Anaerobic Digester Codification

[Alan.Johnson]

We are building free standing Anaerobic Digester plants for dairy farmers and as merchant facilities; they are not associated with waste water treatment plants. But our codification guide has been NFPA 820 and we struggle with how it should be interpretted.
Here is my current question: NFPA 820 says that 10 feet horizontally and 15 feet vertically where there is biogas storage, we are directed to wire everything Class 1 Div 2. The main digester tank is 30 high and is filled with a manure/water mixture; the biogas is gathered in a flexible dome above the water level. We would like to mark as 'unclassified' the area at ground level of the digester tank as the biogas storage well out of the 15 foot perimeter of vertical clearance.
Thanks for your help on this.

 

[rbalex]

The Scope of NFPA 820[2008] and its Tables are specific enough to apply to your installations; however, it would be helpful to know which Table-Row/Line you are applying because I don’t see anything that entirely matches your envelope description.

 

[Alan.Johnson]

NFPA Table 6.2 (a)

NFPA Table 6.2 (a)

Row 15 is the specific row of the table we are reading.

 

[rbalex]

Row 15 is the specific row of the table we are reading.

I am making the (possibly wild) assumption that you are specifically using Table 6.2 Line/Row 15a and 15b. If that is correct, there is a corresponding diagram in NFPA 820, Annex A (FIGURE A6.2a) that illustrates the intent.

In your case, the Division 1/Division 2 envelopes drop to grade from their maximum horizontal extension. If the only Class I material involved were methane (CH4) this would not necessarily be the case since it is lighter than air. However, there is usually a significant hydrogen sulfide (H2S) component in sewage-type gasses. That and various mercaptans give sewage gas its smell. H2S and the mercaptans are also both ignitable and heavier than air, so the envelope extends to grade. H2S is also Group C, but may be treated as Group D for practical purposes in some sewage operations.