Screening is an essential stage of water treatment. Both surface water intended for drinking and wastewater are screened with the aim of protecting the further treatment stages from large objects that could cause obstruction, and improving the efficiency of these later treatment stages.
In this guide, we’ve discussed everything you need to know about screening in water treatment, including what it is, how it works, why it’s used, and the pros and cons of this process.
📌 Key Takeaways:
- Water screening is the first stage of the drinking water or wastewater treatment process.
- Water is screened to remove large debris and coarse solids that could damage later filter stages in the treatment plant.
- The three common types of water screening are fine screening, medium screening, and coarse screening.
Table of Contents
🤔 What Is Water Screening?
Water screening is the process of removing large solids and debris from water.
Depending on the source water, screening may be used to remove the following solids:
Water treatment plants use water screening to prevent these large solids from entering further treatment stages, which could cause clogging and reduce the efficiency of the treatment process.
Because of its importance in supporting other treatment equipment, screening is always employed first, before any other water treatment processes.
📖 How Water Screening Works
There are different types of screening processes used for different feed waters, which produce varying results.
These screening types are:
- Fine screening, which has a spacing between bars of under 10 mm
- Medium screening, which has a between-bar spacing of 10-40 mm
- Coarse screening, which has a between-bar spacing of 40mm+
The smaller the spacing between the bars, the smaller the debris that can be screened out of the water. In some scenarios, multiple screening processes (such as coarse and fine screening) may be implemented together to remove solids of various sizes.
Water is screened by automatic bar screens or rakes. Once separated, the removed debris can be disposed of in landfill sites.
🔎 Why Is Water Screening Used?
Water screening is used for three primary purposes:
- To protect the later filtration stages from large debris that could obstruct some of the units.
- To make it easier and more affordable to implement these water treatment stages.
- To prevent reduced efficiency problems that could be caused by large matter in the water.
Thanks to the initial screening process, less money can be spent on water treatment equipment and chemicals, and less time must be dedicated to implementing further treatment stages.
Plus, removing a potential cause of clogging and blockages will allow water treatment equipment to work as intended, producing better results, allowing systems to work as efficiently as possible, and enabling equipment to last longer before repairs and replacements are needed.
📝 Types of Wastewater Treatment Screening Solutions
There are various types of screens that might be used in the water screening process. These include:
Hand-Cleaned Coarse Screens/ Bar Screens
Hand-cleaned coarse screens (or manually cleaned bar screens) are a common wastewater screening method in small treatment plants.
A manually cleaned bar screen may be used as a standby option during high-flow periods, or as a temporary solution while mechanical screens are being repaired or replaced. Manual bar screens might be basic, but they still offer sufficient protection for the later stages of the water treatment process.
Mechanically Cleaned Coarse Screens
Mechanically cleaned screens (automatic bar screens) are more efficient and are better suited to large water treatment plants. There are four key categories of mechanically cleaned screens:
- Reciprocating Rakes: Or climber screens; reciprocating rakes use a single rake, so they’re not as efficient when dealing with large loads compared to screens with multiple rakes.
- Chain-Driven Screens: This automatic bar screen uses an automatic chain that rakes from downstream or upstream.
- Continuous Belt Screens: These efficient, high-tech screens use multiple rakes to provide continuous screening for both coarse and fine loads, and are also self-cleaning.
- Catenary Screens: These chain-driven screens are front-return and front-cleaned, and prevent clogging from large or heavy debris with built-in internal mechanics.
Microscreens are the smallest types of screens that are used in wastewater treatment. They’re a type of low-speed drum screen lined with filtering fabrics that trap debris as small as 10-35 micrometers.
The solid waste is collected in the drum, while the wastewater passes through to later treatment stages. This collected solid waste can then be disposed of.
Fine Wastewater Screens
Wastewater screening may also employ the use of fine screens, which have openings smaller than 6-10 mm and are designed to remove fine solids. These screens may be made from wedge wire, wire cloth, or perforated plate.
Most wastewater screening applications use one or several of the following fine screens:
- Externally- & Internally-Fed Drum Screens: Or rotary drum screens, use rotating cylinders in the channel of flow.
- Static Wedge Wire Screens: Require plenty of floor space and are usually used by large wastewater treatment plants).
- Step Screens: Use moveable or fixed plates that span across the flow channel.
⚖️ Pros & Cons of Screening in Water Treatment
Now we know how water screening works, let’s take a look at the pros and cons of this treatment method.
- Protects later treatment stages. A drinking water or wastewater treatment plant can use initial screening processes to prevent clogging and reduce the complexity of additional treatment stages.
- Easy to operate and maintain. For the most part, all types of screens are easy to operate and require minimal maintenance. This makes them suitable for use in large-scale water treatment plants.
- Costly to dispose of. The collected debris from screening must be disposed of in landfill, which adds to the overall cost of water treatment.
- Bad smells/attracts vermin. The screened debris may cause poor odors, which could result in complaints from locals. It might also attract vermin and other unwanted pests.
📑 Final Word
Screens are an essential piece of wastewater treatment equipment. There are numerous types of screens, and some may be better suited to certain scenarios than others.
Once water has been screened, it’s ready for further treatment processes, including grit removal, coagulation and flocculation, sedimentation, filtration, and disinfection.
👨🔧 Click here to learn more about the municipal water treatment processes.
❔ Water Treatment Screening: FAQ
Which type of screen is used for water treatment?
There are a variety of screens that might be used for water treatment, from fine screens to coarse screens. Popular screen types include static wedge wire screens, continuous belt screen types, reciprocating rakes, drum screens, and bar screens.
Is screening the same as filtering?
No, screening isn’t the same as filtering and the two processes can’t be used interchangeably. Screening is similar to filtering, but on a much larger scale. The aim of screening is to remove large, visible contaminants (usually at least 10 mm in size) by trapping them in screens. Filtration in large-scale water treatment usually involves the use of sand or gravel filters to remove dissolved particles in water.
What is coarse screen and fine screen in water treatment?
Coarse screen and fine screen simply refers to the size of the screen that is used for screening in drinking water and wastewater treatment plants. Coarse screens have larger gaps between the screening bars and are best for trapping larger debris, while fine screens have smaller gaps between the bars and are best for trapping smaller debris. Coarse screens and fine screens are often both used together for more thorough screening.
What does screening remove from water?
Some of the objects removed by screening include sticks, rags, plastics, trash, and other large solids that may be present in the water. The exact types and sizes of the items removed depends on the source water quality and the size/type of screens used.