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How to Process Biogas Digestate into Organic Fertilizer

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biogas digestate

Digestate/Biogas Digestate
Digestate, anaerobic fermentation residues, is the semi-solid material remaining after biogas fermentation. Digestate usually formed into three forms, which are whole, solid and fibre. The materials of digestate include: manures and slurries, domestic/retail/ commercial food wastes, whisky industry by-products, manures and crops. The plant nutrients (especially the nitrogen) in digestate are more available than compost.
Biogas digestate, as an alternative fertilizer, is widely practiced in many oversea countries. Also the practice is well established, such as in New Zealand, Germany, Danish, Sweden, Germany and other European countries.

The Reason Why Biogas Waste - Digestate can be Processed into Organic Fertilizer?

what is the reason for processing digestate

The digestate is composed of stabilized organic matter and mineralized nutrients. It is an easy product to handle into organic fertilizer and can successfully substitute mineral fertilizers in agricultural applications. The digestate has been applied at agriculture such as in flower, vegetable seeding, edible fungi, breeding earthworms, etc.
Biogas Digestate Composition:
◆ Biogas residue itself contains a lot of organic matters (about 36% -49%), beneficial microorganism, humic acid (10-24.6%), crude protein (5-9%), which can increase soil biological activity and nutrient supply. After processed into organic fertilizer, it has the function of soil improvement, which can improve soil structure and increased water retention capacity.
◆ Digestate includes lots of nitrogen, phosphorus, potassium, which greatly promote the growth of crops. It is mainly used for the base fertilizer in crop production. We all know that nitrogen (N) (0.8%-1.5%) is the major plant nutrient and is the most common plant growth limiting factor of agricultural crops. The higher concentration of P (0.4-0.6%) , K (0.6-1.2%) in biogas waste makes it more suitable for supplement of macro-nutrients in soils.
◆ In the meantime, digestate also contains copper, iron, manganese, zinc and other trace elements, which are the necessary nutrients for the growth of crops.
◆ The use of digestate as compost fertilizer could improve economies and environmental impact of microalgae production by reducing water needs and fertilizer costs. Now biogas residue has been widely used in the agricultural community.

Technology to Process Biogas Digestate into Compost Fertilizer
Digestate is most commonly available in its whole form with a dry matter (DM) content of around 3-10%. If the digestate cannot be directly applied in agriculture in the vicinity of the digestion plant, longer transport distances frequently demand for further treatment, e.g. dewatering, composting etc.
We can conclude the whole processing technology as:
Dewatering (separated solid - 15% useable as fertilizer compost) → Composting (windrow composting of large amount digestate into quality compost fertilizer) → Granulation (pelletize compost digestate into granule ones for sale and easy storage) → Packaging

biogas digestate dewatering

1. Dewatering:
Dewatering of the digestate is essential as often the transportation distances are too far. Producers can separate digestate with a solid-liquid separator machine into liquor (typically 1-6% DM) and fibre fractions (typically 20-40% DM)
The solid-liquid separating process not only reduces the sludge volumes to be handled but also improved reject water quality as it enables easier and more reliable nitrogen removal and subsequent water reuse as process water.

Solid--reprocessed into organic fertilizer
Liquid nutrient rich concentrate (15%) useable as NPK fertilizer
Clean water (70%), direct dischargeable or recyclable into process

2. Composting of the solid phase of digestate from biogas production plant
In the composting process, digestate is broken down under the influence of atmospheric oxygen. Modern biogas waste composting is a multi-step, closely monitored process with measured inputs of water, air and carbon- and nitrogen-rich materials (equalising the C:N ratio to the value 15–25:1).

digestate composting process

Rectangular Agitated Bed Composting is a widely-used method in developing countries, such as India, China, Nepal etc. The whole process is aided by shredding the size of digestate, air oxygen concentration above 15%, adding water and ensuring proper aeration by regularly turning the mixture. When sufficiently frequently turning the fermented material, the safest method of compost aeration and ventilation is the addition of coarse-grained material while inert material such as wood chips, chaff and similar materials can be used.

biogas waste composting

Azeus Groove Type Compost Turner is specially designed for digestate composting. Here is the working video of groove type turner machine:

3. Granulation process
Biogas digestate compost granulation stage occurs after compost crushing and screening process. Digestate compost is granulated to produce a value-added organic-based fertilizer product that recycles nitrogen and phosphorus to reduce dependency on the dwindling phosphorus resources from which inorganic phosphorus fertilizers are produced.In Vehmaa in Finland, Biovakka Suomi Oy Biogas Plant granulated dried digestate. And the composition of nitrogen and phosphorus in granules and granulation residue in the digestate is in average as follows:

3.6 % KA
2.2 % KA
6.8 % KA

Granulation is the key machine in the digestate organic fertilizer production, which directly decides the quality and appearance of finished products. New type organic fertilizer granulator machine is used in biogas waste granulation process. It can process 2-5 tons organic materials in one hour. The new type fertilizer granulator machine is also suitable for other kinds of raw materials’ granulation, such as livestock manure, straw fermentation, sewage etc.
Working video of new type granulator:

4. Packaging
After granulation, the generated biogas digestate organic fertilizer granules are dried, cooled and screened. Packing is the last stage in biogas waste organic fertilizer production process. Automatic Packing machine is a full automatic machine with functions of automatic weighing, bag, filling, package sending and sealing. The machine has the advantages of high efficiency, high accuracy weighing, strong compatibility, realize the automatic feeding and stopping.

Product Name: How to Process Biogas Digestate into Organic Fertilizer