Micro-methanisation: turning urban food waste into energy

On aver­age, each French per­son gen­er­ates 253 kg of resid­ual waste (grey bin waste) per year¹. The good news is that most of it (32.8%) is actu­ally recov­er­able: this is decay­able bio-waste (food waste and green waste). How­ever, 57% of French people still throw their food waste away with house­hold waste, mainly because there is no sep­ar­ate col­lec­tion². A law that came into place on 10^th Feb­ru­ary 2020 on the fight against waste and the cir­cu­lar eco­nomy requires all pro­du­cers to sort bio-waste at source by 1^st Janu­ary 2024.

Micro-meth­an­isa­tion is very bene­fi­cial for urb­an or peri-urb­an use.

So, what should we do with these large volumes of bio-waste? To date, com­post­ing is the most pop­u­lar solu­tion: in France, there are 749 com­post­ing centres com­pared to 17 house­hold waste meth­an­isa­tion units³. For some, micro-meth­an­isa­tion could be a new way of recyc­ling in cities.

Like meth­an­isa­tion, micro-meth­an­isa­tion is based on the fer­ment­a­tion of organ­ic mat­ter in the absence of oxy­gen. It pro­duces meth­ane and a digest­ate that can be used in agri­cul­ture. What is spe­cial about the pro­cess? “Ter­rit­ori­al or agri­cul­tur­al meth­an­isa­tion units treat tens or even hun­dreds of thou­sands of tonnes of waste each year,” explains Anne Trémi­er, dir­ect­or of the OPAALE research unit at INRAE. In a micro-meth­an­isa­tion unit, we do not exceed 1,000 tonnes per year. It should be noted that there is no har­mon­ised defin­i­tion of the term. Some man­u­fac­tur­ers, such as Try­on-environ­nement, pos­i­tion them­selves on the micro-meth­an­isa­tion mar­ket with a treat­ment capa­city of 1,000 to 8,000 t/year. “The reg­u­la­tions do not address micro-meth­an­isa­tion in par­tic­u­lar,” adds Juli­en Thu­al, an expert at Ade­me’s waste recov­ery depart­ment. “At Ademe, we talk more about autonom­ous meth­an­isa­tion because that is what char­ac­ter­ises these units.”

Autonomy is one of the advant­ages of micro-meth­an­isa­tion. For example, for Bee&Co’s Bio­Bee­Box unit, the bio­gas feeds a cogen­er­a­tion tur­bine that pro­duces heat and elec­tri­city. The lat­ter sup­plies the needs of the micro-meth­an­iser, not­ably to heat the hygien­isa­tion and diges­tion tanks. Sur­plus elec­tri­city can be fed into the grid, and the heat pro­duces domest­ic hot water. “Their deploy­ment could be jus­ti­fied in isol­ated areas, where autonomy is essen­tial,” says Juli­en Thu­al. The Pux­in micro-meth­an­iser, for example, is widely deployed in India to pro­duce bio­gas for domest­ic use.

Anoth­er advant­age is mod­u­lar­ity. All the pro­cesses – grind­ing, hygien­isa­tion, anaer­obic diges­tion, cogen­er­a­tion, and com­post­ing – are integ­rated into a con­tain­er. The foot­print is lim­ited to about ten square metres. “This small foot­print makes micro-meth­an­isa­tion very inter­est­ing for urb­an or sub­urb­an use,” says Anne Trémi­er. “Anoth­er factor is the mod­u­lar­ity: it is easy to add or remove con­tain­ers to mod­u­late the treat­ment capa­city.” Research­ers at ITM Atlantic have ana­lysed 15 inter­na­tion­al urb­an meth­an­isa­tion pro­jects (almost all of which treat more than 1,000 t/year)⁴. They observe that it is inter­est­ing to place the treat­ment unit in the heart of new low-dens­ity urb­an districts.

Con­versely, it is prefer­able to relo­cate the meth­an­iser to the out­skirts of very dense neigh­bour­hoods or neigh­bour­hoods already equipped with waste man­age­ment sys­tems. “With the Decis­ive research pro­ject⁵, we have shown that micro-meth­an­isa­tion does not replace oth­er recov­ery solu­tions but is com­ple­ment­ary,” says Anne Trémi­er. “In new dis­tricts, these loc­al units make it pos­sible to rethink the entire recov­ery chain while adapt­ing to needs.” Anne Trémi­er adds, “exist­ing meth­an­isa­tion or com­post­ing facil­it­ies will not be able to increase their treat­ment capa­city. With the oblig­a­tion to treat bio-waste at source, micro-meth­an­isa­tion in peri-urb­an areas can meet these addi­tion­al recov­ery needs.” Finally, one of the Decivise pro­ject pilots has shown the interest of imple­ment­ing micro-meth­an­isa­tion in sparsely pop­u­lated areas, thus redu­cing the trans­port of bio-waste.

How­ever, the pro­cess is still not very wide­spread in Europe. The first units were iden­ti­fied in the early 2000s in eco-dis­tricts in Ger­many and the Neth­er­lands⁶. SEaB Energy, a Brit­ish com­pany, is the European lead­er in urb­an micro-meth­an­isa­tion, and a few French start-ups have emerged in recent years (such as Bee&Co). A report by Meth­a’­syn­er­gie⁷ (the insti­tu­tion­al and pro­fes­sion­al play­ers in the meth­an­isa­tion sec­tor in the PACA region) lists 875 on-farm micro-meth­an­isa­tion units in Europe in 2015, includ­ing 660 in Ger­many and 26 in France. The author car­ries out a more recent census in 2020 in France: 90 units are coun­ted, includ­ing 3 treat­ing bio-waste. Micro-meth­an­isa­tion is widely deployed in agri­cul­tur­al envir­on­ments, and in France only a few pilot units are test­ing the pro­cess in urb­an environments. 

There is no micro-meth­an­isa­tion install­a­tion using bio-waste in France that has yet demon­strated its eco­nom­ic performance.

The reas­on for such a tim­id deploy­ment? Eco­nom­ic prof­it­ab­il­ity. The invest­ment required to install a micro-meth­an­isa­tion unit for biowaste or food industry residues is between €150,000 and €700,000. And the unit con­sumes heat and elec­tri­city: part of the pro­duc­tion is used to power it. As a res­ult, the quant­it­ies of bio­gas, elec­tri­city or heat sold are small. “I don’t know of any micro-meth­an­isa­tion install­a­tions for bio-waste in France that have demon­strated their eco­nom­ic per­form­ance, although we have sup­por­ted many pilots,” points out Juli­en Thu­al. “The eco­nom­ic aspect is under­es­tim­ated, and neigh­bour­hood com­post­ing facil­it­ies are often more afford­able tech­nic­ally and eco­nom­ic­ally.” Some new aven­ues of recov­ery are being explored to ensure the prof­it­ab­il­ity of these sys­tems. In the Decis­ive pro­ject, the digest­ate was used as a sub­strate to pro­duce a biopesti­cide sim­il­ar to a com­mer­cial product. “The pro­cess is less energy intens­ive than the one cur­rently used,” says Anne Trémi­er. “We ima­gine that part of the prof­it­ab­il­ity of micro-meth­an­isa­tion could come from these high value-added products from the digestate.”

In addi­tion, there are reg­u­la­tions. Con­sidered as clas­si­fied install­a­tions for the pro­tec­tion of the envir­on­ment (ICPE), just like lar­ger meth­an­isa­tion units, micro-meth­an­isa­tion units must be installed more than 100 m from dwell­ings. “This is a very import­ant bar­ri­er for these prox­im­ity install­a­tions, and one won­ders wheth­er a spe­cial regime could be put in place,” states Anne Trémier. 

Will micro-meth­an­isa­tion be the solu­tion of the future in cit­ies? “It is import­ant to identi­fy the ter­rit­ori­al con­text: the avail­able sources of waste, the exist­ing install­a­tions, etc.,” con­cludes Juli­en Thu­al. “The idea of micro-meth­an­isa­tion is attract­ive, but it must first demon­strate good per­form­ance, includ­ing eco­nom­ic per­form­ance.” Micro-meth­an­isa­tion can, in the same way as loc­al com­post­ing, bene­fit from great­er social accept­ance. “Pre­vi­ous stud­ies⁸⁹ have shown that home com­post­ing encour­ages loc­al res­id­ents to sort bet­ter and even pro­duce less waste,” con­cludes Anne Trémi­er. “We do not yet have the same exper­i­ence with micro-meth­an­isa­tion, but we hope that it will be as acceptable.”

Anaïs Marechal