- Email: [email protected]
Wastewater treatment: Green technologies rise to the bait
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Wastewater treatment:
Green technologies rise to the bait D
evelopments in wastewater treatment processes enhance a plant’s ability to improve effluent quality;; ensure the robustness and reliability of equipment; and aim for high automation and low maintenance e. In the first feature of our special focus, Anthony Bennett looks at some cutting-edge filtration and separation processes.
Introduction A range of approaches are used to treat wastewater to achieve a required final effluent quality, and manufacturers develop technologies and advanced equipment to effect such treatment. Recent developments in technology also emphasise the need for certain existing treatment facilities to be upgraded.
• Self cleaning filters (SCF) can be used upstream of advanced membrane treatment processes, which are designed to remove fine solids and reduce chemical oxygen demand (COD), usually after appropriate biological treatment;
So what are the latest tools and options available to wastewater professionals?
• And for discharges to sensitive receiving waters, secondary treated effluent can be passed through media adsorption systems for tertiary polishing of solids/COD, metals and phosphorus;
In this feature we describe the main features and benefits of filtration and separation processes as applied to wastewater treatment. We examine the options available to engineers for adopting these processes. And using case studies, we illustrate the successful implementation of technology for several advanced processes.
• When wastewater is to be reused for potable supply, membrane ultrafiltration (UF) systems can supply high quality feed-water for reverse osmosis (RO) demineralisation systems, followed by disinfection (where required) using ozone, hydrogen peroxide, chlorine or UV;
Advanced unit processes
• Sludge generated at municipal wastewater treatment plants is dewatered after thickening, usually by presses or centrifuges. Innovative technologies combine these two processes, making savings in space and coagulant use.
• Many wastewater treatment plants screen to remove larger solids; and protect pipe-work, pumps and more sensitive equipment downstream; • Advanced technology screens, often removing particles >500 µm, are constructed of robust materials such as stainless steel. They are available with automated selfcleaning mechanisms such as brushes or discs, which permit long-term, lowmaintenance, reliable pretreatment;
New technology to save space Treatment requirements change for a variety of reasons. Strict discharge consents set by regulators, notably for nitrogen and phosphorus, may necessitate plant upgrade to improve effluent quality. Hydraulic loads
tend to increase with time requiring capacity expansion through the development or consolidation of intermittent catchment discharges.
Continuous monitoring of the effluent stream to detect treatment deterioration or plant failure, can prevent contamination of the receiving environment from discharges. Many existing municipal wastewater treatment works use conventional activated sludge (CAS), rotating biological contactor (RBC) or trickling filter secondary treatment processes, together with associated sludge treatment technology. But these basic treatment systems will need upgrading in order to achieve stricter limits on nutrient removal and reduced solids and COD. Plant enlargement or the installation of additional treatment units using conventional technology help meet increased capacity demands, but space limitation and the high capital costs entailed often prove prohibitive. Advanced technologies offer a viable alternative.
Technology review 1 – GE’s ZeeWeed UF treatment system The addition of polishing technology to improve effluent quality is one option, and membrane technology – microfiltration
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Focus on wastewater treatment Dealing with process wastewater is a significant challenge that plants in many industries need to come to terms with. Environmental pressure and stringent regulation is one aspect, but plant managers also need to focus on the cost-saving aspect of wastewater treatment. To correspond with the Aquatech and WEFTEC shows, we focus on some aspects of wastewater treatment: Below (p12-p17): Anthony Bennett on some modern options available to wastewater treatment professionals; p18-p19: We highlight a technology that could offer a cost-effective alternative to ion exchange; p20-p21: MBR – we look at pilot trials for wastewater treatment using a thermophilic MBR at an activated carbon plant; p22-p25: We publish some entries for this year’s Aquatech Innovation award.
(MF), ultrafiltration (UF) or nanofiltration (NF) – often fits the bill. This technology can improve effluent solids and COD quality, whilst, in the case of UF/NF, afford a good degree of pathogen removal. One example of this is the ZeeWeed UF treatment system from ZENON Membrane
Solutions (part of GE Water & Process Technologies), which uses hollow fibres. The effluent is passed through a chemically resistant polymer membrane (nominal pore size 0.04 µm) under low-pressure suction. The fibres are gathered as bundles and assembled into cassettes, and may be fitted in-line with current secondary treatment systems. UF is achieved by direct immersion of the cassettes into effluent tanks. The system can cope with variable hydraulic loads, occupies a small footprint, and has low maintenance requirements by employing an outside-to-in filtration system. Rejected solids are kept from the membrane surface, which is less prone to fouling, and the exterior is scoured by continuous oxygen bubbling. It incorporates automated membrane selfcleaning through permeate back-pulsing – as and when UF efficiency problems are detected – and is therefore suitable for employment at remote or unmanned treatment plants. These UF systems are available as modular containers in various sizes, making capacity expansion possible. As they are separate from upstream treatment systems, they also allow independent biological and hydraulic treatment improvements. The membrane separation stage replaces the final clarification stage of conventional CAS, removing the hydraulic load limits determined by settling times. This system also allows coagulants to be added at the aeration stage for phosphorus removal, thus reducing sludge generation. New build, complete membrane bioreactors (MBR) can therefore increase the treatment capacity or be run in parallel with the existing plant. Membrane UF technology therefore, in combination with demineralisation via RO and appropriate disinfection (e.g. UV treatment), provides a way to generate high quality non potable water from a wastewater source that can be safely reused for irrigation, industrial processes, or groundwater recharge. RO requires a high quality feedwater in order to operate at the necessary efficiency over a prolonged period, and membrane UF can meet this need.
Figure 1: GE ZeeWeed UF cassette.
UV treatment using in-pipe technology is a good alternative to ozone or
chlorine disinfection and can deal with variable influent flow. The automated monitoring of influent clarity makes it possible to optimise continuous UV dosing for effective disinfection.
Discharge or recycle? Municipal treatment plants can be subject to strict controls for solids and nutrient contaminants with regard to discharge – especially when sensitive receiving waters are involved. These can include river catchment areas identified as vulnerable to eutrophication, or when discharge is directly into lakes and reservoirs. In such cases, effective tertiary treatment using the UF systems outlined above can consistently produce a permeate with <1 mg/l suspended solids. Because of the highly-efficient particle retention of UF systems effluent when treated with coagulant, even post-biological treatment will be discharged at <0.1 mg/l phosphorus. Tertiary treatment for solids, COD and phosphorus removal can also be achieved using adsorptive media (see ‘adsorptive media filtration’, below).
Case study 1 – SCF and membrane technology in Singapore There has also been renewed interest in reusing wastewater, after appropriate
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treatment. One such approach uses UF followed by RO demineralisation and, where appropriate, ozone or UV disinfection. A facility in Belgium, for example, uses this treatment prior to re-injecting the water into an aquifer, producing 2.5 million m³ of re-used water per year – roughly 40% of the region’s potable water requirement). Another example of the large-scale re-use of wastewater is the Singapore NEWater project, instigated by the Singapore Public Utilities Board (PUB). The aim of the project is to supplement Singapore’s existing potable and industrial water supplies with high quality reused (HQR) water obtained from the treatment of municipal and industrial wastewater. The NEWater project has been influenced by successful water recycling experience elsewhere, for example in the USA at Orange County Water District, South Carolina. Extensive pilot testing at a water recycling plant based in Bedok, Singapore commenced in 1998 and four full-scale water treatment plants (WTP) have been commissioned by the Singapore PUB to date. Since late 2002, WTPs at the towns of Bedok and Kranji have supplied 27 and 41 millions of litres per day (MLD) respectively. The NEWater project was expanded in January 2004 when the Seletar WTP added 24 MLD, and this HQR water has been used for both potable and industrial consumption. Approximately 13.5 MLD HQR water is currently blended with rainwater in reservoirs, meeting 1% of the daily potable water requirement, and the Singapore PUB intend to increase this contribution to 45.5 MLD by 2011, utilising a greatly enhanced HQR water supply upon the opening of the Ulu Pandan WTP in late 2006 (166 MLD, with potential for further expansion by 24 MLD). Advanced filtration and separation technology is used at all of these NEWater WTPs. Amiad’s automated stainless steel brush ABF 10000 is used at all four sites to screen the secondary municipal effluent supply to 200 µm (at Seletar) and 500 µm (elsewhere). MF/UF treatment provides high quality feed-water for RO systems, using technology from various equipment suppliers including Koch, Siemens and GE, while UV disinfection completes the treatment process. Using this technology makes it possible for the NEWater advanced treatment systems to supply a high quality permeate that meets the appropriate World Health Organisation (WHO) and USA Environmental Protection Agency (EPA) water quality standards: [physical – TDS <100 mg/l and conductivity < 200 µS/cm; chemical – sulphate <5 mg/l and nitrate <15 mg/l; and bacteriological – undetectable enterovirus and faecal coliform. See
Figure 2: In-pipe MP UV disinfection unit. Hanovia produces MP UV disinfection equipment that incorporates in-line transmissivity monitors, which increase lamp intensity automatically when water quality deteriorates, for example from increased UV scattering from suspended particles or increased UV absorption from higher organic content. Variable UV lamp intensity can also save energy, since lower inputs can dose clean, high transmissivity water. The Singapore PUB elected to use Hanovia MP UV disinfection in its NEWater project along with the UF technology, after extensive pilot testing and life-costing at Bedok.
www.pub.gov.sg/NEWater_files for details of HQR water quality produced in the pilot study.
Technology review 2 – Virotec’s adsorptive media filtration
phosphate (influent 6-9 mg/L) without the need for pH adjustment, and possessed a phosphorus binding capacity > 5 kg-P/tonne, without reaching ultimate capacity during the 6-month study.
Adsorptive media is used to polish secondary treated water by reducing metals, arsenic and phosphate. One example is produced by Virotec and consists of 18 highly sorptive and acid neutralising reagents which can be produced in a pelletised form. In this form, their reagents have an open, porous skeleton suited to passive flow-through wastewater treatment, which largely comprises aluminium, silicon, iron, calcium and titanium-based minerals. A large internal surface area and small particle size (mainly <10µm) result in good physical and chemical sorption properties and a high sorptive capacity (1500 meq/kg).
Treatment requirements change for a variety of reasons. Strict discharge consents set by regulators, notably for nitrogen and phosphorus, may necessitate plant upgrade to improve effluent quality. Hydraulic loads tend to increase with time requiring capacity expansion due to the development or consolidation of intermittent catchment discharges.
Although Virotec’s reagents are potentially applicable to all contaminated industrial wastewater where metal removal (to >99% and low ppb residual concentrations) and/or acidity treatment are required, this type of pelletised flow-through treatment system is particularly suitable for municipal wastewater polishing. The units are readily retro-fitted in-line with existing systems to improve effluent quality, and can be flexibly configured.
Virotec’s own studies demonstrate a binding capacity of >12 kg-P/tonne. The pelletised reagent also improved effluent solids, COD, colour and odour, and WRc concluded that the ViroFilter technology was suitable for employment at smaller treatment plants without needing chemical dosing, and at larger plants with flocculent addition.
The UK Water Research Council (WRc) conducted independent tests on Virotec’s ViroFilter Technology, assessing its suitability for tertiary treatment (phosphorus removal and polishing). The system removed 80%-85%
Virotec’s ViroSewage technology has also reduced phosphorus releases at Fairfield WTP, Australia, which serves a densely populated suburb of Brisbane. With an inflow of 2.4 MLD, influent total-P concentrations ranging from 7 to 18 mg/L were reduced by an average of 79%. Here, the rapidly-settling reagent was
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dosed into the conventional primary settlement-CAS treatment works. In this case, space limitation prevented the adoption of conventional nutrient treatment, but Virotec’s advanced technology improved effluent quality to meet the Australian EPA phosphorus consent. [For more information on this interesting technology, see ‘technology review’, p10-12, Filtration + Separation, June 2006].
UV disinfection UV disinfection (see figure 2 on page 14 for an example) works by disrupting the replication capability of DNA in micro-organisms, (a process considered to be most effective at around 265 nm, the peak absorbance wavelength of DNA.) Micro-organisms require exposure to a specific intensity of UV light, for a given time, to ensure they are deactivated. Water transmissivity can affect UV dosing and hence disinfection, so an accurate transmittance monitoring system is essential. Both monochromatic (254 nm) low-pressure (LP) and medium-pressure (MP) polychromatic (240-310 nm wavelength range) lamps are available to carry out this process. LP lamps operate at around 30% efficiency, equating to 60W germicidal output per lamp, and while MP lamps require a higher energy input, they also have higher overall useful UV output and possess the equivalent water treatment capability of up to 10 LP lamps – and thus have a lower lifecost. A WTP using conventional LP UV technology can have a large footprint, with
Filtration+Separation September 2006
open channels using many UV lamps in series. In-pipe UV disinfection is a viable alternative as it both suits the variable-flow regimes typical at numerous small and medium WTPs, and can be readily retro-fitted, using, for example, existing chlorine contact tanks. Plant operators are safely shielded from both UV exposure and the inhalation of effluent aerosol by the enclosed system.
Sludge dewatering Sludge dewatering is used for volume reduction and subsequent treatment processes. Flotation, belt presses and centrifuges form the main approaches of dewatering technology, while sludge thickening is normally a separate process prior to the dewatering stage. Recent innovations in sludge treatment have combined the thickening and dewatering processes:
Case study 2 – sludge dewatering at Lerwick WTP Lerwick WTP has specific and unusual requirements for its sludge treatment, and required expansion to meet local demand. The sludge feed is thin and variable, being influenced by the high rainfall and storms characteristic of the Shetlands. Additionally, Lerwick WTP is located close to residential areas, so odour control is particularly important. To meet these specific requirements Lerwick chose technology from Ashbrook Simon-Hartley. The 3-Belt Klampress is designed for a sludge composition of <1.5 % solids. A variable speed gravity deck thickens the sludge whilst the pressure section dewaters it, all in one
combined unit. The sludge requires polymer addition once at the start of the process through a specifically-designed variable orifice mixer, reducing polymer consumption – compared to conventional two-stage processing. The conditioned sludge moves through the dewatering zones, via channels designed to distribute evenly the sludge across the transfer belt. Initial gravity drainage is followed by passage of the sludge through a constricting wedge to increase the pressure in a controlled way. The sludge then passes through a series of preliminary and full pressure rollers; the number of these rollers installed (6, 8, 10 or 12) determines the dewatering efficiency. The plant footprint and installation costs of this single combined unit can be correspondingly smaller than for two separate conventional units. The 3-Belt Klampress installed at this remote location meets Lerwick’s requirements, and uses removable steel shields for housing and odour containment.
Wastewater monitoring As we have seen from the examples in this article, wastewater treatment systems are becoming more reliable. However, the periodic failure of a plant is inevitable, and the consequences of failed treatment can be severe. Continuous monitoring of the effluent stream to detect treatment deterioration or plant failure can prevent contamination of the receiving environment from discharges. Ammonia in wastewater is normally discharged under consent, typically at around 5 mg/l. With many biological nitrification units, deterioration of the treatment system not only inadequately oxidises the ammonia, but also, (as shown in studies conducted at the UK's Cranfield University), has associated releases of the powerful greenhouse gas nitrous oxide (N2O).
Technology review 3 – early warning against nitrification failure Water Innovate has developed the N-Tox continuous gas monitor to detect increases in N2O above treatment channels, offering an early warning system for nitrification failure. A rise in N2O typically occurs about 7 hours prior to eventual discharge of ammonia in the effluent, allowing sufficient time for remedial action by plant managers or alternative effluent storage or bypass disposal.
Figure 3: 3-Belt Klampress combined sludge thickening, and dewatering unit at Lerwick WTW.
N-Tox monitors using “real-time” nondispersive infrared gas analysis in the range 22000 ppm N2O (4-4000 mg/m3), at 1 second intervals (delayed by 8-30 seconds dependent
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effluent quality. Membrane filtration systems, protected by SCF screening systems, can dramatically improve solids and COD effluent quality when installed in-line with the existing plant, while new-build MBR systems installed in parallel with existing plants can allow flexible capacity expansion. Adsorptive media flow-through systems can be used to achieve a tertiary polishing of metals, COD and phosphate, while also improving effluent colour and odour.
Figure 4: Nitrous oxide concentrations measured along a CAS/nitrification treatment channel at a municipal wastewater treatment plant using Water Innovate's N-Tox continuous gas monitor (data courtesy of Water Innovate).
upon sampling line length). This non-invasive technique does not suffer from problems of maintenance, or fouled probes common with alternative ammonia, nitrate and nitrite aqueous phase (or on-line respirometric monitoring systems). The N2O detector is housed with an air sampling pump, autocalibration systems and a data logger. The technology has been used at municipal and industrial wastewater treatment plants. Effective
nitrification occurs in a short distance away from the anoxic zone of the tank, shown by low N2O gas concentrations well below 2 ppm, reduced from approximately 6 ppm in the head-space above the anoxic zone (see figure 4).
Conclusions Wastewater treatment plants requiring upgrade have a range of advanced technologies available to achieve improved
Continuous gas monitoring can be a reliable way to confirm effective nitrification in biological treatment systems, mitigating ammonia discharge consent failure, and combined sludge thickening and dewatering units save space. Using all these new technologies, plants nowadays have a wide range of technologies at their disposal with which to reuse wastewater for potable and industrial consumption, instead of simply discharging it.
•
Author Anthony Bennett Clarity E: [email protected]
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Wastewater treatment:
Green technologies rise to the bait D
evelopments in wastewater treatment processes enhance a plant’s ability to improve effluent quality;; ensure the robustness and reliability of equipment; and aim for high automation and low maintenance e. In the first feature of our special focus, Anthony Bennett looks at some cutting-edge filtration and separation processes.
Introduction A range of approaches are used to treat wastewater to achieve a required final effluent quality, and manufacturers develop technologies and advanced equipment to effect such treatment. Recent developments in technology also emphasise the need for certain existing treatment facilities to be upgraded.
• Self cleaning filters (SCF) can be used upstream of advanced membrane treatment processes, which are designed to remove fine solids and reduce chemical oxygen demand (COD), usually after appropriate biological treatment;
So what are the latest tools and options available to wastewater professionals?
• And for discharges to sensitive receiving waters, secondary treated effluent can be passed through media adsorption systems for tertiary polishing of solids/COD, metals and phosphorus;
In this feature we describe the main features and benefits of filtration and separation processes as applied to wastewater treatment. We examine the options available to engineers for adopting these processes. And using case studies, we illustrate the successful implementation of technology for several advanced processes.
• When wastewater is to be reused for potable supply, membrane ultrafiltration (UF) systems can supply high quality feed-water for reverse osmosis (RO) demineralisation systems, followed by disinfection (where required) using ozone, hydrogen peroxide, chlorine or UV;
Advanced unit processes
• Sludge generated at municipal wastewater treatment plants is dewatered after thickening, usually by presses or centrifuges. Innovative technologies combine these two processes, making savings in space and coagulant use.
• Many wastewater treatment plants screen to remove larger solids; and protect pipe-work, pumps and more sensitive equipment downstream; • Advanced technology screens, often removing particles >500 µm, are constructed of robust materials such as stainless steel. They are available with automated selfcleaning mechanisms such as brushes or discs, which permit long-term, lowmaintenance, reliable pretreatment;
New technology to save space Treatment requirements change for a variety of reasons. Strict discharge consents set by regulators, notably for nitrogen and phosphorus, may necessitate plant upgrade to improve effluent quality. Hydraulic loads
tend to increase with time requiring capacity expansion through the development or consolidation of intermittent catchment discharges.
Continuous monitoring of the effluent stream to detect treatment deterioration or plant failure, can prevent contamination of the receiving environment from discharges. Many existing municipal wastewater treatment works use conventional activated sludge (CAS), rotating biological contactor (RBC) or trickling filter secondary treatment processes, together with associated sludge treatment technology. But these basic treatment systems will need upgrading in order to achieve stricter limits on nutrient removal and reduced solids and COD. Plant enlargement or the installation of additional treatment units using conventional technology help meet increased capacity demands, but space limitation and the high capital costs entailed often prove prohibitive. Advanced technologies offer a viable alternative.
Technology review 1 – GE’s ZeeWeed UF treatment system The addition of polishing technology to improve effluent quality is one option, and membrane technology – microfiltration
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Focus on wastewater treatment Dealing with process wastewater is a significant challenge that plants in many industries need to come to terms with. Environmental pressure and stringent regulation is one aspect, but plant managers also need to focus on the cost-saving aspect of wastewater treatment. To correspond with the Aquatech and WEFTEC shows, we focus on some aspects of wastewater treatment: Below (p12-p17): Anthony Bennett on some modern options available to wastewater treatment professionals; p18-p19: We highlight a technology that could offer a cost-effective alternative to ion exchange; p20-p21: MBR – we look at pilot trials for wastewater treatment using a thermophilic MBR at an activated carbon plant; p22-p25: We publish some entries for this year’s Aquatech Innovation award.
(MF), ultrafiltration (UF) or nanofiltration (NF) – often fits the bill. This technology can improve effluent solids and COD quality, whilst, in the case of UF/NF, afford a good degree of pathogen removal. One example of this is the ZeeWeed UF treatment system from ZENON Membrane
Solutions (part of GE Water & Process Technologies), which uses hollow fibres. The effluent is passed through a chemically resistant polymer membrane (nominal pore size 0.04 µm) under low-pressure suction. The fibres are gathered as bundles and assembled into cassettes, and may be fitted in-line with current secondary treatment systems. UF is achieved by direct immersion of the cassettes into effluent tanks. The system can cope with variable hydraulic loads, occupies a small footprint, and has low maintenance requirements by employing an outside-to-in filtration system. Rejected solids are kept from the membrane surface, which is less prone to fouling, and the exterior is scoured by continuous oxygen bubbling. It incorporates automated membrane selfcleaning through permeate back-pulsing – as and when UF efficiency problems are detected – and is therefore suitable for employment at remote or unmanned treatment plants. These UF systems are available as modular containers in various sizes, making capacity expansion possible. As they are separate from upstream treatment systems, they also allow independent biological and hydraulic treatment improvements. The membrane separation stage replaces the final clarification stage of conventional CAS, removing the hydraulic load limits determined by settling times. This system also allows coagulants to be added at the aeration stage for phosphorus removal, thus reducing sludge generation. New build, complete membrane bioreactors (MBR) can therefore increase the treatment capacity or be run in parallel with the existing plant. Membrane UF technology therefore, in combination with demineralisation via RO and appropriate disinfection (e.g. UV treatment), provides a way to generate high quality non potable water from a wastewater source that can be safely reused for irrigation, industrial processes, or groundwater recharge. RO requires a high quality feedwater in order to operate at the necessary efficiency over a prolonged period, and membrane UF can meet this need.
Figure 1: GE ZeeWeed UF cassette.
UV treatment using in-pipe technology is a good alternative to ozone or
chlorine disinfection and can deal with variable influent flow. The automated monitoring of influent clarity makes it possible to optimise continuous UV dosing for effective disinfection.
Discharge or recycle? Municipal treatment plants can be subject to strict controls for solids and nutrient contaminants with regard to discharge – especially when sensitive receiving waters are involved. These can include river catchment areas identified as vulnerable to eutrophication, or when discharge is directly into lakes and reservoirs. In such cases, effective tertiary treatment using the UF systems outlined above can consistently produce a permeate with <1 mg/l suspended solids. Because of the highly-efficient particle retention of UF systems effluent when treated with coagulant, even post-biological treatment will be discharged at <0.1 mg/l phosphorus. Tertiary treatment for solids, COD and phosphorus removal can also be achieved using adsorptive media (see ‘adsorptive media filtration’, below).
Case study 1 – SCF and membrane technology in Singapore There has also been renewed interest in reusing wastewater, after appropriate
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treatment. One such approach uses UF followed by RO demineralisation and, where appropriate, ozone or UV disinfection. A facility in Belgium, for example, uses this treatment prior to re-injecting the water into an aquifer, producing 2.5 million m³ of re-used water per year – roughly 40% of the region’s potable water requirement). Another example of the large-scale re-use of wastewater is the Singapore NEWater project, instigated by the Singapore Public Utilities Board (PUB). The aim of the project is to supplement Singapore’s existing potable and industrial water supplies with high quality reused (HQR) water obtained from the treatment of municipal and industrial wastewater. The NEWater project has been influenced by successful water recycling experience elsewhere, for example in the USA at Orange County Water District, South Carolina. Extensive pilot testing at a water recycling plant based in Bedok, Singapore commenced in 1998 and four full-scale water treatment plants (WTP) have been commissioned by the Singapore PUB to date. Since late 2002, WTPs at the towns of Bedok and Kranji have supplied 27 and 41 millions of litres per day (MLD) respectively. The NEWater project was expanded in January 2004 when the Seletar WTP added 24 MLD, and this HQR water has been used for both potable and industrial consumption. Approximately 13.5 MLD HQR water is currently blended with rainwater in reservoirs, meeting 1% of the daily potable water requirement, and the Singapore PUB intend to increase this contribution to 45.5 MLD by 2011, utilising a greatly enhanced HQR water supply upon the opening of the Ulu Pandan WTP in late 2006 (166 MLD, with potential for further expansion by 24 MLD). Advanced filtration and separation technology is used at all of these NEWater WTPs. Amiad’s automated stainless steel brush ABF 10000 is used at all four sites to screen the secondary municipal effluent supply to 200 µm (at Seletar) and 500 µm (elsewhere). MF/UF treatment provides high quality feed-water for RO systems, using technology from various equipment suppliers including Koch, Siemens and GE, while UV disinfection completes the treatment process. Using this technology makes it possible for the NEWater advanced treatment systems to supply a high quality permeate that meets the appropriate World Health Organisation (WHO) and USA Environmental Protection Agency (EPA) water quality standards: [physical – TDS <100 mg/l and conductivity < 200 µS/cm; chemical – sulphate <5 mg/l and nitrate <15 mg/l; and bacteriological – undetectable enterovirus and faecal coliform. See
Figure 2: In-pipe MP UV disinfection unit. Hanovia produces MP UV disinfection equipment that incorporates in-line transmissivity monitors, which increase lamp intensity automatically when water quality deteriorates, for example from increased UV scattering from suspended particles or increased UV absorption from higher organic content. Variable UV lamp intensity can also save energy, since lower inputs can dose clean, high transmissivity water. The Singapore PUB elected to use Hanovia MP UV disinfection in its NEWater project along with the UF technology, after extensive pilot testing and life-costing at Bedok.
www.pub.gov.sg/NEWater_files for details of HQR water quality produced in the pilot study.
Technology review 2 – Virotec’s adsorptive media filtration
phosphate (influent 6-9 mg/L) without the need for pH adjustment, and possessed a phosphorus binding capacity > 5 kg-P/tonne, without reaching ultimate capacity during the 6-month study.
Adsorptive media is used to polish secondary treated water by reducing metals, arsenic and phosphate. One example is produced by Virotec and consists of 18 highly sorptive and acid neutralising reagents which can be produced in a pelletised form. In this form, their reagents have an open, porous skeleton suited to passive flow-through wastewater treatment, which largely comprises aluminium, silicon, iron, calcium and titanium-based minerals. A large internal surface area and small particle size (mainly <10µm) result in good physical and chemical sorption properties and a high sorptive capacity (1500 meq/kg).
Treatment requirements change for a variety of reasons. Strict discharge consents set by regulators, notably for nitrogen and phosphorus, may necessitate plant upgrade to improve effluent quality. Hydraulic loads tend to increase with time requiring capacity expansion due to the development or consolidation of intermittent catchment discharges.
Although Virotec’s reagents are potentially applicable to all contaminated industrial wastewater where metal removal (to >99% and low ppb residual concentrations) and/or acidity treatment are required, this type of pelletised flow-through treatment system is particularly suitable for municipal wastewater polishing. The units are readily retro-fitted in-line with existing systems to improve effluent quality, and can be flexibly configured.
Virotec’s own studies demonstrate a binding capacity of >12 kg-P/tonne. The pelletised reagent also improved effluent solids, COD, colour and odour, and WRc concluded that the ViroFilter technology was suitable for employment at smaller treatment plants without needing chemical dosing, and at larger plants with flocculent addition.
The UK Water Research Council (WRc) conducted independent tests on Virotec’s ViroFilter Technology, assessing its suitability for tertiary treatment (phosphorus removal and polishing). The system removed 80%-85%
Virotec’s ViroSewage technology has also reduced phosphorus releases at Fairfield WTP, Australia, which serves a densely populated suburb of Brisbane. With an inflow of 2.4 MLD, influent total-P concentrations ranging from 7 to 18 mg/L were reduced by an average of 79%. Here, the rapidly-settling reagent was
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dosed into the conventional primary settlement-CAS treatment works. In this case, space limitation prevented the adoption of conventional nutrient treatment, but Virotec’s advanced technology improved effluent quality to meet the Australian EPA phosphorus consent. [For more information on this interesting technology, see ‘technology review’, p10-12, Filtration + Separation, June 2006].
UV disinfection UV disinfection (see figure 2 on page 14 for an example) works by disrupting the replication capability of DNA in micro-organisms, (a process considered to be most effective at around 265 nm, the peak absorbance wavelength of DNA.) Micro-organisms require exposure to a specific intensity of UV light, for a given time, to ensure they are deactivated. Water transmissivity can affect UV dosing and hence disinfection, so an accurate transmittance monitoring system is essential. Both monochromatic (254 nm) low-pressure (LP) and medium-pressure (MP) polychromatic (240-310 nm wavelength range) lamps are available to carry out this process. LP lamps operate at around 30% efficiency, equating to 60W germicidal output per lamp, and while MP lamps require a higher energy input, they also have higher overall useful UV output and possess the equivalent water treatment capability of up to 10 LP lamps – and thus have a lower lifecost. A WTP using conventional LP UV technology can have a large footprint, with
Filtration+Separation September 2006
open channels using many UV lamps in series. In-pipe UV disinfection is a viable alternative as it both suits the variable-flow regimes typical at numerous small and medium WTPs, and can be readily retro-fitted, using, for example, existing chlorine contact tanks. Plant operators are safely shielded from both UV exposure and the inhalation of effluent aerosol by the enclosed system.
Sludge dewatering Sludge dewatering is used for volume reduction and subsequent treatment processes. Flotation, belt presses and centrifuges form the main approaches of dewatering technology, while sludge thickening is normally a separate process prior to the dewatering stage. Recent innovations in sludge treatment have combined the thickening and dewatering processes:
Case study 2 – sludge dewatering at Lerwick WTP Lerwick WTP has specific and unusual requirements for its sludge treatment, and required expansion to meet local demand. The sludge feed is thin and variable, being influenced by the high rainfall and storms characteristic of the Shetlands. Additionally, Lerwick WTP is located close to residential areas, so odour control is particularly important. To meet these specific requirements Lerwick chose technology from Ashbrook Simon-Hartley. The 3-Belt Klampress is designed for a sludge composition of <1.5 % solids. A variable speed gravity deck thickens the sludge whilst the pressure section dewaters it, all in one
combined unit. The sludge requires polymer addition once at the start of the process through a specifically-designed variable orifice mixer, reducing polymer consumption – compared to conventional two-stage processing. The conditioned sludge moves through the dewatering zones, via channels designed to distribute evenly the sludge across the transfer belt. Initial gravity drainage is followed by passage of the sludge through a constricting wedge to increase the pressure in a controlled way. The sludge then passes through a series of preliminary and full pressure rollers; the number of these rollers installed (6, 8, 10 or 12) determines the dewatering efficiency. The plant footprint and installation costs of this single combined unit can be correspondingly smaller than for two separate conventional units. The 3-Belt Klampress installed at this remote location meets Lerwick’s requirements, and uses removable steel shields for housing and odour containment.
Wastewater monitoring As we have seen from the examples in this article, wastewater treatment systems are becoming more reliable. However, the periodic failure of a plant is inevitable, and the consequences of failed treatment can be severe. Continuous monitoring of the effluent stream to detect treatment deterioration or plant failure can prevent contamination of the receiving environment from discharges. Ammonia in wastewater is normally discharged under consent, typically at around 5 mg/l. With many biological nitrification units, deterioration of the treatment system not only inadequately oxidises the ammonia, but also, (as shown in studies conducted at the UK's Cranfield University), has associated releases of the powerful greenhouse gas nitrous oxide (N2O).
Technology review 3 – early warning against nitrification failure Water Innovate has developed the N-Tox continuous gas monitor to detect increases in N2O above treatment channels, offering an early warning system for nitrification failure. A rise in N2O typically occurs about 7 hours prior to eventual discharge of ammonia in the effluent, allowing sufficient time for remedial action by plant managers or alternative effluent storage or bypass disposal.
Figure 3: 3-Belt Klampress combined sludge thickening, and dewatering unit at Lerwick WTW.
N-Tox monitors using “real-time” nondispersive infrared gas analysis in the range 22000 ppm N2O (4-4000 mg/m3), at 1 second intervals (delayed by 8-30 seconds dependent
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effluent quality. Membrane filtration systems, protected by SCF screening systems, can dramatically improve solids and COD effluent quality when installed in-line with the existing plant, while new-build MBR systems installed in parallel with existing plants can allow flexible capacity expansion. Adsorptive media flow-through systems can be used to achieve a tertiary polishing of metals, COD and phosphate, while also improving effluent colour and odour.
Figure 4: Nitrous oxide concentrations measured along a CAS/nitrification treatment channel at a municipal wastewater treatment plant using Water Innovate's N-Tox continuous gas monitor (data courtesy of Water Innovate).
upon sampling line length). This non-invasive technique does not suffer from problems of maintenance, or fouled probes common with alternative ammonia, nitrate and nitrite aqueous phase (or on-line respirometric monitoring systems). The N2O detector is housed with an air sampling pump, autocalibration systems and a data logger. The technology has been used at municipal and industrial wastewater treatment plants. Effective
nitrification occurs in a short distance away from the anoxic zone of the tank, shown by low N2O gas concentrations well below 2 ppm, reduced from approximately 6 ppm in the head-space above the anoxic zone (see figure 4).
Conclusions Wastewater treatment plants requiring upgrade have a range of advanced technologies available to achieve improved
Continuous gas monitoring can be a reliable way to confirm effective nitrification in biological treatment systems, mitigating ammonia discharge consent failure, and combined sludge thickening and dewatering units save space. Using all these new technologies, plants nowadays have a wide range of technologies at their disposal with which to reuse wastewater for potable and industrial consumption, instead of simply discharging it.
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Author Anthony Bennett Clarity E: [email protected]
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