Hello everyone,
I am wondering whether any of the board's members has
operated or knows of a municipal WWTW where only WAS is
anaerobically digested ? (i.e. no primary clarifiers, so no primary sludge)

Many thanks
Zaheer

Dear Dwi Handaya,
Thank you for your reply, and apologies for being late responding:

The situation for the client I am working for is that several of their
(nutrient removal) activated sludge plants (from roughly 15 Ml/d to 80 Ml/d in size), running at sludge ages of 15 to 25 days typically, have their (undigested) waste sludges dewatered and trucked to remote disposal facilities.

I am doing cost estimates for development options (factoring in items such as the transport costs etc.), and would like to back-up some of the references from published papers I have read, specifically about gas productions and volatile solids reductions in WAS-only anaerobic digesters .  So I was looking to verify some calculations and cost exercises that we have done against the case studies and anecdotal or 'un-published' data that might be available.

Thanks again, but I realize that this is far less common than Primary Sludge  anaerobic digesters, or mixed PS and WAS.
Regards, Zaheer  

Hi Zaheer,

For field-scale references, there are a few WAS-only plants in Florida (that use a Bardenpho design, e.g., Lee Co).  You can also get to this number by backing out primary solids digestion rates (60-70% VSR) from mixed solids digesters (typically 45-55% VSR).  Another source of information is papers dealing with sludge pretreatment technologies.  Having been through this exercise before, I'd suggest using a VSR value of 30-35% for WAS-only.  Since longer sludge ages translate into poorer digestibility, you might want stay on the lower end of that range.  If your facility can justify sludge pretreatment (or recuperative thickening), you can raise the WAS VSR to 50-75%.  Note that these are Volatile Solids values -- not Total Solids values.  Regards, John