Fundamentals to water qualities

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Chemical Oxygen Demand (mg/L) The Chemical Oxygen Demand (COD) indicates the amount of oxygen that is required for the chemical oxidation with potassium dichromate in the substances contained in water. [1]

Note: The COD is always higher than the BOD5; the ratio COD / BOD5 lies at 1.5 – 2 with well biologically degradable materials.

Biochemical Oxygen Demand 5 (mg/l) The Biochemical Oxygen Demand BOD5 is the amount of oxygen consumed in biochemically oxidizable organic substances contained in one litre of water in 5 days under the corresponding metabolic activity of the microorganisms at 20 °C in dark (in mg O2/l).

Note: If the BOD5 value attains a value of <10 mg / litre (organically biodegradable load), then it is no longer putrefactive under normal conditions. [2]

This value is critical if the water is to be stored for a longer period, without formation of any odour. (For instance, absence for few days).

The Total Organic Carbon TOC (mg/L) content is the amount of organic carbon compounds in a sample. These are burnt during the analysis and the amount of the resulting CO2 is measured. [1]

Note: The TOC is a sum parameter in the water analysis as well as run-off water analysis and reflects the contamination of water with organic matter. Clean spring waters indicate a TOC content of 1 - 2 mg/l. slightly contaminated rivers and streams show values around 2 - 5 mg/l. The mesotrophic lakes already have the values of 5 - 10 mg/l, and a productive carp pond has a typical value around 15 - 25 mg/l. The value many exceed over 100 mg/l in highly contaminated sewage water.

Spectral Absorption Coefficient 245 nm SAK (1/m) The UV absorption is a sum parameter for water pollution by dissolved organic substances such as aromatic compounds and humic substances. The Spectral Absorption Co-efficient and Extinction per meter is converted with a measurement wavelength of 254 nm. [1]

Note: Thus, Spectral Absorption Co-efficient 245nm corresponds as sum parameter for the inorganic salt contamination in approximate electrical conductivity. A number of organic substances absorb ultraviolet light. It is based on DIN 38404-3 (2006), which enables measurement at a wavelength of 254 nm. The light attenuation per meter of water (hence the dimension 1 / m) simultaneously functions as a measure of organic contamination of the water. Thus, it is also possible to convert the measured value into approximately related carbon parameters such as TOC (Total Organic Carbon), COD (Chemical Oxygen Demand) and BOD (Biochemical Oxygen Demand). (State Office for Nature, Environment and Consumer Protection, North Rhine-Westphalia).


As the water becomes colder, the amount of O2 that can be dissolved increases and decreases depending on the salinity or decrease in the atmospheric pressure. Examples:

0 °C, standard pressure, fresh water: 14.6 mg/l = 100% saturation
10 °C, standard pressure, fresh water: 11.3 mg/l = 100% saturation
20 °C, standard pressure, fresh water: 9.1 mg/l = 100% saturation

The following environments can be distinguished depending on the percentage of oxygen in the water:

  • Aerobic environment = Presence of dissolved molecular oxygen (O2) and chemically bound oxygen, > 0.5 mg O2/l
  • Anoxic environment = Presence of a nitrogen bound oxygen (for instance, NO3- ; NO2- ), < 0.5 mg O2/l
  • Anaerobic environment = neither dissolved molecule nor nitrogen bound oxygen is present, < 0.5 mg O2/l

If there is less oxygen in water, then it could be attributed to the presence of certain microorganisms for formation of unpleasant smells and toxic gases. In this process, the oxygen required for respiration of bacteria is separated from oxygen-containing compounds such as sulphates, sulphites, etc. which leads to the formation of sulphides and H2S in significant quantities and are released. [3]

Counter action: aeration, the content of oxygen must always be high in enough quantity.

Conductivity [µS/cm]

The chemically pure water with pH of 7 is dissociated only to a limited extent in the H3O+ and OH-electric charge carriers. Therefore, it has a specific high resistance of 18.2 M Ω •cm (= 1.82×1013 Ω •mm²/m) at 25 °C. This corresponds to a specific conductance value of 54.9 nS•cm−. Thereby the effect of variation of temperature is about 1.5 to 2% per Kelvin. Dissolved salts and acids intensify the concentration of charge carrier. Already the mains water reaches up to about 10,000 times the conductivity of an average of 500 µS•cm−1 depending on the mineral content. [4]

The pH value is a measure of the acidic or basic character of an aqueous solution. The pH value is a non-dimensional unit. The ionic product of water at 25 °C gives following results with autoprotolysis.

KW = c(H3O+) • c(OH−) = 10^(−14) mol²/l²

And divides the pH values of dilute aqueous solutions into:

  • pH < 7 as acidic aqueous solution, here it is cH3O+ > cOH−
  • pH = 7 as neutral aqueous solution, here it is cH3O+ = cOH−; along with a property of pure water
  • pH > 7 as basic (alkaline) aqueous solution, here it is cH3O+ < cOH−

Theoretically, chemically pure water at 22 °C has a pH value of 7 (the equilibrium constant for dissociation of water is about 10-14). This value is specified as chemically neutral. However, chemically pure water has no buffer and hence reacts to the slightest contamination with a significant change in pH value. Therefore due to CO2 solution, a pH value of 4.5 and 5 is immediately set in previously chemically pure water with admission of air. [4]

Turbidity is a unit measuring the proportion of finely divided particles and suspended solids in the water sample. It is determined at 860 nm wavelength. [1]

  • Generally, it deals with safe sediments, undissolved particles, also known as suspended particles
  • Dissolved matter, the microbes serve as a nutritive medium and thereby their growth is fostered
  • They prevent an effective disinfection (chlorine uptake, chlorine attaches to the particles); therefore, turbid water must be filtered first
  • Measuring unit of turbidity: NTU, according to WHO TW (turbidity value) should be <1 NTU (Katadyn Water Guide)

Heavy metals Lead, cadmium, chromium, nickel, copper, zinc, mercury are heavy metals. They are obtained via rainwater in groundwater which is filtered through the rock bed. Heavy metals can accumulate in the human body. (WHO Guidelines for TW [sticky limit]). Heavy metals can be removed from water only via complex processes such as distillation or flocculation.


  1. 1,0 1,1 1,2 1,3 ISA – Institut für Siedlungswasserwirtschaft der RWTH Aachen
  2. Hans Mönnighoff, 1993, Ökobuchverlag
  3. Diplomarbeit Schikowski, 10'1988, Prof. Olschewski
  4. 4,0 4,1 Wikipedia