Biofiltration: Nature's Solution To Purifying Polluted Water

Polluted water can be cleaned using a biofilter, a pollution control technique that uses a bioreactor containing living material to capture and biologically degrade pollutants. The biofilter works by passing the water through a bed of media on which microorganisms attach and grow to form a biological layer called a biofilm. The biofilm then uses biological oxidation, adsorption, and filtration processes to remove the pollutants from the water.

The effectiveness of the biofilter depends on the type of media used, which can be sand, gravel, plastic, or activated carbon particles, among others. The media provides a surface for the microorganisms to grow and form a biofilm, which is essential for the removal of pollutants.

Biofilters have been used to treat wastewater, drinking water, and aquaculture systems, among other applications. They are particularly effective at removing volatile organic compounds, odors, and hydrocarbons from water.

The main challenge in operating a biofilter is maintaining proper moisture throughout the system. The air is typically humidified using a watering system or a bio scrubber before it enters the bed.

Overall, biofilters offer a cost-effective and environmentally friendly solution for treating polluted water, especially when dealing with biodegradable pollutants and large volumes of air or water.

The role of microorganisms in the biofiltration process

Microorganisms in biofilters can be naturally attached to the filter bed or artificially immobilised. Immobilisation can be done through covalent bonding, covalent cross-linking, entrapment, membrane separation, and microencapsulation.

The performance of biofilters depends on the type of filter bed media, nutrients from influent water, flow rate, temperature, and maintenance measures.

The biofilm is an important component of a biofilter to treat the pollutants, which is formed on the surface of the filter bed packing materials. Different groups of microorganisms (e.g. bacteria, yeasts, fungi, algae and/or nematodes) may live together in the biofilm.

The main purpose of biofiltration is the removal of biodegradable organic carbon, but a wide variety of possible filter configurations, together with a broad range of different water types and pretreatment methods, contribute to extremely diverse biofiltration performance results.

The types of filtering media used in biofilters

Biofiltration is a pollution control technique that uses a bioreactor containing living material to capture and biologically degrade pollutants. The media used in biofilters vary depending on the type of filtration required.

Mechanical Filter Media

Mechanical filtration involves physically filtering out suspended or undissolved particulates from water. This is often one of the primary forms of waste removal in an aquarium, as it captures and removes waste and debris before it is broken down into nitrate and phosphate. Examples of mechanical filter media include filter rollers, filter pads, sponges, and filter socks.

Some types of mechanical media are denser than others, with density measured in microns. A dense felt filter measuring 50 microns will capture finer particles compared to a more porous 200-micron filter sock. Coarse mechanical filter media, such as coarse sponges and filter socks, can be reused after rinsing and drying, while fine mechanical filter media, like filter floss and felt polishing pads, are disposable.

Chemical Filter Media

Chemical filtration involves removing dissolved contaminants that change the "chemical composition" of the water. There are several types of chemical filter media that can be used to target various problems.

• Carbon: A broad-spectrum adsorption media that can remove toxins, tannins, and odor-causing compounds. It is effective at clearing up yellow water and keeping aquariums odor-free.
• GFO: Granular Ferric Oxide binds to phosphate molecules so they can be removed from the water.
• Resin Media: Targets specific heavy metals, phosphate, and various dissolved organic waste compounds.
• Mixed (All-In-One) Filter Media: Uses a combination of carbon, GFO, and resin to provide a comprehensive solution.

Biological Filter Media

Biological filtration involves harboring beneficial bacteria to biologically break down waste via the nitrogen cycle. In an aquarium, hobbyists can supply special biological filter media (bio-media) that provide extra surface area for the beneficial bacteria to grow. Examples of bio-media include ceramic rings, bio-blocks, bio-balls, and other inert porous materials.

Bio-media does not need to be replaced and will remain effective as long as it stays wet and the bacteria are alive. However, it should be rinsed regularly to remove detritus and should not be allowed to dry out, as this will kill the bacteria. It typically takes 2-4 weeks or more for bio-media to cycle and become populated with beneficial bacteria.

The advantages of biofiltration

Biofiltration is a cost-effective, eco-friendly, and sustainable way of treating wastewater. It is a biological process that uses microorganisms to remove pollutants from water. Here are some of the advantages of using biofiltration:

• Cost-effective and eco-friendly: Biofiltration is a low-cost method compared to conventional techniques. It is an environmentally friendly process that does not produce any secondary pollutants.
• Simple and flexible: Biofiltration is a simple process that can be adapted to different situations. It can be used for small or large flows of water and can be designed according to space and capital requirements.
• High flow rate and efficiency: Biofiltration has a high flow rate and can achieve a significant percentage of efficiency even for low concentrations of contaminants.
• No external thermal power required: Biofiltration works at room temperature and does not require external thermal power.
• Applicable to various toxic pollutants: Biofiltration can be used to treat a wide range of toxic pollutants, including volatile organic compounds and heavy metals.
• Safe and user-friendly: Biofiltration is a safe and user-friendly process that does not require skilled professionals.
• Less chemical usage: Biofiltration uses fewer chemicals compared to conventional techniques.
• No evolution of secondary pollutants: Biofiltration converts contaminants into biodegradable waste without producing secondary pollutants.

The challenges of biofiltration

The use of biofilters to clean polluted water is a promising and economic alternative to physical and chemical disinfection. However, there are several challenges to their use:

• Microbial community and growth: The performance of biofilters depends on the type of microorganisms in the biofilm and their growth. However, there is a lack of understanding of the metabolic pathways of these microorganisms and how they contribute to the complete removal of contaminants.
• Biofilm layer: When the concentration of contaminants increases, a thick biofilm layer can form, which can lead to inactive microbial biofilm with varying pore sizes. Multi-microbial cultures can be used to improve the removal of various pollutants with less degradation time.
• Contaminant sources and characteristics: The biofiltration equipment or method must be designed and optimised based on the characteristics and quantity of the pollutants present in the wastewater. The treated water and acquired metals should also be considered for safe disposal or recycling/valuable products.
• Temperature: The operating temperature of the biofilter affects the number of active microbes and the degradation of pollutants. Lower temperatures can decrease the removal of organic matter due to changes in the microbial structure and metabolism.
• Moisture content: Moisture content is important for the activity of microorganisms and the formation of biofilms. Organic and inorganic media have different moisture retention properties, with inorganic media retaining more moisture.
• Pressure drop: The relationship between pressure drop and biomass concentration can be used to predict the performance and stability of biofilters.

The use of biofiltration in drinking water treatment

Biofiltration is a water purification technology that plays a crucial role in producing safe drinking water. It is a process that removes fine particles through physical means such as straining and sorption, as well as capturing and degrading contaminants through biological activities. Biofilters are composed of complex microbiomes, primarily shaped by water quality, and they contribute to addressing safety challenges in drinking water.

Biofilters contain filter medium grains (e.g. sand, granular activated carbon) that are covered with biofilms. The biofilm activities break down nutrients (e.g. nitrogen and phosphorus-containing compounds) and organic carbon, as well as capture other unwanted contaminants in the influent water. Water coming out of a biofilter typically undergoes disinfection before entering drinking water distribution pipes or being released into the environment as wastewater effluent.

There are three main biofiltration processes used in drinking water treatment plants (DWTPs): rapid sand filtration (RSF), granular activated carbon filtration (GACF), and slow sand filtration (SSF). These processes are applied together with coagulation, flocculation, sedimentation, and disinfection steps in various combinations.

RSF processes water with a wide range of initial turbidity at a high flow rate (5-30 m/h), which can reduce most physical hazards (particles) and inorganic compounds (e.g. iron, manganese, ammonia, nitrate). GACF is applied to further remove chemical and biological hazards that can break through the RSF and the following ozonation barriers. GACF is primarily designed to remove unpleasant odour, taste, and colour from water caused by natural organic matter (NOM) and organic micropollutants. SSF is one of the oldest and most effective approaches to control microbial contamination (i.e. pathogenic oocysts, bacteria, and viruses) from drinking water. It is effective for reducing turbidity, total organic carbon (TOC), nitrogen compounds, pesticides, pharmaceutical chemicals, and microbial contaminants.

The mechanism of biofiltration processes involves biological removal, which occurs in parallel with physical-chemical removal. When operating a new filter, the removal is initially achieved through physical mechanisms such as straining and adsorption. Over time, biofilms are generated on the surface, and biological removal (e.g. biodegradation) gradually becomes the main removal mechanism.

The composition, activity, and robustness of the biological community in biofilters determine the removal effectiveness of biofiltration units in DWTPs. Bacteria are predominantly present in all DWTP biofilters (including RSF, GAC, and SSF), with Proteobacteria being the most abundant phylum in almost all drinking water biofilters.

Biofiltration has several advantages over other water treatment methods, including no addition of chemicals, low energy input, and high removal efficiency of organic compounds, undesirable taste and odours, and pathogens. It is an eco-friendly and economic technique for removing pollutants and pathogens from drinking water.

Frequently asked questions