Why Anaerobic Digestion will be a powerful vector within the Circular Economy II

Why Anaerobic Digestion will be a powerful vector within the Circular Economy II

Dear colleagues, first of all I must apologise for the delay in supplying with this second part of my former article concerning AD and its role within the referred Circular Economy.

As you may recall, an overall and holistic description was attempted with the purpose to draw a relatively complete picture of the “vector” in consideration. It is now the time to move onto the next stage which covers process optimisation and other appraisals. A certain technical language is unavoidable but I will try to be as succinct as possible (easy for some and difficult for others). Before we getting down, please do not forget that each country or region is different and sometimes the used criteria might not match in the same way.

OK, we know what is AD. Typically, organic matter (commonly measured in VS) coming in and out after a certain degradation work carried out by microorganisms. Difference in VS-in and VS-out provides with the VS-degraded which is directly related to the biogas (and methane) production. As any natural process, AD has limitations and methane production (the main source of energy) is not maximised at all. This is the step where disintegration technologies come to the forefront whenever the hydrolysis stage is one of the most important limitations. It is difficult for the bacterial biomass to hydrolyse complex substrates. It is completely out of this article to involve any commercial implication but it is possible to find technologies within the three main methods used by the industry, namely, Physical (e.g. mechanical or thermal), Chemical (e.g. oxidising) and Biological (e.g. enzymes).

Regardless the method used, it is important to keep in mind that disintegration technologies do not provide with an unitary process by themselves but just improving an existing one. On that basis, it seems logic to evaluate them, in a first instance, through the so-called Energy Return on Energy Invested (ERoEI). If technologies average a range of improvement between 15 to 30% concerning the conventional AD process (referred to the volume of methane yield per mass of VS fed), ERoEI is an useful tool to calculate the net energy produced by the system. Obviously, the higher ERoEI figure is desired and a minimum figure of 3 should be achieved.

Furthermore, if ERoEI procures with the usable energy/power generation, also known as Exergy, it may be rated to the net profit from an OPEX point of view which could be enriched by considering other expenditures (e.g. O&M tasks, consumables, etc.) To complete the figure, CAPEX should be also included since, independently of purchasing costs, some technologies require a huge lump sum as compared with others concerning civil works and installation. A cutting-edge way of evaluation considers all of the above under the name of TOTEX. If interested, I have already deployed an article around this concept.

Thus, ERoEI and TOTEX analysis may predictively draw an optimum scenario from the energy (methane) angle, but that’s not all folks! After having squeezed the full energy-potential of AD, it is the turn of nutrients and other valuable materials recovering as well as biosolids valorisation. The modern term of Bio-resource has risen.  

It is easy to understand now why AD is converting into a Bio-refinery. In fact, AD may be transformed into a factory of Bio-hydrogen and Bio-plastics derived from VFAs (even there is a trend in using the name ‘VFAs factory’, e.g. PHAs, as polyesters obtained by bacterial fermentation, cover 12% of the Bio-polymers market). The phosphorus fraction may be precipitated up to Struvite or also recovered by different Bio-polymers. Ammonia stripping may produce ammonia-salt.

Depending on the interest, digestate may be treated (dewatered and/or dried) up to reach soil conditioner or compost and, lately, pelletized and used as fuelVermiculture (earthworms’ cultivation) might replace microorganisms in composting since there are some worms able to transform wastes into organic fertilisers. Vitrification (generation of glasses resistible to lixiviation) is another promising field for inertising final wastes.

There is a huge work in many countries around the integration of all of these -and other-concepts and it makes sense whether we keep our eyes open around the resources scarcity today. Just considering that such result may be reached from the Organic Fraction of Municipal Solid Waste (OFMSW) is quite enlightening and remarks one of the big advantages of AD as renewable source of energy. We will be able very soon to allocate numbers to this balance as we did for energy (biogas and methane).

Thereby, AD is and may be much more than a source of energy. At the end of the day the transformation of Wastes into Resources is somehow being accomplished and this is crucial to run the Circular Economy concept.



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