A recyclable plastic bulking agent (RPBA) that can be screened and reused was developed to improve
sludge composting and to reduce costs. Two RPBAs were developed: RPBA35 (35 mm in diameter) and
RPBA50 (50 mm in diameter). The objective was to study the influence of size and quantity of RPBA on
temperature, oxygen content, water removal during sludge composting, and phytotoxicity of the
compost. RPBAs of both sizes improved the temperature, oxygen supply, and water removal compared
with the treatment with no RPBA, and obtained phytotoxic-free compost. RPBA50 more effectively
removed water than RPBA35. Oxygen diffusion rate in the composting pile containing RPBA50 was higher
than in the treatment with no RPBA. When the RPBA50: sludge mixture ratio was above 1:1.5, the period
over which the temperature exceeded 55 C was insufficient to meet the harmless treatment
requirement. The water evaporation rate was highest at a ratio of 1:2.