Water treatment in a motorhome: how the water in the tank stays fresh

There is no statutory time limit for how long drinking water may stand in a motorhome tank. Nevertheless, the relevant technical rule provides a figure. The Deutscher Verein des Gas- und Wasserfaches (DVGW, German Technical and Scientific Association for Gas and Water) specifies a fixed interval in its Kompaktinfo information sheet of March 2024. Water in an installation should be replaced every 7 days, preferably every 3 days. From 72 hours of stagnation, the same source expects microorganisms to multiply.

Legally, the fresh water tank in a motorhome is a drinking water installation. The rule therefore applies there too, and under worse conditions than in a house. The tank stands in the sun. Hardly anyone replaces the fresh water completely, and the water point remains an unknown.

The Umweltbundesamt (German Environment Agency) sets the threshold for water in the pipes even tighter. According to the agency, water that has stood for more than 4 hours belongs neither in a glass nor in a cooking pot. It gives two reasons: substances from the pipe and microbial influences. The value applies to the pipes, not to the storage tank, and is therefore not a time limit for the contents of the tank.

The widespread claim of 6 months of germ-free water in the tank has no counterpart in the technical rules. It describes laboratory conditions: sterile tank, sterile pipes. A vehicle in use never achieves them.

§ 2 no. 1 of the Trinkwasserverordnung (TrinkwV, German Drinking Water Ordinance) defines drinking water regardless of how it is transported. It names pipelines, water transport vehicles, drinking water storage tanks and offshore structures. Supply on board land vehicles, watercraft or aircraft is expressly covered. Water in a motorhome is therefore drinking water as soon as it is intended for drinking, cooking or washing.

Section 2 no. 2 letter d supplements this on the system side. Mobile water supply systems are movable systems from which drinking water is drawn. The wording refers to systems on board land vehicles, watercraft and aircraft, in each case including the drinking water installation. The fresh water tank, water pump, pipes and taps together form the installation.

The implementation guidance issued by the state of Baden-Württemberg in May 2024 refers to this category simply as d systems. As further examples, it lists water transport vehicles and mobile sales stands.

There is a separate technical rule for non-stationary systems. DIN 2001-2 covers the planning, construction, operation and maintenance of such systems. It explicitly names vehicles as an example. The 2025-06 edition replaces the January 2018 version and has been adapted to the new Drinking Water Ordinance. Baden-Württemberg's implementation guidance cites the standard in connection with operators' obligations to provide proof. This refers to operators who supply water commercially or to the public. For campers in their own vehicle, this does not create any obligation to provide proof.

Under § 47(1) no. 2, a perceptible adverse change in the water must be reported to the health authority without delay. The wording gives 4 characteristics as examples: colour, odour, taste and turbidity. The list is not exhaustive, and the sentence does not limit the obligation to commercial activity.

§ 48(2) likewise names mobile water supply systems without any restriction. Anyone with indications that their own system has caused a change must do 3 things. They clarify the cause, remedy it and inform the authority of both. Paragraph 1 of the same section contains the restriction to commercial activity; paragraph 2 dispenses with it.

The annual testing under § 29(2) covers 5 parameters. Listed are Escherichia coli, intestinal enterococci, coliform bacteria, Clostridium perfringens and the colony count at 22 and 36 degrees. It applies only to vehicles used commercially. Private owners who want certainty can voluntarily commission the same parameters from an approved testing laboratory. With standing water, it is also worth testing for Pseudomonas aeruginosa, a typical germ of damp environments. § 43 prescribes a separate detection method for it.

In the case of a private vehicle, the health authority in practice only takes action if there is a risk to health. § 54(3) leaves this to its discretion. The first step therefore always remains the same: find the cause and eliminate it.

Annex 3 Part I of the German Drinking Water Ordinance contains a special rule for mobile systems. It concerns the colony count at 22 degrees Celsius. Under the method set out in § 43(3), the following 3 values apply:

• 100 colonies per millilitre at the consumer's point of use,
• 20 per millilitre in the disinfected water directly after treatment,
• 1,000 per millilitre in water storage tanks of mobile water supply systems.

For the tank, a value ten times higher therefore applies. However, the relaxation applies exclusively to incubation at 22 degrees Celsius, i.e. to environmental germs. For the colony count at 36 degrees Celsius, the same annex specifies 100 per millilitre. There is no special rule for mobile systems there. At 36 degrees, it is the germs that colonise the human body that grow.

What has been relaxed, then, is the value for environmental germs, while the value at body temperature remains unchanged. Anyone who concludes from the figure of 1,000 that more is generally permitted in a motorhome is reading only half the table.

Legionella pneumophila serogroups 1 and 6 were detected, as well as L. anisa, L. feeleii and L. quateriensis. The highest single value was 25 colony-forming units per millilitre for L. pneumophila serogroup 1.

The user survey in the same study is revealing. 60% of owners had cleaned the tank in the previous year. 45% drained it before putting the vehicle into storage. Only one in 20 disinfected it with chlorine.

The study is a case report with a small sample from the USA, not a representative German sample. It shows an order of magnitude, not a frequency for the German vehicle fleet. It remains notable that the taps were contaminated more often than the tanks. The route from the fresh water tank to the tap is evidently the more critical section.

With legionella, the route of uptake is also decisive. The bacteria reach the lungs via aerosols, i.e. fine droplets. The shower in the vehicle produces exactly such droplets, whereas drinking hardly does. Anyone who takes a shower first after the water has been standing for 2 weeks at the campsite is choosing the worst possible start. How a legionella filter works and where it makes sense to fit one is explained in a separate article.

Water preservation means adding an agent that is supposed to inhibit germs in the filled tank for weeks. Legally, the only decisive factor is the active substance, not the form in which it comes, whether tablet, powder or solution.

Legally, any addition is a form of treatment. § 18 no. 4 TrinkwV permits disinfection in mobile systems and when storing cold water. The purpose is therefore permissible. The removal of pathogens by filtration is governed by no. 1 of the same section. However, it applies to raw water. The agents are then restricted by § 19(3). Only substances and methods from the list under § 20 are permitted.

In the disinfection part, the list contains 5 substances: calcium hypochlorite, chlorine, chlorine dioxide, sodium hypochlorite and ozone. The methods part names 10 methods, 9 of which are chlorine-based. Silver does not appear in any section.

Chlorine dioxide is therefore at a different level from chlorine. Anyone who applies the 0.2 milligrams per litre from the chlorine dioxide row to chlorine stays a third below the chlorine value. The dose may then turn out too weak.

According to the list, higher values are disregarded if disinfection cannot be achieved otherwise. The same applies if ammonium temporarily interferes with it. For chlorine, the list specifies an addition of up to 6 milligrams per litre and a residual content of up to 0.6 milligrams per litre. This rule is aimed at water suppliers, not at the tank in a vehicle.

In practice, one difficulty remains with chlorine. Without measurement, the content in the tank is unknown. Commercially available products are calculated for a tank volume that is rarely known exactly. On top of this comes the ongoing dilution during the journey.

Effective tank cleaning consists of 4 steps, and the order determines the result:

1. Drain completely. Residual water in the lines and pump dilutes any agent. It immediately re-inoculates the freshly cleaned tank.
2. Clean mechanically. Biofilm is a coating, not a dissolved substance. Where there is an inspection opening, a brush should go in. Without an opening, the only option is chemical cleaning followed by thorough rinsing.
3. Treat the lines as well. The coating also sits in hoses, pump, water heater and taps. A clean tank connected to contaminated lines achieves nothing.
4. Rinse until the smell disappears. Detergent residues are themselves a nutrient.

The water hose deserves the same attention. Between trips, it often lies in the storage compartment and stays damp inside. Then it goes into the tank. A dedicated drinking water hose that is sealed at both ends does more for hygiene than any preservative. This is especially true at campsites, where all campers use the same water point. What matters when filling up with water on the road is covered in a separate article.

An activated carbon filter for water adsorbs dissolved substances. It does not retain bacteria, and if it stays in place for a long time it can itself become colonised. Anyone who needs a germ filter needs the membrane. The overview of filtration technologies puts both methods into context. In the camping series manual, the Nano cartridge is called a germ barrier directly before the water is drawn off.

The order in the system is also set out in the manual and in the installation instructions. In the direction of flow, Active comes before Nano, with the Protect pre-filter module always in first place. The reason is carbon dust. It is released from the activated carbon during the first rinse and prematurely clogs the membrane.

A filter in a motorhome has 2 sensible installation points, plus the combination of both. Each option solves a different problem, and the decision depends on how the vehicle is used:

• Upstream of the tank, the filter cleans the water during filling. Sediment, chlorine taste and particles from the water point stay out. A filter in this position cannot reach what happens in the tank itself.
• Downstream of the tank, it safeguards the point of use. Germs that have multiplied in the tank are retained here. In return, the filling water flows into the tank unfiltered.
• Combining both points covers the entire route. The combination is worthwhile for large tanks and water that stands for long periods, because that is where both risks coincide.

If you empty the tank within 3 days, one stage is enough. If you carry 200 litres and make them last a week, the problem lies in the tank. Then the stage at the point of use is what counts. Using water sparingly lengthens the time it stands in the tank, which should be borne in mind when saving water in a motorhome.

In its 2020 guide, the Umweltbundesamt advises against devices designed to purify the water additionally. The reason is given in the same paragraph: In filters that are not replaced regularly, bacteria can multiply and contaminate the drinking water. A recommendation from 2021 takes the idea further. In a system that is built and operated in compliance with the rules, there is no need for post-treatment on hygiene grounds.

Both statements are aimed at domestic installations connected to a monitored supply network. A motorhome has neither a mains connection nor monitoring. The water point changes, the water is stored in the tank, and nobody monitors the system.

The core of the objection is nonetheless valid. A filter that stays in the housing too long turns from protection into a risk. The replacement interval is therefore not a recommendation. It is the condition under which a filter in a vehicle remains justifiable.

Before the vehicle is laid up, 4 components must be completely empty: tank, lines, pump and water heater, plus the drain valves. Residual water left for months is the starting point for a coating that can no longer be rinsed away in spring.

Filter cartridges should come out of the housing. The camping series manual requires them to be removed and left to dry with the nozzle facing upwards. This is how the manufacturer prevents germ growth and mould. The manual expressly prohibits 3 forms of storage: in the fridge, placed in rice and in closed containers. The time the cartridge spends dry on the shelf does not count towards its service life. According to the manual, the 4 or 6 months refer only to active use; a well-dried cartridge carries on into the next season.

At the start of the season, the order for recommissioning is: first flush, then fill, then filter. For buildings, the DVGW requires the entire installation to be flushed after an interruption in operation. Where water is supplied to the public, a specialist company is also involved. In a private vehicle, the core of this remains: flush before drawing the first glass. After that, the interval of 7 days, or better 3 days, applies again.