- Email: [email protected]
Potable water: sustainable separation treatment
32
Feature
Filtration+Separation May 2006
Potable water:
sustainable separation treatment A
nthony Bennett investigates the latest generation of advanced filtration and separation technologies – focussing on potable water production – but also showing how the technologies can be applied in wastewater treatment and water reuse applications.
Process design Potable water quality requirements are well defined by the World Health Organisation (WHO), and set into legislation by measures such as European Union Directive 98-83. Within such frameworks, all possible water constituents are given maximum admissible concentrations, including a vast and diverse number of physical, chemical, microbiological and radiological parameters. However, this bewildering array of legislative requirements can be simplified by concentrating on various key parameters, as shown in table 1. Potable water must be free from biological pathogens and concentrations of chemical substances that would endanger life, whilst maintaining agreeable taste and smell, without turbidity or colour. In addition to pathogens, table 1 also lists the constituents and aesthetic measurements that are used to characterise the nature and variability of water sources, so that particular contaminants can be removed by suitable unit processes to achieve the necessary standards. Potable water treatment systems are designed by first understanding the composition of the feed water to be treated. Unit process combinations are selected, based on what are determined to be the
critical contaminants in the feed water. Once these have been identified, selection of technologies can essentially be compared on the basis of particle size removal efficiency. The filtration and separation processes commonly used in potable water treatment are shown in table 2. Here, each is categorised by its particle size removal efficiency and included is the approximate maximum particle size that can be treated, together with what remains in the product water from the unit process. For example, if colour related to organics was detected as a major problem, the main component of the process system would include NF or RO as a final process step. Or,
if arsenic levels were high in a groundwater source, then IX, ED or RO would be included to remove this toxin at the end of the process chain. If we wanted to remove Cryptosporidium parvum or Giardia lamblia pathogens then ultracentrifuges, granular/fabric filters or MF technology would be required. The actual processes required would ideally be selected based on a whole life cycle cost evaluation for the specific project.
Hydrological cycle We will now consider how potable water production fits into the hydrological cycle as a whole. We can view the hydrological cycle (see figure 1) at three scales: large, medium
The image shows reservoirs adjoining the Shenzhen City Skyline, in Guangdong Province, China: Many regions in countries such as China are now addressing World Health Organisation (WHO) legislation to clean up their water.
Feature
Filtration+Separation May 2006
and small cycles. The large cycle refers to the global geographical cycle of evaporation, precipitation, surface and groundwater systems. The medium cycle is based around the water catchment and includes the collection, treatment and use of potable water, and the
treatment of wastewater. Because water supply management tends to be managed at this level in municipalities, there is a tendency in the water industry to think of water and wastewater treatment as two distinctive areas. As a result, important opportunities for increasing environmental sustainability are overlooked because water reuse is not considered. Even
Table 1: key drinking water standards Constituent or characteristic
Maximum value
Units
Occurrence, effects and comments
Aluminium
0.2
mg/l
Arsenic
0.05
mg/l
Chloride Colour
250 15
mg/l Hazen Units
Copper
1
mg/l
Hardness
500
mg/l as CaCO3
Iron
0.2
mg/l
Results in coloured water and occurs naturally in many water sources Has well documented health effects Salty taste Caused by presence of metals and/or natural organic matter. The standard is set for aesthetic reasons and requires the water to be virtually colourless Metallic taste and causes bluegreen staining Present in water sources arising from limestone geology with well known scale-producing effects Rusty colour and metallic taste that can cause reddish or orange staining. May be associated with corrosion of unlined iron water mains High amounts are associated with lead supply pipes Black to brown colour and black staining with bitter metallic taste. Occurs along with iron in many water sources Arises from the use of fertilisers in agriculture, and may be minimised by good practice and appropriate controls Nitrite is usually present due to the use of chloramine in water disinfection, which remains in water longer than chlorine Hydrogen sulphide, musty or chemical smell. TON = threshold odour number, a qualitative score from examination by tasting panel. Low pH: bitter metallic taste indicative of corrosion High pH: slippery feel; and soda taste Salty taste Salty taste The TDS is used as a key design parameter Measures general organic content of the water.*no legal guideline exists for TOC, this is indicative. A measure of the cloudiness of the water. NTU = nephelometric turbidity unit. Need < 1.0 NTU to maximise disinfection efficiency. Metallic taste
Lead
0.025
mg/l
Manganese
0.05
mg/l
Nitrate
50
mg/l
Nitrite
0.5
mg/l
Odour
3
TON
pH
6.5 - 8.5
Sodium 200 Sulphate 250 Total Dissolved Solids (TDS) 500
mg/l mg/l mg/l
Total Organic Content (TOC) 3*
mg/l
Turbidity
4
NTU
Zinc
5
mg/l
though wastewater is treated for direct surface water recharge or indirect groundwater recharge at some sites, this is unmanaged reuse – often driven by local economic factors that do not take into account overall medium cycle management issues. By concentrating on the small cycle, we can overcome these problems. We define this as the planned collection and treatment of wastewater so that it can be carefully treated locally and recycled directly and safely to the consumer, thereby shrinking the hydrological cycle. Effective management of the small cycle by municipalities increases environmental sustainability.
Pathogen removal Pathogen removal is a critical consideration when reusing water, but the techniques required highlight the overlap between what can be achieved by physico-chemical and membrane technology in potable water production. For large scale municipal installations, it is important to be able to demonstrate the integrity of water production systems and this feature is often a driving force in specifying MF or UF membrane technology as the final process step. Zenon’s ZeeWeed immersed UF membrane systems, for example, are supplied with a membrane integrity test that estimates the log removal value (LRV) for the system, a measure of the pathogen removal efficiency. A LRV of 4 corresponds to 99.99% removal based on the specified level being present in the feed. In a similar manner to its competitors, Zenon uses a Pressure Decay Test (PDT), which involves pressurising the inside of the membrane’s hollow fibre with air, isolating the permeate and then measuring the rate of pressure decay over time. The rate of decay is then empirically linked to the LRV and calculated using algorithms in the control system. The PDT verifies the integrity of the hollow fibres and all other components in contact with the feed water. The PDT is scheduled automatically via the control system, where an operator is able to select the frequency and approximate time for each PDT to take place. The testing protocols are designed to demonstrate the integrity of the system. Advances such as integrity testing have been driven by legislative aspects – but manufacturing costs have also reduced considerably over recent years while enhancing performance, resulting in whole life costs that are competitive with traditional water treatment processes. Membranes have additional advantages over conventional techniques, including the fact
33
34
Feature
Filtration+Separation May 2006
iron and manganese particles. These characteristics provide improved iron and manganese removal, coupled with typically decreased coagulant dosage requirements.
Equipment design Membrane systems can be engineered into modular systems – allowing expansion up to large flows for municipal applications – and downgrading to smaller flows from mobile equipment. For example, Zenon’s Z-BOX potable water packaged plants are preengineered, modular water treatment systems that bring large-plant performance to small municipal or commercial applications. These types of systems are built on an expandable building-block design. They can be quickly set up and feature scalable treatment capacity that can be quickly increased as demand grows. Figure 1: the hydrological cycle (image courtesy of Veolia Water)
that separation is achieved without a change of state (i.e. liquid to gas as in evaporation/distillation processes); little accumulation takes place in the process (which operates continuously); and low levels of chemical additives are required. To illustrate what pretreatment can be incorporated for membrane systems we now review two applications – brackish/seawater filtration and iron/manganese removal. In addition, other pretreatment solutions can be selected to remove the variety of parameters listed in table 1.
Brackish/seawater filtration Where RO is the final process step, MF and UF technology can be applied effectively as pretreatment for brackish water processing and desalination systems. Power consumption is the primary component of the overall operating cost in desalination systems, and this is proportionally reduced if we can maximise the membrane flux rate and minimise the RO membrane surface area required. Even in brackish water RO systems, energy consumption figures highly in operational cost evaluations. The main advantage of using MF and UF in RO pretreatment is that these technologies provide a positive barrier to solid particles and pathogens, which can significantly reduce the rate of RO membrane fouling and cleaning frequency – resulting in extended life expectancy of RO membranes. Also, reliable production of RO feedwater – regardless of raw water turbidity variations – leads to higher achievable RO membrane flux, greater plant availability and higher capacity in a smaller footprint, and modular
design that has less requirement for pretreatment chemicals such as flocculants, biocides or anti-scalants.
Iron/manganese removal Where UF is the final process step and we need to remove iron and manganese, enhanced coagulation of the feed to the membrane system is required. When raw water supplies contain iron in excess of 5 mg/l, and manganese in excess of 1 mg/l, conventional technologies such as gravity sedimentation or granular filtration, are not always effective due to filter blinding – caused by precipitated iron and manganese, or the formation of an iron bacteria film. If there is surface water intrusion into the groundwater supply, pathogens may also be present, which conventional systems may have difficulty removing. As UF systems remove particles greater than 0.1 microns in size, suspended metals such as iron and manganese can be eliminated. However, iron and manganese also exist in dissolved forms, which can pass through the membrane. To achieve removal of dissolved iron and manganese, these metals must be oxidised from soluble species to insoluble species, which can be subsequently removed by the membrane. Oxidation is normally achieved by aeration or by the addition of oxidising chemicals such as chlorine, chlorine dioxide or potassium permanganate. As settling is not required following membrane-based separation, there is only the need to form pin-sized floc for the membrane to effectively separate oxidised
The modular design results in a number of additional benefits including pre-assembly and factory testing of the equipment, minimisation of on-site construction costs, and accelerated delivery, set up, and start up times. In addition to including integrity monitoring for pathogen removal, these systems are simple to operate and maintain, are highly automated requiring minimal operator supervision, and comprehensive, automatic cleaning capabilities ensure optimised system performance.
Water reuse Regular precipitation is required in most parts of the world to satisfy potable water requirements. In many areas, historical records for droughts and floods have been broken almost every year over the last decade, and increased urbanisation and risks to the supply of imported water reduce potable water availability. Hence, traditional water sources cannot always be relied upon, and the alternative of reusing water becomes a viable and sometimes necessary alternative. Many countries have embraced unmanaged reuse for industrial applications, saline ingress control, groundwater recharge and irrigation. Water reuse is of fundamental importance to the environment and economy; the benefits are numerous and include reduced aqueous pollution into receiving waters and retention of high quality water for potable supply. The use of membrane technology is favoured in water reuse, probably more strongly than in potable water treatment, because membrane technologies can provide product water of a consistent and reliable high quality despite wide fluctuations in feed quality.
Feature
Filtration+Separation May 2006
Table 2: filtration & separation processes in potable water treatment Processes
Input Particle Size (microns)
Output Particle Size (microns)
Typical applications
No limit 200 200 2
0.1 0.1 0.1 0.01
No limit No limit
1 0.1
Removal of suspended solids Removal of suspended solids Removal of suspended solids Removal of large dissolved molecules and suspended colloidal particles Pre-treatment Removal of suspended solids, including micro-organisms
0.01 0.01
0.0001 0.0001
Removal of charged ions Removal of charged ions
0.02
0.001
Removal of soluble species
100 100
0.1 0.01
Nanofiltration (NF)
5
0.005
Reverse osmosis (RO)
5
Removal of suspended solids, including micro-organisms Removal of large dissolved molecules, suspended colloidal particles and some viruses Removal of multivalent ions (such as hardness and organics) and certain charged or polar molecules Removal of inorganic ions
Physico-chemical Gravity sedimentation Hydrocyclones Centrifuges Ultracentrifuges Screens/strainers Granular/fabric Based on ionic charge Ion exchange (IX) Electrodialysis (ED) Based on adsorption Activated carbon Membrane Microfiltration (MF) Ultrafiltration (UF)
Membranes that can be used in wastewater reuse include MF or UF, with either type used in membrane bioreactor (MBR) systems, (depending on the water quality requirement for downstream processes); here, membranes can be immersed in the raw water or included within a pressurised side-stream system. MBR systems provide a superior and economic solution for treating wastewater, to a quality suitable for reusing onsite following additional post-treatment as necessary. The use of UF membranes produces very high permeate quality with virtually zero solids discharge and reduced odour. Operation at high solids levels results in smaller process tanks and lower investment cost, and low sludge loading leads to extended sludge age and less surplus sludge that needs disposing of. Packaged MBR systems can be installed in a modular design similar to the potable water systems described above. This simplifies installation, but also opens up the possibility of being able to retrofit UF membranes into existing wastewater treatment systems.
Process hybridisation For municipalities, the challenge for the future will be optimising power production, utilising short carbon cycle (renewable) energy whilst effectively managing water at the small end of the hydrological cycle. The combination of energy and water policies at the municipal level will help provide for a sustainable future. Hybridisation is an approach that can increase process efficiency by taking advantage of the synergy between strengths and weaknesses of
<
0.0001
different process solutions. These solutions can include competing technologies such as thermal and membrane techniques. The hybridisation of power generation systems is being seriously investigated in order to learn how power stations can improve efficiency by using unused power generation capacity to increase valuable freshwater production when sufficient demand or storage is available. Power stations using long carbon cycle fossil fuels must typically be operated above a minimum threshold due to their thermal processes. As well as this, wind, tidal and solar based short carbon cycle power stations will provide intermittent peaks when a demand from consumers may not exist. Using excess electricity generated, excess freshwater can be produced, which can then be banked as groundwater for future use – to increase security of supply. We would be effectively converting excess electricity into valuable fresh groundwater for future use, and it could be argued that this is an equivalent to energy storage.
Conclusions Advances in membrane technology are impacting on all aspects of the hydrological cycle with various applications of the same technology in potable and wastewater treatment systems. Membrane technologies are increasingly beating competing conventional equipment in tenders due to lower capital, annual and lifecycle costs.
Membrane technology brings many benefits including superior quality drinking water, compact plant footprints, consistent performance regardless of raw water quality, and highly automated operation. Now that cost is less of an issue, it is possible that conventional technologies be will rendered obsolete. In addition to cost savings, the use of MBR systems in water re-use and the possibilities for synergy in process hybridisation provides municipal water and energy planners with a robust set of tools that can now be applied to help move towards a sustainable future. Water reuse can be used to expand potable water supplies in small scale water cycles and reduce the requirement of traditional raw water sources. Future projects can be envisaged where advanced membrane technologies become integrated with short carbon cycle power generation systems to provide process and freshwater for banking and storage when required, making use of hybridisation in the selection of process technologies. The result could be a reduction in overall energy requirement and environmental impact, by making full use of short carbon cycle energy production peaks by balancing periods of low energy demand with increased potable water production.
•
About the Author Anthony Bennett is technical director at Clarity, which provides a range of technical authoring and related services specifically for specialist publications and innovative companies in the environmental sector. Tel: +44 (0) 1535 647200 E: [email protected] Web: www.clarityauthoring.com
35
Feature
Filtration+Separation May 2006
Potable water:
sustainable separation treatment A
nthony Bennett investigates the latest generation of advanced filtration and separation technologies – focussing on potable water production – but also showing how the technologies can be applied in wastewater treatment and water reuse applications.
Process design Potable water quality requirements are well defined by the World Health Organisation (WHO), and set into legislation by measures such as European Union Directive 98-83. Within such frameworks, all possible water constituents are given maximum admissible concentrations, including a vast and diverse number of physical, chemical, microbiological and radiological parameters. However, this bewildering array of legislative requirements can be simplified by concentrating on various key parameters, as shown in table 1. Potable water must be free from biological pathogens and concentrations of chemical substances that would endanger life, whilst maintaining agreeable taste and smell, without turbidity or colour. In addition to pathogens, table 1 also lists the constituents and aesthetic measurements that are used to characterise the nature and variability of water sources, so that particular contaminants can be removed by suitable unit processes to achieve the necessary standards. Potable water treatment systems are designed by first understanding the composition of the feed water to be treated. Unit process combinations are selected, based on what are determined to be the
critical contaminants in the feed water. Once these have been identified, selection of technologies can essentially be compared on the basis of particle size removal efficiency. The filtration and separation processes commonly used in potable water treatment are shown in table 2. Here, each is categorised by its particle size removal efficiency and included is the approximate maximum particle size that can be treated, together with what remains in the product water from the unit process. For example, if colour related to organics was detected as a major problem, the main component of the process system would include NF or RO as a final process step. Or,
if arsenic levels were high in a groundwater source, then IX, ED or RO would be included to remove this toxin at the end of the process chain. If we wanted to remove Cryptosporidium parvum or Giardia lamblia pathogens then ultracentrifuges, granular/fabric filters or MF technology would be required. The actual processes required would ideally be selected based on a whole life cycle cost evaluation for the specific project.
Hydrological cycle We will now consider how potable water production fits into the hydrological cycle as a whole. We can view the hydrological cycle (see figure 1) at three scales: large, medium
The image shows reservoirs adjoining the Shenzhen City Skyline, in Guangdong Province, China: Many regions in countries such as China are now addressing World Health Organisation (WHO) legislation to clean up their water.
Feature
Filtration+Separation May 2006
and small cycles. The large cycle refers to the global geographical cycle of evaporation, precipitation, surface and groundwater systems. The medium cycle is based around the water catchment and includes the collection, treatment and use of potable water, and the
treatment of wastewater. Because water supply management tends to be managed at this level in municipalities, there is a tendency in the water industry to think of water and wastewater treatment as two distinctive areas. As a result, important opportunities for increasing environmental sustainability are overlooked because water reuse is not considered. Even
Table 1: key drinking water standards Constituent or characteristic
Maximum value
Units
Occurrence, effects and comments
Aluminium
0.2
mg/l
Arsenic
0.05
mg/l
Chloride Colour
250 15
mg/l Hazen Units
Copper
1
mg/l
Hardness
500
mg/l as CaCO3
Iron
0.2
mg/l
Results in coloured water and occurs naturally in many water sources Has well documented health effects Salty taste Caused by presence of metals and/or natural organic matter. The standard is set for aesthetic reasons and requires the water to be virtually colourless Metallic taste and causes bluegreen staining Present in water sources arising from limestone geology with well known scale-producing effects Rusty colour and metallic taste that can cause reddish or orange staining. May be associated with corrosion of unlined iron water mains High amounts are associated with lead supply pipes Black to brown colour and black staining with bitter metallic taste. Occurs along with iron in many water sources Arises from the use of fertilisers in agriculture, and may be minimised by good practice and appropriate controls Nitrite is usually present due to the use of chloramine in water disinfection, which remains in water longer than chlorine Hydrogen sulphide, musty or chemical smell. TON = threshold odour number, a qualitative score from examination by tasting panel. Low pH: bitter metallic taste indicative of corrosion High pH: slippery feel; and soda taste Salty taste Salty taste The TDS is used as a key design parameter Measures general organic content of the water.*no legal guideline exists for TOC, this is indicative. A measure of the cloudiness of the water. NTU = nephelometric turbidity unit. Need < 1.0 NTU to maximise disinfection efficiency. Metallic taste
Lead
0.025
mg/l
Manganese
0.05
mg/l
Nitrate
50
mg/l
Nitrite
0.5
mg/l
Odour
3
TON
pH
6.5 - 8.5
Sodium 200 Sulphate 250 Total Dissolved Solids (TDS) 500
mg/l mg/l mg/l
Total Organic Content (TOC) 3*
mg/l
Turbidity
4
NTU
Zinc
5
mg/l
though wastewater is treated for direct surface water recharge or indirect groundwater recharge at some sites, this is unmanaged reuse – often driven by local economic factors that do not take into account overall medium cycle management issues. By concentrating on the small cycle, we can overcome these problems. We define this as the planned collection and treatment of wastewater so that it can be carefully treated locally and recycled directly and safely to the consumer, thereby shrinking the hydrological cycle. Effective management of the small cycle by municipalities increases environmental sustainability.
Pathogen removal Pathogen removal is a critical consideration when reusing water, but the techniques required highlight the overlap between what can be achieved by physico-chemical and membrane technology in potable water production. For large scale municipal installations, it is important to be able to demonstrate the integrity of water production systems and this feature is often a driving force in specifying MF or UF membrane technology as the final process step. Zenon’s ZeeWeed immersed UF membrane systems, for example, are supplied with a membrane integrity test that estimates the log removal value (LRV) for the system, a measure of the pathogen removal efficiency. A LRV of 4 corresponds to 99.99% removal based on the specified level being present in the feed. In a similar manner to its competitors, Zenon uses a Pressure Decay Test (PDT), which involves pressurising the inside of the membrane’s hollow fibre with air, isolating the permeate and then measuring the rate of pressure decay over time. The rate of decay is then empirically linked to the LRV and calculated using algorithms in the control system. The PDT verifies the integrity of the hollow fibres and all other components in contact with the feed water. The PDT is scheduled automatically via the control system, where an operator is able to select the frequency and approximate time for each PDT to take place. The testing protocols are designed to demonstrate the integrity of the system. Advances such as integrity testing have been driven by legislative aspects – but manufacturing costs have also reduced considerably over recent years while enhancing performance, resulting in whole life costs that are competitive with traditional water treatment processes. Membranes have additional advantages over conventional techniques, including the fact
33
34
Feature
Filtration+Separation May 2006
iron and manganese particles. These characteristics provide improved iron and manganese removal, coupled with typically decreased coagulant dosage requirements.
Equipment design Membrane systems can be engineered into modular systems – allowing expansion up to large flows for municipal applications – and downgrading to smaller flows from mobile equipment. For example, Zenon’s Z-BOX potable water packaged plants are preengineered, modular water treatment systems that bring large-plant performance to small municipal or commercial applications. These types of systems are built on an expandable building-block design. They can be quickly set up and feature scalable treatment capacity that can be quickly increased as demand grows. Figure 1: the hydrological cycle (image courtesy of Veolia Water)
that separation is achieved without a change of state (i.e. liquid to gas as in evaporation/distillation processes); little accumulation takes place in the process (which operates continuously); and low levels of chemical additives are required. To illustrate what pretreatment can be incorporated for membrane systems we now review two applications – brackish/seawater filtration and iron/manganese removal. In addition, other pretreatment solutions can be selected to remove the variety of parameters listed in table 1.
Brackish/seawater filtration Where RO is the final process step, MF and UF technology can be applied effectively as pretreatment for brackish water processing and desalination systems. Power consumption is the primary component of the overall operating cost in desalination systems, and this is proportionally reduced if we can maximise the membrane flux rate and minimise the RO membrane surface area required. Even in brackish water RO systems, energy consumption figures highly in operational cost evaluations. The main advantage of using MF and UF in RO pretreatment is that these technologies provide a positive barrier to solid particles and pathogens, which can significantly reduce the rate of RO membrane fouling and cleaning frequency – resulting in extended life expectancy of RO membranes. Also, reliable production of RO feedwater – regardless of raw water turbidity variations – leads to higher achievable RO membrane flux, greater plant availability and higher capacity in a smaller footprint, and modular
design that has less requirement for pretreatment chemicals such as flocculants, biocides or anti-scalants.
Iron/manganese removal Where UF is the final process step and we need to remove iron and manganese, enhanced coagulation of the feed to the membrane system is required. When raw water supplies contain iron in excess of 5 mg/l, and manganese in excess of 1 mg/l, conventional technologies such as gravity sedimentation or granular filtration, are not always effective due to filter blinding – caused by precipitated iron and manganese, or the formation of an iron bacteria film. If there is surface water intrusion into the groundwater supply, pathogens may also be present, which conventional systems may have difficulty removing. As UF systems remove particles greater than 0.1 microns in size, suspended metals such as iron and manganese can be eliminated. However, iron and manganese also exist in dissolved forms, which can pass through the membrane. To achieve removal of dissolved iron and manganese, these metals must be oxidised from soluble species to insoluble species, which can be subsequently removed by the membrane. Oxidation is normally achieved by aeration or by the addition of oxidising chemicals such as chlorine, chlorine dioxide or potassium permanganate. As settling is not required following membrane-based separation, there is only the need to form pin-sized floc for the membrane to effectively separate oxidised
The modular design results in a number of additional benefits including pre-assembly and factory testing of the equipment, minimisation of on-site construction costs, and accelerated delivery, set up, and start up times. In addition to including integrity monitoring for pathogen removal, these systems are simple to operate and maintain, are highly automated requiring minimal operator supervision, and comprehensive, automatic cleaning capabilities ensure optimised system performance.
Water reuse Regular precipitation is required in most parts of the world to satisfy potable water requirements. In many areas, historical records for droughts and floods have been broken almost every year over the last decade, and increased urbanisation and risks to the supply of imported water reduce potable water availability. Hence, traditional water sources cannot always be relied upon, and the alternative of reusing water becomes a viable and sometimes necessary alternative. Many countries have embraced unmanaged reuse for industrial applications, saline ingress control, groundwater recharge and irrigation. Water reuse is of fundamental importance to the environment and economy; the benefits are numerous and include reduced aqueous pollution into receiving waters and retention of high quality water for potable supply. The use of membrane technology is favoured in water reuse, probably more strongly than in potable water treatment, because membrane technologies can provide product water of a consistent and reliable high quality despite wide fluctuations in feed quality.
Feature
Filtration+Separation May 2006
Table 2: filtration & separation processes in potable water treatment Processes
Input Particle Size (microns)
Output Particle Size (microns)
Typical applications
No limit 200 200 2
0.1 0.1 0.1 0.01
No limit No limit
1 0.1
Removal of suspended solids Removal of suspended solids Removal of suspended solids Removal of large dissolved molecules and suspended colloidal particles Pre-treatment Removal of suspended solids, including micro-organisms
0.01 0.01
0.0001 0.0001
Removal of charged ions Removal of charged ions
0.02
0.001
Removal of soluble species
100 100
0.1 0.01
Nanofiltration (NF)
5
0.005
Reverse osmosis (RO)
5
Removal of suspended solids, including micro-organisms Removal of large dissolved molecules, suspended colloidal particles and some viruses Removal of multivalent ions (such as hardness and organics) and certain charged or polar molecules Removal of inorganic ions
Physico-chemical Gravity sedimentation Hydrocyclones Centrifuges Ultracentrifuges Screens/strainers Granular/fabric Based on ionic charge Ion exchange (IX) Electrodialysis (ED) Based on adsorption Activated carbon Membrane Microfiltration (MF) Ultrafiltration (UF)
Membranes that can be used in wastewater reuse include MF or UF, with either type used in membrane bioreactor (MBR) systems, (depending on the water quality requirement for downstream processes); here, membranes can be immersed in the raw water or included within a pressurised side-stream system. MBR systems provide a superior and economic solution for treating wastewater, to a quality suitable for reusing onsite following additional post-treatment as necessary. The use of UF membranes produces very high permeate quality with virtually zero solids discharge and reduced odour. Operation at high solids levels results in smaller process tanks and lower investment cost, and low sludge loading leads to extended sludge age and less surplus sludge that needs disposing of. Packaged MBR systems can be installed in a modular design similar to the potable water systems described above. This simplifies installation, but also opens up the possibility of being able to retrofit UF membranes into existing wastewater treatment systems.
Process hybridisation For municipalities, the challenge for the future will be optimising power production, utilising short carbon cycle (renewable) energy whilst effectively managing water at the small end of the hydrological cycle. The combination of energy and water policies at the municipal level will help provide for a sustainable future. Hybridisation is an approach that can increase process efficiency by taking advantage of the synergy between strengths and weaknesses of
<
0.0001
different process solutions. These solutions can include competing technologies such as thermal and membrane techniques. The hybridisation of power generation systems is being seriously investigated in order to learn how power stations can improve efficiency by using unused power generation capacity to increase valuable freshwater production when sufficient demand or storage is available. Power stations using long carbon cycle fossil fuels must typically be operated above a minimum threshold due to their thermal processes. As well as this, wind, tidal and solar based short carbon cycle power stations will provide intermittent peaks when a demand from consumers may not exist. Using excess electricity generated, excess freshwater can be produced, which can then be banked as groundwater for future use – to increase security of supply. We would be effectively converting excess electricity into valuable fresh groundwater for future use, and it could be argued that this is an equivalent to energy storage.
Conclusions Advances in membrane technology are impacting on all aspects of the hydrological cycle with various applications of the same technology in potable and wastewater treatment systems. Membrane technologies are increasingly beating competing conventional equipment in tenders due to lower capital, annual and lifecycle costs.
Membrane technology brings many benefits including superior quality drinking water, compact plant footprints, consistent performance regardless of raw water quality, and highly automated operation. Now that cost is less of an issue, it is possible that conventional technologies be will rendered obsolete. In addition to cost savings, the use of MBR systems in water re-use and the possibilities for synergy in process hybridisation provides municipal water and energy planners with a robust set of tools that can now be applied to help move towards a sustainable future. Water reuse can be used to expand potable water supplies in small scale water cycles and reduce the requirement of traditional raw water sources. Future projects can be envisaged where advanced membrane technologies become integrated with short carbon cycle power generation systems to provide process and freshwater for banking and storage when required, making use of hybridisation in the selection of process technologies. The result could be a reduction in overall energy requirement and environmental impact, by making full use of short carbon cycle energy production peaks by balancing periods of low energy demand with increased potable water production.
•
About the Author Anthony Bennett is technical director at Clarity, which provides a range of technical authoring and related services specifically for specialist publications and innovative companies in the environmental sector. Tel: +44 (0) 1535 647200 E: [email protected] Web: www.clarityauthoring.com
35