Effects of pre-treatments on the lipid extraction and biodiesel production from municipal WWTP sludge

• One of the first attempts made in Europe to convert municipal sludge to biodiesel.
• First attempt to optimise the yield of lipids extracted using pre-treatments.
• First attempt to compare four sludge of different characteristics from the WWTP.
• Primary sludge has a conversion to FAME of 19% in dried sludge basis.
• Pre-treatments do not affect essentially the conversion and composition of FAME.

Biodiesel production is currently limited due to high raw material costs. The potential of using sludge from municipal wastewater treatment plants as an alternative lipid feedstock was investigated. Four different types of sludge (primary, secondary, blended and stabilised) were tested in lipid extraction by Soxhlet using hexane, and biodiesel production by acid catalysis. To improve the extraction efficiency, the influence of pre-treatment methods (ultrasonic and mechanical disintegration) and duration of these treatments were investigated. Finally, the effect of sludge acidification with concentrated HCl was also evaluated. The pre-treatment methods did not increase significantly the amount of extracted lipid as well as biodiesel yield. Previous sludge acidification showed lower yield of lipids from primary, secondary and blended sludge. However, the amount of saponifiable lipids was higher, giving the overall biodiesel yield almost unchanged. Among the four sludges tested, primary sludge achieved the greatest lipid and biodiesel yields, 27% and 19% respectively, on the basis of dry sludge. The highest biodiesel yields obtained from blended, secondary and stabilised sludge amounted to 15%, 4% and 2% respectively, on the basis of dry sludge. No significant influence of the pre-treatments and acidification on the fatty acid composition was found. At least 8 fatty acids were determined, with a predominance of palmitic (C16:0), stearic (C18:0) and oleic acid (C18:1). The comparison of sludge fatty acids profile with common biodiesel feedstocks showed suitability of WWTP sludge for production of biodiesel.

Figure optionsDownload as PowerPoint slide