Transforming organic waste into hydrogen using marine bacteria

Hydro­gen is an ener­gy source favoured by the French gov­ern­ment to decar­bonise trans­port. To date, 99.3% of the world’s hydro­gen is pro­duced using fos­sil fuels. Only water elec­trol­y­sis gen­er­ates low-car­bon hydro­gen. Of all the known low-car­bon pro­duc­tion meth­ods, the BIOTEC H2 inter­na­tion­al joint lab­o­ra­to­ry – due to open in Ham­mamet in 2022 – is ded­i­cat­ed to the pro­duc­tion of bio­hy­dro­gen by dark fer­men­ta­tion at high temperature.

Hana Gan­noun. This process is based on the recov­ery of fruit and veg­etable waste by acetic fer­men­ta­tion. We use a marine bac­teri­um, Ther­mo­to­ga mar­iti­ma¹. The waste and bac­te­ria are placed with sea­wa­ter in a biore­ac­tor. The biore­ac­tor is heat­ed to 80°C, main­tained with­out light or oxy­gen, and the agi­ta­tion and pH are con­trolled to pro­vide opti­mum con­di­tions for the bac­te­ria to grow. The degra­da­tion of biowaste by bac­te­ria pro­duces dihy­dro­gen (H₂), CO₂ and acetate.

Pierre-Pol Lieb­gott. This process has been known about for twen­ty years, and there are no obsta­cles to pro­duc­ing bio­hy­dro­gen. We have demon­strat­ed the fea­si­bil­i­ty of this process in a 2L fer­menter fed with waste from food mar­kets in Tunisia.

HG. We want­ed to use marine bac­te­ria so that we could sup­ply the reac­tor with salt water. This avoids adding an addi­tion­al use to the fresh­wa­ter resource.

PPL. From the tax­o­nom­ic map of exist­ing micro-organ­isms, we chose Ther­mo­to­ga mar­iti­ma, which is a very spe­cial marine micro-organ­ism: it is poly­ex­tremophilic. This means that it can with­stand very high tem­per­a­tures and high salt con­cen­tra­tions. In nature, this bac­teri­um thrives in under­wa­ter hydrother­mal springs. Why choose these spe­cif­ic char­ac­ter­is­tics? In a sug­ar-rich envi­ron­ment, many con­t­a­m­i­nat­ing bac­te­ria can devel­op and dis­rupt the reac­tion. But at 80°C, no con­t­a­m­i­na­tion can devel­op, ensur­ing that only Ther­mo­to­ga mar­iti­ma is at work.

High-tem­per­a­ture fer­men­ta­tion has anoth­er advan­tage: it is more ener­gy-effi­cient. Fer­men­ta­tion is a process that releas­es heat, and main­tain­ing a fer­menter at a tem­per­a­ture of 20°C requires cool­ing. Cool­ing requires more ener­gy than heat­ing, for which we use a solar water heater.

PPL. Bio­log­i­cal process­es are inex­pen­sive and require less ener­gy. To pro­duce 1 mole of hydro­gen, you need 0.2 moles of ener­gy with a micro­bial elec­trol­y­sis cell. This fig­ure ris­es to 1.7 for water elec­trol­y­sis. Above all, dark fer­men­ta­tion makes it pos­si­ble to utilise an enor­mous amount of organ­ic mat­ter. In France, almost a third of house­hold waste is putresci­ble, and its col­lec­tion is now com­pul­so­ry. In Tunisia, this fig­ure is 70%, and much of this waste is dumped direct­ly into pub­lic land­fill sites.

HG. We are work­ing to improve the way we han­dle waste over the course of the year. We have three study sites: a whole­sale mar­ket in Tunis, a munic­i­pal mar­ket and a hotel. The Tunisian gov­ern­ment wants to set up a man­age­ment sys­tem for this type of waste: our objec­tive is to ensure that the biore­ac­tor oper­ates in a sta­ble and effi­cient way through­out the year.

P‑P. L. Methani­sa­tion is sim­pler to imple­ment, and the process is already rel­a­tive­ly well devel­oped. But methane is less advan­ta­geous from an ener­gy point of view. More­over, hydro­gen is in the process of being adopt­ed more wide­ly in Europe, spurred on by sev­er­al invest­ment plans. By con­vert­ing organ­ic waste into bioH₂, we pro­pose to make the most of the infra­struc­tures – pro­duc­tion, dis­tri­b­u­tion, etc. – that will be implemented.

PPL. The the­o­ret­i­cal yield is around 4 moles of H₂ per mole of sug­ar, but in real­i­ty it is cur­rent­ly less than 3, which is still a good result. In prac­tice, with one tonne of waste, we pro­duce one kilo of bioH2.

HG. We are cur­rent­ly look­ing to improve this yield. To do this, we are study­ing oth­er marine micro-organ­isms and also syn­thet­ic con­sor­tia – mix­tures of sev­er­al bac­te­r­i­al strains.

PPL. The process is still in its ear­ly stages: we’re at TRL 3–4, which cor­re­sponds to a small-scale pro­to­type. We will short­ly be upgrad­ing from a 2 litre to a 10 litre fer­menter. But we’re not aim­ing to use large vol­umes, like large methani­sa­tion units. The aim is to devel­op a unit designed for domes­tic pro­duc­tion of bioH₂ from house­hold waste. This will enable us to tar­get a less com­pet­i­tive mar­ket and offer peo­ple ener­gy independence.

HG. Yes, we’re talk­ing about slur­ry. It’s an aspect we’re work­ing on because we’re try­ing to take a com­plete cir­cu­lar econ­o­my approach. Unlike diges­tate from methani­sa­tion, there are sev­er­al obsta­cles to the recov­ery of slur­ry: it is rich in salt and organ­ic fat­ty acids. It is there­fore not pos­si­ble to use it on agri­cul­tur­al soils. We are work­ing on the sol­id frac­tion of the slur­ry: by com­post­ing it, it would be pos­si­ble to pro­duce enzymes or poly­mers that could be used in packaging.

PPL. If we suc­ceed in treat­ing the fer­men­ta­tion slur­ry, our process will be a com­peti­tor to methane production.

Anaïs Marechal