The polymer is activated as it passes through the high turbulence generated by the orifice. The system operates like a low-pressure homogenizer as it passes through the annular nozzle orifice. The pressure required to operate the chamber is less than 10 pounds per square inch (psi). The orifice opening automatically compensates for solution flow changes. This ensures that the polymer activation energy level remains adequate for the flow range of the polymer feed system.
While the activation technology is vital, two other key components of the polymer feed system—pump technology and control options—were discussed with West Haven. These components govern long-term maintenance, operator interaction and overall satisfaction with the designed system.
Pumps used for metering activated polymer are of a positive displacement design and are either reciprocating or rotary type. The most common pumps used are single diaphragm, peristaltic or progressing cavity. Understanding the pros and cons of each type will help determine the right pump selection and the long-term maintenance required.
For the West Haven application, the progressing cavity pump was selected. A progressing cavity pump uses a single helix rotor through a double helix elastomer core stator. As the rotor rotates inside the stator, it progressively opens and closes displacement cavities from suction to discharge. The pumping motion provides a gentle nonshearing, near pulseless flow. This is beneficial when metering neat or solution polymers.
The progressing cavity pump design can also handle a broad range of viscosities, which might require special options when using other technologies. Since a progressing cavity pump can be easily calibrated for a known fixed displacement, it is easily integrated with flow rate controls and uses speed control via a variable frequency drive (VFD).
The West Haven operators wanted to interact with the system and make planned adjustments with a high degree of knowledge. Historically, they had been operating manually without supervisory control and a data acquisition (SCADA) system. However, having the ability to operate remotely made sense for further advancements.
The system was customized with a programmable logic controller (PLC)- based control panel incorporating a human machine interface (HMI) touchscreen display. Operators were now able to easily calibrate, adjust and confirm both polymer and dilution water allowing them to fine-tune operations.
Understanding the long-term needs of the plant was vital to the success of this project. Today, all three systems continue to meet the demands of the city and the expectation of its wastewater operators.