Grinder/Cutter/Shredder Pump Resolves Sewage Clogs

The variety and popularity of personal disposable items has drastically increased in recent years. The challenge of pumping these items from a sewage collection pit, where they invariably end up, into the main sewer line has become a critical problem. For the most part, the job of evacuating sewage from a collection pit in the basement of a hospital, hotel or high-rise building has been given to a 4-inch or 6-inch solid handling sewage ejector pumping system with motors and stands above ground. A typical installation could comprise a round pit about 4 feet to 6 feet in diameter and possibly 6 feet to 8 feet deep.

The pumps would typically be suspended from a plate covering the top of the pit and positioned so that the suction port rests 12 to 18 inches from the bottom of the pit. A control panel with the aid of floats would turn the pumps on and off at pre-set levels, alternate their operating cycles and sound a high-level alarm if required. These ejector pumps have been and are still doing a great job in many low-traffic, well-maintained installations. However, for high traffic areas, the story is different.

Property managers and maintenance engineers have been faced with frequent and expensive service calls to unclog these ejector pumps. Many just choose to live with the problem. The more proactive managers and maintenance engineers have chosen other options:

• Install a recessed vortex impeller—This usually alleviates the problem temporarily, but when a mop head or a similar bulky item goes through the pump, it will usually get stuck at the check valve.
• Use a carbide-tip impeller—An impeller with carbide tips would cut the solids against stationary hard edges on the suction casing. This also works for a while, but the clogging is only delayed, not avoided, since large stringy items enter the pump anyway.
• Use a submersible chopper pump—This is a good option and extends the time between clogs. However, it is not an ideal solution, because even the more aggressive chopper pumps do not reduce the size of the solids enough to prevent subsequent clogging. They often clog when a large number of disposable items enter the pump in rapid succession, especially if some of these items are long and fibrous.
• Install a macerator—Macerators can be effective against solids, but usually cannot be easily installed if the facility was not designed for them. A macerator will only shred and, therefore, cannot replace a pump.
• Use a grinder/cutter/shredder pump—This may be the best option since, by design, these pumps will reduce sewage solids to a fine slurry that can be easily pumped out without causing clogs or other problems.

Clogged Systems

Dealing with sewage is a nasty business, so the desire to minimize human exposure has always been a prevalent factor when selecting sewage handling equipment. When dealing with a problematic sewage ejector installation that experiences clogs every few months (or worse, every few weeks), some factors to consider are:

• Exposure of service personnel to unknown and potentially life-threatening pathogens
• Cost of pump removal, unclogging and reinstallation
• Disposal of the items removed from the clogged pump
• Cost of pump overhaul after possibly running partially clogged and without lubrication to the lower sleeve bearing
• Reduced end-user confidence and reluctance to pay for the frequent unclogs and repairs, since the end user feels that the service company may not be up to the task of finding a long-term solution
• Potential floods due to overflown sewage, which often happens when the secondary pump clogs while the primary pump is being repaired or maintained
• Frustration of the service crew members, since they feel that they are unnecessarily and repeatedly exposed to a nasty and risky working environment
• Possible lawsuits by inconvenienced tenants that occupy areas close to the sewage collection pits who may be forced to relocate during cleanup

When evaluating a problematic sewage ejector system, property managers and maintenance engineers have two choices: keep fixing the same problem or find a long-term solution. Once the decision is made to look for a long-term answer to the problem, a high-performance submersible, cutter/grinder/shredder pump offers an effective solution.

Clog Elimination

To eliminate clogged pumps and other equipment, install a cutter/grinder/shredder sewage pumping system. The most important steps to follow are:

1. Establish the hydraulic system requirements, such as head and flow.
2. Select the pumps and control panel. Some manufacturers offer the following options—high head, low flow; low head, high flow; or bi-directional. Most high-traffic commercial or institutional facilities would require low-head-high-flow units, since most of them need to pump up just one level to the main sewer line. Other, more problematic installations would benefit from a bi-directional system that offers the advantage of dislodging a possible wad and doubling the life of the cutter.
3. Check the existing electrical service. This is necessary to make sure that it can handle the new pumps. Keep in mind that the new pumps will require additional horsepower to process the solids.
4. Determine alarm modes. Consult with the end user regarding the preferred mode of the high-level alarm—such as a bell, flashing light, text message or combination of them all.
5. Assess the severity of the specific sewage pit. In the event of a particularly problematic sewage collection pit, consider installing a third pump that would discharge in the pit. This would greatly facilitate the processing of the solids and would keep sediments in suspension while one of the other pumps is evacuating the pit. Refer to Figure 1 for a typical three-pump system.

There is great value in doing one’s homework in the process of matching a grinder/cutter/shredder sewage pump with a given sewage collection pit, but nothing can substitute the experience of a service company with a proven track record of solving sewage handling problems.

Figure 1. A three-pump grinder/cutter/shredder system

 

Conclusion

When faced with a costly and problematic sewage ejector installation, choose a long-term solution. Hire an experienced service company that handles a known line of submersible grinder/ cutter/shredder pumps and above all, ask:

• Would this new pump system evacuate my sewage pit even at the highest sewage inflow rate with just one pump?
• Is the impeller built so that fibrous material does not collect on its vanes?
• How long is the warranty?
• Should the existing check valves be replaced with new, full port, ceramic-coated check valves?
• Are the impeller and cutter bi-directional, and is this supported with a control panel?
• Are references available to support the pump system’s performance—such as application, time in operation, size and type of pumps and services required during the life cycle?

Once these questions are answered to  the end users’ satisfaction and they feel confident that they have made the right choice, they should implement the new pumping system. A higher initial acquisition cost will become affordable when the total life-cycle costs are considered.