Q Is plug-flow high solids digestion costly to operate?
A Typically, a plant of 25,000 tonnes per year of source segregated organic waste would produce approximately 750 kW of electricity using the Kompogas process. The plant would consume 10% to 15% of the reactor`s electricity production and 20% to 30% of its heat production.
These plants are manned five days a week by three operators during the day. Nighttime operation is automatic. Reactors have been operated for more than 20 years with minimal component replacement costs. As an example, the slow moving paddle mixer relies on an 11 kW motor and has all assembly components accessible from outside the reactor.
Q Can high solids AD affect the quality of the final compost?
A Anaerobic digestion will not affect the quality of the final product in regard to contaminants (plastics, glass, and metals) but the processing steps before and after digestion will. The level of pretreatment or final compost polishing will be based on the feedstock quality and the intended fraction of quality compost expected.
The same challenges in regards to final product quality are found on composting platforms without AD and similar processing equipment would have to be used.
Most of the AD installations in Europe produce a high fraction of quality compost from typical source separated.
Both Veolia and Kompogas have extensive experience with AD and/or composting of organic municipal solid waste. In these plants, pretreatment will typically segregate 45% of the mixed waste as feedstock to convert into energy. From that approximately one-third becomes quality compost.
Canada is leading the way in North America in respect to organic waste recovery. Certain provinces implemented source separated programs in the 1990s and most have organic recycling as part of their waste management strategy. Plug-flow thermophilic anaerobic digestion technology further increases the benefits of organic waste diversion, efficiently closing the loop on energy and nutrients, while avoiding landfilling useful resources.
Christian Cabral is with Veolia Water Technologies. This article appeared in ES&E’s January/February 2015 issue.