Application of a Multi-stage Washing Process to Biomass Produced During Phytoremediation of Cement Kiln Dust Landfills

Bioenergy is a proposed solution to anthropogenic greenhouse gas emissions. A sustainable deployment will require targeting marginal lands, particularly brownfield and/or degraded lands. Industrial remediation, however, is often uneconomical or even desirable, especially if degraded lands passively return to a nature like state. Degraded lands suitable for remediation include cement kiln dust landfills, containing saline solids. Phytoremediation uses plants to transport pollutants from the soil into standing biomass. After harvest, the biomass, here Phragmites australis, can be processed to separate salt, biomass and process water. Work showed that salt recovery was preferential for large liquid to solid ratios (10x) over longer durations and/or higher temperatures. Elevated temperatures require additional energy while co-leaching more biomass (~ 4 wt%). Repeated soaking with smaller volumes of room temperature water did not single, large volume soaks. A three-stage multi-stage soaking method produced a similar recovery (< 1.5 wt% salt) while consuming 20% less water. A modelled process produces biomass with sufficient energy density for use in the cement kiln at a fuel cost of $3.83/GJ, parity at a carbon price of $42 USD/t CO₂. Remediation of cement kiln dust stockpiles is on the century time scale while co-producing biofuel for the plant would be annual. Each square kilometer planted could displace 4% of the fuel energy while returning the waste to the kiln. This represents an early opportunity for combined remediation, waste valorization and bioenergy.