The membrane system used the existing piping and new filtrate pumps installed in the pipe gallery.
Click here to enlarge imageIn most cases, existing media filter capacity can be greatly increased without building a new facility. Because most submerged membrane systems produce filtration rates greater than 15 gpm/ft2, compared to 5 gpm/ft2 with media filters, the submerged membrane system will deliver up to a three-fold improvement in capacity when retrofitted into the entire existing filter media box. Thus, the capacity expansion is usually limited only by the amount of water that can be brought to the facility and the capacity of the existing infrastructure.
Not all existing media filters are good candidates for retrofit, however, as certain plant configurations may result in complex pipe work and ancillary system designs that may not provide the level of access for maintenance or inspection required. Retrofitting is a good option when the new membrane filters will fit within the existing hydraulic profile with minimum modifications to the existing concrete. It is also important to ensure that the adjacent media filters can continue to operate while the membranes are being installed. With proper planning and project execution, the system retrofit can be completed without any interruption in plant operation.
Case Study
The City of Kennewick, WA, needed to double the capacity at its 7.5 mgd filtration plant that had been built in 1979 to treat the Columbia River source. The plant was designed to augment two collector wells that provided the main sources of drinking water. The city wanted to be able to operate the plant year-round and rely less on the wells. The treated water also had to meet both current and anticipated Safe Drinking Water Act (SDWA) regulations.
The city considered three options: building a mirror-image of its current plant, increasing capacity of the existing plant with a high-rate sedimentation/filtration process, or retrofitting with submerged membranes to replace the existing granular media filters.
After evaluating the three choices, the city decided to retrofit with submerged membranes, as this presented the most cost-effective option and the shortest construction time. The membrane system could be installed in the existing filter basins, could be easily automated, and offered the greatest flexibility to meet future regulations.
The city conducted a side-by-side pilot test on the Memcor CS system and another vendor’s system, and concluded that the CS system would offer the lowest 20-year lifecycle cost. The CS system could be used with the existing piping and new filtrate pumps installed in the pipe gallery, and would double the capacity to 15 mgd without having to expand the footprint.
Since startup, the plant has met its capacity and performance needs. Should it need to be expanded in the future, new modules may be added to the membrane basins in a short time and without a major expense. —m
About the Author:
Stratton Tragellis is Global Director, Product Management, Purified Water Municipal Segment at Siemens Water Technologies in Shrewsbury, MA. He can be reached at [email protected].