PHASED ANAEROBIC CODIGESTION OF AIRCRAFT DEICING RUNOFF AND SLUDGE: PROPIONALDEHYDE AS INTERMEDIATE

A major fraction of the deicer applied to aircraft is presently discharged untreated. Most aircraft deicing fluid (ADF) is composed primarily of propylene glycol or ethylene glycol, which are aerobically biodegradable. Deicer runoff can exert BOD values higher than 250,000 mg/L, due to glycols. Therefore, deicing fluid treatment methods are required. The research reported involves anaerobic codigestion of municipal sludge and ADF. The authors' previous report demonstrates that existing municipal digesters may be employed to treat the seasonally generated deicer runoff with municipal sludge which lends nutrients, alkalinity, and biomass that are lacking in the runoff. In this investigation, two-phased digestion with an initial fermentation reactor followed by a methane-producing reactor was used in an attempt to increase sustainable organic loading rates. Under a majority of the conditions studied, higher deicer organic loadings were sustained by two-phased digesters in comparison to conventional, single-staged digesters operated in parallel. Propionaldehyde was identified as a significant fermentation intermediate when municipal sludge and propylene glycol deicer were codigested. However, propionate was the major product when deicer was fermented without sludge. The toxicity of propionaldehyde was determined using anaerobic toxicity assays, and the IC50 of propionaldehyde to unacclimated aceticlastic methanogens was 430 mg/L. Toxicity issues regarding propionaldehyde in natural and engineered environments are discussed. The US EPA is currently formulating airport stormwater permit limits to protect receiving stream quality, and it is probable that only glycol will be addressed. However, the toxicity of anticorrosion and surfactant additives is significant. In addition, fermentation products and intermediates may be toxic. In the future, the potential toxicity of deicer additives and degradation products should be considered when airport stormwater discharge standards are developed or modified.