Energy From Solid And Liquid Wastes - I

Lecture No: 1 1.1.Waste materials: sources and quantities and characterization Solid waste materials are handled by what is termed the waste management system. Waste management is a tradition that goes a long way back as solid waste has existed for about as long as the human race and waste management has now become a complex task as the amounts and complexity of solid waste materials have increased dramatically especially during the 20thcentury. Optimal waste management depends upon the quantities of waste but also on the composition of the waste materials. This chapter discusses the sources, quantity and quality of solid waste materials. 1.2. The role of solid waste management. Human activities invariable involve the consumption of matter (food, water, metal etc.) and the production of waste products (waste water, air pollution, solid wastes, etc.). Seen in a greater perspective, the human society may be viewed as a device that transforms natural resources into undesirable waste products under the production of desirable products such as energy, food, consumer goods etc. as illustrated in Fig.1.1.

Natural resources

Waste products

Disposal

Solid waste management

Fig.1.1. The role of waste management in the turnover of matter in the human society

These products, however, ultimately end up as waste at some point n time such that the overall process is that of converting natural resources into waste. The depletion of natural resources is undesirable as it reduces the possibility fir future generations to have access to

the same resources as we have now. The production of waste products also has negative side effects in terms of environmental degradation and pollution. The role of sustainable waste management is therefore to reduce the amount of waste that is discharged into the environment by reducing the amount of waste generated and to transform the waste that is generated into a form where it can be recycled to the input side of the society thus, reducing the need for extraction of new natural resources. In other words the purpose of waste management is in addition to provide sanitary living conditions to reduce the amount of matter that enters or leaves the society and encourage the reuse of matter within the society. Of course as it is not practically possible to recycle all f the waste generated or to rely solely on recycled products, one other important aspect of waste management is to dispose of the non-recyclable waste products in an environmentally safe manner. This is discussed further in the next sections. 1.3 The solid waste management system The whole set of activities related to handling, treating, disposing or recycling solid wastes is termed the waste management system. The purpose of the waste management system is to make sure that the waste materials are removed from the source or location where they are generated and treated, disposed of or recycled in a safe and proper manner. The system consists of several steps as illustrated in Fig.1.2. The waste management system consists of four main parts: (1) generation e.g., wasteproduction, (2) collection e.g., collection systems and transport of waste materials, (3) treatment e.g., transformation of the waste materials into useful products, and (4) final disposition e.g., the use of recyclable products or the placement of on-recyclable materials in landfills. Each of these steps is again comprised of several subparts. The following sections focus on the generation of solid waste materials including sources, types, impact of sources, separation, and materials production rates. The remaining steps in the waste management system in will be discussed in further detail in subsequent chapter.

Waste sources

Waste types

Production of waste materials

Source separation

Internal collection Production rates

Collection Collection and transport

Transport Transfer

Shredding Physical treatment

Sorting Compaction

Treatment

Thermal treatment

Incineration Gasification Anaerobic digestion

Biological treatment

Aerobic composting Anaerobic digestion

Final disposition

Recycling

Land filling Fig. 1.2. Components of waste management system. 1.4.The waste management priority list Advanced waste management systems are often based on a prioritized list of management strategies to minimize environmental problems and preserve resources. Waste management strategies are categorized into four areas with respect to their final disposition of

the waste. 1) Minimization or prevention of waste generation. 2) Recycling of waste. 3) Thermal treatment with energy recovery 4) Landfilling Minimization of waste has the top priority anti is generally the responsibility of the waste producer. Environmental certification of industrial production processes is being done by several industries partly because law in several countries requires it and partly because it may be used to promote the products as being environmentally friendly. Often there is also an economic advantage connected to waste minimization (reduced taxes on waste or less consumption of expensive raw materials. Recycling has second priority as it results in the recovery of materials that can be used as raw materials for other purposes than the one where it was generated. Recycling has long been practiced in many regions. The types of waste recycled; however, have been limited to certain easily reusable materials such as metal, glass and paper. In special cases an industry with a specific waste would have an agreement with another industry that would use this waste as input to production with economical gain for both. In countries with advanced waste management more and more categories of waste are now being recycled and legislation to enforce and Increase recycling is being developed. Despite waste minimization and recycling there will always be waste that cannot be recycled either due to significant economical loss or because there is no process at present where it can be used. In such cases an option is to incinerate .the waste and produce energy. Even though the material is lost there is at least energy production, which will save some of the fossil fuels that would otherwise be used. Landfilling of waste is only used as a last resort and is considered the least optimal solution. In special cases wastes that cannot be recycled today but may be recycled in the future are placed in landfills for storage until they can be recycled. This is the case for several types of plastic and wood that is impregnated with copper based chemicals.