Valorization of Gas-To-Liquid Sludge as a Sustainable Soil Organic Amendment for Cotton Cultivation

Gas-to-liquid (GTL) sludge is a specific wastewater treatment by-product generated during the industrial process of natural gas conversion into liquid fuels. This least studied sludge is pathogen?free and rich in organic matter and plant nutrients. Therefore, it can be recycled in agricultural soils as an organic amendment and reduce therefore the potential risks of landfilling. In this open field study, we compared the performance of soil treatments with GTL sludge to the more conventional chemical fertilizers and cow manure compost for the cultivation of cotton under the natural conditions of Qatar. After a complete growing season, GTL sludge application resulted in the enhancement of soil properties and plant growth compared to conventional inputs. As such, there was a significant dose-dependent increase of soil organic matter (4.01% and 4.54%), phosphorus (534 and 1090 mg kg-1 ), and cumulative fiber yield (4.68 and 5.67 t ha-1 ) for GTL sludge application rates of 1.5% and 3%, respectively. The produced fiber quality was adequate for an upland cotton variety and appeared more dependent on the prevailing climate conditions than soil treatments. On the other hand, the adverse effects generally related to industrial sludge reuse were not significant and did not affect the designed agro-environmental system. Accordingly, plants grown on GTL sludge-amended soils showed lower antioxidant activity despite a significant salinity increase. In addition, the concentrations of detected heavy metals in soil were within the standards’ limits, which did not pose environmental issues under the described experimental conditions. Leachate analysis revealed no risks for groundwater contamination with phytotoxic metals, which were mostly retained by the soil matrix. Therefore, the agricultural reuse of GTL sludge can be a sustainable solution to improve soil quality, lower carbon footprint, and mitigate environmental contamination.