It is well known that in the practice of the drinking water as well as of the wastewater treatment especially for the processes of pH-regulation, flocculation, precipitation or disinfection, the dosing of reagents is necessary. The costs of the automatic dosing stations are comparatively high. For the smaller water treatment plants these stations are uneconomical and charged with maintenance problems very often. In many cases the frequently observed highly fluctuating reagents needs and the recess between can cause disruption of the normal exploitation work. These disadvantages can easily be overcome by a device based on permeable or semi-permeable membranes. The use of semi-permeable membranes is favourable especially in the cases of fluctuating water quantities or standstills. It can be effective because of the possibility for their self-regulation during the dosing. When there is no inflow available some kind of concentration equilibrium is established between the internal and external membrane layers. This system is easy to install and it can be easily adapted to the local conditions. The device can be a canister filled with the desired reagent and covered with a lid of a definite semi-permeable membrane. After determining the permeability of the membranes made of different materials only the appropriate membrane area is important to be determined. Developed methods for defining the specific membrane permeability are discussed in the paper. Suggestions for the application of such devices in practice are given as well.

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