ORP in Wastewater Treatment: What Redox Potential Tells You and How to Use It
ORP stands for oxidation-reduction potential, also known simply as redox potential. It is the difference between the redox potential sensed by the indicating electrode and that of the reference electrode in a liquid, and it gives one composite index for the redox state of the entire system.
A low ORP value means that the wastewater system holds a high concentration of reducing substances or organic pollutants and a low concentration of dissolved oxygen — a reducing environment dominates. A high ORP value means the opposite: little organic pollutant, abundant dissolved oxygen or oxidising substances, and an oxidising environment in control.
Why Bother Measuring ORP
Traditional redox-based water treatment has well-known shortcomings: the control conditions are not precise enough, chemicals are wasted and the process is not always environmentally friendly. With an ORP instrument, the ORP electrical signal itself becomes the measurement and control signal, which sharpens the precision of redox control considerably — and therefore improves treatment performance.
The measuring principle is similar to that of pH, and many online pH meters already offer a two-channel design in which one channel is dedicated to ORP. In short, ORP is an important direction for automatic control in wastewater treatment plants and for precise control of anaerobic processes: it saves energy, helps steer the metabolic pathway of anaerobic micro-organisms and improves treatment results.
Why ORP Is Hard to Read Directly
Many redox reactions occur in wastewater treatment, and the factors that influence ORP differ from one reactor to the next, so it is rarely possible to say which factor acting on which substance produced a given shift in ORP. In an activated sludge system, for example, a large number of organic substances are present; a considerable change in organic concentration may produce only a small change in ORP, and it is difficult to identify which organic substance caused it. Before studying what a change in ORP indicates about treatment, therefore, it is necessary to know which factors move ORP in the first place.
1. Dissolved Oxygen (DO)
DO is the concentration of oxygen dissolved in water. In an aerobic tank the DO at the outlet should be held at 2 mg/L, or 4 mg/L where pure oxygen is used for aeration. In an anoxic denitrification tank DO should be 0.5 mg/L. In an anaerobic tank molecular oxygen is essentially absent, and nitrate nitrogen should preferably stay below 0.2 mg/L.
As an oxidant in wastewater treatment, DO is the most direct cause of a rise in system ORP. In pure water, ORP is linear against the logarithm of DO, and ORP rises as DO rises.
2. pH
pH is a key control factor in wastewater treatment. Aerobic micro-organisms and acid-producing fermentative bacteria grow best at pH 6.5 to 8.5, while anaerobic methanogens are happiest at pH 6.8 to 7.2. The appropriate pH is normally held by dosing alkali.
Microbial metabolism of pollutants strongly affects pH. In the acidogenic stage, acid-producing bacteria break down large organic molecules into fatty acids and carbon dioxide, which lowers pH; but the ammonia released while proteins are broken down raises it. In the methanogenic stage, methanogens consume acetic acid to produce methane, which raises system pH.
pH is an important driver of ORP movement: the higher the pH, the lower the ORP; the lower the pH, the higher the ORP. Note, however, that although pH and ORP correlate to some degree in wastewater, ORP is also affected by microbial activity and dissolved oxygen, so the correlation is weaker than it is in pure water.
3. Temperature
Temperature is a very important parameter in wastewater treatment. Aerobic micro-organisms are vigorous between 15 and 30 degrees Celsius; anaerobic micro-organisms work best near 35 degrees Celsius and near 55 degrees Celsius.
In anaerobic treatment, a change in temperature affects the composition and multiplication of the microbial population, the methanogenic rate and the settleability of the sludge. To keep the anaerobic tank stable, wastewater is therefore cooled in a cooling tower or heated with steam before it enters, bringing it to 35 or 55 degrees Celsius.
Practice and research show that the higher the solution temperature, the lower its ORP, and the same holds in wastewater treatment. Part of the reason is that warming reduces the size of water-molecule clusters. Temperature also shifts acidity and alkalinity, gas solubility, biological activity and the inter-phase equilibria of pollutants, all of which feed back into ORP.
4. Composition of the Microbial Community
Biological wastewater treatment systems contain a distinctive ecosystem. In a two-phase anaerobic reactor, acid-producing bacteria and methanogens are effectively separated, which makes the system easier to control and manage. In a UASB reactor dominated by flocculent sludge, acidogens and methanogens are screened out in sequence along the direction of flow. In anaerobic granular sludge and anaerobic biofilm, the dominant population changes from acidogens on the outside to methanogens towards the inside.
In an anaerobic system the DO concentration and the ORP must both be kept very low; in the methanogenic stage in particular, the redox potential must not exceed minus 330 mV. Some DO inevitably enters with the influent, but under this distinctive ecosystem, and through the synergy and symbiosis between aerobic, facultative and anaerobic micro-organisms, system ORP quickly falls into the range that suits methanogens. This low redox potential is not confined to anaerobic reactors: it also appears inside the flocculent sludge of aerated tanks.
5. Microbial Activity
The activity of anaerobic activated sludge can be expressed by the maximum specific methanogenic rate and the maximum specific COD removal rate; the activity of aerobic activated sludge can likewise be expressed by the maximum specific COD removal rate. The more active the biomass, the faster it consumes oxygen and produces reducing substances, and the faster ORP falls.
ORP is a composite index of the macroscopic redox character of a water body, and it has many influencing factors. Besides the five main ones above, pressure, organic matter, solids and the species of micro-organism all play a part. These factors are not independent: they interact and constrain one another, so the redox character of a water body is the result of several factors acting together.
Where ORP Values Sit in a Treatment Train
Redox potential was first applied to industrial wastewater treatment, especially wastewater from metal finishing, and later spread into municipal wastewater treatment plants. A wastewater system contains several variable-valence ions and dissolved oxygen, that is, several redox couples. An online ORP instrument detects the redox potential within a very short time, with no need for laboratory sampling, which shortens the analytical cycle dramatically and raises working efficiency.
The important redox reactions in a wastewater system include the biological degradation of carbonaceous, nitrogenous and phosphorus-containing organic pollutants, the hydrolysis and acidification of organic matter, nitrification and denitrification, anaerobic phosphorus release and aerobic phosphorus uptake.
1. Each treatment stage needs a different redox potential. Aerobic micro-organisms generally grow above plus 100 mV, with an optimum of plus 300 to plus 400 mV. Facultative anaerobes carry out aerobic respiration above plus 100 mV and anoxic respiration below it. Obligate anaerobes require minus 200 to minus 250 mV; obligate anaerobic methanogens require minus 300 to minus 400 mV, with an optimum of minus 330 mV. The normal redox environment of an aerobic activated sludge system lies between plus 200 and plus 600 mV. The ORP ranges that suit the common biological reactions in wastewater treatment are summarised in the table below.
Typical ORP ranges for the common reaction stages of biological wastewater treatment.
Using ORP as a Control Strategy
2. Control parameter for aerobic, anoxic and anaerobic treatment. By monitoring and managing the ORP of the wastewater, operators can steer which biological reaction takes place. They change the environmental conditions of the process — increasing the aeration rate to raise dissolved oxygen, dosing oxidising substances to raise the redox potential, reducing aeration to lower dissolved oxygen, or dosing carbon source and reducing substances to lower the redox potential — and so promote or suppress a given reaction. ORP thus becomes a control parameter for aerobic, anoxic and anaerobic biological treatment, and better results follow.
Aerobic treatment. ORP correlates well with COD removal and with nitrification. Controlling the aeration rate through ORP avoids both under-aeration and over-aeration and secures effluent quality.
Anoxic treatment. ORP correlates to some degree with the nitrogen concentration of the denitrification state, so it can serve as a criterion for judging whether denitrification is complete. Practice shows that when the derivative of ORP with respect to time is below minus 5, the reaction is fairly complete. Residual nitrate nitrogen in the effluent also prevents the formation of toxic and harmful substances such as hydrogen sulphide.
Anaerobic treatment. Whenever reducing substances are produced during anaerobic treatment, ORP falls; when they diminish, ORP rises again and then stabilises over a period.
In short, for the aerobic stage ORP correlates well with the biological degradation of COD and BOD and with nitrification. For the anoxic stage it correlates with the nitrate nitrogen concentration of the denitrification state, which makes it a usable criterion for the end of denitrification.
3. Controlling the Biological Phosphorus Removal Stage
Biological phosphorus removal has two steps. The first is phosphorus release under anaerobic conditions: at an ORP of minus 100 to minus 225 mV, fermentative bacteria produce fatty acids, which polyphosphate-accumulating organisms (PAOs) take up while releasing phosphorus into the water. The second is in the aerobic tank, where PAOs degrade the fatty acids absorbed in the previous stage and convert ATP to ADP for energy; storing that energy requires them to take up excess phosphorus from the water, and the uptake reaction needs an ORP of plus 25 to plus 250 mV in the aerobic tank before biological phosphorus storage can occur.
Operators can therefore regulate the phosphorus removal stage through ORP and improve phosphorus removal performance. Where denitrification or nitrite accumulation must be avoided during nitrification, the ORP has to be kept above plus 50 mV. By the same logic, to prevent odour (hydrogen sulphide) in a sewer system the ORP in the pipe must be held above minus 50 mV, which stops sulphide formation and reaction.
4. Tuning Aeration Time and Intensity to Save Energy
Because ORP correlates markedly with dissolved oxygen in the water, operators can also use ORP to tune aeration time and aeration intensity, meeting the needs of the biological reaction while saving energy and reducing consumption.
In summary, ORP measurement is simple, the instruments are inexpensive, accuracy is high and the readings are displayed in real time. Through online ORP monitoring, operators can grasp the state of the purification reaction and of water pollution from live feedback, and so achieve fine-grained management of the treatment process and efficient management of water environment quality.
As noted above, however, many redox reactions occur in wastewater treatment and the factors influencing ORP differ between reactors. In practice, therefore, operators still need to study how dissolved oxygen, pH, temperature and salinity relate to ORP in their own plant, and to establish ORP control parameters suited to each water body.
