To start off this series this series of blogs is going to look at a basic nitrifying activated sludge plant it will not look at activated sludge plants that are designed to remove nutrients although some of the principles will be the same.
As a general rule the problem with the operation of the activated sludge plants that I have had some experience in is that the control of mixed liquors is, especially on the larger treatment works not what it should be in so far as mixed liquor concentrations are often either too low or too high. This at treatment works that tend not to have an abundant amount of operational staff will be a risk that is prevalent.
The risk to the operation of the treatment works is that the solids flux on the final settlement tanks will be exceeded and there is a potential for blanket loss from the process, either to a downstream process (a sand filter for example) or potentially to the environment. The lesser risk is that energy is wasted because an excess of bacteria consumes more oxygen and if taken to the extreme can cause damage to the mechanical blowers in the long term.
The traditional way of managing this has been to undertake manual sampling of the mixed liquors and for an operator to adjust the sludge wasteage rates to waste more sludge from the activated sludge system. However nowadays where an operator can be in charge of several treatment works things ultimately get missed, a single mixed liquor sample per day does not necessarily represent the actual process depending upon how it is sampled and analysed.
What is the automated and instrument based solution.
Well the first step is to install instrumentation. Firstly a good mixed liquor probe will give a constant reading of the mixed liquor concentration and at least give the operator a view of what is happening to the activated sludge plant. This can form the basis of a good but basic control system (desludging on timer with mixed liquor event points should the concentration drop below or rise above action points). An addition of a suspended solids monitor measuring the Return Activated Sludge can add the potential of wasting solids on the basis of mass of mixed liquors running the system on a mass basis which is what an activated sludge plant arguable should be run on.
With the addition of a RAS suspended solids probe there is also the option to run the timer mode with a mixed liquor trim with a PLC calculating how much to waste to keep within a set tramline for mixed liquor concentration. This can be termed an intermediate level of solids control.
With the addition of a PLC on site and the programming of the site parameters into the PLC then the control of activated sludge solids can be taken a step further and the concept of sludge age can be used, with the PLC calculating how much of solids are contained within the activated sludge plant and an optimal sludge age inputed into the system by an operator then the PLC can quite simply calculate the amount of mixed liquor to waste with a safety factor inputed into the system to prevent the control system from wasting too many solids.
These control systems are shown in the simple diagram below:
So is there anything else? Of course the different control systems could with the addition of influent load monitoring be set up to waste solids on the basis of the incoming load calculating the mass required to treat the incoming load, however practically this would yield little benefit as the response time of the system could not allow react quickly enough to make any significant savings, and the majority of the savings in this respect can be made by controlling the dissolved oxygen concentrations.
So what does all of this mean in real terms, i.e. the energy costs? Taking a simple 2500m3 reactor that is over its ideal solids concentration by 1000mg/L and standard aeration efficiencies this, using empirical formulas amounts to 60kW per day or around 22,000 kW/year.
In terms of a simple payback on the investment of the instrument this would allow a return of investment of about 5 years for the Intermediate Control solution and about 2 ½ years for the Basic Control system. The Advanced Control System would not be financially acceptable to most organisations where a 5 year return on investment is normally the standard. On larger plants where economies of scale can be taken is where the more advanced control systems become viable.
This has been a simple look at how controlling the mixed liquor of activated sludge plants can assist in compliance by keeping the activated sludge solids to within normal operational limits by using control systems to control the concentration of solids. This will be covered in more detail in a later blog. The financial aspects of limiting the activated sludge solids have been covered in a very basic way but it can be seen that the payback of installing simple installation is viable on activated sludge plants , just based on energy consumption down to about 10,000 population equivalents.
In the next blog of this series looking at the activated sludge system the subject of dissolved oxygen control will be examined.