Variations in activities of sewer biofilms due to ferrous and ferric iron dosing Bruno Kiilerich, Pia Kiilerich, Asbjørn H. Nielsen and Jes Vollertsen ABSTRACT Addition of ferrous and ferric iron salts to wastewater is a commonly used practice for sulfide abatement in sewer force mains. When iron is added to wastewater where sulfate respiration takes place, it produces ferrous sulfide precipitates with the formed sulfide. The effect of iron addition has traditionally been focused on solely from the perspective of reaction stoichiometry. Possible influences on the microbial communities in biofilms growing in force mains have largely been neglected. In this study the activity and microbiome was examined in three pilot scale force mains conveying real wastewater, two subjected to iron treatment and one operated as an untreated control. Activity was measured on suspended biofilm samples extracted from the experimental setup. The microbiome of the biofilm was analyzed by V3 þ V4 16S rDNA sequencing. Correlation analysis of chemical composition of the biofilms and activity measurements for operational taxonomic units of relevance to sulfide and methane production were performed. In conclusion, it was found that both ferrous and ferric treatment reduced sulfate reduction and methane production, and that both iron salts induced significant changes to force main biofilm microbiomes. Bruno Kiilerich (corresponding author) Asbjørn H. Nielsen Jes Vollertsen Department of Civil Engineering, Aalborg University, Thomas Manns Vej 23, DK-9220 Aalborg Ø, Denmark E-mail: bkiilerich@grundfos.com Bruno Kiilerich Grundfos Holding A/S, Poul Due Jensens Vej 7, DK-8850 Bjerringbro, Denmark Pia Kiilerich Statens Serum Institut, Artillerivej 5, DK-2300 København S, Denmark Key words | 16S rDNA sequencing, methanogens, microbiome, sulfate-reducing bacteria, sulfide abatement, wastewater INTRODUCTION During wastewater conveyance, available organic and inor- ganic compounds are somewhat degraded or transformed by suspended wastewater biomass and the microbial biofilm communities growing on the surfaces of the pipelines (Hvitved-Jacobsen et al. ). Because of topographical differences, the wastewater must sometimes be pumped through force mains. Here the pipeline runs full of waste- water, thus obstructing the reaeration that otherwise would have occurred, had the wastewater been conveyed in a gravity sewer (Boon ). The lack of reaeration and the ongoing microbial processes result in available oxygen being quickly depleted. When oxygen and, if present, nitrate and ferric iron are depleted, anaerobic conditions prevail and sulfate is the preferred electron acceptor used for respir- ation by the microorganisms (Hvitved-Jacobsen et al. ). Respiration using sulfate results in the formation of sulfides, which has detrimental effects when wastewater is dis- charged from force mains. These effects concern odor nuisances when sulfide accumulates in the sewer atmos- phere, microbial mediated corrosion of sewer assets with derived cost for rehabilitation, and health and safety issues for utility workers due to its toxicity (Boon ). If sulfate ultimately gets depleted in the biofilms, anaerobic respir- ation will result in formation of methane, which, when escaping the wastewater, can act as a greenhouse gas (Guisasola et al. ). Due to the immediate detrimental effects of sulfides, abatement of these are widely implemented. Precipitation of the formed sulfides with iron salts of the ferrous (Fe(II)) or ferric (Fe(III)) type is one of the most widespread methods (Ganigue et al. ). Fe(II) reacts directly with sul- fides to form insoluble ferrous sulfides (FeS). Fe(III) on the other hand must first be reduced either biotically or abioti- cally to Fe(II) which then precipitates available sulfides. During the initial reduction of Fe(III), sulfides are oxidized to elemental sulfur. FeS precipitation is easily recognized in natural environments by a blackening of the water, biofilms, and sediments (Barton ). This distinct blacken- ing is also observed in sludge, which indicates that FeS accumulates here (Hao et al. ). The amorphous iron 845 © IWA Publishing 2017 Water Science & Technology | Bonus Issue 3 | 2017 doi: 10.2166/wst.2018.261 Downloaded from http://iwaponline.com/wst/article-pdf/2017/3/845/253472/wst2017030845.pdf by guest on 11 May 2021