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Why do we need water treatment?
Drinking water is the most important of all foodstuffs. It is irreplaceable. In ancient times "well poisoners" were mercilessly pursued and faced harsh penalties - often death.
Today water, and drinking water in particular, is protected by laws and ordinances. Public welfare demands comprehensive protection of the water supply to prevent contamination and other problems.
Difficulties with groundwater can arise particularly in densely populated areas, in which the catchment areas of water treatment systems are used with increasing intensity. Here, the natural protective and purification effects of the layers above the groundwater are no longer sufficient in all cases to maintain the quality of the groundwater in the long term.
So that sufficient drinking water of perfect quality is always available, it is therefore often necessary to fall back on impure water and to employ problem-orientated water treatment.
Assessing water quality
Depending on its natural origins, water can contain many different impurities. The Drinking Water Ordinance therefore defines the physical and chemical requirements for the water that is used to supply drinking water. However, simply measuring and assessing a few important sum parameters allows conclusions to be drawn about water quality and the usability of a particular water as drinking water.
Redox potential
In every reduction oxidation process of the type that also occurs in water, a state of equilibrium is established after a time. This state can be measured electrically: the redox potential is measured as an electrical voltage between the water and a reference electrode. If the redox potential is negative, reduced substances predominate. If the redox potential is positive, the oxidized substances in the water predominate.
In ventilated groundwater, the redox potential is, for example, between 200 mV and 300 mV, and in treated water it is between 700 mV and 900 mV.
A high redox potential means that the pure water now contains hardly any reduced and/or assimilable substances.
For example, with a redox potential of 740 mV and a chlorine concentration of 0.1 mg/l, water can be regarded as sufficiently disinfected - but not if it has a redox potential of 600 mV and a chlorine concentration of 0.6 mg/l. Provided that the system configuration is suitable, the redox potential can be used for the process control of a water treatment system.
Oxygen concentration
Low-oxygen ground water is known as reduced water. Before it can be used as drinking water, it must be treated by oxygen enrichment.
Adding oxygen to a reduced water results in divalent iron (Fe II) oxidizing into the trivalent form (Fe III), enabling it to be filtered out. The addition of oxygen also allows manganese-binding bacteria to accumulate in a downstream filter bed in which dissolved (divalent) manganese is oxidized to form low-solubility brownstone. Oxygen also allows the accumulation in the filter bed bacteria of the species nitrosomonas and nitrobakter, which oxidize ammonium to form nitrate.
Oxygen always has a positive effect on corrosion protection in pipelines. In order to form a sufficient lime rust protective coating in pipeline systems, the oxygen concentration should not fall below 5-6 mg O2/l.
Capability of oxidation (KMnO4 consumption)
The capability of oxidation (KMnO4 consumption) shows the behaviour of the water with respect to oxidisable substances such as oxygen, chlorine, chlorine dioxide, ozone, permanganate and chromate.
The capability of oxidation of the water is measured by the consumption of added potassium permanganate (KMnO4). A consumption rate of 4 mg/l KMnO4 corresponds to a consumption rate of 1 mg/l O2. The limit value for the oxygen consuming capacity is 5 mg/l O2.
This process for determining capability of oxidation reacts relatively unspecifically. Some substances oxidize completely, while others display inert behaviour or are only incompletely mineralised. In order to determine the water's organic substances concentration, it is therefore necessary to use other parameters. This is particularly the case if raw water with an anthropogene load is involved.
In addition to the analytical determination of the Dissolved Organic Carbon (DOC) and of the Total Organic Carbon (TOC), a UV absorption measurement can also be carried out at a wavelength of 254 nm.
SAC value at 254 nm
Numerous organic substances have absorption bands in the UV range. This property can be used to draw initial conclusions about oxidisable organic water impurities, particularly when the composition of the impurities is relatively constant.
The SAK value is rounded to 0.1 and is given in m-1. Drinking water regulations contain no statement on the SAK value at 254 nm. However, the SAK value correlates with the KMnO4 consumption and the DOC. Continuous SAK measurement allows statements to be made about the trends displayed by the DOC value pattern.
Coloration (436 nm)
Drinking water should be colourless. Coloration and turbidity are quality shortcomings. A high concentration of organic substances in the groundwater, particularly humates, colours the water yellow to yellowish brown. Sewage entering the groundwater or source water, or other physical and chemical impurities, also colours the water.
A colour measurement can be advantageous wherever surface water with a high humate concentration is used.
The colour of the water is measured in an absorption measurement in the visible range with a wavelength of 436 nm, and is output in m-1 (colour) or as a Hazen colour in mg Pt/l. The limit value is 0.5 m-1 or 20 mg Pt/l.
When used as a quality parameter in a water treatment system, coloration shows how effective the oxidation stage breaks down and oxidizes the substances colouring the water.
Turbidity (FNU)
Turbidity in the water arises as a result of mineral or organic solid particles.
Water with a high degree of turbidity is always grounds for concern from a microbiological point of view. Drinking water regulation demands that drinking water should not contain any undissolved substances. The limit value is 1.0 FNU (formazine nephelometric units) at the waterworks discharge.
Turbidity in the groundwater and source water in most cases occurs:
- if there is insufficient soil filtration
- when snow melts
- after prolonged heavy rainfall.
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If soils contain iron or manganese, undissolved substances can also be contained in water from springs.
In the case of UV disinfection, the turbidity may not exceed values of 0.2 - 0.3 FNU.
By means of a turbidity measurement, it is possible to regulate the dosing of flocculants and / or monitor filter stages.
It must be noted that very small gas bubbles can influence the turbidity measurement in saturated waters. |
Demands places on a modern water treatment system
The drinking water regulations demand that to obtain drinking water, it is only permissible to use bodies of water from which perfect water can be obtained on a long term basis.
This means that modern water treatment must provide drinking water that is equivalent to natural drinking water in terms of quality.
The main tasks of water treatment plants include:
- Hygiene processing (disinfection) of water contaminated by bacteria
- Removing all substances such as turbidities, iron oxide, manganese oxide etc. that represent a physical problem
- Oxidation of reducing substances such as NH4+, As3+, Fe2+, Mn2+...
- Removing the assimilable, organic carbon
- Removing humates and similar compounds
- Removing all highly molecular organic human impurities (CHC, Pesticides...)
- pH-adjustment (deacidification/neutralisation)
Humates, compounds similar to humates and other highly molecular organic components can be reduced by water treatment. However, since these two groups of materials are in most cases biologically persistent, they are first split by oxidation into biologically degradable fragments and are then mineralised in a biologically active treatment stage.
Following the biological mineralisation of the organic carbon compounds, the water is low in nutrients, which effectively reduces the risk of it being contaminated by germs on its way to the consumer.
The bacteriological safety of drinking water is defined by, among other things, the limit values for colony-forming units at 20 °C and 36 °C on its way to the consumer. The hygiene processing mainly also requires the removal of the nutrient potential during the water treatment. In water that is low in nutrients, this means that there is no need for chlorination even in more widely branched networks (example: the water supply for the city of Munich).
Disinfection of the water completes the treatment process. The water can be disinfected by a UV system installed at the filter outlet or by an additional oxidation stage using ozone.
During final oxidation with ozone, the following processes run simultaneously:
- Disinfection of the water
- Compensation of any oxygen deficit that may have arisen in the filter bed
- Oxidation of nitrite.
The so-called safety chlorination or transport chlorination contravenes both regulations and the Minimum Law, and is not permitted for the treatment of drinking water. This means that the addition of chlorine is restricted to emergency cases. Drinking Water Standards limit the concentration of low-molecular chlorine compounds (trihalogen methane) (THM) - typical reaction products of chlorination - to 50 µg/l.
The disinfection of water with UV light is a physical process. Under favourable conditions, the UV light damages the microorganisms in the water in such a way that they can no longer produce infection. However, the UV light does not change the water quality. Sufficient treatment is therefore necessary prior to disinfection of the water with UV light.
In a drinking water treatment system, raw water is treated in a process with active physical, biological and chemical mechanisms in such a way that at the end of the treatment process, drinking water is available that conforms to the requirements of Drinking Water Standards.
The stipulations standards also include that the available raw water must remain as natural as possible and that only the harmful substances may be removed or reduced to the smallest possible amounts during treatment.
The treatment of drinking water must not be equated with the treatment of process water, in which water is altered to meet industrial needs..
Natural drinking water treatment must be orientated towards the processes that also take place in nature for the purification of water. Three main process steps can be recognized here:
- Oxidation
- Filtration
- Hygiene processing/disinfection accompanying the process
What is known as the HYDROZON® process is a natural treatment method that employs the latest knowledge in order to make drinking water from raw water contaminated by geological or human sources. |
Overview
