In our last post, we explained how plastic media filters remove contaminants during secondary treatment. Today we’re going to talk about the next level of treatment, which removes ammonia through a process called tertiary nitrification. As you can see from the picture above, it’s probably not what you imagine when you think of water treatment!
Although ammonia is highly concentrated in urine in a form called “urea,” the concentration in the raw wastewater at Kline’s Island averaged only 17.7 parts per million in 2018. Ammonia can be toxic to fish and other aquatic life, and is strictly regulated by the federal EPA and Pennsylvania’s Department of Environmental Protection. It is removed through nitrification: a biological process in which the ammonia is converted to nitrite, and then to nitrate.
Some wastewater plants — including Kline’s Island — are using an efficient, tried and true treatment filtering method that’s over 100 years old: rock media trickling filters (RMTFs). In fact, the filters at the Kline’s Island Wastewater Treatment plant date from 1931 — and, like the plastic media trickling filters we talked about in our last post, they are covered in a zoolegal film that oxidizes the ammonia.
Here’s how it works: Clarified wastewater that’s been through secondary treatment is sprayed over more than five acres of rock media trickling filters — which is what’s going on in the picture above. The spraying serves two functions: It regulates the amount of wastewater that contacts the filter media, and it also increases the amount of dissolved oxygen in the wastewater. Without that oxygen, the bacteria couldn’t do their work.
The filter media consists of 1-inch to 3-inch graded quarry stone that’s 10 feet deep. As the water drips through the rocks, the zoogleal film attached to the rock uses the ammonia as food — oxidizing it first into a form of nitrogen called nitrite, and then turning it to nitrate.
In addition to bacteria, there are worms, snails, microscopic animals called rotifers, and protozoa that make a home on the rock media and also use the bacteria as a food source. The biological treatment processes are completed when the wastewater sprayed over the rock causes some of the organisms to slough off and be carried, by gravity, from the trickling filters to one of the 10 final settling tanks for removal.
Like the other settling processes at Kline’s Island, these final settling tanks also use gravity to clean the nitrifying bacteria and the microorganisms and macroinvertebrates from the nitrified wastewater.
The sludge goes to the gravity thickeners for further treatment, and the settled wastewater then continues by gravity to the final treatment step — the chemical disinfection process — before it’s released into the Lehigh River.
We’ll discuss this final disinfection process in an upcoming post.
For the rest in our Behind the Scenes series, please see: