Integrating wastewater reuse with seawater desalination
The ability to turn wastewater and seawater into high quality process water is key to the expansion of downstream oil and gas in water short areas
WILLIAM CELENZA and JOHN HERRING
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Industry accounts for roughly 20% of total freshwater withdrawals globally, with the agriculture and domestic sectors accounting for the remaining 70% and 10%, respectively (see Figure 1). Water use for energy has most often been embedded in ‘industry’. However, the International Energy Agency (IEA) has estimated global water withdrawals for energy production in 2010 accounted for roughly 15% of the world total,1 or roughly 75% of all industrial water withdrawals. The Organisation for Economic Co-operation and Development (OECD) Environmental Outlook to 2050 predicts that global water demand for manufacturing, not including thermal electricity generation, will increase by 400% from 2000 to 2050, the largest increase projected for any sector.2 Therefore, the 5% of total global freshwater withdrawals by the industry sector in 2000 for manufacturing is projected to increase four-fold by 2050. These figures vary considerably across countries, with more developed countries having a much larger proportion of freshwater withdrawals for industry than less developed countries, where agriculture dominates.3 For comparison, a recent US DOE analysis4 of 2005 data, shown in Figure 2, indicated that industry, including thermal electricity generation, was 47% of the total US fresh water withdrawals, with the agriculture and domestic sectors accounting for the remaining 40% and 13%, respectively. Thermal energy use itself was 40% of US fresh water withdrawals, equalling agriculture. Also in the US, fresh water thermal energy withdrawals in 2005 accounted for 70% of all thermal energy water withdrawals. Thus adoption of water reuse options for industry, in particular the thermal energy sector, offers large potential to conserve fresh water sources in developed countries today and
an increasing potential in undeveloped countries as manufacturing expands.
Freshwater shortages are forcing industrial customers to rethink water supply options. The 2014 World Water Assessment Prog-ramme by UNESCO highlights that withdrawing freshwater from a surface or groundwater source, using it, and disposing of it – known as a ‘linear approach’ – is problematic going forward. Future development requires approaches that minimise resource consumption and focus on resource recovery,3 rather than focusing mainly on a linear approach based on compliance with discharge standards. Although using non-freshwater sources, such as sea- or wastewater, can be challenging, they offer a great potential for reducing demands for freshwater.3 Many large companies have made considerable progress in evaluating and reducing their water use and that of their supply chains.5 Most companies have a supply chain water footprint much larger than their operational one; and more than 80% to 90% of the footprint, with most of its water risks, may be beyond its direct operations.6 A water footprint is ‘the total volume of water used in the production of the goods and services produced by a business’7 and is usually quoted in terms of cubic metres of water per unit product. For example, the average total water footprint for operations that make crude oil available as an energy carrier is estimated at 3.8 m3/MWh.8 In particular, for the oil refining step the range of water footprint has been reported as 5-10% of this total; the crude recovery and stabilisation steps, along with carbon dioxide considerations, contribute most of the difference towards the total.
Industrial facilities located near oceans or saline aquifers around the world are relying more on desalination technology to provide freshwater. Industrial facilities are also facing stricter discharge regulations that can favour development of non-conventional water resources, such as wastewater effluent and produced water. Therefore, innovations in technologies that improve wastewater treatment to meet requirements for reuse may ultimately lower costs and allow for production expansion without the need to secure more freshwater or increase wastewater treatment capacity.
Seawater desalination and wastewater reuse are now common in coastal regions that experience freshwater shortages. This aticle reviews a recent application of desalination and industrial wastewater reuse utilising membrane filtration (MF) and reverse osmosis (RO). This discussion describes the methodology used in designing an integrated freshwater production plant with a wastewater reuse unit at a large grassroots industrial facility in the Middle East. The characteristics of the treated wastewater and reuse requirements and their impact on the overall system are discussed. Production of potable water by RO is not considered in this article.
Although the energy costs associated with desalination remain a concern, advances in system design, energy recovery and operating knowledge have resulted in lower overall costs. The cost for implementing desalination technologies is site specific. For a large desalination plant with a capacity over 100000 m3/day, which is typical for a city or a large industrial application, the cost per cubic metre is inversely proportional to the production capacity. The economic considerations discussed in this article compare the cost of increasing seawater desalination capacity by RO technology to installing and operating a membrane bioreactor (MBR) based wastewater reuse treatment system capable of producing plant utility water. Included in the cost comparison are a wastewater treatment process designed only for ocean discharge and an upgraded system designed for feed to a post-treatment effluent reuse system, such as an RO, that utilises an MBR unit as the biological treatment step.
Integrating wastewater reuse into an industrial facility
Meeting industry’s need for water will require an evolution from the common linear approach to total water management. Closed loop systems that recycle water, and integrate water, storm water and wastewater elements, can offer superior environmental performance.9 Also, integrated closed loop systems (see Figure 3) have the potential to satisfy the economic and environmental goals of sustainability by recovering a resource such as low salinity water through reuse instead of being wasted.
Figure 3 illustrates a closed loop system for an industrial facility that primarily sources water needs from seawater desalination. Note that the treated water supply is segregated into specialty water and utility water users, such as process, fire water and cooling water supply. Overall, condensed atmospheric moisture from the site’s dedicated air separation unit combined with plant storm water run-off supply less than 3% of the fresh water needs; however, these water sources offer the lowest total dissolved solids (TDS) options available. Though it is a small portion of the overall plant capacity, this low salinity source helps improve the overall cost structure of the facility.
Industry has typically viewed wastewater as a compliance issue rather than a strategic resource. The end point of conventional wastewater treatment has been the production of a treated wastewater effluent suitable for discharge to the environment.10 The main advantage of wastewater reuse (see Figure 3) is a direct offset of the amount of seawater needed to be desalinated, since the total dissolved solids content of wastewater (~1000 mg/l) is much lower than that of seawater (35 000 to 45 000 mg/l). A wastewater treatment plant (WWTP) in a reuse scheme is subject to a two-tiered treatment specification requirement. First, the treated effluent quality must be suitable for direct use as feed to other technologies, such as an RO unit. In addition, the treated wastewater must meet ‘discharge to coastal water’ specific performance standards. It is arguable which aspect will define the final design of the wastewater system. In reuse applications, parameters that are not overly restrictive (such as TSS, disinfection, dissolved hydrocarbons) in discharge scenarios can significantly impact the performance of the reuse facility. Likewise, the flexibility of the reuse system can address a wide variety of waste stream parameters (high salinity, specific constituent excursions such as heavy metals), which would otherwise be problematic for discharge to coastal waters.
When undertaking such a system design, certain factors must be considered. In the event of a treatment upset at the WWTP, depending on the extent of disruption and the duration, the WWTP effluent may need to be detained on-site or discharged to the seawater outfall if circumstances permit. Also, process upsets upstream of the WWTP in a reuse scheme could have a direct economic impact on water sourcing costs as well as increased treatment and environmental compliance costs. For instance, streams that are mostly oil should be collected separately from the oily water sewer system and managed outside the WWTP. Also, the repeated recycling of water through an RO unit may result in the build-up of trace constituents, such as silica, some volatile organic compounds and boron, which must be managed to maintain the quality of the water for its intended uses. As such, a more robust or redundant treatment system should be considered and additional discipline on managing a site sewer system would be warranted. The WWTP effluent must maintain a delicate balance to accommodate the most restrictive of these requirements under a wide variety of operating and upset conditions.
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