Abstract: A biofilter load comprises a stack of blocks of a fired porous clay material having passages extending vertically therethrough for flow of sewage. The passages may be molded in the blocks or may be formed between adjacent blocks by the method of stacking. The blocks are stacked in a frame which is placed in the biofilter. The blocks are manufactured from 8-12 mm clay granules placed in a mold in a layer of 60-80 mm. The clay is heated to the temperature at which the granules begin to swell, is maintained at a temperature in the range of swelling of the clay for a set period of time, and cooled in multiple stages. The process produces blocks with deep branched open pores to provide optimum conditions for development of microorganisms on the pore surfaces and bacteria-denitrification in the deep pores to improve the efficiency and productivity of sewage treatment.