Analysts determine water quality by testing for specific chemicals. Most often, the type of water being tested determines what parameters, or analytes, the analyst looks for. For example, chlorine is an important parameter in finished drinking water, but is not usually a factor in natural water. This section lists common water quality parameters important in drinking water, wastewater, and natural water. Many parameter listings include descriptions of the effects of analyte levels on living organisms. 


Ammonium (NH4+)

Ammonium (NH4+) — or its uncharged form ammonia (NH3) — is a form of nitrogen which aquatic plants can absorb and incorporate into proteins, amino acids, and other molecules. High concentrations of ammonium can enhance the growth of algae and aquatic plants. Bacteria can also convert high ammonium to nitrate (NO3-) in the process of nitrification, which lowers dissolved oxygen.

Blue-Green Algae

Blue-green algae (BGA) is actually cyanobacteria, and can range in colors from blues, greens, reds, and black. BGA can reduce nitrogen and carbon in water, but can also deplete dissolved oxygen when overabundant. Monitoring BGA is important because they pose a serious threat to water quality, ecosystem stability, surface drinking water supplies, and public health through toxin production and the large biomass produced in algal blooms.

Biochemical Oxygen Demand – BOD

Biochemical oxygen demand, or BOD, measures the amount of oxygen consumed by microorganisms in decomposing organic matter. BOD also measures the chemical oxidation of inorganic matter (i.e., the extraction of oxygen from water via chemical reaction). A test is used to measure the amount of oxygen consumed by these organisms during a specified period of time (usually 5 days at 20 C). The rate of oxygen consumption is affected by a number of variables: temperature, pH, the presence of certain kinds of microorganisms, and the type of organic and inorganic material in the water. BOD directly affects the amount of dissolved oxygen in rivers and streams. The greater the BOD, the more rapidly oxygen is depleted in the stream. This means less oxygen is available to higher forms of aquatic life. The consequences of high BOD are the same as those for low dissolved oxygen: aquatic organisms become stressed, suffocate, and die. Sources of BOD include topsoil, leaves and woody debris; animal manure; effluents from pulp and paper mills, wastewater treatment plants, feedlots, and food-processing plants; failing septic systems; and urban stormwater runoff. BOD is affected by the same factors that affect dissolved oxygen. BOD measurement requires taking two measurements. One is measured immediately for dissolved oxygen (initial), and the second is incubated in the lab for 5 days and then tested for the amount of dissolved oxygen remaining (final). This represents the amount of oxygen consumed by microorganisms to break down the organic matter present in the sample during the incubation period.


The chloride (Cl-) ion is a component of salt found natuarally in minerals and oceans. Chloride salts are typically soluble in water, and high concentrations of chloride in water bodies can negatively impact fish and aquatic communities. the medium.


Chlorophyll in various forms is bound within living cells of photosynthetic organisms, such as phytoplankton and cyanobacteria (blue-green algae). The amount of chlorophyll found in a water sample is used as a measure of the concentration of phytoplankton. These measurements contribute to the understanding of the general biological “health” of the system, such as its trophic status or primary production. Chlorophyll measurements can also identify algal bloom events and their effects on water quality and anticipate toxic algal blooms. Chlorophyll fluoresces when irradiated with light of a particular wavelength (435-470 nm). For field measurements, in-situ fluorometers induce chlorophyll to fluoresce by shining a beam of light of the proper wavelength into the water and then measuring the higher wavelength light which is emitted. These real-time chlorophyll measurements complement extractive lab analysis.


Electrical conductivity is an indicator of water quality. Conductivity data can determine concentration of solutions, detect contaminants and determine the purity of water. Conductivity sensor usually measures conductivity by AC voltage applied to nickel electrodes. These electrodes are placed in a water sample (or other liquid), where the current flows through the electrodes and the sample. Current level has a direct relationship with the conductivity of the solution.


Depth of water is measured by a non-vented strain gauge. A differential strain gauge transducer measures pressure with one side of the transducer exposed to the water and the other side exposed to a vacuum. Depth is calculated from the pressure exerted by the water column minus atmospheric pressure. Factors influencing depth measurement in water include barometric pressure, density, and temperature. Calibration in the atmosphere zeros the sensor with respect to the local barometric pressure. A change in barometric pressure will result in errors unless recalibrated or compensated.

Dissolved Oxygen

Every species on our planet depends on water and oxygen. For aquatic species, adequate dissolved oxygen is of prime importance to their continued survival. Since dissolved oxygen levels are directly related to good water quality, the two are highly interdependent. Many factors can affect DO levels, and an understanding of these levels in order to make informed decisions concerning wastewater treatment operations, hypoxic zones, aquaculture facilities or large-scale ecosystems is essential.


Nitrate (NO3-) forms in water when bacteria use dissolved oxygen to oxidize ammonium. Nitrate is mobile and may seep into streams, lakes and estuaries from ground water enriched by animal or human wastes or commercial fertilizers. High concentrations of nitrate can enhance the growth of algae and aquatic plants.

Oil – Hydrocarbon

Hydrocarbon monitoring in water is of growing importance near oil platforms, marinas, shipping channels, mines, and oil-spill affected waters. For oil monitoring and mapping applications, a crude oil sensor with a near UV LED source and a refined fuels sensor with a deep UV LED source are useful. The crude oil sensor can detect oil throughout the water column in offshore monitoring. Closer to shore, where colored dissolved organic matter (CDOM) concentrations can be high and cause interfere with crude oil readings, a refined fuels sensor is recommended.

Photosynthetically Active Radiation – PAR

PAR measures irradiance–or the amount of sunlight or ambient light that diffuses through water compared to surface light. PAR focuses on the dynamics of the photic zone, typically 1-5 meters below the surface, and leads to an understanding of photosynthesis, toxic algae blooms, and eutrophication (nutrient loading). Scientists studying phytoplankton and photosynthesis and measuring chlorophyll need to measure PAR in order to resolve the absorption rate. High levels of PAR can indicate photoinhibition (limiting photosynthesis in shallow waters), which affects submerged aquatic vegetation and certain aquatic species.


Rhodamine is a red dye which measures the TOT (Time of Travel) for surface and ground water. It indicates how water moves, and is useful for tracing pollutants, studying aeration and dispersion, as well as waste buildup and flushing in estuaries and bays and waste water retention and flushing in wetlands. Rhodamine fluoresces when irradiated with light of a particular wavelength. For field measurements, in-situ fluorometers induce rhodamine to fluoresce by shining a beam of light of the proper wavelength into the water and then measuring the higher wavelength light which is emitted.


Temperature of water is one of its most basic properties, and many other parameters depend on temperature for accuracy. With temperature data, we can monitor thermal loading or discharge and determine changes in the thermocline, which affect the health of aquatic species and organisms. Many aquatic organisms are sensitive to high temperatures. The solubility of oxygen is lower in warmer water, thus limiting oxygen supply.


Turbidity is the measurement of water clarity. Suspended sediments, such as particles of clay, soil and silt, frequently enter the water from disturbed sites and affect water quality. Suspended sediments can contain pollutants such as phosphorus, pesticides, or heavy metals. Suspended particles cut down on the depth of light penetration through the water, hence they increase the turbidity — or “murkiness” or “cloudiness” — of the water. High turbidity affects the type of vegetation that grows in water.


The Oxidation-reduction potential, ORP in short, is a measure for the concentration of oxidizing and reducing agents in water. Its value is influenced both by pH and temperature. ORP is a sum parameter that gives no information on the concentration of a single substance in a mixture. ORP measurements are used to monitor chemical reactions involving electron transfer. In drinking water treatment it can be found in Ozone treatment and the removal of iron, manganese, and nitrate as well as in disinfection steps. ORP measurements are used as a hygiene parameter and decrees maximum and minimum values for fresh water, pool water, and salt water. In waste water treatment ORP is measured in the denitrification process and in detoxication of industrial waste water.


The calcite solubility is the amount of calcite a water sample of a defined volume can dissolve at 25°C. Calcite is the dominant modification of calcium carbonate under the temperature and pressure conditions usually found in water treatment plants. Responsible for the calcite solubility is excess carbonic acid, which can even cause corrosion of metal pipes or cement-based materials. An important measure to prevent corrosion is the deacidification, i. e. the physical or chemical removal of excess carbonic acid, which leads to a rise in pH. The upper limit for the pH is defined by the carbonate balance: If the pH is raised above the balance value, scaling will occur, causing problems both in the water conduits and in the treatment processes. The former German drinking water ordinance – TrinkwV – listed the delta-pH value with a set point of 0 to ensure that the treated water is in a balance state. The current drinking water ordinance lists the calcite solubility instead. The water is sufficiently deacidified if the calcite solubility at the outlet of the treatment plant is not higher than 5mg/l. According to TVO this condition is fulfilled if the pH-value is higher than 7.7. Excess carbonic acid can be removed physically by means of a stripping process using aeration. With high-performance aeration systems, the aeration efficiency can be changed by adjusting the volumetric flow rate of the air. The flow rate is adjusted to react to changing carbonic acid concentrations caused, for example, by changing water sources, mixtures, or volumes. A variety of parameters can be used as command variable for an automatic adjustment. Besides the water volume flow and the pH value, the delta-pH value is a very useful parameter for precise deacidification control. It is an expression for the deviation between the current pH value of the water and the pH value of the same water at calcite saturation. The delta-pH value allows a both technically and economically optimized operation of the aeration process. Since the energy consumption of the aeration system is proportional to the square of the air volume, a precise control provides a very noticeable saving of energy and therefore cost.


The pH value is an intrinsic value of all aqueous solutions: Each aqueous solution has a pH value. At 25°C pure water contains H+ and OH- ions in equal amounts.This state is called neutral with a pH value of 7. The pH scale covers values between 0 and 14. Acids have pH values towards 0, caustic solutions have pH values towards 14. The pH value is important in almost any area concerned with water: The European drinking water directive demands a pH value of 6.5 – 9.5 for drinking water both for hygienic and technical reasons. Water with pH lower than 6.5 can cause corrosion of metal pipes and give rise to high concentrations of heavy metal ions in drinking water. Corrosion is also a very important issue for cooling towers and boiler feed water and in fact for any water in contact with metal pipes and tanks. For swimming pools hygiene and health are the main reason for restricting the pH value to 6.8 – 7.6. Too high values affect the disinfection process, and too low values may lead to skin irritation. Industrial waste water has to be neutralized prior to release into the sewer system. In waste water treatment the pH value is controlled to remove heavy metals or to optimize ORP reactions such as Chromate reduction, followed by neutralization. In process water the pH value is one of the most important parameters. It can affect product quality and production efficiency. It might even be vital for a chemical reaction such as polymerisation.

Hydrogen Peroxide

Hydrogen peroxide as a pure substance is an explosive liquid. Commercially available are more stable colourless solutions of approx. 30% H2O2 in water. Light and catalysts such as manganese dioxide, platinum, or even blood or dust cause hydrogen peroxide to decompose and release oxygen gas, which is why it is sometimes called “active oxygen”. Hydrogen peroxide is a weak acid and reacts as an oxidizing agent towards most substances. However, its oxidizing strength is much less than that of the other common disinfectants, and the concentrations used for disinfection accordingly much higher. Its effect is due to the production of highly reactive oxygen, with the advantage that there is no disinfection byproducts and no disagreeable odour. Since hydrogen peroxide decomposes in contact with solid materials, leaving only oxygenated water, waste water from rinsing and disinfecting of pipes and tanks can easily be discarded on the ground without risk for health or environment. This and the easy use of the liquid make it a popular disinfectant despite the high costs and comparatively low efficiency. Hydrogen peroxide can be used as a bleaching agent in pulp industries. In food&beverage industries it is used to disinfect PET bottles in aseptic facilities. In waste water it can be used to oxidize specific ingredients. It may be used as an oxidizing agent with severe restriction of the residual concentration. On the other hand, it is a popular disinfectant for drinking water pipes and tanks. In medical applications hydrogen peroxide is widely used for disinfection due to its good compatibility and odourless and colourless application.


Ozone is an instable molecule of three Oxygen atoms and a very strong oxydising agent. At room temperature it is a gas. Due to its instability it cannot be stored in pressurised cylinders and has to be prepared on site. Ozone is an eco-friendly disinfectant. However, its great disinfection strength can only be used to good advantage in suitable reactors with a sojourn time of at least three minutes. The long-term effect of Ozone is only a few minutes. Ozone is commonly used for the treatment of drinking water and swimming pool water, to reduce the organic load, often in combination with filtration, and to remove inorganic substances such as iron and manganese. Ozone is also used for the disinfection of process water, air washers and air conditioning. In industrial applications it is sometimes used to detoxicate difficult waste water containing chelated metals and cyanides.

Chlorine Dioxide

Chlorine dioxide is an instable, non-storable, toxic gas with a characteristic scent. The molecule consists of one Chlorine atom and two Oxygen atoms – represented in the chemical formula ClO2. It is very reactive. To avoid the risk of spontaneous explosions of gaseous Chlorine dioxide or concentrated solutions, it is generally handled in dilution with low concentrations. Chlorine dioxide is soluble in water, but tends to evaporate quickly. Typically it is prepared on site, for example from hydrochloric acid and sodium chlorite. The procedure provides solutions with approx. 2 g/l ClO2 that can be safely handled and stored for several days. The disinfection effect of Chlorine dioxide is due to the transfer of Oxygen instead of Chlorine, so that no chlorinated byproducts are formed. Chlorine dioxide is used as disinfectant against biofilm, bacteria, spores, and viruses. Today it is believed that the molecule´s unpaired electron is transferred to the DNA of the microorganism which cracks and causes cell necrosis. Chlorine dioxide has a long-term effect of several days. In contrast to Chlorine, the disinfection strength of Chlorine dioxide does not depend on pH, and neither does the measurement show a pH influence in the range of 6-9 pH. Chlorine dioxide is used as a disinfectant in cooling towers and air washers, in cleaning processes for bottles, fruit and vegetables, and in bleaching processes for paper and textiles.


The expression „free Chlorine“ represents Chlorine dissolved in water, and that covers three different Chlorine compounds that form depending on pH: Chlorine as Cl2 gas can only be found in acidic solutions. With increasing pH Chlorine reacts with water to form Hypochloric acid – HOCl. At pH 2 and higher almost all Cl2 has reacted to HOCl. At approx. pH 6, neutralisation starts, and the Hypochloric acid is transformed into Hypochlorite ion – ClO-. At pH 9 and higher almost all Hypochloric acid has turned into Hypochlorite salt. Set aside from the free Chlorine is the organic bound Chlorine that results from reaction of Chlorine with nitrogen- or carbon-containing substances, often as unwanted disinfection-byproducts. Most prominent among these are monochloramine and trichlormethane. The pH-effect described above has consequences for the disinfection strength: with increasing pH the disinfection strength decreases. Hypochloric acid is a hundred times more efficient than Hypochlorite salt. On the other hand, due to the chemical reaction with water and the formation of Hypochlorite salt, Chlorine can be stored in alkaline solutions for quite long periods and shows a long-term effect of several hours. Chlorine is used as disinfectant, for example for swimming pool water, and is part of many household detergents. The chemical industries use Chlorine both for processes and and for products, such as PVC, bleaching agents, for the synthesis of polycarbonate.