Wastewater Treatment Nutrient Removal and Energy/GHG Nexus

The implementation of increasingly stringent nutrient discharge permits, namely nitrogen and phosphorus, has largely focused on receiving water quality and has ignored the corresponding impact on treatment sustainability. 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; <0.02 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 that reflects potential algal growth, 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 emissions values ranged from 4,260 to 12,950 CO2 equivalent metric tonnes per year (CO2 eq mt/yr) as follows: Level 1 (4,260), Level 2 (5,600), Level 3 (6,600), Level 4 (7,580), and Level 5 (12,950). The 70 percent increase from Level 4 to 5 is attributed to sidestream reverse osmosis (RO) plus RO reject management required to meet the stringent level. The primary contributors to GHG emissions are energy related (aeration, pumping, mixing) at upwards of 65 percent total GHG emissions. The GHG emissions increase from Level 3 onwards relates to chemicals demand (e.g., external carbon source, metal salt, and polymer) to compliment biological limitations and filtration requirements. The water quality surrogate, algal production, showed that 95 percent potential algal savings is achievable for from Level 1 to 3. To remove an additional 4 percent (total 99 percent removal with respect to Level 1) translates to a nearly doubling of GHG emissions (Level 3 to 5). As for cost, the capital increases 70 percent from 7.9 million to 13.3 million 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 600 percent increase from Level 1 to 5 (222/MG treated to 1,365/MG treated, respectively). Rather than focus our attention strictly on point source dischargers that require 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.