DIG-L: The scum layer problem in CSTR systems

["AHHM Schomaker" <ts@HASKONING.nl>]

A persistent problem in anaerobic treatment of sewage sludge and 
manure in CSTR systems is the formation of scum layers. Causing 
blockage of outlet pipes and ineffective operation of mechanical 
mixers, these layers usually lead to high maintenance costs and 
labour input for these CSTR's. 

The abatement of a scum layer usually consists of common 
removal of the layer at specified times. Sometimes intensive 
blending of the CSTR surface layer with the scum using a propeller 
mixers is applied. 

Besides my wish to discuss all relevant topics on sewage sludge 
digestion I am in particular interested in starting a discussion on 
the former subject. In this respect the following questions are 
relevant to my opinion:
- what is known about the (average) composition of scum layers in 
sewage sludge digesters (fat, proteines) ?
- what causes scum layers and which components makes them 
float (differences in density, gas attachment to fibres) ? 
- which part of the scum layer (in VS, COD or BOD) is still 
biodegradable using digestion, taking the average composition into 
account ?
- what could be the amount of biogas generated from this extended 
biodegradation of these layers ?
- which specific measures could be taken and which technical 
arrangments could be made for optimum extended biodegradation 
of these layers ?
- what will be the expected volume reduction of the scum after this  
biodegradation ?
- what other additional measures could be taken to prevent the 
formation, to increase the breakdown or to improve the removal of 
these layers ?

I have posted this message to the small group of people that was 
interested in setting up a discussion group on sewage sludge 
digestion (SeSDiG), with a copy to the digestion discussion list of 
crest.org (what I am intending to do with future messages also). 
Anyone who is interested in specific discussions on sewage 
sludge digestion technology, is invited to put forward his e-mail 
address to me for participation in SeSDiG.

It is my intention to make an internet site of SeSDiG in time with 
all relevant information on the subject (technical/technological data, 
contact lis bank, technical papers, discussion forum, related 
internet sites and so on). Anyone who has suggestions in this 
respect is welcome to send them to me. 

I am anxious to hear your reaction on this message.

Kind regards,

Ton Schomaker

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