PTQ Q3 2026 Issue

Microbial contamination in diesel storage tanks

Long-term case study and mitigation strategies at a Brazilian refinery show how biocide treatment plays a key role in maintaining high-quality diesel during storage

Gabriela Feix Pereira, José Danilo Haick Tavares, and Gustavo Artur Silva Santos Dorf Ketal Brasil Alex Sandro Santos Campos Acelen, Brazil

D espite advances in quality management practices and operational control, diesel storage remains a critical stage within the fuel logistics chain. During storage and handling, the fuel may be exposed to physico- chemical and biological factors that can influence its stabil - ity over time. In this sense, storage should be viewed not as a passive step, but as a dynamic operational stage where fuel properties, infrastructure conditions, and the presence of contaminants interact. Potential quality deviations may arise during this period. However, these can be identified and managed within the internal logistics chain, for example, through the application of longer settling times to ensure that the fuel delivered to customers meets the required specifications. In this context, the presence and accumulation of water in storage systems is widely recognised as one of the main factors influencing diesel stability during storage. Microorganisms are intrinsically present in the environment and can enter fuel systems through air, water, or operational handling, making their presence in storage infrastructure difficult to avoid completely. Once water is present, it provides favourable conditions for microbial proliferation, initiating processes that may affect operational performance. Over time, microbial activity can lead to biomass formation, sediment accumulation, and increased turbidity. Furthermore, microbial contamination is associated with other relevant degradation mechanisms, including diesel biodeterioration, as microorganisms may uti- lise fuel components as sources of carbon and energy. This process promotes changes in diesel composition and stability. Microbial activity is also directly related to microbiologically influenced corrosion (MIC), resulting from biofilm formation on the internal surfaces of storage tanks and piping, which creates localised corrosive microenvironments. In addition, the release of biosurfactants as byproducts of microbial metabolism may promote the formation and stabilisation of water-oil emulsions, hindering phase separation and repre- senting an additional challenge for the management of fuel storage systems.¹ , ² Figure 1 presents a laboratory microcosm simulating a diesel storage tank, in which microbial biomass is predom- inantly located at the water-oil interface. The diesel used in this experiment was obtained from a generic commercial

Diesel

Water

source to be used as a laboratory standard. In this scenario, strategies such as the efficient removal of free water from storage tanks and the application of biocides with appropri- ate monitoring become essential to mitigate microbial pro- liferation, preserve asset integrity, and ensure the quality of stored diesel. This article presents a case study conducted at the Mataripe Refinery, owned by Acelen, located in the Brazilian state of Bahia, which experienced a decline in diesel qual - ity due to microbial contamination and sludge formation. Laboratory analyses confirmed high microbial levels, lead - ing to the development of a chemical treatment based on a broad-spectrum biocide. The treatment was applied and monitored over a 10‑month period, resulting in a reduction of more than 40% in the time required for water decantation, solid sedimentation, and fuel stabilisation prior to certifica - tion. This improvement enabled faster batch release and enhanced compliance with quality specifications. Microbial development Stored diesel is susceptible to biodeterioration when water accumulates at the bottom of storage tanks. Water is con- tinuously introduced into storage tanks during receipt of new diesel batches and may be present as free water, dis- persed water, or emulsified. In addition, condensation of Figure 1 Laboratory microcosm simulating diesel storage conditions (six months at 25°C), showing microbial bio - mass located at water-oil interface. System prepared fol- lowing ASTM E1259 guidelines for biocide evaluation in diesel Source: Dorf Ketal Brazil R&D Laboratory

PTQ Q3 2026