The methods of filtration used for water treatment vary greatly. Filtration includes all processes that mechanically separates a mixture of liquid and solid or dissolved substances into liquid (filtrate) and concentrated substances (retentate). The selection of the filtration process or multiple process steps is determined by the quality of the raw water and the requirements for the pure water (drinking or process water). The minimum size of the solids to be held back is defined via the separation limit of the filter.
Problem substances occurring often in raw water are:
- Sand, solids, turbidities and particles, broken out incrustations from pipelines
- Dissolved substances such as iron, manganese and arsenic
- Aggressive carbonic acid
- Pesticides, chlorinated hydrocarbons (CHCs) etc.
- Dissolved salts (sea water)
- Impurities from circulation of water (e.g. condensate/steam cycle)
- Accumulation through concentration at circulation water
Filtration via activated carbon or other adsorbing filter materials (adsorption) and chemical deacidification are also classed as filtration processes.
This overview shows a rough correlation of the filtration processes and their use depending on the separation limits.
When using drinking water from the community water supply in large quantities, the raw water from kilometres of pipe networks can be contaminated with rust, sand, broken out incrustations and other solids. These solids can be filtered out with cartridge filters.
Cartridge filters are not suitable for removing larger quantities of dirt. In case of large quantities of dirt, the agglomeration of solids on the cartridge surface can lead to a rapid increase in the differential pressure.
In nanofiltration and reverse osmosis, cartridge filters are normally used as protective filters.
Gravel filtration and multi-layer filtration
In contrast to cartridge filtration, gravel filtration, in particular the multi-layer filtration variant (filter gravel + filter carbons), represents a deep-bed filtration.
With multi-layer filtration, the water passes through various layers of filter material with increasing fineness in the direction of filtration. Dirt is agglomerated in the various layers of the filter depending on its size.
Multi-layer filters can take up large quantities of solids. Gravel filtration or multi-layer filtration, in conjunction with flocculation, is used in particular in the treatment of river water. In these cases filtration is often preceded by a sedimentation stage as the initial process step.
Gravel and multi-layer filters are cleaned regularly via back-flushing.
Pressure filter plants
In addition to single-layer and multi-layer filtration, pressure filters are used for iron removal, manganese removal, deacidification and adsorption.
Ozone bio-filtration is a natural method for treating water containing humates. The method uses the biological mechanisms in the filter bed for reduction of TOC / DOC. With ozone as an oxidising agent the organic carbon compounds with a high molecular weight are split, colouration is massively reduced and the concentrations of the compounds with a low molecular weight are increased. Some of the compounds created in this way are biologically available and can be degraded in the downstream bio-filtration stage.
This method is especially environmentally friendly as - in contrast to nanofiltration systems or reverse osmosis - almost no substances which are harmful to the environment are created and energy consumption is minimised.
Membrane filtration techniques are used for many different applications in water treatment. The following processes are grouped together under membrane filtration:
- (MF) Microfiltration
- (UF) Ultrafiltration
- (NF) Nanofiltration
- (RO) Reverse osmosis