May 4, 2018

Ductor in Bioenergy Insights May/June 2018

The global production of chicken meat is growing and seems set to continue doing so. But chickens are not without feathers and these are mostly considered as waste. There are millions of tonnes of feathers around the world. Although there are a number of products made from chicken feathers, like feather meal, diapers, fibres, biodiesel and biogas, many producers have a problem with feather waste and some even pay a considerable amount of money to have it burned. Moreover, the EU is likely to tighten the rules of using wastes for animal feed. Poultry slaughterhouses produce waste other than feathers, such as offal and litter/manure, but the feathers seem to be the most difficult for anaerobic digestion. Chicken feathers are formed of β-keratin, which contains disulphide bonds, hydrogen bonds and hydrophobic interactions, resulting in a structure resistant to most proteolytic enzymes. There has already been a lot of research published on this subject.Although the theoretical methane potential of feathers is high — 400 m3/tonne — the obtained values are just a fraction of it, around 21 m3/tonne. The reason for this is the high ammonia concentration in the digester arising from the high protein content (98% of dry matter) of feathers. Anaerobic digestion also reduces the bacterial content of the feathers.

A microbial population developed by Ductor was found to be very effective in releasing ammonia from organic material like chicken litter or manure. The same population was tested on chicken feathers in laboratory scale fermentation and the feathers were solubilised almost completely in just seven days at 52°C (see photo). The crushed feathers were obtained from a slaughterhouse. The tests were carried out at 6% total solids concentration (133 g feathers/l) with duplicated runs. After the ammonification fermentation the slurry was hygienised at 95°C for one hour. Stripping was carried out at 83°C at pH 11. Prior to the methane potential analysis the pH of the slurry was adjusted back to the level measured after ammonification.The methane potential analysis was conducted in thermophilic conditions (55°C) using an AMPTS II device for 34 days. The concentration of volatile solids in the test flasks was 25 g/l, 33% of which originated from the feathers, and the rest from the inoculum. The inoculum was obtained from a thermophilic reactor fed with chicken litter; from Ductor’s pilot plant. In seven days an average 60% of the total nitrogen was ammonium in the liquid phase. The highest yields were 80% when using longer fermentation times. After the fermentation, ammonia was stripped away and the methane potential was measured to be 310 m3/tonne dry matter.In conclusion, after the ammonia fermentation and ammonia stripping, the chicken feathers yielded almost the whole theoretical methane potential without any risk of ammonia inhibition. There is, however, also a high concentration of hydrogen sulphide which is not removed from the liquid by stripping. The process needs to use a hydrogen sulphide removal unit before a CHP or gas cleaning system. The ammonia stripped is recovered as ammonium sulphate, which is a good nitrogen fertiliser.

This example reveals that there are nitrogen containing waste streams that can be converted to biogas compatible through a microbiological fermentation process. This means you could drive about 11 000 million kilometres using the 2 million tonnes of chicken feathers. It is a long way!

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