In the second in this series of blogs looking at controlling the problems experienced in the activated sludge process within the wastewater industry Oliver Grievson looks to contentiously talk about dissolved oxygen control. He says contentiously because everyone has an opinion on this and those opinions are usually pretty strong.He is going to suggest ways of simple control all the way up to some advanced control methods.
So what is actually the problem and why does it need some element of control? Dissolved Oxygen through forced aeration is something that is required usually through surface aeration or diffused aeration, the problems experienced is that too much air can cause the development of undesirable filamentous bacteria if it is not controlled over long periods of time and secondly is the cost. Arguably not actually a problem more a fact of life but considering that aeration has historically been quoted as 50-60% of all the aeration costs it is certainly something that needs to be controlled. I once worked in a treatment works where there was no control but then again there was no on-line dissolved oxygen monitoring so we didn’t know the problem that was being caused. The first thing that I did was install dissolved oxygen monitoring and from there build the business case for a control system, it was a pretty easy sell!
There are of course two types of aeration, surface aeration and diffused aeration.
Firstly to briefly mention surface aeration, there are limited control mechanisms for this depending upon whether the surface aerators are single, dual or variable speed. The basic methodology of control will be to measure the dissolved oxygen and, if possible vary the speed to suit the dissolved oxygen requirements and at low requirements switching off the surface aerators and running and dwelling their operation. The problem being is that a minimum amount of surface aeration is required for mixing of the solids in the aeration lane.
Secondly is the diffused aeration where the potentials for control are much more abundant. So what are they? For this I will be split the methodologies into what I think are basic, moderate and advanced. This is my opinion and the right control strategy will vary depending upon the treatment works. On some plants a basic system is all that is needed and 80% of the cost savings can be achieved this way and a more advanced system is not cost beneficial. On other treatment works the “Rolls Royce” of control system maybe appropriate because of the sheer size of the treatment works. The system that is utilised is up to the water company and engineers that are designing the system. So enough procrastinating…..what are they?
Firstly to explain the two aeration delivery systems that I am going to consider, there is direct delivery which is where the dissolved oxygen probes are directly linked to the control system which controls the blowers. The second delivery mechanism is indirect and the blowers are used to pressurise the aeration header pipework to a defined pressure, the pressure is controlled by aeration control valves and the degree that the valves are open is governed by the dissolved oxygen probes. For the sake of this article I will consider both systems as the same, not quite right but this article would get very technical otherwise
A basic dissolved oxygen control system is a simple control loop (whether PID or enough type of control), this is similar to that described above with surface aeration with measurement of the dissolved oxygen in the aeration lane. A high dissolved oxygen concentration requires less aeration to be delivered and a low dissolved oxygen concentration requires more aeration. The benefits system are that it is very simple, it usually has a controllability of +/- 0.5mg/L but typically for the sake of safety the dissolved oxygen is set to 2 mg/L.
A lot of water companies take the risk and reap huge benefits by challenging this historical set point and set it lower than this but then compliance is risked by potentially having insufficient dissolved oxygen available and for the process performance to suffer as a result.
This brings us onto a more advanced control methodology that is arguable and that is ammonia control. The theory of this is that if there is insufficient dissolved oxygen available then the ammonia removal will be the first part of the process performance that will suffer. This control system is a layer on top of the basic dissolved oxygen control system. So how does it work?
Typically this is a feed-back control system that measure the ammonia at the effluent point to the aeration lane or collection point of the flows if there is more than one lane. The ammonia concentration is measured and is used to either increase or decrease the dissolved oxygen setpoint for the basic system. So if the ammonia concentration is high the dissolved oxygen would need to be increased and the setpoint increased from let’s say 2mg/L to 2.5mg/L. The rate of the increase depends upon the algorithm that is used for the control system. This is where the intelligence of the control exists. Some water companies opt for a nudge and wait system. The ammonia concentration is measured, a set adjustment made, a time period elapses and the ammonia concentration measured again. An alterative to this is instead of having a set adjustment having a proportional adjustment depending upon how far away from the ammonia setpoint is. The alternative to this would be having continuous measurement of the ammonia and having a PID loop similar to that used for the basic control also for ammonia control so basically having a coarse dissolved oxygen control loop with an ammonia control fine tuning loop.
However the arguments against ammonia control is that the ammonia removal is not proportional to the amount of dissolved oxygen concentration and there is actually a tipping point that exists to the extent that below a certain amount of dissolved oxygen then the ammonia concentration starts to rise and above the tipping point then near enough all of the ammonia is removed.
The last and most complex control methodology for dissolved oxygen control is full organic load control and basically looks at the organic load that is entering the treatment works and calculates how much dissolved oxygen the treatment works actually requires and adds a safety factor but controls the dissolved oxygen at much lower excess concentration. This has been done both by respirometric measurement and off-gas measurement. The benefits of this is that a very small proportion of excess dissolved oxygen is provided and wastage is minimised.
So what system is correct, as mentioned earlier it is very much “horses for courses” and will depended upon the size of the treatment works, the aeration system, and the resources available. If the treatment works is small then the most basic of dissolved oxygen systems will suffice, if the treatment works is a very large biological nutrient removal treatment works then a system more complex than briefly mentioned here is probably more appropriate. The choice is for the water company and the engineers involved.
In the next in this blog series I will be talking about another area where control systems can be used to minimise the amount of electricity consumed and that is Return Activated Sludge Control.