RefCom: A Test-Bed Converting Municipal Solid Waste to Methane

Mr. Don Walter should be commended for his technical paper dealing with the real-world problems of preparing anaerobic digester feeds from a difficult raw material, municipal solid waste (MSW). The information presented in the paper is not readily available in the published litera­ ture, and will be valuable to engineers interested in MSW processing and digestion. The author discusses some of the pitfalls that may be commonly encountered in front-end and digestion process design and operation, and offers solutions for problems faced at the Pompano Beach proof­ of-concept biodigestion plant. The paper indicates that the problem of developing an optimum MSW digestion process is far from being solved. This writer feels that to develop a cost-effective MSW digestion process it is necessary to develop and apply innovative digestion modes and novel digester designs. In addition, new designs must be developed for inflow-out­ flow structures, scum-control devices, and reactor vessels to achieve high solids residence times (SRT's). We feel that complete-mix CSTR digesters and conventional mechanical or gas mixing are unsuitable for MSW diges­ tion, particularly when large particle size and high solids­ content feeds are used. The literature is replete with information showing that, • in general, higher rates of reaction and product yields are obtained with smaller feed particle size. The effect of particle size on MSW digestion has not been studied in detail. The Pompano Beach digesters are operated with large particle size MSW because it is felt that no particular advantage is gained by reducing the feed particle size. On the cor.�rary, our preliminary studies indicated that .both the methane yield and the production rate decreased dur­ ing mesophilic high-rate digestion at an HRT of 12 days as the refuse feed particle size was increased (Fig. 1) [1]. Similarly, the benefits of fme-particle MSW digestion could significantly outweigh the cost of producing the fme particles (Fig. 2). Finally, we feel that unusually large MSW particles aggravate the mixing and scum problems in a digester. The Pompano Beach digesters are operated at a thermo­ philic temperature. As shown in Fig. 4, thermophilic digestion exhibits lower net energy production than meso­ philic digestion if it has the same or lower methane yield than that of the latter process. Thermophilic digestion can be justified only if it affords a higher methane yield and is operated with a more concentrated feed than those of the mesophilic process (Fig. 5) [2] .