At first glance, the treatment of municipal wastewater may seem straightforward. Fouled water flows through a wastewater treatment facility, where harmful impurities or particulates are removed. Once the water has reached a predetermined level of cleanliness, it is either released to the environment or sent for reuse.
In reality, treating wastewater is a complicated process involving critical steps that must be performed to exact criteria by highly technical equipment. The system is only effective when the proper chemicals are used to treat the water, and these chemicals can cover a wide range of pH levels, viscosities, material compatibilities and handling characteristics.
A few of the more commonly used chemicals in wastewater treatment applications and their characteristics are outlined below.
- Sodium hypochlorite: Widely known as bleach, this liquid is most commonly used for disinfection at the treatment plant. This chemical is one of the most difficult to handle because it corrodes most metals it comes in contact with, and it cannot be mixed or stored with ammonia or other acids, organics and reducing agents used at the plant. It is also difficult because of its tendency to off-gas, which can cause metering pumps to become gas-bound. To prevent this, special vent valves are available for diaphragm pumps. Peristaltic pumps, which do not allow this phenomenon to occur, also can be used.
Sulfuric acid: Used for pH adjustment, this chemical is provided either as a concentrate or dilute. Concentrated solutions can be handled in many situations with cast iron, steel materials or Alloy 20, while dilute solutions require plastics such as PVC or Kynar.
- Sodium hydroxide: Also used for pH adjustment, this chemical is often provided in solution strengths from 25 to 50 percent. Metering pump elastomers, such as Viton (a material often used for O-ring seals within a metering pump), are not compatible with sodium hydroxide, so they require special care. Especially in higher solution strengths, this chemical tends to gel in the pump if the pump is idle for a period of time. Flush valves or special pump heads can assist with this issue.
- Sodium bisulfite: This is one of the most commonly used dechlorinating agents at treatment plants. A mixer is usually required to keep this chemical in solution in the metering pump supply tank. Suitable materials for this chemical include plastics, such as PVC and Kynar, and metals, such as 316 stainless steel.
- Emulsion polymer: Fed as a coagulant to assist the dewatering equipment within a wastewater treatment facility, this chemical is shear-sensitive once hydrated and is often extremely viscous. Diaphragm pumps with high-viscosity head designs or peristaltic pumps are often required.
Faced with a myriad of chemicals that have unique handling and implementation characteristics, plant operators must be certain that they have selected the proper metering pump technologies for their dosing applications.
Because there is no “one-size-fits-all” solution, creating the most efficient, effective and safe chemical-handling operation in wastewater treatment likely requires the use of different types of pump technologies, all of which bring their own set of benefits to the operation.
Outlined below are four pump technologies that, if implemented properly, can play a key role in an optimized wastewater treatment operation.
1. Mechanical Diaphragm Metering Pumps
Mechanically actuated diaphragm metering pumps are easy to implement and operate, which makes them an attractive alternative for many applications. They typically have a lower initial cost than other motor-driven metering pump designs, especially at higher flow rates, but they can have higher operating costs.
Because the diaphragm is attached to the piston for a positive return usually aided by a spring, these pumps offer excellent suction-lift capabilities and can handle liquids that off-gas, such as sodium hypochlorite, and more viscous chemicals.
These pumps are somewhat limited in discharge pressure capabilities, with many being around 100 to 150 pounds per square inch (psi) (7 to 10 bar) max. Repeatable accuracy is typically about +/-2 percent.
They are generally not offered with an on-board relief valve, so an external relief valve is usually required to prevent damage to the pump in an over-pressurized situation.
2. Hydraulic Diaphragm Metering Pumps
As a low-maintenance pump designed for 20 years of service, hydraulically actuated 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 bath of hydraulic fluid and because the diaphragm is hydraulically balanced.