Screens vs. Grinders...or is it time for something new?
The composition of municipal wastewater is changing. We are seeing improved efficiency in appliances – low-flow toilets and high-pressure showers – leading to a more concentrated wastewater. This, in combination with high volumes of fibrous material such as ‘flushable’ wipes is forcing suppliers, designers and end-users alike to focus on a major challenge – how do we keep it moving?
In response to this new demand on collection systems, a call for more innovative and robust technologies is sounding. However, before we look ahead at what is new, an understanding of the ‘now’ is key to knowing how we can improve. Here is a quick look at the benefits and considerations for the two most commonly used technologies available that try to address the issues of congestion and blockages in wastewater collection systems.
In small to medium sized pumping stations, grinders can be an effective means to reduce the size of solids passing through the system. They are typically installed upstream of the pumps, on the inlet to the station, with the idea to shred everything that enters the station. It is possible to reduce plugging of pumps by use of a grinder, but this does not come without sacrifice.
Deferring the problem
From a process standpoint, shredding up rags and debris may eliminate the problem at source, but it does not solve it holistically. By shredding up solids to a smaller size, it may cause larger rag balls to form downstream or potentially reduce the capture efficiency of screens and headworks equipment, at the plant.
In conventional activated sludge or extended air plants, screens commonly have larger openings. That means that rags which would otherwise be captured and removed from the process, may slip through and work their way into the plant, potentially plugging more sensitive equipment – or at a minimum, taking up space in the plant that would otherwise be used for treatment.
Cost of Maintenance and Ownership.
Grinders are a sacrificial piece of equipment – that is their purpose. They take the brunt of the wear with hopes that the pumps downstream will continue to operate smoothly. In applications with sand and grit present, grinders will last 2-5 years before a general overhaul is required. As grinders are difficult to source – i.e. carried by only a small list of reputable suppliers – rebuilds or replacement units are extremely costly. For the most basic grinders – rebuilds or cartridge exchanges can run in excess of $15,000 CDN. Furthermore, grinders add to the power consumption of a station with motors typically no smaller than 5 Hp. These motors often run continuously, 24-7, even when the pumps are not running.
Mechanical Screens in Pumping Stations / Collections
Not as commonly used as grinders, screens serve the same purpose, to eliminate pump related issues at the source. Screens, like grinders are typically installed up-stream of the pumps, extending from the bottom of the inlet channel to above grade, discharging to a bin or hopper, which is located at grade level. From a process standpoint, the big advantage is that screens will actually remove inorganics from the system rather than passing the problem downstream. However, with a majority of stations being remote or unmanned, this poses a new set of challenges.
What to do with the screenings?
In the most straightforward applications of screens in pumping stations, screenings removal is required. Screenings are commonly taken to landfill for disposal. However, landfills will rarely accept sanitary waste, or wet screenings, so more ancillary equipment may be required to bring screenings in compliance with landfill requirements. Even basic removal of screenings can be a challenge in remote locations and adds to the cost of operating a station. In locations near residential areas, consideration for odour control may also be required. These challenges are often enough to eliminate screens as a viable option for pumping stations within collection systems.
CAPEX vs. OPEX
Another reason that screens are rarely found in pumping stations is their cost. The capital expenditure to procure and install a screen far exceeds that of a grinder (for the same station), but the running costs are much lower. There are fewer wear parts when compared with grinders and they are driven with motors that do not typically exceed 3Hp. They run intermittently, and are commonly controlled to maintain a calculated head loss across the unit.
Perhaps, time for something new?
In consideration of the advantages and drawbacks for each technology, KSB has designed something new that leverages the advantages of a screen but eliminates the issue of waste management.
KSB’s AmaDS3 Pumping Station is a revolutionary new concept whereby small inclined perforated plate screens capture solids before they ever have to pass through the pumps. During the run cycle of the station, the screens are flushed clean and solids are passed out of the station alleviating the need for large bins and disposal of waste.
Stopping solids from passing through the pumps also adds a level of reliability and addresses an issue that has traditionally limited the versatility of wastewater pumps in high flow, low head applications. Requirements for large solids free passages in wastewater pumps lower the efficiency of the pump and also limit the head, especially in low flow applications. The AmaDS3 Pumping Station addresses this issue and allows KSB to use smaller, more efficient pumps with lower solids free passages, for the same application.
Check out the animation below to see how this revolutionary approach works.
KSB's solids separation solution: the AmaDS