“The sludge shall be presumed to frequently contain rags, scrap metal, textiles, sticks, hair, broken glass, sand, silt and that vast variety of other materials often found in a sanitary or combined storm and sanitary sewage system. In addition, there is always a carryover of abrasive silt from the grit chamber, which will settle out with the various sludges.”
This definition of sludge from the city of New York pretty well describes one of the more difficult pumping applications that can be encountered. However it is defined, sludge is tough stuff that presents problems not generally encountered elsewhere. Because it is such a challenging service, the chances of selecting the wrong pump are considerable, and the consequences are expensive.
Evaluating the hydraulics of sludge flow is difficult because:
- Sludge is nonhomogeneous and has a high degree of variability. Viscosity cannot be treated as a constant in pressure drop calculations, and special methods must be used to calculate the friction loss.
- Sludge flow can be either laminar or turbulent.
- Users must understand complex fluid mechanics and not rely on rule-of-thumb approaches.
- Because sludge is so viscous compared to water, it takes a lot of energy to move it and keep it moving.
Sludge Characteristics
When considering a pump system for sludge service, it is essential to know the specific components of the media. Different media call for completely different pumping strategies. The components of sludge can be divided into roughly three categories:
- Grit or abrasive products
- Large solids and potential clogging-type solids
- Thick and viscous organic solids
Each represents a pumping challenge, and taken together, they can seem unmanageable. Even so, the sludge must be moved, so a suitable pump must be found for the service. First, prioritize the major pumping challenges. Next, identify pumps with features that address the challenges, examine the features for crossover potential and select the best pump for the application based on that research.
General Sludge Pumping Concerns
General concerns include:
- If grit handling is poor and there is a heavy carryover of grit into the system, wear and tear will be the main challenge. These severe duty services must use abrasion-resistant centrifugal pumps and components. If the grit removal facilities are good, positive displacement pumps can be used without concerns about excessive maintenance.
- Gravity-thickened sludges should be pumped using abrasion-resistant materials. No matter how effective the grit removal process is at the front of the plant, fine particles will always be carried along in the system. The higher investment cost will balance over time, with less time and budget wasted on system maintenance.
- Other problems include entrained gas if the sludge starts to ferment, creating variation in sludge consistency that can cause flow control problems. Concentrations can range from 2% to 10%, depending on whether the sludge is from the clarifier or thickener operation.
- Sludge consistency changes are a major influence on pump selection. Concentrations of raw, digested or digesting solutions of up to 8% solids can be handled by centrifugal pumps, provided their net positive suction head (NPSH) requirement is not exceeded, and their speed can be varied to accommodate changing sludge consistencies. Another problem to watch for is “rat-holing” or funnel flow in the sludge. Above 8%, positive displacement lobe pumps are recommended as standbys, and above 10%, they should be used exclusively.
IMAGE 3 (right): Typical sludge and water flow curves
System & NPSH Analysis
To determine if a sludge can be pumped, it is necessary to reasonably predict its flow characteristics so a system head curve can be generated and NPSH evaluations can be done. Because many factors contribute to sludge flow loss characteristics, careful system analysis should be done before pump selection. Most references do not cover sludges above 10% consistency because sludges of that thickness are not considered “flowable” with the standard positive suction heads available in treatment plants. Centrifugal pumps are usually not considered for these services, and positive means of feeding positive displacement pumps must be examined.
Most users agree that calculating total dynamic head is the greatest challenge they face in heavy sludge media. Image 1 shows a typical system head curve of a 4-inch diameter sludge line with a 10-foot positive suction head, 10-inch static lift and piping length equivalent to 1,000 linear feet at various consistencies.
Wastewater sludges behave as non-Newtonian fluids for which frictional or dynamic head loss is a function of the flow properties of the sludge, the pipe diameter and the flow velocity. Dynamic head losses between five and 20 times those expected for water are common for sludge flowing through pipelines. Below the lower critical velocity, flow is laminar. Above the upper critical velocity, the flow is turbulent, and the material acts as a Newtonian fluid (Image 2). Head loss for turbulent sludge flow is much more sensitive to changes in velocity. For non-Newtonian fluids, techniques are available to predict head loss in both laminar and turbulent flow.
Suction Side Critical
All this implies that a great deal of attention should be paid to the suction side of the pump when working with sludges. Here are a few important guidelines:
- Ensure adequate NPSH is available from the system, no matter which pump is selected.
- Suction piping should be kept as short and straight as possible.
- Sludge velocities should be kept above 4-5 feet per second (ft/sec) to avoid sedimentation in the line and to reduce grease buildup.
- Derivation and analysis of the system head curve should be thorough.
- A range of expected losses at a given consistency should always be evaluated—that is, normal and worst case at a specific concentration of solids. A range of concentrations should be evaluated, since the concentration will be heaviest at the beginning of the pumping cycle and lighter toward the end of the cycle.
- No matter what pump is selected, positive displacement or centrifugal, a variable speed device should be available. These can range from simple, adjustable V-belts to a variable frequency drive (VFD).
Pumping cycles should be performed as frequently as is practical. It is better to withdraw sludge often at either a reduced flow rate or a reduced time duration than to have infrequent, long pumping cycles. Primary clarifier design can also be important because its operation often dictates that a pump with a specific head capacity curve be used.
Because of its organic makeup, sludge is always susceptible to breakdown by bacterial action. This action produces gases that can become trapped as voids in the sludge. This usually is not a concern in positive displacement pumps, because these designs will displace a measured volume of mass, whether gas, sludge or otherwise. Centrifugal pumps, on the other hand, can have great difficulty handling this type of media.
Sludges present significant challenges to pumping systems. Due to the unusual and complex fluid mechanics associated with sludges, system designers and users must treat each application as unique based on the specific sludge characteristics.
For more on sludge, visit pumpsandsystems.com/tags/sludge.
