Quick & Dirty Digester Sizing

[sjohns4]

I'm doing some preliminary planning and evaluation for an anaerobic digester at an existing WWTP. The plant design flow is 8 MGD and treats almost entirely domestic flow with nothing too exotic.

Are there any rules of thumb to come up with a tank volume / footprint that could be used to get me in the ball park...at least to see if it's worth it for the client to have us dive a bit deeper into it.

Thanks,

Mike

 

[bimr]

My understanding is that anaerobic digesters are not cost effective unless you have plant larger than 10 MGD or if you have primary sludge.

 

[sjohns4]

I suspect you may be right, I wasnt sure where the threshold was but was also of the understanding anaerobic was better suited for larger plants. This paticular plant has primary and secondary clarifiers. Currently the sludge is mixed and sent to a small thickner / holding tank before being pressed.

 

[bimr]

Most smaller facilities are going with aerobic digesters at the present time. People are also moving away from the use of primary clarifiers.

You will have to check with the solids handling regulations from the agency in your locale for proper sizing.

You should also include a sludge thickener (gravity dewater screen) to reduce the size of the facility.

Here is an example from one state:

http://www.ipcb.state.il.us/documents/dsweb/Get/Document-12042/

Empirical Design
When such calculations are not submitted to justify the
design based on the above factors, the minimum combined
digestion tank capacity outlined below will be required.
Such requirements assume that the raw sludge is derived from
ordinary domestic wastewater, a digestion temperature is to
be maintained in the range of 85 to 95 F (29 to 35 C), 40
to 50 percent volatile matter in the digested sludge, and
that the digested sludge will be removed frequently from the
process. (See also subsection (a)(1) above and Section

370.860(a)(1).)
A) Completely Mixed Systems
For digestion systems providing for intimate and effective mixing of the digester contents, the systemmay be loaded up to 80 pounds of volatile solids per 1000 cubic feet of volume per day in the active digestion units.

 

[Lan123]

Design criteria for a high rate digestors (From Wastewater Treatment Plant Design WPCF & ASCE)

Solids Retention Time 10 to 20 Days
Solids loading Lb VSS/Day/Cu.ft 0.1 to 0.40
Volume
Primary Sludge + Waste Activated Sludge 2.67 to 4 Cu.Ft/ Capita

Combined primary plus waste biological sludge feed concetration % solids (dry basis) 4 to 6
Digestor underflow concentration % soldis (dry basis) 4 to 6