Resource Recovery and Conservation最新文献

After summarizing data on the rate of generation and the composition of Japanese municipal solid waste, current modes of collection and disposal are briefly reviewed. Industrial waste disposal in Japan is still a problem; its solution can be exemplified by Toyota's Motomachi integral disposal center, featuring waste segregation and pretreatment, incinerators, and wastewater cleaning. The remarkable Japanese effort in the development of elementary techniques and integrated systems for sorting municipal waste is illustrated by a few examples of such techniques and systems. A large part of Japanese refuse is incinerated in municipal incinerators inspired by W. European practice. Specific Japanese developments in the fields of incineration, gasification and pyrolysis are discussed and show an emphasis on fluidized bed techniques and on the problems of waste plastics and rubber disposal.

A computer simulation of a hypothetical independent wastepaper dealership was used to test the effects of price forecasting and formal management policies on profitability. Several forecasting techniques were tested as were several management policies. Policies combining forecasting with price/inventory controls or with six-month contracts (between dealers and collectors) produced higher contributions to profit than a policy of maintaining a constant margin between price paid to collectors and price paid by mills.

This paper presents an attempt to construct an economic model of the U.K. waste paper industry. A primary determinant of the demand for waste paper was found to be the output of the packaging industry. Neither mechanical pulp prices nor waste paper prices appeared to have any significant effect on waste paper demand. Imports of waste paper appear to be determined to some extent (stock levels also being important) by the gap between forthcoming home supplies and forecasted usage requirements. On a limited price data base, it seems from our investigations that (with one exception in the period 1974/1975) the supply elasticity of waste paper in the United Kingdom is low, at least in the poorer quality grades. This would seem to be an important finding as it suggests that the opportunities for recycling waste paper might be substantially less than many have hoped.

Bioconversion of food processing wastes is receiving increased attention with the realization that waste components represent an available and utilizable resource for conversion to useful products. Liquid wastes are characterized as dilute streams containing sugars, starches, proteins, and fats. Solid wastes are generally cellulosic, but may contain other biopolymers. The greatest potential for economic bioconversion is represented by processes to convert cellulose to glucose, glucose to alcohol and protein, starch to invert sugar, and dilute waste streams to methane by anaerobic digestion. Microbial or enzymatic processes to accomplish these conversions are described.

A theoretical concept is presented for the description of the separation process in a vertical gravitational air classifier. Particle movements within the classification zone are described by two types of transport: a convective transport, which represents the average movement of the particles and is characterized by the mean absolute particle velocity u; and a mixing transport, which comprises all transport due to deviations from the average particle displacement and is characterized by the mixing coefficient E.

The main assumptions of the model are that particle inertia is negligible and that the transport parameters u and E are constant along the height of the classification zone. The removal of the particles from the classification zone is described by rate equations. At the heavy fraction exit the removal rate is assumed to be linearly proportional to the fall velocity of the particle and the relative particle concentration at the heavy fraction exit. At the light fraction exit the removal rate is assumed to be linearly proportional to the superficial air velocity and the relative particle concentration at the light fraction exit.

Expressions are derived for the separation curve and for the mean residence time of the particles. The mean residence time of the particles is an indirect measure of the throughput capacity of the classifier. The relation between separation efficiency and mean residence time of the particles is calculated. The theoretical concept is experimentally verified for particles with small inertia at low particle concentrations in the classification zone. On the basis of the relation between separation efficiency and mean residence time of the particles conclusions are drawn with regard to the design of vertical gravitational air classifiers for separating usable components from mixed municipal waste. Obviously, these conclusions are valid only within the assumptions made in the model. Examples of possible designs illustrate these conclusions. It can be concluded from theory that suppression of particle mixing and accelerated removal of the particles from the classification zone yields the highest separation efficiency at comparable particle residence times.

A technique for the dry separation of mixed plastics has been developed. The process involves triboelectrically charging polymer particles by friction on the wall of an air-cyclone, followed by differential deflection in an electrostatic field. The effect of ambient relative humidity on the charge acquired by five commercial polymers has been investigated leading to the separation of a nylon-66 from artificial mixtures containing a polymethylmethacrylate, two kinds of polyethylene and polyvinylchloride (PVC) at room relative humidities and temperatures. Products with a purity above 96% have been achieved with recoveries ranging from 60 to 70%. Recoveries could be increased to about 90% on repassing the material through the apparatus. Such a dry separation process may find application in plastics recycling and recovery.