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