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?


A critical subject for any biogas digester is pressurizing the gas. To be able to use the gas, pressure must be applied in one or another way.

Usually, ballast is applied on the digester tank or separate gasbag to push the gas out. However, you need quite a lot of ballast to apply enough force and you need this ballast to be spread out nicely, otherwise you will end up with pockets of gas not being pressurized.



Ballast on a gasbag.


One of the directions we’ve been exploring is the behavior of a flexible tank on flat ground. Main question here would be: can a flexible tank placed on flat ground supply enough pressure on the gas inside?

We expect the shape of the tank and the pressure inside are subject to the gravitational force of the ‘slurry’ inside being lifted. When the tank blows up and gets rounder, the slurry inside is lifted which means its stores potential energy.



A 9,5L tank for testing


Shape and volumes measured


And then we started testing. We made a small tank (about 9,5L) and filled it up with different amounts of water. Then we added air until full and during this process we measured pressure and the shape of the tank. Results are below. The lines represent different water volumes in the tank.


test graph.jpg


We think the steep (darkblue) curve of the tank without water represents the elastic stress of the tank. Interesting is to see the curve shifting to the left as the tank is filled up with more water at the start. The tank with 8.2/9.5 L has a pressure of 4 mbar as soon as it contains a little bit of gas.

The image below are stills of the tank with 6.2 L water; clearly visible is the lifting of the waterlevel.


video stills 6-2 L.jpg

Stills of the video made from the 6.2L filled tank.


This first test brings up a lot of questions:

  • How is this effect scalable? (does it translate to a 4000-liter tank?) and;
  • Are pressure curves this steep allowable?

Please comment! Any ideas or suggestions are welcome.

Sprint session in the Netherlands

In the last week of September, all the partners in the project joined forces to start with the design of the flexible digester. The goal was to generate three concept directions at the end of the week, as a starting point for the design process.


When all the product challenges were defined, we could start the chaos of brainstorming and idea generation. We circled around posters with each a challenge to generate ideas and built upon other’s ideas. Together we created a large amount of ideas on yellow post-its.


To share one of the design challenges: How to maximize the temperature of the slurry in the bag?

There are several designs of a greenhouse on the market. Do we know what the effect of a greenhouse is? We were thinking of using UV stabilized bubble wrap instead of a greenhouse, why do you think that would or wouldn’t work?

Field research

Author: Lia Bardoel, SimGas BV

What to do to gather all the information you need to start designing? Off to the field! Talking to our sales staff and technicians about their experiences with (potential) customers, interviewing flexbag owners and visiting potential customers for the flexbag. In this blogpost, we will share the outcomes of these interviews and games!

Workshops with technicians and sales people

IMG_20170610_143042Who knows the farmers better than our SimGas staff? During these interactive workshops with games, brainstorms and design activities they have taught us more about potential customers for the flexbag. The main insights were:

  • Farmers want a product that works always. Reliability is key!
  • Farmers want to have a completely finished product. All parts connected to the Flexbag should be designed and delivered by the company.
  • Farmers want to invest in a professional branded product.
  • Building up a trust relationship with the customer is key for sales.

Visit of customers with a flexible digester or potential customers


We have visited flexbag owners and potential customers in the area where we are selling biogas digesters. Some main insights were:

  • Some farmers mentioned the footprint of the digester to be important.
  • Farmers do not want to worry about the functioning of the product.
  • Flexbag leakages are rare.
  • The problem that sometimes occurs is scum formation.


With all the information gathered, we cannot wait to start the design process!