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By Herb Werner
|Figure 1. In addition to reciprocating, the piston also simultaneously and continuously rotates in one direction.|
Releases of wastewater effluent with high nitrogen concentrations into bays and watersheds are of great environmental concern, as they can have a devastating effect on water ecosystems. As is well known, nitrogen is an end-product of the bacterial degradation of ammonia, which is present in high levels in untreated wastewater.
In its various forms, nitrogen can deplete dissolved oxygen in receiving waters, stimulate aquatic plant growth and exhibit toxicity toward marine life, as well as present a public health hazard. Wastewater effluents containing nitrogen can cause eutrophication, the excessive growth of plant and algae blooms in lakes, streams and rivers. This can deprive oxygen and sunlight from marine life. Nitrate is a primary contaminant in drinking water and a cause of the human health condition called Methemoglobinemia, in which the oxygen-carrying property of hemoglobin is altered.
No one doubts that nitrogen levels created during the wastewater treatment process must be significantly reduced before being discharged into the environment. The challenge, however, is to apply a method that not only reduces the nitrogen present in the effluent, but if possible, does so at an economical cost.
Through a process known as "denitrification," water treatment facilities convert the excess nitrate into nitrogen gas, which is then vented into the atmosphere. Any of several processes are commonly used to accomplish this, most of which use pre-treatment basins, aeration tanks and blowers. However, for nearly 200 wastewater treatment plants in the U.S., the answer is a process that involves the addition of methanol into the effluent, accelerating the activity of anaerobic bacteria that break down harmful nitrate.
Methanol is a volatile, light, colorless, flammable and biodegradable liquid readily available from suppliers. It’s also estimated that methanol denitrification costs are about one-eighth of the cost associated with other methods used.
In most cases, methanol is added into the effluent stream using a metering pump. The type of pump used can vary but needs to meet certain application requirements. These include chemical compatibility of wetted parts and electrical ratings for the area where the pump is being installed. Methanol is an alcohol that is considered an organic polar solvent. Therefore, all wetted parts need have a degree of chemical resistance. Methanol is also both volatile and flammable, and therefore the metering pump used will need to meet approvals for use in hazardous locations.
Diaphragm pumps are traditionally used to meter methanol for wastewater treatment applications. However, an alternative metering technology that has only one moving part in the fluid path eliminates the internal check valves present in diaphragm as well as other reciprocation pump designs.
|Figure 2. As the angel of the pump head increases above zero, the piston reciprocates and fluid moves through the pump.|
The “CeramPump” relies on only one moving part — a rotating and reciprocation ceramic piston — to accomplish both the pumping and valving functions. Given its precision, it works particularly well in low-flow volume wastewater treatment operations.
Sapphire-hard, dimensionally stable ceramic internal components allow for the precision manufacturing of internal components with extremely tight clearances. This, as well as the elimination of multiple check valves, results in a metering pump that can self-prime down to the micro liter range and never lose prime due to air bound or check valves that don’t seal well enough to prevent backflow at very low flow rates.
For metering methanol for denitrification, the pump is driven by a 1/3 HP hazardous-duty motor rated for Class I, Group C,D; Class II, Group E,F,G. The photo below is an installation in a non-heated outdoor enclosure at a community college wastewater treatment plant.
The low-volume piston pump uses one moving part to accomplish both the pumping and valving functions, eliminating check valves present in reciprocating syringe, diaphragm, bellows and piston pump designs. It uses a rotating and reciprocating ceramic piston, moving within a precision-mated ceramic liner to accurately pump fluid in one direction without allowing backflow. The reciprocation action of the piston acts very similar to a standard piston pump. As the piston moves back, it draws fluid into the pump chamber. As it moves forward, fluid is pushed out of the pump.
However, what is unique is that in addition to reciprocating, the piston also simultaneously and continuously rotates in one direction. The piston has a flat cut into the end closest to the inlet and outlet port (See figure 1). As the piston rotates, the flat is alternately aligned with the inlet and outlet port, essentially functioning as a valve. At no time are the inlet and outlet ports interconnected, and therefore the need for check valves is eliminated. One complete synchronous rotation and reciprocation is required for each suction and discharge cycle as shown in figure 1.
The piston displacement (or volume pumped per stroke) is variable and controlled by the angle of the pump head to the drive. When the pump angle is zero, the pump head is in straight alignment with the drive, and the flow is zero. In this situation, there is no reciprocation and the piston is only rotating. As the angle of the pump head increases above zero in either direction with respect to the drive, the piston reciprocates, and fluid is moved through the pump (See figure 2). The greater the pump head angle, the greater the displacement (piston stroke) per cycle.
Adjustment is infinite between zero and 100% and a flow-rate indicator provides for accurate and simple linear calibration. The pump is designed so that at any angle and flow rate, the piston always bottoms for maximum bubble clearance. This is especially important at very small dispenses and flow rates, as the presence of even a minute bubble will significantly affect accuracy.
Typically, four check valves are present in diaphragm, bellows, and traditional piston pumps. Even during normal operation, these will wear over time and not seal properly, allowing backflow. When this occurs, accuracy will begin to drift, and minimally the pumps will need periodic recalibration. Eventually, the check valves need to be serviced.
|Figure 3. Precision is the range- or degree-of-variation from dispense to dispense.|
With sapphire-hard ceramics for both the piston and mated liner, the FMI pump components are dimensionally stable. They will not change shape or dimension over time. They are also wear-resistant and chemically inert. As a result, the volume of the pumping chamber remains stable for millions of dispenses, eliminating the need for recalibration.
Consistency in dispensing can be measured both by the accuracy and precision of dispenses. Accuracy is a comparison of the average value of the dispense volume compared to the desired or target value. Precision is the range- or degree-of-variation from dispense to dispense. The FMI CeramPump maintains a precision of 0.5% or better (See figure 3).
The pump is available in several configurations, including stand-alone production-dispensing systems and miniature OEM models for medical, analytical and process instrumentation.
Besides its use in methanol metering for effluent denitrification, other chemical metering applications of the valveless pump include the following:
Herb Werner has been the marketing manager for Fluid Metering, Inc. for 19 years and has more than 35 years fluid-control experience in chemical-process, water-treatment, medical & analytical instrumentation, pharmaceutical, and semiconductor industries. He has a B.S in Environmental Biology and is an active member of ISA, AWWA, & WEF societies.
Located on Long Island about 30 miles from New York, Fluid Metering, Inc. patented the first rotating and reciprocating valveless pump and has been providing fluid-control solutions for medical, analytical, chemical process, and industrial applications for over 50 years. In its markets, FMI pumps can be found in laboratories and on production floors, in OEM equipment and instrumentation and in process-control and field installations.