How To Choose Wastewater Screen Options

Screening is a critical component of wastewater treatment headworks and is used in a wide range of other applications, including cooling systems, stormwater, flood prevention, and more. The most common screening technologies include static screens, vertical bar or perforated plate screens, rotary drum screens, and augers. Each has distinct advantages, so engineers and plant managers should design a headworks system that best fits their needs.

Understanding Screen Options

The four most common screening systems include:

1. In-Channel Screens (perforated plate or bar screens). These traditional systems use vertical or inclined bars to trap debris directly within the flow of the channel. A mechanical rake or brush periodically combs the bars to remove solids, making them a robust choice for handling high-volume municipal flows and heavy trash loads.
2. Augers. The combination of a perforated basket and an internal screw allows these systems to capture, lift, and dewater solids. This can save space in applications where compaction and dewatering are essential, as well as reduce the size and weight of waste.
3. Rotary Drum Screens. This technology uses a rotating cylinder of fine mesh or wedge wire to remove solids and self-clean. Because the drum is always presenting a clean surface to the influent, it is highly effective at removing fine particles and greasy materials that might clog fixed screens.
4. Static Screens. With no moving parts, static screens rely entirely on gravity to pull water through a stationary screen face. They are the most cost-effective and low-maintenance option available, though they are best suited for consistent low-flow operations to ensure manual cleaning is manageable.

Factors To Consider

In order to choose the right screening technology, the technical and operational factors must match the specific needs of the operation. This includes:

Flow delivery and volume. The first question is whether the influent is delivered by gravity or if it is pumped. For example, drum screens are typically excluded from gravity-fed applications due to their limited bypass capacity. In addition, static and auger screens are generally suited for lower volumes, typically less than 5 to 7 million gallons per day (MGD) per unit. By comparison, a single internally fed drum screen can handle larger volumes, ranging from 10 to 13 MGD.

Capture rate efficiency (CRE). Required screening is another important factor. Standard technologies like auger and vertical bar screens, which have CREs of 50% to 60%, are often used for biological processes where ultra-fine screening is unnecessary. However, more sophisticated operations may require perforated plate screens such as band or front-loaded screens, or internally fed drum screens.

Debris type and loading. Wastewater plants that are prone to rags and wipes should avoid augers and static screens, as this material can clog brushes used to clean them. Other operations, such as surface water intake or stormwater management, may involve large debris like logs, weeds, or animals, which require specialized trash racks or automatic bar screens for effective removal.

Footprint and hydraulics. Available space plays a key role in determining the appropriate headworks technology. Augers, for example, require a certain amount of “runway” in a channel and are often installed at steep angles (35° to 45°). Meanwhile, drum screens can often fit into smaller spaces and in some cases reduce the need for channels.

Maintenance and operating costs. While no screen is 100% maintenance free, it is imperative to understand the operating costs associated with each option. Augers and some in-channel screens, for example, use brushes that can wear out, requiring regular inspection and replacement. Similarly, many high-efficiency screens rely on spray wash water for cleaning, increasing both freshwater and electricity consumption, which drive up utility costs. However, static screens and some vertical bar screens do not use brushes or wash water, resulting in comparatively low operating costs.

Total cost of ownership (TCO). While upfront costs are always a consideration, it is critical that plant managers and other decision makers take the TCO into account as well. In many cases, investing in high-quality headworks screening is often a “pay now or pay later” proposition; in other words, if a system fails to remove trash effectively, it can impact downstream systems and result in greater overall operational costs.

Ultimately, the ideal choice for screening depends on a delicate balance of hydraulic capacity, debris characteristics, and long-term operational requirements. While a static screen might offer the lowest upfront cost for low-flow scenarios, a high-efficiency rotary drum or perforated plate screen can save thousands in downstream maintenance by protecting sensitive biological processes from fine debris. By evaluating these factors, along with the TCO, engineers and plant managers can ensure they are investing in the long-term health and efficiency of their operation.