When making decision about long-term capital investments, stakeholders typically consider traditional metrics, like cost and if the technology is validated in the industry. With building controversy regarding the effects of climate change and increased public interest in mitigation efforts in areas vulnerable to sea level rise, a new metric has emerged: greenhouse gas emissions.
This report was prepared to provide an analysis of potential secondary treatment options at the Sand Island Wastewater Treatment Plant (SIWWTP) in Honolulu, Hawaii, and a comparison between treatment levels and sustainability, with a focus on greenhouse gases. While the current federal administration has rolled-back progressive regulations on climate change and greenhouse gases, Hawaii is moving forward with the intention that climate change is not something political and must be addressed. Hawaii is especially vulnerable to the symptoms of a changing climate, including ocean warming and sea level rise. Per a 2018 Mayor’s Proclamation, Honolulu is committed to a 100 percent renewable energy future by 2045, which includes developing a community greenhouse gas inventory.
MME identified, assessed, and compared the carbon footprint of the existing baseline primary treatment facilities with the secondary treatment alternatives under consideration for implementation at the SIWWTP. The results helped to guide the selection of technologies for secondary treatment expansion of the plant, provide input on the selection of technologies for future process, and supported the environmental impact study.
This study used a design model run for the two secondary treatment options, step feed activated sludge (SFAS) and membrane bioreactor (MBR), based on the plant influent flow rate of 89 MGD.
The results showed that secondary treatment to the plant process will more than double the carbon emissions of the plant. Both the MBR and SFAS process require significantly more electricity than primary treatment only, resulting in additional emissions of almost 30,000 CO2e/yr., or approximately 40%.
One metric ton of CO2e is equivalent to the greenhouse gas emissions generated from driving 2,434 miles in an average car per year.
These results provided a metric for evaluating secondary treatment options, beyond just cost. Additionally, GHG emissions are a common topic that broadens the audience beyond scientists and engineers and makes it accessible to the general public, giving them an understanding behind selection of technology at the plant.
The wastewater treatment industry is well positioned to lead the way in reducing GHG emissions because of the beneficial uses of biogas, which can be used for onsite production of renewable energy.
Further research and reporting will result in encouraging utilities towards using co-gen and becoming energy-independent. The benefits of this are two-fold. First, they are raising capital by not having to buy power and selling excess to the grid, and additionally they are offsetting the costs associated with emissions.