Resource Recovery and Conservation最新文献

Principles of economic evaluation previously developed are applied to a case study of the available options for waste disposal or resource recovery from solid waste. The dominant feature in any such analysis at a preliminary stage of planning is the uncertainty in the cost and revenue estimates. It is shown how this uncertainty can be explicitly included, with sensitivity analysis used to isolate the critical parameters and risk analysis to examine the range of probable costs. The results of the case study suggest that, of the thirty options studied, landfill at a local site or the use of pulverized waste directly as a fuel are currently the cheapest, followed by indirect landfill or the production of a solid refuse-derived fuel. Other options, including incineration and pyrolysis, appear currently uncompetitive in economic terms.

This paper introduces a new parameter for describing air classifier performance, “Total Efficiency”, found by first calculating the fractions recovered of both light and heavy fractions at specific air speeds and then multiplying the two fractions. Total Efficiency can be plotted against air speed, and such plots yield information about peak efficiencies and sensitivity to air flow rate variations. This method allows the comparison of different air classifiers and also leads to a technique in which the data can graphically express contamination of the light or heavy fraction at different air speeds. These curves can then be used to determine the optimum air speed in terms of market value of the product produced. Data on three different throat designs — the zigzag throat and two variations of the zigzag, are used to illustrate the method.

Four aquatic biomass species were anaerobically fermented to methane as part of an evaluation of these biomass as potential energy resources. Two freshwater weeds (Duckweed (Lemna sp.) and Hydrilla verticillata) and two marine algae (Gracilaria ceae and Ulva lactuca) were evaluated. Volatile solids, ash content, calorific values, and elemental analyses are reported for these biomass. All four were fermented at mesophilic (37°C) conditions in 50 1 CSTR units using a rich nutrient feed of essentially equal parts by weight sewage sludge and aquatic biomass in an approximately 5% solids concentration slurry and with a 26-day retention time. In addition, the two freshwater weeds were evaluated in a similar manner at thermophilic (60°C) conditions. Bioconversion efficiency was based on measured energy out, as methane, and measured energy in, as calorific values of the biomass. It was found that 25 to 34% of the energy value in the freshwater weeds was recovered at mesophilic conditions, a low value perhaps due to the fact that steady-state conditions were not reached in the fermenters. For the marine species, 27 to 45% of the energy value was recovered under the same conditions. Conversion of the freshwater weeds at thermophilic conditions, however, was from 32 to 46%. It was found that by assuming all total volatile solids in the seaweed had an oxidation state equivalent to cellulose the same bioconversion efficiency was obtained as measured directly, appearing to indicate a high fraction of biodegradable polysaccharides. Freshwater weeds, however, demonstrated a much lower conversion based on calorimetric values than with the assumption that all volatile material was cellulosic in nature. This may indicate that bioconversion of a cellulosic fraction occurred, but that residual higher energy components in the biomass such as lignin were nonbiodegradable under these conditions. Results of the bioconversion of alkaline pretreated (saturated lime) Duckweed were approximately equal to those with the untreated biomass. An inhibition investigation to explain lower than anticipated bioconversion was conducted on the two marine species based on their high degree of sulfonation of polysaccharides. This hypothesis proved to be invalid, and slow acclimatization of innoculating microorganisms was given as a possible explanation of observed results. Further, in situ or batch fermentation were carried out at mesophilic conditions using minimal inoculum. Both freshwater aquatic biomass were evaluated in 2 1 units, while Hydrilla was also evaluated in a 50 1 unit. It was found that 80% of the methane was evolved in the first two months of operation. Moreover, bioconversion performance in these simple mesophilic in situ units were equal to that in the CSTR units at thermophilic conditions, namely 34–46% conversion. Obtaining baseline biomass conversion results in simple in situ units appears most practical. Pretreat