How do you regulate aeration in terms of oxygen levels?
asked 27 March 2015 asked by shaman (900 points)
A PI-type controller (proportional-integral controller) will work well enough.
Hello, to be more specific, I am looking to regulate using Redox potential, while assessing its effectiveness by measuring the level of oxygen.
In order to use Redox, there is no textbook value you'll be able to put quickly in automatic mode. We recommend a manual mode and a learning curve where you study the behavior of your plant for at least a month to find your value (the knee bend in the curve). Keep in mind that this setpoint will change with the seasons and you'll need to do this exercice again as the temperature changes. Another more accurate way to control aeration is to use ion selective sensors and measure ammonia and nitrate to see when the nitrification occured and redure the blowers. You keep your DO readings as limits to avoid going too high or too low in O2.
Oxy-redox, of course… Start aeration at a low redox threshold (-50 mV) and cut it at a high oxygen threshold (2 mg O2/l) This should be done according to a sequence: On Maxi aeration: 90 mn On Mini aeration: 15 M Stop Maxi aeration: 60 M Stop mini aeration: 15 mn
Hello, redox potential with an O2 verification level and an O2 fallback level in case of malfunction of the redox sensor. Stopping at an O2 level can be tricky especially with industrial effluents. I had it happen recently. The station was verifying O2 with stop threshold of 3 mg/l, following a load increase, nitrification was not complete; we had to go to redox mode and boost the threshold to between 240 and 250 mV to obtain the desired effluent water quality. Meanwhile O2 was between 4 and 5 mg/l.
Actually we don’t use a controller but rather a PLC. A controller tries to maintain a set point. For aeration, as Leo said, we rely on aerobic stages (high oxygen threshold) and phases of anoxia (aeration stopped by low redox threshold) using min and max on/off times. This is easily managed with either a PLC or a micro-controller (or even via remote control in the case of a smaller plant)