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