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Nitrogen in Sewage Systems: The Chemical Element & Human Impact To Fresh Water

Why the concern over a natural element, one that is critical to plant and animal life? There are three related reasons: sewage favours the formation of nitrate, excessive nitrate has negative effects on aquatic environments, and a high concentration of nitrate in drinking water has potential negative human health consequences.
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Nitrogen: The Element

This article is about nitrogen, a universal element and an essential component of the earth’s makeup and of all living things. I’ll discuss how septic systems play an important role in mitigating pollution by some forms of nitrogen, particularly nitrate because nitrate pollution can adversely affect both environmental health and human health.

It’s best if we start with a little bit of chemistry. “Chemistry! Oh no!” you say? But you are a curious person; after all, you’re reading this blog post about septic systems. So, I promise that this chemistry lesson will be painless, and might even leave you with something interesting to chat about the next time you’re standing in line at the grocery store. And it will certainly leave you understanding the importance of good septic system design and installation.

Nitrogen is a very cosmopolitan chemical element; it occurs in many forms in the physical environment, notably the nitrogen gas that makes up about 78% of the atmosphere. It also combines with oxygen to form oxides and ionic molecules like nitrate and nitrite, and with hydrogen to form ammonia. 

Nitrogen is also a major component of the complex bio-molecules that make up all living things. Organic nitrogen is a key component of proteins, amino acids and DNA – the very stuff that makes a human or a fish or a tree. Plants need nitrogen to grow – that’s why it’s a component of fertilizers. We and all animals need nitrogen to grow too – we get it from the food we eat.

While the many forms of nitrogen are “all natural,” the overall balance of these forms is critical to a healthy environment, and in turn human health.

In previous posts, we’ve learned about the water cycle – how water moves and transforms throughout the environment, like groundwater, in lakes and rivers, the oceans, clouds, rain and the water contained in all living things. Nitrogen, like all elements, cycles through the environment too, and it is constantly transitioning between its many organic and inorganic forms.

Nitrogen Process

The nitrogen cycle has been carefully characterized by scientists and is well understood because it’s so important to understanding healthy, balanced biological and environmental systems. Nitrogen, along with carbon, hydrogen and oxygen, is by far the main elements that make up all living things, from humans and trees to sea slugs and the bacteria that live on sea slugs’ skin. But the main source of nitrogen in the environment is atmospheric nitrogen gas, which is inert – not useable by living things! Uh-oh. But, not to worry – nitrogen fixation is the process of converting nitrogen gas (N2) to ammonia and other compounds, which are useable by living things. Bacteria in soil do the main job of nitrogen fixation, although lighting does a good job of “fixing” nitrogen in the atmosphere, too.

In soil, nitrification converts the ammonia (or ammonium, when it’s in a charged state) to nitrites and then to nitrates, and it’s the nitrates that plants use as food, converting the nitrate into organic bio-molecules.

In addition to atmospheric deposition, nitrogen enters the environment as waste from living things and from the decomposition of dead plants & animals. Animals excrete organic nitrogen (urea, uric acid and creatinine) into the environment, and when they die, through decomposition release these and other forms of organic nitrogen. The decomposition of plants also returns organic nitrogen to the soil.

Organic Nitrogen and Inorganic Nitrogen

Organic nitrogen is converted, mainly by bacteria, to ammonium through a process called mineralization. This process is very important because plants can only use inorganic nitrogen – such as ammonia and nitrate – as “food,” so the organic nitrogen that enters the environment from living (or formerly living) things is not useable by plants. At least, not until soil bacteria have worked it into inorganic forms through mineralization and then nitrification.

(In organic and subsistence farming, urine and feces or manure are considered cheap and effective fertilizers. Urine is also useful for leather tanning, making gunpowder and dyeing fabrics, though thankfully these are historical uses!)

Nitrate in soil that is not taken up by plants eventually is released to the atmosphere as nitrogen and nitrous oxide gases. Atmospheric nitrogen can return to soil and surface water, and all forms of soil nitrogen can make their way to groundwater and surface water.

So this sums up the basics of the nitrogen cycle. Now comes the important part – nitrogen is a big part of the animal waste, meaning that it’s a big part of our waste, meaning there’s probably a lot of it churning through our septic systems. How much? As a point of reference, Environment Canada estimated that in the late 1990s, approximately 80,000 metric tons of nitrogen were released annually into Canadian surface waters, just from municipal waste! The US EPA has estimated that about 11 grams of nitrogen per person per day is a typical septic input. That’s about 16 kg annually for a family of four, enough to potentially impact the local environment. The other major sources of nitrogen from human activities are chemical fertilizers and farm animal manure.

But, why the concern over a natural element, one that is critical to plant and animal life? There are three related reasons: sewage favours the formation of nitrate, excessive nitrate has negative effects on aquatic environments, and a high concentration of nitrate in drinking water has potential negative human health consequences.

Sources of Nitrogen in Wastewater

Septic systems collect organic nitrogen: urea, uric acid and creatinine from urine and feces, and other organic forms from food waste. In the septic tank and in the leaching field, organic nitrogen is mineralized to ammonia and ammonium, which in turn is nitrified to nitrite and nitrate.

nitrogen and septic systems

Nitrate is pretty stable, very water soluble and the most common form of nitrogen found in water. So, nitrate formed in the environment following agricultural application of fertilizers and leaching of septic effluent can easily reach surface water, either from overland flow or from leaching through the soil to groundwater that then feeds surface water. (When there are many sources of pollution, such as a number of agricultural fields or a number of septic systems, impacting a water body, this is called “non-point source pollution.”)

Just as nitrate is food for agricultural crops, it is also food for aquatic plants and algae. Excess nitrate in surface water (along with phosphorous and other nutrients) can lead to explosive growth of algae, and this leads to extreme fluctuations in dissolved oxygen. Even though algae photosynthesize and add oxygen to the water, aquatic bacteria feast on all that algae and so use up oxygen, leading to anoxic conditions.

How Does Eutrophication Occur?

When excess nutrients lead to excessive growth, the process is called eutrophication and results in severely diminished water quality. Although the aesthetics of turbid, green water are unsightly, the most significant impact of eutrophication is the inconsistent and depleted levels of dissolved oxygen that makes it difficult or impossible for fish and aquatic invertebrates to survive.

Nitrate is food for plants, but too much in surface water disturbs ecological balances and impairs the health of aquatic systems. Too much nitrate in drinking water is a major concern of public health and water system professionals because it can have negative effects on human health.

High Levels of Nitrogenous Waste in the Blood

Nitrogen in water,  the health effects: Likely you’ve never heard of “blue baby syndrome,” or methaemoglobinaemia, and that’s a good thing. It occurs when an infant is exposed to high levels of nitrate in drinking water. Nitrate interferes with how blood carries oxygen, and as the name suggests, it results in babies literally turning blue from a lack of oxygen. In extreme cases, methaemoglobinaemia can be fatal, and even adults who are exposed to unusually high levels of nitrite can experience decreases in blood pressure, increased heart rate, reduced ability of the blood to carry oxygen to tissues, headaches, abdominal cramps, vomiting, and death. So nitrate, even though it is a naturally-occurring compound, is on a list of closely monitored chemicals in drinking water.

In British Columbia, in natural water that is not impacted by agriculture or other human activity, nitrate levels are low, less than 1 milligram per litre (mg/L). Nitrate levels above this baseline are a reliable indicator of human impact on the water source, and the legal limit (the Maximum Acceptable Concentration or MAC) for nitrate is 45 mg/L. Nitrate concentrations above this level indicate a serious and potentially damaging problem – for the environment and for human health.

Nitrogen Loading From Septic Systems

Now, back to our septic system. We’ve learned in previous posts that a well-designed septic system provides suitable primary treatment in the septic tank (i.e., settling and bacterial decomposition of waste) and secondary treatment of tank effluent in the leaching field. The raw sewage entering the tank is mostly (about 75%) organic nitrogen and the rest is mostly ammonium.  

Remember that a septic tank is anaerobic (no oxygen), so anaerobic bacteria continue the mineralization of organic nitrogen to ammonium as part of primary treatment in the tank. The effluent leaving the tank and headed to the leaching field is about 70 to 90% ammonium, with the rest being unconverted organic nitrogen. What happens to all this nitrogen?

denitrification

The ideal fate of nitrogen waste is for it to be converted to inert nitrogen gas and returned to the atmosphere. In order for this conversion to take place, the organic nitrogen in septic waste has to be 1, mineralized (to ammonium), then 2, nitrified (to nitrite and then nitrate) and then 3, denitrified to nitrogen gas. Following so far?

Mineralization is done mainly in the septic tank by anaerobic bacteria.  Some ammonia can escape the tank as a gas. The rest stays in the effluent as ammonium.

Nitrification is done mainly in the upper, aerobic zones of the leaching field by aerobic bacteria. Some nitrate can be taken up by plants and used by bacteria in the leaching field. The rest stays in the treated effluent and heads down towards groundwater.

Denitrification by anaerobic bacteria happens mainly below the leaching field, in the anaerobic saturated zone above the water table. Nitrogen gas will eventually volatilize from the soil to the atmosphere. Nitrate that is not denitrified in the saturated zone enters groundwater and eventually surface water.

Inorganic forms of nitrogen (ammonia, nitrite, nitrate) are very water soluble. They don’t stick to soil particles, so that means they move quickly through the leaching field, leaving little time for bacteria to do their work in decomposition.

Nitrogen Reducing Septic System

Prevention is the best form of pollution control. A well-designed septic system controls the fate of nitrogen by incorporating sufficient capacity, regulation of effluent inputs and flow rates and site selection of the leaching field in relation to the saturated and unsaturated zones (that is, the anaerobic and aerobic zones). 

This allows the full range of nitrogen conversions to take place; mineralization, nitrification, denitrification. Although nitrate formation is a necessary part of the decomposition of nitrogenous waste, good septic design enhances denitrification to minimize the level of nitrate that reaches ground or surface water. Of course, all these considerations are a major design concern in municipal wastewater treatment plants, too.

Just as we understand the water cycle and our role in water conservation, it’s also important for us to understand and control our influence on the nitrogen cycle, to limit negative impacts on the environment and our own health.

There! You’ve had a bit of a chemistry lesson! I hope you’ve enjoyed it. 

 

 

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