Without a doubt, each pump technology can provide a number of options and advantages to wastewater treatment plant technicians, the challenge is how to choose which metering pumps are best for each wastewater treatment chemical dosing application.
At first glance, wastewater treatment may seem simple. Dirty water passes through a wastewater treatment facility, where impurities or particulate pollutants are removed. Once the water has reached a predetermined level of purification, it is released into the environment or sent for reuse.
Chemicals and dosing pumps for waste water treatment
In reality, wastewater treatment is a complicated process involving critical steps that must be performed to exacting standards by highly technical teams. The system is only effective when the right chemicals are used to treat the water, and these chemicals can cover a wide range of pH levels, viscosities, material compatibilities and handling characteristics.
Some of the most commonly used chemicals in wastewater treatment applications, their characteristics and how they influence metering pumps are described below.
Sodium hypochlorite
Widely known as bleach, this liquid is most commonly used for disinfection at the outlet of the sewage treatment plant. This chemical is one of the most difficult to handle because it corrodes the metals it comes into contact with and cannot be mixed or stored with ammonia or other acids, organics and reducing agents used in treatment plants.
It is also difficult because of their tendency to off-gas, which can cause dosing pumps to overheat. To prevent this, special vent valves are available for diaphragm pumps. Peristaltic pumps, which do not allow this phenomenon to occur, can also be used.
Sulphuric acid
Used for pH adjustment, this chemical is supplied in concentrated or diluted form. The use of metals should be avoided for parts in contact with the acid, as they will quickly corrode. Plastic materials such as PVC or PVDF should be used.
Sodium hydroxide or caustic soda:
This chemical, which is also used for pH adjustment, is typically supplied in concentrations of 25 to 50%. Metering pump elastomers such as Viton (a material often used for metering pump O-rings) are not compatible with sodium hydroxide, requiring other materials such as EPDM or PVDF.
Especially in highly concentrated solutions, this chemical tends to crystallise in the pump if the pump remains idle for a period of time. Special valves or pump heads can help to solve this problem. Alternatively, frequent maintenance of these elements would be necessary.
Sodium bisulphite
It is one of the most widely used dechlorinating agents in water treatment. A mixer is normally needed to keep this chemical in solution in the supply tank of the dosing pump. Suitable materials for this chemical are plastics, such as PVC and PVDF, and metals, such as stainless steel AISI 316.
Emulsion polymer
Fed as a flocculant to assist sludge dewatering equipment within a wastewater treatment facility, this chemical is extremely viscous. Diaphragm pumps with high viscosity head designs, piston pumps or peristaltic pumps are often required.
The role of plant technicians
When faced with multiple chemicals with unique handling and application characteristics, plant operators need to be sure they have selected the right metering pump technologies for their metering applications.
Since there is no one-size-fits-all solution, establishing the most efficient, effective and safe chemical handling operation in wastewater treatment is likely to require the use of different types of pumping technologies, all of which bring their own set of benefits to the operation.
Main dosing pumps for wastewater treatment
There are four main technologies. Plant operators can optimise operations by selecting the best combination of these options.
Four pumping technologies are described below that, if applied correctly, can play a key role in an optimised wastewater treatment operation.
1. Mechanical diaphragm metering pumps
Mechanically driven diaphragm metering pumps are easy to implement and operate, making them an attractive alternative for many applications. They tend to have a lower initial cost than other motor-driven metering pump designs, especially at higher flow rates, but can have higher operating costs.
Because the diaphragm is attached to the piston for positive return, usually assisted by a spring, these pumps offer excellent suction lift capability and can handle gaseous liquids, such as sodium hypochlorite, and more viscous chemicals.
The discharge pressure capacity of these pumps is somewhat limited, as many have a maximum pressure of 7 to 10 bar. Accuracy is typically +/- 2%.
They are generally not offered with a built-in safety relief valve, so an external safety relief valve is often necessary to prevent damage to the pump in an overpressure situation.
2. Hydraulic diaphragm metering pumps
As a low-maintenance pump designed for 20 years of service, hydraulically driven diaphragm metering pumps are ideal for operation in the harshest chemical handling conditions. Their longevity and minimal maintenance requirements are possible because most of their moving parts are submerged in a hydraulic fluid bath and because the diaphragm is hydraulically balanced.
Hydraulically balanced means that the hydraulic fluid is on the oil side of the diaphragm, while the chemical being pumped is on the other side (wet or process side) of the diaphragm. The piston, internal to the pump, pushes the oil against the diaphragm, but never comes into direct contact with it. These pumps are capable of pumping with extremely high back pressures, so long delivery lines are not a problem.
The hydraulic diaphragm pumps are equipped with an adjustable internal relief valve that prevents an overpressure situation, which could damage the pump. They also offer a dosing accuracy of +/- 1%.
3. Electromagnetic metering pumps
Solenoid or solenoid driven metering pumps are an economical choice in wastewater treatment operations for low flow/low pressure chemical dosing applications. They are typically available up to a maximum of about 100 litres per hour. At these capacities, maximum pressures are less than 1 bar.
For low flow rates (3-4 l/h) they can reach up to 20 bar. They offer a repeatable dosing accuracy of +/- 3%. They normally require the installation of an external safety relief valve to prevent damage to the pump due to overpressure.
4. Peristaltic pumps
This simply designed pumping technology is ideal for handling viscous and abrasive chemicals. As the design includes a rotor with shoes or rollers that squeeze the hose or tube and push the liquid to the outlet. Peristaltic pumps can run dry, a feature that other technologies cannot offer because their components can be damaged in dry operating conditions.
Peristaltic pumps also do not have valves that can become clogged. This is an important consideration when the pump has to be stopped during the operation of a product. The absence of valves in a peristaltic pump eliminates any clogging problems, especially when handling viscous liquids or those that solidify or gel on standing. This also makes the peristaltic pump ideal for handling abrasive or corrosive chemicals.
Support from manufacturers
While these pumping technologies can provide a list of advantages to wastewater treatment plant operators, the challenge is knowing which pump is best for which chemical dosing application. This is where the pump manufacturer can lend a hand.
Many pump manufacturers also design and manufacture many of the components that go into a complete chemical feed system, e.g. safety and back pressure valves, calibration columns, tanks, mixers, injection fittings and control panels.
Some offer the complete chemical feed system as an option and can build a system to meet the specific needs of the treatment plant. While the task of properly equipping a treatment facility can be daunting, the manufacturers of the various components that will be used to optimise the system, or the manufacturer of a complete system, can help determine the best way to equip the plant.
Optimisation of the operation of metering pumps for wastewater treatment
Ensuring that communities have the cleanest possible water puts a great deal of pressure on water and wastewater treatment facilities to perform their work as effectively, efficiently and safely as possible. The various chemicals that may be required in the different treatment processes only increase the risk to the plant operator.
An operation will only achieve the maximum of adequate performance if the best pumping technologies are chosen for each of the many critical chemical dosing processes.
