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Radu Popa's research group at Portland State University

Acid Fermentation of Cellulose (ADC)

The search for sustainability in carbon recycling. Cellulose is the most abundant organic material on Earth. Millions of tons of cellulose-rich residues derived from plants in the form of paper products, wood chips, sawdust, cut grass, leaves, hay, etc. are thrown away as garbage, or burned. Cellulose, itself, is a polymer made of sugar residues (thousands of molecules of glucose arranged in long chains) that is only slowly degraded by microbes when buried directly in soil. It can sometimes take years to breakdown cellulose-rich materials when they are placed directly in soil. Because almost all modern methods of composting cellulose waste use oxygen to break cellulose into smaller molecules, much of the energy and carbon stored in cellulose is lost in the atmosphere as heat and carbon dioxide (CO₂). Current technologies for composting and recycling garbage also produce large amounts of methane (CH₄), a strong green-house gas which, together with CO₂ is most responsible for global warming. The burning of wood, and the biodegradation cellulose-residues with oxygen, account for the single largest waste of recyclable materials and energy in our society. True sustainability calls for finding and implementing more efficient methods of recycling residues, specifically methods having the least carbon footprint on the Earth's atmosphere.

• ADC is an alternative process for breaking down cellulose; ths method is fast, efficient, gives off very little CO₂ and has zero methane emission. ADC is a combination of controlled fermentation plus respiration and breaks down cellulose using bacteria common in soil.

• Because no toxic chemicals are used or produced in ADC, this method is well suited for organic farming, non-toxic household recycling, and offers significant advantage as an alternate recycling biotechnology in in large scale commercial recycling of waste materials.

• The ADC-research group from PSU studies the scientific principles, costs, benefits and practical applications of ADC.

The secret is in the mix (and smart manipulation). Many variants of the ADC biotechnology are known. In Europe a relatively similar process has been used for composting hay and sawdust. American farmers use a related process in making silage. In Japan local farmers call it Bokashi fermentation. A number of providers exist on the internet offering "Bokashi-fermentation" supplies. Last but not least, many scientific publications reported the contribution of soil bacteria to the degradation of cellulose.

Whereas the application of ADC seems simple, achieving proper results requires a specific mix of cellulose-rich materials, nutrients, water, microorganisms and specific incubation conditions must be applied. When ADC is applied correctly a community of bacteria is established that out-competes other species of microbes. Under these conditions the microbes become very efficient in degrading cellulose by producing extracellular enzymes, blocking oxygen respiration and the formation of methane, fixing nitrogen from air, and establishing conditions that will not result in bad odors which are so common in garbage decay and conventional composting.

The main steps of ADC.

• Step 1. Obtaining microbes giving the best ADC performance. These microbes are induced to make spores and can be stored dried or frozen. This step is done in microbiology labs and rich cultures are then made available to users.

• Step 2. Initial fermentation, takes about 1-6 weeks (depending on materials and conditions) and results in partially degraded cellulose (with very little CO₂ and zero methane production).

• Step 3. Final decomposition. The acidity of the fermented product is buffered and ammended and the mixture is finally decomposed in soil mixtures to create an energy-rich, carbon-rich and mineral-rich compost which is used as fertilizer in soil (pots, gardens, crop fields and orchards).

ADC can be applied at the micro-scale (household), mini-scale (organic farming) and macro-scale (commercial and urban landfill composting). About 25-30% of the present city garbage is recyclable by ADC.

ADC RECIPES

1) ADC using food scraps, leaves and grass clippings

ADC should not be confused with conventional composting. In conventional composting a random mixture of organic materials of various types and sizes are stored in a bin, pile or composting box. Given enough time (this may last anywhere from six months to more than a year), and with the help of microbial oxidation a slow degradation will occur. Disadvantages of conventional composting include the long time needed to obtain a stable end product, high processing costs (in commercial composting) for turning over the decaying waste so that it is properly aerated, bad smells, the generation of large quantities of methane and carbon dioxide (two potent greenhouse gases), and loss of a significant portion of energy and organic carbon in the process. ADC is a controlled two step microbial biotechnology that eliminates these problems, though to be successful proper raw materials, bacteria, and procedures must be observed.

ADC-recyclable materials

The amendments added to the overall mixture vary based on the proportion between the different start materials. The recipe given here is adjusted for mixtures of kitchen vegetable scraps, leaves, grass clippings and very small amounts (< 5 %) of shredded paper. Egg shells can also be added, but not the eggs content. For decomposing mixtures rich in paper, cardboard boxes, hay, straws, mulch and sawdust a different recipe is needed. DO NOT USE animal waste, any significant amounts of oil, bones, meat, plastics, metals, unshredded wood or cloth (except for pure shredded cotton). Because ADC is based on active microbes chemical household products that may be toxic and inhibit the growth of bacteria (such as detergents, soap, paint, gasoline, oil, antifreeze, litter box contents, etc.) should be avoided.

Fragment size

Raw materials used in ADC have to be fragmented into small pieces. Crumbling or cutting up food waste into bite size pieces works very well. Tearing a piece of bread into two or three pieces also works well. For a whole melon, apple, cucumber, or other spoiled fruit or vegetable, cutting up the produce into strips or sections as if it were to be served on a plate is recommended. For paper, or leaves, a small wood chopper or paper shredder makes excellent size fragments. If a wood chopper is not available, leaves and grass will fragment easier if they are dried first, then torn or broken up into small fragments. For shorter fermentation it is highly recommended for grass clippings to be dried first. Paper coffee filters do not need to be shredded because they are already very thin and porous and will degrade without difficulty, and can be processed right along with coffee grounds held inside the filters.

The fermentor

The fermentation step of ADC occurs in a bucket or drum called a fermentor. Fermentors have an internal lid that rests on the mixture (see the figure above). Do not allow too much space between the internal lid and the sides of the fermentor (¼ of an inch or ~0.5 cm space is acceptable). ADC fermentors are specifically designed to restrict air access to the fermenting mixture. If a garbage bin is used to construct a fermentor, make certain to also include an inner lid for closing off access of the air space inside the bin. The fermentation step is anaerobic (or at most microaerobic). This means that it occurs in the absence of air, or at the very least it must occur at very low concentrations of oxygen. ADC fermentors need to be drained. During fermentation liquid will accumulate in the fermentor and percolate to the bottom. This liquid should not be allowed to accumulate for lengthy periods. It will inhibit the fermentation. A small drain for stopping and starting drainage is placed at the bottom of the fermentor. Using this system, the fermentor can be periodically “bled” of excess liquid as it accumulates. Draining the small amount of fluid that accumulates (about a cup per day in a 5 gallon bucket) every other day ensures efficient operation of the fermentor. Alternatively, if there is a convenient place to continuously drain the fermentor, that works, too. The fluid can be safely flushed down the drain, or diluted and used as liquid fertilizer. Some internet providers sell customized fermentors (called Bokashi fermentors) of different sizes. An improvisation made from a garbage bin also works well, but in this case a drainage hole has to be made at the lowest point in the bin. The inner lid should not be very heavy to avoid compaction. It can be improvised from wood, plywood, plastic or aluminum, or ceramic materials. Very large, especially tall, garbage bins with the capacity of holding ~60 gallons (~250 liters) are too heavy to manipulate. They will also show compaction toward the bottom, and give poorer results.

STEP 1. OBTAINING THE STARTER BACTERIA

A starter has to be used containing microbes with high ADC efficiency. In research laboratories this is done by selecting bacteria from nature. Some Bokashi providers offer a mixtures of microbes and fermented cellulose called “effective microorganisms”. Some people use wheat bran as a starting inoculum. Wheat bran is (in most cases) rich in spores of microbes from soil including Clostridium, which are important in the fermentation step.

STEP 2. FERMENTATION

• Add a grid at the bottom of the fermentor. This is very important to allow the extra water formed during fermentation to drain.

The preparation depends on composition.

• If the raw fermentable materials are dominated by kitchen waste start adding layers of fermentable materials in the fermentor, lightly dust them with the starter bacteria (wheat bran or the “effective microorganisms powder”). Some shredded paper can also be added but no more than 5 %.

• If the mixture is dominated by grass clippings, leaves and shredded paper, apart from the starter the mixture should be sprayed with a solution of ~1 % molasses dissolved in water. (Alternatively sugar can be used at a final concentration of about 1 gram per liter; this means one spoon of sugar to every 10 liters or ~ 2.5 gallons of water). It is better for the grass clippings and leaves to be dried before they are used.

•Add 5 g of sea salt to each 10 L of water (this is approximately one teaspoon of salt to every 10 liters or ~2.5 gallons).

• Do not compact the mixture in the fermentor. The mixture should be moist and fluffy, with lots of gas spaces.

• Add the inner lid and allow it to rest on top of the fermenting waste.

• Cover with the top lid.

•As more materials need to be fermented keep on adding them under the inner lid and treat them with the starter, molasses, salt, as shown above. Avoid exposing the fermenting mixture to air for extensive periods of time.

• When one fermentor is filled keep it closed for 2-6 weeks to complete the fermentation; meanwhile begin filling a second fermentor. In a typical household setting two fermentors of 5 gallons each works well.

• Avoid too much water. If excessive water is present the fermentation will slow down, acidity develops slower, methane and hydrogen sulfide could form, and the fermented product begins to stink. Bad odors are also a sign of improper raw materials in the initial mixture.

Incubation conditions

Fermentation starts very fast (two or three days). Low temperature slows this process. Do not allow the fermentor’s content to freeze – it will stop the activity of the microbes. Do not expose the fermentor to high heat. For households, the best place to store the fermentor is inside the house in a convenient location such as the kitchen, or in a pantry or garage where access is easy and it can be drained and serviced with little effort. On a farm in the summer the best place to place the fermentor is in the shade. In winter, a garage or a tool shed are best.

The evolution of fermentation

In early steps some microbes from the community use the sugar (or molasses) and consume most of the oxygen (but not all). Then bacteria called clostridia germinate from spores and start fermenting the remaining sugar (or molasses) while producing small organic acids. This acidity (in combination with the low oxygen concentration) inhibits other microbes and allows clostridia to become dominant. When the concentration of sugars is near zero (generally after a couple of days), the Clostridium microbes start producing enzymes called cellulases. These enzymes degrade the cellulose into smaller fragments and make more sugar available to the microbes. Monitor the evolution of fermentation. The mixture should have a sweet vinegary fruity odor. If the mixture starts to stink the fermentation is not working correctly (this may mean too much water, animal products, wrong acidity, wrong compaction, wrong microbes, etc). Stop immediately and bury.

Draining

During fermentation liquid forms and drains toward the bottom. This liquid is acidic, very rich in organic carbon and in energy and should not be used directly to water plants with it. Similar to other nutrient-rich liquid fertilizers, because it is very concentrated it may kill plant roots if added directly to the soil. Dilute it first about ten times in water.

STEP 3. DECOMPOSITION

Final decomposition occurs in mixtures with soil. The fermented materials produced in Step 2 (see figure) will preserve their general shape and size, but inside these materials the degradation of cellulose is advanced already. This fermented mixture cannot be used as compost; it is too concentrated, and the decomposition is not yet finished.

• Dig a hole in the ground about 0.5-1 foot deep (~15-30 cm) and add an amount of fermented material. Add loose soil to obtain a about 50/50 mixture and mix with a spade or shovel.

• Cover up the mixture with loose soil and leave to decompose for at least one week.

While in the soil other microorganisms will start degrading the smaller organic molecules and then continue with the partly degraded cellulose.

Fertilization with the AFC product

After a week or two in the soil decomposition is almost finished and the mixture can be used directly as a fertilizer in flower beds, garden, or added at the root of trees. Remember that this is a carbon recycling method. Because it is rich in organic carbon this fertilizer is most efficient if it is incorporated in soil rather than added on top. Alternatively, the soil-fertilizer mixture can be stored as a compost pile or dried, bagged and used later. This mixture is very rich in nutrients and energy and will have a strong positive effect on plant growth.

On average about 25-30 % of a household’s garbage can be treated by ADC. In gardens this percentage will be higher, because leaves and grass are often produced in larger amounts. One great benefit is that the materials used in ADC are recycled instead of being thrown away as garbage.

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