At various times, I have addressed potential issues with the discharge of nitrogen in septic tank effluent. There have been some recent discussions, developments, and questions about treating nitrogen, so I thought it would be a good time to look at the issues again.

In our industry, nitrogen is important for several reasons. There are concerns around nitrogen from the perspective of the potential for groundwater contamination and human health impacts, discharge to estuary and wetland environments and impacts on aquatic life and ecosystems, and as valuable plant nutrient when land-applied properly.

In septic tank effluent, about 75 percent of the nitrogen is in the ammonium (NH4+) form and about 25 percent in organic nitrogen. As the effluent moves into soil through the biomat, the organic fraction is degraded, resulting in the formation of additional NH4-nitrogen. For a time, ammonium can be held up on the soil exchange sites; the time varies depending on the soil capacity to adsorb the ammonium and whether aerobic conditions occur in the soil.

NITROGEN HEALTH RISKS

Soil capacity to absorb ammonium is increased with the presence of organic matter, such as the area of the biomat. When monitoring for nitrogen in the soil below the system, scientists may not see expected levels of nitrogen because until the exchange sites are filled, the ammonium will not continue to move. If the soil under the biomat is not aerated, the nitrogen will stay in the ammonium form. This does not mean the nitrogen is gone or reduced — just that it has not yet been nitrified to the nitrate-nitrogen state. These are also the reason for conflicting reports about nitrogen removal in systems.

The process for the conversion to nitrate (NO3-) is called nitrification, and it occurs relatively quickly as effluent moves into an aerated area of the soil, which is exactly what we want to occur beneath our soil dispersal trenches to help with removal of bacteria and viruses. Once nitrogen is in the nitrate form, it is highly soluble in water and moves with water through the aerated zone into groundwater.

Here is where nitrate becomes a health concern. If the groundwater is used as a drinking water source, elevated nitrate levels higher than 10 mg/L can pose health risks to infants drinking the water directly or in their formula. Nitrate interferes with oxygen movement in the bloodstream, which, if left unchecked can be fatal. In estuary and wetland environments, impacts on organisms are not as well documented. But it appears that effects in those environments begin to happen at lower levels — less than 3 mg/L.

Without anything else occurring, dilution as effluent moves through the soil into groundwater is the primary treatment mechanism. The largest risk from nitrogen moving into the groundwater would be in areas of dense population, in very uniform permeable soils and shallow groundwater systems.

REDUCING NITROGEN IN SYSTEMS

There are two potential pathways to capture and reduce nitrogen. One is through biological uptake, and the other is through the process of denitrification. These can occur naturally in and around our systems, or we can engineer or construct systems that take advantage of these processes to remove nitrogen from effluent before release to the environment.

Plants can remove significant amounts of nitrogen from soil. Nitrogen is, after all, an essential plant nutrient for growth. In shallow drainfield systems, this uptake or removal by plants is obvious where you see very green grass strips over the area of drainfield trenches. Uptake in most conventional systems is small because systems are often deeper than where the majority of plants roots exist. It is one of the major reasons for keeping systems shallow.

Take advantage of this uptake as much as possible. Constructed wetland systems have been demonstrated to remove 60 to 90 percent of nitrogen from wastewater using plants as a part of the system. Captured nitrogen ends up in the plants, which must be harvested periodically as a part of routine system maintenance to keep the high removal rates.

Denitrification is the process where nitrate-nitrogen is reduced to nitrogen gas and is released to the atmosphere. For this process to occur, the nitrates must pass through an aerobic environment into an anaerobic environment with a source of carbon or organic matter present. This can occur naturally in soils with different anaerobic zones and sufficient carbon for the reactions to occur. This is one of the many reasons tracking nitrogen from systems is difficult.

In sensitive areas, wellhead protection zones, densely populated areas with septic systems, and uniform sandy soils with shallow groundwater, additional treatment may be called for to ensure nitrogen levels are reduced. We see this happening in Florida, the Chesapeake Bay watershed, in Long Island, New York, and in California. The problem can be addressed by installing and maintaining systems that reduce nitrogen.

CONTINUAL RESEARCH

There are a number of these systems. Soil treatment mounds have been demonstrated to reduce nitrogen levels between 40 to 70 percent. Denitrification occurs as the effluent moves from the aerated sand below the pressure distribution network into the original soil that has a ready supply of organic matter in the former topsoil. Anaerobic sites are created in the soil and denitrification occurs.

Recirculating media filters can remove 40 to 60 percent of the nitrogen. This happens by aerating the effluent in the filter, running the effluent back through the anaerobic septic tank several times before final soil discharge.

Additional research is being conducted on passive approaches to nitrogen removal, such as trenches filled with wood chips or other organic material that effluent moves through following aeration. These methods have been used successfully to treat stormwater and animal waste.

We continue to have more options available as research continues. This is good for us in the industry; it is good for the environment; and it is good for our health!