Chemical Conditioning Technologies and Reaction Mechanisms Based on Sludge Properties

Author:Yang Peng

Supervisor:wang dong sheng zhang wei jun


Degree Year:2019





Activated sludge process is one of the greatest inventions in the field of environmental engineering and plays a crucial role wastewater treatment.However,wastewater treatment with activated sludge is accompanied with generation of a large amount of surplus sludge which is characteristics of high moisture content,high organic content and contains various trace pollutants and pathogen.Therefore,the water treatment problem has been transformed into solid waste treatment in traditional activated sludge process.Sludge is high hydrated and heterogeneous colloidal system,it is always negatively charged and relatively stable due to presence of electrostatic repulsion.The extracellular polymeric substances(EPS)are in the form of gel-like network and bind with water molecules though steric hindrance,so the sludge is viscoelastic and difficult to dewater.Chemical conditioning is an indispensable step in sludge deep dewatering.The sludge conditioning is driven by a successive reaction in solid-liquid interface between sludge particles and conditioners,leading to changes of gel-like structure and water release from flocs.Since the sludges from different sources differ in physicochemical properties,selection of traditional conditioning techniques did not consider the difference in sludge properties,leading to low conditioning efficiency in sludge treatment.Thus,it is important for highly effective and stable operation of sludge deep dewatering system by pertinently develop the sludge conditioning methods based on interactions between sludge properties and conditioners.This thesis aims to(a)investigate the sludge physicochemical properties of typical sludges(surplus sludge,anaerobic sludge and thermal hydrolysis-anaerobic sludge);(2)propose the conditioning methods based on different sludge properties and optimize the conditioning reaction parameters;(3)explore the variations in sludge floc morphology and EPS properties and understand the conditioning mechanisms.The main findings of this work include:(1)The changes in sludge physicochemical properties in typical treatment process were investigated and migration and transformation behaviors of harmful heavy metals were revealed.The results indicated that sludge solid organic content was reduced from 64.25%to 40.20%after thermal hydrolysis,and the proteins(PN)and polysaccharide(PS)were destroyed,PN and PS concentrations were decreased from 31.73 mg/gTSS and 15.02 mg/gTSS to 10.23 mg/gTSS and 5.54 mg/gTSS respectively.Sludge zeta potential was increased at the same time.The PN and PS in sludge were further degraded after thermal hydrolysis anaerobic digestion,and alkalinity and NH4+-N were increased from 305 mg/L and 31 mg/L to 6952 mg/L and1811 mg/L respectively.Meanwhile,the mobility of Pb,As,Cd and Zn was enhanced after thermal hydrolysis,while that of Ni increased.With further anaerobic digestion,the mobility of Cd,Cu and Ni was increased while that of Pb and As was reduced;(2)For surplus with high organic content,traditional flocculation conditioning was less effective to cause bound water release,so it is necessary to reduce the water binding capacity of sludge flocs by destroying the EPS fractions.In addition,a novel ferrous ions catalyzed sodium hypochlorite was established to solve the safety problems of Fenton reagents and persulfate,and the process parameters were optimized.The results showed that the sludge conditioning efficiency of ferrous ions catalyzed sodium hypochlorite was enhanced by reducing solution pH,the chlorine radicals(·Cl)were responsible for EPS degradation at pH>2,while hydroxyl radicals(·OH)were dominant at pH of 2.Therefore,at pH≤2,ferrous ions catalyzed sodium hypochlorite performed better in EPS degradation and the high molecular weight of biopolymers was converted into low molecular weight of biopolymers of less than1350 Da.After oxidation,the ferric produced was responsible for sludge flocculation,oxidation and coagulation showed a synergistic effect in sludge conditioning.Under neutral condition,the four organic acids oxalic acid(OA),tartaric acid(TA),citric acid(CA)and EDDS were used to chelate with ferrous and induce catalytic effects on sodium hypochlorite.Organic acids chelated ferrous ions catalyzed sodium hypochlorite were also able to oxidize the EPS fractions of sludge,the oxidation ability was EDDS>OA>CA>TA,the optimal molar ratios of OA,TA,CA and EDDS and ferrous ions were 1:1,2:1,1:1 and 1:2.Oxalic acid chelated ferrous ions catalyzed sodium hypochlorite exhibited best conditioning effect in sludge dewatering enhancement,since the oxalic acid could form precipitates with ferric ions and function as flocculation and skeleton builders,while other organic acids decrease the ferric hydrolysis;(3)Sludge solution chemistry is highly changeable;especially the alkalinity of sludge liquor greatly increased after anaerobic digestion,high alkalinity will affect the hydrolysis reactions and consequently flocculation behaviors.Therefore,with regards to anaerobic digestion sludge of high alkalinity,this chapter investigated the effects of alkalinity on hydroxyl aluminum conditioning and unraveled the molecular mechanisms on interactions between EPS and hydroxyl aluminum with different speciation.The results indicated that hydroxyl aluminum conditioning efficiency was greatly reduced with increase of alkalinity.The medium hydrolyzed aluminum was rapidly converted into aluminium hydroxide,meanwhile the flocculation mechanisms is changed from charge neutralization and complexation adsorption to sweeping,reducing the sludge conditioning efficiency of hydroxyl aluminum.In addition,the interactions between hydroxyl aluminum with different speciation and EPS were also considered.Medium hydrolyzed aluminum(Al13)and high hydrolyzed aluminum(Al30)showed the stronger binding ability with PN and PS,Al(OH)3 showed the weakest effects.Al13 and Al30 interacted with EPS through complexation and electrostatic interaction,while Al(OH)3 interacted with EPS by weak link(hydrogen bond);(4)As for advanced anaerobic digestion sludge with low organic content,high alkalinity,high ammonia nitrogen and high phosphate concentrations.,the magnesium salt was added to form struvite minerals with ammonia nitrogen and phosphate,these struvite minerals can act as skeleton builders and enhance sludge floc strength,and the fine sludge particles were further flocculated with addition of organic polymers.The results indicated that the magnesium ammonium phosphate can bind with EPS and reduce the sludge compressibility coefficients.In addition,the molecular structure and charge property of organic polymers have important effects on sludge conditioning efficiency;the sludge flocs flocculated with organic polymers with plentiful branch were characteristics of large floc size and high structural strength.The preparation of Mg-CPAM gel conditioning achieved conjunction of crystallization and flocculation,thus showing a synergistic effect on sludge conditioning.This technique promoted coupling of chemical conditioning and nutrients recycling,alleviate the alkalinity loss of sludge liquor and improve the process stability of Anammox and fertilizer efficiency in sludge land use.