Composting of Silicone Polymer Under Different Moisture Conditions, and with the Addition of Various Amounts of Soil

Abstract

Silicone polymers (polydimethylsiloxanes, or PDMS) are used in many down-the-drain consumer products and thus enter wastewater treatment facilities, after which they can be composted with the resulting sludge. Our previous experiments showed that PDMS would not harm the composting process, and although it does not degrade in moist compost, it will degrade (60% in 4 months) after compost is added to soil. The present experiment investigates the fate of PDMS in compost which is allowed to dry to various extents. PDMS (350 cst) was added to moist compost to yield 2000 mg PDMS/kg of (dry weight) compost. Compost (180 g dry weight) was mixed with moist soil (either 0, 18, or 54 g dry weight) and 30 g (dry weight) of microcrystalline cellulose. Samples were incubated at 58°C for one month using four different moisture regimes, varying from continuously moist to dry. The release of CO₂ from cellulose degradation was monitored as an indication of microbial activity in the four moisture regimes. After incubations were completed, compost samples were extracted and assayed for evidence of PDMS degradation. PDMS degradation increased with increasing severity of compost drying. Little degradation (>90% recovery) was found in continuously moist samples, while samples exposed to partial drying/rewetting showed 40-90% recovery. Dry samples had 20-30% recovery. Some (maximum <13%) degradation products (mostly dimethylsilanediol) were found, but the majority had biodegraded or volatilized. The addition of soil had little effect on PDMS degradation, and the compost itself was active enough to degrade PDMS.