Process Automation and Control, Smart Water Grid, wastewater treatment

Controlling the Water Industry – Activated Sludge Part IV – Whoops! Where did all those solids go?

In this last post on Activated Sludge Oliver Grievson is going to talk about what can be done within the process that can protect the overall permit of the treatment works and attempt to sum up what he has  said so far.

As we all know of all the process that are used in wastewater treatment it is the activated sludge process as advanced as it is does trip some operators and operational companies up sometimes. Of all the processes in all the operating companies in the globe I am sure the majority of them would say that of all the failures, near misses and pollution incidents that they may or may not have the majority is with the Activated Sludge Process. So what happens and why is this.

The majority of the failures in the process are to do with the solids and the loss of the solids to the receiving watercourse. This issue is usually to do with the way the treatment works is being treated and an issue with settlement has been caused. Part III of this blog series talked about normalizing the RAS process and this certainly helps but is not the only cause of problems. So what can be done?

The design of wastewater treatment works varies from country to country but the settleability of the sludge is usually take into account when designing the settlement tanks. Most days operators go out and take samples and if the settleability is below 90mL/g then everything is absolutely fine and no one worries anymore, if above this the situation is monitored. What can be done? Well there are commercial systems tht measure the SVI (rather the SSVI) and these can be used as a measurement of settleability. The alternative is to give the operator a system where they can dial in the settleability and from then on an automated system can take on the responsibility.

So what is this system doing?

Well, if you take the current measured SSVI and work out the solid flux of the system in its current state and divide this by the design state then a solid flux ratio can be derived. This ratio can then be used to fluctuate the ratio of the incoming flow to the flow of the RAS so that the treatment works remains at a solid flux ratio at or near 1 whilst also taking into account the thickness of the RAS suspended solids and the sludge blanket level. If either of these factors increase to an undesirably high value then the resulting ratio can be decreased until satisfactory results are taken into account. Couple this with an effective surplusing strategy as controlled by the systems described in the second blog then the issues surrounding solids can effectively be controlled.

What else is there? Well without going into every scenario of activated sludge plants which more qualified people that I have already have done my top hit list would be

  • Toxic shock  – Which can effectively be monitored by organic load monitoring on the inlet of the works controlling the dissolved oxygen system which won’t cure some toxic shock events it can certainly help. There are also instruments out there that will measure the health of the actual bacteria which will certainly help in diagnosing the problem.
  • Settlement of Solids (into the lane –  One that I have come across in my time on wastewater treatment works which is settlement of solids in the lanes. Different from filamentous bacteria development. Difficult at best to manage but to my mind it can be monitored by looking at the MLSS versus the surplussing regime or actually measuring the mass of air delivered versus energy consumption giving the aeration efficiency. Gross solids settlement should affect this measure and when it deteriorates to such an extent then this would indicate a problem.
  • Filamentous Bacteria Development – This is the development of filamentous bacteria within the activated sludge lanes. This is not something that can be monitored automatically (as yet) but the manual testing of SSVIs should pick up a problem and it can be managed automatically (to an extent) by using the solid flux ration
  • Loss of Nitrification –  I am sure that most of the readers will have run a plant where nitrification has been lost, usually through the MLSS being too low or toxic shock/nitrification inhibition or settlement of solids.See all of the above for solutions.

I certainly haven’t been all inclusive with the various problems that the activated sludge process has but I think (I am sure I will be corrected) that I have covered at least some of the most common problems and shown that instrumentation in the right place can help the operators that run the wastewater treatment works of today in the way that they need to be run to get the best out of the treatment process,

To summarise this series of blogs on the activated sludge process is difficult at best but I think it really all comes down to managing the solids in the process as best as you can, It is after all a mass based process. By managing the solids and using automation to push the treatment process to its limits we can use instrumentation to get the most appropriate treatment (not over or under treating) at the best possible efficiency and in this way protect both the environment and the customer’s pockets.


About noahmorgenstern

Entrepreneurial Warlock, mCouponing evangelist, NFC Rabbi, Innovation and Business Intelligence Imam, Secular World Shaker, and General All Around Good Guy


2 thoughts on “Controlling the Water Industry – Activated Sludge Part IV – Whoops! Where did all those solids go?

  1. Expensive in terms of both capital and O&M costs, requires a constant energy supply, needs trained operators who can monitor the system and react to changes immediately, and the availability of spare parts and chemicals may be an obstacle. The track record of activated sludge plants in the developing world is very poor, and few operate as designed or intended.

    Posted by water industry | July 23, 2012, 1:03 pm
  2. All,

    I hate to acknowledge this, but I have visited WwTPs that have never wasted solids. And I mean NEVER. Where did they go to? Over the clarifiers every time it rained.

    Posted by John B Cook | July 23, 2012, 1:43 pm

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