In the third post of this blog series on the problems associated within the activated sludge process and how to control them Oliver Grievson is going to look at the Return Activated Sludge system.
So what problems are seen and how can they be fixed? A particular one that I have seen and this especially on smaller activated sludge plants where there is little or no control there has been an issue with settlement. Now you can query the fact that what on earth has the amount that you re-circulate solids around the system got to do with the way in which the sludge settles and the answer is of course the floc loading within the start of the process especially in plants that do not tend to have an anoxic zone (particular the case with oxidation ditches). It is the variation from very high floc loading to very low floc loading that causes problems in sludge settlement
The civil engineering solution would be too install an anoxic zone or in an oxidation ditch control aeration to create anoxic zones within the ditch (again an alternative method of control). The way of controlling this with instrumentation and control is to install a method of RAS pumping control. The second “problem” is that of energy consumption, not really a problem but after aeration pumping is the next highest energy consumer on activated sludge plant.
The problems that I have seen regard on treatment works is on small treatment works where the rate of return is controlled by the level that a bellmouth is manually set at so as flow into the works varies so does the return rate. The tendency being to set the RAS too high (the bellmouth at a lower setting) as the settlement is poor (due to the lack of RAS control) and if flow rates increase (due to inclement weather) then solids will be lost to the environment. The other problem that I have seen is fixed speed return pumping, something that can be managed if the rate of flow to the pumping station managed but causes a problem if it isn’t This is particular the case on larger treatment works that have Archimedes screw pumps.
Fortunately control systems can resolve both of these problems depending upon on what RAS system is in place on the treatment works and whether sludge is removed from the final tanks by pump or by bellmouth and how the sludge is returned to the treatment works but the general principle is the same. To define the problem and solve the first problem, that of inconsistent RAS causing settlement problems is relatively simple. In the UK at least if a treatment works has a permit to discharge to the environment it must have a flow meter measuring the flow, for control purposes of the treatment works the best position of this meter is measuring the treatment flow to the works (for a single secondary process works serial secondary process works). For the sake of this blog I am going to assume that the incoming flow is that received by the Activated Sludge Process. If not then the flow measured into the process needs to be measured. The flow measured by the flow meter can be used to ensure that the rate of return to the RAS return point or points (if the Eckenfelder floc loading is being used) is proportional to the incoming flow into the process. This gives the ability to stabilize the floc loading into the process and thus stabilize the process as a whole. In order to do this all that is needed is the incoming flow rate that should be there anyway and the measurement of the RAS flow. If these flow rates are stabilized so that they follow each other then the first problem can be resolved. This is not say that the flow rates have to equal each other, just be proportional to each. This is where the second problem, that of energy consumption, can be addressed but this also increases the amount of risk to the wastewater operator.
If you have a system in place as described above so that the incoming flow and RAS flow are both measured then the simple way to control the process is to divide the RAS flow by the incoming flow and calculate the RAS ratio. As the ratio decreases the amount of RAS that is returned is lower and power savings are made. However as the RAS ratio decreases sludge blankets get thicker and deeper and as such it is advisable to measure the RAS solids and the sludge blanket level to mitigate the operational risk. These two instruments can also be used in other control systems, RAS solids in sludge wastage control (part I of this blog) and sludge blanket level in protecting the permit (part IV of this blog, next week). As the solids get thicker then there is also a risk that bellmouths will block so an alert limit on the thickness of the RAS.
So what are the benefits, using the two examples I used earlier. The smaller treatment works which was an oxidation ditch the benefit of installing a control system was more for compliance purposes only. The treatment works had SSVI3.5 concentrations of 220mL/g upon installation of the control system and stabilizing the floc loading the SSVI3.5 concentrations dropped to 80mL/g and an extreme case and an extreme decrease but shows the benefits of installing a control system. The second case is a much larger treatment works where three screw pumps were returning the RAS to the activated sludge process. Examination of the flows and loads showed that only 1 of the screw pumps needed to be running for 16 hours of the day with the second pump running for the remaining 8 hours. The third screw pump was only needed as a standby pump. Considering that these pumps were rated at 80kW each the overall saving on a monthly basis for this particular case was about 440MW per month, a significant operational saving.
In the next in the blog series I will examine what can be done on activated sludge plants to resolve some of the compliance issues that activated sludge plants cause and I will also attempt to sum up this series of blogs.