metal removals from the sludge cake were investigated.After the hybrid electrochemical conditioning processes with Fe(II)addition,heavy metal removals of 78.49%for Cu,84.26%for Zn,and 36.99%for Pb were achieved from sludge samples,respectively.A lower removal efficiency of Pb was achieved compared with Cu and Zn because Pb mainly existed in the form of residual fraction(57.03%)in the sludge,The distribution of heavy metals in the sludge cake,filtrate and on the electrode surface was further investigated.Results showed that 62.54%of Cu and 26.02%of Zn in sludge existed in the electrode plates,respectively.This might be ascribed to the fact that Cu had a higher redox potential than Zn and could be reduced and enriched on the cathode more easily.The mechanism of heavy metal removal with the hybrid conditioning method promoted the release of heavy metals into the liquid phase with the generated acidity and the oxidizing environment of the system.Then,heavy metals released to the liquid phase of sludge could transfer toward the cathode by electromigration under the applied electrical field and attach to or be reduced on the cathode surface,thus achieving an excellent removal efficiency.3.The integration of electrocoagulation and calcium hypochlorite oxidation was investigated for sludge deep dewatering and phosphorus fixation.The integration of electrocoagulation and calcium hypochlorite oxidation for the enhanced sewage sludge dewatering and phosphorus fixation were proposed.After the hybrid conditioning process,the CST and SRF were decreased by 87.62%and 91.76%relative to raw sludge,respectively.Furthermore,99.47%of the phosphorus in the sludge liquid was transferred and fixed to the sludge cake and the electrode surface.In addition,the non-apatite inorganic phosphorus(NAIP)and apatite phosphorus(AP)in the sludge cake increased.The mechanism of sludge deep dewatering and phosphorus fixation with the hybrid conditioning method mainly included extracellular and intracellular oxidation of sludge followed by the electro-flocculation process.To be specific,the former were achieved through electrochemical oxidation and active chlorine oxidation,resulting in the disintegration of EPS structure and the destruction of the microbial cell envelopes,which could promote the release of sludge bound water and phosphorus.Then,the electro-flocculation could benefit the reconstruction of sludge floc structure and the fixation of phosphorus in the sludge cake.Moreover,the p H was maintained near neutral(7.45),which favored the fixation of NAIP and AP during the electro-flocculation process,In addition and avoided acid and alkali adjustment of sludge before land applications.4.Mechanisms on the inactivation of pathogenic bacteria in sludge by the hybrid electrochemical conditioning processes were investigated.With the pretreatment of sludge by the hybrid electrochemical oxidation with Fe(II)addition,the content of living cells decreased to 2.90%,and the total coliform decreased significantly to 950 MPN/g TS in municipal sludge.The oxidizing substances and acidic environment produced by the hybrid conditioning method could effectively destroy the microbial structure in the sludge.Besides,the sludge temperature increased to about 70°C due to the Joule heating produced by the hybrid conditioning processes,which could further inactivate the pathogenic bacteria in the sludge and achieve good sludge stabilization.With the pretreatment of sludge by the hybrid electrocoagulation with Ca(Cl O)2 addition,the content of living cells decreased to 2.04%,and the total coliform decreased significantly to422 MPN/g TS in municipal sludge.The mechanism of pathogen inactivation with the hybrid conditioning method included the disintegration of the EPS structure and the destruction of the microbial cell envelopes induced by intracellular and extracellular oxidation and active chlorine oxidation.The paper is oriented to the key processes of sludge dewatering,heavy metal removal,pathogenic bacteria inactivation and nutrient element utilization in the process of sludge recycling,and proposes hybrid electrochemical conditioning strategies to achieve efficient sludge pretreatment and pollution control,which provide theoretical foundation and technical support for sludge resource utilization. |
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