Biodiesel production from sewage sludge: new paradigm for mining energy from municipal hazardous material

This work demonstrates that the production of biodiesel using the lipids extracted from sewage sludge (SS) could be economically feasible because of the remarkably high yield of oil and low cost of this feedstock, as compared to conventional biodiesel feedstocks. The yield of oil from SS, 980,000 L ha(-1) year(-1), is superior to those from microalgal and soybean oils, 446 and 2200 L ha(-1) year(-1), respectively. According to the case study of South Korea, the price of the lipids extracted from SS was approximately $0.03 L(-1) (USD), which is lower than those of all current biodiesel feedstocks. This work also highlights the insight of a novel methodology for transforming lipids containing high amounts of free fatty acids (FFAs) to biodiesel using a thermochemical process under ambient pressure in a continuous flow system. This allowed the combination of esterification of FFAs and transesterification of triglycerides into a single noncatalytic process, which led to a 98.5% ± 0.5% conversion efficiency to FAME (fatty acid methyl ester) within 1 min in a temperature range of 350-500 °C. The new process for converting the lipids extracted from SS shows high potential to achieve a major breakthrough in minimizing the cost of biodiesel production owing to its simplicity and technical advantages, as well as environmental benefits.