Geothermal plants are often affected by corrosion caused by microbial metabolites such as H2S. In the Bad Blumau (Austria) geothermal system, an increase in microbially produced H2S was observed in the hot (107 °C) and scaling inhibitor-amended saline fluids and in fluids that had cooled down (45 °C). Genetic fingerprinting and quantification revealed the dominance, increasing abundance and diversity of sulfate reducers such as Desulfotomaculum spp. that accompanied the cooling and processing of the geothermal fluids. In addition, a δ34S isotopic signature showed the microbial origin of the H2S that has been produced either chemolithotrophically or chemoorganotrophically. A nitrate addition test in a test pipe as a countermeasure against the microbial H2S formation caused a shift from a biocenosis dominated by bacteria of the phylum Firmicutes to a community of Firmicutes and Proteobacteria. Nitrate supported the growth of nitrate-reducing sulfur-oxidizing Thiobacillus thioparus, which incompletely reduced nitrate to nitrite. The addition of nitrate led to a change in the composition of the sulfate-reducing community. As a result, representatives of nitrate- and nitrite-reducing SRB, such as Desulfovibrio and Desulfonatronum, emerged as additional community members. The interaction of sulfate-reducing bacteria and nitrate-reducing sulfur-oxidizing bacteria (NR-SOB) led to the removal of H2S, but increased the corrosion rate in the test pipe.

Aquifer thermal energy storage may result in increases in the groundwater temperature up to 70 °C and more. This may lead to geochemical and microbiological alterations in the aquifer. To study the temperature effects on the indigenous microbial community composition, sediment column experiments at four different temperatures were carried out and the effluents were characterized geochemically and microbiologically. After an equilibrium phase at groundwater temperature of 10 °C for 136 days, one column was kept at 10 °C as a reference and the others were heated to 25, 40 and 70 °C. Genetic fingerprinting and quantitative PCR revealed a change in the bacterial community composition and abundance due to the temperature increase. While at 25 °C only slight changes in geochemical composition and gene copy numbers for bacteria were observed, increasing concentrations of total organic carbon in the 40 °C column were followed by a strong increase in bacterial abundance. Thermophilic bacteria became dominant at 70 °C. Temporary sulfate reduction took place at 40 and 70 °C and this correlated with an increased abundance of sulfate-reducing bacteria (SRB). Furthermore, a coexistence of SRB and sulfur-oxidizing bacteria (SOB) at all temperatures indicated an interaction of these physiological groups in the sediments. The results show that increased temperatures led to significant shifts in the microbial community composition due to the altered availability of electron donors and acceptors. The interplay of SRB and SOB in sedimentary biofilms facilitated closed sulfur cycling and diminished harmful sulfur species.

At geothermal plants, process failures often occur due to corrosion and scaling processes. Especially after heat extraction, sulfate reducing bacteria contribute to corrosion processes by producing reduced sulfur compounds. In biofilms containing scales such as iron sulfides, corrosion processes are enhanced. In a mobile bypass system located at the geothermal plant in Neubrandenburg (North German Basin), the influence of biofilm formation on corrosion and scaling was investigated. Short-term heat shocks were successfully tested in the bypass system in order to reduce biofilm formation and thus to diminish corrosion and scaling processes.

The temperature affects the availability of organic carbon and terminal electron acceptors (TEA) as well as the microbial community composition of the subsurface. To investigate the impact of thermal energy storage on the indigenous microbial communities and the fluid geochemistry, lignite aquifer sediments were flowed through with acetate-enriched water at temperatures of 10, 25, 40, and 70 °C in sediment column experiments. Genetic fingerprinting revealed significant differences in the microbial community compositions with respect to the different temperatures. The highest bacterial diversity was found at 70 °C. Carbon and TEA mass balances showed that the aerobic degradation of organic matter and sulfate reduction were the primary processes that occurred in all the columns, whereas methanogenesis only played a major role at 25 °C. The methanogenic activity corresponded to the highest abundance of an acetoclastic Methanosaeta concilii -like archaeon and the most efficient degradation of acetate. This study suggests a significant impact of geothermal energy storage on the natural microbial community and various metabolic activities because of increased temperatures in sediments with a temperature-related sediment organic matter release.

The microbial biocenosis in highly saline fluids produced from the cold well of a deep geothermal heat store located in the North German Basin was characterized during regular plant operation and immediately after plant downtime phases. Genetic fingerprinting revealed the dominance of sulfate-reducing bacteria (SRB) and fermentative Halanaerobiaceae during regular plant operation, whereas after shutdown phases, sequences of sulfur-oxidizing bacteria (SOB) were also detected. The detection of SOB indicated oxygen ingress into the well during the downtime phase. High 16S ribosomal RNA (rRNA) and dsrA gene copy numbers at the beginning of the restart process showed an enrichment of bacteria, SRB, and SOB during stagnant conditions consistent with higher concentrations of dissolved organic carbon (DOC), sulfate, and hydrogen sulfide in the produced fluids. The interaction of SRB and SOB during plant downtimes might have enhanced the corrosion processes occurring in the well. It was shown that scale content of fluids was significantly increased after stagnant phases. Moreover, the sulfur isotopic signature of the mineral scales indicated microbial influence on scale formation.

In geothermischen Anlagen können Biofilme die Mineralbildung und die Injektivität von Bohrungen sowie die Materialbeständigkeit beeinträchtigen. In drei bezüglich Temperatur und Salinität sehr unterschiedlichen Anlagen waren Organismen des Schwefelkreislaufs an Betriebsstörungen beteiligt: Die erhöhte Abundanz von Sulfat-reduzierenden Bakterien (SRB) auf der kalten Seite eines Wärmespeichers wies auf deren Beteiligung an der Korrosion und der Abnahme der Injektivität hin. In allen Anlagen führte der Zutritt von Sauerstoff bzw. der Eintrag von Nitrat zu einer temporären Zunahme Schwefel-oxidierender Bakterien (SOB) und hat vermutlich Korrosionsprozesse beschleunigt. Außerdem hatte in einem Kältespeicher die temporäre Zunahme der SOB ein Filterclogging zur Folge. Aufgrund ihrer entscheidenden Rolle bei mikrobiell induzierter Korrosion (MIC) weisen Änderungen in der Abundanz von SOB und SRB auf die Ursachen mikrobiell bedingter Störungen hin. Zur Beseitigung der Störungen wurden temporäre Erhöhungen der Temperatur, Säuerungen sowie die Zugabe von Wasserstoffperoxid (H2O2) oder Nitrat in den Anlagen getestet und aus mikrobiologischer Sicht bewertet.

This paper describes microbial metabolic processes that are considered to be relevant for the technical reliability of a geothermal heat store. The study reports on changes of the microbial community composition in geothermal well fluids of different temperatures and after plant downtimes monitored by genetic fingerprinting. Stagnant conditions favored the enrichment of bacteria, sulfate reducers (SRB), and sulfur oxidizers (SOB) in the well. Furthermore higher concentrations of DOC, SO42-, H2S, and H2 were detected in the first fluids produced after plant downtime. The increased abundance of SOB indicated oxygen ingress during plant downtime. The interaction of SRB and SOB might have further enhanced corrosion and scaling processes. A mobile bypass system installed at the site will help to understand the processes occurring in the well and to study biofilm formation and corrosion rates at different temperatures.

The microbial diversity of a deep saline aquifer used for geothermal heat storage in the North German Basin was investigated. Genetic fingerprinting analyses revealed distinct microbial communities in fluids produced from the cold and warm side of the aquifer. Direct cell counting and quantification of 16S rRNA genes and dissimilatory sulfite reductase (dsrA) genes by real-time PCR proved different population sizes in fluids, showing higher abundance of bacteria and sulfate reducing bacteria (SRB) in cold fluids compared with warm fluids. The operation-dependent temperature increase at the warm well probably enhanced organic matter availability, favoring the growth of fermentative bacteria and SRB in the topside facility after the reduction of fluid temperature. In the cold well, SRB predominated and probably accounted for corrosion damage to the submersible well pump and iron sulfide precipitates in the near wellbore area and topside facility filters. This corresponded to lower sulfate content in fluids produced from the cold well as well as higher content of hydrogen gas that was probably released from corrosion, and maybe favored growth of hydrogenotrophic SRB. This study reflects the high influence of microbial populations for geothermal plant operation, because microbiologically induced precipitative and corrosive processes adversely affect plant reliability.