Loading calculations

Loading calculations

By Jose Alfredo López   3:03   1 comentario:

Once you have acquired your biodigester, this must be loaded with organic matter and water to allow the proliferation of methanogenic bacteria. These bacteria are supplied by the manure and are necessary to produce biogas. 

Summarising the previous blog post (click here to read it) I acquired an 8.5 m3 biodigester and determined that it should be fed daily with 19 kg of pig manure and 41 kg of water for a 100 days Hydraulic Retention Time, 8.0 % dry matter and an Organic Loading Rate of 0.63 kg organic matter/m3/day. Now we need to determine how to load it.

The first step is defining the useful volume. We already did that in the Feeding Calculations blog post by defining a 70 % useful volume (6.0 m3). Then, we determine how much manure and water is needed to fill the six cubic metres of useful volume. But first of all, let's define that the mix of water and manure has a 10 % dry matter. 

Why cow manure? Can't we use any other substrate to load the biodigester?

Cow manure is ideal because ruminants faeces contain huge amounts of methanogenic bacteria, which means that we will not need to wait too much to obtain a flammable biogas. We can also use other manures, although I haven't, but I presume they may take a bit longer to produce a flammable biogas. So for a household biodigester, we can use dog faeces. This may not be rich in methanogenic bacteria, but it has some, for sure. or you can use the household wastewater, but you may not want to mess up with that. It is also suggested to add ruminal content (10 % of the total volume, approx), this is extremely rich in methanogenic bacteria. The problem is that ruminal content can only be acquired in slaughterhouses which are usually located outside the cities.

Do I need to do laboratory tests? Are they expensive?

If you want to be sure what you are doing is right, do it. Dry matter and organic matter determination are the cheapest analysis. You can also follow the classic water:manure proportion of 2:1 (in kilograms), but this only works for fresh manure. For slurries (manure mixed with water), the proportion might be 1:1 or even 1:2, and for dried manure (including rabbit, guinea pig and horse fresh manure) it can be 3:1 - 5:1. The density is easy to determine by adding a determined amount of manure into a volumetric bucket of water and recording the water displacement.

Dry matter determination

What happens after the biodigester is loaded?

I will tell you the whole biodigester story in the next blog post, but once the biodigester is loaded, this must be left resting as a if it were a batch system (no feeding) until it reaches a pH of 6.8 or starts producing a flammable biogas. Once you get to this point, you start feeding the biodigester progressively. During this resting period, the organic matter of the manure will be fermented and methanogenic bacteria will start reproducing without the feeding pressure. So every time, more and more methane will be produced (click here to know about the conversion of organic matter into methane). A flammable biogas contains from 50 to 70 % of methane, many factors affect this proportion. 

Read More

Feeding calculations

Feeding calculations

By Jose Alfredo López   22:23   No hay comentarios:

In this post, we are going to size a biodigester for a small farm. Where I come from, Latin America, the most popular biodigester is the plug-flow tubular biodigester because it lasts up to 10 years, it is relatively cheap, and it is easy to install. So, let's start:

This example is a real case; it was the second tubular biodigester I installed. The smallest tubular biodigester size available at the store was 8.5 m3.

8.5 m3 Plug-flow tubular biodigester

Just as you see the biodigester in the image above, it is easy to determine some measures like the length and its width. If we multiply the width by two, this is the measure of the circumference perimeter. In the image below, the measures in black colour are the ones we already know. In red colour, we apply some basic geometric formulas to determine the radius (r) of the circumference and the area

Knowing the measures of the biodigester we can now determine how much it can be fed daily using pig manure. For this, we will use the parameters stated in this blog post: Parameters of design (click here).

As said in the post, we will leave a 30 % space for the gas and the rest will be the space occupied by the digestate.

The digestate space is also the useful volume. Why does it have to be 30 % and not less? That depends on you, take into consideration that the gas space in the biodigester also works as a gas reservoir. Sometimes I also use 20 % but no less than that because the bubbles formed in the upper layer of the biodigester can block the gas pipes.

Now we need to determine the Hydraulic Retention Time (HRT), in the Parameters of Design Post, I said it is comprehended between 30 and 100 days.  Here is a consideration to take into account when working with manures: the lower HRT, the more contaminated with pathogens (coliform bacteria) the digestate will result (Al Seadi, 2008). This researcher also claims that the Hydraulic Retention Time should be chosen taken into account the atmospheric temperature. The colder the temperature, the higher the Hydraulic Retention Time should be. Therefore, because I want to obtain a clean digestate and the average temperature of my city is 20 degrees Celsius, let's choose 100 days of retention time.

Dividing the useful volume by the Hydraulic Retention Time (6000 litres / 100 days) it results in a daily feeding of 60 litres.

60 litres of feed is not only pig manure, it is also water. As stated in the Parameters of Design post,the dry matter of the digestate should be lower than 10 %. Therefore, the pig manure + water mix should have a dry matter content lower than 10 %. If higher, the biodigester can get obstructed. Let's use 8 % dry matter.

In the laboratory I determined the following:

Pig Manure dry matter: 25.3 % of total weight

This goes beyond the calculation, but the 19 kg of manure should be entered into the biodigester progressively. I mean, during the first week of feeding you will use 5 kg of manure and 10 kg of water, during the second one, 10 kg of manure and 20 kg of water. Keep doing that until you reach the 19 kg. I usually suggest doing this progressive feeding during five weeks.

As you can see from the calculation, the proportion water: manure is 2:1, now you know where this classic feeding balance comes from. However, this is not a rule because it can change based on our assumptions for dry matter and HRT. So far, we have determined that our 8.5 m3 biodigester will be fed with 19 kg of manure and 41 kg or litres of water. Now let's calculate the Organic Loading Rate (my favourite parameter). For this we need the organic matter of pig manure:

Pig Manure organic matter: 20.25 % of total weight

A 0.64 kg Organic Matter per cubic metre of useful biodigester per day is a good parameter to start, but It could be better. We could try with a higher value like 0.8 kg OM/m3/day to obtain a greater amount of biogas. Now we can start our calculation all again but state the amount of OLR we want (maximum 1.0, this is my personal suggestion) and we will see that the HRT and the dry matter will change. Here we should not forget that the maximum amount of dry matter of the manure + water mix is 10 %. You will notice something interesting like it is not possible to get a lower HRT and high OLR without exceeding the 10 % dry matter rule. As stated before this will have an effect on the digestate microbiological quality and the biogas production. So what did I chose?

I selected a high OLR (1.0) with a little HRT of around 30 days. Of course, I obtained a pretty contaminated digestate, but this one was only applied to the soil and not plants (except for grass). The decisions you make will depend on your purpose. Of course, if you are planning to sell your digestate, make sure it is clean, or you will get into trouble. You can also use the biogas to boil the digestate if you do not mind about the biogas, this is up to you. As you can see, there are many things involved, and you should wisely choose your digestion parameters based on your goals (produce biogas or produce digestate).

Final Result. The bag on top is a 2 m3 gas storage

The final result.

Before starting to feed the biodigester with the 60 litres of manure + water mix, it should be loaded with digestate. The calculations are much simpler, but we will see this in the next blog post.

Read More

The four stages of anaerobic digestion

The four stages of anaerobic digestion

By Jose Alfredo López   23:43   No hay comentarios:

Knowing the four stages of anaerobic digestion will allow you to explain why biodigesters fail when they are overloaded, how to fix this issue, why biogas do not burn in the beginning and many other things. So let's start:

In the early years of anaerobic digestion research, Omelianskii (1906) described the Methanobacillus omelianskii as the responsible bacteria for converting organic matter into methane. By this date, methane was a well-known gas because people related it with the awareness of not lighting a fire in swamps. Anyway, later in 1967, Bryant found out that the Methanobacillus omelianskii was not a single bacteria, but an association of two microorganisms. Indeed, two types of bacteria participate in the conversion of organic matter into methane and carbon dioxide: acidic and methanogenic bacteria.

The following stages occur all at the same time; however, they are called stages because the products released by bacteria in one phase is substrate for the bacteria participating in the next one.

Source: Van Lier (2014)

Hydrolysis: This phase is slow and consists in the breakdown of all the big and complex molecules (polymers) into smaller units (monomers and oligomers). This breakdown is done by enzymes released by hydrolytic bacteria. Proteins are broken down into amino acids, fats into long chain fatty acids, and sugars into monosaccharides.

Acidogenesis: This is the quickest stage. Here, acidogenic bacteria convert all the monomers and oligomers into volatile fatty acids (VFA). During the process carbon dioxide is released. Because carbon dioxide does not burn, a non-flammable biogas means this stage is prevailing in the biodigester.

Acetogenesis: Some authors combine this stage with Acidogenesis. In this phase, volatile fatty acids are converted into acetate. Carbon dioxide and hydrogen are also released during the process.

Methanogenesis: Methanogenic bacteria are very slow to duplicate which is a disadvantage compared with acidogenic bacteria which need few hours to replicate, methanogenic require days.There are many ways in which methane is formed, but two routes prevail. The biggest part of the methane comes from the acetate (70%), and the rest of the carbon dioxide and hydrogen conversion (30 %).

The main aspects from the fours stages of anaerobic digestion are:

- All these phases occur at the same time in the biodigester

- Acidogenic bacteria duplicate quicker than methanogenic bacteria

So, we can now easily explain what happens in a Batch System. Once the substrate is mixed with the water inside the biodigester, the different types of bacteria that the substrate contains start degrading the organic matter. In the beginning, hydrolysis will prevail because complex molecules mostly compose substrates like manure. However, there are also simpler molecules which are substrate for acidogenic bacteria. The release of carbon dioxide from this stage is the main constituent of the biogas after the first day of digestion. As days pass by, most of the complex molecules of the manure are already broken into simpler molecules, and acidogenic bacteria start processing them. Methanogenic bacteria are also working since the very beginning because there is also acetate in the manure, but as days passes by, more acetate, carbon dioxide, and hydrogen are formed, and more methanogenic bacteria are produced to deal with them to generate methane. 

How do you think the biodigester will work if you feed it daily? Don't you think a non-flammable biogas is more likely to be produced if acidogenic bacteria have an advantage over methanogenic bacteria because of the duplication rate) Well, we can avoid this by regulating the amount of feed we put inside the biodigester. If we feed the biodigester with moderate amounts of substrate, then methanogenic bacteria can reach an equilibrium with acidogenic bacteria and a flammable biogas (rich in methane) will continuously be produced. Now, you may be thinking how much is low and how much is high? The anaerobic digestion parameters (HRT and OLR) will help us with that, but we will see this in another post.

References

Van Lier, J. 2014. Anaerobic Fundamentals and COD Balance. TU Delft. [Retrieved from https://www.youtube.com/watch?v=Yr6LIu379Yw&list=PLrwuNGSwGLHcrfxlTiJ2zTGWi6-fEZV8m&index=5]

Read More

Design Parameters of Biodigesters

Design Parameters of Biodigesters

By Jose Alfredo López   0:35   No hay comentarios:

In this post I will talk to you about five must-know parameters for biodigesters design.  These are essential to know before going any further with biodigesters

1 - Useful Volume

2 - Dry Matter of the substrate 

3 - Organic Matter of the substrate

4 - Dry Matter of the digestate

5 - Hydraulic Retention Time

6 - Organic Loading Rate

From now on, every time you read a paper about biodigesters, evaluate the functionality or compare the operation between biodigesters, compare these parameters. They tell you a lot about if a biodigester is working or not or why a biodigester produces more biogas than another.

1 - Useful Volume: The whole biodigestion chamber represents the total volume. Inside the chamber there is a space for the gas (20 - 30 % of the total volume) and the rest is the space occupied by the digestate. The digestate space is the useful volume. In the image below, you can see that the useful volume is limited by the horizontal tube on the right. It is impossible that the digestate reaches a higher volume because it will leak through the horizontal tube. 

Total and Useful volume in a household biodigester.

2 - Dry matter of the substrate: The substrate is the feed to be used in the biodigester. Every substrate contains water, everyone, even the straw. The dry matter is the difference between the total weight of the substrate and the weight of the water. The dry matter is represented as percentage of the total weight. This parameter is determined in a laboratory or you can use a literature value.

3 - Organic matter of the substrate: The dry matter of a substrate is composed by minerals and organic matter. The organic matter is represented as percentage of the total weight or the dry matter weight. This is also determined in a laboratory or you can use a literature value.

4 - Dry matter of the digestate: Same concept as (2). It should be lower than 10 %.

5 - Hydraulic Retention Time: This parameter is determined with a very simple formula. By analysing the formula below, the Hydraulic Retention Time means the time the substrate is left in the biodigester for fermentation. This parameter is usually set up between 30 and 100 days. Yes, it is a large range; however, this value will also vary depending on the dry matter of the digestate and the Organic Loading Rate. We will see this in a future post exercise.

6 - Organic Loading Rate: This is the amount of organic matter to be fed to the biodigester per cubic meter of useful biodigester volume per day. The higher this parameter, the more biogas you will produce, but if it is too high it can inhibit methanogenic activity. In a small-scale context I suggest not to be more than 1.0. After using this OLR you can try with higher amounts and see if the system is still working alright.

After knowing these parameters, we are ready to see the continuous system. 

Read More

How does a biodigester work?

How does a biodigester work?

By Jose Alfredo López   0:59   1 comentario:

Biodigesters are the environmental technology that everybody wants to know about, but few know how to. Certainly. the implementation of a biodigester requires more knowledge and money than composting and to operate it efficiently you need to know a bit more than the basics. Therefore in this blog, I'll teach you how to build a small one where it will be easy for you to learn the basics and gain the confidence to build a bigger one.

The methane generated in a biodigester concentrates a lot of energy which can even be used to generate electricity

A biodigester is an anaerobic chamber where several groups of bacteria oxidise the organic matter into carbon dioxide and methane. This means that inside a biodigester, there are different types of bacteria waiting for you to feed them with manure, food waste, fats, wastewater or even cannabis (for real!, soon I'll show that research here) to produce biogas which is a flammable gas that contains methane.  As you might be guessing, or already know, the coolest thing of this process is that this biogas can be used in your kitchen to substitute the natural gas you normally use. One key aspect of this process is that oxygen should not enter the biodigester. Well, actually, there is always a tiny small amount of oxygen inside but it does not interfere with the whole process. Just remember, no oxygen allowed.

Most publications describe two system designs for biodigesters:

1 - Batch: This is basically, just feeding once and leave it for fermentation. This is only useful to generate the digestate which can be used to fertilise plants.

2 - Continuous: Under this system a biodigester is fed constantly and a continuous production of biogas and digestate is possible.

So, let's start with the batch type. Here is a picture of the simplest biodigester ever:

Batch system

So, as you can see in the picture, a batch system can be made by joining two water drums with a gas hose. The water drum to the left is filled up to 3/4 of its volume with manure and water. In the image I used one part of cow manure and two parts of water. Then, this water drum is sealed with a stopper that has a small tube attached to hose whose extreme is inserted into water. So, instead of the water drum to the right you can just use a bucket with water. The fact that the hose is inserted into water is to prevent the air from entering into the hose and reach the water drum to the left. One day after the system is set up, you will be able to see bubbles coming from the water. Those bubbles are the biogas which is making pressure through the water column to escape; however, this biogas is mainly carbon dioxide which is not flammable. More time is needed for the methane to appear, and burn.

Biogas escaping from water

I suggest to keep running this system up to one month, after which, most of the biogas will already be released. Of course you can also capture the biogas in life guard rings, but the amount will be so small that it is not worth to use it. So, if the biogas is lost, what is this system for?

1 - To produce digestate: After one month you will have a digestate that can be used to fertilise the plants in the garden or any crop you have (except crops whose leafs are eaten). Do not worry, it does not have a bad smell. You can dilute it with water (one part of digestate and two parts of water).

2 - To produce inoculate: The digestate generated is also a good inoculate because it is rich in methanogenic bacteria.

As you can see this system is very simple, but do not look at it over the shoulder, you may want to try this system first because you will need the inoculate to produce biogas in your future biodigester. You can give it a try using dog poop. In the next post we will see how to make a continuous system.

Read More

Windrow composting

Windrow composting

By Jose Alfredo López   7:09   1 comentario:

Windrow composting is the most common composting method. Basically, the method consists in accumulating the biodegradable material in large narrow piles above the ground, moisten and turn it every two weeks or three weeks. In the next lines I'm going to show you the experience at the Agrarian University (Peru):

The Waste Treatment Centre of the Agrarian University collects all the garden waste (carbon source) generated at the university and compost it mixing it with the manure generated at the University farms. The mixing is done in similar weight proportions: 50 % carbon source and 50 % manure. This is the common proportion applied to compost manure because under this proportion is possible to reach the 20:1 - 30:1 carbon:nitrogen proportion. So, if we want to treat 250 kg of manure, we mix it with 250 kg of carbon source. To do windrow composting I suggest to use at least 250 kg of both materials combined  because with less volume there will be heat loss and composting will take an eternity.

Garden waste (carbon source)

Manure (nitrogen source)

A 500 kg composting pile

Temperature monitoring is suggested to see if composting is going all right. A soil thermometer is very cheap. 

As you may see in the figure above, the composting pile is placed in a concrete base. This is not necessary in small-scale. Large scale composting does require a concrete floor because of the leachate percolation which can contaminate the groundwater.

Water needs to be added frequently, perhaps once every two days during the first weeks. This varies depending on the moisture content of the organic material and weather conditions. There is not a specific water amount, but the important thing is to keep it humid. Many people complain that their composting pile does not reach enough temperature, this can be easily solved by adding more water or adding nitrogen-rich material like manure.

The first turning should be done at the day 15. The reason is very simple, if water has been added constantly and the composting pile has enough nitrogen-rich material, one can be sure that by this day the temperature has reached enough temperature to kill animal and vegetable pathogens (70 degrees for three days or 55 degrees for ten days). When you turn the composting pile you are aerating the organic material which is good because this is an aerobic process (requires oxygen). Because turning cools the composting pile, this should not be done constantly during the first two weeks (I even doubt you will even try it because 500 kg is very heavy to turn) or the pile will not reach the 70 degrees Celsius desired temperature. In the image below you can see the first turning, the material is well degraded by the second week.

The first turning is done at the day 15

The next turnings can be done every two or three weeks, and water does not need to be added so constantly because the compost is acquiring the capacity to retain water.  A mature compost can be obtained at the fourth month.

Just a final screening (optional) and the compost is ready to be added to the soil! Mature compost does not have smell, has a cool temperature, has a coffee colour and seeds are capable to germinate on it. 

Read More

Which is the most suitable way to treat food waste at home?

Which is the most suitable way to treat food waste at home?

By Jose Alfredo López   5:45   1 comentario:

You, as well as me, might be worried about food waste. Food waste is the primary source of greenhouse gases and leachate at landfills. If we divert food waste from landfills, they could last longer, and most of its pollution issues would be solved.

If you are willing to take action against food waste, you should know that you can do something at your home. However, you need to have a clear idea about what to do with the sub-products from food waste treatment. Three things can be obtained depending on the method used:

1 – Biogas: This can only be produced using a biodigester. It can substitute the natural gas you use at home for cooking.

2 – Liquid fertilizer: This can be generated through vermicomposting, Bokashi composting, biodigesters and lactic fermentation. Liquid fertilizer can be used in the garden to provide nutrients to the soil making your plants look better.

3 – Soil improver: Compost, Bokashi, and humus are soil improvers that increase the quality of your garden soil keeping it healthy.

Depending on your conditions and the sub-product you are most interested in, there is a suitable food waste treatment for you. Take a look at the following diagram!

Click on picture to enlarge

Carbon source: By carbon source I mean green waste high in fibre or dehydrated like dry wild herbs, dry leafs, coffee chaff, wheat bran or even paper.

Lactic fermentation: Probably not available everywhere, at least in Peru it is. Lactobacillus bacteria are used to fermentate liquid waste under anaerobic conditions to create a nutrient-rich digestate with a very sweet aroma (for real!).  I will talk more about in another post.

Vermicomposting: Worm farming.

Read More