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

["Chuck Steiner" <h2osmart@ix.netcom.com>]

----- Original Message -----

2.18.00

Dear Dr. Doelle and Mr. Gornall:

Please permit me to clarify one = comment as well  as elaborate on the several points raised.
Table 1 does contain an error in that = the design  parameter ought to have been Ability to accommodate
low suspended solids rather than high = suspended  solids.  Conventional High Rate ramains "no" whereas
Recommended Design remains = "yes".  I had  ample opportunity to catch this error but did not--sorry.

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.  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.  I'll  explain.  There are two fundamental phases of treatment  which
simultaneously occur in anaerobic  digesters.  In single phase digesters they occur in the same  vessel.
In two phase digesters, they occur in = separate  vessels (or compartments).  The first phase utilizes
hydrolytic, acetogenic, and = homoacetogenic  bacteria which together convert carbohydrates, fats,
proteins, hemicellolose, and cellulose = in  descending preference to organic acids with acetates = representing
the ultimate fatty acid = produced.  These  bacteria are collectively called acidogenic bacteria and
the associated phase of treatment is = referred to  as the acidogenic phase.  These bacteria have a
metabolic rate which is at least = seven  times that of the following second phase which reduces
acetates to principally methane = gas.   Whenever a single phase digester, mesophilic or otherwise,
go acidic, i.e., the flare goes out, = it merely  reflects that the first phase has accelerated past = the 
second phase resulting in for = formation of fatty  acids faster than the conversion of fatty acids to
methane.  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.
That only way a single = phase digester,  mesophilic or otherwise, can be controlled so that both = phases
can survive without the first phase = dominating  is by humans controlling the organic feed to the digester.  If it is overfed, the pH drops.  = If it is  fed a correct amount, the pH stays about 7.0.  If it is = slightly  underfed,
grossly underfed, or never fed for = several  months, the pH rises but very slightly.  While it is true = that
the first phase lowers the pH while = the second  phase raises the pH due to the release of alkalinity, these
two offsetting reactions occur at = rates which  differ by at least a factor of 7.  Self-regulating, I think = not.
In two phase treatment, one permits = the bacteria  to go crazy reaching pH values of 3.5 to 4.0 where they
are able to optimize their ability  to efficiently degrade hemicellulose, cellulose, and = lignocellulosic  materials.
At this low pH, methane forming = reactions cannot  occur and consequently the carbon dioxide gas formed
is extremely pure and therefore may be = beneficially used for a variety of commercial applications.  The  slow
moving methanogenic reactions in the = second  phase at a pH of around 8.2 are somewhat anticlimatic as its
feedstock is extremely uniform = compared with the  first phase.

The article's reference to raw = solids was  intended to mean additions such as sawdust, cardboard, grass  clippings,
food wastes, and a variety of other = non-sanitary  type organic based wastes.  The Pollution = Engineering magazine
had the municipal marketplace as its  main focusfor many years.  Only in the last five years has = it  expanded its
vision to include industrial waste = treatment  matters.  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.

The paper does not assert that high = rate  digesters are not a valid choice for agricultural wastes or animal  by-products.
It does state that two-phase treatment = will  accommodate these wastes.  It does not state that single phase = will  not.  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
     2.  Twice the = biogas  production due to the ability to achieve twice the reduction of = volatile  solids
     3.  Two = separate and  rather pure gas streams rather than the 65-35 mixture
     4.  And = smaller  footprint--about 1/10th the volumetric size

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.  Regards-Chuck = Steiner

from: <3d.htm>doelle 
To: <3d.htm>Les. <3d.htm>Gornall
Cc: <3d.htm>digestion@crest.<3d.htm>org
Sent: Friday, February 18, 2000 = 3:52  PM
Subject: Re: DIG-L: = Re:'Understanding  Anaerobic Treatment' - longish message

Many thanks for the extensive evaluation of Chuck = Steiner's  article, which saves me writing it. I fully agree with all = comments.
The  need for the addition of lime shows very clearly that the digester is = not  properly run, that is too much VS or other acids are being formed = because the  pH sinks below 7.0 This normally occurs when there are too much carbon = in the  loading mix. If the correct C/N ratio is observed, there is no problem = with  the pH as Les said below and no lime is required. I have visited = digesters up  to a 2000 m3 size and never did they require any lime additoin.
I = have,  however seen UASB in Thailand being used for agricultural and other = wastes,  which run at an enormous cost because of the lime addition = requirement. We  investigated this and clearly found that UASB may be very good for = industrial  waste, but s questionable for agricultural waste. Manure type of = waste, one  cannot use UASB in any case because of flow probles. This is my  experience.
It is very simple, as Les said below, if you observe = the proper  rules of microbiology and feed the proper C/N ratio, there is = absolutely no  problem. If you want, however to digest predominantly lignocellulosic  material, you have to watch out. In these cases we suggest composting = and  mushroom production as being much more suitable.
If you want to get = energy  from lignocellulosic materials, you are much better off to use = cogeneration  and/or gasification. Whenever any type of human or animal manure is = involved,  you HAVE to use anaerobic digestion. Our good microbes can do a lot of = things  and good things for mankind, but not everything.
Horst = Doelle

At  02:03 18/02/00 +0000, you wrote:
 >
 >
 >The thought = provoking  article promoted on this discussion forum
 >"Understanding = Anaerobic  Treatment" by C.G. Steiner ( Pollution
 >Engineering Online, = February  2000) contains a number of misleading or
 >confusing statements = which  could be a dis-insentive to potential
 >investors in several AD  technologies which have been successfully refined
 >over many = decades  and are technically proven, commercially viable and  >appropriate for  their designed feedstocks and human = resources.
 >
 >Describing  "Current practices" Steiner observes that it is " not uncommon  >to add  lime to a digester when the flare goes out". In 25 years of  >digestion  experience I have not added lime (or the more sensible sodium 
 >bi-carbonate) . If the flare goes out, the digester has either = been 
 >overloaded organically - in which case the best policy is to = stop  feeding
 >the digester for a few hours until the methanogenic = population  catches
 >up, or the methanogens have been poisoned - this = requiring  more complex
 >analysis, removing the source of the toxicity and = either  seeding or
 >balancing the feed rate or both. The 'golden rule' = of  digestion is "if
 >the flare goes out stop feeding the digester" = -  headlined on the front
 >page of many operator training  manuals.
 >
 >The paper then states " efforts to control pH = .. are  practically
 >non-existant, even though anaerobic microbiology = is rather  pH sensitive"
 >The reason for this is that in simple CSTR's the = most  common type of
 >digester, pH is self regulating. This is due to = the  typically massive
 >bi-carbonate alkalinity in most digesters = (thousands  of mg/l) and in a
 >healthy digester a very low Volatile Fatty = Acid  concentration
 >(150-350mg/l). Using an inherent = carbonate-bicarbonate  buffering sytem
 >the digester maintains itself at pH 7 + /- 0.4 = even if  the operators
 >inject very high VFA feedstocks into the = digester  accidentally. It is
 >this inherent robustness that has made the = anaerobic digestion route the
 >choice of the waste water = industry for  the past century. Also the
 >maintenance of a pH control loop on = a  digester is predicated on the pH
 >probe being maintained and  calibrated. Whilst not difficult to do, it
 >demands from the = operator a  rigid, inflexible routine which is often
 >difficult to achieve = in  practise especially where there is only one
 >operator who needs = a  holiday or becomes sick.
 >
 >"Nutrient addition" = furthermore is not  required in most systems since the
 >ratio of available carbon = to  nutrients to the microbial system for most
 >wastes provides an = excess  of nutrients. The landfill leachate example in
 >the paper is a = good  example of the exception proving the rule. It is
 >worthwhile = visiting  digester sites and noting the number of redundant
 >nutrient = dosing and  pH correction equipment on over-complex AD
 >installations = especially in  the industrial sector.
 >
 >According to C.G. Steiner "..raw = solids  and grease are universally feared
 >constituents in anaerobic = digester  feed." If by "raw solids" the author
 >means high total solids = or  materials like poultry manure with a cake like
 >consistency = then this  is not the experience in Europe. In Northern
 >Ireland, there = are  several agricultural waste digesters feeding 11-22%
 >total = solids  mixtures of 10% TS cattle manure and 50%TS dry chicken waste  >into high  end mesophilic digesters and the goal is to increase this  >feedstock TS  further - the problem is not inside the digester but in  >materials  handling prior to the digester - pumping 20 tonnes of 11-22%  >total  solids damp cake containing half bricks and terylene string by way  >of  fertiliser bags down an 8" pipe is an art in itself but not impossible  >and not theoretical since there is already 15 years of daily  experience
 >of this on many plants.
 >
 >As for = grease being  "universally feared" this is incomprehensible since
 >in Denmark = (e.g.  Vaarst - Fjellerad digester) amongst many places
 >industrial = fatty  wastes and greases are PURCHASED by digester operators
 >to = enhance the  gas production - the V-F digester produces 7 volumes of
 >gas = per volume  of digester per day and is quite economical enough to pay
 >for = itself  and heat the local village. Fats and Volatile Fatty Acids are  >closely  related and the latter are pre-requisits of biogas production. I 
 >relish fatty and greasy wastes and enjoy the gas production = that  follows.
 >
 >It is true that for dilute wastewaters, fixed = film  digesters enable high
 >loading rates with the caveat that if = the waste  is cold and contains less
 >than 3% total solids there may not = be  sufficient carbon in the feedstock
 >to heat the digester even = with  final effluent heat recovery. However,
 >whilst there are many  substrates for which fixed film digesters are
 >ideal or even = the only  sensible route (e.g. the liquor fraction of
 >centrifuged or = screened  wastes), on heavier wastes anaerobic filters
 >have a history of = blocking up. The devil is in the detail and there a
 >few good = units  around which do not block and these are worth looking  at.
 >
 >Several hybrid digesters ( CSTRs with anaerobic = filters)  have been
 >successfully operated on industrial wastes to good = effect  but the table 3
 >in the paper states that conventional high = rate  digesters are not a valid
 >choice for agricultural wastes or = animal  by-products yet these feedstocks
 >are manifestly the foundation = of the  huge investments being made in the
 >European AD industry. = Restaurant  wastes and more especially the organic
 >fraction of Municipal = Solid  Wastes are the growth areas in the industry,
 >building on the = great  success (and also learning from some failures) of
 >the = Mesophilic and  Thermophilic digester programme over the past two 
 >decades.
 >
 >"Plant start up can be achieved in = weeks  instead of months" is not a
 >valid reason for going two stage = nor a  reality. Since even the humble
 >mesophilic varieties of = methanococus or  methanosarcina will double in 3
 >days. A typical digester start = up  programme begins with a 120 day
 >retention time, it only takes = 3 days  to halve the RT to 60 days, three
 >more to 30 days and three = more to 15  days - total 9 days! Given that an
 >operator might be = conservative and  half the maximum loading rate then the
 >digester can usually be = started  in under a month even after being
 >temporarily stalled whilst  commissioning new equipment or waiting for
 >supplies of = feedstock to  regularise. Indeed at the Goddards Green
 >digester (UK) a 600 = tonne per  day sludge processing plant with 2 x 2000
 >cubic meter = mesophilic  digesters was built by Siemens in 13 months to
 >construction = completion  in 1999, and commissioned in 28 days to client
 >take over. This = is not  'months' of digester start up but = 'weeks'.
 >
 >
 >
 >The  discussion of the merits of two phase digestion over single phase 
 >digestion leaves a number of unanswered questions. Firstly = what was  the
 >reason for the lack of uptake on two phase digestion in = the 1970s  when
 >several universities around the world tried this and = failed? The  biggest
 >problem was stabilising the first stage acetogenic = environment  to keep it
 >pure enough to operate without methanogenisis.  Unfortunately the
 >methanogens being injected in the feedstocks = (particularly from animal
 >wastes) had other ideas and as often = as not  the two phase system became a
 >two vessel single phase system = with  methanogenesis taking part in both
 >vessels. Perhaps this = problem has  been resolved now. Of more concern
 >is the basic reduction of = complex  and fixed hydrocarbons to VFA. This
 >first step still has to = overcome  basic macro scale barriers - how is
 >hemi-cellulose bonded to = lignin to  be removed quickly in smaller vessels?
 >Steam, strong acids and = strong  alkali's have all been tried without
 >commercial success. This = will be  a necessary giant leap for mankind if
 >the sawdust in table 3 = is to be  reduced to less complex organic materials
 >for subsequent  methanogenesis.
 >
 >Overall, if the acetogenic phase plus = the  methanogenic phase require
 >equipment inventories costing the = same as a  simpler but larger
 >Continuously Stirred Tank Reactor then one = would  surely choose the
 >simpler system. In the digester business, = the long  term picture that has
 >evolved is that it is the waste that = produces  the gas results not the
 >digester. Consistency of supply and = quality is  probably more important
 >than any other factor. Materials = handling  upstream and downstream of the
 >digester and the ability of the = people  on site to manage the system are
 >no less important. At ten = volumes of  gas per volume of digester per day
 >an operator has about 15 = minutes to  respond to a digester foaming
 >incident before the contents of = the  digester are deposited on the ground
 >outside - sometimes a bus = is a  more reliable way of travelling a long way
 >than an F1 sports = car -  especially if the driver previously only drove 
 >tractors.
 >
 >This author would like to see more = very high  rate digesters where these
 >are appropriate - the 10-20 hour = retention  time UASB has made a very
 >significant contribution to the = digestion of  industrial effluents and I
 >feel sure that other high rate = solutions  will also find their niche in
 >the market but solid, proven, = well tried  solutions with a long life,
 >managable engineering and low = lifetime  costs due to equipment and
 >management simplification also have = a place  in the control of pollution
 >even if the tank has a larger  volume.
 >
 >
 >
 >Best  Regards
 >
 >
 >
 >
 >Les.  Gornall
 >Director
 >Practically Green
 >Environmental = Services
 >Solar House
 >Magherafelt
 >BT45 = 6HW
 >Northern  Ireland
 >+44 1648 32615  = Tel./Fax
 >
 >http://www.practicallygreen.com
 >
 >Ho= mepage  of "Practically Green" and "Practically Green = News"
 >
 >DIGESTION  List Sponsors, Archive and  = Information
 >http://www.crest.org/renewables/digestion-list-archive >http://www.crest.org/renewables/biomass-info/
 >Beginners  Tour of  = Biogas
 >http://WWW.roseworthy.adelaide.edu.au/~pharris/biogas/begin= ners
 >
 >
Horst  W.Doelle, D.Sc., D.Sc. [h.c.]
Chairman, IOBB
Director,  MIRCEN-Biotechnology
FAX: +617-38783230
Email:  doelle@ozemail.com.au DIGESTION List Sponsors, Archive and Information = http://www.crest.org/renewables/digestion-list-archive  http://www.crest.org/renewables/biomass-info/ Beginners Tour of Biogas = = http://WWW.roseworthy.adelaide.edu.au/~pharris/biogas/beginners