Membrane Pilot Testing at the City of Tempe Water Treatment Plant

City of Tempe, 
Arizona

Membrane Pilot Testing at the City of Tempe Water Treatment Plant

The Memcor 3M10C microfiltration pilot plant
The Memcor 3M10C microfiltration pilot plant is equipped with 15 m2 of polypropylene membranes with a nominal pore size of 0.2 µm and is operated in a dead end mode.

Carollo Engineers, in partnership with the City of Tempe, is conducting a study to evaluate low pressure membrane filtration for treatment of Arizona’s Salt River Project canal water. In addition to providing an absolute barrier against pathogens such as Cryptosporidium and Giardia, microfiltration and ultrafiltration membranes are being used in combination with powdered activated carbon (PAC) to remove taste and odors. Two pilot units are being tested side by side, while a third pilot unit is being used to evaluate ceramic microfiltration membranes as an alternative for recovery of filter waste washwater. Hydraulic performance data along with physical and microbial water quality data developed from this study will be used to develop design criteria for full-scale implementation of membrane filtration.

For this study Carollo is developing the experimental plans, setting up the pilot units, and performing PAC testing. Carollo is also responsible for acquiring, interpreting and reporting all data. This task involves the use of an online data acquisition and master programmable logic controller system that allows remote monitoring and operations adjustment. Carollo is also in charge of pilot plant operations, including performing membrane chemical cleaning when required. The City of Tempe is assisting Carollo in setting up the units and providing daily maintenance and monitoring of the pilot plants. The City is also conducting water quality analyses.

Pilot Plant Process Description

Raw water is pumped from Tempe’s South Water Treatment Plant bar screen area. Before entering the pilot area, PAC is added to the raw water feed line, ensuring that both units receive the same dose.

Microfiltration Pilot Plant Process Description

Raw water for the Memcor microfiltration unit is fed to a 200-gallon storage tank. Raw water is then fed to the outside of the hollow membrane fibers through the upper and lower headers of the membrane modules. The water is forced across the membrane producing a clean permeate which travels through the lumen of the fibers. Permeate flow is adjusted with a diaphragm valve providing back pressure.

This unit utilizes an air backpulse backwash coupled with raw water scour. During backwash, high pressure air is vented through the membrane causing a forceful outward release that loosens the cake of particles accumulated on the membrane’s outer surface. Raw water is rushed along the outside of the fibers, scouring away loosened particles. The membrane flux and the backwash frequency depend upon raw water quality and PAC dosage. These operating parameters are being optimized during the study.

Ultrafiltration Pilot Plant Process Description

The Aquasource ultrafiltration pilot plant The Aquasource ultrafiltration membrane pilot plant treats the same water source as the Memcor unit. Downstream of the feed pump, a 200 μm prefilter retains the larger debris and prevents clogging of the hollow fiber lumen. Prefiltered raw water enters the concentrate loop and a recirculation pump produces a velocity of 3 ft/s inside the membrane in an attempt to maintain the rejected material and PAC in suspension. A fraction of the permeate is collected and stored for backwashing the system.

The Aquasource ultrafiltration pilot plant utilizes two 7.2 m2 modules of cellulosic membranes with a nominal pore size of 0.01 μm and is operated in a crossflow mode.

Backwash is performed periodically using permeate at a pressure of 36 psi and for a 30-second duration. Because the membranes are constructed of cellulose acetate, they are susceptible to biological fouling and attack. To inhibit microbial growth, chlorine is added to the backwash water before entering the modules to achieve a free chlorine residual of approximately 1.0 mg/L.

Ceramic Pilot Plant Process Description

A ceramic membrane filtration system is being used to treat filter waste washwater to drinking water quality standards. Ceramic membrane technology was chosen due to its ability to handle high solids concentrations and the compatibility of these membranes with all chemicals used in water treatment.

Water Quality Monitoring

Raw and finished water is being monitored for pH, turbidity, particle counts, ultraviolet absorption at 254 nanometers, and total organic carbon. Both raw and finished waters are also being monitored for the presence or absence of total coliforms, while total bacteria are enumerated by heterotrophic plate counts.

Conclusion

The data developed by this study will be utilized by the City of Tempe to determine the effectiveness of low pressure membrane filtration to meet future water quality and regulatory standards, as well as aiding in the cost-effective implementation of this technology at their existing facilities.