Influence of physico-chemical stress on biodegradability of car engine lubricating oil

Biodegradability of used and unused lubricating oil was assessed with a mixed culture of bacterial isolated from the used oil. Flow rate determination indicated that used oil flowed faster at both low (30°C) and high (70°C) temperatures than unused oil. Hear transfer experiments showed that used oil conveyed heat more readily than unused oil so when the temperatures of used oil were 38, 67 and 97°C, the corresponding temperatures of unused oil were 35, 55 and 72°C, respectively. Utilisation of used oil by the mixed culture resulted in higher total viable counts (20 × 10⁷CFUml⁻) and pH drops (7·3–5·5) than those of unused oil (c.11 × 10⁷CFUml⁻¹andpH 7·3–6.0). Analysis by computerised gas chromatography confirmed that used oil has been modified while in service and is more biodegradable than unused oil. The formation of several novel compounds which could be detected by gas chromatography resulted from microbial growth on used oil.