Producers in Greece Generate Electricity with Olive Mill Wastewater

Following the pro­duc­tion of bio­gas from live­stock waste, anaer­o­bic diges­tion tech­nol­ogy is now emerg­ing as a solu­tion for man­ag­ing olive mill waste­water in Greece.

With the first bio­gas plants now pro­cess­ing liq­uid waste pro­duced through olive oil extrac­tion, a small group of olive oil pro­duc­ers in north­ern Greece can rid them­selves of the respon­si­bil­ity of dis­pos­ing of the toxic mate­r­ial while con­tribut­ing to the pro­duc­tion of green energy.

However, there are still some chal­lenges involved. The quan­ti­ties of olive mill waste­water cur­rently feed­ing bio­gas plants are extremely low in rela­tion to the total amount pro­duced across the coun­try.

However, experts point to Italy, sug­gest­ing there are sim­i­lar prospects for apply­ing the tech­nol­ogy to those of their neigh­bor.

No need for evap­o­ra­tion tanks

For the Kyklopas SA olive mill, in Evros, in north­east­ern Greece, the dis­posal of olive mill waste for bio­gas pro­duc­tion is a wel­come solu­tion in the man­age­ment of the toxic residues.

“Our coop­er­a­tion with the pro­duc­tion unit of Biogas Komotini SA has taken a bur­den off our backs,” Niki Kelidou, of Kyklopas SA, said. ​“Despite the fact that we have a license to dis­pose of the residue in our unit, the man­age­ment of waste­water is a job that requires sig­nif­i­cant effort and costs as the quan­ti­ties we pro­duce are high.”

The fam­ily-owned olive mill pro­duces 3.4 mil­lion liters of liq­uid waste­water annu­ally as a byprod­uct of its annual pro­duc­tion of 260 tons of olive oil.

“The cost of build­ing the reser­voir where the liq­uid waste was deposited for evap­o­ra­tion was €40,000 and we built it just two years before we started col­lab­o­rat­ing with the bio­gas plant,” Kelidou said. ​“Had we struck the deal ear­lier, we would not have built the tank.”

In addi­tion to con­struc­tion costs, the main­te­nance of the dis­charge basin incurs addi­tional expenses.

“Every two to three years, the tank must be emp­tied and the thick sludge has to be scraped,” Kelidou said. ​“Moreover, we incurred trans­port costs because the tank is located at a dis­tance of 20 kilo­me­ters from the olive mill.”

An addi­tional fac­tor that enabled the coop­er­a­tion with the bio­gas plant is the short dis­tance that sep­a­rates them, which keeps trans­porta­tion costs rel­a­tively low.

“The Komotini bio­gas plant is only 50 kilo­me­ters from the mill,” Kelidou said. ​“This is cru­cial because, dur­ing the oper­a­tion of the mill, about five tanks are loaded on a daily basis and trans­ported to the bio­gas plant.”

A great envi­ron­men­tal haz­ard

The prob­lem of olive mill waste­water man­age­ment is acute as the quan­ti­ties pro­duced amount to sev­eral hun­dred thou­sand tons.

In fact, the prob­lem con­cerns all olive oil-pro­duc­ing regions in the Mediterranean. The annual pro­duc­tion of olive mill waste­water across the basin is esti­mated to exceed 30 mil­lion cubic meters, of which 50 per­cent con­sists of liq­uid waste.

More than half of this amount is chan­neled into streams, which usu­ally end up in ground­wa­ter. The pol­lu­tion caused in ground­wa­ter by phe­nols cre­ates tox­i­c­ity prob­lems, while the con­t­a­m­i­na­tion of sur­face waters is also high.

The remain­ing vol­umes of the olive mill liq­uid waste are chan­neled to the ground (20 per­cent) and the sea and rivers (12 per­cent). Only 10 per­cent are directed to evap­o­ra­tion tanks and other infra­struc­ture which is appro­pri­ate for the dis­posal of this toxic liq­uid.

Green energy from olive oil waste

Thessaloniki-based Biogas Lagada SA, which has been in oper­a­tion since 2015, receives approx­i­mately 4,000 tons of olive mill liq­uid waste annu­ally. This cor­re­sponds to five per­cent of the total waste received by the unit.

“We receive 80,000 tons of waste annu­ally, which con­sists mainly of live­stock waste mat­ter, pro­duc­ing 8,400 MWh of elec­tric­ity,” said Odysseas Koupatsiaris, the direc­tor of Biogas Lagadas SA. The pro­duc­tion of the unit cov­ers the elec­tric­ity needs of about 1,500 house­holds.

Utilizing the mil­l’s waste can also con­tribute towards the cir­cu­lar econ­omy in addi­tion to green energy pro­duc­tion.

“At the Lagada bio­gas plant, we also pro­duce 75,000 tons of organic fer­til­izer, which is sup­plied to grow­ers at a com­pet­i­tive cost of €2.00 to €2.50 per ton and is mainly applied to wheat or corn­fields,” Koupatsiaris said.

A small-scale solu­tion

However, there are sig­nif­i­cant lim­i­ta­tions and the dis­posal of olive mill waste for bio­gas pro­duc­tion can­not be a large-scale solu­tion.

“One dif­fi­culty con­cerns the sea­sonal flow of raw mate­r­ial dur­ing the period of olive oil pro­duc­tion, which only stretches across a few months a year,” Koupatsiaris said. ​“The waste could poten­tially be stored in the premises of the bio­gas unit to feed sup­ply through the year, but this means that appro­pri­ate stor­age should have been planned dur­ing the design of the bio­gas plant facil­i­ties, which is not the case.”

Additionally, olive mill waste can­not account for the total amount of waste. It must be com­bined with large vol­umes of other types of waste, such as manure, to be digestible.

“I esti­mate that olive mill waste can­not be more than 30 per­cent of the mate­ri­als processed in the unit,” Koupatsiaris said. ​“This is because some of the waste com­po­nents cre­ate a prob­lem and can dis­rupt the chem­i­cal process of bio­gas pro­duc­tion.”

“Thus, to cor­rect the prob­lem cre­ated by the lev­els of car­bon and fats, it is mixed with waste from pig farms, poul­try farms and mink units, as this cre­ates the nec­es­sary bal­ance of car­bon to nitro­gen,” he added.

Positive prospects

However, the lim­i­ta­tions fac­ing the dis­posal of olive mill liq­uid waste do not can­cel the fact that the tech­nol­ogy has promis­ing prospects, experts say.

“In Greece, the liq­uid waste of olive mills is con­sid­ered as a sec­ondary source of bio­mass, but in Italy, there are bio­gas units that are designed to run on olive mill waste exclu­sively and belong to com­pa­nies in the olive oil sec­tor,” said Costas Alexandridis, mem­ber of the Board of the Hellenic Association of Biogas Producers.

“The tech­nol­ogy is not well known in Greece, which per­haps explains the low lev­els of its appli­ca­tion,” he added.