Grease Deposits in Sewer Systems

Fat, oil, and grease deposits (FOGDs) are solid masses that form when fats, oils, and grease from household, food service, and industrial wastewater react with suspended solids (SS) during transport through sewer pipes and eventually settle. These deposits hinder the smooth flow of wastewater, leading to urban flooding and environmental pollution. Additionally, they promote microbial proliferation within pipelines, resulting in the production of greenhouse gases such as methane (CH₄) and hydrogen sulfide (H₂S), increasing carbon emissions and posing safety risks including pipeline explosions. This study begins with an overview of the hazards posed by FOGDs in drainage systems and employs bibliometric analysis to review how physical, chemical, and biological factors in sewer pipes influence the formation and characteristics of these deposits. By analyzing the transport, chemical reactions, microbial adhesion, and biodegradation of fats, oils, and grease within pipes, the study further explores the physical, chemical, and biological mechanisms underlying FOGD formation. Finally, it outlines future research directions to support pollution and carbon reduction, as well as improvements in the quality and efficiency of urban wastewater management systems.

Fat, oil, and grease deposits (FOGDs) are solid masses formed when fats, oils, and greases in household, catering, and industrial wastewater react during transportation through sewage pipes, aggregating with suspended solids (SS) in the water and eventually settling. These deposits accumulate in urban drainage systems, leading to pipe blockages and reduced flow capacity, which can result in urban flooding and environmental pollution. Additionally, the high organic content of these sediments may promote the production of methane (CH₄) and hydrogen sulfide (H₂S), thereby increasing safety risks within the pipeline. In the context of enhancing the quality and efficiency of urban drainage systems and advancing critical urban infrastructure projects, understanding the influencing factors and formation mechanisms of FOGDs in sewage pipes is crucial. Such insights can support the scientific development of prevention and control strategies, ultimately improving the overall efficiency and safety of drainage systems. Researchers have carried out a series of studies on the formation and influencing factors of FOGDs in sewage pipes and have made notable progress.

It is generally believed that FOGDs are primarily formed through saponification reactions between calcium, magnesium, and other ions in the water and fatty acids (FAs). The resulting metal salts can further adsorb other suspended particles, metal ions, and grease present in the sewage, aggregating to form larger deposits. Therefore, all water chemistry factors and environmental conditions that influence the saponification reaction and aggregation process will impact the formation of FOGDs. Recent studies have also shown that physical and biological factors, such as the hydrodynamic conditions within the pipeline, the concentration of suspended particles in the wastewater, and the microorganisms attached to particle surfaces, can significantly influence the formation and evolution of FOGDs.

FOGDs in sewage pipes are lipids composed of fatty acids, triglycerides, and fat-soluble hydrocarbons, which form solid deposits by adhering to the surfaces of suspended particles. These deposits originate mainly from two sources: oily wastewater discharged by the food and catering industry and residential households, and suspended particles in the sewage that bond with lipids to form solid deposits. The hazards posed by FOGDs in sewage pipes are primarily due to their obstruction of water flow, which leads to overflow pollution and the release of hazardous gases, thereby compromising the safety of the pipeline system. Over time, FOGDs accumulate and harden, adhering to the inner walls of sewage pipes. These hardened solids can further attract and bind other suspended particles, forming larger grease masses. This process narrows the effective cross-sectional area of the pipe, reduces the flow capacity of the sewage system, and increases the risk of sanitary sewer overflows (SSOs). Studies have shown that 25% to 70% of overflow pollution events in drainage systems are closely linked to the accumulation of FOGDs. In combined sewer systems, heavy rainfall with high recurrence intervals can cause FOGDs to overflow into receiving water bodies, leading to more severe water pollution. In separate sewer systems, small initial flow rates, narrow pipe diameters, and limited gradients reduce flow velocity and eliminate rainwater flushing, creating conditions that are more conducive to FOGD formation.

Table 1 Current Status and Causes of Blockages in Sewage Pipes in Different Cities

Under the hypoxic to anoxic conditions commonly found in sewage pipes, fat, oil, and grease deposits (FOGDs) significantly promote microbial growth and lead to the production of hazardous gases such as methane (CH₄) and hydrogen sulfide (H₂S) through biochemical reactions, posing serious risks to pipeline safety. Specifically, floating oil on the water surface impedes gas–liquid exchange, while the hydrophobic lipid film formed on the surface of FOGDs can reduce oxygen mass transfer efficiency into the sediment by 40–60%, thereby creating a stable anaerobic microenvironment. Additionally, FOGDs contain high concentrations of organic matter (COD > 15,000 mg/L), which provide abundant metabolic substrates for methanogens (e.g., Methanosaeta) and sulfate-reducing bacteria (e.g., Desulfovibrio). Studies have shown that the methane yield from FOGDs can reach 60.3–77.8 mL/(g·d) (based on volatile solids), which is significantly higher than the 1.56 ± 0.14 g/(m²·d) observed in typical pipeline sediments. This is primarily because, under anaerobic conditions, fats and oils are rapidly broken down into short-chain fatty acids, alcohols, and other intermediates through β-oxidation and related pathways, and are subsequently converted into CH₄ via acetoclastic and CO₂-reduction pathways.

Using China National Knowledge Infrastructure (CNKI) and the Web of Science Core Collection as data sources, the keywords “fat, oil, and grease” and “sewer” were used to retrieve literature related to FOGDs in sewage drainage systems published between January 2007 and June 2024. After deduplication and manual screening of the search results, a total of 76 valid publications were obtained. These documents were analyzed using the bibliometric software CiteSpace, and a keyword co-occurrence network map was generated.

The main keywords in current research on this topic are primarily concentrated on the key components of FOGDs—such as fats, fatty acids, and oils—as well as on the formation processes, including pollutant degradation and particle aggregation. This indicates that existing research still largely focuses on the influence of chemical components on FOGD formation. The appearance of keywords such as bacteria, anaerobic digestion, and co-digestion also suggests that the role of microorganisms in FOGD formation has begun to attract increasing attention in recent years. Based on the research trends in the field of pipeline sediments, this article will focus on the key physical, chemical, and biological factors; review and evaluate their impact on FOGD formation; analyze the underlying formation mechanisms; and explore future research directions.