Aldehyde amenability to biodegradation is indicated by high biochemical oxygen demand (BOD) levels reported by several investigators. At a low test concentration, formaldehyde, acetaldehyde, butyraldehyde, crotonalde-hyde, furfural, and benzaldehyde all exhibited substantial biooxidation (Heukelekian and Rand 1955; Lamb and Jenkins 1952). An olefinic linkage in the position usually renders the material inhibitory (Stack 1957). The levels inhibitory to unacclimated microorganisms for acrolein, methacrolein and crotonaldehyde were 1.5, 3.5, and 14 mg. per liter (mg/l), respectively, whereas levels for ac-etaldehyde, propionaldehyde and butyraldehyde were 500 mg/l or above. Formaldehyde was inhibitory at 85 mg/l.
Bacteria can develop adaptive enzymes to allow biological oxidation of many potentially inhibitory aldehydes to proceed at high influent levels. Stabilization by acclimated organisms of several organic compounds typical of petrochemical wastes has been investigated (Hatfield
1957). For organisms acclimated to 500 mg/l formaldehyde, approximately 3 hr aeration time was required to bring the effluent concentration to zero. However, effluent organic concentration after this interval was still high, indicating oxidation to formic acid or Cannizzaro dismutation to methanol and formic acid. Eight to ten hr of aeration were required for the effluent BOD to approach zero. Removals of acetaldehyde (measured as BOD) were from an initial concentration of 430 to 35 mg/l after a 5 hr aeration time. Propionaldehyde removals were from 410-25 mg/l after five hr. The oxidation pattern of paraformalde-hyde, the polymer of formaldehyde, resembled its precursor.
Data collected through Warburg respirometer studies using seed sludges from three waste treatment plants (Gerhold and Malaney 1966) showed that aldehydes were oxidized to an extent second only to corresponding primary alcohols. Only formaldehyde exhibited toxicity to all three sludges. Branching in the carbon chain increased resistance to biooxidation.
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