Reverse osmosis

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Updated – February 3, 2024

Introduction to reverse osmosis

Who doesn't know it, and for owners of cherry orchards it is a horror, the bursting of ripe cherries after a rainstorm.

The reason for this is the work of the osmosis (Greek for “penetration”): Rainwater carries only a few dissolved particles. It therefore has a high chemical potential.
The sugar-rich water in the cherry pulp contains many dissolved substances. It therefore has a low chemical potential.
In order to equalize the potential differences, the pulp absorbs the rainwater, which increases the volume of the cherry and ultimately the skin can no longer withstand the pressure and bursts.

The Reverse osmosis goes the opposite way: water with a lot of impurities should be converted in its concentration of different (harmful) substances into water with few (or no) impurities.
For this purpose, the contaminated water is pressed through fine membranes at high pressure. The substances to be removed remain stuck there. Depending on the fineness of the membrane pores, the emerging water is now free of dirt and unwanted substances such as bacteria, viruses, hormones, antibiotics, etc.
To overcome the osmotic pressure of drinking water of around 2 bar, the applied pressure must be at least 50% higher. As the salt content increases, the osmotic pressure also increases; salt water requires a much higher pressure: around 350 bar must be used in saturation.

Reverse osmosis systems for drinking water production

Producing drinking water from rain, river or pond water was the original idea behind reverse osmosis technology. Systems based on this process are in use in crisis areas, in space travel or in developing countries.

With increasing contamination of drinking water, especially through harmful drug residues, the importance of these cleaning processes is also becoming increasingly important in the private environment.

The demands on filter technologies have grown accordingly. While previously it was only necessary to remove dirt particles > 10 µm, today far more difficult tasks have to be carried out.
Filters that retain, for example, arsenic, lead, cadmium, sodium, sulfate, calcium, magnesium, phosphates, chlorides, fluorides, nitrates, nitrite, radioactive substances, bacteria, viruses, dioxins, organic substances, chlorine, pesticides, insecticides and drug residues work with pore widths of just 0.02 µm. For comparison, a fine human hair measures between 0.02 and 0.04 mm, which is 1,000 times thicker. Bacteria are between 0.1 and 700 µm in size and viruses are between 0.015 and 0.44 µm in size.

The filter performance of reverse osmosis systems is given in GPD (gallons per day). 100 GPD corresponds to a daily production of 380 liters of pure water.

Commercially available systems usually produce between 1,500 and 2,250 liters per day (400 to 600 GPD), or around 1.1 to 1.5 liters per minute and cost in 7-stage filter structure 300 .. 400 euros, including a replacement filter set.

Hygiene aspects

Since only viruses smaller than 0.02 µm can still pass through the finest filter, you should be able to assume that there are no concerns about hygiene.

During holiday periods when no water is being drawn, bacteria can settle in the filters. However, these would have no chance of passing the 0.02 µm filter. In addition, automated flushing intervals ensure that such colonization is prevented.

99.99% sterility is guaranteed by downstream UV sterilization systems. Here the water flows along a quartz glass cylinder, inside which a UV lamp is installed. UV light kills bacteria and viruses and prevents algae formation.

With a 16W UV lamp At around 130 euros, with the flow rate mentioned above, an IRAY dose of around 4,000 J/m³ Realizable - ten times the value of what is considered standard recommendation in practice.

Chlorine and chloramines are broken down with sufficient UV-C doses. However, in order to achieve the required dose, a slow (!) passage of the water to be purified in the UV-C clarifier is required.

Ideally, for everyday use at home, the system should be run via a dosing system with a low flow rate (approx. 1 liter per minute). The water is taken from the pipe network and filled via the dosing pump through the UV-C clarifier into a hygiene drinking water tank (capacity approx. 30 liters, or adapted to daily drinking water consumption).
Due to the extremely slow flow rate, the resulting water is free of chlorine and germs.

sewage

Since contaminated wastewater is also produced during the treatment of pure osmosis water, the effective water consumption increases by around 100% or more.

The 7-stage system mentioned above uses 1 liter of wastewater for 1 liter of osmosis water.

In this respect, a wastewater pipe is always required when installing a reverse osmosis system. If necessary, this wastewater can also be put into a cistern from which the toilet is flushed, provided there are two water circuits installed in the house.

Ongoing costs

From a line pressure of 3 bar, a reverse osmosis system can be operated without electricity. However, if you don't want to do without automatic flushing and a UV hygiene station, you need a 230V connection.

A Filter change every six months costs around 50 euros.

UV-C lamps have a lifespan of 8,000 to 10,000 hours, i.e. about a year, and cost around 20 euros.

In total, there are 12 to 15 euros in monthly fixed costs for operation.

Economics

Anyone who wants to pay attention to economic considerations in addition to the hygiene aspect will not want to let the water used for rinsing disappear unused down the drain.

However, it should be borne in mind that this rinse water contains the “pollutants” that are rinsed out in a higher concentration than normal water, regardless of its origin, and should therefore not be used for hygiene-critical tasks!

However, there is nothing wrong with having a separate water supply for flushing the toilet, for example.