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Solid and Liquid Digestate from Anaerobic Digestion – Uses and Odor Prevention

liquid digestate potatoes

Comparison of harvest with digestate as fertilizer for potatoes – Image by Sustainable sanitation via Flickr

Introduction

Household waste is inherently putrescible (compostible), and provides a natural material for decomposition by methane bacteria. It will ferment naturally, in a landfill, which even when lined and capped is at comparatively little cost. But, landfills do bring with them considerable environmental and safety hazards. They require extensive gas abstraction and leachate control systems to protect neighboring crops, neighboring properties and underground water supplies. Even then possibly 30% to 60% of the methane produced leaks out and can’t be collected.

Biogas Plant Odor Meme

Biogas Plant Odor Meme

Household waste will ferment rapidly in anaerobic digestion plants to provide a convenient source of biogas as fuel and a stabilized “digestate” for landfilling, or refining into useful soil conditioners and soil substitutes for agriculture, forestry or land reclamation. . It will capture almost 100% of the methane produced, and this methane can also be processed into a automotive biofuel. It is carbon negative (helps reduce carbon emissions) and “green”. Anaerobic Digestion technology may not be as reliable as other processes and the cost is high at present, but further investment in research into the processes will almost certainly be able to improve reliability very rapidly.

Anaerobic digestate is produced by the method in which microorganism breaks down the biodegradable material without the presence of oxygen and this digestate has many advantages as a fertilizer and soil improver. Anaerobic digestate is used mostly for agricultural and sometimes also industrial purposes.

Solid and Liquid Digestate

For Anaerobic Digestion (biogas digestion) to really take off economically it is essential that both the liquid and solid products – compost and liquid digestate fertilizer – can be marketed and sold. There has been recent success in this area as well.

Liquid digestate is drained-off and either sold as a fertilizer as-is, or can be put through an evaporation system and sold in crystalline form.

Solid digestate is fibrous and consists of lignin and cellulose. This digestate sometimes also contain remains of bacteria and minerals. Also if checked then there is high moisture retention properties in the acidogenic digestate.

The quality of the solid part of the digestate is graded against chemical, physical and biological aspects. It acts as solid conditioner as it provides soil the organic content. It provides nutrients that are required for the growth of the plant. Digestate can, when applied correctly, help plants with resistance towards diseases. It also stimulates the biological activity of the soil.

The solid digestate is placed in piles to aerate for about two weeks. Once the digestate has been aerated it can be used as a soil improver or growing media constituent in the same way as compost. A key point you will want to remember here is going to be that there may be regulations which limit the use of digestate due to concerns about possible transmission of infection. The reason why this will be significant is,for example, this limits the uses and value of digestate. If the material is derived from mixed wastes sources additional sorting may also be required to remove contaminates.

Avoid Odor When Preparing Liquid Digestate for Recycling Its Use Within AD Plants

Avoid odors from the AD pre-sorting plant. Proprietary designs for biogas digesters can require that the operator carries out mixing with digestate to enable sorting of inorganic and organic material to be carried out. Always ensure that the possible odor production from this activity can be controlled, again. this may require enclosing the space and filtering the vented air to remove odors. This is because the pre-sorting stage can be the source of further unpleasant smells. Alternatively, just enclose the whole digestion process in a large building, ensure that the air pressure through any door openings is always held below the air pressure outside by extraction fans, and filter all vented air to remove all significant odor before it is discharged.

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