[Digestion] Pasteurization: 1. HX , 2. When?

[Duncan Martin]

*Heat Exchange*
Nice idea - but bear in mind that heat exchange works best on continuous 
sterilization (rare in AD) or where you have an endless succession of batch 
processes and several pasteurizers.  The minimum is a spare tank the same 
size. Otherwise, if you only have one pasteurizer, you have nowhere to put 
the warmed inflow until you have completely drained the previous batch.  Not 
a huge expense however.

(Discharge to sewer must be pretty rare though?)

*When to pasteurize*
There is a logic to BEFORE - but if the feedstock contains solids it can be 
(a) hard to pass through heat exchangers, so process economy suffers and (b) 
hard to ensure full treatment because of slow heating inside the larger 
lumps.

Pasteurization AFTER as a "feel good thing" to make regulators happy may 
well be the honest truth - thanks Stan! But it's the kind of wasteful folly 
that should start us calculating the biogas yield from a batch of nice plump 
regulators! Well macerated, of course!!

I know of one small plant in Ireland, on Vicky Heslop's farm in County 
Waterford, that has opted for pasteurization BETWEEN (only) - in a two-stage 
digester. There may be others in UK. I understand the regulators are happy. 
It avoids the above BEFORE problem - and also that of producing a sterile 
"broth" liable to reinfection, as mentioned in my posting a day or two ago.

Duncan.