Dorset, UK based autoclave manufacturer, Aerothermal Group, has published research which is claimed to prove that pre-treating black bag municipal waste in an autoclave before sending it to anaerobic digestion could increase methane generation by over 300% and substantially reduce the amount of material requiring disposal.
The research was carried out by the firm’s senior research manager, Dr. Zhengjian Wang, and its technical director and principle engineer, Dr. Nigel Baily at a test facility in Poole, in the South West of England.
Aerothermal explained that as part of the research, experiments were conducted in triplicate on two waste streams. The first waste stream consisted of a sample of MSW feedstock with large items removed.
Tests on this were said to demonstrate how much extra material can go to the anaerobic digestion (AD) system as well as the increased gas yield per tonne of material fed to the AD system.
The second waste stream was a sample of MSW feed stock used to demonstrate the improvements that autoclaving can achieve on a like-for-like basis.
The two waste samples were each divided into four portions. One portion of each waste was processed each day, four days per week. 50% of each was processed via the autoclave route and the other half was processed via a non-autoclave route.
After loading, the autoclave was initially evacuated using a vacuum pump to remove incondensable gases, which were filtered out in activated carbon filter beds. The autoclave was then brought up to pressure and temperature (6.2 bars abs/160 degrees C) through the injection of steam. When the temperature had equalised throughout the autoclave the load was left to “cook” for 45 minutes.
Twelve cylindrical constantly stirred tank anaerobic digesters, each of which has a working volume of 1.8 litres, were used for this trial. To start the process 1.8 litre of seed inoculums was first added into each digester. When the digesters reached the required temperatures a daily feeding, semi-continuous operating regime was started according to defined organic loading rates.
The researchers explained that the biogas production was monitored daily and the gas composition was measured once per week. The pH of the digestate was also monitored daily and a weekly composite digestate sample for each single digester was collected and the properties such as DS, VS, ammonia and VFAs were analysed once per week.
According to the company, test results on the first waste stream (where large items had been removed) revealed that autoclaving gives a gas yield of about 150m3/tonne which is more than double the target of 65m3/tonne of waste.
It also claimed that autoclaving greatly increased the fraction of the waste that could be digested, as well as the gas yield from each kilogram of waste sent to the digesters which substantially reduced the amount of material requiring disposal.
Tests on the second (like-for-like) waste stream were said to have revealed that autoclaving gives a substantial improvement in gas yield, achieving again a figure in excess of 150m3/tonne of waste processed with a HRT of 27 days.
Both waste streams were claimed to show that autoclaved materials were less susceptible to the development of high VFA concentrations than un-autoclaved material and steam consumption was in close agreement with predictions.
“We’re very excited that our research has proved autoclaving can make black bag municipal waste disposal more efficient, cost effective and less damaging to the environment,” commented Christian Toll, AeroThermal’s CEO.
“Now we’ve got the scientific proof, we’re looking forward to working with waste disposal companies across the UK and beyond to help them use autoclaves to improve the way they dispose of their waste,” he concluded.
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