Re: DIG-L: Re:'Understanding Anaerobic Treatment' - longish mes

["Les. Gornall" <ae198@dial.pipex.com>]

Chuck,

My thanks to you for your cogent reply

and the correction to table 1


>  Table 1 does contain an error 


- we all make them!


>My observation on lime addition whenever the flare goes out was made 
>only for the purpose of illustrating the almost universal lack of process 
>controls utilized in anaerobic digestion.  I wasn't advocating lime over 
>other eligible chemicals nor was I supporting the practice of controlling 
>pH only when the flare went out. 

I am comfortable with this but my point is that in a CSTR it is not 
possible to use pH to control the digester nor is pH control required due 
to the inherent buffering.
 
>
>In the regard the Gornall suggestion to "stop feeding the digester for a 
>few hours until the methanogenic population catches up" reflects an 
>inarticulate appreciation of the actual biology at work.  
>The additional statement "that in simple CSTR's, the most common type of 
>digester,  the pH is self regulating." further augments the serious lack 
>of understanding of the biochemistry of anaerobic digestion treatment. 


Firstly, I am only to deeply aware of the microbiology of anaerobic 
digesters
I just thought the practical end result was more succinct than a process 
description for the purposes of this forum.  The role of pH and digester 
*control* in a CSTR needs to be clarified and I believe we are really 
talking about the dynamic response of the digester to de-stabilising 
influents.  I can speak from practical experience.

THE MODEL.  A CSTR in which a more or less steady state condition exists 
between feed and digester contents [for example in a digester which has 
been operated for say a year] with a regular feed 24 hours a day from a 
buffer tank 7 days a week.

A typical shock overload might be induced by say a delivery of old very 
highly acidic organic waste that has been sitting for some time in a 
remote tank at high ambient temperatures (ensiled if you will).  This 
material is put into the digester feed buffer tank and pumped into the 
digester continuously (24 hour feeding) - what happens?

The first reaction is that acetate created in the external tank is 
absorbed directly by the cell wall of the methanogens and the methane gas 
production increases dramatically in less than 15 seconds. This reaction 
is so quick that gas bubbling in the sludge side of the heat exchangers 
can be heard to turn on and off in synchronisation with the digester feed 
pump. (I hear Chuck cheering already as the two phase image is invoked! 
:-) )  The total soluable VFA in the digester does not change appreciably 
above the sum of the average VFA level in the digester (typically 
250mg/l) plus the injected VFA (say a further 100mg/l) Within an hour to 
6 hours the acetogenic bacteria are doing their work on the remainder of 
the organics in the partly acidified feedstock and gas production further 
increases but this time mainly from CO2 production and much CO2 is lost 
from solution causing a drop in methane levels on the gas quality meter, 
a further rise of soluable VFA to 400 or 500 mg/l (almost double the 
steady state level) is likely by the end of the day. The internal 
buffering of the digester is such that the pH does not change a tenth of 
one pH unit. The digester will then respond in one of three ways 
depending on the relative size of the organic overload compared to the 
normal feed rate.

     a.) the VFA level will fall back to 250mg/l as VFA is transmuted to 
biogas. In a 2000 tonne digester with 2 x 10 e6 Litres this will amount 
to half a tonne of biogas and the physical integrity of the digester will 
be determined by the ability of the stirring system to strip out half a 
tonne of gas in a short time without foaming. This is normally not a 
problem.  The Methane fraction of the digester gas may increase for a few 
minutes then fall back to about 58% methane. The boiler stays on, the 
flare stays on.

     b.) The VFA level will remain constant as the feed batch runs into 
the digester via the buffer tank which dilutes the incoming feed with 
less reactive organics.  This produces the same result as above but 
continuously for  2- 3 days until the methanogenic population increases - 
there will be a sudden dop in VFA when the VFA food supply is exceeded by 
the developing methanogenic population food demand.  During this time the 
pH does not change more than 0.2 pH units. The methane % may fall to 56%, 
the boiler will struggle but the flare is more tolerant.

     c.) The VFA continuously rises. When the VFA level reaches 1000mg/l 
the pH still does not move more than 0.2 - 0.4 pH units, the biogas 
production increases by more than 50% and may double but the methane 
level falls below 52 %, the finely tuned boiler fails through excess air 
blow out and the flare pilot light fails to ignite every time and alarms. 
  In this case the operator stops feeding the digester and allows the 
methanogenic population to develop.  At 1000 mg/l TOTAL VFA. there is no 
inhibition of methanogenic bacteria which are producing more methane than 
normal by way of assistance. The digester has an extra tonne of methane 
to remove as the VFA level collapses.  End to end play continues as 
normal when the VFA levels are normalised.

The CONTROL of the digester microbiology is done entirely by feedrate 
control (operator controlled.)

The first control signal is always the total gas production and the gas 
quality.
The second control signal is the daily or "per shift" VFA reading of the 
digester contents.
Action bands are set at say 400 mg/l to slow digester feed by 1/3 and 
600mg/l to slow digester feed by a half if these levels are exceeded for 
24 hours. If the digester boiler fails stop feeding the digester for a 
shift or until the digester gas quality improves. If the flare goes out, 
stop feeding the digester until gas quality improves and expect foaming 
stress on the plant.

Because the digester is so well buffered he pH has no role to play 
whatsoever 
in *Controlling* the process.  

If the digester pH falls to 6.5 - it died three days ago! 

>If left alone by the mandate to STOP FEEDING THE DIGESTER, its 
>pH will continue to drop
>  to perhaps 3.5 to 4.0.  It will continue to exhibit this pH permanently 
>until adjusted upward.  This is
>  precisely what happens in a silo containing silage.  It goes acidic and 
>stays that way, permanently.

I disagree - this method has controlled digesters for decades.


>My perspective 
>was therefore primarily directed at municipal
>  digester practice.  The associated reference to grease also being feared 
>was correct.  Vegetable oils do not 
>  dissolved at mesophilic temperatures whereas at thermophilic 
>temperatures they do dissolve.  I would therefore
>  expect that the V-F digester is operated at the higher temperature. 

Of course! I'm the greatest fan of 'going thermophilic' where appropriate.


>
>  The paper does not assert that high rate digesters are not a valid 
>choice for agricultural wastes or animal by-products.

OK

>The gist of the entire article was to espouse the treatment benefits of 
>two-phase fixed growth thermophilic as far more 
>  efficient treatment than single phase suspended growth mesophilic 
>digestion namely:
>
>      1.  Half the cost for same organic load

This has been on my wish list for some years!

>      2.  Twice the biogas production due to the ability to achieve twice 
>the reduction of volatile solids

Please provide comparative examples.

>      3.  Two separate and rather pure gas streams rather than the 65-35 
>mixture

Useful 

>      4.  And smaller footprint--about 1/10th the volumetric size

This is on my wish list as well!

>
>  What bothers me somewhat is the spontaneous willingness on the part of 
>veterans of anaerobic treatment to preserve
>  obsolete, highly inefficient, costly, and process troublesome technology 
>just because it been around since Adam and Eve.  
>  Technical improvements occur in all fields, even anaerobic treatment. 
>Rather than attack the new, your customers would
>  better benefit from an acquisition of more process efficient and 
>cost-effective treatment expertise. Single phase
>  mesophilic anaerobic treatment is clearly obsolete technology. Your 
>defense of it won't change the ultimate
>  domination of two-phase thermophilic digestion, at least until something 
>better is developed.  

Go for it Chuck :-)

I hope we can meet to discuss this some more especially the role the two 
stage process can play with some organic liquors I have in mind.

Now I must go and crank up my old steam car and it takes a few hours to 
get the pressure up!



Best Regards




Les. Gornall
Director
Practically Green 
Environmental Services
Solar House
Magherafelt
BT45 6HW
Northern Ireland
+44 1648  32615 Tel./Fax

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