Anaerobic Digestion at an ENR Plant: Implications for Energy, Carbon, and GHG

Many publically owned treatment works (POTWs) in the USA are increasingly required to comply with more and more stringent effluent nutrient criteria; in particular for nitrogen (either as ammonia or total nitrogen) and phosphorus. Enhanced nitrogen removal (ENR) plants are often required to produce effluent with total nitrogen levels near or below 3.0 mg/L.Recent congressional stimulus and Department of Energy funding opportunities have encouraged the production of renewable power from wastewater solids. One of the most proven methods has been anaerobic digestion with combined heat and power (CHP) for the produced digester gas (or “biogas”). One complication associated with the addition of digestion is the solubilization of nutrients under anaerobic conditions that were already removed by the POTW's liquid treatment processes. These nutrients are returned to the treatment process from dewatering processes that concentrate the solids for land application or other uses or disposal. The “recycle streams” that are returned in these situations usually constitute between 20 and 25 percent (and some times as high as 50 percent in special situations) of the total ammonia load to the plant.More recently, added emphasis has been placed on the greenhouse gas (GHG) emissions associated with operating these same POTWs. Treating nitrogen has a variety of potentially significant emissions, including:Scope 1 emissions of nitrous oxide (N2O) though nitrogen removal processes.Scope 1 N2O emissions associated with total nitrogen discharged to the receiving water, of which a small fraction evolves as N2O in the environment.Scope 2 carbon dioxide equivalent (CO2e) emissions from electricity used for treatment and/or offsets from renewable power that replaces currently-used, fossil-fuel-derived power.Scope 3 emissions for the manufacture of methanol that is used for denitrification.A final Scope 1 emission (that is not currently required to be reported under GHG reporting protocols: i.e. LGOP, GRP, IPCC) is the CO2 emission associated with respiration of aerobic treatment of anthropogenic carbon sources like methanol for denitrification. At present, neither the natural gas purveyor nor the methanol manufacturer take the GHG “hit” for un-sequestering the natural gas carbon used to manufacture methanol. As such, it may be prudent for the end user to account for this Scope 1 emission.This paper evaluates options for treatment of digestion recycle ammonia in the context of DC Water and Sewer Authority's (DC Water) Blue Plains Advanced Wastewater Treatment Plant (AWTP). The relative weight of each of the above factors on the AWTP's overall GHG emissions inventory will be presented.