For friends who work in water treatment or biochemical reactions, when it comes to ORP (oxidation-reduction potential), they may feel overwhelmed - this thing is invisible and intangible, with values jumping back and forth. Sometimes, even though the indicators seem to be correct, when ORP collapses, the entire system will have problems. In fact, there is no need to treat ORP as a "mysticism". Its essence is the "thermometer" of the "redox environment" in the biochemical system. To control ORP is to create comfortable "living conditions" for microorganisms and let them work well. Today, let's talk in plain language about how to control ORP, from "why control it" to "how to operate it specifically". Let's explain step by step.
Firstly, we need to understand: what exactly is ORP? We don't need to remember the technical terms "electron transfer potential energy". Simply put, a high ORP value indicates that there are "more oxidants" in the system and the environment is biased towards "oxidation"; A low value means "more reducing agents" and an environment that tends to be "reducing". And the microorganisms in the biochemical system are the "masters of choosing the environment" - aerobic bacteria prefer environments that are biased towards oxidation (ORP is generally positive by tens to hundreds of mV), anaerobic bacteria have to work in strong reducing environments (ORP is usually negative by hundreds of mV), and even facultative bacteria have to adjust their "working mode" according to changes in oxygen, carbon, nitrogen and other things in the environment. So ORP is not an optional indicator, it is a key signal for us to judge whether microorganisms are comfortable living or not, and whether they are working or not. For example, if the ORP in the aerobic tank suddenly drops, it is most likely due to insufficient aeration, causing aerobic bacteria to "suffocate due to lack of oxygen"; When the ORP of the anaerobic tank reaches a positive value, it's over. Oxygen leaks in, and the anaerobic bacteria "strike" directly, and methane production stops.
What is the core logic for controlling ORP? Just one thing: "Adjust as needed" - first clarify what your biochemical system is supposed to do (is it to degrade COD? Or is it denitrification and phosphorus removal? Or produce biogas? ）Then determine which microorganism is needed to "dominate the work", and finally stabilize ORP in the corresponding range based on the needs of the microorganism. It's not about saying 'the higher the value, the better', nor 'the lower the value, the better'. For example, during denitrification, aerobic bacteria are needed for nitrification (ammonia nitrogen to nitrate nitrogen), and ORP needs to be controlled at+200~+400mV; during denitrification (nitrate nitrogen to nitrogen), facultative bacteria need to be replaced, and the environment needs to be reduced to -50~+50mV. If ORP doesn't decrease at this time, denitrifying bacteria won't work at all, and nitrate nitrogen will accumulate in the water. So the first step is to clarify the "target range", which is the "navigator" that controls ORP. Without this, the subsequent operations will be just fooling around.
Next is the most practical: how to adjust ORP specifically? Let's talk about different scenarios, after all, the gameplay of aerobic, anaerobic, and anaerobic systems is different. Let's take it one by one.
First, let's talk about aerobic systems, such as aerobic tanks and biological aerated filters. The core is "oxygen control" because oxygen is the main oxidant here, and ORP and dissolved oxygen (DO) are almost "tied together". Many friends make a mistake: they think that the larger the aeration, the higher the dissolved oxygen (DO), and the more stable the ORP will be - in fact, if the DO is too high, the ORP will soar too high, which not only wastes electricity but may also inhibit certain aerobic bacteria (such as those that degrade difficult to degrade organic matter); If DO is too low, ORP will fall down again, aerobic bacteria cannot breathe, COD cannot decrease, and ammonia nitrogen cannot be nitrified. How should we adjust it?
Firstly, we need to closely monitor the relationship between DO and ORP. The situation of each system is different. For example, in some aerobic tanks, when DO is between 2-3mg/L, ORP just stabilizes at+250~+300mV. So let's control DO within this range, and ORP will naturally stabilize. How to control DO? The most direct way is to adjust the opening of the aeration valve or the frequency of the aeration fan - now many water plants use "DO-ORP linkage control", for example, setting the ORP target to+300mV. When the ORP is below 280mV, the system automatically turns on the aeration; If it is higher than 320mV, reduce aeration, no need for people to monitor and adjust, it is convenient and accurate.
In addition, the carbon nitrogen ratio in the aerobic system can also affect ORP. For example, if the COD of the incoming water suddenly increases and microorganisms "eat more", oxygen consumption will increase. At this time, even if aeration is not activated, DO will still decrease and ORP will also decrease. In this situation, it is not enough to rely solely on adjusting aeration, but also to look at the inflow load. If COD continues to be high, it may be necessary to adjust the inflow (such as diluting a portion of the treated water with reflux), or supplement some nutrients (such as adding urea or potassium dihydrogen phosphate if nitrogen and phosphorus are not enough), so that microorganisms can "eat evenly" and oxygen consumption is stable, and ORP will not fluctuate.
Speaking of anaerobic systems, such as UASB and IC reactors, the goal is to stabilize ORP at -200~-400mV (methane production stage). The key here is to "prevent oxygen" and "control carbon sources", because anaerobic systems are all "oxygen sensitive". A little bit of oxygen enters, and ORP will skyrocket, directly "poisoning" microorganisms.
Firstly, it is necessary to do a good job of "sealing", which is the foundation of the foundation. Many friends' anaerobic tanks have unstable ORP, and after checking, it was found that there is air leakage in the inlet pipe or the top cover plate of the reactor is not tightly closed, causing air to seep into the tank. Therefore, after each maintenance, it is necessary to check the sealing condition, and it is best to add a "water seal" to the inlet pipe to prevent air from entering with sewage. Also, if devices such as reflux pumps and agitators in anaerobic systems require air cooling, it is important to be careful not to let air leak into the water, otherwise it will truly be like a 'thousand mile embankment destroyed by ant nests'.
Then there is the control of carbon source and pH. When anaerobic microorganisms degrade organic matter, they produce methane and carbon dioxide, which are reducing agents that can maintain a reducing environment. If the COD of the incoming water is too low, microorganisms will not be able to eat it, and the reducing agent will not be enough, causing ORP to float upwards; If the COD is too high, microorganisms will "eat up" and produce too many volatile fatty acids (VFA), leading to a decrease in pH. When the pH is below 6.5, methane producing bacteria will stop working and ORP will also become chaotic. So it is necessary to regularly measure the COD of the incoming water and the VFA and pH in the pool. If the COD is not enough, add some carbon sources (such as glucose, methanol, or high concentration organic wastewater). If the VFA is too high, add alkali (such as sodium hydroxide, sodium carbonate) to adjust the pH. Generally, the pH is controlled at 7.0-7.5, and ORP is less likely to have problems.
There is another small detail: when the anaerobic system is started, ORP is particularly difficult to control because the microbial population is small at the beginning and the reduction environment has not been established. Don't worry, slowly add low concentration wastewater to allow the microorganisms to multiply little by little. At the same time, you can also add some "inoculated sludge" (such as sludge from other anaerobic tanks) to accelerate the establishment of the reduction environment. When ORP stabilizes below -200mV, gradually increase the inlet load, otherwise it is easy to "start up failure".
Finally, let's talk about anaerobic systems, such as denitrification tanks, where the target ORP is generally between -50~+50mV. The core here is "carbon source control and oxygen prevention", because denitrifying bacteria need carbon sources as "food" and there must be no oxygen interference (otherwise they will prioritize oxygen over nitrate nitrogen).
Many friends cannot lower the ORP of their denitrification tanks, so the first thing to check is whether there is oxygen leakage - for example, if the aerobic tank in front of the denitrification tank has too much aeration, DO carries sewage to the denitrification tank, or if the agitator in the denitrification tank is "aeration agitation" (which is the most difficult and directly oxygenates the tank), even if a carbon source is added, the ORP cannot be lowered. So the stirring of the denitrification tank must use "mechanical stirring" (such as blade stirring), and cannot use aeration stirring; If the DO of the effluent from the aerobic tank is too high, a "degassing tank" should be added in front of the denitrification tank to remove some of the oxygen in the water.
Then there is' the amount of carbon source must be sufficient '. When denitrifying bacteria degrade nitrate nitrogen, they need a carbon source (such as COD) as an electron donor. If the carbon source is insufficient, even without oxygen, they will not have the strength to work and ORP will not be stable. How to determine if the carbon source is sufficient? The carbon to nitrogen ratio (C/N) can be calculated. Generally, denitrification requires a C/N ratio of 5~8:1. For example, if the nitrate nitrogen in the influent is 50mg/L, the COD must be at least 250~400mg/L. If it is not enough, carbon sources such as methanol, sodium acetate, or COD from domestic sewage must be supplemented. When supplementing, do not add too much at once, otherwise COD will remain in the later system. It is best to "add a small amount multiple times" and monitor the changes in ORP and nitrate nitrogen. If ORP remains stable at around 0mV and nitrate nitrogen continues to decrease, it indicates that the carbon source is added precisely.
In addition to these specific operations, there are also several "general tips" that can be used in aerobic, anaerobic, or anaerobic systems, which can help you avoid many detours.
The first one is' Don't just focus on ORP as one indicator ', it should be linked with other indicators. For example, if the ORP of the aerobic tank decreases, you need to check if DO has decreased, COD has increased, and ammonia nitrogen has not decreased; When the ORP of the anaerobic tank increases, it is necessary to check whether the pH is low, whether the VFA is high, and whether there is oxygen leakage - ORP is a "signal soldier", not a "cause", only looking at ORP cannot find the problem, and it needs to be analyzed together with indicators such as DO, pH, COD, ammonia nitrogen, and VFA to accurately find "where to adjust".
The second is to "set a reasonable range of fluctuations" and not pursue "absolute stability". The biochemical system itself has fluctuations (such as changes in inlet water quality and temperature), and it is normal for ORP to fluctuate slightly. For example, the ORP of the aerobic tank is set at+300mV, allowing it to oscillate between 280-320mV. As long as it does not exceed this range, microorganisms can adapt and do not need to adjust it too much whenever there is a fluctuation, otherwise it will make the system more unstable. For example, when the aeration valve opens and closes intermittently, the dissolved oxygen (DO) fluctuates between high and low, leaving microorganisms at a loss.
The third one is "regularly calibrate the instrument", don't let the ORP electrode "deceive you". The ORP electrode may age or be covered by pollutants in the water (such as oil stains and biofilms) over time, and the measured values may be inaccurate - for example, if the actual ORP is+200mV and the electrode displays+100mV, you may think that the aeration is not enough and turn up the aeration, but the ORP actually surges to+300mV, which may actually cause problems. So it is generally recommended to calibrate the ORP electrode once a week, using a standard buffer solution (such as a pH 7.0 buffer solution, with an ORP of about+200mV, depending on the buffer solution instructions), wiping off any dirt on the electrode to ensure that the measured values are accurate, so that control is meaningful.
Finally, to summarize: controlling ORP is not a "high-precision technology", the core is to "first clarify the target range, then identify the influencing factors, and finally adjust as needed". The aerobic system focuses on DO and carbon nitrogen ratio, the anaerobic system focuses on sealing and pH, VFA, and the anoxic system focuses on carbon source and leak proof oxygen. Combined with other indicators, regular calibration of instruments can basically stabilize ORP. Dealing with biochemical systems is actually like making friends with microorganisms. You can understand their temperament (what ORP environment they like), create comfortable conditions for them, and they will naturally work well. Once the system is stable, we also have peace of mind.
