Your question is very good.

The answer is that you don’t have to alternate the water residue from anaerobic to aerobic. The only think you have to do is to study and decide the way of application, the irrigation dose and the irrigation frequency of this water in order to sustain aerobic conditions in the soil.

In brief you have to decide:

1.      The way of water application. (Eg drip irrigation maintains soil in unsaturated condition and so it is easier for the aerobic microorganisms to work, on the other hand application by flooding makes the soil profile saturated and so anaerobic conditions will prevail until the gravity water is moved deeper and so the aerobic microorganisms start again their work)

2.      The irrigation dose: by taking in mind soil physical properties, in order to avoid deeper percolation of the irrigation water. By this you will avoid possible aquifer contamination. For avoiding this possibility you can install a drainage system.

3.      The irrigation frequency by taking in mind soil physical and chemical properties, meteorological data (temperature and so evapotranspiration rainfall etc)

4.      The chemical analyses of the used water. These analyses and the soil chemical and physical properties will lead you to understand which the limiting designing factor will be. I think that you will find more details in any book analyzing land treatment systems and especially slow rate systems type 1 or type 2  

Hello, George. Thanks for your advice about using liquid digestate.

Now a related question comes to my mind: If we use the semi-solid anaerobic digestate as feed for earthworms (Eisinia foetida) is a pre-treatment needed to avoid making the worm-bed anaerobic and possibly killing the worms?

If you are aiming to produce compost from the solid residue, you have to analyze this residue for water content, carbon, nitrogen and heavy metals. After this you have to find other residues from other procedures (eg straw, grasss, or plant pruning etc) to make a mixture that will meet the requirements of composting. At the next table you can find acceptable and optimum ranges for a number of parameters concerning composting

(Keener, Dick, & Hoitink, 2000)

Parameter                                          Acceptable range                   Optimum range

C/N                                                     25:1-40:1                                 30:1-40:1

Water content (% by weight)              45-65                                       50-60

Particle Size (cm)                                0,8-1,2                                     Depends upon materials

Porosity                                               30-50%                                    36-45%

Bulk density (kg / m3)                         640                                          -

pH                                                       6.0-9.0                                     6,5-8,0

Concentration O2                                >1%                                         >>5%

Temperature (oC)                                 45-68                                       54-65

Keener, H. M., Dick, ,. W., & Hoitink, H. A. (2000). Composting and Beneficial utilization of Composted by-Product Materials. Στο W. A. Dick, Land Aplication of Agricultural, industrial and Municipal By-Products. Madison, Wiscovnsin, USA: Soil Science Society of America Book Series 6.

So if you have the analyses of all the possible materials you can calculate the quantity of each material for making the mixture by conducting the calculations presented at the

Rynk, ,. R., van de Kamp, M., Willson, G. B., Singley, M. E., Richard, T. L., Kolega, J. J., . . . Brinton, W. F. (1992). On-Farm Composting Handbook. ΝΥ: Northeast Regional Agricultural Engineering Service. (Table 3.3 pp 19)

I think that more information about composting and sludge co-composting can be found in the following book:

Tcobanoglous, G., Theisen, H., & Samuel, V. A. (1993). Intergrated Solid Waste Management. Engineering Principlesand Management Issues. New York: Mc Graw-Hill. Chapter 14

In the above mentioned three books you can also find typical analyses for many materials used for composting mixtures. I hope that I helped

G. Bourazanis

Thank you, George. Very helpful indeed.

Your sentence seems incomplete: "So if you have the analyses of all the possible materials you can calculate the quantity of each material for making the mixture by conducting the calculations presented at the ... "

You are wright "the following reference:

Rynk, ,. R., van de Kamp, M., Willson, G. B., Singley, M. E., Richard, T. L., Kolega, J. J., . . . Brinton, W. F. (1992). On-Farm Composting Handbook. ΝΥ: Northeast Regional Agricultural Engineering Service.

(Table 3.3 pp 19)

Article On-Farm Composting Handbook

Thanks for your update, George. 

Thanks also for offering full-text; I found the handbook downloadable from http://bit.ly/1OIhrfJ .

Table 3.3 seems very useful. (The index shows page 19 but on the downloadable version it is on page 35.)

Kind regards and best wishes,   John.

Comments from Dr Elaine Ingham follow the introduction below.

One of our interests is to achieve increased crop yields by improving the soil at all scales of application but specifically on do-it-yourself for subsistence- and backyard food production.

Low-cost inputs are important. Anaerobic digestion (AD) is a process under which organic material (vegetable or animal) can be decomposed to a raw fuel-gas and an effluent. The effluent, called "digestate", can be liquid or semi-solid. Multiple organisations refer to the use of digestate as a fertilizer. That piqued our interest because digestate is a waste stream of somewhat consistent characteristics. But we are conscious that vermiculture and agriculture in general require AEROBIC conditions, with ANAEROBIC conditions leading to severe problems.

We sought advice from Dr Elaine Ingham, President of Soil Foodweb Inc. Our query and Dr Ingham's response are set out below.

1) OUR REQUEST TO DR INGHAM:

Our project involves teaching subsistence farmers, urban farmers etc how they can improve their crop yields by using synergy between:

• VERMICULTURE (DIY source of nutrients and microorganisms)

and

• BIOCHAR (DIY source of soil amendment to keep these beneficial nutrients and microorganisms in their soil season after season.)

We call the procedure “VERMICHAR”.

Now there is a diversion:

Multiple advocates of biogas synthesis by anaerobic digestion (AD) tout the digestate (liquid and semi-solid) as being suitable for use as a fertilizer.

AD is indeed an interesting intervention generating biogas as an energy source for subsistence farmers and similar poverty-stricken communities living hand-to-mouth

In several of your interesting SOIL FOOD WEB lectures on the internet, you make it clear that anaerobic soil conditions will lead to the development of undesirable microorganisms and toxins.

Therefore I presume that the digestate from AD will contain these undesirable organisms.

Our questions:

• Is the presumption above correct?

By implication, if the poor folk apply digestate can they damage the fertility of their soils?

• How can we make digestate suitable for use as fertilizer?

For example, will it be adequate to aerate the liquid digestate mechanically every 30 minutes or so say, 12 hours?

Or will it be necessary to somehow provide a source of AEROBIC microorganisms before aeration?

2) DR INGHAM REPLIED AS FOLLOWS: 

QUOTE:

One of the things you have probably noted is that worms don't like anaerobic conditions; if the vermi-compost becomes too compacted, or if the microbes growing on foods added to the worm bin drive the material anaerobic, the worms will leave that part of the bin, and if things get really bad, the worms will leave the bin altogether.  Worms require aerobic conditions in order to thrive, no different from plants, or humans. 

The scientific literature is very clear that anaerobic processes result in materials that can be highly toxic, and not at all beneficial to plants. Reading all the literature on the subject can be very time consuming, so if you don't mind, I'll summarize it here. 

First: Most pathogens ARE anaerobic organisms.  Most pathogens grow BETTER in anaerobic conditions.  

When oxygen drops below a threshold of 6 ppm, or mg oxygen per kg of any material, then the conditions promote growth of pathogenic bacteria, such as E. coli, Salmonella, Shigella, and disease-causing fungi, like mildew, crown rot, blight, etc.  Beneficial bacteria and fungi that compete with, consume and inhibit pathogens will not grow in low oxygen conditions.  They may be present in dormant stages, but will not be able to function until the material is fully aerobic again.  

Second:  Many, many species of bacteria and some fungi convert any plant available N, P, and S into gaseous forms.  Ammonia, phosphine gas and hydrogen sulfide will be produced when oxygen drops into low concentrations, and these gases will be volatilized.  This means all the available N, P, and S needed by a plant will be lost from anaerobic digestates.  I know that people who don't understand the important differences between aerobic and anaerobic metabolism will say that because CO2 is volatilized during the decomposition process, then all other nutrients were concentrated.  But this is NOT TRUE, because the conditions released methane, and CO2 converted N, P and S into ammonia, phosphine and hydrogen sulfide.  All the plant-available N, P and S are gone from an anaerobic digestate.  

Digestate as fertilizer?  Are these folks really that ignorant?      

Third: Some very toxic compounds known to man are produced during anaerobic decomposition.  Formaldehyde is a good example, but there are probably hundreds of other toxic aldehydes that can only be made during anaerobic metabolism. Formaldehyde can be produced at a high enough concentration in an anaerobic digestate that it can make people in sewage treatment plants extremely sick.  There are reasons that ventilation is extremely important when dealing with anaerobic conditions.      

Fourth: Anaerobic organisms produce an enormous number of strong organic acids, such as vinegar, butyric acid, valeric acid, and putrescine.  And smelly (along with the ammonia and hydrogen sulfide smells). The pH of these materials can be as low as 2 or 3, depending on the length of time anaerobic metabolism was allowed to go on, along with what temperature, moisture, etc was maintained. The low pH will kill just about any terrestrial plant.  Now, this only indirectly relates to whether digestate is a fertilizer or not, but low pH materials will destroy root systems.  Plants will not grow.  People want fertilizers to grow plants.  No fertilizer and extremely toxic materials.......... good luck growing plants.    

Fifth:  The nutrients that did not volatilize as gases (so, Ca, B, K, Mg, Fe, etc) will be in highly reduced forms (acid conditions discussed in point four), which terrestrial plants cannot use.  Perhaps marsh or swamp plants might get by, because they have mechanisms to get aerobic biology to grow around their roots, and to convert nutrients back into something these swamp plants can use.  But terrestrial plants don't have these mechanisms.   Again, where's the fertilizer?    

I just have to shake my head when schemes like this are brought up.  The focus of the product promoters is so narrow, so short-sighted.  Greed or ignorance?  But given a flush enough advertising budget, they will convince some people to buy it.    

If perchance anaerobic digestate was dried and aerated, to return to aerobic conditions, and if the toxic compounds, like formaldehyde, could be removed, and soluble nutrients added back in, and the low pH neutralized, the pathogens are in high numbers.  Without adequate competition, and no protection for the plant, diseases will take out the plants.  Nothing left in that..... ah ..... stuff, to stop the pathogens.    

Do some reading on anaerobic metabolism and the anaerobic waste products.  Most introductory microbiology text books will cover it in reasonable detail.  Read up on Clostridium, a genus of anaerobic bacteria which flourish at low oxygen concentrations in materials coming from anaerobic digestors.  

So, anaerobic digestate as a fertilizer?  The simple answer is...... no.

Elaine R. Ingham

President, Soil Foodweb Inc.       Soil Life Consultant

UNQUOTE

On 2016.06.16 I edited my question as a result of some of the answers. I added a subsidiary question: " Can we convert digestate to aerobic nature to avoid pathogens?"

Again with input from Dr Ingham, it seems this is not difficult: "With aeration, the digestate could be resuscitated and used to grow worms and make worm cast.  Just remember that some manure or source of nitrogen, phosphorus and sulfur will likely need to be added, such as green plant materials, since all the soluble N, P and S was lost a gas during the anaerobic digestion."

In summary, if the digestate can be exposed to aerobic conditions and if a source of aerobic microorganisms is provided, the aerobic species will out-compete the anaerobic species.  A concept block flow diagram shown below.

Dear John,

In relation to your two original "departure points", you are right on both points! the thing is anaerobes do not like oxygen and die quickly when exposed. In fact they are fragile to a number of other environmental conditions, including temperature and pH. The digestate is a rich source of nutirents for plants; and assuming it is not full of contaminants from other sources, it should serve your plants well. I dont think that you would expereince any yield deficit from the presence of a few short-surviving anaerobes.

I hope that this helps. Robert

Thanks, Robert.

I am a chemist; not a soil scientist, nor a microbiologist, so it's easy to confuse me.  ;-)

I find a disconnect between:

(a) your "...  digestate is a rich source of nutrients for plants ..."   and   

(b) Dr Ingham's "... all the soluble N, P and S was lost a gas during the anaerobic digestion ...".

Care to comment?

Further, have you any ideas about my Question 3 (beginning of the thread)?

Hi John,

You will know better than I then, as i am neither a biologist, chemist. Biological and chemical processes are dynamic, but I have seen little literature to show that the overarching issues described here.

I have completed a lot of research into AD, which is highly referenced (link attached). Might I suggest you have a look, in particular to chapter 2, which will set out some of the nutirent dynamics based on the current literature.

I see you are looking at micro-scale digesters. If they digest their feedstock thoroughly, the digestate should not be able to support the remaining anaerobes and the population will naturally die - they will also not produce methane (a pollutant) and also I assume the reason for the process (to gain energy for cooking). However, it will not require a great deal of O2 to have the same effect.

happy reading.

robert

Thesis Assessing the multi-functional role of anaerobic digestion in England

Thanks, Robert. I'll study it.

Great. Let me know if it was useful or have any queries.

Once the digestate is applied to the field/soil, there has been a shift from anaerobic to aerobic conditions, thus wouldn't it seem plausible that the environmental conditions have now shifted to favour the aerobic bacteria that are indiginous to the soil - and the digestate derived bacteria will die?

I would hypothesise your AD bacteria will die in aerobic soil conditions, and thus no harm to plant performance.   

I am designing an AD trial on a vineyard that will/shouldl answer this question, we should have a definitive answer for you in 9 months.

Thanks, Bede. My primary focus is domestic-scale digesters, with presumably a low ratio of available soil to digestate quantity. Now I wonder about industrial-scale digestate disposal, where availability of land for digestate disposal may be a limiting factor, along with capacity to convert from anaerobic to aerobic conditions.

Good wishes for your initiative; I look forward to hearing the results of your work.

There are facultative and obligate anaerobes. Facultative anaerobes can survive under low-oxygen conditions, obligate anaerobes cannot. When these microorganisms die, they provide valuable nutrients to other forms of life: everyone eats, everyone is eaten.

Thus, if the conditions in the soil are such that they are in the range to which the specific plant species under consideration is well adapted, then that plant will do well. If those conditions are, in addition, such that either facultative or obligate anaerobes can do well, then they, likewise, will survive, and if they are not, then the anaerobes will die and provide nutrients to the plants: and the vast majority of plants require an aerated soil.

Note that if any fertilizer is over-used-- _any_ fertilizer, from any source-- then it is likely to cause harm to plants. Where, for example, digestate or effluent from a biodigester is used as a fertilizer, it must be used within limits (again, exactly like any other fertilizer), or the probability is tat it will cause harm to plants. Similarly, overwatering can cause harm to plants.

I have almost 400 peer-reviewed articles in my far more extensive collection of subjects pertaining to biogas which are directly regarding the use of digestate in agriculture. These studies have shown that, properly applied, biodigester effluent is an excellent fertilizer, and does not harm plants. _Of course_ the overuse of digestate can produce anaerobic conditions in the soil. But the balance is clear: it it took an hectare of land to produce the plant matter that was directly (or indirectly in the form of manure) used to produce the effluent, then it should be spread on approximately the same area: an hectare.

I would also mention that, unfortunately and as a matter of science, not commerce, Dr. Ingrahm is simply wrong in several of the statements she has made. For example, I know of no reports in the relevant literature of phosphene gas as a component of biogas at greater than ppm or even ppt levels. The amount of sulfur in the feed material converted to hydrogen sulfide gas depends entirely on the pH of the digestate, and in the pH range needed for healthy digestion, it can often be a single-digit percent of the organic sulfur in the feed material. Biogas from the great majority of substrates has very low (ppt) amounts of hydrogen sulfide. And finally, ammonia is a rare constituent of biogas. The result of these facts is that effluent generally has essentially all the phosphorus, sulfur and nitrogen in it that was in the original substrate.

As regards effluent being acidic, then as with all biological processes, anaerobic digestion is homeostatic, and a healthy digestion process cannot produce other than a mildly acidic effluent (>6.8). indeed, proper digestion produces a strong buffer system. As compared with aerobic compost, every reference I can recall asserts on the basis of analysis that there is _more_ plant-available N in effluent, because ammonia is also produced in aerobic composting, and where the pile is well-aerated, the ammonia tends to volatilize. I make all these statements, again, on the basis of a vast, peer-reviewed, scientific literature.

And I apologize for the mention, but the gratuitous ad hominem comments surely have no place in a science-based discussion, and may tend to make one think that the necessary dispassionate perspective required for good science has been lost.

David House, The Complete Biogas Handbook, completebiogas.com

I acknowledge the criticisms of @David William House and have drawn Dr Ingham's attention to them. I apologise to all researchers who have been offended by any argumentum ad hominem in which I may have indulged.

I appreciate your response Dr. Hofmeyr. Vermiculture is not my area of expertise, and I have not done vermiculture, but with regard to your question, asked so long ago, about aeration of effluent before vermicomposting, it seems to me self-evident that when water is drained from effluent that still has solid constituents, air will replace water in any void that has been evacuated. Mixing coarse sawdust with effluent before or after draining promises to increase that effect. Indeed, I would venture that the differences between, say, fresh soft manure and effluent as regards the ease or difficulties of aeration would be similar: when bedding made from one needs to be turned, likely bedding made from the other will need to be turned. Effluent may have an additional advantage, which is that it would be less likely to overheat, given that so many of the labile constituents would have already been broken down.

In any case, I know based on the literature that vermicomposting following digestion is fairly common in many areas of the world where digesters are also commonly used, such as India and China, since it is often listed in the relevant references as providing an additional source of income, particularly for women, through the sale of worms and compost/fertilizer.