Unity Filtration Details
Filter Area
The filtration area is the most important performance factor. In filtration, generally slower is better. This allows the particles to adhere to the filter media. At high flow rates, the velocity actually can wash the filtered material through the media. The Unity filters offer a total of 63 square feet of filter areas.
(This is 61/2 or 10 times more than the screen filters)
Filter Depth
Volume, A critical factor in filter operation is the dirt holding capability of the media. If the dirt is only held on the surface, any upset can cause it to release. Upsets include higher flow rates, or stopping flow rates, or soft particles which deform and squeeze through the media, or particle loads which build up so rapidly that pressure loss becomes continuously too high. All of these things can happen in these buildings. High flow rates happen if a second pump comes on. Low flow happens anytime the pumps shut off. The primary source of dirty water problems in NYC water is soft, adherent (sticky) particles, and high dirt loads can occur anytime there is an upset in the mains. This was demonstrated on 9/11, when the demand for water to fight the fires was so great that the water mains unloaded. The Unity filters which are installed in Southern Manhattan were strained, but handled that incredible dirt load without failure. It took several scheduled backwashes before they were back to normal, but they did not pass the dirt and they did not shut down their buildings.
The multilayer principle is entirely different. The largest filter media is on the top. Beneath it is a finer, denser layer, with another extremely fine denser layer below that. (All supported on very dense under bed). As water and dirt flow into the filter, the dirt particles pass down through the media until they reach a point where they can't go any further. But if conditions change, for example higher flow rates, all that happens is that some particles are driven further in to the bed. But, they don't get driven out the bottom. The lowest layer is only impacted by particles which have gotten through the larger media above. The lowest layer doesn't clog but it doesn't unload either.
Filtered Water Quality
Experience has shown that filtration below 10 micron invariably produces excellent water quality in NYC applications. Because there is little control over the quality of the water in the reservoirs, for a number of reasons, it is important that a filtration system be able to deal with water quality problems summer and winter (cold and not so cold) and water which can have a significant amount of dissolved iron and manganese. The Multi-Layer filter takes advantage of well known principles of oxidation and coagulation to give even better water quality than straight particle filtration can give. These principles and the capability of not unloading are only available through Multi-Layer filtration.
Adherent Dirt
Sand has always been a major problem for wells and of course, is critical for the operation of irrigation nozzles. Screens are the best way to deal with sand and other hard particles. However, if the material being filtered is sticky or fibrous, then it causes problems of another kind. The filtered material actually plates onto the screen, obviously causing the holes to clog or get smaller. Backwashing does not clean them, they must be chemically, (often acid) cleaned. In a domestic water supply, this is a critical problem. And since it is dependent upon what is in the water, it probably becomes a maintenance problem for the onsite personnel.
Cost
The Unity system has been competitively priced since its inception. It is a large system, but that is unavoidable, it has a lot of work to do. It is supplied with multiple tanks for redundancy, and also to provide an uninterrupted supply of filtered water. It is critical to the successful filtration of the water supply that no unfiltered water pass into the building. If unfiltered water gets into the building, it may not show up immediately, but at some point, during a critical time, such as a holiday, or an emergency, suddenly the dirt which is within the plumbing system will break loose and the building will have a dirty water episode. A properly designed and sized system will prevent that kind of problem.