From Trash to Treasure

Turning agriculture waste into fertilizer

Rachel Lee
26 min readNov 5, 2022

The population of Africa is expected to reach 2.5 billion by 2050, which will account for 27% of the global population. [1]

More people equals more mouths to feed, which means we must produce more food.

To produce enough food to meet the needs of 2.5 billion people, Africa must triple crop yields to support it’s rapidly growing population. [2]

While this seems incredibly daunting, it is actually very simple. Africa has the resources to produce this amount of food:

  • Africa is home to 60% of the world’s arable land. [3]
  • 61% of Africans are farmers. [4]
  • 16.9% of the African population are young people who need employment. [5]

Africa has the potential to produce 3 times more cereal grains (2.6 million tons of food), enough food for Africa to support itself and become completely self sufficient. [6] So the question becomes, why isn’t Africa leveraging these resources to unlock it’s full agricultural potential?

Why does Africa’s full agricultural potential remain untapped?

Africa has the lowest agricultural productivity in the world. It is the only country in which per capita food production has decreased over the last 30 years. [7]

In the 1960s, the average cereal yield in Africa was 57% of the world average, and by the 1990s the gap had widened and yields were only 47% of the world average. The yields today remain at the 1990s level. [8]

Since the green revolution, crop yields around the world have skyrocketed. Everywhere but Africa.

Crop yields in the U.S, Brazil, Russia, Asia, India and Africa of the 3 major crops in the world: maize, rice and wheat, from 1961 to 2020. For all three crops, Africa’s yields are far below the rest of the world. source

African yields have remained stagnant, refusing to budge — or in the case of rice, yields have actually decreased. So this begs the question WHY. Why is agricultural productivity and crop yields so low in Africa?

It boils down to a list of root causes:

  • Low quality seeds
  • Lack of proper storage facilities
  • Poor transportation infrastructure
  • No farming technology
  • Little to no agriculture education
  • Extreme and unpredictable weather
  • Poor soil health
  • Lowest fertilizer application rates in the world

In this article I am going to focus on the importance of fertilizer for increasing crop yields in Africa.

Let’s take a step back

What even are fertilizers and why are they important?

Fertilizers are like the vitamins you take — you can’t get enough vitamins and minerals on your own so you take supplements.

The same goes for plants: plants create their food by harnessing the power of the sun to create sugar from carbon dioxide and water (photosynthesis). Fertilizer maintains healthy soil by replenishing the soil with nutrients that the plants have depleted.

In order to thrive, plants need 16 essential minerals. When there is a shortage of these minerals, productivity decreases and crop yields decrease. Fertilizer is used to replenish the soil with the minerals and nutrients plants need to grow and provide beneficial elements (like additional nutrients and microbes) to help the growth of plants. [9]

How fertilizers work

Three of the 16 elements, carbon, hydrogen, and oxygen, are provided by water and carbon dioxide, which means they are always available to plants in abundance. The remaining 13 essential elements are absorbed through the roots of a plant from the soil. These 13 mineral elements are divided into three categories:

  1. Micronutrients: are only used in very small amounts by plants, and deficiencies of these nutrients are very uncommon. Fertilizer usually doesn’t contain micronutrients since plants use such a small amount of them. Micronutrients include boron, zinc, manganese, chlorine, copper, iron, nickel and molybdenum.
  2. Secondary nutrients: plants need larger amounts of secondary nutrients than micronutrients, but less than macronutrients. There are only three secondary nutrients: calcium, magnesium, and sulfur. (Acidic soils with a low pH are because of low calcium levels).
  3. Macronutrients: are the main elements plants need and are used by plants more than any other nutrients, which means that the soil is usually deficient of these nutrients. The primary macronutrients are nitrogen, phosphorus and potassium which are used in very large quantities by the plant.

All fertilizers have a specific NPK ratio to provide plants with the optimal percentage of each nutrients. (NPK stands for the 3 macronutrients: nitrogen (N), phosphorus (P) and potassium (K)). A 16–16–16 fertilizer will contain an equal balance of all three macronutrients. A 25–4–2 contains 25% nitrogen, 4% phosphorus and 2% potassium. [10]

Nitrogen is the most important nutrient that is essential for plant growth. The air we breath is 78% nitrogen (and only 21% oxygen), but plants are not capable of absorbing nitrogen from the air, so they must uptake nitrogen from the soil. [11] This is the main goal of fertilizer — to provide plants with nitrogen in a usable form.

The amount of fertilizer needed

The amount of fertilizer a plant needs depends on a lot of variables — the kind of plant, the plant’s growth rate, the amount of fruit it produces.

As a general rule of thumb, the faster a plant grows and more fruit it produces, the more fertilizer it requires.

(Fun fact: Corn, the most important and widely produced crop in the world, is also the hungriest crop, meaning it needs the most fertilizer. 5.6 million tons of nitrogen are applied each year to corn fields in the USA. [12])

The global average amount of fertilizer applied per hector is 100.78kg/ha. The optimal amount of fertilizer is within a range of 50–100kg of fertilizer per hector of land.

Africa is WAY behind this target, applying an average of 15.88kg/ha, and in many Sub Saharan African countries the average is less than 7kg/ha. [13] African fertilizer application rates are almost four fifths below global average.

Average nitrogen fertilizer usage in Africa compared to the global average. source

Now that you’re an expert on what fertilizers are made of and what they do, we can dive into the different types of fertilizers.

Fertilizers can be places under two broad categories: inorganic or organic.

Inorganic fertilizers

Inorganic fertilizer (also called synthetic fertilizer or chemical fertilizer) is the most common type of fertilizer (and the fertilizer that is responsible for feeding half of the global population — 3.5 billion people— ).

Synthetic fertilizer is derived from chemicals like nitric acid, natural gas, sulfur, and atmospheric nitrogen. Synthetic fertilizers are created by the Habor Bosch process, an extremely energy intensive process that combines nitrogen with hydrogen to produce ammonia, a critical component of nitrogen fertilizers.

This is done by applying extremely high pressure (200–400 atmospheres) at extremely high heat (450° to 650° C) to methane, water and air. The result of the process is a liquid ammonia that can then be used to create synthetic fertilizers. [14]

6 tons of carbon are emitted into the atmosphere for every ton of ammonia produced, representing 1.8% of global greenhouse gas emissions. [15] (AKA this is NOT an environmentally friendly process. It is actually the exact opposite).


  • Synthetic nitrogen fertilizer doubles crop yields, increasing total yields by 57%. [16]
  • EXTREMELY efficient. Synthetic fertilizer is the most important farming tool that has dramatically increased yields, reduced poverty and created enormous amounts of food.
  • Very fast acting — since synthetic fertilizer is water soluble it can be sprayed onto plants and be taken up very quickly. For depleted soils that need nutrients immediately, synthetic fertilizer is incredible at quickly providing plants with the nutrients they need.
  • The Habor Bosch process is extremely cheap, averaging at $160 per ton of ammonia produced. [17] (It is the inputs that makes synthetic fertilizer so expensive.)


  • Synthetic fertilizers must be reapplied very frequently to maintain results. (Think of synthetic fertilizer like a shot of espresso — it is an extremely concentrated and highly effective short burst of nutrients that wears off quickly).
  • Only half (50%) of synthetic fertilizer applied to crops makes it to the plants it was intended for — the other half runs off into bodies of water creating massive dead zones, releasing nitrate and methane into the atmosphere. [18]
  • Applying too much too quickly leads to fertilizer burns and overapplication which can stunt plant growth.
  • 1.8% of global greenhouse gas emissions are from nitrogen fertilizer production. Producing nitrogen fertilizer requires a lot of energy and is extremely harmful to the environment. [19]
  • Depletes soil nutrients in the long term.
  • Causes toxic buildup of dangerous chemicals like arsenic, cadmium, and uranium.

Organic fertilizers

Organic fertilizers get nitrogen, phosphorus and potassium from plant or animal sources (aka organic matter) like blood meal, compost, bat guano, manure, seaweed, and worm castings.

This organic matter naturally contains NPK in a usable form that can be directly used by plants.


  • Organic fertilizers are typically cheaper when buying through a retailer. When producing organics yourself, costs are practically nothing!
  • Organics do not release chemicals or toxins that are harmful to plants.
  • Organic fertilizers provide plants with the three essential micronutrients, but also contain bacterium, microbes, and microorganisms that are extremely beneficial for the soil.
  • The organic matter improves soil structure, reduces erosion and improves water retention (which is especially useful in hot arid conditions where drought is widespread).
  • There is no pollution that happens during production of organic fertilizers
  • Almost no runoff! The majority of organic fertilizer applied to crops makes it to the plants it was intended for. If a small amount of the fertilizer does get washed away by rain, it will not harm ecosystems or negatively effect the environment.


  • Organic fertilizer is not nearly as effective as synthetic fertilizer. Since organic fertilizers are made from organic materials, they contain the macronutrients, but in very low amounts. So the the soil bacterium has to break down the micronutrients and convert it into larger quantities that the plant needs. (Organic fertilizer is not nearly as effective in the winter/cold weather because the soil bacterium are not as active in cool temperatures).
  • Organic fertilizers have to break down into the soil before they can be absorbed by plants. It usually takes 3–5 months for organic fertilizers to fully break down and become usable to the plant.
  • Pests and bugs are more drawn to organic fertilizer, which can cause pest infestations. (Especially for organics like manure or compost which is not fully broken down).
  • Organic fertilizers are much less common/popular than synthetic fertilizer, meaning that availability is limited.

The fertilizer problem in Africa

No innovation in the 20th century has been as impactful as synthetic nitrogen fertilizer. Fertilizer is responsible for increasing crop yields at a rate of 120% per year from 1904–1980, reducing global poverty by 20% and increasing farmer incomes. [20] (AKA fertilizer is one of the most important tools for increasing crop yields and reducing poverty).

Despite the enormous importance of fertilizer, it remains unused in Africa.

I call this the fertilizer problem.

Fertilizer is this incredible tool that can solve so many problems in Africa (like creating more food, creating jobs, increasing yields, improving soil health, mitigating climate change), but yet this resource is ignored.

It remains an untapped resource to solve some of the biggest problems that African countries face, and it will remain an untapped resource until something is done about the fertilizer problem.

The fertilizer problem can be broken down into three buckets, or root causes:

  1. Price
  2. Environmental impact
  3. Accessibility


Outrageous prices is the single biggest reason why fertilizer use is so low in Africa. The cost of fertilizer in Sub Saharan Africa is four times higher than the rest of the world. It doesn’t make economical sense for a farmer to spend a large portion of their income on fertilizer, when they need to pay for their child’s education or electricity for a light bulb. [21]

There are a few causes for these ridiculously high fertilizer prices:

  • One of the largest causes is extremely poor transportation infrastructure which makes it almost impossible to transport fertilizer to mainland countries. Bribes and tariffs at each country boarder are estimated to increase the final cost of goods by 18%. [22] (Transporting fertilizers from an African seaport to a farm 100km inland can cost more than shipping the same fertilizers from North America to the African seaport.) As a result, African smallholders pay 2–4 times the average world price for fertilizer. [23]
  • Africa imports 80% of it’s fertilizer, meaning that it is directly reliant on a supply chain of another country. Even a minor fluctuation in the supply chain or price of fertilizer will have a massive ripple effect on the importing cost of the fertilizer for Africa. [24]
  • The Russia Ukraine war has increased fertilizer prices everywhere, hitting Africa the hardest. Africa imported 40% of it’s fertilizer from Russia and Belarus — since the war in Ukraine, exports from both countries has been stopped. [25] Global fertilizer prices have increased by 312% since the fall of 2022, forcing farmers to go without fertilizer. Since the war in Ukraine, fertilizer prices have doubled in Africa. Because of this, Sub Saharan Africa is facing a fertilizer deficit of 350,000 metric tons. [26]

“The war in Ukraine has had a really big affect on us farmers. All around me here, my neighbors planted without using fertilizer, because they could not afford it. Their crop has failed this time around.” — Benard Mwenja, Kenyan maize farmer, NPR

Since the beginning of COVID (when the global supply chain shut down), Sub Saharan Africa fertilizer consumption fell 30% because farmers could not afford the rising prices. If this trend were to continue, it would translate to 30 million metric tons less food produced, which is equivalent to the food needs of 100 million people. [27]

Environmental impact

Fertilizer is extremely taxing to the environment, from production to application, an immense amount of resources are consumed and emissions are released.

  1. Nitrogen (one of the key ingredients of fertilizer) emits 5 tones of carbon into the atmosphere for every one tone of nitrogen produced.
  2. Once the nitrogen is produced, it is fed through the Haber Bosch process, an extremely energy intensive process that turns nitrogen and hydrogen into ammonia that can then be used to make fertilizer. The Haber Bosch process accounts for 1.8% of global CO2 emissions. [28]
  3. Next the fertilizer travels to farmers all over the world and is applied to fields. Only 50% of the fertilizer applied to fields makes it to the crops it was intended for. The other half is washed away into streams and rivers, causing eutrophication and algal blooms, destroying ecosystems and wildlife.
  4. The half of the fertilizer that remains on the plants gets broken down by microbes in the soil, releasing nitrous oxide, a greenhouse gas 300 times more potent than CO2. [29]

So yes, fertilizers are extremely harmful to the environment.

Another angle to look at the environmental effect of fertilizers is through the lens of soil health.

75% of Sub Saharan Africa’s soils are degraded, worth the equivalent of $4 billion annually. [30] It is a tricky situation in Africa, because applying fertilizer to already degraded soil sucks the soil of the remaining nutrients it has. But under application will produce very low yields. Applying fertilizer to already degraded soils:

  • Causes nitrification, where ammonium-nitrogen is converted to nitrate-nitrogen, releasing hydrogen which causes soil acidification and raises soil pH levels. Low pH levels prevents plants from absorbing critical nutrients.
  • Fertilizer contains a lot of salt which causes the ‘toxicity effect’ to germinating plants, stunting their growth, and in turn decreasing the health of both the soil (because it is now overloaded with salt) and the plant.
  • Reduces the organic matter in soils, which in turn reduces the soil’s ability to store and hold carbon. Over time, carbon that has been stored by microbes and organic matter in the soil are released into the atmosphere. [31]
  • In the long term, fertilizer actually causes soil degradation. Long term application of synthetic fertilizers adds too much of one kind of nutrient to the soil, eliminating the soil’s ability to hold and produce other essential nutrients. [32]


In the 2006 Abuja Declaration, the African government stated that a minimum of 50kg of fertilizer must be used per hector of land. Africa is not even close to this target, averaging 10–15kg/ha. [33]

It is estimated that only 8% of farmers in Africa apply over the recommended (50kg/ha) amount. [34] And these 8% of farmers are large scale farmers who receive large subsidies from the government so they can afford the high price of fertilizers.

The rest of the small scale farmers suffer from low fertilizer application rates because of a lack of access to fertilizers:

  • Currently, a massive fertilizer deficit across Africa is due to the war in Russia and Ukraine. Africa imports 40% of it’s total fertilizer from Russia and Ukraine, both countries have halted exports due to the war, leaving Africa in an undersupply of fertilizer. [35]
  • The African government has stopped all fertilizer subsidies for small scale rural farmers — only some large scale farmers are eligible to receive subsidies. Combining this with the high price of fertilizers, 89% of farmers can not afford the price of fertilizers, or do not choose to take the risk of investing in fertilizer. [36]
  • The average distance that small holder farmers live from central towns and markets is 40+km — too far to walk. Most farmers will take “boda bodas” (motorcycle taxis) to get into town to pick up fertilizer. Due to the rising fuel prices, the cost of taking boda bodas has rose 30%, making it no longer economical to make the trip into town. [37]

So, to recap the problem with fertilizers:

  • Very expensive
  • Long term damage to the soil
  • Harmful to the environment
  • Farmers have to travel long distances to purchase fertilizers

Exploring solutions

To date, several solutions and programs have been put in place to address the fertilizer problem in Africa. In this section we are going to explore some of these solutions and what is missing from them.

01: Expand fertilizer manufacturing across Africa

It is estimated that Africa is home to 9% of the world’s natural gas reservoirs [38] — so it has tremendous power to utilize the abundance of natural gas to create a booming fertilizer business that could not only service Africa, but could also become a global fertilizer supplier.

Producing fertilizer in Africa would allow the country to become self sufficient and un reliant on other countries for fertilizer imports. This would also greatly decrease costs by eliminating transportation — one of the key factors of fertilizer price in Africa.

02: Implement fertilizer subsidies

Fertilizer subsidies have been shown to be one of the most effective ways to increase fertilizer accessibility and increase yields.

In April 2020 Sir Lanka shut down the fertilizer subsidy that had been in place for decades. It’s exports of rice fell by 40%, and it’s tea exports (Sri Lanka’s most grown crop) decreased 15%. [39][40] The government quickly reinstated the subsidy in 2021, but not before it had undermined the season’s crops and lost hundreds of millions dollars in revenue.

Implementing an intense fertilizer subsidy like the one in Sri Lanka, will allow African farmers to afford fertilizer. (To learn more about the potential impact of fertilizer subsidies in Africa, check out this study).

03: Farming training programs

Only one in five African farmers have received agricultural training or education programs. Of those farmers who did participate in programs, 85% were male headed. [41] (Even though 80% of African farmers are women). [42]

At the end of the 5 year agricultural program, the farmers had increased knowledge, improved agricultural practices, better equipment, resulting in higher yields compared to when the farmers started the program.

Farmers knew how much fertilizer to apply, which fertilizer to use, when to apply fertilizer, when to till the soil and how to detect pest infestations.

Another example of the benefits of farmer training programs is a 10-year study done in China that found when 21 million farmers were trained in better management practices, average yields for staple crops rose by 11% and nitrogen fertilizer use decreased by 18%.

What is missing

While some of these solutions have shown promising outcomes, none have produced long lasting benefits to help farmers become self sufficient.

What I don’t see is solutions that:

  • Allow farmers to become completely self sufficient and not have to rely on a central source to buy inputs from.
  • Target lowering fertilizer prices.
  • Heal the degraded soils and don’t cause long term damage to soil health.
  • Cut down the distance farmers have to travel to market to buy inputs and sell products.
  • Decrease the environmental impact of fertilizer production.

Trash to treasure

One solution that checks all the boxes is turning agricultural waste into fertilizer.

Agricultural waste includes manure, grass, crop trimmings, stocks of plants and wood chips. The biggest draw of using agricultural waste as an organic fertilizer is that agri waste contains 73% nitrogen and 13% ammonia, two of the most critical components of fertilizers. [43]

Turning this waste into fertilizer can be done in many different ways: regular composting, vermicomposting, fermentation processes and anerobic or aerobic digestion.


But before we dive into turning agricultural waste into fertilizer, we need to ask where this agricultural waste is coming from. Does Africa have enough agricultural waste to produce fertilizer for the entire continent?

The short answer is yes.

57% of the total waste produced in Africa is organic waste. This includes agriculture scraps and trimmings, rotten food, manure, human waste, and slaughterhouse waste. [44]

In South Africa, 10.3 million tons of edible food is thrown out each year, costing the South African government $700 000. [45] 90% of this waste is incinerated, releasing thousands of tons of CO2 and methane into the atmosphere. [46]

What if instead of burning all this organic waste, we could extract the nutrients from it and convert it into fertilizer, to be re applied to fields and increase yields?

This is the potential that organic fertilizer has in South Africa.

Anaerobic digestion

Anaerobic digestion (AD) is a process where bacteria breaks down organic matter (like agriculture waste, manure, and food waste) without oxygen. The bacteria used in anaerobic digestion are acidogenic bacteria.

If I were to summarize AD into one sentence, I would say that anaerobic digestion is a process that uses microorganisms to break down organic matter into a nutrient sludge that can be used as an extremely nutrient dense organic fertilizer.

This all happens in big reactors:

Anaerobic reactors in California source

The process of AD can be wet or dry, mesophilic or thermophilic, and single or multistage. The most efficient model for producing fertilizer is wet, mesophilic, single-stage:

  • In wet AD, the waste is first broken down into smaller pieces, making a sludge, before it can be processed. This method is much better for products where you want a liquid output at the end (which is the end goal of fertilizer produced by AD). In dry AD, the waste is processed in it’s solid form, needing minimal mechanical sorting.
  • Anaerobic digesters using bacteria that live between 35–40°C are known as mesophilic, and ADs using bacteria that live at 55–60°C are called thermophilic. In thermophilic systems, the energy input is higher, since more energy must be used to keep the environment at that high temperature. Thermophilic systems are commonly used when processing human waste, because the high temperatures can help to sterilize the waste and speed up the process of producing biogas.
  • In single stage systems, all the reactions happen within one reactor or tank. In multistage, each of the four reactions happen in different tanks, which takes up more space and increases cost.

Now let’s jump into the 4 steps of anaerobic digestion, understanding the inputs, outputs, and key processes for each step. 😎


Organic matter is made up of complex polymers, which are inaccessible to microorganisms. So before the anaerobic digestion can start, the matter must be broken down down into very small components: macromolecules.

The bacteria convert:

  • Carbohydrates into sugars
  • Lipids into long chain fatty acids, and
  • Proteins into amino acids

Sometimes certain substrates (lignin, cellulose, and hemicellulse) are not fully broken down by the bacteria, so enzymes are added to the mixture to cut the large molecules up into smaller molecules.

Once the matter is broken down into small enough molecules, the molecules diffuse through the cell membrane of acidogenic bacteria.

This process happens at 30–50 °C, the optimal temperature for the bacteria to function.

The biggest issue with hydrolysis is that it is a very lengthy process. It can sometimes take weeks for the waste to be broken down into small enough molecules. Hydrolysis is also one of the most crucial steps; if the matter is not broken down into small enough molecules, all the rest of the process will be negatively effected. Because of this, hydrolysis is called a “rate limiting step”, meaning that if hydrolysis goes wrong, the entire process will be effected. [47]


The term “acidogenesis” is derived from the word “acidogenic”, which is the bacteria used in anaerobic digestion.

The acidogenesis step relies solely on the bacteria’s ability to convert the small molecules of waste biomass into fatty acids.

Once the acidogenic bacteria absorb the small molecules created during hydrolysis into their cells, they produce intermediate volatile fatty acids (IFAs). IFAs are the building blocks of organic compounds. They are short chain mono-carboxylate compounds which are a precursor to producing biofuel. [48]

Lastly, the acidogenic bacteria break down the remaining amino acids into IFAs. During the amino acid breakdown process, ammonia is produced as a by product. (Ammonia is one of the key elements in fertilizer. This will be important later😉).

Acidogenesis is the fastest of all the steps in anaerobic digestion, taking less than 36 hours from start to finish.


The acidogenic bacteria convert the VFAs into acetate, producing hydrogen as a by product.

This hydrogen is used to create a syntrophic relationship where H2 molecules are passed like ping-pong balls between the acetate molecules, providing acetate with extra charge. [49]

At the same time:

  • Lipids are converted into acetate
  • Glycerol is converted into acetate
  • Fatty acids are converted into propionate
  • Long chain fatty acids with an even amount of carbon atoms degrade to acetate
  • Long chain fatty acids with an odd number of carbon atoms degrade to propionate

So at the end of this process we are left with a lot of acetate.


Next, the methanogenic microorganisms come in to play — methanogenic microorganisms are a type of archaea that convert CO2 to methane. [50]

The methanogenic bacteria consume the acetate and propionate, producing methane.

The methanogenesis stage is often very problematic:

  • Methanogenic bacteria are extremely sensitive to oxygen, with 99% of them dying within 10 hours of mild exposure to oxygen. [51] This makes this stage very finicky, since absolutely no oxygen must be allowed to get into the reactor (which is very hard).
  • Methanogenic bacteria is very slow, taking 10–16 days to fully turn CO2 to methane.
  • The methanogenesis process needs a higher pH level to operate than the other steps in anaerobic digestion. Increasing the pH throughout the entire process will be inefficient for the other stages, but the low pH during methanogenesis decreases the amount of methane the bacteria can produce.
Anaerobic digestion flow chart source

Making fertilizer from this

The main product produced from this process is biogas — a renewable gas made of 60% methane and 40% CO2. This biogas is a renewable resource which can be used to fuel vehicles, heat homes and generate electricity. Currently, the most common use of biogas is electricity.

Digesting 1 ton of food waste through anaerobic digestion produces approximately 300 kWh of energy, enough energy to power 4 incandescent lightbulbs for 10 hours. [52]

The part of anaerobic digestion that is useful for organic fertilizer is the ‘slurry’ or ‘sludge’ of broken down organic waste left behind by the process. This sludge is called digestate. Digestate is extremely rich in phosphorus, potash, ammonia, urea, and most importantly, nitrogen. [53]

The nitrogen content of digestate is almost 30% higher than the nitrogen content of the organic waste into it’s raw form. The fermentation and processing steps of AD break down the nitrogen into a form that is usable to the plant, so the digestate can be applied directly to crops, and the plants can easily uptake the nutrients. [54] [55]

Most companies making digestate fertilizer apply the raw digestate directly to the field. Digestate applied directly to crops have been shown to increase yields by 1.2 times. [56]

The problem with applying digestate directly to crops is that it is unrefined, and contains only a base amount of nutrients. The nutrient concentration of the digestate can increase by 70% when it is processed. [57]

So the best option to get the most out of the digestate is to process and refine the digestate to increase nutrient concentration.

The technology of digestate processing is very new, so there are many different ways to do this.

Digestate processing

The different digestate processing methods and options use cheap and simple technologies that can easily be coupled with anaerobic digestion and require little to no extra energy.

In this section we are going to cover some of the most basic digestate processing and nutrient enhancement methods (there are many more methods than this, but we are just going to cover the basics):

  • Liquid separation
  • Evaporation
  • Reverse osmosis

Liquid separation

All digestate processing starts with the same step: separating the liquid from the solid. This can be done by physically filtering out the solids though a net or sieve, or running the digestate through a filtration device (which is much more expensive and requires extra power to operate).

The solid part contains most of the phosphorus, and the liquid part contains most of the nitrogen. [58]

From here the nitrogen liquid can be supplemented with a few drops of phosphorus and potassium to give the plants all the NPK they need. Then the liquid fertilizer can be applied as a foliar fertilizer to the crops. And the solid fertilizer can be applied as a compost to the soil.


The goal of evaporation (or thermal drying) is to eliminate water and conserve the highest amount of carbon in the solid digestate, which will help to improve the pH of the soil. The process takes 7–10 days.

This could simply mean spreading the solid digestate out on the ground and allowing the sun and air to evaporate the liquid and naturally improve nutrient concentration. Or, for more industrial applications, this looks like large scale exhaust gas cleaners, belt dryer, drum dryers, solar dryers and volatiles which mechanically evaporate the water from the solid digestate.

The industrial methods take less time, but require much more energy and infrastructure. Letting the sun and air work their magic on the digestate works just as well.

Once most of the water is evaporated, the dried digestate can be applied to plants as a thick compost. [59]

Reverse osmosis

Reverse osmosis is a kind of membrane technology used to isolate the important nutrients in the liquid digestate. The goal is to extract all of the nutrients and add it to the solid digestate, using the purified clean liquid as irrigation water.

In reverse osmosis, a semipermeable membrane (that only lets H2O molecules pass) is sandwiched between osmotic water and the liquid digestate. The osmotic water pushes the liquid digestate through the membrane, filtering out all the tiny tiny nutrients and minerals from the liquid digestate.

These nutrients can then be added to solid digestate to increase it’s nutrient concentration, or collected and applied directly to crops as pellets.

The biggest draw about using reverse osmosis is that it doesn’t use expensive, high pressure valves or extensive equipment. It also decreases the volume of the digestate by 42%, which is very important in reducing transportation costs, and increased nitrogen content by 4.2 times and phosphorus by 4.4 times. [60]

To sum up

Let’s sum all this up:

  • Anaerobic digestion takes organic waste and uses bacteria and microorganisms to break it down into a nutrient sludge called digestate
  • This happens in large anaerobic digestors
  • Before anaerobic digestion can start, the organic matter goes through the ‘pre treatment’, where it is broken down into macromolecules
  • The end product of anaerobic digestion is biogas
  • The leftover product of anaerobic digestion is digestate which contains massive amounts of nitrogen and phosphorus
  • Digestate can be applied directly to plants as fertilizer, or processed to increase nutrient concentration before application
Beautiful diagram of the steps in anaerobic digestion source

Problems with organic fertilizers that agri waste fertilizer can solve

Turning agriculture waste into fertilizer is pretty cool. It a great source of NPK, dramatically reduces environmental impact, reduces costs and increases accessibility of fertilizer for rural farmers.

It is the only organic fertilizer that:

  • Contains a high amount of nitrogen (and NPK)
  • Produces high yields
  • Is extremely cost effective

Nitrogen content

One of the largest limiting factors of organic fertilizer is the low nitrogen content. Organic fertilizers contain 40–60% less nitrogen than synthetic fertilizers, which dramatically reduces yields by 26%. [61]

The biggest benefit of using digestate as fertilizer is that it has a high nitrogen content. The ratio of nitrogen to other elements in digestate is roughly 50:50, which surpasses all organic fertilizers currently on the market. [62]


On average, organic fertilizers produce 30% smaller yields compared to synthetic fertilizers. [63] To make organic fertilizers an attractive replacement to synthetic fertilizers, organic fertilizers must be able to produce the same yields.

Because of the high nutrient content of digestate, it has the potential to increase yields up to 44% compared to traditional organic fertilizer. [64]

Nitrogen use efficiency (NUE)

NUE is a very important detriment of fertilizer efficiency. It is the ratio of nitrogen the plant uptakes compared to the amount of nitrogen applied to the plant. The optimum NUE rate is 70–80%. [65] The NUE of traditional nitrogen fertilizer is extremely low, at 32–42%. [66]

The NUE of organic fertilizer is 50–60%. [67]

The NUE of digestate knocks both synthetic and other organic fertilizers out of the park with an impressive 80% nitrogen use efficiency. [68]

Time to activate

Regular organic fertilizer takes an average of 2–5 months to break down into usable nutrients that the soil can use.

Since all nutrients in digestate are already broken down into a usable form, digestate takes less than 3 weeks to be absorbed into the root system of the plant. [69]

Pests + bugs

Organic fertilizer attracts bugs and pests because most organic fertilizers contain pieces of organic matter (like half of a rotting apple or potato skins), which is great food for pests and bugs.

Digestate is much different than compost-like organic fertilizer because anaerobic digestion dully breaks down all parts of the waste. The final digestate is a fine fertilizer mix that has no chunks of organic matter in it, eliminating the problem of pest infestations.

Big problems with digestate

Despite all the amazing benefits of digestate fertilizer, there are quite a few big problems with the process.

  • Hard to separate waste inputs: it can be super hard to separate and clean the agriculture waste coming in. If the organic matter is contaminated with pollutants, pollution (spills, physical debris like tools and metal), pieces of cloth and string, or any physical object or material, the entire anaerobic digestion process will be contaminated. (This process of separating the waste also takes a long time).
  • Heavy metal content: agricultural waste is the scraps of crops — crops which have had synthetic fertilizer applied to them. These synthetic fertilizers leak large amounts of heavy metals into the soil and consequently into the plants themselves. This means that the anaerobic digestion will contain large amounts of heavy metals, which then applied to the soil, will introduce more heavy metals into the soil. Heavy metals are extremely toxic to plants and very harmful to soil health. Separating and filtering out heavy metals from digestate is almost impossible.
  • Size: anaerobic digestors are very big. Meaning they take up a lot of space, and are very expensive to build/buy and keep running. Anaerobic digestors are 100 feet in diameter and 110 feet in height, which is very large, and only works for an industrial scale. [70]
  • Price: anaerobic digestors are not cheap. Including building, installation, and maintenance costs, the average price of a single anaerobic digestor is 1.2 million. This is WAY out of the budget of a small scale African farmer. [71]
  • Methane emissions: if the lid of the anaerobic digestor is not covered or fully closed, the methane produced during AD will be released into the atmosphere. 2,750,886 cubic meters of methane are produced per one cycle of AD. If the lid was not properly closed and this methane was released into the atmosphere, it would have a potentially disastrous effect on global emissions. [72]
  • Storage: there are two main problems with digestate storage. The first is that the longer digestate sits, the lower the nutrient concentration becomes. Nutrient concentration can decrease up to 40% by reacting with oxygen (aka being stored in containers). [73] The second problem is that digestate takes up a lot of space. Over 90% of the agriculture waste fed into the digestor will represent the end volume of the total digestate produced. The volume of digestate does not decrease after AD. This makes storing digestate very challenging since it takes up so much space. [74]
  • Decrease soil pH: early tests show that over a 3 year period, digestate decreased yields 0.005%. For a 3 year time period, this had no effect on plant growth or yields, but over a longer period of time, this decrease in pH will compound and have a negative effect on both plant development and soil health. [75]

Companies in the space

Some very cool companies working on leveraging the power of anaerobic digestion to produce biogas, fertilizer, compost and water:

  • Anaergia — Using anaerobic digestion to produce syngas, clean water and both solid and liquid fertilizer from wastewater, municipal waste and agricultural waste.
  • GFL Ag — Creating a sulfur based fertilizer by turning food scraps into class 1 compost with soil microbes.
  • The Waste Transformer — Has created a commercial anaerobic digestor to turn food scraps and waste into soil compounds which they upcycle into carboard and textiles.

The big picture

Anaerobic digestion of agricultural waste has the potential to completely transform how fertilizers are produced and applied.

Digestate can improve soil health and quality, supply plants with all the NPK they need, eliminate fertilizer emissions and runoff and increase yields.

But by far the most exciting part is the potential of digestate as a fertilizer for small holder farmers.

Small scale anaerobic digestors can be created to provide farmers with on farm fertilizers. So farmers can transform agricultural waste (that they would have incinerated) into fertilizer. This demolishes the transportation issue of fertilizer in Africa and greatly reduces cost the cost of fertilizers. While at the same time increasing yields, improving soil health and quality, boosting agricultural productivity, and solving the problem of organic waste.

All this is possible from turning trash into treasure.

I’m Rachel, a 16 year old biotech lover, ultra runner and podcaster. Check out my other work and connect with me:

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Rachel Lee

Building the skills to one day build solutions to some of the biggest problems in the world |