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Membrane filtration: using rainwater as drinking water

The per capita demand for drinking water in Germany has been declining for years and most recently reached a level of around 121 l per inhabitant per day for household-related activities in 2012 (source: Federal Statistical Office). The development of water-saving techniques in households and the steep rise in drinking and wastewater charges over the past 20 years have favored this trend. Of the 121 liters of drinking water, around 51 liters are used for flushing the toilet (40), cleaning the rooms (6) and watering the garden (5), and another 17 liters for washing clothes. We use the largest part with 44 l for daily personal hygiene, only 3 l are used for cooking, eating and drinking.

In a water-rich country like Germany, water scarcity is usually not (yet) an economically convincing argument for the need for increased use of rainwater. But in many parts of the world things look very different - also against the backdrop of impending climate changes and increasing urbanization.

More than half of the world's population already lives in metropolitan areas, and in just under ten years this proportion should increase to 60%, that is five billion people. Special attention should therefore be paid to the effects of increasing urbanization on the demand for drinking water. The emergence of so-called megacities in emerging and developing countries around the world with their rapidly growing settlement conglomerates is exacerbating the water supply and disposal situation. The concept of a communal water supply in central systems turns out to be completely inadequate. In addition to the cost of creating and maintaining the water infrastructure, there are usually high operating costs and the difficulty of being able to provide sufficient drinking water at all. This makes it all the more important to close the cycle, i.e. decentralized processing of rainwater into drinking or at least process water and its increased use, e.g. as toilet flushing, washing water or for irrigation. This can significantly reduce the consumption of high-quality drinking water in urban areas.

Even in regions without sufficient groundwater resources, rainwater harvesting and the establishment of smaller water cycles can make economic and ecological sense. The same applies to remote houses that are not connected to the drinking water network and do not have their own well or are located in areas with little groundwater, e.g. mountain huts, rainwater harvesting offers a valuable option for water production.

For us, the use of rainwater as an ecological supplement in the household, e.g. for watering gardens and green areas and for flushing toilets in water-saving sanitary facilities, has become considerably more important. In this way, 51 of the approximately 121 liters of valuable drinking water per day can be saved. With restrictions or under certain conditions, rainwater can also be used for washing clothes (17 l drinking water daily) and washing dishes (7 l daily). But for this not only all pathogens, but also the dirt and fine dust must be removed from the water.

Filtering the rainwater before storing it

The use of rainwater in the household requires a high standard of hygiene, which is defined for Germany in the Drinking Water Ordinance (TrinkwV) of 2012. In order to guarantee permanently good water quality, rainwater must be cleaned and sterilized in a rainwater cistern before it is reused or before it is collected. The quality of rainwater depends on the uptake of substances on their way through the atmosphere and on the place of impact. Rain not only washes particles and fine dust out of the air, but also picks up other dust, dirt and excrement from birds and other animals on the way over sealed surfaces or the roof, for example. Rainwater that runs off is microbially contaminated with pollutants and germs, and often has an unpleasant smell or a noticeable color / cloudiness.

The Upper Bavarian company Seccua, an expert in the field of drinking water treatment, offers with UrSpring a residue-free removal of all pathogens without the use of chemicals, but through ultrafiltration. The UrSpring filter system is installed at the transfer point from the rainwater cistern behind the pressure booster system. It is matched to the required flow rate for service water for the quick filling of cisterns, washing machines or for watering the garden.

The filter elements, which are also used to disinfect well water, have nano filter pores with only 0.015 µm (15 millionths of a millimeter), i.e. Seccua membrane filtration removes all particles from the water that are larger than 0.015 micrometers. This also completely removes the fine dust that has not yet settled in the water. In addition, Seccua guarantees full retention of bacteria, parasites and viruses, which the UrSpring filters have proven in tests according to the strictest US EPA guidelines. Ultimately, the UrSpring delivers crystal-clear, germ-free water from rainwater to every connected tap.

In contrast to conventional filter solutions such as sand or gravel filters, the UrSpring's membrane filter can be backwashed. The dirt particles and germs retained by the UrSpring collect on the dirty water side of the filter and, depending on requirements, can be rinsed out of the filter by simple manual or automatic rinsing. Only a few liters of water are required to rinse the filter. The UrSpring not only produces significantly better water quality, but also achieves a considerably longer filter service life than any conventional filter.

Soft water from the clouds

Rainwater is very soft water. Not only garden plants like this, but this low water hardness also extends the life of the heating elements in washing machines and dishwashers and prevents calcification of the toilet and cistern. From the point of view of water protection, the use of rainwater for washing laundry would be desirable, as softeners could be dispensed with due to the low water hardness, and 20% less washing powder could be used.

In combination with the UrSpring BioFilter, the system removes unpleasant smells or stains from the rainwater and thus prevents the formation of edges on the ceramic or discoloration of the laundry.

While supplying the toilets with rainwater in a detached house is clearly unprofitable, the use of rainwater is particularly worthwhile in large residential complexes or public buildings such as schools. A life cycle assessment of drinking water and rainwater use prepared on behalf of the Swiss Federal Office for the Environment, Forests and Landscape has shown that the cost-benefit ratio of a rainwater treatment system becomes more favorable the more individual consumers are supplied with process or drinking water from rainwater.

Rainwater harvesting systems for the household require constant monitoring and care. A current study “on the hygienic control of rainwater harvesting systems in Germany” came to the conclusion that, after several years of operation, 70% of the rainwater harvesting systems were objectionable. On the other hand, rainwater harvesting systems that were installed, operated, maintained and hygienically checked according to the regulations - i.e. those that were planned, built and operated according to the generally recognized rules of technology - were not hygienically objectionable even after several years of operation. The Fachvereinigung Betriebs- und Regenwassernutzen e.V. assumes around 500,000 rainwater harvesting systems nationwide, around 50,000 new systems are installed every year.

Dipl. Ing. (FH) Michael Hank, founder and managing director of Seccua GmbH, Steingaden

Runoff rainwater is microbially contaminated with pollutants and germs.

Tests according to the strictest US EPA guidelines have proven that the membrane filter retains bacteria, parasites and viruses.