Pumps in Dosing Systems

Demand-dosed and timed-dosed systems require different approaches to pump control and high-water alarm configurations
Pumps in  Dosing Systems
There are two ways to deliver effluent: through demand-dosing or by using a timer to control when the effluent is delivered to the next location in the system. Shown are systems from SJE-Rhombus.

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Last month we wrote about systems in which a pump delivers effluent or sewage to a location where the rest of the flow is by gravity. This month, we look at systems where a pump is used for some type of pressure distribution and explore approaches to effluent delivery.

When we started in the industry, the most common applications were pumping to distribution laterals in a set of trenches or to the rock bed of a mound system. Today, there are many more pump applications, and often there is more than one pump in the system – as in pumping to a pretreatment media filter, and then to the final soil treatment and dispersal area.

 

Two delivery methods

There are two ways of delivering the effluent: through a demand-dosing regimen or by using a timer to control when the effluent is delivered to the next location in the system. Again, when we started, almost all effluent was delivered through demand dosing. But with the advent of more technology, some systems require or perform best with a timed system.

Under demand dosing, the pump starts when a volume of effluent fills the pump tank to a predetermined level. A system can deliver doses as many times per day as the use in the home fills the tank. Each time the pump starts, a volume of wastewater is delivered to the pressure distribution network.

Usually, the amount is determined by taking the estimated daily sewage flow and dividing by four. So, for a 600 gpd system, the pump would be set to run for each 150 gallons of sewage. The actual amount of effluent and the number of times the pump runs depends on the actual use on a given day.

 

Float control

The most basic form of demand dosing is a float-operated, motor-rated switch into which the pump is plugged. The float is a single wide-angle or differential float control, called a piggyback control. These systems should be equipped with cycle counters or time meters to indicate the number of times the pump has operated and the length of time it has run.

This information allows a service provider or installer to know how much effluent the pump has delivered. This means there should be a control panel to facilitate obtaining the information on pump operation. In addition, in any pumping situation, a high-water alarm float should be wired into a separate circuit to alert the owners to a pump malfunction.

Because a single float has limited range of motion, the more common setup in systems today uses two separate floats for turning the pump on and off. These floats are attached to a float tree to make them easier to remove and to allow removal without taking out the pump at the same time. When two floats are used, the pump starts when the effluent rises to the “on” float elevation, pumps down to the “off” float elevation, and turns off. Here again, there should be a separate high-water alarm float.

In systems where flows are high enough and storage is limited, duplex pumps can be used. When the effluent rises to the pump “on” elevation, the first pump starts and delivers the dose volume. The next time, the second pump delivers the effluent.

If one pump fails, or if the use of the system is excessive, the effluent rises to the level of a lag switch, which starts the resting pump. To give warning that one pump has malfunctioned or that there are excessive flows, an alarm switch is positioned below the lag switch, or the two can be combined. There needs to be an alarm to alert the user to problems.

 

Timed dosing

For timed dosing, an adjustable timer controls the pump rest interval and run time for specific dose volumes and times. Peak flows (morning and evening) from a residence are stored and then dosed evenly throughout the day. Using this approach rather than demand dose allows the effluent to be spread out more equally during the day. For many media filters and aerobic treatment units, this improves treatment performance.

Timed dosing also uses floats to control operation. However, the float switch is a signal float instead of a motor-rated switch. When the effluent rises to the preset level, the float sends an electric signal to the control panel. This enables the timer.

After the prescribed rest interval, the pump delivers the specified volume of effluent by operating for the time it takes the pump to deliver the amount of the dose. Often, other devices, such as pressure transducers, are used instead of floats.

Many configurations are possible with timed dosing. One method involves the use of a separate redundant “off” float. When this float is in the “off” position, it protects the pump from running if not enough effluent has been generated. The timer enable and the redundant “off” function can be controlled by the same float.

Another option is a peak enable float, installed between the enable and alarm floats. When activated during high-flow events, this float allows the pump to run, resulting in more dosing events during the day. These functions can also be controlled by a single float, but it is important to set them so that the flow does not exceed the capacity of the next part of the system.

Timer-override floats should not be used. This configuration changes the function of the timed system to a demand-dosed system until the effluent in the tank returns to normal operating levels. If the system functions continuously because flows are routinely exceeded, this will hydraulically overload the system. Again, there should be an alarm wired on a separate circuit – one the user cannot easily ignore.

 

Equalizing flow

One management concept that is being used more often is flow equalization. Simply put, this reduces stress on the system during peak flows by storing effluent and delivering it at lower-use times during the day. Usually, the flow is equalized over 24 hours, or longer if necessary.

In any case, if the effluent is to be stored, there must be enough tank capacity, and a timer system is required. For single-family homes, the pump tank capacity should be at least 1,000 gallons, or twice the estimated daily sewage flow. For other systems, the design flow and the required storage should be determined from real flow data. Usually, the data needs to be collected for 45 to 90 days. Next we will discuss pump installation.



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