Jos Luis Campos Gmez Anuska Mosquera Corral Ramn Mndez Pampn and Yung Tse Hung

CONTENTS

19.1 Generation of Wastewater 759

19.1.1 Production Process 759

19.1.2 Characterization of the Effluent 762

19.2 Biological Treatment 762

19.2.1 Biological Processes and Strategies 762

19.2.2 Interactions between Biological Processes and Compounds 764

19.3 Technologies for Wastewater Treatment 771

19.3.1 Anaerobic Treatment 771

19.3.2 Aerobic Treatment 772

19.3.3 Treatment Combining Nitrification and Denitrification Units 773

19.4 Guidelines for the Design of a Wastewater Treatment Plant for Wastewater

Containing Formaldehyde and Urea 775

19.4.1 Decision Tree Structure 775

19.4.2 Recommendations 776

References 777

19.1 GENERATION OF WASTEWATER

19.1.1 Production Process

Approximately one million metric tons of urea-formaldehyde resin are produced annually all over the world. More than 70% of this urea-formaldehyde resin is consumed by the forest products industry. The resin is used in the production of an adhesive for bonding particleboard (61% of the urea-formaldehyde used in the industry), medium-density fiberboard (27%), hardwood plywood (5%), and as a laminating adhesive (7%) for bonding furniture case goods, overlays to panels, and interior flush doors, for example.

Urea-formaldehyde resins are the most prominent examples of the thermosetting resins usually referred to as amino resins, comprising ca. 80% of the amino resins produced worldwide. Melamine-formaldehyde resins constitute most of the remainder of this class of resins, with other minor amounts of resins being produced from the other aldehydes or amino compounds (especially aniline), or both.

Amino resins are often used to modify the properties of others materials. These resins are added during the processing of diverse products such as textiles (to impart permanent press

characteristics), automobile tires (to improve the bonding of the rubber to the tire cord), paper (to improve its strength, especially when wet), and alkyds and acrylics (to improve their cure). Amino resins are also used for molding products, such as electrical devices, jar caps, buttons, dinnerware, and in the production of countertops.

Urea-formaldehyde resins are used as the main adhesive in the forest product industry because they have a number of advantages, including low cost, ease of use under a wide variety of curing conditions, low cure temperatures, water solubility, resistance to microorganisms and to abrasion, hardness, excellent thermal properties, and a lack of color, especially in the cured resin.

The major disadvantage associated with urea-formaldehyde adhesives as compared with the other thermosetting wood adhesives, such as phenol-formaldehyde and polymeric diisocyanates, is their lack of resistance to moist conditions, especially in combination with heat. These conditions lead to a reversal of the bond-forming reactions and the release of formaldehyde, so these resins are usually used for the manufacture of products intended for interior use only. However, even when used for interior purposes, the slow release of formaldehyde (a suspected carcinogen) from products bonded with urea-formaldehyde adhesives is observed.

19.1.1.1 Chemistry of Urea-Formaldehyde Resin Formation

The synthesis of urea-formaldehyde resin takes place in two stages. In the first stage, urea is hydroxymethylolated by the addition of formaldehyde to the amino groups of urea (Figure 19.1). This reaction is in reality a series of reactions that lead to the formation of mono-, di-, and trimethy-lolureas. Tetramethylolurea does not appear to be produced, at least not in a detectable quantity. The addition of formaldehyde to urea takes place over the entire pH range, but the reaction rate is dependent on the pH.

The second stage of urea-formaldehyde synthesis consists of the condensation of methylolureas to low-molecular-weight polymers. The rate at which these condensation reactions occur is very dependent on pH (Figure 19.2) and, for all practical purposes, occurs only at acidic pHs. The increase in the molecular weight of the urea-formaldehyde resin under acidic conditions is thought to be a combination of reactions leading to the formation of the following:

1. Methylene bridges between amido nitrogens by the reaction of methylol and amino groups on reacting molecules (Figure 19.3a)

2. Methylene ether linkages by the reaction of two methylol groups (Figure 19.3b)

3. Methylene linkages from methylene ether linkages by the splitting out of formaldehyde (Figure 19.3c)

FIGURE 19.1 Formation of mono-, di-, and trimethylolurea by addition of formaldehyde to urea.

NHCH2OH

NHCH2OH HOH2CHN

NHCH2OH HOH2CHN

NHCH2OH

Addition reaction

\ Condensation reaction \

9 10 11 12

FIGURE 19.2 Influence of pH on the rate constant k of addition (solid line) and condensation (dashed line) reactions of urea and formaldehyde.

4. Methylene linkages by the reaction of methylol groups splitting out water and formaldehyde (Figure 19.3d)

The difference between the pH profiles of the two stages of urea-formaldehyde resin synthesis is taken advantage of in the production of these resins (Figure 19.2). In general, the commercial production of urea-formaldehyde adhesive resins is carried out in two major steps. The first step consists of the formation of methylolureas under basic conditions (pH 8 to 9), to allow the methylo-lation reactions to proceed in the absence of reactions involving the condensation of the methylolureas.

RHN NHCH2OH H2N NHR RHN NHCH2HN NHR

RHN NHCH2OH HOH2CHN NHR RHN NHCH2OCH2HN NHR

RHN NHCH, OCH2HN NHR RHN NHCH2HN NHR H H

RHN NHCH2OH HOH2CHN NHR RHN NHCH2HN NHR H H

FIGURE 19.3 Condensation reactions of methylolureas to form (a) methylene bridges between amido nitrogens, (b) methylene ether linkages, and (c) and (d) methylene linkages. Reactions of these types produce higher molecular weight oligomers and polymers.

In the second step, the reaction mixture is brought to acid conditions, at ca. pH 5, and the condensation reactions are carried out until the desired viscosity is reached. The reaction mixture is then cooled and neutralized.

An acidic-cure catalyst is added to the urea-formaldehyde resin before it is used as an adhesive. Ammonium chloride and ammonium sulfate are the most widely used catalysts for resins in the forest products industry. A variety of other chemicals can be used as a catalyst, including formic acid, boric acid, phosphoric acid, oxalic acid, and acid salts of hexamethylenetetramine.

Resin cure is normally conducted at a temperature of 120°C and pH < 5. The reactions that occur during the final cure of the resin are thought to be similar to those that occur during the acid condensation of the methylolureas. These reactions lead to the formation of the crosslinked polymeric network characteristic of the hardened, cured resin.

19.1.2 Characterization of the Effluent

The effluent generated during the production of the resins arises from different operations within the factory. The effluent of the production processes comes mainly from cleaning operations of reactors, storage tanks, filters from the towers of formaldehyde production, and the filters from the reactors. Another source for disposal comprises the spills occurring during the transfer of the resins from the reactors to the storage tanks and from these to the truck used to distribute them to other factories.

Because of the processes carried out in the plant, the expected compounds in wastewater are formaldehyde, urea, and polymers of these compounds. The global effluent of this kind of factory is characterized by a high chemical oxygen demand (COD) (due mainly to formaldehyde), relatively high values of nitrogen (arising from urea and copolymers) and a low content of phosphorus and inorganic carbon. The main characteristics of the effluent of a resin factory are showed in Table 19.1.

19.2 BIOLOGICAL TREATMENT 19.2.1 Biological Processes and Strategies

The wastewaters generated by the adhesive industries contain high concentrations of both carbon and nitrogen compounds. The process chosen to treat these wastewaters will depend on their COD/N ratio. When the COD/N ratio is high, an anaerobic treatment is the best option as it will save costs

TABLE 19.1

Characteristics of the Effluent from a Resin Factory

TABLE 19.1

Characteristics of the Effluent from a Resin Factory

Vidal et al.1

Garrido et al.2

Garrido et al.3

Eiroa et al.4

COD (g/L)

0.46-3.9

1.1-4.1

0.46-4.0

0.12-6.85

Formaldehyde (g/L)

0.22-4.0

0.20-2.8

0.22-4.0

0.007-2.7

TKN (g/L)

0.12-0.81

0.13-0.70

0.11-0.80

0.056-1.46

N-NH+ (g/L)

0.003-0.018

0.006-0.36

TSS (mg/L)

19-150

12-664

VSS (mg/L)

16-140

pH

7.1-11.2

6.5-9.6

TOC (g/L)

0.30-2.08

Alkalinity (mg CaCO3 /L)

167-2000

P-PO43- (mg/L)

0.1-31

COD, chemical oxygen demand; TKN, total Kjeldahl nitrogen; TSS, total suspended solids; VSS, volatile suspended solids; TOC, total organic carbon.

COD, chemical oxygen demand; TKN, total Kjeldahl nitrogen; TSS, total suspended solids; VSS, volatile suspended solids; TOC, total organic carbon.

(less energy, less sludge production). In this process, formaldehyde is degraded to methane and carbon dioxide and urea is hydrolyzed to ammonium:

Anaerobic degradation of formaldehyde

Urea hydrolysis

Generally, the sole use of an anaerobic stage is not enough to reduce COD sufficiently to reach the required concentration for disposal, and the concentration of nitrogen compounds remains practically constant. Therefore, to remove the nitrogen compounds and the remaining COD, a posttreatment based on the nitrification-denitrification process is necessary. This process can be used in a post-denitrifying or predenitrifying configuration (Figure 19.4).

19.2.1.1 Postdenitrifying Configuration

In this case, the wastewater is fed to the aerobic reactor where the remaining formaldehyde is oxidized to CO2 (Equation 19.3) and urea is hydrolyzed to ammonia. This ammonia is then oxidized to nitrate (Equation 19.4). Nitrate goes to the denitrifying unit where it is reduced to dinitrogen gas in the presence of an electron donor, which is generally provided by organic matter (Equation 19.5). Because formaldehyde is oxidized in the first unit, methanol is commonly added to carry out this process, which produces an increase in operational costs.

Aerobic degradation of formaldehyde

Postdenitrification

Wastewater Formaldehyde Urea

Methanol

Predenitrification

Wastewater Formaldehyde Urea

Formaldehyde

Formaldehyde

Wastewater Formaldehyde Urea

NH+

Urea hydrolysis

Nitrification

Denitrification Formaldehyde removal

Formaldehyde removal

i

NO-

FIGURE 19.4 Postdenitrification and predenitrification configurations for the treatment of wastewaters containing formaldehyde and urea.

Effluent

FIGURE 19.4 Postdenitrification and predenitrification configurations for the treatment of wastewaters containing formaldehyde and urea.

Nitrification

Denitrification

4 NO" + 5 CH2O ^ 2N2 + 5 CO2 + 3 H2O + 4 OH" (19.5)

19.2.1.2 Predenitrifying Configuration

Wastewater is supplied to the anoxic unit, where the nitrate recycled from the nitrifying unit is denitrified using the formaldehyde as the electron donor. When the COD/N ratio of the wastewater is high, the anaerobic degradation of formaldehyde and denitrification can occur in the same unit, this last process having preference for thermodynamic reasons.3 The hydrolysis of urea is also carried out in the anoxic reactor. The wastewater containing ammonia and a low concentration of formaldehyde is fed to the aerobic tank, where ammonia is nitrified to nitrate and the remaining formaldehyde is oxidized. The disadvantage of this configuration is the dependence of the percentage of nitrogen removal on the recycling ratio between the aerobic and anoxic units:

where ^ is the percentage of nitrogen removal and R is the recycling ratio between the aerobic and anoxic units.

If the COD/N ratio of the wastewater is low, a better option is the use of a nitrification-denitrification stage without a previous anaerobic digestion in order to preserve organic matter for denitrification.

19.2.2 Interactions between Biological Processes and Compounds

Maintaining the stability of a biological treatment of wastewaters containing formaldehyde and urea is complicated because some compounds exert a toxic effect on the processes involved. Figure 19.5 shows the possible toxic interactions between the different compounds and processes.

Compound Process

Compound Process

FIGURE 19.5 Compounds and intermediates of wastewater treatment, with arrows indicating the inhibitory effects of them on the different processes.
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