Are Biological Trickling Filters Right for Your Municipal Wastewater Treatment Facility?

With a history stretching back well over 100 years, trickling filters are a well-known type of biological wastewater treatment technology that is still used widely over the world. Biological trickling filters have a reputation for reliability and efficiency in secondary wastewater treatment applications, and is particularly well-suited for small to mid-sized municipalities.

In this article, we’ll take a look at the major pros and cons of biological trickling filters for municipal wastewater treatment applications in particular, and explore how this technology might fit your wastewater or sewage treatment needs.

Benefits of biological trickling filters

The main benefits of a biological trickling filter include a simple design, small footprint, good organics removal, and energy efficiency. Read on to learn more about these benefits.

Simple design

Biological trickling systems feature a relatively simple design, consisting of just a handful of components, including a container or tower filled with filtration media like crushed rock, gravel, or shredded plastic, as well as a distributor (typically a rotating arm with sprinklers), and an under-drain system. As biological trickling systems are relatively straightforward from a mechanical standpoint, they can be somewhat simpler and more affordable to repair and maintain than other wastewater treatment technologies. 

Small footprint

Conventional wastewater treatment plants often use man-made ponds or lagoons that can take up large areas of land.  By comparison, biological trickling filters are much more compact than other wastewater treatment systems. This is because a trickling filter system uses particulate filtration media that is designed to maximize surface area. As a result, the media is able to house a larger volume of biomass in a smaller area. As such, biological trickling systems can be a good fit for urban areas or other locations where larger areas of land are costly or simply not available for construction of conventional wastewater treatment ponds.

Good organics removal

The main function of biological trickling filters is to reduce the organic content of a wastewater stream, which is typically measured as biological oxygen demand (BOD). Biological trickling filters are excellent for rapid reduction of BOD levels in streams with moderate to high levels of organic content, and can also provide some suspended solids removal as well. The ability to handle high concentrations of organics does, however, depend on the filtration media used in the system. Filtration media are typically classed as low-, intermediate-, or high-rate, and a system design should leverage media appropriate to the BOD loadings of the wastewater stream, the target effluent quality, and any accompanying primary and tertiary treatment technologies. When properly designed, though, a biological trickling filter can produce effluent of high enough quality for compliant discharge. For facilities where stringent effluent standards apply, tertiary treatment may also be needed for additional removal of pathogens or other contaminants.

Energy efficiency

Biological trickling filters are a type of aerobic wastewater treatment, meaning that the biomass will need a supply of oxygen to thrive and do the work of breaking down organic constituents. Most aerobic systems will require moderate energy consumption for aeration, agitation, or maintaining system pressure, but trickling biofilters require energy mainly just to pump water to the distributor mechanism. For this reason, typically biological trickling systems consume less energy than other aerobic treatment technologies, like activated sludge, moving bed bioreactors (MBBRs) and membrane bioreactors (MBRs). If your municipal facility is looking to minimize energy usage, then biological trickling systems can be a great solution.

Drawbacks of biological trickling filters

It’s true that biological trickling filters are a proven technology with some great benefits, but like all wastewater treatment technologies, they are not without their downsides. The main disadvantages of biological trickling filters include clogging and flow rate inflexibility, although there are generally strategies to mitigate these issues.

Clogging

These systems are prone to clogging, which can happen when solids enter the system and become lodged in the distributor mechanism, filtration media, or underdrain system. Clogs can interrupt the normal flow of air and water through the system, which can reduce the capacity of the system, and can also compromise the biofilm by depriving it of the conditions needed for proper growth and performance. Facilities typically mitigate clogging risks by adopting primary treatment processes ahead of the biological trickling filter, which can include screening and sedimentation. That said, municipalities considering biological trickling systems should be aware that a primary treatment step will most likely be needed to ensure optimal system function.

Flow rate inflexibility

Biological trickling filters depend on a consistent flow of water through the system. This ensures that the biofilm remains moist and aerated at all times so that it can continue to perform as expected. Without proper moisture, the system can be prone to filter fly infestations. Conversely, when overloaded with excess organics, portions of the biomass may become anaerobic, which can lead to issues such as unpleasant odors and excess sloughing of biofilm.  As such, municipalities that experience high variability in organic loading and/or wastewater volume will need to carefully select filter media according to their specific treatment needs, and may also need to take additional steps to regulate flow to a biological trickling filter unit.

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