Twenty years ago I operated a biofilter for wastewater treatment. I set layers like this (Pic_004_b):

Superficial layer: earth with live earthworm (Eisenia foetida).

Layer 1: sawdust

Layer 2: gravel diam 3/8"

Layer 3: gravel diam 1/2"

The remove BOD5 (biochemical oxygen demand in five days) efficiency was 79.6 %, but chemical oxygen demand (COD) for both influent and effluent was the same.

Then I suspected was lignin from wood. Can someone confirm o reject it? and why?