Unveiling the Mechanics of Trickling Filters

What is trickling filter

Trickling filters serve as biological reactors within wastewater treatment facilities designed to eliminate organic matter and/or ammonia from wastewater. In contrast to the activated sludge process, where microorganisms are suspended in a mixed liquor, in trickling filters, these microorganisms are affixed to a stationary bed surface.

How trickling filter works

Modern-day trickling filters typically employ structured plastic fill media to provide the fixed bed surface, although historically, rock, gravel, and hardwood were used. Wastewater from primary clarifiers is consistently dispersed over the initial layer of plastic fill media through a rotary distributor. As the wastewater travels downward through the trickling filter, a cross-corrugated pattern within the plastic fill media facilitates effective water-air mixing.

    Operating under aerobic conditions, trickling filters typically operate without forced ventilation or aeration. The tower-like structure of these filters prompts air to be drawn into the tower through slots at the filter's base, a phenomenon known as the chimney effect. This effect is caused by the difference in indoor and outdoor air density due to temperature and moisture variations, leading to air buoyancy.

    A trickling filter comprises high-specific-surface-area materials, such as shredded PVC bottles, crushed rocks, gravel, or specially designed preformed plastic filter media, to create a biofilm. Microorganisms grow within this biofilm on the media's surface. They adsorb the organic load in the wastewater and stabilize it through aerobic metabolism, converting it into water and carbon dioxide, thus removing oxygen-demanding substances. Wastewater is sprayed onto the media's surface, where it trickles downward, coming into contact with thin layers of microorganisms, further purifying the water.

How to choose trickling filter media

When considering plastic media for trickling filter applications, it's crucial to adopt a forward-thinking approach, aiming for a 20-30 year lifespan. Frequently, the initial cost significantly influences media selection. Opting for cheaper or poorly manufactured media increases the risk of premature failure such as media collapse, delamination, frequent blockages, or sagging, all detrimental to treatment performance and longevity. Essentially, settling for the minimum expectation often leads to minimal performance. A minimum service life of 20 years should be anticipated, but investing more time in the specification and procurement process can yield media lasting 30+ years, resulting in long-term cost savings.

Determining the structural integrity and durability of the selected media is essential. We've compiled a checklist of the top five indicators of module strength to consider when specifying media for your trickling filter application:

Material Properties 

    Detailing plastic properties in the media specification is critical. Tensile strength and flexural modulus are pivotal indicators of a material’s strength. Tensile strength defines a polymer's stress resistance without permanent deformation, while flexural modulus measures a material’s stiffness when force is applied perpendicular to its length.


    The gauge, or thickness of an individual sheet, typically denotes a stronger module when thicker. However, fluctuations in strength might exist despite meeting specific flexural modulus criteria. While module weight is significant, it should not solely determine strength.
Considering gauge requirements involves examining how that specific gauge has been utilized and the design load it sustains. Gauge measurements differ between pre-forming and post-forming thickness due to plastic stretching during thermoforming. Both measurements should be taken into account, considering the ratio between them.


    An accurately aligned pack distributes weight evenly, ensuring structural integrity. Insist that a percentage of modules be checked on-site for face, width, and length tolerance to avoid potential performance inefficiencies or failure.

Compression Testing

    Compression testing determines module strength, provided the modules are appropriately designed to withstand the load and creep over time. This method indicates both the strength and lifespan of the media. Plastic behavior varies based on formulations from different suppliers, making compression testing vital to confirm if a media pack meets its designed conditions.

It's crucial to verify that the selected media not only meets the system's design load but also endures the anticipated conditions.

trickling filter media products

Trickling filter products refer to various materials or structured media specifically designed for use within trickling filter systems in wastewater treatment. These products are engineered to facilitate the growth of microorganisms and the treatment of wastewater. Some of the trickling filter products commonly used include:

Structured Plastic Filter Media: Specially designed plastic media that offers a large surface area for the attachment and growth of microorganisms. These structured media provide an optimal environment for biofilm formation and wastewater treatment.

Shredded PVC Bottles: Recycled plastic bottles that are shredded into smaller pieces and used as a cost-effective filler material. They provide surface area for biofilm development and are an eco-friendly alternative.

Ceramic or Porous Media: Specially crafted ceramic or porous materials with intricate designs that encourage microbial attachment and biofilm formation. These products come in various shapes and configurations to optimize treatment efficiency.

Pre-formed Plastic Filter Media: Specifically engineered plastic media with unique shapes and surface textures that aid in providing an increased surface area for biofilm growth and enhanced treatment capabilities.

Composite or Hybrid Media: Some modern trickling filter products are combinations of various materials, such as plastics, ceramics, or other specialized composites, aiming to optimize biofilm formation and enhance treatment efficiency.

These products are intended to create a favorable environment for the growth of microorganisms, allowing them to attach and form biofilms for the effective treatment of wastewater in trickling filter systems. The selection of these products is based on factors such as surface area, durability, treatment efficiency, and overall system performance.


Straightforward and dependable biological process.
Suitable in areas with limited land availability for extensive treatment systems.
May meet comparable secondary discharge standards.
Effectively treats high concentrations of organics, dependent on the choice of medium used.
Resilient against Shock Loads and Toxins. As it's not a mixed system, toxins affect localized areas. Recirculation helps average out shock loads.
Ideal for small to medium-sized communities.
Rapidly reduces soluble BOD5 in incoming wastewater.
Waste sludge is easily dewatered, primarily from the sloughed-off bio-layer. Settles and dries better compared to activated sludge.
Efficient for nitrification processes.
Durable process components.
Low energy requirements, typically not needing blowers.
Requires low maintenance and has few moving parts.
Moderate level of skill and technical expertise is required for system management and operation

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