Friday 28 September 2012

Calrecovery: Options for Solid Waste

George Savage, Executive VP, Calrecovery pointed out that solid waste management must minimize the use of landfill, avoid pollution and minimize climate change. The key waste characteristics to be considered were the elements present, the microbial content, the moisture content, the putrescible organic matter, and any traces of toxic chemicals or plastic contamination.  The disposal options were recycling into new materials, bio-processing, thermal processing and in last resort, landfill. 
Bioprocessing occurred at temperatures below 150oC and could be aerobic (composting), anaerobic (composting or digestion) or enzymatic hydrolysis.  Composting sequesters carbon rather than releasing it to the atmosphere but the process requires energy (aeration of the “windrows”) and odor-controlling semi-permeable covers if used in built-up areas. 
Anaerobic digestion or biogasification is advanced in Europe where funding has been provided.  The US lags 10-15 years behind the EU in this technology. 
The outputs are methane, compostable residues and water which needs purifying.  New systems use less water and are more economical.  A dry system or tunnel digester is available for solid organics.  50-80% of EU food waste goes to biogasification or composting whereas US food waste treatment is in its infancy.  Japan uses mainly thermal methods.

Thermal processing could be a) Incineration, where oxidation occurs and flue gas cleaning is required, b) Gasification, or burning in little oxygen to recover e.g methane for energy generation, c) Pyrolysis or high temperature treatment in the absence of oxygen or d) Plasma Treatment which was now being used in Japan (5000+oC) 

One tonne of municipal solid waste incinerated to give 200kgs ash which would have to be landfilled.  Pyrolysis on the other hand could yield gas and oil and carbon black all of which were reusable.  This process remained to be developed on the large scale required by the authorities and all the by-products would need purifying before reuse.

(from a paper given at the INDA WOW Conference, Chicago, June 2012)

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