Backwashing is less costly than changing cartridges!
Backwashing a 30” UNITY Multi-Layer filter @ 75 gpm for 12 minutes uses 900 gallons. Divide this by 7.5 equals 120 cubic feet. The New York City water department charges a combined water and sewer fee of 5.99 per 100 cubic feet of water. Multiply $5.99 by 1.2 and this will give you a cost of $7.18 per tank. Based upon on the filter backwashing once a week for one year the cost per tank is $373.00. A 30” three tank system would cost $1,120.00 per year.
A UNITY 36” Multi-Layer filter @ 105 gpm for 12 minutes uses 1260 gallons. Divide this by 7.5 equals 168 cubic feet. Multiplying $5.99 by 1.68 will give you a cost of $12.06 per tank for an annual cost of $462.00. A 36” three tank system would cost $1,881.00 per year.
Filter cartridge changing for a cartridge type filter, based on the same 200 gpm demand as a 30” three tank backwash system, could be as high $448.00 per change. Changing the filters just once a month could bring an annual cost of $5,376.00. Comparing a 36” three tank backwash system to the cartridge type filter with both having the same 300 gpm demand the cost of the cartridge system would be $598.00 per change at an annual cost of $7,176.00. Both based on the assumption of only changing once a month. Knowing the volume of dirt that is in the water, changing them only once a month is an extremely generous assumption.
Ask the cartridge filter people to guarantee, in writing, there claim of how often their filters need to be changed. See what they say, then if it’s done more often let them pay for it.
Another key factor is the man hours used to change the filters plus the fact the filters are off line not supplying filtered water to the building.
Opening and inserting filters into the tank without sanitizing also poses a possible sanitary problem from unwashed hands, airborne bacteria etc.
Detailed Notes
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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 bag, cartridge & screen filters)
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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, as in bag, cartridge & screen filters 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.
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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.
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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. Both bag and cartridge filters need to be changed and replaced when they load up. In a domestic water supply, this is a critical problem. Opening and inserting cartridge filters or bags into the tank without sanitizing also poses a possible sanitary problem from unwashed hands and airborne bacteria etc. And since it is dependent upon what is in the water, it probably becomes a maintenance problem for the onsite personnel. Another key factor is the man hours used to change the filters plus the fact the filters are off line not supplying filtered water to the building.
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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.
Comparison Of Filtration Methods For New York City Residential Buildings
Job-site conditions:
| Flow rate | 380 - 400 gpm | |
| Water Supply | NYC water, dirt loading variable, estimated 5 - 50 parts per million | |
| Product | Unity Automatic Multilayer | Screen backwashing screen |
| Media filters Configuration | 3 filters in Parallel sharing the flow | One filter (or two) sharing the flow, depending upon which system is chosen |
| Filter Area | 63 square feet 3 filters each 3 feet in diameter, each with 3 filtration layers |
930 (or 1400) square inches, 6.46 square feet (or 9.72 square feet) |
| Filter depth | Filtration in depth using several layers of media, each progressively finer down through the bed to prevent passage of particles and unloading | Surface filtration on one screen surface |
| Water Quality | Removal of ALL particles 10 micron and larger and about 50% of particles between 5 and 10 micron. | Removal of virtually all particles larger than 25 micron (Removal of virtually all particles larger than 10 micron) |
| Pressure loss | 4 psi With one pump running 8 psi with both pumps running | Unstated. Depends upon screen mesh size |
| Frequency of Backwash | Usually once per week, but can be adjusted more or less often if conditions change | Unstated. Can be continuous under severe conditions |
| Backwash water | 4,725 gallons per week is basic schedule | 110 gpm for 40 seconds, or as necessary (total unknown) |
| (Backwash frequency is based upon estimated incoming water consumption of 40,000 gallons per day. The Unity system is capable of removing the dirt from 750,000 gallons of water per backwash. Actual capability of the screen system is unknown.) | ||
| Electric Power Consumption | 18 watts continuously to operate 3 timers | Unknown (230-460 volt panel to operate a fractional horsepower motor for backwash, plus a booster pump to supply the extra water needed for backwash, plus power for a 4" solenoid valve) |
| Point of Installation | (Recommended) On incoming water supply to use city pressure for control and backwash | (Recommended) On pumped water supply to use high pressure water for backwash |
| Pressure capability (Depending on need) | Up to 200 PSI | Up to 200 PSI |
| Operation | Fully automatic based upon calendar setting, with manual override | Automatic based on pressure loss sensing, with manual override |
| Operator attention | Observe conditions once per day | Observe backwash when possible |
| Maintenance | Annual PM to clean gauges and test ports, every 5-7 years replace diaphragms (Under maintenance contract) |
Scheduled maintenance unknown Unscheduled maintenance. |