Formaldehyde wastewater

When the same experiment was repeated with cresols at the initial concentration of the individual cresols of 100 mg Jiii , then complete removal of all cresols was achieved after 5—6 h. Upon the treatment of the phenol-formaldehyde wastewater, the initial concentration of phenol was equal to 1550 mg dm After 14 h of permeation in the experimental system, the phenol concentration in the feed solution dropped below 0.01 mg dill , and thus 99.9% removal was achieved. 

By contrast, formaldehyde is unstable in SCW, with its decomposition rate being strongly temperature-dependent. At temperatures above the critical, formaldehyde completely decomposes to methanol, formic acid, carbon oxide, and carbon dioxide. The main decomposition product at low temperatures is methanol, which gives way to carbon monoxide at high temperatures. At 300 atm, 300—500 °C, and a residence time of 2 min, formaldehyde decomposes almost completely. The rapid decomposition of formaldehyde seems to be the main reason for its absence in the reaction products in a number of works. The Cannizzaro reaction, leading to the formation of methanol and CO, makes it possible to purify dilute formaldehyde wastewater to form easily separable methanol [233]. 

Many electroless coppers also have extended process Hves. Bailout, the process solution that is removed and periodically replaced by Hquid replenishment solution, must still be treated. Better waste treatment processes mean that removal of the copper from electroless copper complexes is easier. Methods have been developed to eliminate formaldehyde in wastewater, using hydrogen peroxide (qv) or other chemicals, or by electrochemical methods. Ion exchange (qv) and electro dialysis methods are available for bath life extension and waste minimi2ation of electroless nickel plating baths (see... 

Adequate ventilation is necessary for aH process lines to ensure worker safety. Electroless copper baths must have good ventilation to remove toxic formaldehyde vapors and caustic mist generated by the hydrogen evolution reactions and air sparging. Electroless nickels need adequate ventilation to remove nickel and ammonia vapors. Some states and municipalities requite the removal of ammonia from wastewaters. A discussion of printed circuit board environmental issues and some sludge reduction techniques is avaHable (25). 

Biological Treatment Processes for Urea and Formaldehyde Containing Wastewater... 

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. 

In this case, the wastewater is fed to the aerobic reactor where the remaining formaldehyde is oxidized to C02 (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. 

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

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. 

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) ... 

Zagomaya 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 30mg/L... 

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. 

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

In systems treating formaldehyde, the loading rates of removed nitrate ranged from 0.44 kg/m3 d to 0.94 kg/m3 d N-NO,. 33-49 These values are in the range of denitrifying loading rates obtained for other kinds of wastewaters (1.1 kg/m3 d or 1.5kg/m3 d N-NO,),50-51 which means formaldehyde can be used efficiently as an electron donor for denitrification. 

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). 

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

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). 

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 ... 

Garrido, J.M., Mendez, R., and Lema, J.M., Treatment of wastewaters from a formaldehyde-urea adhesives, Water Sci. Technol., 42, 293-300, 2000. 

Glancer-Soljan, M., Soljan, V., Landeka, T., and Cacic, L., Aerobic degradation of formaldehyde in wastewater, Food Technol. Biotechnol., 39, 197-202, 2001. 

---