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

[doelle <doelle@ozemail.com.au>]

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
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>
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>
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Horst W.Doelle, D.Sc., D.Sc. [h.c.]
Chairman, IOBB
Director, MIRCEN-Biotechnology
FAX: +617-38783230
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