Biofilms in Environmental Engineering

Biofilms — surface-associated microbial communities embedded in a self-produced extracellular polymeric substance (EPS) matrix — are the dominant mode of microbial life in most environmental and engineered systems. In environmental engineering, biofilms are simultaneously essential and problematic: they drive many biological treatment processes, but they also cause biofouling of membranes, pipes, and heat exchangers. Understanding and managing biofilms is therefore a core competency of the field.

The biofilm lifestyle offers microorganisms several advantages over planktonic growth: protection from environmental stresses and biocides, retention in favourable microenvironments, opportunity for syntrophic interactions, and access to nutrients adsorbed to surfaces. These same properties make beneficial biofilms robust treatment agents and problematic biofilms difficult to eradicate.

Modern biofilm research combines microscopy (confocal laser scanning microscopy, atomic force microscopy), molecular biology (fluorescence in situ hybridization, sequencing), modelling (individual-based models, continuum models), and engineering (reactor design, hydrodynamic studies). Hrisana Journal welcomes contributions across this methodological spectrum.

Moving bed biofilm reactors (MBBR) use plastic carriers retained in aerated reactors to support biofilm growth. The carriers provide a large protected surface area for biofilm attachment, allowing high biomass inventory and long sludge age without the need for sludge recirculation. MBBRs are used for BOD removal, nitrification, denitrification, and Anammox applications, often in compact configurations suitable for retrofits or space-constrained sites.

Trickling filters and biotowers — fixed-film biofilm reactors packed with rock or plastic media — have been used for municipal wastewater treatment for over a century. Wastewater is distributed over the media and percolates down, contacting biofilms that grow on the media surface. Modern plastic media with high specific surface area have revitalized the technology, and integrated fixed-film activated sludge (IFAS) processes combine suspended and attached growth to boost capacity within existing tankage.

Biofilm-based systems are also central to drinking water treatment (biologically active sand filters remove biodegradable organic matter and ammonia), odor control (biofilters treating hydrogen sulfide and VOCs), and bioremediation (permeable reactive barriers with biofilm-coated media). In each application, the biofilm's retention of biomass and high cell densities enable efficient transformations in compact reactor volumes.

Biofouling — the unwanted accumulation of biofilms on surfaces — affects membrane systems, heat exchangers, piping networks, and sensors. In membrane systems, biofouling is the most intractable form of fouling because biofilms are viscoelastic and can reform rapidly after cleaning. Once established, biofilms are difficult to remove completely:EPS protects cells from shear, biocides, and disinfectants, and surviving cells can repopulate the surface within hours.

Prevention is more effective than cure. Strategies include pre-treatment to remove assimilable organic carbon (AOC) from feed water, dosing of biocides such as chloramine at sub-lethal concentrations to suppress biofilm growth, optimization of membrane hydrodynamics to limit mass transfer to the biofilm, and selection of membrane materials with anti-biofilm properties. Periodic cleaning-in-place (CIP) with caustic, acid, and oxidant solutions removes accumulated biofilm and restores membrane performance.

Monitoring biofouling remains challenging. Pressure drop and permeate flux decline are lagging indicators. Advanced monitoring tools — optical sensors, ATP measurement, fluorescence spectroscopy — can provide earlier warning but are not yet widely deployed. Research into rapid, sensitive, and preferably online biofilm monitoring is an active area with significant practical impact.

Biofilm research employs a wide range of methods. Laboratory biofilm reactors — the CDC biofilm reactor, the drip flow biofilm reactor, the rotating disk reactor — allow controlled study of biofilm formation under defined hydrodynamic and nutrient conditions. Microfluidic platforms enable high-throughput, real-time observation of biofilm dynamics at single-cell resolution. Confocal laser scanning microscopy with fluorescent stains reveals biofilm structure, viability, and EPS composition in three dimensions.

Molecular methods have transformed biofilm analysis. Fluorescence in situ hybridization (FISH) with taxon-specific probes identifies and locates specific organisms within biofilms. Sequencing of biofilm communities reveals composition and diversity. Metatranscriptomics identifies which genes are expressed, providing insights into active metabolic processes. Mass spectrometry imaging maps the spatial distribution of metabolites within biofilms at micron scale.

Mathematical modelling complements experimental work. Individual-based models track the growth and division of individual cells within a biofilm, capturing emergent structural and compositional patterns. Continuum models treat biofilms as a biomass continuum and are computationally more efficient for large-scale simulations. Both approaches contribute to our understanding of biofilm dynamics and to the design of biofilm-based technologies.

Several research directions are shaping the future of environmental biofilm engineering. Quorum sensing — the bacterial communication system that regulates biofilm formation — can be manipulated to either enhance beneficial biofilms or disrupt problematic ones. Engineered biofilms with designed microbial consortia are being explored for applications ranging from wastewater treatment to bioproduction. Bioelectrochemical systems use biofilms as catalysts at electrode surfaces, enabling energy recovery from waste.

Climate change and water scarcity are driving interest in water reuse, which relies heavily on membrane processes vulnerable to biofouling. Better biofouling control would reduce the energy and chemical costs of water reuse, expanding its applicability. Similarly, the energy transition is driving interest in bioelectrochemical systems and biofilm-based production of biofuels and biochemicals.

Hrisana Journal invites submissions on all aspects of biofilms in environmental engineering — from fundamental biofilm biology to full-scale process applications. Our peer-reviewed, open-access format ensures your work reaches the research and practitioner communities worldwide. Visit our Submit Manuscript page to begin your submission.