fixedfilm

Fixed-film refers to a method of biological treatment in which microorganisms grow as a biofilm on a stationary support or media rather than remaining in suspension in the water. The biofilm forms on surfaces such as plastic, ceramic, or rock media, providing a large surface area for microbial attachment and activity.

In operation, wastewater is brought into contact with the media so that microorganisms metabolize organic matter and nutrients as the water flows past or over the biofilm. Mass transfer occurs by diffusion of substrates from the bulk liquid into the biofilm, while oxygen or electron acceptors diffuse in from the surrounding liquid or air. The fixed nature of the biofilm provides residence time for biodegradation and can permit slow-growing organisms to dominate.

Common fixed-film configurations include trickling filters, biofilters, rotating biological contactors (RBCs), and packed-bed or fixed-bed reactors. Trickling filters use a spray or trickling flow over a bed of media; RBCs rotate discs partially submerged in wastewater; packed-bed systems contain media fixed in a reactor through which wastewater percolates.

Fixed-film systems are widely used for municipal and industrial wastewater treatment, especially for biological removal of organic matter and nitrification. They are valued for lower energy requirements and robustness to flow variations, and can operate with simple controls. They also support biofilm-based processes at a range of scales from small facilities to large plants.

Limitations include potential clogging or headloss due to biofilm growth, difficulty controlling biofilm thickness, slower start-up for new systems, and reduced performance at high or highly variable loads. Maintenance often involves media inspection or replacement and periodic cleaning.

Historically, fixed-film treatment has roots in early trickling-filter devices developed in the mid-20th century, with later innovations such as rotating biological contactors expanding design options for modern wastewater treatment.