- Email: [email protected]
Clean water unit operation design
CHAPTER
Clean water unit operation design: biological processes
9
CHAPTER OUTLINE Introduction ............................................................................................................111 Discouraging Life ....................................................................................................111 Disinfection ...............................................................................................111 Sterilization ...............................................................................................115 Encouraging Life .....................................................................................................115 Biological Filtration ................................................................................................115 Further Reading ......................................................................................................116
INTRODUCTION Nowadays fewer biological processes are used in modern clean water treatment (though ironically in the days of slow sand filtration, there was more biology than chemistry in clean water treatment). This is mainly because clean water usually has fewer of the nutrients that are needed to sustain life. The most important aspect of life, as far as clean water treatment is usually concerned, is how to eradicate it. There are three levels of cleanliness: cleaning, disinfection, and sterilization. Cleaning water (and equipment) involves removing gross contamination. Disinfection involves removing pathogenic organisms (those which cause disease). Sterilization involves removing or inactivating every single viable or potentially viable organism. Cleaning is done by the standard water treatment processes, and by maintenance activity, whilst disinfection and sterilization are distinct unit operations. Drinking water is merely disinfected, but water and equipment used in pharmaceutical manufacture must be sterile. Let us start with the more common process.
DISCOURAGING LIFE DISINFECTION The addition of either chlorine or ozone is almost always undertaken to disinfect municipal water supplies before they are sent to supply. An Applied Guide to Water and Effluent Treatment Plant Design. DOI: https://doi.org/10.1016/B978-0-12-811309-7.00009-6 © 2018 Elsevier Inc. All rights reserved.
111
112
CHAPTER 9 Clean water unit operation design: biological processes
Which is better? British engineers tend to favor chlorine, whereas French engineers favor ozone. Why favor chlorine? The number one reason (other than its far lower price) is that if you dose enough of it, it persists as a chlorine residual that disinfects the main all the way to the tap, unlike ozone. Why favor ozone? Chlorine reacts with organic compounds to produce suspected carcinogens [of which the most common are trihalomethanes (THMs)] as well as things that can taste unpleasant. Ozone is a more powerful oxidizing agent, which destroys organics, leaving no nasty by-products from reaction with organics (though there can be problems with other by-products, e.g., chlorate and bromate). Chlorine gas is the cheapest and still the most common disinfectant, but the standard 1 tonne drums are a great deal of poisonous gas to be storing under pressure. They are a magnet for saboteurs, a fact that plant designers should bear in mind. Domestic supplies are sometimes disinfected with UV light, which has the advantage of not requiring the storage of chlorine or its dangerous compounds, and the disadvantage of not producing a disinfectant residual. Membrane filtration can also be used, with much the same disadvantages and advantages as UV. Note that for manufacturing QA reasons, UF membranes are better at disinfection than RO membranes with far smaller pores.
Chlorination Chlorination of water involves adding a controlled amount of free or combined chlorine to water at a controlled pH, and retaining the mixture for a controlled minimum time before release to supply. Care must be taken not to chlorinate highly colored natural waters, as this will lead to excessive concentrations of disinfection by-products such as THMs, suspected human carcinogens. Chlorine gas, sodium, and calcium hypochlorite are used to add chlorine to water to disinfect it. Alternatively, sodium hypochlorite can be produced on-site from brine via electrolytic chlorination. Chlorine gas is the cheapest option, but the standard 1 tonne drums of chlorine gas are arguably not intrinsically safe. Lots of small sites use 33, 50, or 70 kg cylinders, and on-site hypochlorination or electrochlorination (OSE) may be an option at large plants, especially those close to the sea’s free supply of brine, although issues with brine purity, strength, and bromate concentration would have to be considered. Sodium hypochlorite solution is a practical alternative at small- and medium-sized plants, and solid calcium hypochlorite may be used at the smallest plants. This last is known as “HTH,” and is rarely used in the United Kingdom, probably because operational staff hate dealing with the solids handling and dissolution kit. Chlorine gas is depressurized to vacuum conditions in chlorination equipment to reduce leakage potential and consequences. It is mixed with water using an eductor known as a chlorinator, to produce hypochlorous acid and hydrochloric acid: Cl2 1 H2 O"HClO 1 HCl HClO2H1 1 OCl2
Discouraging Life
Above pH 8 the hypochlorite ion (OCl2) predominates. Hypochlorous acid is a much stronger disinfectant than hypochlorite ion. pH control is therefore crucial to effective chlorination. Sodium and calcium hypochlorite solutions are dosed using dosing pumps. These are modified to handle the bubbles that can form in hypochlorite solutions due to decomposition of hypochlorite producing oxygen, which can cause gas locking, especially in low flow conditions. When it comes to ensuring sufficient contact time, the WRc publication TR60 “Disinfection by chlorination in contact tanks” (see Further Reading) is a useful starting point. Chlorine contact tanks (CCTs) are essentially a plug flow reactor, and TR60 is mostly about how to design them hydraulically to get a tight range of residence times. While TR60 is now arguably quite outdated the following recommendations still hold good: • • • • • •
Using inlet baffles, or more simply (and lower in headloss), introducing flow to the tank at right angles to the tank flow direction Using a rectangular tank, baffled to produce a long, narrow serpentine flow path A channel length to width ratio of 40:1 Ensuring linear flow rates are high enough to prevent sediment settlement (minimum 1 m/s) Ensuring 0.1 0.2 ppm chlorine residual at outlet after 30 minute contact time Ensuring pH in the range 6 7 throughout CCT
The chlorine dose rate can be controlled using flow-paced feedback from the chlorine residual, though clearly there will be a long lag in the control loop. Alternatively, chlorine can be deliberately overdosed (known as superchlorination), and the chlorine residual “trimmed” after the CCT by dechlorination to give far tighter control of the residual. A common UK standard is to have a chlorine analyzer close to the chlorine dose with the set point cascaded from another analyzer downstream of the contact tank. Overdosing is these days usually only employed for ammonia, manganese or iron removal or to compensate for inadequate contact time. This stable residual trace of chlorine is the big advantage of chlorine as a drinking water disinfectant. It disinfects the distribution system all the way to the customer’s tap. All UK water companies now use the Ct concept to design and operate chlorine dosing systems, taking account of the residence time distribution (e.g., t0 or t10) and the hypochlorous acid concentration.
Ozonation Ozone is a stronger oxidizing agent than chlorine and is also more toxic and corrosive. This affects the selection of construction materials for ozonation systems. It also affects how the excess air carrying the ozone with its traces of ozone is handled after it has been passed through the water to be treated. Activated carbon is very effective in removing ozone. Ozone is made on-site from air or from pure oxygen, most commonly by “corona discharge,” in which very high voltages are used to produce ozone from
113
114
CHAPTER 9 Clean water unit operation design: biological processes
hot electrical sparks. The process also produces oxides of nitrogen, and the air is usually dried before being subjected to corona discharge to prevent the formation of nitric acid in the air handling system.
Dechlorination Chlorine can also be removed very reliably with activated carbon from either air or water, though this can be expensive at the scale of municipal water treatment. The carbon requires replacement after a couple of years and the process is not suitable for partial removal of chlorine. Chlorine can be removed more cheaply by reduction with sulfur dioxide or bisulfite. The reactions are: 1 SO2 1 H2 O-HSO2 3 1H 22 HOCl 1 HSO2 -SO 1 Cl2 1 2H1 3 4 1 SO2 1 HOCl 1 H2 O-Cl2 1 SO22 4 1 3H
Gaseous sulfur dioxide is handled and dissolved in the same way as chlorine. Dechlorination is most commonly required after chlorine has been added in excess as an oxidant to remove reduced metal ions or ammonia. Just sufficient bisulfite is added to reduce the excess of chlorine after the reaction has taken place. There may be a requirement to operate the disinfection process at a different pH from earlier coagulation stages, and then adjust the pH again afterwards. Historically, dechlorination was commonly achieved by flow-paced feedback controlled dosing of the reducing agent into a static mixer at a dose rate controlled by chlorine residual analyzers or redox probe, although some older works may still use “flash” mixers. The dechlorination reaction is rapid and complete, so mixer hydraulic retention time (HRT) is not a practical concern. The sulfur dioxide dose rate can be controlled using flow-paced feed forward and/or feedback for a sample point just upstream of the dosing point. This is usually controlled by chlorine analyzers nowadays rather than redox probes though a combination of chlorine and sulfite monitors may be used for nominally zero chlorine zero sulfite discharges to a watercourse.
Ultraviolet light Water clarity is key in ensuring efficient disinfection by UV. The process involves passing the water to be disinfected through a chamber illuminated by UV lamps. It is now applied to drinking water at all scales of supply. UV needs at least as low a solids concentration as chlorine disinfection (i.e., ,1 NTU), so it is fine for both surface and borehole water, but it works best with borehole water, due to its innately low suspended solids concentration. UV is also much used in pharmaceutical water treatment, as it does not involve adding chemicals, which might contaminate or react with pharmaceutical ingredients. Potable water that has been demineralized and UV treated is commonly referred to in the pharmaceutical industry as “purified water.”
Biological Filtration
Membrane filtration Ultrafiltration (UF) membranes have pores that are too small (5 20 nm) to allow bacteria or even viruses to pass through and, if configured correctly, can be an effective means of disinfection largely by size exclusion. Work in the 1990s by the UK’s WRc showed virtually complete virus removal of some wastewaters despite the UF membranes having a pore size much larger than the viruses. Coarser membranes used for microfiltration with pore sizes of 20 nm 1 µm remove only a percentage of these microorganisms.
STERILIZATION Water can be sterilized by heating and by the same agents used for disinfection, but with multiple stages, higher doses, longer residence times, and posttreatment filtration.
ENCOURAGING LIFE The only commonly used engineered biological process in clean water treatment is biological filtration. It can be used to remove natural and synthetic organics, iron, manganese, nitrate, and ammonium ions. Slow sand filtration (see Chapter 7: Clean Water Unit Operation Design: Physical Processes) is the most frequently used process and removes all these substances to varying degrees, along with suspended solids. Removal of organics in granular activated carbon (GAC) filters may also be to some extent biological, especially if ozone activated (BAC). In practice, only iron, manganese and ammonium ions tend to be removed by dedicated biological filters at commercial scale, and even these are uncommon processes.
BIOLOGICAL FILTRATION Dedicated biological filters can be used to remove up to 1 mg/L of ammonium, as well as a few milligrams per liter of reduced manganese and iron from drinking water. Surface loadings from 10 to 34 m/h and operating pH around 7 are reported in full-scale plants. Operational experience suggests that individual air injection to each filter should be used, and good quality air blowers. Research indicates that the dissolved oxygen (DO) (or redox potential) needs to be less than saturation to achieve the conditions required for the right biology to develop. The process is not completely robust and pilot plant testing is strongly recommended if this technology is being considered.
115
116
CHAPTER 9 Clean water unit operation design: biological processes
If the ammonium concentration is less than 1.0 mg/L, nitrification will reliably take place on a conventional sand filter, though with concentration higher than 1.5 mg/L a discrete biological nitrification stage of aerated filters is required.
FURTHER READING McNaughton, J. G., & Gregory, R. (1977). Water Research Centre Technical Report disinfection by chlorination in contact tanks, TR60. Marlow: Water Research Centre. Mouchet, P. (1992). From conventional to biological removal of iron and manganese in France. JAWWA, 84(4), 158.
Clean water unit operation design: biological processes
9
CHAPTER OUTLINE Introduction ............................................................................................................111 Discouraging Life ....................................................................................................111 Disinfection ...............................................................................................111 Sterilization ...............................................................................................115 Encouraging Life .....................................................................................................115 Biological Filtration ................................................................................................115 Further Reading ......................................................................................................116
INTRODUCTION Nowadays fewer biological processes are used in modern clean water treatment (though ironically in the days of slow sand filtration, there was more biology than chemistry in clean water treatment). This is mainly because clean water usually has fewer of the nutrients that are needed to sustain life. The most important aspect of life, as far as clean water treatment is usually concerned, is how to eradicate it. There are three levels of cleanliness: cleaning, disinfection, and sterilization. Cleaning water (and equipment) involves removing gross contamination. Disinfection involves removing pathogenic organisms (those which cause disease). Sterilization involves removing or inactivating every single viable or potentially viable organism. Cleaning is done by the standard water treatment processes, and by maintenance activity, whilst disinfection and sterilization are distinct unit operations. Drinking water is merely disinfected, but water and equipment used in pharmaceutical manufacture must be sterile. Let us start with the more common process.
DISCOURAGING LIFE DISINFECTION The addition of either chlorine or ozone is almost always undertaken to disinfect municipal water supplies before they are sent to supply. An Applied Guide to Water and Effluent Treatment Plant Design. DOI: https://doi.org/10.1016/B978-0-12-811309-7.00009-6 © 2018 Elsevier Inc. All rights reserved.
111
112
CHAPTER 9 Clean water unit operation design: biological processes
Which is better? British engineers tend to favor chlorine, whereas French engineers favor ozone. Why favor chlorine? The number one reason (other than its far lower price) is that if you dose enough of it, it persists as a chlorine residual that disinfects the main all the way to the tap, unlike ozone. Why favor ozone? Chlorine reacts with organic compounds to produce suspected carcinogens [of which the most common are trihalomethanes (THMs)] as well as things that can taste unpleasant. Ozone is a more powerful oxidizing agent, which destroys organics, leaving no nasty by-products from reaction with organics (though there can be problems with other by-products, e.g., chlorate and bromate). Chlorine gas is the cheapest and still the most common disinfectant, but the standard 1 tonne drums are a great deal of poisonous gas to be storing under pressure. They are a magnet for saboteurs, a fact that plant designers should bear in mind. Domestic supplies are sometimes disinfected with UV light, which has the advantage of not requiring the storage of chlorine or its dangerous compounds, and the disadvantage of not producing a disinfectant residual. Membrane filtration can also be used, with much the same disadvantages and advantages as UV. Note that for manufacturing QA reasons, UF membranes are better at disinfection than RO membranes with far smaller pores.
Chlorination Chlorination of water involves adding a controlled amount of free or combined chlorine to water at a controlled pH, and retaining the mixture for a controlled minimum time before release to supply. Care must be taken not to chlorinate highly colored natural waters, as this will lead to excessive concentrations of disinfection by-products such as THMs, suspected human carcinogens. Chlorine gas, sodium, and calcium hypochlorite are used to add chlorine to water to disinfect it. Alternatively, sodium hypochlorite can be produced on-site from brine via electrolytic chlorination. Chlorine gas is the cheapest option, but the standard 1 tonne drums of chlorine gas are arguably not intrinsically safe. Lots of small sites use 33, 50, or 70 kg cylinders, and on-site hypochlorination or electrochlorination (OSE) may be an option at large plants, especially those close to the sea’s free supply of brine, although issues with brine purity, strength, and bromate concentration would have to be considered. Sodium hypochlorite solution is a practical alternative at small- and medium-sized plants, and solid calcium hypochlorite may be used at the smallest plants. This last is known as “HTH,” and is rarely used in the United Kingdom, probably because operational staff hate dealing with the solids handling and dissolution kit. Chlorine gas is depressurized to vacuum conditions in chlorination equipment to reduce leakage potential and consequences. It is mixed with water using an eductor known as a chlorinator, to produce hypochlorous acid and hydrochloric acid: Cl2 1 H2 O"HClO 1 HCl HClO2H1 1 OCl2
Discouraging Life
Above pH 8 the hypochlorite ion (OCl2) predominates. Hypochlorous acid is a much stronger disinfectant than hypochlorite ion. pH control is therefore crucial to effective chlorination. Sodium and calcium hypochlorite solutions are dosed using dosing pumps. These are modified to handle the bubbles that can form in hypochlorite solutions due to decomposition of hypochlorite producing oxygen, which can cause gas locking, especially in low flow conditions. When it comes to ensuring sufficient contact time, the WRc publication TR60 “Disinfection by chlorination in contact tanks” (see Further Reading) is a useful starting point. Chlorine contact tanks (CCTs) are essentially a plug flow reactor, and TR60 is mostly about how to design them hydraulically to get a tight range of residence times. While TR60 is now arguably quite outdated the following recommendations still hold good: • • • • • •
Using inlet baffles, or more simply (and lower in headloss), introducing flow to the tank at right angles to the tank flow direction Using a rectangular tank, baffled to produce a long, narrow serpentine flow path A channel length to width ratio of 40:1 Ensuring linear flow rates are high enough to prevent sediment settlement (minimum 1 m/s) Ensuring 0.1 0.2 ppm chlorine residual at outlet after 30 minute contact time Ensuring pH in the range 6 7 throughout CCT
The chlorine dose rate can be controlled using flow-paced feedback from the chlorine residual, though clearly there will be a long lag in the control loop. Alternatively, chlorine can be deliberately overdosed (known as superchlorination), and the chlorine residual “trimmed” after the CCT by dechlorination to give far tighter control of the residual. A common UK standard is to have a chlorine analyzer close to the chlorine dose with the set point cascaded from another analyzer downstream of the contact tank. Overdosing is these days usually only employed for ammonia, manganese or iron removal or to compensate for inadequate contact time. This stable residual trace of chlorine is the big advantage of chlorine as a drinking water disinfectant. It disinfects the distribution system all the way to the customer’s tap. All UK water companies now use the Ct concept to design and operate chlorine dosing systems, taking account of the residence time distribution (e.g., t0 or t10) and the hypochlorous acid concentration.
Ozonation Ozone is a stronger oxidizing agent than chlorine and is also more toxic and corrosive. This affects the selection of construction materials for ozonation systems. It also affects how the excess air carrying the ozone with its traces of ozone is handled after it has been passed through the water to be treated. Activated carbon is very effective in removing ozone. Ozone is made on-site from air or from pure oxygen, most commonly by “corona discharge,” in which very high voltages are used to produce ozone from
113
114
CHAPTER 9 Clean water unit operation design: biological processes
hot electrical sparks. The process also produces oxides of nitrogen, and the air is usually dried before being subjected to corona discharge to prevent the formation of nitric acid in the air handling system.
Dechlorination Chlorine can also be removed very reliably with activated carbon from either air or water, though this can be expensive at the scale of municipal water treatment. The carbon requires replacement after a couple of years and the process is not suitable for partial removal of chlorine. Chlorine can be removed more cheaply by reduction with sulfur dioxide or bisulfite. The reactions are: 1 SO2 1 H2 O-HSO2 3 1H 22 HOCl 1 HSO2 -SO 1 Cl2 1 2H1 3 4 1 SO2 1 HOCl 1 H2 O-Cl2 1 SO22 4 1 3H
Gaseous sulfur dioxide is handled and dissolved in the same way as chlorine. Dechlorination is most commonly required after chlorine has been added in excess as an oxidant to remove reduced metal ions or ammonia. Just sufficient bisulfite is added to reduce the excess of chlorine after the reaction has taken place. There may be a requirement to operate the disinfection process at a different pH from earlier coagulation stages, and then adjust the pH again afterwards. Historically, dechlorination was commonly achieved by flow-paced feedback controlled dosing of the reducing agent into a static mixer at a dose rate controlled by chlorine residual analyzers or redox probe, although some older works may still use “flash” mixers. The dechlorination reaction is rapid and complete, so mixer hydraulic retention time (HRT) is not a practical concern. The sulfur dioxide dose rate can be controlled using flow-paced feed forward and/or feedback for a sample point just upstream of the dosing point. This is usually controlled by chlorine analyzers nowadays rather than redox probes though a combination of chlorine and sulfite monitors may be used for nominally zero chlorine zero sulfite discharges to a watercourse.
Ultraviolet light Water clarity is key in ensuring efficient disinfection by UV. The process involves passing the water to be disinfected through a chamber illuminated by UV lamps. It is now applied to drinking water at all scales of supply. UV needs at least as low a solids concentration as chlorine disinfection (i.e., ,1 NTU), so it is fine for both surface and borehole water, but it works best with borehole water, due to its innately low suspended solids concentration. UV is also much used in pharmaceutical water treatment, as it does not involve adding chemicals, which might contaminate or react with pharmaceutical ingredients. Potable water that has been demineralized and UV treated is commonly referred to in the pharmaceutical industry as “purified water.”
Biological Filtration
Membrane filtration Ultrafiltration (UF) membranes have pores that are too small (5 20 nm) to allow bacteria or even viruses to pass through and, if configured correctly, can be an effective means of disinfection largely by size exclusion. Work in the 1990s by the UK’s WRc showed virtually complete virus removal of some wastewaters despite the UF membranes having a pore size much larger than the viruses. Coarser membranes used for microfiltration with pore sizes of 20 nm 1 µm remove only a percentage of these microorganisms.
STERILIZATION Water can be sterilized by heating and by the same agents used for disinfection, but with multiple stages, higher doses, longer residence times, and posttreatment filtration.
ENCOURAGING LIFE The only commonly used engineered biological process in clean water treatment is biological filtration. It can be used to remove natural and synthetic organics, iron, manganese, nitrate, and ammonium ions. Slow sand filtration (see Chapter 7: Clean Water Unit Operation Design: Physical Processes) is the most frequently used process and removes all these substances to varying degrees, along with suspended solids. Removal of organics in granular activated carbon (GAC) filters may also be to some extent biological, especially if ozone activated (BAC). In practice, only iron, manganese and ammonium ions tend to be removed by dedicated biological filters at commercial scale, and even these are uncommon processes.
BIOLOGICAL FILTRATION Dedicated biological filters can be used to remove up to 1 mg/L of ammonium, as well as a few milligrams per liter of reduced manganese and iron from drinking water. Surface loadings from 10 to 34 m/h and operating pH around 7 are reported in full-scale plants. Operational experience suggests that individual air injection to each filter should be used, and good quality air blowers. Research indicates that the dissolved oxygen (DO) (or redox potential) needs to be less than saturation to achieve the conditions required for the right biology to develop. The process is not completely robust and pilot plant testing is strongly recommended if this technology is being considered.
115
116
CHAPTER 9 Clean water unit operation design: biological processes
If the ammonium concentration is less than 1.0 mg/L, nitrification will reliably take place on a conventional sand filter, though with concentration higher than 1.5 mg/L a discrete biological nitrification stage of aerated filters is required.
FURTHER READING McNaughton, J. G., & Gregory, R. (1977). Water Research Centre Technical Report disinfection by chlorination in contact tanks, TR60. Marlow: Water Research Centre. Mouchet, P. (1992). From conventional to biological removal of iron and manganese in France. JAWWA, 84(4), 158.