Study On The Effect Of Calcium Peroxide Pretreatment On Anaerobic Digestion Of Sludge From Wastewater Treatment Plant

Large amounts of waste activated sludge are produced from municipal wastewater treatment processes.Being a sustainable and efficient method of sludge reduction and resource recovery,anaerobic digestion（AD）has attracted wide attention.However,the stage of sludge hydrolysis is the limited step of AD,which will seriously affect the performance of sludge digestion.In recent years,calcium peroxide（CaO₂）,a kind of slow-release oxygen material,is expected to promote sludge hydrolysis and increase methane production,but the effect of CaO₂ on anaerobic environment and gas production in AD process remains unclear.In view of this,this study studied the effect of CaO₂pretreatment on AD of the waste activated sludge produced from A²/O process.Firstly,the optimal temperature of AD after CaO₂ pretreatment was optimized,and then the optimal total solid（TS）of waste sludge in AD process after CaO₂ pretreatment was explored.Finally,the comprehensive influence of CaO₂ pretreatment on AD process was discussed under the conditions of optimal temperature and TS.The aim is to provide technical support for the development of CaO₂ pretreatment sludge AD.The main results and conclusions of this paper are as follows:（1）After the waste activated sludge was pretreated with CaO₂ for 48 h,the p H value,H₂O₂ concentration and soluble chemical oxygen demand（SCOD）concentration of the system were significantly increased,indicating that CaO₂ played an alkaline and strong oxidizing role and promoted the destruction of floc and cell structure of the sludge.After pretreatment,batch tests of medium temperature anaerobic digestion（MAD）and high temperature anaerobic digestion（TAD）were carried out.It was found that the maximum methanogenic potential（P₀）and maximum methanogenic rate(R_max)in the MAD process were increased by 23.86%and 121.22%compared with the control group,respectively.The effective methane production time(T_ef)was reduced from 28.17 days to 15.77 days in the control group,which significantly increased the methane production rate.In TAD,P₀ and R_max were slightly lower than that of the control group.Therefore,CaO₂pretreatment sludge is not suitable for TAD,but MAD can not only improve methane production and rate,but also stabilize AD process.（2）After the dewater sludge with different TS（4%,6%,8%and 10%）was pretreated by CaO₂ for 48 h,it was found that the higher the TS,the higher the SCOD concentration.However,the increase of SCOD concentration began to decline when the TS of waste sludge was higher than 6%.In addition,both nitrate nitrogen and nitrite nitrogen concentrations increased rapidly and then decreased gradually during the pretreatment process.So it was believed that the nitrification potential of sludge itself would be activated by the addition of CaO₂.When different TS sludge was pretreated for MAD,P₀in 4%and 6%TS reached 129.90 m L/g VS and 155.64 m L/g VS,respectively,which was higher than that of the control group,but lower than that of the control group when TS was 8%and 10%.Meanwhile,R_max of different TS systems also gradually decreased with the increase of TS.On the other hand,pretreatment significantly increased the initial acetic acid concentration of MAD,but the higher the TS of sludge,the more complex the VFAs component,which was not conducive to the utilization of methanogens.In addition,pretreatment reduced the concentration of PO₄^3--P of MAD supernatant.The analysis of digestate showed that pretreatment increased the total nutrient content of biogas residue and avoided heavy metal accumulation,and also reduced the biodegradability of biogas slurry with TS of 4%and 6%.In conclusion,CaO₂ pretreatment sludge with TS of 6%has the best effect on MAD methane recovery.（3）When the sludge with TS 6%was pretreated with CaO₂ for 48 h before MAD,it was found that the oxidation-reduction potential（ORP）was high at the initial stage and then gradually decreased,indicating that CaO₂ released oxygen slowly at the initial stage of MAD,and there were more reactive oxygen species（ROS）in the system.With the extension of AD time,the oxygen release decreased and ROS was consumed,which led to the anaerobic environment gradually formed.In addition,modeling analysis showed that CaO₂ could promote the conversion of non-degradable substances into degradable parts of sludge,reducing the non-degradable substances from 69.17%to 41.11%the control group,and the degraded part increased by 17.94%compared with the control group,which enhanced the overall digestion performance of MAD reactor.（4）Microbial community analysis on the 14th day of MAD after sludge pretreatment showed that pretreatment could significantly increase the abundance of facultative bacteria such as Actinobacteria（21.53%）and Firmicutes（16.04%）,promote hydrolysis and acidification,and rapidly consume oxygen in the system.Moreover,Nitrospirota（0.24%）also shows adaptability to low-oxygen environments.In addition,compared with the control group,CaO₂ pretreatment increased the abundance of Methanobacterium and Methanosarcina and decreased the abundance of Methanosaeta,suggesting that the addition of CaO₂ enriched necessary functional microorganisms and created a unique co-existence environment for facultative bacteria and methanogenic archaea.It also provides strong support for the application of CaO₂ in MAD system.