Abstract: Methods and apparatus are disclosed for treating biodegradable waste products. A liquor of waste products is introduced into a reactor mixing zone comprising a column of fixed media material having a high surface area to volume ratio, a low resistance to liquid flow and a surface condition receptive to aerobic microbial growth. The liquor is circulated in a continuous flow path including the fixed media column. Preferably, a mechanical surface aerator is positioned in the continuous flow path immediately above the mixing zone to aerate the liquor and distribute it uniformly over the top of the fixed media column. The aeration, distribution and circulation of the liquor are accomplished in a manner to promote homogeneous mixing throughout the mixing zone and to encourage aerobic microbial growth on the fixed media while minimizing agitation of a nature tending to dislodge the aerobic microbial growth from the fixed media and develop flocculent aerobic microbes.