The Effects And Mechanisms Of Poly Aluminum Chloride (PAC) On Sludge Anaerobic Process And Its Regulation

Poly aluminum chloride(PAC)is a kind of inorganic polymer flocculants developed from the traditional aluminum salt in 1960s.PAC is made by partial hydrolysis of acid aluminum chloride solution using a specific reactor,is an inorganic coagulant and generally consists of Al monomers such as Al(OH)²⁺,Al(OH)₂⁺,dimer(Al₂(OH)₂⁴⁺),trimer(Al₃(OH)₄⁵⁺),Al₁₃(Al O₄Al₁₂(OH)₂₄(H₂O)₁⁷ ₂⁺)and inert large polymer with molecular weight normally larger than 3000Da.During experiments,PAC can be divided into Ala,Alb and Alc according the hydrolyzed species.PAC has been applied in industrial and domestic wastewater throughout the world because the characteristics of high efficiency and low toxicity.The widespread use of the PAC inevitably led to its massive accumulation in waste activated sludge(WAS).It was documented that Al content in WAS reach to 5-30 mg Al/g TSS.With the increase requirements of wastewater treatment and sludge dehydration by transportation,treatment and disposal,PAC's content in WAS may continue to increase.Sludge anaerobic process is one biological technology that can realize the stabiliz ation,harmless,reduction and resource recovery of organic matter and produce resources such as(SCFAs,hydrogen and methane),has been widely used in the WAS treatment.WAS anaerobic process is driven by a series of biochemical processes or functional microorganisms,and the presence of PAC is likely to have a certain impact on the microbial activities.Moreover,in sludge anaerobic process,microbial has its bias due to the different target products,especially p H environment.Besides,PAC as a kind of inorganic polymer flocculants is greatly affected by the environment alkalinity.Therefore,PAC's influence on microorganisms in sludge anaerobic reactors with different target products will be more complex.However,to date,the effect of PAC on sludge process has not been systematically revealed.Firstly,this paper investigated the effect of different PAC concentration on the SCFAs production at p H uncontrolled environment,studied the impact of PAC on typical intermediate metabolites of sludge fermentation,and explore the effects of Al species on SCFAs production,and offered feasible suggestion about PAC sludge fermentation.Experimental results showed that with the increase of PAC addition from0 to 40 mg Al per gram of total suspended solids,SCFA yield decreased from 212.2 to138.4 mg COD/g volatile suspended solids.Mechanism exploration revealed that PAC benefited the aggregates of sludge flocs and caused more loosely-and tightly-bound extracellular polymeric substances remained in sludge cells.Besides,it was found that the hydrolysis,acidiogenesis,and methanogenesis processes were all inhibited by PAC.Although three types of Al species,i.e.,Ala(Al monomers,dimer,and trimer),Alb(Al₁₃(Al O₄Al₁₂(OH)₂₄(H₂O)₁₂⁷⁺),and Alc(Al polymer molecular weight normally larger than 3000 Da),were co-existed in fermentation systems,their impacts on SCFA production were different.No correlation was found between SCFA and Ala,whereas SCFA production decreased with the contents of Alb and Alc.Compared with Alb,Alc was the major contributor to the decreased SCFA production(R²=0.5132 vs R²=0.98).In practical application,a combination of PAC with low Basic and environment with high alkaline was recommended.Secondly,the paper also investigated the effect of PAC on hydrogen accumulation from WAS at p H 9.5,and studied the relevant mechanism,analysis the impact of Al species on hydrogen accumulation,and microbial community was also investigated.Experimental results showed that with an increase of PAC addition from 0 to 20 mg Al per gram of total suspended solids(TSS),the maximal hydrogen yield from alkaline fermentation(p H 9.5)increased from 20.9 m L to 27.4 m L per gram volatile suspended solids(VSS)under the standard condition.Further increase of PAC to 30 mg Al/g TSS didn't cause a significant increase of hydrogen yield(p>0.05).The mechanism explorations revealed that although PAC reduced the total short-chain fatty acid(SCFA)production,this reduction was mainly enforced to propionic acid fermentation type,which did not contribute hydrogen production.PAC suppressed all the microbial processes relevant to anaerobic fermentation to some extents,but its inhibition to hydrogen consumption was much severer than that to hydrogen production.Among the three types of Al species present in the anaerobic fermentation systems,Ala(monomeric species)significantly affected the maximal hydrogen production potential while Alb(medium polymer species)and Alc(species of sol or gel)posed impacts on hydrogen production rate and the lag time.Illumina Miseq sequencing analysis revealed that PAC did not affect the populations of SCFA and hydrogen producers,but the two hydrogen consumers,Acetoanaerobium and Desulfobulbus,were almost washed out by PAC.Thirdly,the effect of PAC on sludge anaerobic digestion produce methane at p H7was investigated,and mechanism was revealed.The results showed that with the increase of PAC concentration from 0 to 30 mg Al/g TSS,the cumulative methane yield decreased from 152.99 to 120.37 m L/g VSS.Mechanism research showed that the existence of PAC would increase sludge apparent activation energy.It is observed that compared with blank reactor,the apparent activation energy(AAE)of 30 mg Al/g TSS PAC sludge increased from 47.88 to 66.35k J/mol,which implies a crucial role in anaerobic sludge digestion,suggested that more energy was needed for organic dissolution.In the sludge anaerobic reactor,the existence of PAC would benefited the aggregates of sludge flocs and increase sludge partial size,and change BSA structure,make it more difficult to be used by microorganism.Illumina Miseq sequencing analysis revealed that PAC decrease the methanogenic archaea such as methanobacterium and methanosaeta.Based on above research,methane is one of the most important product during anaerobic process,to reduce the inhibition of PAC on sludge anaerobic digestion,different pretreatment were selected to investigate the effect on methane production.In the experiment,ultrasound,thermal and alkaline pretreatment was selected to investigate the solublization of organic from PAC and blank sludge,besides,first order model and modified Gompertz equation has been used to fitting methane production data.At last,the dewaterability of sludge anaerobic digested effluent was studied to evaluate the performance of sludge treatment and disposal.The result showed that three pretreatments all have increase the solublizaiton of blank and PAC sludge,but thermal and alkaline pretreatment only stimulate the solublization of PAC sludge.And the cumulative methane production of PAC sludge in thermal and alkaline reactor was higher than in ultrasound reactor,methane production potential,hydrolysis coefficient and maximum biogas production rate all have increased.And the coagulation by PAC efficiently decrease the residue of organic,zeta potential in reactor and finally reduce the capillary suction Time(CST).Based on the above,thermal and alkaline pretreatment was more appropriate on PAC sludge anaerobic process.The study comprehensively investigated the effect behavior of PAC in the process of WAS anaerobic digestion,mechanism and microbial characteristics,and explored the feasibility of pretreatment technologies to regulate the PAC's effect on WAS anaerobic process,filling a fundamental gap between PAC and sludge anaerobic process,providing some necessary theoretical basis and technical references for the control of WAS anaerobic digestion in the footer,which are of certain theoretical and practical significance.