Pumps & Systems, May 2008
In trench-type wet wells, the pump intakes are placed near the bottom of a deep, narrow trench coaxial with the inlet pipe but considerably below it (see Figure 1). The closely confining walls of the trench prevent cross currents, which creates a good hydraulic environment for the pump intakes. The trench works very well for both water and wastewater. The ramp shown in Figure 1 is omitted in clear water applications, and the row of pumps starts immediately downstream from the end of the inlet pipe. Pumping stations with capacities in the range of as little as 2.5-Mgal/d to as much as 220-Mgal/d for both water and wastewater are currently in service and performing well. Some of these installations are more than 40 years old with no indication of poor or unacceptable pump performance.
The feature that makes the trench-type wet well so attractive to wastewater operators who have used one is its ability to self-clean by using only the main pumps. Other types of wet wells must be cleaned manually or by vacuum truck. During the cleaning cycle in a trench-type wet well, all scum, sludge, grit, gravel, rags and other trash are swept into the last pump by water accelerated to a high velocity by flowing down the curved ramp. Cleaning can be completed in a few (typically between one to four) minutes with no manual labor beyond the touch of a button. The process can be automated to occur at any desired time intervals, such as once every three days or once per week. Because the trench can easily be kept clean and odor-free, this type of wet well can be placed anywhere (in residential, commercial or business districts) without adverse odor impact.
Trench-type wet wells are gaining popularity in the Midwest, South and East Coast of the United States, and they are already so popular on the West Coast that some large utilities have adopted them as standard. Recommended dimensions in terms of D (outside diameter of the pump intake suction bell) are given in American National Standard for Pump Intake Design (ANSI/HI 9.8-2007) , reproduced in the figure below. They were developed from years of both model and prototype testing. However, some recommendations are still occasionally ignored, and the result is usually a flawed product that will not give satisfactory performance.
The purpose of this paper is to assist designers by giving the reasons for the recommendations in ANSI/HI 9.8.
This subject is covered thoroughly in both second and third editions of Pumping Station Design, Chapter 12 [2, 3]. In addition, some considerations specific to trench-type wet wells are described below.
Types of Pumps
Any type of water or wastewater pump (dry pit centrifugal, vertical turbine solids handling or both dry pit and wet pit submersible non-clog pumps) can be used in trench-type wet wells. Dry pit pumps can be used by installing a flare for the suction bell followed immediately by an elbow (preferably long radius) and a horizontal pipe to the dry well. Submersible pull-up pumps can be used by casting a recess for the discharge elbow in the side of the trench; after the elbow and discharge pipe are placed, the recess can be filled with lean concrete. Column pumps are ideal in trenches.
Number of Pumps
An ideal number is three-two to carry the maximum load and one for a standby. If they are variable speed units, the lowest flow rate that can be pumped is usually about 25 percent (half the capacity of a single pump) of the station capacity. Check with the manufacturer; some machines can pump at lesser capacities. However, all centrifugal pumps have a lower limit, and below the tolerable flow rate for a single pump, the pump must be turned off and on as with constant speed pumps. That may be a problem for downstream processes as sedimentation basins are upset by sudden changes in incoming flow. Other processes, such as chlorination and dechlorination, may not meet requirements at all with sudden changes in flow rates. Confer with the treatment plant designer.
If the disparity between peak wet weather flow and average dry weather flow is large, energy use can be decreased and flow matching improved by installing more pumps and in two different sizes so as to place most of the expected average dry weather flow rates in the pumps' Preferred Operating Region. Using more pumps, however, increases capital and maintenance costs, and decreasing the size of the last pump adversely affects cleaning. Decisions here depend on the designer's artfulness.
Effect of Number of Pumps on Cleaning