Water. desalination + reuse

PROJECTS | 20 | Desalination & Water Reuse | November-December 2014 The snack foods project approach was to specify treatment components and a detailed, pre-selected treatment process for use in the RFP. Prior to developing the RFP, environmental staff evaluated and visited a different but similar type of production facility where they had third party pilot tests conducted for sizing and selection of the treatment components. The approach taken by the potato processing project's environmental and procurement staff was to specify only influent and effluent water quality in the RFP. They then left selection of the treatment process and components to the preselected and prequalified bidders. They hired an owner's engineer to help select the best proposal for the facility. Both approaches worked, however the pre-selected technology approach provided much more comfort to plant and corporate environmental staff and required significantly less evaluation of alternative technologies proposed by various qualified bidders. Common to both projects was a requirement for a performance warranty from the design and build firm. And both demanded one week of performance testing to demonstrate continuous daily attainment of water quality analytical limits for reuse. The selection of treatment technologies was dependent upon the influent water chemistry and constituents as well as the required reuse water quality and capital and operational costs. Effluent limits for the water to be reused were established at the snack foods facility as "must meet EPA primary and secondary drinking water standards." This was only slightly different to the "equivalent to EPA primary and secondary drinking water standards" limits adopted by the potato processor. As neither plant required human consumption quality from its reuse water, the "equivalent to" definition was related more to regulatory definition than actual water quality. However, for recycling or reuse of water in both food plants, the technology selection decisions were driven by other factors besides the water quality. These factors included: higher influent temperatures; the potential limit of ultrafiltration membranes used in the MBR process; and concerns about the RO membranes. And with temperatures nearing 38°C, the biological aeration systems had to be able to supply sufficient oxygen/ aeration at high water temperatures. This was exacerbated in the snack foods facility due to ambient desert temperatures encountered at its Arizona location. BiOlOgiCal TREaTmEnT Biological treatment involved activated sludge with biological nutrient removal. While phosphorus was not a limiting factor for the snack foods facility, higher concentrations of phosphorus in the potato process wastewater required biological removal of phosphorus in addition to the nitrogen treatment. Biological nitrification and denitrification was performed for both facilities by segmenting the aeration tanks into zones and controlling the dissolved oxygen concentrations. To address its phosphorus removal issue, the potato wastewater treatment used enhanced biological nutrient removal (EBNR). This involved not only control of phosphorus uptake from the water stream into biosolids, but also control of biosolids for removal of the phosphorus prior to re-release into the treated water flow. As a back-up, more redundant system, the addition of ferric chloride for chemical precipitation of phosphorus was implemented and installed at the potato processing facility. Parallel aeration systems were designed and built for each of the processing wastewater treatment plants so that aeration diffusers could be maintained and loadings adjusted by the plant operators. A common-wall construction was used for the potato plant's two aeration tanks to reduce construction costs. Surface wasting was included in the potato processing treatment system to counter the build-up of floating foam generated in some EBNR aeration tanks. Surface wasting enables plant operators to limit the build-up of surface floating solids and is a preferred option over wasting from the return activated sludge (RAS) piping. For blower applications there is a range of options and it largely comes down to client preferences and the characteristics of the site. At the snack food facility, positive displacement Aerzen blowers were constructed in an outside covered sunscreen canopy with weatherproof and external sound enclosures. This reduced costs compared to building an enclosure and sound insulation inside the building. Meanwhile, the five, 100-horsepower Neuros spiral blowers deployed at the potato processing facility run quietly enough to be located in a common room without hearing protection (figure 2). In both facilities the membrane bioreactor (MBR) filtration systems used GE Zenon ultrafilter membranes in different set ups. The snack food factory used removable, Vendor-prefabricated membrane racks in steel, prefabricated tanks sited above ground. The potato processing facility deployed custom-designed, concrete MBR tanks covered with aluminum decking and an overhead crane for removal (figure 3). Larger systems generally will use concrete tanks. Treatment technologies Similar treatment processes are utilized in the plants. They include: • starch recovery, screening, oil segregation and removal; • emergency spill control; • equalization; • pH adjustment; • primary clarifiers; • biological nutrient removal (BNR); • activated sludge in bioreactors; • membrane bioreactors (MBR); • reverse osmosis; • chlorine disinfection and water stabilization; and • chemical feed systems. Figure 1. Aeration tanks – the one on the left is being water-tested.

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