Does anybody know of papers that describe the kinetic of sorption and desorption of pharmaceuticals and other micro-pollutants to and from activated sludge in conventional activated sludge sewage treatment plants or laboratory simulations?
It appeared that the fact that sludge flocs are not solid particles but aggregates of smaller particles with some stagnant water between the particles makes sorption and desorption so slow that it can affect the biodegradation and overall fate of the chemical in the treatment process, but it is hard to find any studies that deal with the actual kinetics.
The problem is described at some level in the supporting information of the linked paper but we didn’t get to the solution.
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What about adding some inhibitors to suppress the biodegradation so that only adsorption can be expected.
We have used different methods of inhibiting biodegradation of chemicals in sludge for determining sorption equilibrium of chemicals. I prefer dry heat of freeze dried sludge over chemicals and the same methods could properly be used for investigating the kinetic of the sorption process.
I have not made the experiment there it is measures how fast the equilibrium is established.
My question is if there are papers that have done this that I didn't catch in my literature search.
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@Henrik, it is a pity that I don't have any paper you want.
However, it is still an very interesting topic to discuss. As you pointed out, 1) sludge contains a lot of water; 2) adsorption of some pollutants might be so slow that biotransformation may influence. Therefore, the adsorption test should use actual sludge (no dewatering, no heating) but the bioactivity should be inhibited. That is why I suggested to use chemical inhibitors whose interference should also be considered.
I also would like to see more suggestions from you since you already compared different methods.
I have for a long time been planning to make a comparison of methods for inhibiting biodegradation that are usable for sorption studies with activated sludge.
There are already several studies that show problems with obvious chemical methods like adding sodium azide which interferes with the sludge structure and sorption properties and additionally does not inhibiting the respiration of sludge completely.
The inhibition methods are easy to find but there remains some considerations on how the effectiveness should be evaluated. Bioactivity should be inhibited for very variable chemicals. Sludge structure should be preserved. Sorption properties should not be changed by large increase in ion strength or any strong redox activity etc.
On your original question, have you read this paper?
http://www.sciencedirect.com/science/article/pii/S0043135405000357
I have been reading extensively about PPCP sorption to activated sludge, and it's the only one that I can recall that might be relevant for you.
@Amanda: Thanks. The paper you suggest is the type of paper I am looking for and this one I didn’t know. It is more focused on sorption into sludge and my idea is that it is the desorption kinetic that is particular difficult. Traditionally we would find the desorption kinetic from the equilibrium Kd = ksorption / kdesorption but this seems not to work so well for sludge flocs.
@Michael: I know the paper you mention. I like the proof that sodiumazide disturbs sorption equilibrium of chemicals, but I am really after the kinetics of sorption and limiting factor to determine this isn’t to inhibit biodegradation. We can just choose model chemicals that are not biodegraded.
The last sentence I am not completely sure that I understand. You are right that my work on the determination of sorption and realisation of the problems of non-equilibrium of sorption of some chemicals during the passage of activated sludge treatment plants originates from my work with the Poseidon coordinator (Poseidon was an EU project 2001-2004 and Thomas Ternes who now works with you at BfG was the co-ordinator). We spend some time considering the perfect sorption experiment in that project and it ended up in several papers. After rejecting chemical preservation one concept was to measure the true equilibrium by analysing both water and sludge (Ternes et al. 2004. A rapid method to measure the solid-water distribution coefficient (Kd) for pharmaceuticals and musk fragrances in sewage sludge. Water Res. 38, 4075-4084.) and the other was to ensure bacteria inactivation through physical methods (Andersen, 2005, see link) . Are you saying that you and Thomas are working on a paper on sorption kinetic or bacteria inactivation now that I should know about?
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