Issues with scum

An issue known to all working in biogas: a hard dry layer of scum will form on top of the slurry in your biogas digester. This scum layer can give your digester issues as clogging of the gas, inlet or outlet pipes or reduce biogas production.

During our visit to Kenya last January-February, we started to investigate how different kinds of feed and feeding ratios are creating (scum) layers in a biogas digester. Some of the results are displayed below.

Question 1: How do different feeding ratios have an effect on creating layers in the digester?

We are advising our customers to mix cow manure and water to a porridge like substance. Talking about ratio’s like water : manure = 1:1 has proven a difficult measure, as there is a great variety in the watercontent of cowmanure, and this ratio-mixing can therefore give a great variety of results.

A simple test showed us the following: we filled up transparent buckets with the porrige like mixture. When leaving this porrige like mixture for 38 hours, the slurry didn’t form any layers but remained a seemingly homogeneous substance.

When we raised the water content in the mixture, layers are formed already after a few hours, and will be clearly visible after 24 hours. From top to bottom: thick dry layer (15%), water layer (35%), slurry layer (50%).

This could mean that when customers are feeding with more water than manure, the chances on scum formation are higher than when the mixture contains more than 50% manure, as the floating layer will emerge. On the other hand, when digestion processes start, the mixture will get an higher watercontent. This could cause the layer to start floating inside the digester after some time.



Question 2: How does layer creation differ with cow or pig manure?

We tested with cow manure, pig manure and a mixture of cow and pig manure. In all cases there were layers formed, they were only much different from each other:

  • The cow manure (1:1) has a 4-5 cm layer of thick scum on top, followed by a 3 cm layer of water. The rest is solids. The difference in substance between the scum and the water seemed small.
  • The 10 cm layer on top of the pig manure (1:1) is very dry and contains more fat than with cow manure. The water layer below that was about 4 cm, and the rest was water.
  • The mixture of cow and pig manure made the top layer about 20-25 cm, the water layer below that 5 cm and the solid layer 10 cm.

We can conclude that pig manure tends to form layers more than cow manure does; the forming of layers goes more rapid and the layers are more extreme in density and thickness. This supports the claim of our technicians that on digesters fed with pig manure have a bigger risk of scum formation.



What are your experiences with scum? And do they align with our findings?

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