Published on: May 27, 2026

Concentration vs. Load – Measuring Performance of On-Site Nitrogen Reducing Systems

Written by MASSTC Project Assistant, Bryan Horsley,

Our many bays, estuaries, and coastal waters around Cape Cod are polluted with excess nitrogen, mostly from domestic wastewater. Efforts to quantify that pollution and pinpoint its sources were conducted as part of the Massachusetts Estuaries Project (MEP), which was a collaborative nitrogen modeling effort by MassDEP, UMASS Dartmouth, and the towns in which their work was focused.  

Today, towns across the Cape rely on the MEP reports to guide their efforts to reduce nitrogen pollution and restore water quality. Each impaired estuary was assigned a Total Maximum Daily Load (TMDL) based on the MEP reports, which is the maximum amount of nitrogen an estuary can receive daily while still maintaining acceptable water quality. Those TMDLs are communicated in units of mass load, i.e. kilograms or pounds (Concentration (milligrams per liter) X Volume (liters) = Load (kilograms). A town wanting to reduce nitrogen pollution can then measure what the existing daily load of nitrogen is to a particular estuary, subtract the TMDL for that estuary and calculate how much they need to reduce the daily nitrogen load to restore water quality.

Simple enough right?

The TMDL approach makes sense in that it provides a clear target and a known quantity of nitrogen to be removed. Towns are now using this approach to quantify nitrogen reductions needed to restore water quality and are using a variety of tools to do so. Many towns are beginning to require installation of advanced nitrogen reducing (NR) onsite septic systems at various regulatory triggers (home sales, addition of bedrooms, etc.) to help reduce nitrogen pollution and to meet TMDL targets.

When measuring performance of NR septic systems samples are collected at the outlet of the system (before the treated water is disposed of through the soil absorption system or leaching field) and analyzed for concentration of nitrogen in milligrams per liter (mg/L). Based on MassDEP’s analysis of Best Available Nitrogen Reducing Technologies (the BANRT list) the targeted nitrogen concentration emitted from best performing NR septic systems is 10 mg/L or less. Town watershed planners can then use that concentration value to calculate estimated nitrogen load reductions and progress toward their TMDL targets.

Using concentration as a measure of performance becomes problematic when we consider using alternative toilet systems to reduce nitrogen pollution. Alternative toilet systems are another tool that can be used to reduce nitrogen pollution and include things like composting toilets, urine diverting toilets, and incinerating toilets.

These alternative toilet systems can reduce domestic nitrogen pollution by 90% or greater, but they also reduce water usage substantially, which creates an issue when comparing their performance with NR septic systems. Despite significantly reducing the nitrogen load (kg) it’s likely that the concentration (mg/L) would not show that same reduction because of the reduction in water usage.

Excessive water usage effectively dilutes the amount of nitrogen leaving a building and results in a lower concentration measurement. On the other hand, water conservation and reduced water usage (as expected with waterless and micro-flushed alternative toilet systems) would concentrate the amount of nitrogen leaving a building and result in a higher concentration measurement despite that the load may be significantly reduced. For this reason, it is essential to use load to measure the true performance of alternative toilet systems. Since many towns see alternative toilet systems as a potential alternative option to NR septic systems it is also important that those NR septic systems are also measured using load so they can be equitably compared with the alternative toilet systems.