In 2008, the City’s three water treatment facilities produced approximately 230 million gallons per day (MGD) on average, to sufficiently supply the 1.8 million consumers in the City of Baltimore and the metropolitan area. As a part of the overall “multiple barrier” treatment process, the City employs the Rapid Sand Filtration technique, a widely-used and accepted water treatment system dating back to the early part of the 20th century.

This water purification method principally involves running water through a filter bed of sand media which separates impurities and other suspended solids from water and allows clean water to percolate through the tiny voids of the fine sand particles. The amount of suspended solids in drinking water is tightly regulated by the Maryland Department of the Environment and closely monitored at the filtration plants. Water clarity is used as one indicator of the potential for water to harbor harmful microorganisms; the clearer the water, the better the water. Though the sand filtration process is technically complex in terms of engineering, chemical, and mechanical aspects, it is conceptually simple and involves running water through a bed of sand media to allow water through the media pores and to extract and separate very small particles on the sand bed surface.

In the current water treatment system, processes upstream of the filtration process in the treatment train help in the removal of a significant portion of larger particles and other impurities through chemical application and extended detention times in settling tanks. These upstream processes reduce filter loading and extend individual filter runtimes during the filtration process, assuring that the treatment plants can meet both quantity and quality demands. The rapid sand filtration system has proven to be effective, simple, cost-effective, reliable, and easy to maintain and operate and provides water of high clarity. However, with continued increased emphasis on water clarity, and in an effort to ensure that “raw” water of varying clarity can be successfully treated, the metropolitan water system is working with Baltimore area engineers and scientists to explore a newer and more technologically advanced method of filtration: membranes.

As part of a study to design a water filtration plant that can treat raw water from the Susquehanna River, membrane filtration is being evaluated as a part of the treatment train. Membranes are very thin hollow tubes that allow water to pass through while retaining extremely small particles ensuring that very high water clarity can be achieved.  They are more suitable for raw water which can change quality very quickly. The process of membrane filtration involves forcing water through the membrane wall to separate impurities. (Figure 1). Filtration takes place from the outer surface of the fiber to the hollow inner core. Filtered water passes through the wall of the fibers while particulates in the feed stream are retained on the outside of the fiber wall.

Membrane configurations come in the pressure mode filtration system in the submerged membrane filtration system. The submerged membrane systems, as illustrated in Figure 2, operate in open tank design. Feedwater typically flows by gravity into the membrane cell. A suction pump draws filtrate water through the membranes. Submerged systems are ideal for retrofitting existing basins and increasing capacity in a small footprint.

Pressurized membrane systems (Figure 3) operate in a closed environment. Feedwater is pressurized through the membranes. Both the submerged and pressurized systems have fully automated processes including backwash, cleaning, and membrane integrity testing.
The City along with its water partners in the surrounding jurisdictions will continue to explore promising treatment techniques to provide high-quality water to regional consumers.