Household Water Post-Treatment System Based on Reverse Osmosis
Clean drinking water in an apartment or house largely determines the quality of life. The centralized drinking water supply system does not always ensure the delivery of high-quality drinking water to apartments and houses. Although such water should meet the requirements of SanPiN “Drinking Water,” this does not always happen. Even when the water complies with the regulations, its taste remains mediocre.
These circumstances occur for many reasons related to the organization of the centralized drinking water supply system. Primarily, this is due to worn-out pipeline networks. Therefore, even if the water meets all standards after the treatment plant, by the time it reaches the consumer, having passed through deteriorated pipes, it may acquire secondary contamination.
This problem is solved by installing a drinking water post-treatment system at the consumer’s location. Currently, two types of water post-treatment systems can be distinguished. These are systems that use filtration through various granular materials and systems that use reverse osmosis membranes. In turn, systems using filtration through granular media can be divided into “pitcher”-type systems (non-pressurized) and systems using pressurized filtration (under the pressure of tap water).
The main drawback of filtration through granular media is the need to maintain a relatively low filtration rate. Therefore, for small household filters, the productivity of such systems should not exceed 0.5 liters per minute, which is extremely inconvenient in the operation of such systems. When attempting to achieve an acceptable flow rate of purified water at 2 liters per minute, the filters begin to work very inefficiently because the necessary contact time between the water and the granular media is not ensured.
For ordinary filtration, it is better to use a pitcher filter.
This issue is discussed in more detail in [1].
To obtain truly high-quality drinking water, it is recommended to use systems with reverse osmosis membranes. During reverse osmosis desalination, 90–95% of dissolved salts are removed from the water, while organic matter and larger contaminants are completely removed.
The weak point of reverse osmosis is the membrane’s vulnerability to poor-quality feedwater, as well as the membrane’s inability to retain gases. If “dirty” water is supplied to the reverse osmosis membrane, it will quickly fail. Therefore, high-quality preliminary water treatment is required before the reverse osmosis stage.
Another drawback of reverse osmosis water purification systems is their relatively low productivity in terms of purified water.
The source water from the drinking water supply system enters the household water purification system. First, the water undergoes pretreatment. This allows the membrane to operate under optimal conditions. Pretreatment consists of three stages. At the first stage, the water is filtered through a polypropylene cartridge with a filtration rating of 1–20 microns. The second and third stages of pretreatment are filter housings with a standard size of 10 inches, filled with activated carbon. In one filter, the activated carbon is loose, while in the other, it is compressed.
Thus, at the pretreatment stage, mechanical and suspended substances, iron, as well as some organic compounds and gases are removed from the water. Dissolved contaminant gases such as hydrogen sulfide and ammonia should not reach the membrane, as they easily pass through it and, accordingly, enter the purified water. Therefore, it is necessary to ensure the longest possible contact between the water and activated carbon at the pretreatment stage. This will remove contaminant gases from the water. At the same time, the salt composition of the water remains unchanged during pretreatment. Almost all salts, including heavy metals, will be removed at the reverse osmosis desalination stage.
After this pretreatment, the reverse osmosis membrane can only become clogged with hardness salts. Here, it should be noted that CaCO₃ salts will deposit on the membrane if the water’s carbonic acid balance is shifted toward the precipitation of these salts. However, water from the drinking water supply that has undergone coagulation at the central treatment plant has a carbonic acid balance shifted toward the dissolution of precipitates, i.e., in the opposite direction of precipitation. In this case, the increasing concentration of calcium bicarbonate on the membrane surface during reverse osmosis desalination will gradually shift the carbonic acid equilibrium toward precipitation. It is important to adjust the concentrate flow from the membrane so that the carbonic acid equilibrium remains balanced, where there is no precipitation of salts but also no dissolution.
As a result, after the membrane, high-quality water can be obtained, but with one drawback: this water will have a relatively low pH value.
Why does this happen?
Water contains carbon dioxide (CO₂), which easily passes through the membrane into the purified water. The product of CO₂ dissolution in water is the bicarbonate ion (HCO₃⁻). The more bicarbonate there is in the water, the less CO₂ dissolves in the water. The membrane retains 90–95% of bicarbonate, while CO₂ passes through the membrane completely. As a result, in the purified (partially desalinated) water after the membrane, CO₂ begins to dissolve rapidly because there is almost no bicarbonate to restrain it.
The dissolution of CO₂ in water leads to the formation of carbonic acid (H₂CO₃), and the pH of the water decreases. Moreover, the pH of water from the drinking water supply can drop below 6.0. According to SanPiN “Drinking Water,” the pH of drinking water should be in the range of 6.0 to 9.0 pH units.
To raise the pH of the purified water, it must be passed through a mineralizer. The simplest mineralizer is a cylindrical filter loaded with calcite (CaCO₃).
The acidic purified water after the osmotic membrane gradually dissolves the calcite and saturates the water with calcium bicarbonate. At the same time, the water’s carbonic acid is consumed to dissolve the calcite. As a result, the concentration of carbonic acid in the water decreases, and its pH rises.
Example:
Drinking water from the city pipeline entering an apartment has the following composition:
Bicarbonate ion (HCO₃⁻) — 2.0 mg-eq/L (2.0 × 61 = 122 mg/L)
CO₂ — 0.4 mg-eq/L (0.4 × 44 = 17.6 mg/L)
Let us calculate the pH of the source water:
The pH value of 7.05 complies with regulatory requirements.
After the osmotic membrane, the bicarbonate concentration in the water was 0.12 mg-eq/L (0.12 × 61 = 7.32 mg/L), while the CO₂ concentration remained the same (CO₂ = 0.4 mg-eq/L).
Let us calculate the pH of the osmotic water:
This pH value does not meet regulatory requirements. Only if the bicarbonate concentration in the osmotic water is 0.2 mg-eq/L will the pH of such water be 6.0.
In any case, osmotic water will have a carbonic acid balance significantly shifted toward the dissolution of precipitates, i.e., it will be acidic. To neutralize the carbonic acid balance of osmotic water, a mineralizer must be installed after the reverse osmosis membrane.
Another important question arises: Where exactly should the mineralizer be placed?
Due to the low productivity of household reverse osmosis membranes in terms of purified water, the purified water is not directed immediately to the clean water faucet but to a storage tank. The storage tank maintains excess pressure and allows for a clean water flow rate at the faucet of about 2 liters per minute, which is an acceptable value. At the same time, a standard membrane provides a productivity of about 0.15–0.2 L/min. Therefore, operating a household purification system without a storage tank is extremely inconvenient.
The mineralizer must be placed before the storage tank. In this case, the water flow rate through the mineralizer will be 0.15–0.2 L/min. If the mineralizer is placed after the storage tank, the water flow rate through the mineralizer will be 2 L/min (10 times higher!). At such a high flow rate, the carbonic acid in the water practically does not have time to react with the calcite. As a result, the pH of such water hardly changes. Installing the mineralizer after the storage tank is almost pointless.
When the mineralizer is installed before the storage tank, the process of removing carbonic acid and saturating the water with calcium proceeds quite efficiently.
For the example above, placing the mineralizer after the tank reduced the CO₂ concentration in the osmotic water from the initial 0.4 mg-eq/L to 0.35 mg-eq/L. After placing the mineralizer before the tank (immediately after the membrane), the CO₂ concentration dropped to 0.05 mg-eq/L, and the pH of such water was 6.73 pH units. In this case, the water reached a neutral carbonic acid balance, corresponding to the standard state of clean natural surface water.
Based on the above data, let us assemble a post-treatment scheme for drinking water for household needs, assuming that the source water comes from a high-flow surface source (a river) and undergoes coagulation treatment at the central water treatment plant.
Figure 1 shows a schematic of the household post-treatment system for centralized drinking water supply. The source water from the drinking water pipeline enters the pretreatment stage under pressure. This stage consists of three cartridge filters installed in series. At the first stage (1), mechanical filtration is performed using a polypropylene cartridge. Mechanical impurities up to 1–20 microns in size are removed from the water.
Next, the water passes through cartridges with loose (2) and compressed (3) activated carbon. At these stages, some organic matter, petroleum products, and aggressive gases are removed from the water.
Important! The water flow rate through the pretreatment system must not exceed 0.5 L/min. The fact is that activated carbon works effectively only at relatively low linear filtration rates. That is, it is necessary to ensure relatively long contact between the water and the carbon. The linear filtration rate for carbon should not exceed 10 m/hour. Then the water flow rate (V) through the carbon cartridge will be:
V = 10*f =10*0.0028 = 0.028 m3/ hour = 28 L/hour = 28/60 = 0.47 L/min
Where:
f — cross-sectional area of the cartridge, m²
f = 0.06 * 0.06* 0.785 = 0.0028 ,m²
where 0.06 m is the diameter of the cartridge.
After pretreatment, the water enters the reverse osmosis purification stage. At this stage, the water is divided into two streams by a semipermeable membrane: clean (permeate) and dirty (concentrate). Part of the water passes through the semipermeable membrane, while the remaining part, saturated with salts and other contaminants (concentrate), is discharged into the sewer.
To ensure that some of the water passes through the membrane, resistance must be created in the concentrate line. If no resistance is created, all the water from the membrane will be discharged into the sewer. Therefore, a regulating valve is installed on the concentrate line after the membrane and before the sewer discharge.
Using the regulating valve, the concentrate flow rate must be adjusted so that the discharge to the sewer is approximately 60% of the total water flow entering the membrane. Accordingly, the percentage of purified water will be 40%. Standard reverse osmosis membranes provide a purified water flow rate of 7–15 L/hour at standard drinking water pipeline pressure (3–5 bar). Therefore, the source water flow rate to the membrane should be:
12/0.4 = 30 L/hour
where:
12 L/hour = 0.2 L/min — purified water flow rate from the membrane
0.4 — fraction of purified water
Then the concentrate flow rate should be:
30 * 0.6 = 18 L/hour = 0.3 L/min
Using a stopwatch and a measuring cup, the regulating valve should be adjusted to set the concentrate flow rate to approximately 0.3 L/min. At the same time, with sufficient pressure of the source water after the membrane, purified water can be obtained at a flow rate of about 0.2 L/min.
Under these conditions, there will be no excessive discharge of water to the sewer via the concentrate line, and no calcium carbonate deposits will form on the membrane.
If a post-treatment system consisting only of the first three stages (1, 2, 3) is used, the water flow rate through these filters will be very high, and full purification will not occur.
It is important to install a siphon before discharging the concentrate into the sewer. For this, it is sufficient to create a loop in the flexible tubing. Otherwise, contaminant gases from the sink drain (where the concentrate is discharged) will enter the membrane through the concentrate line during periods when the purification system is idle (especially during prolonged idle periods). The gases will pass freely through the membrane and dissolve in the purified water. When the purification system is idle, with no clean water being drawn and the storage tank full, the concentrate line remains without water due to the automatic shutoff valve. Therefore, contaminant gases can reach the membrane and, accordingly, the clean water. For this reason, a simple water trap must be installed on the concentrate line.
After the membrane, the clean water enters the mineralizer at a flow rate of no more than 0.2 L/min. These conditions allow for a significant reduction in carbonic acid concentration and bring the carbonic acid balance to a neutral, natural (original) state.
The fully treated water then enters the storage tank and, after the tank, is supplied to the clean water faucet installed in the kitchen sink at a flow rate of 2 L/min.
Figure 1 Schematic diagram of a household post-treatment system based on reverse osmosis
This system produces high-quality water with a pleasant taste. In my subjective opinion, 9 out of 10 people who try such water will no longer want to drink any other.
It is necessary to provide several recommendations for the initial startup of this water purification system and for startup after replacing the filter media.
During the initial startup and after replacing the filter media in filters 1, 2, and 3, they must be flushed. For this, the regulating valve on the concentrate line should be fully opened. The same should be done after replacing the membrane. After flushing the filters, the required concentrate flow rate must be re-established using the regulating valve.
When installing or after replacing the mineralizer, it must also be flushed. Flushing it with the permeate flow from the membrane is very time-consuming and inefficient. For flushing, the mineralizer should be connected after the tee diverting flow to the storage tank (position 5″ in Figure 1). At the same time, the storage tank should first be completely filled with clean water. There is a shutoff valve at the tank inlet. Close this valve, install the mineralizer after the tee, open the valve on the tank, and then open the clean water faucet. Flush the mineralizer with clean water from the storage tank. Then install the mineralizer in its standard working position before the storage tank and start the system.
It should be added that there are currently mineralizers on the market that contain both calcite and activated carbon. This helps eliminate residual contaminants (primarily contaminant gases) and ensures more stable and reliable operation of the entire water treatment system. This system specifically considers a mineralizer with calcite and activated carbon.
It should be noted that installing a filter with only activated carbon after the storage tank is also a highly debatable solution, as a small filter will operate under relatively high water flow rates compared to its size.
In conclusion, I will take the liberty of providing an approximate economic calculation of the cost of clean water produced by this system for household consumption.
Initial data for the calculation (August 2025):
Cost of a household drinking water post-treatment system according to Fig. 1:
Approximately 10,000 RUB.
Cost of replacing filter elements in filters 1, 2, and 3:
Approximately 200 + 200 + 250 = 650 RUB.
Cost of a mineralizer: 500 RUB.
Drinking water and wastewater tariffs for the city of Saratov (from 07/01/2025):
Drinking water: 37.8 RUB/m³
Wastewater: 20.74 RUB/m³
Let us calculate the cost of one liter of purified water, assuming the post-treatment system produces 10 liters of water per day.
Calculation:
For clarity, we will calculate over a 2-year horizon, i.e., until the membrane is replaced. During this time, the pretreatment system’s filter elements will be replaced 3 times (once every 6 months), and the mineralizer will be replaced once.
Costs over 2 years for water treatment equipment:
10,000 + 650 × 3 + 500 = 12,450 RUB.
Costs for drinking water and wastewater:
The source water consumption to produce 1 liter of clean water will be:
1/0.4 = 2.5 liters = 0.0025 m³.
Then the cost of drinking water to produce 1 liter of clean water will be:
0.0025 × 37.8 = 0.095 RUB/liter.
The wastewater discharge to produce 1 liter of clean water will be:
2.5 − 1 = 1.5 liters = 0.0015 m³.
Then the wastewater costs will be:
0.0015 × 20.74 = 0.03 RUB/liter.
Let us calculate the total volume of water produced by the system over 2 years:
10 × 365 × 2 = 7,300 liters = 7.3 m³.
Then the cost of one liter of purified water will be:
(12,450 + 0.095 × 7,300 + 0.03 × 7,300)/7,300 = 1.83 RUB/liter.
For example, if 10,000 liters of clean water are produced over 2 years, the cost per liter will be:
(12,450 + 0.095 × 10,000 + 0.03 × 10,000)/10,000 = 1.64 RUB/liter.
Thus, the cost of a five-liter canister will be no more than 10 RUB, while the price of high-quality water in a five-liter canister in stores exceeds 75 RUB.
The benefit is obvious, and at the same time, consistently high quality of purified water is ensured.
In conclusion, I would like to note that obtaining high-quality drinking water at home is currently a relatively inexpensive process, but it requires careful immersion in the topic and thorough consideration of all aspects related to post-treatment of drinking water in household conditions. Otherwise, you may pay money and receive relatively low-quality purified water.
List of references:
- Water treatment for drinking purposes. Tikhonov I.A. – https://tiwater.info/water-treatment-for-drinking-purposes/+
