Wastewater stripping

Air emissions may arise from fugitive propane emissions and process vents. These include heater stack gas (carbon monoxide, sulfur oxides, nitrogen oxides, and particulate matter) as well as hydrocarbon emission, such as fugitive propane and fugitive solvents. Steam stripping wastewater (oil and solvents) and solvent recovery wastewater (oil and propane) are also produced. 

This chapter provides a state-of-the-art review of HFMST including a general review of hollow fiber membrane contactors, operating principles, design consideration, commercial availability of hollow fiber membrane, and module for scale-up and large-scale studies. Application of HFMST in pharmaceutical, biotechnological, gas absorption and stripping, wastewater treatment, and few latest studies of metal ion extraction are described in detail. 

The urea produced is normally either prilled or granulated. In some countries there is a market for Hquid urea—ammonium nitrate solutions (32% N). In this case, a partial-recycle stripping process is the best and cheapest system. The unconverted NH coming from the stripped urea solution and the reactor off-gas is neutralized with nitric acid. The ammonium nitrate solution formed and the urea solution from the stripper bottom are mixed, resulting in a 32—35 wt % solution. This system drastically reduces investment costs as evaporation, finishing (priQ or granulation), and wastewater treatment are not required. 

The problem in reducing the NH and urea content in the wastewaters to below 100 ppm is because it is difficult to remove one in the presence of the other. The wastewater can be treated with caustic soda to volatilize NH. However, in a more efficient method, the urea is hydrolyzed to ammonium carbamate, which is decomposed to NH and CO2 the gases are then stripped from the wastewater. 

Ammonium Ion Removal. A fixed-bed molecular-sieve ion-exchange process has been commercialized for the removal of ammonium ions from secondary wastewater treatment effluents. This application takes advantage of the superior selectivity of molecular-sieve ion exchangers for ammonium ions. The first plants employed clinoptilolite as a potentially low cost material because of its availability in natural deposits. The bed is regenerated with a lime-salt solution that can be reused after the ammonia is removed by pH adjustment and air stripping. The ammonia is subsequentiy removed from the air stream by acid scmbbing. 

Chevron s WWT (wastewater treatment) process treats refinery sour water for reuse, producing ammonia and hydrogen sulfide [7783-06-04] as by-products (100). Degassed sour water is fed to the first of two strippers. Here hydrogen sulfide is stripped overhead while water and ammonia flow out the column bottoms. The bottoms from the first stripper is fed to the second stripper which produces ammonia as the overhead product. The gaseous ammonia is next treated for hydrogen sulfide and water removal, compressed, and further purified. Ammonia recovery options include anhydrous Hquid ammonia, aqueous Hquid ammonia, and ammonia vapor for incineration. There are more than 20 reported units in operation, the aimual production of ammonia from this process is about 200,000 t. 

EPA has also developed pretreatment standards for industrial faciHties that discharge directiy to pubHcly owned treatment works (POTWs). The three types of pollutants of principal concern are pollutants that interfere with the operation of the POTW, pollutants that contaminate the sludges produced in the POTW, and pollutants that pass through the POTW or that are otherwise incompatible. One particular concern is volatile contaminants that can be stripped into the air during conventional wastewater treatment and become air pollution problems. These pretreatment standards are included in the effluent guidelines for the different industries. 

Process water streams from vinyl chloride manufacture are typically steam-stripped to remove volatile organics, neutralized, and then treated in an activated sludge system to remove any nonvolatile organics. If fluidized-bed oxychlorination is used, the process wastewater may also contain suspended catalyst fines and dissolved metals. The former can easily be removed by sedimentation, and the latter by precipitation. Depending on the specific catalyst formulation and outfall limitations, tertiary treatment may be needed to reduce dissolved metals to acceptable levels. 

Process and environmental air is compressed and passed through activated beds to reduce air emission levels to <5 ppm. Process wastewater is air stripped to remove CCl. The solvent containing air is also passed through the activated carbon beds. The total air flow through the beds averages about 3965 mVmin (140,000 SCFM). 

Liquid-liquid extraction is used primarily when distillation is imprac-tic or too costly to use. It may be more practical than distillation when the relative volatility for two components falls between 1.0 and 1.2. Likewise, liquid-liquid extraction may be more economical than distillation or steam-stripping a dissolved impurity from wastewater when the relative volatility or the solute to water is less than 4. In one case discussed by Robbins [Chem. Eng. Prog., 76 (10), 58 (1980)], liquid-liquid extraction was economically more attractive than carbon-bed or resin-bed adsorption as a pretreatment process for wastewater detoxification before biotreatment. 

Certain refinery wastewater streams are treated separately, prior to the wastewater treatment plant, to remove contaminants that would not easily be treated after mixing with other wastewater. One such waste stream is the sour water drained from distillation reflux drums. Sour water contains dissolved hydrogen sulfide and other organic sulfur compounds and ammonia which are stripped in a tower with gas or steam before being discharged to the wastewater treatment plant. 

Air stripping is used to remove 90% of the toluene (molecular weight = 92) dissolved in a 10 kg/s (159 gpm) wastewater stream. The inlet composition of toluene in the wastewater is 500 ppm. Air (essentially free of toluene) is compressed to 202.6 kPa (2 atm) and bubbled through a stripper which contains sieve trays. In order to avoid fire hazards, the concentration of toluene in the air leaving the stripper is taken as 50% of the lower flammability limit (LFL) of toluene in air. The toluene-laden air exiting the stripper is fed to a condenser which recovers almost all the toluene. A schematic representation of the process is shown in Fig. 2.11. Calculate the annual operating cost and the fixed capital investment for the system. The following physical and economic data are available ... 

Toluene is to be removed from a wastewater stream. The flowrate of the waste stream is 10 kg/s and its inlet composition of toluene is 5(X) ppmw. It is desired to reduced the toluene composition in water to 20 ppmw. Three external MSAs are considered air (S2) for stripping, activated carbon (S2) for adsorption, and a solvent extractant (S3). The data for the candidate MSAs are given in Table 3.6. The equilibrium data for the transfer of the pollutant from the waste stream to the yth MSA is given by... 

Fixed cost of stripping system associated with S4, = 270,000 (Rowrate of wastewater, kg/s/- ... 

The solution indicates that to reduce the CE content of the terminal wastewater stream to 7 ppmw, air stripping can be used to intercept the bottom product of the first scrubber (lu = 2) to 2.7 CE at a cost of 713,108/yr. Since air stripping... 

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