To sustain the performance of a land treatment system, soil hydraulic conductivity must be maintained. Biological clogging of soil pores tends to decrease hydraulic conductivity. The changes in soil biological properties and hydraulic conductivity were examined in repacked soil cores receiving weekly additions of wastewater with three C:N ratios for 28 weeks. We tested whether addition of a nitrification inhibitor (dicyandiamide, DCD) and polysaccharidase enzyme preparation (PEP) with wastewater would maintain hydraulic conductivity. Increases in wastewater C:N ratio increased soil microbial biomass, carbohydrate, and nematode population and decreased nitrate leaching and hydraulic conductivity, by up to 80%. While neither DCD nor PEP treatments lowered soil carbohydrate, both had increased hydraulic conductivity (in the range of 280-290%) in cores irrigated with high C:N wastewater, relative to ＇nil＇ treatment wastewater.