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Striking a Balance Between Nutrient Removal and Sustainability

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The implementation on increasingly stringent nutrient discharge permits, namely nitrogen and phosphorus, has largely focused on receiving water quality and has ignored the corresponding impact on sustainability from treatment. This report was prepared to provide a bench-top analysis on the balance between nutrient removal and sustainability. The objective is to determine if a point of “diminishing returns” is reached where the sustainability impacts of increased levels of nutrient removal outweigh the benefits of improved water quality. Five different hypothetical treatment trains at a nominal 10 mgd flow were developed with variable treatment objectives: Level 1 (base plant; 30 mg/L BOD; 30 mg/L TSS), Level 2 (8 mg N/L; 1 mg P/L), Level 3 (4–8 mg N/L; 0.1–0.3 mg P/L), Level 4 (3 mg N/L; 0.1 mg P/L), and Level 5 (<2 mg N/L; <0.05 mg P/L). Given that sustainability is a broad term, emphasis was placed on the following parameters while comparing the treatment levels: GHG emissions, a water quality surrogate, capital and operational costs, energy demand, and consumables (e.g., such as chemicals, gas, diesel, etc.)

The parameters were evaluated independently. The results suggest that a point of diminishing return was reached for all sustainability parameters at Level 4 (3 mg N/L; 0.1 mg P/l) or greater. The GHG values ranged from 1,850 to 10,220 CO2 equivalent metric tonnes per year as follows: Level 1 (1,850), Level 2 (3580), Level 3 (3960), Level 4 (4750), and Level 5 (10,220). The doubling from Level 4 to 5 is attributed to sidestream reverse osmosis plus brine management required to meet the stringent level. The primary contributors to GHG are energy related (aeration, pumping, mixing) at upwards of 78 percent total GHG. The GHG increase from Level 3 onwards relates to chemicals demand (e.g., external carbon source, metal salt, and polymer) to compliment BNR. The water quality surrogate, algal production, showed that >90 percent potential algal savings is achievable for Level 3. To remove an additional 8 percent (Level 5) translates to a doubling of GHG. As for cost, the capital doubles from 8 million to 14 million when moving from Level 1 to 2, followed by a nearly tripling of cost when moving from Level 1 to 5. The operational cost discrepancy between levels is more pronounced than capital with a six-fold increase from Level 1 to 5 (190/MG treated to 1,200/MG treated, respectively). Rather than focus our attention strictly on point source dischargers and requiring Levels 4/5 treatment, a combination of Level 3 treatment complimented with best management practices on non-point sources might be a more sustainable approach at achieving comparable water quality.
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Keywords: Algae Surrogate; Greenhouse Gas; Sustainability; Triple Bottom Line

Document Type: Research Article

Publication date: 01 January 2011

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