Biofilm colonizing the Nam Theun 2 power plant Penstock (Lao PDR) - mechanism and potential evolution

Biofilms are the most common bacterial life mode on Earth. These tri-dimension bacterial structures occur at a substratum-liquid interface. Due to their intrinsic properties (niche for pathogens, resistance to biocide treatments, etc.), they cause major problems in various industries. In water systems, the physical and chemical characteristics of biofilms (viscoelastic behavior, roughness) may lead to the lowering of flow velocity. A rough biofilm has developed in the Penstock of the Nam Theun 2 hydropower plant (Khammouane Province, Lao PDR). This biofilm is thought to lead to additional head losses and to slightly affect the power production. The mineral, chemical and microbiological compositions of the biofilm were investigated in order to propose solutions to reduce its effect. Samples were taken during two water drainages in 2011 and 2012. In order to complete the knowledge from the water quality monitoring, major elements, trace elements and rare earth element (REE) contents in samples were measured using ICP-AES and ICP-MS. Crystalline phases were identified and quantified by X-ray diffraction (XRD). The microbial composition of the biofilm was first assessed by culture (2011) and then monitored according to the location and the time after water drainage by molecular biology methods (2012). Results show that the chemical composition of the biofilm is dominated by ferric iron Fe3+ and its mineralogy is mostly constituted of lepidocrocite and magnesioferrite. The bacterial population was dominated by beta-Proteobacteria but population profiles varied strongly according to the layer of the biofilm, the nature of the substratum and the time during which the biofilm was subjected to the conditions of the water drainage. These observations are concordant with the modification of the biofilm properties and the reduction in head losses when returning to functioning regimes in the Penstock. Les biofilms représentent la forme de vie bactérienne la plus répandue sur Terre. Ce sont des structures bactériennes tridimensionnelles qui se forment la plupart du temps aux interfaces substrat-liquide. Du fait de leurs propriétés (niche pour micro-organismes pathogènes, résistance aux traitements biocides, etc.), les biofilms causent des problèmes majeurs dans diverses industries. Dans les réseaux d’eau, les caractéristiques physico-chimiques des biofilms (comportement viscoélastique, rugosité) peuvent conduire à une diminution du débit. Un biofilm rugueux s’est dévelo