Research Of Structure Characteristics And Corrosion Property Of Microbial Communities In A Simulated Reclaimed Water Distribution System

Reclamation of wastewater is one of the most effective ways to alleviate water resource scarcity.Unfortunately,reclaimed water has more adverse effects on pipe walls owing to its complicated water quality conditions(ie.higher content of salt,organics,nutrients and variety of water quality)compared with drinking water,such as a more serious microbiologically induced corrosion problem.The corrosion mechanism of the whole development processes which from initial stage of corrosion to layering of corrosion products to growth of cast iron corrosion tubercles were researched comprehensively and systematically.O ne area of focus was the effect of microbial communities on the formation and structure characteristics of cast iron corrosion tubercles by the contrast analysis of the morphological and physicochemical characteristics of corrosion layers in raw and disinfected reclaimed water.Main conclusions were as follows:(1)The biofilm biomass maturing within 14 days and remaining at 105CFU/cm2.The content of Exopolyssachride,protein,nucleic acids increased over time before 30 days.(2)Microbial diversity increased over time(0~90d)at the phylum,class,and genus level).The microbial communities in the corrosion layers were found to be dominated by Proteobacteria,Bacteroidetes,and N itrospira.SRB has always been existed,which with the relative abundance increased from 0.005% to 1.83%.7~30d,the relative abundance of IRB range from 2.01% to 3.75%,and IO B has higher abundance before 30 days(6.34%~9.86%)and then decrease.Geobacter was found in corrosion layers at any time point(0.03~0.18%).The relative abundance of magnetospirillum peaked at 30 days(2.51%).Moreover,the abundance of sequences affiliated to NH3-producing microorganisms was relatively high and Lactobacillus were detected.(3)The bacterial communities in the tubercles were found to be dominated by Proteobacteria(44.36%),Firmicutes(14.47%)and Acidobacteria(7.78%).The abundance of sequences affiliated to iron-oxidizing bacteria(mainly Gallionella),nitrate fixing bacteria(mainly Azospira)and N itrobacteria(mainly N itrospira)was relatively high(6.42%~9.62%)in the top layer of tubercles.In the shell-like layer and core layer of tubercles,iron-oxidizing bacteria is relatively abundant(averaged at 3%)after iron-oxidizing bacteria,besides,with 0.2% Magnetospirillum in the shell-like layer.(4)Corrosion tubercles were found in raw reclaimed water,which consisted of a top layer composed mainly of CaCO3,a-FeOOH and Fe2O3;a shell-like layer comprised of more Fe3O4;a porous core layer comprised of more green rusts and contained cavities;and a corroded layer at the base of the tubercle and is the source of almost all of the iron that comprises the tubercle.However,thin corrosion layers formed in disinfected reclaimed water,which consisted of a top layer,a middle layer,and a corroded layer.The main components of corrosion products in the tubercles in raw reclaimed water and the thin corrosion scale in disinfected reclaimed water showed the similarity,but the proportions showed some variance.(5)The formation of layered structures of corrosion tubercles is independent of presence of microorganisms.However,activities of microorganisms indeed accelerate corrosion and the layering process and formation of corrosion tubercles.The possible mechanisms of the whole development processes of corrosion tubercles and the effect of microbial communities on the layering process and formation of cast iron corrosion tubercles in reclaimed water might include:a.Stage I(Initial corrosion,0~14d)Give priority to with electrochemical corrosion.Microorganisms accelerate corrosion by forming biofilms that promote electrochemical corrosion and some corrosive bacteria(ie.SRB)accelerate corrosion and the layering process of corrosion products via triggering pitting corrosion.b.Stage II(layering of corrosion layers,14~30d)The oxygen concentration gradient is the main cause of the formation of the la yering structure of tubercles.The formation of oxygen concentration differences is due to both blockage of DO diffusion caused by the biofilms and corrosion layers and DO consumption result from bacterial respiration and chemical reaction inside the corrosion layers.Electrochemical corrosion promotes this process and result in localized corrosion by combination of oxygen concentration cells and the localized corrosion is the only reason for tuberculation.The formation position of shell-like layer is dominated by the depth of penetration of DO into the corrosion layers and it formed within 30 days.At this stage,microorganisms accelerate corrosion and layering by enhancing Electrochemical corrosion,Acid attack,anowires and EMIC.c.Stage III(Growth of tubercles,after 30d)The top layer is characterized by its unstable and loose structure,and this structural features is closely associated with flow of water,water quality and the abundant IOB.The formation of Fe3O4 in the shell-like layer may have the following reasons: Some aerobic bacteria(IOB,SO B,Nitrobacteria and NFB)result in the environmental conditions with low ORP and high pH that in favour of the formation of Fe3O4;IRB and Magnetospirillum could promote the formation of Fe3O4.IRB promote formation of green rusts in the core layer and the possible causes of the formation of the porous core layer that contained cavities are as follows: the effect of gas produced by gas-producing bactera(ie.SRB),the acid attack produced by some heterotrophic microorganisms(ie.NH3-producing bacteria,Lactobacillus,SRB).