Effects Of Heavy Metals In Electroplating Wastewater On The Performance Of H/A/O-MBR And Its Bioaugmentation

In the electroplating industrial park,wastewater is usually collected and treated by the combination of physicochemical and biochemical processes.However,there is a problem that the effluent of biological treatment unit cannot reach the emission standard steadily due to long-term exposure of low concentration heavy metals.To solve this problem,the hydrolytic/anoxic/oxic-membrane bioreactor(H/A/O-MBR)was used as the biological treatment equipment.Representative metal ions and nanoparticles including Pb²⁺,Cu²⁺and n-Cu O in electroplating wastewater were selected as the research objects.Focus on the problems in existing studies,such as underestimating the cumulative toxicity of low concentrations of heavy metals,studies on the migration and speciation transformation of heavy metals are not comprehensive,the influence of heavy metals on membrane fouling lacks theoretical support,and the lack of bioaugmentation methods for compound pollution.The long-term toxic effects of heavy metals,membrane fouling mechanism and bioaugmentation were studied.Firstly,the degradation of characteristic pollutants in simulated electroplating wastewater by H/A/O-MBR was investigated.The degradation efficiencies of sodium formyl sulfonimide(brightener)was the highest,followed by sodium dodecylbenzene sulfonate(surfactant),octylphenol polyoxyethylene ether(emulgator),and liquid paraffin(grease).The main degradation products were alkanes and phthalates.The long-term toxic effects of 2 mg/L and 6 mg/L Pb²⁺,1mg/L and 3 mg/L Cu²⁺,3 mg/L and 10 mg/L n-Cu O were investigated in three parallel H/A/O-MBR(exposure to each concentration for 75 d),respectively.Pb²⁺,Cu²⁺and n-Cu O caused continuous decreases in the removal efficiencies of COD,NH₄⁺-N and TN,and enhanced the inhibition of heterotrophic bacteria,nitrifying bacteria and denitrifying bacteria,among which nitrifying bacteria had the weakest tolerance.Most of Pb,Cu and n-Cu O were trapped in the sludge.Pb and Cu were removed by extracellular adsorption,intracellular enrichment and chemical precipitation in sludge.Chemical precipitation gradually dominated with the increase of heavy metal enrichment,while extracellular adsorption and intracellular enrichment decreased with the saturation of adsorption sites and the decrease of microbial activity.n-Cu O was removed by aggregation precipitation and biosorption,and the aggregation precipitation increased with the increase of n-Cu O enrichment.Pb in sludge mainly existed in residual and reducible state,while Cu mainly existed in residual and oxidizable state.The bioavailability of Pb and Cu in sludge were increasing.When microorganisms were exposed to Pb²⁺,Cu²⁺and n-Cu O,more EPSs were secreted by cells to bind metal ions and nanoparticles,the binding strength of EPSs to cell surface was weakened.Part of TB-EPSs were transformed into LB-EPSs,PN/PS ratio of EPSs increased,and Pb²⁺and Cu²⁺inhibited the production of EPSs in the later stage.The protective effects of protein against the toxicity of heavy metals and nanoparticles was stronger than that of polysaccharid,and its peptide chain structure was changed.The functional groups of EPSs participating in the adsorption of Pb²⁺and n-Cu O were carboxyl,phosphate,hydroxyl and amino group,while those participating in Cu²⁺adsorption were carboxyl,hydroxyl and amino group.The conformational changes of cell membrane components led to the rupture of cell membrane.Then,Pb²⁺was taken as an example to explore the effects of heavy metal ions on membrane fouling of H/A/O-MBR.After long-term exposure to low concentration of Pb²⁺,the membrane fouling rate increased at first and then slowed down.The concentrations of polysaccharides and proteins of soluble and colloidal organic matters(SCFs)in the sludge supernatant increased,and the increase of medium and low molecular weight SCFs accelerated the membrane pore blocking rate.The membrane pore blocking mechanism conformed to the standard blocking.The decrease of LB-EPSs concentration and PN/PS resulted in weaker hydrophobicity of sludge flocs and enhanced sludge aggregation ability,which reduced the cake layer fouling.With the increase of Pb²⁺concentration in the influent,the membrane fouling rate continued to accelerate.The increase of high molecular weight SCFs promoted the formation of gel layer,reduced the blocking of membrane pores,and the mechanism of membrane pore blocking is consistent with cake filtration.The increase of LB-EPSs concentration and PN/PS resulted in the enhanced hydrophobicity of the sludge flocs and reduced sludge aggregation capacity,which aggravated the cake layer fouling.Finally,the H/A/O-MBR subjected to n-Cu O exposure was bioaugmented.Five strains with strong Cu²⁺tolerance and organic degradation ability were screened from the activated sludge of electroplating wastewater treatment plant.The Cu²⁺adsorption process of L1-L5 followed pseudo-second-order kinetics and the isotherms fit well to Langmuir isotherm model,which had the characteristics of rapid binding of monolayer on cell surface.L2-L4 showed higher Cu²⁺adsorption capacity than that of L1 and L5.Their maximum Cu²⁺adsorption capacities were 34.15 mg/g,45.68 mg/g and 26.72 mg/g,respectively.The organic degradation performance of L2-L4 increased with the increase of p H value,and was less affected by the concentration of organics.Their COD removal efficiencies were more than 70%.L2-L4 were identified as Enterobacteria sp.,Meyerozyma guilliermondii and Rhodotorula mucilaginosa,respectively.The bioaugmentation effects of single inoculation and repeated inoculation in H/A/O-MBR were sustainable when L2-L4were mixed in equal proportion,and the average removal efficiencies of COD were increased by 5.7%and 12.9%,respectively.The results showed that the repeated inoculation system had better removal effect on the compound pollution,and could reduce the effluent COD concentration below the discharge standard.Repeated inoculation promoted the diversity and evenness of fungi and the diversity of bacteria in the community.L2-L4 became the dominant flora and promoted the succession of indigenous microbial community,which enhanced the system performance.The best compound proportion of L3 and L4 was 1:2.03.