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Industrial effluent treatment unit operation design
CHAPTER
Industrial effluent treatment unit operation design: physical processes
17
CHAPTER OUTLINE Introduction ............................................................................................................231 Gravity Oil/Water Separators....................................................................................231 Coalescing Media ...................................................................................................233 Hydrophobic Media: Packed Beds ................................................................233 Hydrophilic Media: Nutshell Filters ..............................................................233 Removal of Toxic and Refractory Compounds ................................................234 Adsorption ..............................................................................................................234 Membrane Technologies: Oily Water ........................................................................235 Membrane Technologies: Removal of Dissolved Inorganics .......................................236 Further Reading ......................................................................................................236
INTRODUCTION Many of the processes used in industrial effluent treatment are the same as those used in municipal clean or dirty water treatment. Space constraints on industrial sites tend to mean that high-intensity processes like dissolved air flotation (DAF) are favored over low-intensity alternatives such as sedimentation, though design quality is often low in industrial effluent treatment. The main difference, other than the use of specific chemicals to remove or enhance treatability of specific chemicals present at high concentrations in the effluent, is the use of various kinds of separators for oils and other light nonaqueous phase liquids (LNAPLs). These separators are the main physical process used in industrial effluent treatment which is not used in municipal treatment.
GRAVITY OIL/WATER SEPARATORS Removal of LNAPLs from mixtures is a fairly common requirement in the treatment of industrial effluents. LNAPLs may be hydrocarbons, or they may be biologically derived fats, oils, and greases. The units that carry out this process are known as oilwater separators or interceptors (see Fig. 17.1). An Applied Guide to Water and Effluent Treatment Plant Design. DOI: https://doi.org/10.1016/B978-0-12-811309-7.00017-5 © 2018 Elsevier Inc. All rights reserved.
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CHAPTER 17 Industrial effluent treatment unit operation design
FIGURE 17.1 Oilwater separator (green tank (dark gray in print version) right of picture). Courtesy: Expertise Ltd.
The most commonly accepted standard for the design of Oil Water Separators is the API 421 Monograph “Design and Operation of Oil-Water Separators” (see Further Reading). The standard covers conventional rectangular channel units and parallel plate separators. It is the parallel plate separators that are most commonly associated with this standard. The standard gives an equation (Eq. (17.1)) which can be rearranged to show that required area for separation. Separation Area Calculation Derived From API 421 AH 5
Qm SoÞ 0:00386 ðSw 2 µ
(17.1)
where Qm 5 design flow in cubic feet per minute AH 5 horizontal separator area in square feet Sw 5 specific gravity of the wastewater’s aqueous phase (dimensionless) So 5 specific gravity of the wastewater’s oil phase (dimensionless) µ 5 wastewater’s absolute (dynamic) viscosity in poise The product of this equation represents the required area of tank surface, plus the projected plate area required to effect separation of 60-µm oil globules. The monograph also gives design details, and the results of much practical experience of operation of these separators. It is therefore recommended that
Coalescing Media
anyone considering designing one of these units obtains the monograph, and discusses their application with suppliers of the plate packs to be used in the design. If we require a higher performance than API 421 demands, we can use Stokes Law to determine the required surface area, based on the rise rate of the smallest droplet we wish to remove. We can then see how many plates are required using the “projected area” method. This method involves assigning an effective surface area for settlement of each plate in the stack of its projection onto the base of the tank. The plates should be separated by 1937 mm, and the plate inclination angle should be 3045 degrees from vertical.
COALESCING MEDIA We can enhance the removal of very fine oil droplets with media which capture droplets impinging on their surface. If the media have a hydrophobic surface (as is the case with many plastics) the oil sticks to it and, over time, large droplets are formed which float away. If we use more hydrophilic media, (such as crushed nutshell) we can wash accumulated oil off the media after a collection period.
HYDROPHOBIC MEDIA: PACKED BEDS Passing oily wastewaters with low suspended solids through a bed of random hydrophobic packing, such as 25 mm polypropylene pall rings or saddles, enhances the collection of fine droplets of oil by coalescence. The absence of any backwashing facility makes this unsuitable for water with significant solids content, which is why nutshell filters (see “Hydrophilic Media: Nutshell Filters” section) are more popular for produced water and other oil and gas industry wastewaters.
HYDROPHILIC MEDIA: NUTSHELL FILTERS Depth filters filled with graded crushed nutshells (usually walnut or pecan) have a long record of accomplishment as the final stage of oily water treatment. They can produce a very high-quality water, suitable for reinjection when used with produced water. There are now five generations of designs. Each has dealt in successively more sophisticated ways with the main problem of these filters, namely, the separation of the collected oil from water and nutshells. For the fifth-generation designs, maximum feed oil levels of 100 ppm and surface loadings of 1320 gpm/ft2 are recommended for 98% removal of NAPL and SS . 2 µm, though process engineers usually only specify such filters, with the detailed design being completed by their proprietary suppliers. I strongly recommend this
233
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CHAPTER 17 Industrial effluent treatment unit operation design
approach, having seen in my expert witness practice the results of unwise attempts to design these units from first principles.
REMOVAL OF TOXIC AND REFRACTORY COMPOUNDS There are two types of refractory organic compounds which escape conventional treatment. Firstly, there are the substances which are of genuine concern because they have been shown to be of risk to people or the environment. Then there are the enormous number of substances which have not been shown to have any harmful effects, and are often present at concentrations far below those shown to have any detectable effects in scientific tests. The first class of substances, and any others which we might have rational reasons to suspect might be classed among them, must be reliably removed from wastewater to a level below that which causes harmful effects. The second class of substances should, at least in my opinion, be of no concern to engineers, although they may well have political mileage and be potentially fruitful for academic research. There are many specific techniques used in industrial effluent treatment to remove specific compounds, but in general, the essence of an effective technique in effluent treatment is nonspecificity. The techniques used therefore tend to fall into two categories: adsorption and oxidation. Adsorption is usually achieved by means of granular activated carbon (GAC), although powdered activated carbon (PAC) is also used. In applications where something is known about the composition of the effluent to be treated, isotherms are available for specific chemicals (binary mixtures might also be analyzed using isotherms). In the more common case of unknown or more complex mixtures, empty bed contact times, hydraulic loadings, and depths of carbon are employed in designing nonspecific absorption columns. The units may be downflow, and therefore incorporate solids removal, or alternatively upflow.
ADSORPTION The most common adsorption process in industrial effluent treatment is GAC treatment, though other proprietary adsorbents are also used. Activated carbon is produced by charring coconut or other nutshells, coal, or wood in a controlled manner. This process results in a very open-textured carbon material, full of macroscopic and microscopic channels throughout its bulk. The action of the carbon in removing dissolved substances from solution, and attaching them to its surface happens mostly within the microscopic channels which make up the bulk of its surface area. The material is used either as a fine powder (PAC) or alternatively in a granular form (GAC). PAC tends to be added directly, and GAC held within a vessel,
Membrane Technologies: Oily Water
and contacted with a flow of wastewater passed through the vessel. The retention of GAC means that it may be recovered, and regenerated for further use, whereas PAC used is lost, usually in sludge production. Onsite regeneration is however rare. Instead, at a site level, “spent” GAC is normally replaced with virgin material. The adsorption process is one of nonselective adsorption of all substances that are adsorbed by GAC, until the available binding sites are exhausted, and contaminants break through the end of the bed. The “time to breakthrough” is the most important design parameter, and is theoretically determined by means of adsorption isotherm data available from GAC suppliers. An additional design consideration in industrial applications may be the lack of availability of isotherms reflecting the absorption characteristics of activated carbons for the specific contaminants present. As discussed previously, empty bed contact time and superficial velocity form the main design considerations. Adsorption can be combined with a gravity OWS, and coalescing media, as in the design in Fig. 17.2. In Fig. 17.2, the inclined plate pack is on the left, the coalescing media is in the solid red (gray in print version) container, and on the far right, bags of GAC remove the final traces of oil to allow direct discharge to environment.
MEMBRANE TECHNOLOGIES: OILY WATER Treatment of oily waters arising at petrochemical refineries has traditionally been performed using a mixture of physical, chemical, and biological processes. API oilwater separators and DAF are used to reduce oil levels, and then the
FIGURE 17.2 Cross section through a combined adsorption and gravity OWS. Courtesy: Expertise Ltd.
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CHAPTER 17 Industrial effluent treatment unit operation design
wastewater is aerated along with activated sludge, the treated effluent settled, and sand filters used to polish the final effluent. There can be advantages to replacing the sand filters with membranes. More radically, both the final settlement tank and the sand filters can be replaced by membranes. The water produced by this process is reliably clean enough to be fed to RO membranes for recycle to process as high-purity water, suitable as boiler feed. Careful pretreatment of oily waters going into membrane bioreactors (MBRs) is required. There are several case studies of inadequate treatment of the feed to MBRs, resulting in highly variable outlet chemical oxygen demand levels and in the production of an oily foam requiring expensive tankering away. One such site trialed the spinning membrane system which is currently being promoted by several lab-based investigators, but found its power cost made it uneconomic compared with sponge-ball cleaned tubular membranes as a pretreatment.
MEMBRANE TECHNOLOGIES: REMOVAL OF DISSOLVED INORGANICS There must be a good reason to remove dissolved organics. The required techniques tend to be expensive to install and/or run. Most are essentially techniques borrowed from drinking water production and usually require extensive pretreatment. Membrane processes have different names, but are characterized by the size of the holes in the membrane. Tight membranes (with small holes) such as those used for reverse osmosis have very high head losses (perhaps 15 bar), but they can remove molecules selectively. Ultrafiltration membranes have larger holes, and remove colloidal matter, and large organic molecules only. Head losses are of the order of perhaps 1 bar. Membranes may be combined with electrochemical processes, as in electrodialysis, with electrical flux separating ions across a semipermeable membrane. These processes however tend to be unreliable, especially in effluent treatment applications.
FURTHER READING American Petroleum Institute. (1990). API 421 (withdrawn) Management of water discharges: Design and operation of oil-water separators. Washington, DC: API. Peeters, J., & Theodoulou, S. (2005). Membrane Technology Treating Oily Wastewater for Reuse. Corrosion. Houston, TX: NACE International.
Industrial effluent treatment unit operation design: physical processes
17
CHAPTER OUTLINE Introduction ............................................................................................................231 Gravity Oil/Water Separators....................................................................................231 Coalescing Media ...................................................................................................233 Hydrophobic Media: Packed Beds ................................................................233 Hydrophilic Media: Nutshell Filters ..............................................................233 Removal of Toxic and Refractory Compounds ................................................234 Adsorption ..............................................................................................................234 Membrane Technologies: Oily Water ........................................................................235 Membrane Technologies: Removal of Dissolved Inorganics .......................................236 Further Reading ......................................................................................................236
INTRODUCTION Many of the processes used in industrial effluent treatment are the same as those used in municipal clean or dirty water treatment. Space constraints on industrial sites tend to mean that high-intensity processes like dissolved air flotation (DAF) are favored over low-intensity alternatives such as sedimentation, though design quality is often low in industrial effluent treatment. The main difference, other than the use of specific chemicals to remove or enhance treatability of specific chemicals present at high concentrations in the effluent, is the use of various kinds of separators for oils and other light nonaqueous phase liquids (LNAPLs). These separators are the main physical process used in industrial effluent treatment which is not used in municipal treatment.
GRAVITY OIL/WATER SEPARATORS Removal of LNAPLs from mixtures is a fairly common requirement in the treatment of industrial effluents. LNAPLs may be hydrocarbons, or they may be biologically derived fats, oils, and greases. The units that carry out this process are known as oilwater separators or interceptors (see Fig. 17.1). An Applied Guide to Water and Effluent Treatment Plant Design. DOI: https://doi.org/10.1016/B978-0-12-811309-7.00017-5 © 2018 Elsevier Inc. All rights reserved.
231
232
CHAPTER 17 Industrial effluent treatment unit operation design
FIGURE 17.1 Oilwater separator (green tank (dark gray in print version) right of picture). Courtesy: Expertise Ltd.
The most commonly accepted standard for the design of Oil Water Separators is the API 421 Monograph “Design and Operation of Oil-Water Separators” (see Further Reading). The standard covers conventional rectangular channel units and parallel plate separators. It is the parallel plate separators that are most commonly associated with this standard. The standard gives an equation (Eq. (17.1)) which can be rearranged to show that required area for separation. Separation Area Calculation Derived From API 421 AH 5
Qm SoÞ 0:00386 ðSw 2 µ
(17.1)
where Qm 5 design flow in cubic feet per minute AH 5 horizontal separator area in square feet Sw 5 specific gravity of the wastewater’s aqueous phase (dimensionless) So 5 specific gravity of the wastewater’s oil phase (dimensionless) µ 5 wastewater’s absolute (dynamic) viscosity in poise The product of this equation represents the required area of tank surface, plus the projected plate area required to effect separation of 60-µm oil globules. The monograph also gives design details, and the results of much practical experience of operation of these separators. It is therefore recommended that
Coalescing Media
anyone considering designing one of these units obtains the monograph, and discusses their application with suppliers of the plate packs to be used in the design. If we require a higher performance than API 421 demands, we can use Stokes Law to determine the required surface area, based on the rise rate of the smallest droplet we wish to remove. We can then see how many plates are required using the “projected area” method. This method involves assigning an effective surface area for settlement of each plate in the stack of its projection onto the base of the tank. The plates should be separated by 1937 mm, and the plate inclination angle should be 3045 degrees from vertical.
COALESCING MEDIA We can enhance the removal of very fine oil droplets with media which capture droplets impinging on their surface. If the media have a hydrophobic surface (as is the case with many plastics) the oil sticks to it and, over time, large droplets are formed which float away. If we use more hydrophilic media, (such as crushed nutshell) we can wash accumulated oil off the media after a collection period.
HYDROPHOBIC MEDIA: PACKED BEDS Passing oily wastewaters with low suspended solids through a bed of random hydrophobic packing, such as 25 mm polypropylene pall rings or saddles, enhances the collection of fine droplets of oil by coalescence. The absence of any backwashing facility makes this unsuitable for water with significant solids content, which is why nutshell filters (see “Hydrophilic Media: Nutshell Filters” section) are more popular for produced water and other oil and gas industry wastewaters.
HYDROPHILIC MEDIA: NUTSHELL FILTERS Depth filters filled with graded crushed nutshells (usually walnut or pecan) have a long record of accomplishment as the final stage of oily water treatment. They can produce a very high-quality water, suitable for reinjection when used with produced water. There are now five generations of designs. Each has dealt in successively more sophisticated ways with the main problem of these filters, namely, the separation of the collected oil from water and nutshells. For the fifth-generation designs, maximum feed oil levels of 100 ppm and surface loadings of 1320 gpm/ft2 are recommended for 98% removal of NAPL and SS . 2 µm, though process engineers usually only specify such filters, with the detailed design being completed by their proprietary suppliers. I strongly recommend this
233
234
CHAPTER 17 Industrial effluent treatment unit operation design
approach, having seen in my expert witness practice the results of unwise attempts to design these units from first principles.
REMOVAL OF TOXIC AND REFRACTORY COMPOUNDS There are two types of refractory organic compounds which escape conventional treatment. Firstly, there are the substances which are of genuine concern because they have been shown to be of risk to people or the environment. Then there are the enormous number of substances which have not been shown to have any harmful effects, and are often present at concentrations far below those shown to have any detectable effects in scientific tests. The first class of substances, and any others which we might have rational reasons to suspect might be classed among them, must be reliably removed from wastewater to a level below that which causes harmful effects. The second class of substances should, at least in my opinion, be of no concern to engineers, although they may well have political mileage and be potentially fruitful for academic research. There are many specific techniques used in industrial effluent treatment to remove specific compounds, but in general, the essence of an effective technique in effluent treatment is nonspecificity. The techniques used therefore tend to fall into two categories: adsorption and oxidation. Adsorption is usually achieved by means of granular activated carbon (GAC), although powdered activated carbon (PAC) is also used. In applications where something is known about the composition of the effluent to be treated, isotherms are available for specific chemicals (binary mixtures might also be analyzed using isotherms). In the more common case of unknown or more complex mixtures, empty bed contact times, hydraulic loadings, and depths of carbon are employed in designing nonspecific absorption columns. The units may be downflow, and therefore incorporate solids removal, or alternatively upflow.
ADSORPTION The most common adsorption process in industrial effluent treatment is GAC treatment, though other proprietary adsorbents are also used. Activated carbon is produced by charring coconut or other nutshells, coal, or wood in a controlled manner. This process results in a very open-textured carbon material, full of macroscopic and microscopic channels throughout its bulk. The action of the carbon in removing dissolved substances from solution, and attaching them to its surface happens mostly within the microscopic channels which make up the bulk of its surface area. The material is used either as a fine powder (PAC) or alternatively in a granular form (GAC). PAC tends to be added directly, and GAC held within a vessel,
Membrane Technologies: Oily Water
and contacted with a flow of wastewater passed through the vessel. The retention of GAC means that it may be recovered, and regenerated for further use, whereas PAC used is lost, usually in sludge production. Onsite regeneration is however rare. Instead, at a site level, “spent” GAC is normally replaced with virgin material. The adsorption process is one of nonselective adsorption of all substances that are adsorbed by GAC, until the available binding sites are exhausted, and contaminants break through the end of the bed. The “time to breakthrough” is the most important design parameter, and is theoretically determined by means of adsorption isotherm data available from GAC suppliers. An additional design consideration in industrial applications may be the lack of availability of isotherms reflecting the absorption characteristics of activated carbons for the specific contaminants present. As discussed previously, empty bed contact time and superficial velocity form the main design considerations. Adsorption can be combined with a gravity OWS, and coalescing media, as in the design in Fig. 17.2. In Fig. 17.2, the inclined plate pack is on the left, the coalescing media is in the solid red (gray in print version) container, and on the far right, bags of GAC remove the final traces of oil to allow direct discharge to environment.
MEMBRANE TECHNOLOGIES: OILY WATER Treatment of oily waters arising at petrochemical refineries has traditionally been performed using a mixture of physical, chemical, and biological processes. API oilwater separators and DAF are used to reduce oil levels, and then the
FIGURE 17.2 Cross section through a combined adsorption and gravity OWS. Courtesy: Expertise Ltd.
235
236
CHAPTER 17 Industrial effluent treatment unit operation design
wastewater is aerated along with activated sludge, the treated effluent settled, and sand filters used to polish the final effluent. There can be advantages to replacing the sand filters with membranes. More radically, both the final settlement tank and the sand filters can be replaced by membranes. The water produced by this process is reliably clean enough to be fed to RO membranes for recycle to process as high-purity water, suitable as boiler feed. Careful pretreatment of oily waters going into membrane bioreactors (MBRs) is required. There are several case studies of inadequate treatment of the feed to MBRs, resulting in highly variable outlet chemical oxygen demand levels and in the production of an oily foam requiring expensive tankering away. One such site trialed the spinning membrane system which is currently being promoted by several lab-based investigators, but found its power cost made it uneconomic compared with sponge-ball cleaned tubular membranes as a pretreatment.
MEMBRANE TECHNOLOGIES: REMOVAL OF DISSOLVED INORGANICS There must be a good reason to remove dissolved organics. The required techniques tend to be expensive to install and/or run. Most are essentially techniques borrowed from drinking water production and usually require extensive pretreatment. Membrane processes have different names, but are characterized by the size of the holes in the membrane. Tight membranes (with small holes) such as those used for reverse osmosis have very high head losses (perhaps 15 bar), but they can remove molecules selectively. Ultrafiltration membranes have larger holes, and remove colloidal matter, and large organic molecules only. Head losses are of the order of perhaps 1 bar. Membranes may be combined with electrochemical processes, as in electrodialysis, with electrical flux separating ions across a semipermeable membrane. These processes however tend to be unreliable, especially in effluent treatment applications.
FURTHER READING American Petroleum Institute. (1990). API 421 (withdrawn) Management of water discharges: Design and operation of oil-water separators. Washington, DC: API. Peeters, J., & Theodoulou, S. (2005). Membrane Technology Treating Oily Wastewater for Reuse. Corrosion. Houston, TX: NACE International.