Water contamination affects all life forms and is a critical concern today. Pollutants from human-induced as well as natural sources flow through groundwater and into drinking water.
Oxidation reduction potential (ORP) is an important method for testing water quality. Oxidation occurs when a substance lacking electrons — known as an oxidizing agent — looks for electrons from other substances. On the other hand, substances that have extra electrons to donate are known as reducing agents.
ORP classifies substances as oxidizing or reducing agents and is measured in millivolts (mV) by an ORP meter. A high ORP reading indicates the presence of an oxidizing agent. A low reading indicates a reducing agent. Water pollution levels tend to increase with low ORP readings and decrease with high readings. Scientists consider the ORP of water as an important indicator of pollution levels. Safe drinking water, for example, is an oxidizing agent with an ORP range between +200 and +600 mV. Chlorinated pool water, on the other hand, should have a much higher ORP between +650 and +750 mV.
Scientists, therefore, consider the ORP of water an important tool and indicator when evaluating water quality. It is useful in testing a wide range of water systems, including groundwater, municipal tap water, and wastewater treatment systems. It is also used in combination with other testing methods that measure biological changes in the environment.
ORP and Surface Water Infiltration of Groundwater
Surface water infiltration affects the quality of groundwater. Waste and other pollutants often have their first point of entry in surface water, where they seep into groundwater. In a reductive environment, metals such as arsenic and iron are released from minerals in the aquifers and contaminate groundwater. Low ORP readings are therefore a strong indicator of water contamination in this situation. Arsenic and iron can also originate from industrial waste, mining and other human activities. Metal corrosion is another source of iron.
Depending on the pollutant, a specific ORP level is required to maintain water quality. For example, the removal of phosphorus requires an ORP between +25 and +250 mV. The removal of nitrogen, known as nitrification, occurs at ORP levels between +100 and +350 mV. Phosphorus and nitrogen often originate from agricultural sources, such as feedlot waste, and fertilizers. On the other hand, pollution increases at lower ORP levels. Biological phosphorus levels increase at -100 to -250 mV, and methane production occurs at -175 to -400 mV.
Sometimes, however, conditions other than the ORP of water can affect the infiltration of groundwater. For example, manganese salts can leach into groundwater from a nearby reservoir, increasing manganese levels even in an oxidizing environment. It can also originate from mining and industrial waste. It is important, therefore, to use ORP along with other factors in determining water contamination levels.
ORP in Wastewater Treatment Optimization
Wastewater treatment facilities maintain acceptable levels of certain substances including nitrogen and phosphorus in water. The treatment process requires very low levels of dissolved oxygen (DO) so that bacteria can remove excess nitrogen and phosphorus from the water. An environment with little or no DO is known as an anoxic environment. Water aeration occurs during the treatment process, and must periodically stop for the proper length of time to maintain anoxic conditions.
Taking ORP readings during wastewater treatment is a useful tool in determining whether conditions are right for nitrogen and phosphorus removal. Although a DO reading may indicate anoxic conditions, the ORP level may still be too high. A delay in aeration may be necessary to give ORP levels of water time to decrease. When ORP levels are low enough, the bacteria can then do their job. A combination of DO and ORP meter readings can measure environmental conditions more accurately than DO readings alone.
ORP readings can help scientists evaluate the overall effectiveness of wastewater treatment systems. For example, high ORP readings can indicate whether contaminants from wastewater bypasses are entering a river. A 2016 study published in the Mesopotamia Environmental Journal found low ORP readings near two bypass points on the Douala River near Baghdad, Iraq. Other readings that are typically used in wastewater treatment optimization are biochemical oxygen demand (BOD), total suspended solids (TSS), and pH levels.
ORP in Groundwater Quality Characterization
ORP is an important factor in determining groundwater quality, particularly in detecting metallic contaminants. It is a useful tool for groundwater that has high levels of iron, arsenic and other metals. These metals may enter the groundwater system from natural sources, such as rocks and sediment. Other metals have human-induced sources, such as products of industrial waste, feedlots, and landfills.
The “redox potential” of groundwater refers to its capacity for oxidation or reduction. Groundwater ORP readings typically range between -400 mV and 800 mV. ORP levels help evaluate groundwater that has high levels of oxidizing substances such as chlorine and reducing substances such as sulfite ions. It is important, however, to consider other factors, such as the breakdown of biological matter in the groundwater system. For example, a low ORP reading between -100 and -400 mV may indicate the release of biological phosphorus or acid from fermentation.
Importance of ORP Water Contamination Testing
Water contamination comes from a variety of human-induced and biological sources. ORP is a valuable indicator on its own and when used with other tools. It is important to understand the overall environment of the water being tested since factors other than ORP can affect water quality. For example, the location of nearby reservoirs, farms, or rock formations can affect pollution levels.
On the other hand, factors such as DO and pH often must be analyzed in light of ORP readings. When used along with ORP, other testing methods help determine the effectiveness of wastewater treatment systems. A combination of ORP and DO levels, for example, can help engineers determine whether aeration should be delayed in wastewater treatment.
ORP water contamination testing evaluates the health of groundwater and its environment. It helps scientists and engineers evaluate and optimize wastewater treatment systems. Whether it’s used for testing tap water, groundwater or chlorinated pool water, ORP helps keep our water safe for public use.
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