<![CDATA[Indonesia's B40 mandate has intensified the search for low-cost biodiesel feedstocks. Waste cooking oil (WCO) is limited by a high free fatty acid (FFA) content and by catalyst and purification costs. This study integrates two agricultural wastes into one route: robusta coffee bean husk (Coffea canephora L.) activated carbon serves as both a de-acidification adsorbent and a dry-washing agent, while calcined rice field snail shell (Pila ampullacea) supplies a CaO nanocatalyst for microwave-assisted transesterification (MAT). Each stage was optimised by response surface methodology with a Box–Behnken design. Adsorption with 6 % (m/v) adsorbent for 175 min at 40 mesh lowered the FFA content from 2.2 % to 0.8 % (63.86 % removal); particle size was the only significant single factor (p = 0.0015; R² = 0.9575). MAT with 3 % (m/v) CaO nanocatalyst (98.79 % purity), 11 min and 220 W gave a crude yield of 97 %; microwave power (p = 0.0056) and its interaction with time (p = 0.0179) were significant. Dry washing at 78.9 mesh, 2.18 % (m/v) and 59.2 °C gave a purified yield of 75.33 %. Dry washing cut the water content from 2.69 % to 0.012 % and the acid value from 0.995 to 0.531 mg-KOH g⁻¹, so the fuel met SNI 7182:2015 for density (867.5 kg m⁻³), water, sediment and iodine value; the acid value stayed marginally high and the methyl ester content, 66.69 % (m/m) by GC–MS, below the 96.5 % minimum, showing that conversion rather than purification limits the route.]]>
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