The influence of plant parameters on effluent organic nitrogen was investigated for two advanced wastewater treatment plants in the Chesapeake Bay region. The study investigated the influence of process start-up, reactor configuration, solids retention time (SRT), temperature, chemical dosing and digester recycles. The study concluded that activated sludge process treatment efficiency, as defined by removal of degradable constituents, may have a dominant role in maintaining low concentrations of effluent organic nitrogen. Factors influencing treatment efficiency include SRT, temperature, reactor hydraulics (reactors-in parallel versus reactors-in-series) and plant perturbations. These same factors all impact effluent organic nitrogen. For example, longer SRT, reactors-in-series and higher temperatures all produce lower effluent organic nitrogen and are known to produce higher treatment efficiency. Plant perturbations also impacts treatment efficiency by increasing residual organic nitrogen concentration. It is still unclear what fraction of the organic nitrogen material is degradable within the activated sludge process and is a matter for further study.
Regions across North America are being regulated to limit of technology (LOT) total nitrogen levels of 4 mg/L or less. The basis for these low limits is to prevent eutrophication in nitrogen limiting receiving waters. In most of these regulated cases, the dissolved organic nitrogen (DON) component is included within the total nitrogen limits. As treatment plants are being designed to remove inorganic nitrogen to such low levels, organic nitrogen becomes a predominant fraction in treated effluents.
Many facilities are already being regulated for this constituent in their effluent TN permit limit, and many other facilities in Virginia are cumulatively spending in excess of $1 billion for upgrades to meet stringent total nitrogen effluent regulations. There are no conventional methods known to treat or maintain low DON levels in treated effluents. An ability to treat or influence this fraction during treatment would therefore be useful for operators and designers to minimize total nitrogen levels in their effluent.
This study is an evaluation of the impact of plant parameters on effluent organic nitrogen for two advanced wastewater treatment plants in the nitrogen limiting Chesapeake Bay drainage basin. Alexandria Sanitation Authority operates a 54 mgd advanced wastewater treatment plant using a step-feed nitrogen removal configuration with chemical phosphorus removal using iron and aluminum. The District of Columbia Water and Sewer Authority operates a 370 mgd advanced wastewater treatment plant using a two-sludge configuration. Both plants use filtration for particulate solids removal to maintain effluent total phosphorus limits. The effluents from these plants are therefore comprised of a smaller fraction of particulate organic nitrogen and a larger fraction of the dissolved organic nitrogen.
An evaluation of these two advanced plants provides a picture of some of the factors that may be used to control organic nitrogen during design and operations of wastewater treatment plants.
The objective of this study was to evaluate the influence of plant parameters on organic nitrogen, specifically:
- process start-up
- reactor configuration
- solids retention time (SRT)
- chemical dosing
- digester recycles