A fractional factorial field experiment to study the decomposition of municipal solid wastes stored in wrapped bales

Wrapped bales can be temporarily used to store municipal solid wastes (MSW) prior to treatment or final disposal. The degree of transformation of wastes within the bales can determine the method to treat MSW after their storage. Goal of the research described in this paper was to monitor the decomposition of the organic fraction of MSW stored in wrapped bales. The monitoring was based on the momentary volumetric concentrations of O2, CO2 and CH4 that were regularly measured in the centre of 23 low density polyethylene (LDPE) wrapped bales over a 617-day period. The field experiment was designed at two levels and four factors, based on the principles of fractional factorial designs. The factors that were studied were: shredding, waste particle size and waste density, thickness of wrapping layers, and type of wrapping film. Two bales were randomly chosen and opened after one year of storage and their solid contents were analysed. Results indicate that carbon dioxide was the prevalent decomposition gaseous product that started from around 80% (v/v) and gradually diminished to around 20%. A non-steady methanogenic phase was established after 150 days from initial wrapping and lasted for approximately 150 days. Methane concentration never exceeded 17%, on average, in the interior of the bales. A 40% dry mass reduction was measured after one year of storage in two bales that contained 74% organics. The factorial analysis revealed that particle size/density and film type were the two main factors that significantly affected methane and carbon dioxide generation.

► We monitored CH4, CO2, O2 contents in the centre of 23 bales over a 620-day period.
► A 40% dry mass reduction was calculated in two bales after 1-year of storage.
► Particle size and type of wrapping film significantly affected waste decomposition.
► Both aerobic and anaerobic processes occurred in the interior of the bales.
► Methane was never detected in contents greater than 32% v/v within the bales.