How to Choose a Water Filter: Start with the Task, Not the Price

A filter is selected based on the composition of the water and the specific task. Price appears at the end of the calculation, not at the beginning. Below is a breakdown of which analysis is needed, which classes of equipment exist, and why a single box cannot cover every task at once.

Analysis First, Equipment Second

Without a water analysis report, filter selection turns into guesswork. The minimum set of parameters for a residential project is: total hardness, total iron, turbidity, total dissolved solids, pH, chlorides, sulfates, nitrates and residual chlorine. For a well, add manganese, hydrogen sulfide, ammonium, permanganate oxidizability and microbiology.

The sample is taken after letting the water run for several minutes, into a clean container, and delivered to the laboratory the same day. Iron and pH change quickly in the bottle. If the sample sits for a day in a warm place, part of the iron will precipitate and the result will understate the real load on the equipment.

Drinking water standards in the republic are set by the O'z DSt 950:2011 standard and the sanitary rules SanPiN RUz 0211-06. Under SanPiN 0211-06, the maximum permitted values for domestic and drinking water supply systems are: total hardness 7 meq/L, iron 0.3 mg/L, turbidity 1.5 mg/L, total dissolved solids 1000 mg/L. With the agreement of the sanitary authority, 10 meq/L, 1.0 mg/L, 2.0 mg/L and 1500 mg/L, respectively, are allowed for individual water supply systems.

Why "Within the Limit" Does Not Mean "Soft"

One meq/L of hardness equals 50 mg/L expressed as calcium carbonate. So the 7 meq/L limit is 350 mg/L CaCO₃. In its background document on hardness, WHO classifies water above 180 mg/L CaCO₃ as very hard. Water that fully complies with the sanitary limit can be twice as hard as the lower boundary of the "very hard" category.

The reason is simple. The limit was written for human health. WHO states explicitly that no health-based guideline value is set for hardness because of insufficient data on harm. The service life of a boiler, heat exchanger, heating element and faucet is not regulated by a sanitary document. That is the engineer's job.

Four Tasks, Four Types of Equipment

• Mechanical filtration. Removes sand, scale, rust and fibers. A strainer of 100-500 µm at the inlet, followed by cartridges of 20, 10, 5 or 1 µm. This protects everything installed downstream.
• Softening. Ion exchange resin replaces calcium and magnesium with sodium. This is the principal working method of reducing hardness, rather than masking it.
• Iron removal. Dissolved ferrous iron is first oxidized with air or a reagent, then captured on a catalytic filter media. A mechanical cartridge does not work on dissolved iron at all.
• Point-of-use drinking water treatment. Carbon, a membrane, or a combination of the two under the sink. The task concerns taste, odor, chlorine, organics and salts.

These are four different physical processes. Combining all of them in one housing means that each of them is only half done.

How to Calculate the Cost

The price of the unit is only the first of three cost items. Calculate as follows:

• capital costs for equipment and installation
• consumables over five years: cartridges, tablet salt, reagents, membranes
• losses from inaction: replacing faucets, heating elements and circulation pumps, cleaning the boiler heat exchanger

A cartridge housing at the inlet of a house with hard water does not eliminate scale. It captures suspended solids and looks like a solution. Two years later the owner pays for a heat exchanger.

Certification as a Screening Tool

NSF/ANSI standards apply to residential water treatment equipment. Standard 42 covers aesthetic parameters, including chlorine, taste and odor. Standard 53 covers parameters that affect health. Standard 44 applies to cation exchange water softeners. Standard 58 applies to reverse osmosis systems. Standard 401 covers a group of emerging contaminants, including pharmaceutical residues.

A certificate under the relevant standard means that the claimed reduction was verified by an independent laboratory using a defined method. The absence of a certificate does not make a unit bad, but it shifts the burden of verification to you.

Typical Selection Mistakes

• A water softener without an iron analysis. Iron deposits on the resin and reduces its capacity.
• Reverse osmosis on hard water without pretreatment. The membrane becomes fouled with carbonate scale.
• Any expensive equipment without mechanical filtration ahead of it.
• Selection by average daily consumption without accounting for peak flow. At peak demand, hardness breaks through the resin bed.
• Installation without a bypass, pressure gauges and sampling points. Servicing then means shutting off the water supply to the whole house.

What this means for a project in Tashkent

• In our practice, in approximately 85 percent of the districts of the city and in all other regions of the republic, the water is hard and of low quality. There are individual districts with an acceptable composition, but they are the exception, not the rule.
• Even where the water at the intake is normal, old mains cause secondary contamination. Iron, scale and suspended solids reach the apartment from the pipe, not from the source. A mechanical stage at the inlet is needed in such districts too.
• Start with an analysis of the specific point of use, not with the figures for the district. The spread within a single block can be significant.
• Separate the tasks by circuit. Softening for the hot water circuit and the boiler, mechanical filtration for the entire inlet, point-of-use drinking water treatment for a single point at the sink. This is cheaper and more reliable than installing one universal station for the whole house.