Nitrogen-Spewing Cesspools Give Onsite Treatment a Bad Name

Antiquated seepage pits dominating New York’s Long Island unfairly give septic systems a black eye. Here’s how to slow damaging nitrogen loading.
Nitrogen-Spewing Cesspools Give Onsite Treatment a Bad Name
Jim Anderson

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About a year ago, I explained why all Pumper readers should be aware of efforts in the Chesapeake Bay region looking at septic system contributions to elevated nitrate-nitrogen levels in estuaries and evaluating nitrogen reduction strategies.

Then, a few months ago, an article appeared about nitrogen causing reduction in salt marsh grass and associated environmental problems in the waters around Long Island, New York. The bottom line was that septic systems were identified as the major contributor to these elevated nitrogen levels. The author had the following to say in the opening paragraphs:

“Even properly functioning systems are flawed. Basic septic units are not designed to remove nitrogen. When too many households with septic are clustered too densely, nitrogen levels spike. Converted into ammonium in the tank and then nitrate in the soil, nitrogen seeps into groundwater and ends up in rivers, lakes and bays, where it feeds a menagerie of algae species. The algae, in turn, starve the water of oxygen, kill fish, and turn clear water — an asset for tourism, recreation, commercial fishing, health and property values — into a putrid mess.”

THE REST OF THE STORY

This statement would seem to provide a clear condemnation of the systems we work on. But as is often the case, it does not provide the complete story. In the story there is reference to cesspool systems. This has been the predominant system used in Long Island. Cesspools, as you recall, discharge raw sewage directly into a pit. These are usually somewhat deep, which in Long Island in general puts them near or in contact with the shallow surface aquifers.

Nitrogen in raw sewage is predominantly in the ammonium or organic form, and the deep pits provide little ability to nitrify the nitrogen to the nitrate form. So there is no denitrification to convert nitrate to nitrogen gas, which would reduce the levels of nitrogen moving into the aquifers. Instead, as the nitrogen moves out of the cesspool, it does become nitrified to the nitrate form without much reduction. So, from this perspective the article provides an accurate picture of what can happen.

What can be done to address the problem? In many places across the country, the first approach was to provide a septic tank in front of the cesspool. This makes the system a seepage pit by definition. Here, septic tank effluent is delivered to the pit. The larger solids and some organic matter have been reduced because they are contained in the tank. But in terms of other treatment — pathogens, nitrogen, etc. — there is not much improvement.

CESSPOOLS OUTLAWED

The Chesapeake Bay Group assigned a 5 percent reduction in nitrogen in the septic tank largely because it is assumed that the tank is periodically cleaned, taking some organic nitrogen out of the system. This is probably generous, so not much of a treatment advantage. This is why the State of Minnesota in 1978 labeled cesspools as failing systems and banned them from new construction, and when discovered, required replacement with another system. It is also why cesspools and seepage pits are properly labeled “disposal systems” and not treatment systems.

The next system in continuing sequence is to have septic tank effluent flow by gravity or pressure to a series of treatment trenches. (I skipped over beds because they are less efficient.) As the effluent moves out of the trenches into the soil, the ammonium is converted to nitrate in the presence of oxygen and soil bacteria. Once the conversion has occurred, the nitrate is free to move with the water through the soil and into the aquifer.

As the effluent moves out of the trenches, biomat is formed, tying up some of the organic nitrogen. This leads to about a 20 percent reduction in nitrogen; so not spectacular but definitely an improvement if all those cesspools and seepage pits used this simple technology. In addition, there is an opportunity for additional reductions because the trenches are much shallower in the soil, where there is biological activity and the opportunity for dilution of the nitrogen with enough area around the system. This works well as long as the system density is not too high, a luxury we have in less-populated Minnesota and Wisconsin.

My one trip to Long Island was eye-opening to say the least! More than 3 million people and all of the infrastructure that goes with it. Converting seepage pits and cesspools to trench or mound systems is not realistic. Mounds, by the way, get credit for about a 50 percent reduction in nitrogen. This means additional pretreatment through the use of recirculating media filters, constructed wetlands or ATUs. To Long Island’s credit, it is looking at these technologies and promoting their use. When properly installed and maintained, they can reduce nitrogen inputs by up to two-thirds. This would significantly reduce onsite system contribution to the nitrogen problem.

GET WITH THE PROGRAM

These technologies must be properly used and maintained. This is where our industry comes in, to make sure they are operating correctly and providing the desired treatment. Without proper observation, the systems will do no better job of treatment than the cesspool or seepage pit over time. A question to ask yourself as a service provider: Are you up to speed enough on these systems that you can be part of a solution if nitrogen problems emerge in your area?



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