Water Journal : Water Journal September 2012-1
potable reuse refereed paper technical features 60 SEPTEMBER 2012 water than 20 years (e.g., membranes). Labour and annual repair and replacement costs were specifically not included to avoid errors often introduced when using percentages for these values. Power costs were estimated by calculating the equipment and building electrical draw, and applying a unit power cost of $0.056/ kWh. The cost for consumables (e.g. chemicals) was estimated based on the calculated annual average usage times a unit cost for each consumable. The capital and O&M costs included in this paper should only be used for comparison between the two specific IDWR treatment trains and should not be applied to any actual projects. The costs are considered accurate for comparison purposes, but could vary significantly in different locations of the world. In addition, the costs do not reflect actual costs at the full-scale plant since they have been adjusted to an equivalent flow for accurate comparison. The construction and annual operation and maintenance (O&M) costs are shown in Figures 3 and 4, respectively. Inspection of these figures reveals the following: • Construction costs: - The construction cost of the RO- based train is more than 50% higher than the GAC-based train ($109M versus $69M); - The membrane-based processes are much more costly than the filtration and GAC processes included in the GAC-based approach. • Annual operation and maintenance costs: - The annual O&M costs for the RO- based train are more than 350% higher than the GAC-based train ($3.74M versus $1.05M); - All three of the major unit processes included in the RO-based train (MF, RO and UVAOP) consume large amounts of power, which increases O&M costs; however, other significant cost items included with these processes are membrane replacement, chemicals (e.g. antiscalant, hydrogen peroxide), and lamp, sleeve and ballast replacement. - Chemical costs represent a large fraction of costs for both treatment trains. The most costly for the RO- based train is ferric chloride because of its 40mg/L average dose. The most costly for the GAC-based train is lime because of its 140 mg/L average dose. - GAC regeneration costs are based on a regeneration frequency that results in 12.5% GAC media replacement per year. This frequency could be increased for better organics removal if so desired. Figure 4 shows the annual O&M costs for an increased frequency of 100% GAC media replacement per year. As shown in the figure, the annual O&M costs are still significantly lower than the RO-based approach ($3.74M for RO-based approach and $1.77M for GAC-based approach). - The RO-based treatment train consumes large amounts of power and consequently its O&M costs are very dependent on the cost of electricity. An electricity rate of $0.056/kwh was used in this analysis, which represents an average industrial rate in the US. However, electricity rates vary significantly across the US and the world. For example, California and the New England states have rates as high as $0.09/kwh and $0.13/kwh, respectively. The O&M cost difference between the two trains would increase in these geographic locations. Greenhouse Gases Figure 5 presents the average greenhouse gases (GHG) for the RO-based and GAC- based approaches. As shown, the GAC approach is estimated to produce less than half the equivalent CO2 emissions than the RO approach. The basis for these calculations includes the power estimates for the electro-mechanical equipment, the regeneration energy required for the GAC, and the chemical manufacturing and delivery. As shown, there is a significant GHG sustainability impact (penalty) for the RO approach. RO facilities located inland would likely have even higher GHG emissions because of the difficulty in disposing of the RO concentrate. In addition, the water supply is decreased Chlorine Contact Basin Lamella Claifier Gravity Thickener Lamella Claifier GAC Pump Station Gravity Thickener Centrifuge Filters Lime Clarifier Chemicals Backwash Supply & Waste GAC Chemicals UV AOP MF RO GAC-Based RO-Based Centrifuge Rapid Mix Flocculation BreakTank Figure 3. Construction cost for 70MLD (18.5 mgd) indirect potable reuse plant. Hypo Centrifuge Centrifuge Hypo FeCl3 UV AOP Lime MF RO GAC Lime GAC Lime Centrifuge Hypo RO-Based GAC-Based GAC Regen Freq increased to 1x/yr Figure 4. Annual O&M costs.
Water Journal November 2012-1
Water Journal August 2012