Different pathways have been proposed to explain the anaerobic biodegradation of formaldehyde according to the intermediate products observed.5-7
Gonzalez-Gil and colleagues5 carried out anaerobic activity tests using formaldehyde as the only carbon source and found that part of this compound was readily transformed into methanol. These authors could recover all substrate COD as methane when assayed for initial formaldehyde doses of 200 and 600 mg/L COD, but no methane production was observed for an initial dose of 1400 mg/L COD due to the toxic effect of the formaldehyde on the reaction. Nevertheless, the conversion of formaldehyde into methanol was not inhibited. During formaldehyde conversion, a peak of hydrogen was observed, this peak being related to the initial amount of formaldehyde dosed. It is likely that formaldehyde was first oxidized to formate and then reduced to methanol. Considering that all formaldehyde is converted into methanol and formate, the following reactions are proposed:
Omil and colleagues6 also carried out anaerobic activity tests to study the biodegradation of formaldehyde in the presence and absence of cosubstrate. In the absence of cosubstrate, these authors suggest that the hydrogen generated during formaldehyde removal was consumed both for direct methane conversion and for methanol generation. In Table 19.2, two possible formaldehyde degradation reactions are shown (reactions i and ii). Both are dependent on hydrogen concentration. Although degradation via methanol is thermodynamically favored in standard conditions, this pathway would imply, for the complete mineralization of formaldehyde, the synergistic action of methylotroph methanogenic bacteria (reaction iii) which suggests a situation of certain competition between reaction iii and the consecutive reactions iv and vi. In this way, when autotrophic methano-genic bacteria become inhibited and hydrogen concentration begins to accumulate in the medium, methanol generation becomes more favorable.
In the presence of a cosubstrate, formaldehyde removal was highly enhanced especially by acetate (vii) but not by propionate (viii) and butyrate (ix). The effect that acetate degradation exerts on formaldehyde removal may be related to the inorganic carbon generated, which allows autotrophic bacteria to convert hydrogen into methane (reaction vi) under more favorable conditions. When low concentrations of volatile fatty acids (VFA) and methanol are present, the direct conversion into methane by methylotrophic methanogenic bacteria should be the most favorable pathway for methanol degradation (in competition with the acetogenic bacteria (v)), and autotro-phic methane generation would not be favorable; therefore, both direct conversion into methane and the acetogenic pathway should be considered for methanol consumption. Neither are hydrogen dependent.
Oliveira and colleagues7 found intermediate compounds with 2 to 5 carbons during the degradation of formaldehyde (with 12% methanol) as the sole carbon source and attribute this to a chemical reaction as formaldehyde can form polymers in aqueous solution. The reactions are rapid in the absence of methanol, which is added to formaldehyde solutions to prevent such polymerization. In aqueous solution, when methanol is consumed, formaldehyde is almost completely hydrated to
Estimated Free Energy Changes of Selected Biological Reactions Involved in the Anaerobic Degradation of Formaldehyde and Methanol
(iii) 4 CH3OH ^ 3 CH4 + HCO3-Hydrogen generation
(v) 4 CH3OH + 2 HCO-^ 3 CH3COO- + H+ + 4 H2O Hydrogen and VFA
(viii) CH3CH2COO- + 3 H2O ^ CH3COO- + HCO,3 + H2 + H+
VFA, volatile fatty acids.
methylene glycol, which may polymerize to form a series of polyoxymethylene glycols. These authors suggested that the intermediate compounds come from the anaerobic degradation of the formed polymers. Another possibility is an aldo condensation, which occurs in the presence of weak bases, forming glycolic aldehyde and carbohydrates.
The toxicity of formaldehyde during anaerobic treatment has been reported by several authors.58 Its toxicity depends of several different parameters:
1. Nature of the cosubstrates
2. Operational mode (batch or continuous)
3. Type of reactor (suspended or attached growth systems)
4. Formaldehyde/microorganisms ratio
5. COD/formaldehyde ratio
The importance of the nature of the cosubstrate was shown by Todini and Hulshoff Pol,9 who determined the specific activity of formaldehyde-degrading microorganisms with different cosub-
strates such as hydrogen, sodium butyrate, and sucrose. They obtained the highest degradation rates with sucrose. Vidal and colleagues1 treated wastewater containing formaldehyde in an upflow anaerobic sludge blanket (UASB) reactor using glucose as the cosubstrate, as this compound enhances the reduction of aldehyde to methanol, which is less toxic for the bacteria. Values of 80 mg/L of formaldehyde were reported when acetate was used,10 whereas Todini and Hulshoff Pol9 reported a 50% inhibition at 238 mg/L of formaldehyde when sucrose was fed as the main cosubstrate.
Bhattacharya and Parkin11 studied the influence of the operational mode on formaldehyde degradation. They showed that higher formaldehyde concentrations were tolerated when they were added continuously to acetate and propionate enriched systems rather than when slug doses were used, thus indicating that continuous operation is more favorable for bacterial acclimation.
Sharma and colleagues12 evaluated the anaerobic biodegradation of petrochemical wastewater containing 4.5 g/L of formaldehyde, and found a higher resistance to microorganisms when biomass concentration was increased by immobilization using biomass-supporting particles. Formaldehyde was observed to exert toxicity at 375 mg/L in the reactor working with these supporting particles, whereas only 125 mg/L was tolerated in the control reactor with suspended biomass.
The formaldehyde/biomass ratio is also reported to be a key factor, as demonstrated by de Bekker and colleagues,13 who determined that 0.89 g formaldehyde/g VSS in batch operation exerted a complete inhibition of anaerobic bacteria. They also reported that anaerobic treatment of wastewa-ters containing formaldehyde is possible only when the COD/formaldehyde ratio is higher than 1000.13 However, other authors such as Parkin and colleagues14 achieved a stable operation with lower COD/formaldehyde ratios (about 6), using acetate as the main substrate and a continuous addition of 400 mg/L formaldehyde to an anaerobic filter. They found that formaldehyde exhibits reversible inhibition, recovery being accelerated by removing the toxicant from the liquid phase. These results agree with those of Gonzalez-Gil and colleagues,15 who found that the inhibition exerted by formaldehyde on the methane production rate is partially reversible when formaldehyde concentrations in the reactor are lower than 22 mg/L C-formaldehyde.
There is no information about the mechanisms of formaldehyde toxicity, and the information available in the literature about formaldehyde toxicity in batch and continuous systems is difficult to extrapolate for design purposes (Tables 19.3 and 19.4).
The hydrolysis of urea releases ammonia to the liquid bulk, which can cause inhibition of the methanogenic sludge.21 Nevertheless, these authors found a toxic effect at 1300 mg/L N-NH3, this value being higher than the nitrogen concentration of these wastewaters and no inhibitory effects could be expected.
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