Mechanical separation of impurities in biowaste: Comparison of four different pretreatment systems.

Impurities in biowaste, such as plastics, glass, metals and inert material, negatively impact the operation of anaerobic digestion plants and compost quality, and have to be removed prior to the anaerobic digestion process. Different mechanical pretreatments are available for this purpose. However, data on the removal efficiencies of pretreatment systems for different types of biowaste and for different kinds of impurities are still scarce. This study aims to determine the efficiencies for impurity removal of four biowaste pretreatment plants (BTPs) at real scale - (1) drum-screen + shredder + piston press; (2) shredder + piston press + screw press; (3) separation-mill; and (4) pulper + drum-screen. BTP 1 treats mixed food and garden wastes, while BTP 2, 3 and 4 treat mostly food waste. The efficiency of the pretreatment systems was influenced by the type of pretreated biowaste. The recovery of organics by the press was more efficient when pretreating food waste (BTP 2, 93%) than for treating mixed food and garden wastes (BTP 1, 77%). BTP 3 presented the highest recovery of biogas (up to 98%), but also the highest transfer of inert particles to the substrate. On the contrary, BTP 4 was the most efficient for the removal of inert particles; however, this system also presented 18% loss of biogas potential.

A multi-criteria decision analysis of management alternatives for anaerobically digested kraft pulp mill sludge.

The Multi-Criteria Decision Analysis (MCDA) procedure was used to compare waste management options for kraft pulp mill sludge following its anaerobic digestion. Anaerobic digestion of sludge is advantageous because it produces biogas that may be used to generate electricity, heat and biofuels. However, adequate management of the digested sludge is essential. Landfill disposal is a non-sustainable waste management alternative. Kraft pulp mill digested sludge applied to land may pose risks to the environment and public health if the sludge has not been properly treated. This study is aimed to compare several recycling alternatives for anaerobically digested sludge from kraft pulp mills: land application, landfill disposal, composting, incineration, pyrolysis/gasification, and biofuel production by algae. The MCDA procedure considered nine criteria into three domains to compare digested sludge recycling alternatives in a kraft pulp mill: environmental (CO2 emission, exposure to pathogens, risk of pollution, material and energy recovery), economic (overall costs, value of products) and technical (maintenance and operation, feasibility of implementation). The most suitable management options for digested sludge from kraft pulp mills were found to be composting and incineration (when the latter was coupled with recycling ash to the cement industry). Landfill disposal was the worst option, presenting low performance in feasibility of implementation, risk of pollution, material and energy recovery.