Complexity in Solid Waste Processing

Automated solid waste processing is a very compli­ cated task: complicated equipment should be consid­ ered despite the fact that it can cause inordinate downtime if misapplied. Examples from laboratory re­ search and development are compared with reports of �eld experience to produce guidelines for distinguish­ mg between the types of equipment complexity that are and are not acceptable. A renaissance is beginning for automated solid waste processing. As opposed to the original concepts of single plants performing the entire waste-segregation task, processing is being considered for many different ?has�s of waste manageme . nt. Increasingly, processing IS bemg called upon as simplistic waste management systems fail to meet objectives set by ambitious legis­ lators. This discussion begins by exploring the reasons for inadequate performance of selected earlier efforts' covers current tasks required of waste processing, and the modem implementation of waste processing unit operations; discusses laboratory-developed changes in unit operations; and presents resulting development and design principles. DIFFICULTIES IN EARLY EFFORTS Initial efforts to mechanically process solid waste began in the late 1960s when it was realized that the 427 solid waste problem was part of growing national en­ vironmental concern. At that time, it was recognized that waste quantities were increasing, that the com­ plexity of the waste stream was increasing, and that management practices might be inadequate to protect the environment. Resource recovery, in the original use of the term, began at that time. Mechanical unit operations were installed to extract recyclable materials from the waste stream. The production of refuse-derived fuel, or RDF, was viewed as one of many possible product streams, if often the principal one. Separation equipment was borrowed from other fields and adapted to resource recovery with very little research or modification, not­ withstanding the clear increase in the complexity of the separation due to the increased heterogeneity. Few pilot-scale or demonstration facilities were constructed on which the industry could gather information about behavior of unit operations and scale-up properties, let alone conducting laboratory research to develop new devices [1]. . Politically, the solid waste crisis has re-emerged. On thiS go-round, however, there is much less confidence in � "technological fix." Individual states are passing sohd waste legislation at a furious pace. Much of that legislation mandates waste reduction. Lacking alter­ nati�es, solid wa�te managers are pursuing source sep­ aratIon or curbside collection. Several problems have been addressed with modest success: participation rates, even in ideal neighborhoods, do not meet ex­ pectations; multi-family dwellings and economically disadvantaged areas are not considered; recovered ma­ terials fail to find markets; and the systems prove very expensive. The results of these difficulties are that solid waste managers are having difficulty meeting their states' waste minimization requirements. In efforts to solve these problems, material recovery facilities, or MRFs, have been added to curbside col­ lection systems. The majority of these facilities use hand-sorting, employing persons of limited career out­ look to remove easily-identifiable components from the waste stream (pickers). Such a facility is very labor intensive. Training costs are incessant, as turnover is high. Recovering, for example, HDPE by instructing pickers to look for 2 L soda bottles will leave a lot of HDPE in the waste stream; a thorough waste char­ acterization will yield complaints that HDPE removal is inadequate, and overall waste reduction will be min­ imal. Despite these difficulties, mechanical processing con­ tinues to be considered a poor choice for waste man­ agement systems. The history of mechanical processing has led to a great deal of skepticism. A waste man­ agement official, although very interested in the po­ tential of automated processing, might feel that it could not be recommended to his superiors given the history of the industry. It is very important that failures be examined so that lessons could be learned. This paper considers learning general design principles from the study of selected difficulties experienced in the field and labo­ ratory research. THE ROLE OF UNIT OPERATIONS IN PLANT