Taking water reuse into the future
Anthony Bennett of Specialist Technical Solutions focuses on developments in the application of membrane technology for industrial wastewater reuse in the UK and abroad
The major impetus for reusing wastewater comes from the increasing pressure on natural water resources, as evidenced by the effects of climate change and shifting weather patterns. Wastewater reuse can help conserve the supply of freshwater, and this presents clear advantages with respect to environmental protection.
Professor Blanca Jiménez, chair of the International Water Association's water reuse sub-committee said "&water reuse concepts, as well as applications, are very different around the world. In regions where industrial water reuse is important, it is not only because of the lack of water but also because of government enforcement".
Jiménez went on to add legislation is important in developed countries but that "&in mainly developing countries, water reuse is practised for other purposes, mainly agriculture, while industry focuses more on water consumption reduction practices".
On the other hand, Professor Simon Judd, head of Cranfield University's School of Water Sciences said that in addition to legislation "&wastewater reuse tends to arise from the potential savings in related supply and discharge costs that can be realised."
Judd has recently co-authored Membranes for Industrial Wastewater Recovery and Reuse. Here, he advises that between 1980-1995 water supply and discharge costs trebled in the UK and increased by more than an order of magnitude in California. The economic motivation for reuse of industrial wastewater can be a major driving force because many industries consume large volumes of water, as shown in Table 1. Reuse of wastewater is particularly attractive in cases where the quality of the supplied freshwater, predominantly of potable standard, either exceeds that demanded by the industrial process or else demands further processing.MEMBRANE USAGE
The use of membrane technology is favoured in industrial wastewater reuse because these technologies can provide product water of a consistent and reliable high quality despite wide fluctuations in wastewater quality. Membrane technologies have various advantages over conventional techniques, including the fact separation is achieved without change of state (for example, 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. Membranes that can be used in industrial wastewater reuse include microfiltration (MF) for the removal of suspended solids, including micro-organisms.
Ultrafiltration (UF) is used for the removal of large dissolved molecules and suspended colloidal particles. Either MF or UF membranes are used in membrane bio-reactor (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. Nanofiltration (NF) is utilised for the removal of multivalent ions (such as calcium or magnesium) and certain charged or polar molecules, and the removal of 'dissolved' inorganic ions is achieved by either reverse osmosis (RO) or electro deionisation (EDI).
In both these processes concentrated ions are transferred to the waste stream and low conductivity water (LCW) is produced. Judd advises advances in membrane technology and significant improvements in its efficiency, and therefore cost effectiveness, have greatly increased the competitiveness of recycling wastewater over discharge to the environment.FOOD INDUSTRY
Up to 1.4Ml/d of wastewater is processed at Geest's Bourne Salads factory in Lincolnshire by using an Aquabio MBR system. This comprises separate aeration tank and UF membrane system including X-Flow modules. The membranes are installed in a separate pressurised side-stream system. MBR product is treated further by RO to provide 45% of the requirement for potable water on site.
The remainder that is not reused is discharged to the local watercourse making use of an Environment Agency (EA) consent to discharge 1.0Ml/d. UF permeate not processed by the RO is combined with RO reject. This reduced the dependence on the local sewer and significantly reduced water disposal costs.
In addition, obvious savings on water purchase costs were realised and the small footprint of the plant allowed ease of installation into the available space and reuse of existing infrastructure on site. The plant has now been operating for 12 months and the final reusable water quality has been fully compliant with UK water quality regulations.POWER INDUSTRY
The Flag Fen project was established by the Anglian Water Group and incorporates MF and RO membrane technology. Municipal effluent from Flag Fen WwTW is reused and processed into LCW for feeding into nearby Peterborough power station, owned by Texas Utilities.
The power station previously utilised potable water in steam raising systems but now 1.25Ml/d of potable water is saved, which has reduced the power station's total water use by 11%. ACWa Services installed and commissioned the plant in 2000. The major challenge with using sewage effluent as the feed to the plant was its quality, which varies both seasonally and diurnally. Despite this, Judd has concluded the permeate quality from both MF and RO systems has been reliable since the plant went on stream and in line with values determined from pilot studies. Permeate quality from the RO plant has resulted in LCW of typically 39µS/cm conductivity, and this has enabled ultra pure water production at the power station to increase by 20%. Also, a reduction of more than 90% in the costs of ion exchange regeneration has been realised.OIL REFINING
Tertiary treatment of wastewater from Pemex's Minatitlán Oil Refinery has been achieved by a 26.5Ml/d Zenon MBR system, installed in 2001 at Veracruz, Mexico (see Figure 2). Refinery wastewater is processed by MBR and RO technology to produce LCW for cooling tower make-up and to satisfy various other critical uses within the refinery processes.
In early 2000, following extensive pilot trials, the Zenon ZeeWeed UF system was selected primarily because of the effective pre-treatment afforded to RO systems - a dead-end barrier, protecting RO membranes from contaminants (such as organics, colloidal particulate and bacteria) and consistent production of high-quality treated water, regardless of fluctuations in feed water quality. The MBR system runs at 95% recovery and consistently produces RO feedwater with TSS<1mg/l and turbidity <0.1NTU. The small footprint of the plant reduced installation costs and allowed the system to be retained in the existing space available onsite.Industrial use
NEWater is Singapore Public Utilities Board's high-quality LCW, produced from secondary treated effluent using MF, RO and UV technology. NEWater meets the US Environmental Protection Agency and World Health Organisation drinking water standards.
The first two NEWater factories at Bedok and Kranji (see Figure 3) were commissioned in 2002 and since then NEWater has been supplied to semi-conductor wafer fabrication plants, power generation companies, electronics companies and commercial buildings (for process and air cooling).
The major use of NEWater is by industries as the high quality characteristics are suitable for their direct non-potable use. The total demand for this non-potable use in Singapore amounts to 66Ml/d. Approximately 13.5Ml/d of NEWater is blended into surface water reservoirs for indirect use in potable water treatment plants. This accounts for about 1% of Singapore's daily potable water requirement. The amount is to be increased progressively to 45.5Ml/d by 2011.
Demand for NEWater has been growing. Early last year the third NEWater factory at Seletar was commissioned. The total capacity of the three NEWater factories is now 92Ml/d. A fourth NEWater factory (to go online at the end of 2006) will serve the industrial hub in the western part of Singapore, as well as commercial buildings in the city area.POTABLE USAGE
Orange County Water District (OCWD) in California has been recharging aquifers by injecting reclaimed LCW since 1976. Currently, OCWD is constructing the new Groundwater Replenishment (GWR) System. OCWD's Advanced Water Purification Facility will provide 265Ml/d by using MF, RO and ultra-violet/hydrogen peroxide technologies to provide treated water with a TDS of less than 100mg/l. Half of the water produced will be injected to expand the seawater intrusion barrier - the remainder will be sent through a 22km pipeline to percolation ponds where it will naturally enter and recharge the groundwater basin. OCWD said the GWR System will save money by providing a new drought-proof water source, while using about half the energy compared to the alternative of importing potable water, as well as improving the quality of the water in the groundwater basin. The case studies demonstrate industrial wastewater recycling is technically feasible and can provide economic savings. In general terms, economic viability is likely to depend on regulatory restrictions on supply and discharge combined with the perceived or actual technical and financial risk.
Judd advises that while economic profitability is obviously important, this seems to be insufficient a driver in its own right in most cases. Instead, legislative or water scarcity issues are generally the primary drivers, which then impact on economics.
The key characteristics of all our case studies is the ability of the membrane systems installed to withstand wide variations in wastewater quality, while producing a stable water quality suitable for reuse. It is important to note, however, treated water quality from membranes is usually excellent - the main concern is achieving the necessary throughput without incurring excessive cost. This is reflected in the need for effective pre-treatment for RO membranes. In the examples including RO, either UF or MF technology is utilised utilised as RO pre-treatment. As a result, RO membrane fouling is reduced, RO membrane module life is extended, downtime reduces, and lower maintenance costs and increased throughput are achievable. However, Judd points out a common problem in evaluating potential industrial reuse schemes is a lack of detailed and relevant data describing the water quality.
Although easily remedied, data scarcity can be a major barrier to uptake of membrane technology for the recovery of wastewater. Overall, the case studies have shown the suitability of membrane technologies for industrial wastewater reuse. The ability to produce reclaimed water of sufficient quality is clear.
However, the throughputs are quite different between our case studies and other reuse schemes - and this importantly shows each scheme is unique, involving problems that have not been encountered in other industries and other countries. Organisations such as the IWA and Cranfield University can help to disseminate experience gained so all potential industrial reuse opportunities can be effectively evaluated.
To sum up, Jiménez said: "It is true water reuse is being practised differently around the world but from the technological point of view, options are not so varied and, considering reliability, high-efficiency and low sludge production, there will be a lot more membrane applications in the future."