Chemical inhibitors for biosolids pellets to promote stability during transport and storage

Abstract

The Massachusetts Water Resources Authority (MWRA) serves the metropolitan Boston area and has been heat-drying and pelletizing its biosolids for beneficial reuse since 1991. Episodes of self-heating, associated with rewetting of the pellets in rail cars that are not rain-tight, have been observed periodically during rail car shipments when the pellets arrive at their destination smoldering. Biological activity may play an important role in these self-heating incidents because rewetted pellets contain significant amounts of readily utilized organic material. This study identified the critical moisture level at which microbial activity is significantly initiated and examined several additives that would delay the onset of biological activity during transport. Through the use of respirometric techniques, microbial activity was measured. Propionic acid, citric acid, and sodium sulfite, food additives that can significantly inhibit microbial growth without presenting human health or ecological risks at the typical levels used in foods, and Dustrol 3010 (Arr-Maz Products, Inc., Winter Haven, Florida), a dust-control agent currently being used by MWRA, were investigated. The critical pellet moisture content that induces measurable oxygen uptake is 9% by weight. All four additives delayed the onset of microbial oxygen uptake, and dosages ranging from 20 to 88 mg additive/g pellets reduced oxygen uptake rates by 80 to 90% for a period of 5 to 7 days.