On the basis of incubation experiments and field observations, it has been suggested that the isomeric composition of LABs provides information regarding the degree of biodegradation [6,23,25]. In detergents and untreated sewage, the abundance of each isomer with a given alkyl chain length is almost evenly distributed,whereas in treated sewage effluent, internal isomers dominate over external isomers [7]. This is due to the
selective biodegradation of the external isomers relative to the internal isomers. Takada and Ishiwatari proposed the internal/external ratio for the C12 homologs (I/E ratio; a ratio of sum of 6- and 5-C12LAB relative to sum of 4-, 3-, and 2-C12LAB) to quantitatively evaluate the degree of LAB degradation. A high I/E ratio indicates a high degree of LAB degradation. Consistent with previous reports [2], I/E ratios of
influent samples ranged from 0.9 to 1.1, whereas those of effluent samples increased to the range of 3.0–5.6
(Fig. 3).