From waste to water filter: olive-mill leftovers show 99 % heavy-metal removal

Olive oil’s golden reputation now extends beyond the kitchen. A new analysis published in the Journal of Bioresources and Bioproducts reveals that the industry’s most troublesome leftovers—pomace, stones and mill wastewater—can be turned into powerful, low-cost adsorbents that clean polluted water as effectively as commercial materials. Researchers systematically examined 66 peer-reviewed studies from 1996 to 2024 and found that raw or only minimally treated olive residues removed more than 90 % of copper, lead and toxic dyes from genuine wastewater streams. Olive-pomace biochar achieved lead uptakes of 146 mg g⁻¹, while untreated olive stones adsorbed 625 mg g⁻¹ of the model dye methylene blue—figures that equal or exceed activated carbon benchmarks.

The review highlights that Mediterranean countries produce over 40 million tonnes of these residues yearly, often piling them up or burning them. Converting them into adsorbents tackles two crises at once: reducing agro-waste mountains and offering water utilities a sustainable tertiary treatment option. Importantly, several studies demonstrated success in real olive-mill effluent and urban sewage without costly chemical activation, underscoring a ready-to-deploy circular-economy solution. The authors call for pilot columns and regulatory validation to move from lab triumphs to full-scale installations.