Potable water is the most important food on earth and is not replaceable. The natural element of water is the basis of earthly existence. In Germany, water is the purest and most monitored food. It is constantly tested for quality and ingredients. The daily consumption of water in Germany is 120 liter per citizen – this corresponds to an annual consumption of 4 billion cubic meters. Such huge amounts of water are not required simultaneously throughout the day and are therefore not taken from the water system at the same time. Therefore water storage systems are needed to cover the consumption peaks and the operational standstill periods during the water transportation.
The quality of potable water is legally regulated in the Drinking Water Ordinance. Fundamental basis of the Drinking Water Ordinance is its direct connection to the generally accepted rules of technology like for instance the DVGW regulations and the DIN standards. The harmonized European standard DIN EN 1508 “water supply – requirements on systems and components of the water retention” comprises the normative foundations for storing drinking water. In addition, the DVGW worksheet W 300 “water retention – design, construction, operation and maintenance of water supply tanks within the drinking water supply” reflects detailed regulations of the actual state of knowledge in Germany. Both regulations were originally created for storage systems made of concrete but are also applicable to storage systems out of other materials. It is not intended to change existing tanks but to correspond to the standard, it is intended to “support“ the production of new water storage tanks.
Polyethylene has been used successfully in the gas-and drinking water supply in form of pipes, molded parts and shafts for many decades. But there is also an increasing demand for PE for the renovation of potable water storage tanks. Therefore it was only a matter of time until the demand for PE for the new construction of drinking water storage systems appeared. The advantages of Polyethylene in comparison to concrete are the resistance to external environmental influences and the maximum chemical resistance, the low specific weight and thus the simple handling of the work site, the opportunity for prefabrication of large parts in the workshop to ensure a rapid completion on site and the simple cleaning of the very smooth surfaces. Further advantages, with regard to other materials which are used for the construction of potable water storage systems, are for example the comparatively simple processing, the enhancement possibilities, the possibility of modification and the possibility of recycling. The PE 100 raw materials which are used for the construction of potable water storage systems are accredited for the transport and the storage of potable water by the DVGW and KRV and are equivalent to the requirements of the DVGW worksheet W 270 “multiplication of microorganisms on materials intended for use in drinking water systems – examination and evaluation”. The profiled pipes from FRANK & Krah GmbH which have been used throughout the construction, have a general technical approval of the DIBt with which the basic requirements for the construction of potable water storage systems out of PE 100 helically wound pipes are fulfilled.
The worksheet DVGW W 300 defines potable water storage systems as a closed unit for potable water, which includes water chambers, a control house and operation equipment, provides opportunities to access, includes operating reserves, guarantee pressure stability and is therefore compensating fluctuation of consumption. As the regulations do not make any restrictions in the design of tanks, it opens the door for numerous opportunities. The imagination knows almost no bounds in this respect. Nearly everything is possible – however, the highest aim is always to build a technically sophisticated solution conforming to standards and which is economically attractive. By producing potable water storage systems which are manufactured out of PE 100, it must be noticed that the current maximum inside diameter is 3500mm. Therefore the storage capacity has to be realized through appropriate pipe lengths thus it has to be ensured that sufficient space is available. The transport of the pipes to the construction site also must be considered. The graphical material is showing a potable water storage system with an inside diameter of 3500mm, which was planned by the engineering consultants Kiendl & Moosbauer from Deggendorf with technical assistance from Frank GmbH. As the access route to the existing and still in use storage system was very narrow, the new PE water chamber was delivered in individual segments. The assembling and installation to the existing building was realized in only one week. After the commissioning of the new PE 100 pipe container, the second water chamber will be redeveloped with HydroClick plates out of PE 100.
Generally, the potable water storage system consists of two separate water chambers to ensure a
proper execution of inspections and cleaning intervals.
The map detail is showing the left (green marked) water chamber of the potable water storage system, which was produced out of PE 100 pipes. In the middle (pink marked) there is the control building where the de-acidification machine is located. To the right of it (grey marked) there is the second masoned water chamber, which will be redeveloped with Hydroclick plates out of PE 100. The pipelines of the water chambers were guided through the control house so that the water chamber walls can be inspected at any time. The control house should be carried out in such a way that an easy handling and cleaning of the storage system can be ensured. The access to the water chambers must be secure and must allow an easy operation. The openings required must comply with the effective UVV regulations and shall be large enough to transport materials and equipment for repair and maintenance purposes through it. Special facilities for taking samples in the control house for any increasing and extraction line are most useful in order to ensure that operation is possible without the need of entering the water chambers. The aerator and venting facilities for the water chambers and the control house must be separated technically. The water chambers must be equipped with a bypass to connect the supply system with the discharge water system. The overflow must be designed in such a way as to enable the function of draining excessive water. Therefore the overflow has to be appropriately dimensioned and cannot be equipped with an isolating device. The surface of the stored water must be visible all the time in order to control the water chambers in the best way. To do this, it is reasonable to install special inspection glasses in between of the control house and the water chambers, different lightning options in the water chambers can also be installed. The VDE-regulations for “damp and wet environments” must be considered here.
Appropriate lightning protection equipment should also be considered.
The water chamber is a closed part of the potable water storage system with separate supply-, extraction,- overflow,- and draining devices which can be operated independent of other water chambers. The access to the water chamber should not be possible in filled condition so that a pollution of the drinking water can be excluded. Normally, it is possible to have access to the water chamber through the control house. The main controls and instruments, pumps and monitoring devices are also housed in the control house.
The size of the potable water storage system is determined by the summation curves of the supply and draining device and by the operating reserve. Water storage systems with a maximum daily demand of less than 1000 cubic meter, have a cubic capacity of 35% of the maximum daily demand according to the DVGW worksheet W 300.
As the currently largest diameter for helically wounded pipes is DN 3500 mm, the maximum active storage is approximately 9,2 cubic meter per meter. With two separated water chambers with a length of 35 m length each, there will be an active storage of 650 cubic meters of potable water. For stabilization of the water pressure a water depth of 3,5 meter was proven to be useful. Therefore lying PE 100 pipes with a diameter of DN 3500 mm are representing a very attractive alternative in comparison to other materials like concrete or GFK (glass fiber reinforced plastic).
The wetted surfaces which are needed for the construction of water chambers must be made of
materials which shall meet the test requirements. Especially for additives, which are required in the
use of concrete and cement mortar,it must be examined if they meet the requirements for potable
water storage systems. When plastics are used it is important to ensure that the plastics meet KTWrecommendations
whose suitability must be demonstrated from a microbial point of view according
to D VGW W 270.
The surfaces of the used materials need to be as smooth as possible and of the lowest possible porosity to ease subsequent cleanings and to avoid bacterial growth. With the use of PE 100 pipes and plates this demand will be accomplished. Mineral construction materials like concrete or cement mortar have to be high-quality coated subsequently or have to be lined. Similarly, corrodible metal parts must be protected accordingly to avoid contamination of the drinking water. In the planning and construction of water chambers special care is needed to ensure that there are no areas in which the water stagnates. A constant circulation of the water avoids the risk of scaling on the walls of the water chamber. Fluid flow caused by the inclined water is often already sufficient to produce adequate circulation and mixing. Here, round containers are providing aerodynamic benefits in comparison to angular containers as the covered surface is more even circulated at the same storage volume.
Ventilation installations in the water chambers are needed to allow air movements which are caused by changes in water level. The aerator- and venting facilities are being implemented for hygiene and taste-related reasons. Their dimensioning follows the outgoing through flow or rather the maximum limit for the air speeds in the ventilation facilities. Filtering and water straining equipment is highly recommended to prevent drinking water contamination. For this reason, the openings of the water chambers should not be above the free water surface. Two projects with receiver tanks which were carried out by the public utility company in Bühl, were equipped with efficient air-and air escape valves as the storage capacity of in total 100 m³ per day was handled up to 25 m³. The complete design and planning of the entire system was carried out by the engineering company Eppler in Bühl.
In order to ensure a static load carrying capacity of the storage systems, to facilitate the integration of storage systems in the landscape and to keep the maintenance costs as low as possible, the high above ground level should not exceed one meter. Permanent and variable impacts need to be taken into account with the static design of water tanks. Permanent impacts are for instance soil loads, pressure by groundwater, dead load of the building as well as the weight of the operational installations. The major variable impacts are for example the weight and the pressure of the stored water, snow loads, wind loads and impacts resulting from maintenance work. Drainages must be installed in the bottom and on the side of the tank in order to restrict impacts by existing ground water to a minimum. Thus, the water tank can be built on grounds with a sufficient load capacity. However, it must also be kept in mind that the ground is not contaminated in order to avoid pollution of drinking water through diffusion of toxic substances through the tank walls. The same applies to the filling material.
Prior to commissioning of potable water tanks, a tightness test as well as a cleaning and disinfection of the storage system must be carried out. The tightness test is considered passed if a visible water outlet is not determined and if there is no measurable decrease in water level within a testing period of 48 hours. During the cleaning of the storage tank the use of chemical cleaning products have to be limited to a minimum. The applied materials for the water tank must be free from any damages through the use of chemical cleaning products. The products also must be investigated toxicologically and for drinking applications before using. As Polyethylene features a good resistance against chemicals, there is no reason to fear restrictions in use in case of damage of the inner surface. The use of disinfecting agents should be organized in accordance with the EU directives as well as with the national and local regulations. Recommendations to this are stated in the standard DIN EN 805 “Requirements to water supply systems and their prefabricated parts outside of buildings”. The standard also describes the permitted disinfection methods of pipelines for water supply and how to disinfect potable water storage systems. After the disinfection the microbiological harmlessness must be proved. If the microbiological harmlessness has been proven, the disinfected potable water pipes or storage tanks must be put into operation as quick as possible in order to avoid further contamination.
Polyethylene represents a very good technical and economical alternative in comparison to the so far used materials in the field of new development of small and medium sized potable water storage systems. There are many advantages by using PE in such applications, like for example the long-life cycle, the very good chemical resistance, the low specific weight and the possibility of variable prefabrication in the workshop. All these advantages can lead to considerable time and cost savings.
After successful usage over decades of the raw material PE (approved by DVGW) for potable water piping systems, the material is also finding its way into new fields of application like for instance the new development of potable water storage systems. New areas of application for the material PE are also exploited through new innovations and developments by the industry.
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