For consulting engineers tasked with planning, designing and supervising projects for a wide range of industries, advanced water filtration technology – particularly automatic scraper strainers – offers numerous advantages over traditional choices such as backwash filters and basket strainers.
Various industries including wastewater treatment, power generation, food processing and chemical manufacturing depend on industrial strainers to separate unwanted suspended solids from liquids and slurries. Strainers are also used in the treatment of seawater and wastewater, as well as to remove debris from process and cooling tower water.
Consulting engineers are often involved in specifying strainers, particularly in applications that require bid specs or system-level design decisions. While some projects go directly through buyers, engineering firms frequently play a central role in strainer selection.
Backwash strainers and manual basket strainers are widely accepted technologies and are often the default option during specification. However, conventional strainer designs can present reliability challenges and require frequent maintenance, particularly in applications where large debris or high volumes of suspended solids are present.
In contrast, automatic scraper strainers—cleaning the screen through direct mechanical contact using brushes and blades—are less commonly known. Many consulting engineers are unaware that self-cleaning systems utilizing mechanical brushes are available. These mechanical scraper systems offer a simpler design, eliminate the need for auxiliary piping or external water sources and provide a more robust and adaptable operational profile.
“Automatic scraper strainers are frequently overlooked due to limited familiarity, leading to default specifications of backwash or basket strainers—even in scenarios where a scraper strainer would offer superior performance,” explained Philippe Ellison, Project Manager, Acme Engineering Products, a North American manufacturer of industrial self-cleaning strainers.
This common oversight can result in reduced efficiency, higher maintenance requirements and subpar system protection.
Ellison offers the following guide to help consulting engineers understand the advantages of scraper strainers versus traditional filters. The guide highlights key advantages, ideal uses and selection basics to help engineers specify the right solution with confidence.
By offering a mechanically robust, self-cleaning design that functions effectively under a broad range of conditions, automatic scraper strainers provide tangible advantages over traditional filters.Acme Engineering Products
Strainer Selection and Sizing
Selecting the appropriate strainer begins with understanding the application’s process requirements, including the type and size of solids, solid loading and what needs to be filtered.
Strainer sizing involves balancing flow rate, particle size and solid concentration. Higher solid loading requires a larger filtration area and vessel size. As flow rate and particle concentration increase, so must the strainer’s capacity. Proper data on particle size distribution and operational conditions is critical for accurate equipment selection and sizing as well.
According to Ellison, consulting engineers may specify the wrong type of strainer if they lack detailed information about the operating conditions.
“For example, backwash strainers are sometimes specified in applications where the solids are large, sticky, or difficult to remove – conditions that backwash systems struggle to manage effectively. In these cases, scraper strainers are the better option, as their mechanical cleaning action is specifically designed to handle challenging debris,” explained Ellison.
Oversizing backwash strainers can also reduce cleaning efficiency. Contrary to assumptions, bigger is not always better. Correct flow rate and pressure data are essential for designing effective backwash systems.
In addition, no fluid processing or filtration system remains static. Treatment conditions continually change due to variable factors such as pressure, particle size, solids loading and even the presence of sticky biologicals.
“Flow rate and the amount of suspended solids in a fluid can vary significantly based on production demands, equipment in use, time of day, day of the week and even seasonal conditions,” explained Ellison. “A properly selected strainer must be able to handle the full range of operating conditions to ensure consistent and reliable performance.”
Manual Basket Strainers
Basket strainers are manual filters used to remove large solids or debris from a fluid stream, typically in water or process piping systems. They consist of a pressure vessel housing that contains a perforated or mesh-lined basket. The basket acts as a screen to capture and retain particulates while allowing the fluid to pass through.
In water filtration applications, basket strainers are used to remove coarse materials such as leaves, sand, rust, scale and other solids that may be present in the water. This helps protect downstream equipment such as pumps, valves, meters and more sensitive filters from clogging or damage.
In continuous flow processes that cannot be shut down for cleaning purposes, duplex basket strainers are often installed. This type of strainer employs two distinct chambers that function independently. When one chamber needs cleaning, the flow is seamlessly diverted to the alternate chamber, enabling the removal and cleaning of the first basket.
Cleaning is a messy, laborious process that involves equalizing pressure between the baskets, diverting flow to the off-line chamber, opening the cover, manually removing the clogged basket and cleaning it before refitting the basket, ensuring the seal and tightening the fasteners.
If an operator fails to adequately clean the basket strainers for any reason, both strainers can become clogged at the same time. This can result in quality issues or unexpected downtime until the problem is resolved. For many processors, this can occur simply due to having insufficient personnel to keep basket strainers clean, along with their other duties.
When corrosive environments and process fluid temperature raise concerns about material compatibility, automated scraper strainers are available in materials such as Fiber-Reinforced Plastic (FRP).Acme Engineering Products
Backwash Systems
Backwash filters are used in water filtration systems to remove suspended solids, sediment and other particulate matter from water. They are designed to operate continuously with minimal manual intervention by automatically cleaning themselves through a backwashing process.
In normal operation, dirty fluid flows through the filtration screen, trapping suspended solids, allowing the effluent to pass through the outlet. Over time, these trapped particles accumulate and begin to restrict flow, increasing the pressure drop across the strainer.
To restore performance, the backwash process is initiated. During backwashing, the drain valve opens, causing a reverse in flow across the section of the screen which is isolated by the backwash cleaning mechanisms openings. This dislodges the accumulated solids, which are then pulled into the backwash cleaning mechanism and flushed out through a drain. Once the filtration screen is clean, the system returns to normal filtration mode.
Consulting engineers are generally most familiar with backwash systems, which often leads to their default specification even in cases where scraper technology would offer a more effective solution. This tendency to standardize around known designs can result in missed opportunities for performance and efficiency improvements.
Backwash filters rely on a substantial amount of flow and constant pressure, which can compromise reliability if not always available. “Backwash units do not operate well in backwash mode below 30 PSI. To compensate, some utilize complex, pressure-inducing tactics, but these do not always resolve the issue,” said Ellison.
Additionally, conventional backwash units are not designed to effectively remove larger or irregularly shaped solids.
“Backwash systems are only recommended when filtration requirements are below 50 microns and solid loading is low,” said Ellison.
Backwashing is not ideal for removing large solids from the screen elements. The problem is that the backwash arm must be quite close to the screen to function properly and that prevents the passing of larger particles.
Backwash systems are also more complex and require additional control valves, instrumentation and sometimes external water sources. Scraper strainers eliminate these needs and operate more flexibly through programmable control panels.
Acme Engineering offers a backwash filtration system that stands apart from conventional designs. A key distinguishing feature of Acme’s backwash filters is that they do not require an external source of cleaning water. Instead, they utilize the system’s own process fluid for cleaning, which simplifies installation and reduces water consumption. Additionally, these filters are engineered to operate effectively at lower differential pressures, enhancing system efficiency and extending component life.
Another notable advantage is the internal cleaning mechanism: the suction arm assembly rotates during backwash cycles, while the filter screen itself remains stationary. This design minimizes wear on the screen and maintains structural integrity over time.
ACME is the only manufacturer that produces automatic filtration equipment that can be converted between Backwash and Scraper easily in the field. The filtration housings are designed to accommodate both cleaning mechanisms, allowing operators to switch one for another if the wrong type was specified.
Automatic Scraper Strainers
Automatic scraper strainers are a viable alternative to backwash systems in many scenarios. Unlike backwash strainers, scraper strainers do not rely on a pressurized backwash to remove solids from the screen. Instead, blades and brushes provide more reliable cleaning under varying conditions.
The automatic scraper strainer from Acme Engineering, for example, is a motorized unit designed to continuously remove both large and fine suspended solids. This process is managed by a fully automatic control system.
These scraper strainers are offered with three screen types, selected based on the specific application. Reverse-formed wedge wire screens are the standard choice, valued for their durability and compatibility with brush cleaning systems. For applications requiring finer filtration, multilayer sintered metal mesh screens are recommended. In fibrous processes such as those in the pulp and paper industry, perforated screens with round holes provide optimal performance.
Four blades/brushes rotate at 8 RPM, resulting in a cleaning rate of 32 strokes per minute. The scraper brushes get into wedge-wire slots and dislodge resistant particulates and solids. This approach enables the scraper strainers to resist clogging and fouling when faced with large solids and high solids concentration. It ensures a complete cleaning and is very effective against organic matter “biofouling.”
Scraper strainers allow the solids to accumulate at the bottom of the vessel, where the blowdown valve will open periodically to clear them out. Blowdown occurs only at the end of the intermittent scraping cycle when a valve is opened for a few seconds to remove solids from the collector area. Liquid loss is well below 1% of total flow.
If additional pressure is required to clean the screen, Acme Engineering can add an inexpensive trash pump to the blowdown line to assist in removing the solids from the strainer sump.
“Since the solids are small, a little trash pump can pressurize the blowdown line to evacuate solids from the strainer,” said Ellison.
For applications with high solids loading that are prone to clogging, a macerator can be installed upstream of the automated scraper strainer to break down large solids into smaller fragments. This combination of proven technologies is already in use for some of the most demanding and debris-laden straining applications, including wastewater debris, power plant boiler water slag, asphalt transloading and meat processing waste streams.
While standard carbon steel or stainless steel construction is suitable for typical applications, corrosive environments such as those involving seawater, erosive slurries or aggressive chemicals can rapidly degrade conventional equipment. This deterioration can create risks related to safety, quality and regulatory compliance, as well as cause production downtime due to the need for premature replacement of strainer components.
When the chemical properties and temperature of the process fluid raise concerns about material compatibility, automated scraper strainers are available in other materials such as Monel, D2205, SD2507 and even Fiber-Reinforced Plastic (FRP). The internal mechanism and wetted components can be manufactured from super duplex or similar high-performance steels.
Consulting engineers face increasing pressure to deliver strainer systems that are high performing and cost-effective as well as reliable and low maintenance.Acme Engineering Products
Cost Comparison
Manual basket strainers are the lowest-cost option but require frequent human intervention. Scraper strainers are a mid-priced solution and operate automatically. Backwash strainers are typically the most expensive due to system complexity, additional control valves, instrumentation and sometimes external water sources.
According to Ellison, the cost-benefit ratio of scraper strainers should factor into reduced infrastructure requirements, wear item replacement costs and ongoing maintenance.
For example, a single, automated scraper strainer can replace multiple manual basket strainers as well as the associated piping. Basket strainers require regular maintenance, with manual models requiring cleaning several times daily. The process is messy and undesirable, making it a task operators are often unwilling to carry out.
The frequency of part replacement often depends on the severity of the conditions and how often cleaning is required for the application.
“While scraper strainers contain a few more wear components, such as brushes and blades, these parts are relatively inexpensive. In contrast, backwash systems can feature fewer wear parts but require more costly replacements. Over time, the maintenance expenses for both systems tend to balance out,” said Ellison.
Consulting engineers face increasing pressure to deliver systems that are not only high-performing and cost-effective but also reliable and low maintenance. The adoption of advanced scraper strainer technology represents a significant opportunity to meet these demands.
By offering a mechanically robust, self-cleaning design that functions effectively under a broad range of operating conditions, automatic scraper strainers provide tangible advantages over traditional basket and backwash filters, particularly in high-solids or variable-load applications.
“As consulting engineers become more familiar with scraper technology’s capabilities, supported by application data, installation success stories and performance metrics, they are more likely to consider it a primary option in system design,” said Ellison.