Modern Trash Plagues the Wastewater Industry

Concern about the flushability, transportation and decomposition of non-woven products in wastewater systems first appeared in a technical paper during an environment conference in 1993.¹ The authors wrote that flushable non-woven products might not be compatible with the federally legislated reduced flush volume for residential plumbing fixtures to 1.6 gallons that became law in 1994.

Today, the increasing problems these products cause in maintaining our wastewater collection systems have proven their assertion correct. Sometimes termed “modern trash,” non-woven products—including flushable wipes, synthetic sheets used as carriers for cleaning agents, and other plastic goods—are flushed into the public sewer system where they accumulate and create problems.² Modern trash neither decomposes nor flows through the plumbing system in a normal manner. Low areas in collection systems, such as manholes and pump wells, are particularly susceptible to an accumulation of these products.

Submersible pumps, submerged in the liquid in these locations, are particularly affected. Submersible pump stations collect wastewater and pump it up and out of a well, into the force main and to a treatment facility. As this trash accumulates, normal water flow through the system can become restricted. Pump capacity is reduced, eventually clogging the pump, which shuts down the system, requiring service. Repairing a pump under these conditions is time-consuming and can potentially impact the environment.

In recent years, articles have been written detailing the problems, costs and inconvenience caused by modern trash, prompting pump and lift station manufacturers to create new products designed to mitigate the problem.

There are several factors that must be considered in creating solutions for the problems of modern trash in pumping systems. One aspect that is often overlooked, yet must be accounted for, is the impact that wastewater speed flowing through a system has on situations where modern trash is a factor. Historically, a rule of thumb used in the pumping industry is that a velocity of 2 feet per second (ft/s) is a rate at which solids will stay suspended in a line carrying wastewater. An increase in this rate of flow, described as gallons per minute (gpm), and its associated velocity, expressed in ft/s, for the pipe size, may assist in alleviating some clogging problems.

Case Study

A significant clogging issue that occurred a few years ago provides an excellent example of velocity importance to a pumping system that must handle modern trash. The problem involved a 5 horsepower (hp) grinder pump lift station located downstream from a large nursing home. The system’s pump had to be pulled numerous times to remove clogged debris before returning it to operation.

The pump would run for a few weeks but would clog again three to four weeks later. With assistance from a determined and curious plumber, the station was observed to find the cause of the reoccurrence. It was discovered that the solids being pulled into the pump were ground up and passed properly, but they were not moving along with the liquid waste into the force main. Much of the ground solid waste remained in the discharge riser pipe while the clearer liquid waste moved through the system. The system was functional until an accumulation of the solid discharge waste clogged the pump and discharge elbow.

The pump was moving liquid at approximately 80 gpm through a 4-inch riser and force main, with an acceptable velocity of nearly 2 ft/s. However, based on observation, that velocity was not adequate in this case. It was determined that about 20 feet of the 4-inch riser pipe needed to be replaced with 3-inch pipe. The smaller diameter riser pipe would increase the velocity to more than 3 ft/s.

Decreasing the size of the riser pipe had the intended effect and the frequent clogging stopped. After this experience, the significance of velocity in a wastewater line began to gain importance. The 3 ft/s became the preferred velocity to use as a minimum for more wastewater applications.

This application example demonstrates two things. First, when working with a wastewater application where non-decomposing solids are anticipated, a velocity of 2 ft/s may not be sufficient. Second, while a grinder pump may sufficiently cut up and pass solids, its lower flow rate may still allow clogging problems in some wastewater collection systems.

Conclusion

In the February 2014 issue of WE&T, the Water Environment Federation’s magazine, an article summarized the study that a group of Hazen and Sawyer engineers conducted on this subject.³ The potential problems of designing a system for a 2 ft/s velocity was noted and a regular flow rate of 3.5 ft/s was advised in many systems not only to keep solids suspended, but also to prevent the accumulation of gas pockets and pipeline deterioration.

Although there can be negatives associated with these higher flow rates, such as greater friction losses and higher operating costs, these may offset repeated service calls and potential unit replacement costs, making it something to be considered. Since there are inconsistencies in the industry as to what velocity to use in designing a pump station, one is often left to rely on intuitive knowledge from past experiences based on the type of application being evaluated.

Many other considerations come into the equation as well: solids content (anticipated amount of modern trash) in the system, the type of pump being considered for the station, line length, depth of wet well and even the system voltage. All these variables can affect the long-term dependability of the system. A few items to consider when designing a system where high-solids content can be troublesome are:

• Single phase voltage systems. Some problem-prone applications, such as multi-unit apartment complexes and similar locations, have single-phase power and have installed grinder pumps. A vortex pump may be a more dependable option when single phase voltage is provided at these type locations.
• The use of variable frequency drives (VFDs). VFDs provide added protection to the pump and decrease energy use. Before committing to their use, however, plot a system curve against the pump curve. While the VFD may offer a 2-to-1 turndown ratio, 60 to 30 Hertz, it is not advisable to run a pump at a lower range that enables the flow rate to drop below 2 ft/s. Misapplying a VFD in a manner that enables the pump to operate at a slower speed has been responsible for some clogging issues.
• Pump selection. Select the best pump for the application. No one pump is best for every application. Consult a pump manufacturer with a variety of pump types and a local representative who can assist in evaluating each system on its merit and determine which submersible pump (standard two vane solids handling, vortex, grinder or chopper) is best suited to the system’s needs.

References:

1. “Flushability, Transportation and Decomposition of Nonwoven Products” - T. Konen and C. Christodoulatos – Environment 1993 Conference.
2. “The New Wastewater” – R. Domkowski, ITT WWW USA – Flygt – Pumps & Systems October 2011.
3. “Flow Woes” – B. Copeland and S. O’Rourke – WEF.org Magazine February 2014.