Biogas Slurry: Why It is a Better Fertilizer Than Fresh Animal Manure?

Description: The fertilizing effect of biogas slurry surpasses that of fresh manure – This is a well-established fact. In this article, you’ll learn why this is so. You’ll also discover the best practices you should adopt during application and storage of biogas slurry.

Biogas slurry, also called digestate, is obtained from the production of biogas in an anaerobic digester. In this article, we’re looking at 5 advantages that biogas slurry has over fresh animal manure as a source of organic fertilizer. We are also going to look at precautionary measures to take to avoid loss of nitrogen when applying the slurry and during its storage. We kick off with the 5 advantages of using biogas slurry as fertilizer.

1. Odours Reduction

The offensive odour from fresh animal manure is as a result of substances like volatile acids, phenol and phenol derivatives. A digester can reduce the odour in manure by up to 80%, so long as the manure remains inside the digester for up to 30 days and at an average temperature greater than 30oC. The composition of the odour also changes as the biogas slurry or digestate smells more like ammonia. Another improvement is that if the digestate is stored for a long time, the odour problem will not resurface again. It has also been observed that the odour almost disappear 12 hours after application in the field.

2. Sanitation

The animal manure used in digesters often contains pathogenic bacteria, parasites and viruses. As a result of this, there is a need to ensure that the pathogens in the digestate/biogas slurry have been killed before application on farm lands. If this is not done, pathogens will find their way into human food chain through farm products.

The chemical condition and temperature that exist during digestion can kill a large part of the viruses, bacteria and parasites in the manure. This process is called sanitation and its efficiency depends on the following factors:

  • The retention time (RT) – This is the total length of time the animal manure remains inside the digester. The higher the retention time, the better the sanitation efficiency.
  • The process temperature – Digesters that operate at thermophilic temperatures (heated to 50-55°C) have better sanitation efficiency because the high temperature will kill pathogens.
  • The stirring technique – When the slurry in the digester is stirred, fresh animal manure will contaminate the digestate. As a result of this, some fresh manure will escape along with the digestate untreated. This decreases the sanitation efficiency. The stirring technique influences the amount of untreated manure that can escape.
  • Digester type – High sanitation efficiency occur in digesters whose configurations make it impossible for untreated manure to escape. An example is an elongated plug flow digester.

The sanitation efficiency is not a function of only one of these factors. They all work together to influence the sanitation efficiency. For example, a plug flow digester that works at thermophilic temperature and has a high retention time can achieve 99% of pathogen reduction.

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