Water Innovations gives Water and Wastewater Engineers and end-users a venue to find project solutions and source valuable product information. We aim to educate the engineering and operations community on important issues and trends.
Issue link: http://wateronline.epubxp.com/i/672151
As part of its planning process, the Authority retained RETTEW, a firm specializing in environmental engineering, to evaluate, design, permit, and bid a cost-effective disinfection process upgrade featuring better control of biofouling, along with other water treatment advantages. Together, the partners selected a new disinfection process to reduce operational costs, improve disinfection performance, and replace the membranes. The previously existing biofouling control method consisted primarily of injecting chlorine followed by dechlorination, as well as the periodic application of a biocide in the water stream prior to entry into RO. Operational costs for the previous planning year are shown in Table 1. Advanced Oxidation Processes One method to control biofouling is AOP, which destroys contaminants by oxidation using reactions with hydroxyl radicals. These radicals are one of the strongest oxidants that can be applied in water and are 200 percent more effective in oxidation than sodium hypochlorite. Hydroxyl radicals can oxidize most compounds present in water, including organic compounds, trichloroethylene (TCE), endocrine disrupting compounds (EDCs), N-Nitrosodimethylamine (NDMA), pesticides, and pharmaceuticals. Hydroxyl radicals are generally produced using an oxidant with an energy source, such as the combination of ozone or hydrogen peroxide (H2O2) with UV light to produce highly reactive hydroxyl radicals. However, research indicates that ozone alone can provide AOP treatment when applied to secondary wastewater effluent. The ozone reacts with the organic matter in the secondary effluent, resulting in hydroxyl radicals. This reaction eliminates the need for the additional energy from UV light typically required to produce the hydroxyl radicals. AOP Alternatives Evaluation As part of the review of technologies for biofouling control, UAJA and RETTEW contacted various manufacturers that provide ozonation, UV disinfection, chlorine dioxide, or hydrogen peroxide for technical and economic considerations. RETTEW took samples from the microfiltration effluent and provided them to various manufacturers to test for effectiveness of each manufacturer's systems and to determine key performance indicators to dictate operational economics. As the primary goal was to provide pretreatment for RO, the limited application of hydrogen peroxide in this location did not provide sufficient experience to merit further consideration and was not included in the bench-scale testing. The process used heterotrophic plate counts removal as the key indicator of biofouling control. The bench-scale testing indicated that UV, ozone, and chlorine dioxide performed equivalent to the Authority's existing biofouling controls and could reduce the plate counts to nondetect. RETTEW then reviewed each alternative for implementation and economic impacts (see Table 2). All options were determined to be more cost-effective than the current treatment system. The Authority and RETTEW also considered the potential benefits of applying technologies that could also be modified to an AOP. While the PADEP currently does not require AOP for indirect potable reuse, national trends indicate it could be dictated in the future, as the method is quickly becoming a required technology in water reuse applications similar to the Authority's reuse system. Additionally, the U.S. EPA Water Reuse Guidance Manual suggests AOP for implementation on indirect potable reuse applications, which the Authority would ultimately pursue in later phases of its Beneficial Reuse Project. As AOP could provide this subsidiary benefit consistent with wateronline.com n Water Innovations 9 FILTRATION Parameter Baseline Conditions UV Ozone (A) Ozone (B) Chlorine Dioxide Equipment Capital Cost $275,000 $105,000 $405,000 $520,000 $143,000 Installation Cost $55,000 $124,000 $162,000 $162,000 $8,200 Total Capital Cost $330,000 $229,000 $567,000 $682,000 $152,000 Sodium Hydroxide (pH) Annual Cost $34,000 $34,000 $34,000 $34,000 $34,000 Salt (Dis) Annual Cost $14,000 - - - - Reverse Osmosis Clean-In-Place Annual Cost $10,000 $10,000 $10,000 $10,000 $10,000 Biological Control Annual Cost $262,000 $10,000 $10,000 $10,000 $10,000 Electricity Annual Cost $19,000 $4,800 $7,800 $8,000 $4,000 Other Consumables Annual Cost - - $10,500 $24,000 $6,000 Maintenance Annual Cost $9,100 $3,000 $8,500 $8,500 $29,000 Total Annual Cost $348,100 $61,800 $80,800 $94,500 $93,000 Total 20-Year Cost $7,292,000 $1,465,000 $2,183,000 $2,572,000 $2,012,000 Savings Over Baseline (Total) - $5,827,000 $5,109,000 $4,720,000 $5,280,000 Savings Over Baseline (Annualized) - $291,350 $255,450 $236,000 $264,000 Table 2. Opinion Of Probable Costs