By Rick Ross & Marcus Allhands
LOUISVILLE, KY, March 31, 2005 -- Heavy solid particles in a city's water supply can cause problems such as disruption in yard hydrants, heat exchangers, pump seals and other systems. But effectively removing solids can be costly and require a lot of man-hours to manage. In this case, the city's waste treatment plant needed to find a low-maintenance product that could filter the particles for effluent reuse.
In February 2000, the city of Carmel, Ind., was experiencing heavy solids loading that was affecting auxiliary equipment such as yard hydrants, heat exchangers, spray bar nozzles and pump seals. Persistent basket strainer and belt press spray-bar cleaning (labor cost) and seal replacement (material and labor costs) were the main targets when looking at a new "strainer" system.
Carmel's onsite water use was 300 gpm under peak demand with a minimum system pressure of 50-60 psi. Its water sources were potable municipal water, a non-potable well located onsite and secondary effluent. Very high iron concentrations in the non-potable well had caused problems in the past. The reuse of secondary effluent was the best use of resources and proved to be the most cost-effective. The secondary effluent typically contained 10-15 ppm of total suspended solids with values above 30 ppm during clarifier upsets.
Working through Cleland Environmental Engineering, the city's water treatment facility approached Amiad Filtration Systems, of Oxnard, Calif., and R.A. Ross & Associates, of Louisville, Ky., with a filtering challenge. A project involving the installation of new centrifuges had already been approved, which provided an opportunity for Ed Wolfe, Plant Superintendent, and his staff to re-engineer their water supply system within the plant.
Efficient, solution for reuse
After lab analysis of the solids in the secondary effluent, Amiad engineers recommended a filtration degree of 80 microns. The decision was made to install an Amiad 4" SAF-4500 Automatic Self-Cleaning Filter with an 80-micron screen and PLC controller. An Amiad 4" Super Manual Filter with an 80-micron screen was placed in a bypass line as a backup system.
There were other types of automatic strainers available to the user, which were generally classified as "back-flushing" filters. The cleaning cycle is initiated once a threshold differential pressure is reached across the screen element. However, once the combined area of clean openings in the screen equals the inlet and outlet cross-sectional areas, velocities through the inlet, outlet and through the screen element are all equal. This means the pressure differential across the screen is nearly zero and there is no more energy to clean the rest of the screen.
Typical filters have screen "holes" adding up to 5-10 times more square inches than the cross-sectional area of their inlets and outlets. In other words, for a typical "back-flushing" filter, the cleaning process often removes only a tenth to a fifth of the debris during a cleaning cycle, therefore never using more than a small portion of the screen surface for filtering. Amiad utilizes focused back-flushing with a series of small nozzles cleaning less than 2 square inches of the screen element at a time. These nozzles are attached to a central hollow tube called a "suction scanner" that hydraulically connects to atmospheric pressure (zero gauge pressure).
Advanced screening
Taking just 12-30 seconds per cleaning cycle, the suction scanner and its series of nozzles slowly rotates in a spiral pattern inside the 316-L stainless steel filter screen element moving the small cleaning area across the entire screen surface. The motorized scanner is rotated at a constant 24 RPM, thus ensuring that every square inch of the screen surface is completely cleaned of debris.
The Amiad product handled the 50 fps velocities created by this focused backflushing by incorporating the best screen designs available. Amiad's four-layer 316-L stainless steel screens protect the actual filtering layer from damage by rocks, pieces of metal or other hard foreign objects, yet are designed so that each small opening in the filtering layer is available for the filtration process.
After the waste treatment plant's personnel installed the system in May 2002, the crew was able to go from having to clean the filter press spray bar nozzles every three weeks, to performing the task just twice a year. The city is now successfully reusing effluent throughout the plant, which equates to savings for the treatment facility, the city and the taxpayers.
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