Pretreating food waste with a combination of steam and pressure in an autoclave can double the throughput of an anaerobic digester and significantly reduce the amount of ammonia concentrations by denaturing proteins, according to new research by autoclave manufacturer, AeroThermal Group
The company explained that to date its trials have been run over 580 days and that the increase in performance of the digesters was maximised when the autoclave operated at a temperature of 140oC and the digesters were run at an Organic Loading Rate (ORL) of >5kgVS/m3/day
According to AeroThermal, its experiments have shown that autoclaving has the effect of producing a feedstock for AD which allows stable digestion at very high loading rates, without the addition of trace elements which could be harmful to the environment.
Stable operations were also said to have been achieved at a loading rate as high as 10 kg VS/m3/day with food waste autoclaved at 140oC, while the digesters operating on un-autoclaved feedstock failed at a loading rate of 5 kg VS/m3/day.
The manufacturer attributed the significant increases in digester performance to the denaturing of protein during the autoclaving pre-treatment, which has the effect of reducing the quantity of ammonia generated.
However, the company added that the process also produces a small loss in biogas generation due to this denaturing of protein and the caramelisation of sugars but said that autoclaving at 140OC is probably the optimum temperature as only limited caramelisation will occur and any potential gas loss is more than offset by the advantages of operating stably at higher loading rates.
For the plant operator, AeroThermal said that this means that by using an autoclave, they can potentially double the throughput of the plant and in doing so, they would double revenues for only a marginal increase in operating costs, since the autoclave uses waste heat from the CHP process.
According the firm an autoclave can also facilitate the co-processing and blending of different types of waste, since it pasteurises, homogenises and hydrolyses the feedstock in one simple and highly effective process.
“In the past there has been a lot of speculation on the efficacy of pre-treating food waste by an autoclave,” Christian Toll, AeroThermal’s CEO.
AeroThermal’s testing facility at its laboratory and demonstrator autoclave in Poole, Dorset, is led by Dr Nigel Bailey and Dr Zhenjiang Wang.
AeroThermal added that the programme of experiments builds its work on the EU funded Valorgas project, in conjunction with Southampton University, where optimisation of food waste AD plant performance was explored.
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