Aerobic Degradation of Formaldehyde

Biodegradation pathway

The aerobic degradation of formaldehyde in wastewater has been studied by different authors in both continuous22 and batch experiments.23-25 The degradation can occur by two possible paths (see Equations 19.10 and 19.11):

1. Initiated by a dismutation reaction, yielding formic acid and methanol as products, if the microorganism has a formaldehyde dismutase enzyme

2. Via formic acid if the microorganism has the enzymes formaldehyde and formate dehydrogenase24

The biodegradation of the metabolites starts after exhaustion of formaldehyde in the medium.

Formaldehyde

Formaldehyde Formate

HCHO dehydrogenase > HCOOH dehydrogenase > CO2 (19.11)

Toxic effects

Zagornaya and colleagues22 reported the complete biodegradation of 2300 mg/L of formaldehyde in wastewater treated in an activated sludge plant, whereas Gerike and Gode26 observed that 30 mg/L

TABLE 19.3

Formaldehyde Studies in Batch Systems at 35°C

Biomass

Anaerobic digested sludge

Domestic wastewater

Sludge treating water from seafood processes Granular sludge from a UASB Activated sludge from a plant treating wood-processing-industry wastewater

Main Substrate

Sucrose Glucose

Tested HCHO (mg/L)

10-100 1-10,000 50-200

2-400

200 125

254 300

Reference

IC50, 50% inhibition concentration; VFA, volatile fatty acids.

formaldehyde inhibited oxygen consumption in activated sludge. Eiroa and colleagues25 studied the inhibitory effect of formaldehyde in batch tests; they found no inhibition and also that high concentrations of formaldehyde up to 3890mg/L could be removed using it as the single carbon source. When the same formaldehyde concentrations in the presence of methanol as cosubstrate were tested, higher formaldehyde biodegradation rates were obtained. This possibility of formaldehyde biodegradation despite the presence of an alternative readily metabolizable carbon source is a characteristic of significant practical interest when formaldehyde needs to be removed in environments containing other carbon sources, as in the case of wastewaters from synthetic resin-producing factories. Glancer-Soljan and colleagues24 also found no inhibitory effects of formaldehyde biodegradation in batch assays with an initial concentration of 1000mg/L using a mixed culture containing two bacterial strains.

19.2.2.3 Urea Hydrolysis

A wide variety of aerobic and anaerobic microorganisms are able to express the enzyme urease (urea amidohydrolase), which catalyses the hydrolysis of urea to ammonia and carbon dioxide.27 So far,

TABLE 19.4

Some Results from Literature Obtained in Continuous Systems Treating Formaldehyde-Containing Wastewater

Tested HCHO (mg/L)

Reactor

Anaerobic filter CST

CST immobilized biomass

Chemostat

EGSB

EGSB

UASB

UASB

HAIB

100-400

100-1110

200/400/600 50-2000 95-950 26-1158

Limiting Dose (mg HCHO/L)

400 125 375 1110

100 380 Not observed

Formaldehyde Removal Efficiency (%)

85-88

88-95

99.9

High

Reference

CST, continuous stirred tank; EGSB, expanded granular sludge blanket; HAIB, horizontal-flow anaerobic immobilized biomass.

most authors have preferred anaerobic conditions for the biological treatment of high-strength urea wastewaters with urea concentrations of up to 2g/L.1,28 Also, wastewaters containing high loads of urea together with ammonia and formaldehyde have been treated under anoxic conditions,3 and an aerobic urea hydrolysis has been described by Gupta and Sharma29 and Hamoda.30 Rittstieg and colleagues,31 treating an industrial wastewater containing high concentrations of urea and sulfate, proposed the use of the aerobic process to avoid the production of sulfide if an anaerobic stage were used.

There are no clear results as to which microorganism causes hydrolysis in aerobic conditions. Prosser32 reported that Nitrosomonas or Nitrospira were not ureolytic, which agrees with the conclusion of Campos and colleagues,33 who observed no degradation of urea when this compound was fed to a nitrification reactor. However, Koops and Chritian34 pointed out that the five genera of ammonia-oxidizing bacteria might use urea as an ammonia source. Gupta and Sharma29 and Hamoda30 observed hydrolysis of urea and high nitrification percentages when they treated effluents from fertilizer industries aerobically. Recently, Sliekers and colleagues35 have observed that anaerobic ammonium oxidation (anammox) bacteria did not hydrolyze urea by themselves.

Toxic effects

Different effects of formaldehyde on the hydrolysis of urea are reported. On the one hand, Garrido and colleagues,3 applying anoxic conditions, observed that an inhibitory effect started at 50 mg/L formaldehyde and the levels of inhibition were 50% and 90% for concentrations of formaldehyde of 100 mg/L and 300 mg/L, respectively. Similar effects were found by Campos and colleagues,33 working with an anoxic USB, who observed that formaldehyde concentrations in the reactor of 250 to 300mg/L caused an inhibition of around 53%. This inhibition on the ureolytic activity was also reported by Walker.36 On the other hand, Eiroa and colleagues37 carried out batch assays at different initial urea concentrations from 90 to 370 mg/L N-urea in the presence of 430 mg/L formaldehyde. They observed that a complete hydrolysis was achieved and initial urea hydrolysis rates remained constant.

Eiroa and colleagues37 operated a denitrifying granular sludge blanket with inlet urea concentrations between 100 and 800 mg/L N-urea, and always maintained the efficiency of the hydrolysis in spite of the presence of concentrations of ammonia up to 730 mg/L N (110 mg/L N-NH3). The ammonia levels in the effluent corresponded to ca. 77.5% of the amount of urea fed, the unaccounted portion being attributed to microbial assimilation. However, Garrido and colleagues,3 when increasing the urea loading rate in a multifed upflow filter (MUF) by increasing the inlet concentration, observed that fully hydrolytic efficiency was maintained for a short period of time but later decreased to 55%. These authors attribute the loss of ureolytic activity of the sludge to the higher ammonia concentrations.

19.2.2.4 Nitrification

Nitrification is a two-step process where ammonia is first oxidized to nitrite by ammonia-oxidizing bacteria (Nitrosomonas, Nitrosococcus, Nitrosospira, and so on) and the produced nitrite is finally oxidized to nitrate by nitrite-oxidizing bacteria (Nitrobacter, Nitrospina, Nitrospira, etc.) (Equation 19.12 and Equation 19.13). Both ammonia- and nitrite-oxidizing bacteria are autotrophic microorganisms, which supposes a low growth rate, nitrification being the limiting process during nitrogen removal.

Generally, ammonia oxidation is slower than nitrite oxidation and, therefore, no nitrite production is observed. However, when the amount of carbon source available in the effluent is not high enough to complete the denitrification process (low COD/N ratio), the addition of external organic matter is necessary, which produces an increase in treatment costs. In this case the partial nitrification of ammonia to nitrite reduces not only the oxygen requirements for the oxidation, but also the amount of added organic matter required for denitrification.3

Toxic effects

Osislo and Lewandowski38 studied the effects of several organic compounds on nitrification (acetone, methanol, formaldehyde, and glucose) and found that formaldehyde was the most inhibitory. This inhibition was not due to heterotrophic growth, but to a toxic effect. Campos and colleagues33 shocked a nitrifying system with different concentrations of formaldehyde (100, 200, and 300 mg/L formaldehyde) over 3h. These shocks caused ammonia to appear in the effluent for a short time, but nitrite was never detected. These authors observed a linear tendency between formaldehyde concentration in the reactor and the decrease in the nitrification rate. They also found that most of this compound was consumed in the reactor. Eiroa and colleagues25 studied the effect of formaldehyde on nitrification in batch assays. These authors found that initial concentrations of formaldehyde above 350 mg/L start to decrease the nitrification rate, with complete inhibition at an initial concentration of 1500 mg/L. An increase in the lag phase before nitrification started was also observed. When the authors repeated the experiments in presence of methanol, they found that the inhibitory effect was greater at lower formaldehyde concentrations. In the presence of methanol, at initial formaldehyde concentrations of 175 mg/L, nitrification started to decrease and was completely inhibited at 500 mg/L. The authors explained the differences by the fact that the COD/total Kjeldahl nitrogen (TKN) ratio was higher in the assays with formaldehyde and methanol as carbon sources than in assays without methanol. Therefore, the competition between heterotrophic bacteria and nitrifiers for oxygen and ammonium was higher. However, Eiroa and colleagues39 observed that the simultaneous removal of formaldehyde and ammonium may be carried out in an activated sludge unit, maintaining a nitrification efficiency of 99.9%.

Anthonisen and colleagues40 found that free ammonia (NH3) is an inhibitory compound for both steps of nitrification, nitrite oxidation being more sensitive. Concentrations of this compound depend on dissolved NH+ and pH; therefore, for a certain concentration of NH+, pH can be a suitable parameter to control inhibition by the substrate. Gupta and colleagues,41 treating wastewaters containing both ammonia and urea, found nitrite in the effluent due to the inhibition of nitrite oxidation. Eiroa and colleagues,25 during batch assays with wastewaters containing ammonia and formaldehyde, observed the transitory accumulation of nitrite, probably as a result of the high initial free ammonium (3.9 mg/L N-NH3).

19.2.2.5 Denitrification

The denitrification process is carried out by heterotrophic bacteria such as Pseudomonas, Acinetobacter, Paracoccus, Alcaligenes, and Thiobacillus. The route of nitrogen reduction is showed in Equation 19.14. Generally, dinitrogen gas is the final product, but nitrous oxide may be the final product of denitrification if the denitrifying microorganisms lack N2O reductase,42 at low pH values,42 or in the presence of toxic compounds.43 The presence of low dissolved oxygen concentrations during denitrification also causes the accumulation of N2O44:

NO- —— NO- —— NO —— N2O —— N2 (19.14)

Garrido and colleagues,43 treating wastewaters containing formaldehyde and urea, observed a relation between the formaldehyde concentration in the reactor and the percentage of nitrous oxide produced in the gas phase, which indicates that, probably, the reduction of nitrous oxide to nitrogen is inhibited by the presence of formaldehyde. Therefore, nitrous oxide measurement might serve to check for the presence of formaldehyde or other toxic or inhibitory compounds in denitrifying reactors and consequently to advise the plant supervisor about a possible failure in the system.

As trace gases concentration in biological processes changes rapidly with operating conditions, nitrous oxide could serve to monitor denitrifying systems as well as it was proposed for hydrogen or carbon monoxide for monitoring methanogenic systems.

Because wastewater may contains a low COD/N ratio, the oxidation of ammonia to nitrite during nitrification contributes to decrease the amount of organic matter needed during denitrification:

The theoretical formaldehyde requirements for denitrifying nitrite or nitrate, if biomass production is not considered, are 0.64 and 1.07 kg C/kg N-NOJ, respectively. Garrido and colleagues3 found C/N ratios of 0.8 and 1.3 kg C/kg N-NOJ for denitrification of nitrite and nitrate, respectively, these values being 20% higher than the theoretical ones.

Toxic effects

A negative effect of formaldehyde on the denitrification process has been observed by several authors.333 Campos and colleagues33 found a decrease of 85% in nitrate consumption when formaldehyde accumulated in a denitrifying USB reactor (up to 300mg/L formaldehyde) with an increase of the formaldehyde loading rate. The efficiency of denitrification was totally restored after the formaldehyde accumulation was eliminated by decreasing the loading rate, showing a reversible inhibitory effect. However, Garrido and colleagues3 found only a slight decrease in the denitrifica-tion efficiency, from 90 to 80% at concentrations of 700mg/L of formaldehyde, during the operation of a MUF. Nevertheless, these authors detected nitrous oxide in the off-gas at concentrations higher than 100 mg/L of formaldehyde, this probably being related to a partial inhibition by this compound in the last step of denitrification.

Eiroa and colleagues37 carried out batch denitrifying assays with an initial concentration of 430 mg/L of formaldehyde. They found that formaldehyde was completely biodegraded in less than 30 h, but the denitrification process lasted several days. Therefore, formaldehyde was transformed into other organic compounds (methanol and formic acid), which were then used as carbon sources for denitrification. These authors operated a denitrifying granular sludge blanket reactor at different COD/N-NO3j ratios and at formaldehyde inlet concentrations up to 5000 mg/L, and obtained a mean denitrification efficiency of 98.4%. This high efficiency can be related to the low formaldehyde concentration in the reactor (below 10.3 mg/L), even when the formaldehyde inlet concentrations were increased. Meanwhile, Zoh and Stenstrom45 carried out batch tests to determine the denitrifying kinetics of nitrite using different carbon sources. These authors found that acetate and formaldehyde showed similar rates.

Denitrification can be affected by free ammonia, but this inhibition does not appear up to 300 to 400 mg/L NH3.46 This high concentration can justify that no inhibition of the denitrification process has been reported for this kind of wastewater.3,4 Eiroa and colleagues37 observed that nitrate was eliminated much faster at higher initial urea concentrations. However, they also found an increase of nitrite accumulation, which was later removed, due to high urea concentrations.

19.3 TECHNOLOGIES FOR WASTEWATER TREATMENT

Different kinds of bioreactors and configurations have been used to treat wastewater containing formaldehyde and urea, and three different kinds of treatments can be applied: anaerobic treatment, aerobic treatment, and combined nitrification and denitrification treatments.

19.3.1 Anaerobic Treatment

Anaerobic treatment is recommended for highly concentrated COD wastewater, as the amount of methane generated can compensate for the energy cost in maintaining the temperature of the reactor.

Moreover, this process produces less sludge compared to aerobic treatment. During the anaerobic process, formaldehyde is converted to CO2 and CH4 and urea is hydrolyzed to ammonia; therefore, this process only removes organic matter and a small amount of nitrogen due to ammonia assimilation by anaerobic microorganisms. Most of the time, in order to fulfill disposal targets, a posttreatment to remove nitrogen and the remaining organic matter is necessary.

Different kinds of reactors have been used at the laboratory scale to anaerobically treat waste-water containing formaldehyde. Qu and Bhattacharya,19 using a chemostate, treated a synthetic influent with formaldehyde concentrations up to 1100 mg/L. These authors obtained efficiencies for formaldehyde removal of 99% at volumetric loading rates up to 0.38 kg/m3 ■ d CH2O. Vidal and colleagues1 and Garrido and colleagues3 used a UASB reactor and a MUF to treat synthetic influents with formaldehyde and urea. Vidal and colleagues,1 using glucose as cosubstrate, managed to treat up to 3kg/m3d of formaldehyde, while Garrido and colleagues3 removed 0.5kg/m3d of formaldehyde. The discrepancies between the values might be due to the presence of the cosubstrate, which favors the reduction of the aldehyde to methanol, which is less toxic to the biomass. Nevertheless, the volumetric hydrolytic rates of urea achieved in both systems were similar (0.46 kg /m3d N-urea3 and 0.58kg/m3d N-urea1), being lower than the value of 1.5kg/m3d obtained by Latkar and Chakrabarti47 in a UASB.

At an industrial scale, Zoutberg and de Been20 treated wastewaters from a chemical factory containing up to 10 g/L of formaldehyde and 40 g/L of COD. These authors used a Biobed® EGSB (expanded granular sludge blanket) of 275 m3 with a hydraulic retention time (HRT) of 1.25 d, achieving efficiencies up to 98% (Figure 19.6). To avoid the inhibitory effect of high concentrations of formaldehyde, they operated at a recycle ratio of 30, that is, a superficial upflow liquid velocity of 9.4 m/h, which is rather higher than the 1 m/h used in conventional UASBs. The effluent of the Biobed EGSB was posttreated in a low loaded carrousel to meet the strict demands (overall COD efficiency higher than 99.8%).

19.3.2 Aerobic Treatment

During aerobic treatment formaldehyde is oxidized to CO2 and urea is hydrolyzed, the generated ammonia being oxidized to nitrate if the operational conditions are suitable for nitrification. During this treatment organic matter can be removed, but only a small amount of nitrogen is removed by assimilation; therefore, this treatment is not good enough to fulfill disposal requirements with regard to nitrogen compounds.

To carrousel

Wastewater

Buffer tank

Conditioning tank

Biobed reactor

FIGURE 19.6 Schematic of a plant to treat wastewater containing formaldehyde.

Wastewater

Buffer tank

Conditioning tank

Biobed reactor

FIGURE 19.6 Schematic of a plant to treat wastewater containing formaldehyde.

Canals48 managed to treat wastewater from a petrochemical factory at concentrations of up to 2000 mg/L of formaldehyde using an activated sludge reactor, and Zagornaya and colleagues22 obtained a good removal of this compound when treating resin wastewater in an aerobic reactor.

Garrido and colleagues2 treated wastewaters from a formaldehyde-urea factory using three activated sludge units operating with solids retention times of 10, 17 and 25 d. These authors applied an organic loading rate (OLR) between 0.2 and 1.2kg/m3d COD and obtained removal efficiencies of 80 to 95% and 99.4% for COD and formaldehyde, respectively. Their system achieved a nitrification rate (0.1 kg N-NO-,r/m3 ■ d), the percentage of TKN removal being 45 to 65% due to the biomass growth.

19.3.3 Treatment Combining Nitrification and Denitrification Units

In order to fulfill disposal requirements the best option to treat wastewaters containing formaldehyde and urea is the combination of nitrification and denitrification units in a predenitrifying configuration. In the denitrifying tank, nitrate recycled from the nitrifying unit is denitrified using formaldehyde as the electron donor and urea is hydrolyzed to ammonia. In the nitrification unit, ammonia and the remaining formaldehyde are oxidized to nitrate and CO2, respectively. The nitrogen removal percentage will depend on the recycling ratio between both units.

Garrido and colleagues3 operated a MUF under anaerobic and anoxic conditions and achieved, under anoxic conditions, the treatment of up to 2 kg/m3d of formaldehyde and a hydrolysis rate of up to 0.37 kg/m3 ■ d N-urea. These authors observed that formaldehyde biodegradation is more stable under anoxic conditions than under anaerobic conditions, but only 80% of urea was hydrolyzed in an anoxic environment while a complete conversion occurred under anaerobic conditions. Eiroa and colleagues49 obtained similar values operating a denitrifying granular sludge blanket reactor with synthetic wastewaters containing formaldehyde and urea. They applied up to 2.8 kg/m3d of formaldehyde and 0.44 kg/m3 ■ d N-urea, obtaining efficiencies of 99.5 and 77.5% for formaldehyde removal and urea hydrolysis, respectively. Campos and colleagues,33 using an anoxic USB, achieved a loading rate of hydrolyzed urea of 0.94 kg/ ■ m3 ■ d N-urea and a loading rate of 2.35 kg/m3 ■ d for formaldehyde.

In systems treating formaldehyde, the loading rates of removed nitrate ranged from 0.44kg/m3d to 0.94 kg/m3 ■ d N-NO^.3349 These values are in the range of denitrifying loading rates obtained for other kinds of wastewaters (1.1kg/m3 ■ d or 1.5kg/m3 ■ d N-NO3),5051 which means formaldehyde can be used efficiently as an electron donor for denitrification.

Garrido and colleagues2 used an activated sludge nitrification-denitrification system to treat wastewater from a formaldehyde-urea adhesive factory (Figure 19.7). The treated wastewater contained 590 to 1545 mg/L COD, 197 to 953 mg/L formaldehyde and 129 to 491 mg/L TKN and was also characterized by the presence of polymers with a molecular weight higher than 8000 g/mol, which are not biodegradable. The system was capable of achieving removal efficiencies of 99, 70 to 85, and 30 to 50% for formaldehyde, COD, and TKN, respectively. The COD removal percentage was

Anoxic reactor Aerobic reactor Settler Ozonation tank

FIGURE 19.7 Schematic representation of a nitrification-denitrification activated sludge plant.

Anoxic reactor Aerobic reactor Settler Ozonation tank

FIGURE 19.7 Schematic representation of a nitrification-denitrification activated sludge plant.

VT^i

Anoxic USB

I Influent |

Urea Formaldehyde

FIGURE 19.8 Plant for the integral treatment of wastewaters containing formaldehyde and urea.

NH+ ^ NO, i + r r not related to the operational conditions but to the percentage of COD from the formaldehyde. COD removal essentially took place in the anoxic stage, as was the case for formaldehyde, and only nitrification was carried out in an aerobic reactor.

To remove urea and formaldehyde from synthetic wastewater, Campos and colleagues33 operated a coupled system consisting of a biofilm airlift suspension (BAS) reactor to carry out nitrification and an anoxic USB reactor to carry out the denitrification and urea hydrolysis (Figure 19.8).

These authors studied the effect of the recycling ratio (calculated as the ratio r/i of the flows) for different fed C/N ratios (0.58, 1.0, and 1.5 g C-formaldehyde/g N-NH4+), always using a constant urea inlet concentration of 400 g/L N-urea. The nitrogen removal percentages achieved are shown in Table 19.5. The maximum nitrogen removal percentages were achieved at a C/N ratio of 1.0 g C-formaldehyde/g N-NH4+ for both recycling ratios. When this ratio is lower (0.58) not enough organic matter is present to remove nitrate in the anoxic stage, whereas a fed C/N ratio of 1.5 caused a decrease in the efficiency of the system with respect to nitrogen removal, due to the presence of formaldehyde in the BAS reactor, which decreased the nitrification.

When the system was operated at a high inlet C/N ratio, part of the formaldehyde was not removed in the anoxic reactor and entered the nitrification reactor. This led to a heterotrophic layer

TABLE 19.5

Percentages of Nitrogen Removal r/i C/N Nitrogen Removal (%)

Settling tank

Chlorination channel

Effluent

Chlorination channel

Effluent

Purge ^

FIGURE 19.9 Industrial plant for the integral treatment of wastewaters from an adhesive factory.

Purge ^

FIGURE 19.9 Industrial plant for the integral treatment of wastewaters from an adhesive factory.

being formed around the nitrifying biofilm, which consumed formaldehyde, and depleted the oxygen for the nitrifiers. The loss of nitrification capacity caused a snowball effect, as no nitrate was available for denitrification, which caused the presence of higher concentrations of formaldehyde in the anoxic system and, then, instability of the denitrification and urea hydrolysis processes. These negative effects of formaldehyde can be reduced by operating at higher recycling ratios, because the increase of the recycling ratio causes a dilution effect in the streams, the formaldehyde concentration in the reactors being lower.

Canto and colleagues52 operated an integrated anoxic-aerobic treatment of wastewaters from a synthetic resin producing factory (Figure 19.9). These authors managed to treat up to 2.01 kg/m3d COD and up to 0.93 kg/m3d TKN with removal efficiencies of 80 to 95% and 58 to 93% for COD and TKN, respectively.

As wastewater from resin-producing factories contains recalcitrant compounds, the removal efficiencies achieved by means of the nitrification-denitrification systems could not reach the required disposal values and a posttreatment, such as ozonation, would be necessary to enhance the biodegradability of those compounds.2,53

19.4 GUIDELINES FOR THE DESIGN OF A WASTEWATER TREATMENT PLANT FOR WASTEWATER CONTAINING FORMALDEHYDE AND UREA

19.4.1 Decision Tree Structure

The technology chosen to treat wastewater containing formaldehyde and urea will basically depend on the COD concentration and COD/N ratio. The following decision tree structure can be used in the choice of an approach for wastewater treatment (Figure 19.10).

Equalization tank

Wastewater \

Wastewater \

FIGURE 19.10 Decision tree structure.

19.4.2 Recommendations

Because formaldehyde is the most toxic compound present in this kind of wastewater, to control its concentration in reactors is important in order to maintain the stability of the wastewater treatment plant. For this reason the following are recommended:

1. To use an equalization tank to minimize the possible inlet of a peak of formaldehyde.

2. To use anaerobic digesters with high internal recycling ratios to maintain a low concentration of formaldehyde inside the system.

3. To maintain high recycling ratios between the nitrification and denitrification units. This recommendation is also useful to increase the efficiency of nitrogen removal.

When denitrification via nitrate is not possible (COD/N ratios lower than 3.5) there are two possible options to remove nitrogen:

1. To control the dissolved oxygen in the nitrification unit to obtain a partial oxidation of ammonia to nitrite

2. To add an external carbon source

As the adhesive factory will consume a large amount of methanol in its processes, the addition of this compound to carry out nitrogen removal would have a low cost, and is one of the most feasible options.

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