Wastewater Screening Versus Filtering

JWCE | April 7, 2026 | Municipal Wastewater

Among the many steps essential to wastewater treatment, screening and filtering are the two most commonly confused and sometimes used interchangeably. Both are necessary to prepare the water for further treatment, but they differ significantly.

Screening generally comes before filtering, and for good reason. Without appropriate removal of solids, it’s impossible to filter water with finer particulates without damaging sensitive equipment.

In this article, we’ll explain how these two early stages of wastewater treatment work together to prepare the treated stream for purification, settling, and more.

Defining Screening

Screening is usually set up as the preliminary stage in wastewater treatment. This is especially true at municipal plants that handle mixed sewage flows, but it may also apply to industrial processing waste. Screening is used to remove the majority of debris mixed into wastewater, including everything from large items like branches and rags to small particles like grit and sand.

Some grinding processes use grinders to break down large debris before it’s screened out, allowing the use of finer screens to handle everything. In other systems, progressive sets of screens from coarse to fine handle a wide range of sizes without clogging. It’s all a matter of matching the screening equipment to the waste stream and the desired level of grit removal.

Particle Size

There are two types of screens: coarse and fine, each designed to filter particles of different sizes. The largest particles, including food wrappers, toys, and rags, are captured using coarse screens with typical openings of around 6 mm (0.25 in) or larger.

Comparatively, fine screens can be designed with perforations as small as 1.5 mm (0.06 in), allowing the capture of shredded debris along with larger grit. If the screening process needs to remove sand and the finest particles possible, very fine screens use openings under 1.5 mm to target them.

How the Process Works

Raw, mixed wastewater enters the screening equipment, often cascading over simple racks, flowing through stepped or progressive bar screens, or tumbled in a rotating screen. Screening equipment not only captures debris that could clog or damage more sensitive equipment further down the treatment process, but also aids dewatering, which helps reduce transportation and disposal costs because the waste weighs less.

After the screening equipment captures particles, excess water is returned to the waste stream for further processing, leaving behind the grit and particulates that could cause problems in sensitive membranes, filters, or reactors.

Mechanism

Screening is largely a physical process, relying on screens with mesh or punched openings of specific sizes, which allow rapid separation of a wide variety of particles based on size alone, especially when combined with the centrifugal force created by a rotating drum.

Available Technologies

Wastewater screening equipment has come a long way from the basic rack-style screens that were once commonly used. There are now a number of screening technologies to choose from, many of which are designed to resist clogging for low-maintenance operation. These designs include:

Bar screens, which can provide coarse or progressive screening, depending on how they’re built.
Fine screens, offered in rotary drum, band, auger, and static designs.
Microscreens, for removing fine particulate to prevent fouling in filtration steps.
Grinders and macerators, which reduce larger debris to finer particulate, making fine screens more effective and less likely to clog.

Defining Filtering

While screening is performed as early as possible in the wastewater treatment process, filtering is a secondary and/or tertiary treatment applied only after you have removed the majority of the debris. Without proper screening first, these filters won’t perform as expected.

Most filters rely on sensitive materials, such as active bacteria-rich sludge, bacteria-populated membranes, or sand filters that provide both active and passive filtration. All of those materials are easily damaged or disturbed when large particles and solid debris clog the surface.

Filtering is used to remove everything from suspended and fully dissolved solids to chemical contaminants, oils, greases, sludge, and more. If too much sand or grit, in particular, fills the filtration area, the flow slows down to a trickle and the entire treatment process becomes delayed.

Appropriate screening creates a relatively particle-free waste stream that’s ready for more intensive processing, just like filtering prepares wastewater for chemical disinfection and sedimentation.

Some in-depth filtration methods, such as the intermittent sand filter system, provide a combination of physical, biological, and chemical treatment.