How Microbes Remove Nutrients at Rushmere Reclamation

The Rushmere Water Reclamation Facility is a fictional plant that cleans the wastewater from a mid-sized town. Most of that cleaning is done not by chemicals but by microbes. Billions of bacteria live in the plant's tanks, and the operators' real job is to keep those bacteria fed, aerated, and healthy so they do the work.

The first target is organic waste. Its strength is measured as biochemical oxygen demand, or BOD, the amount of oxygen microbes need to break the waste down. Raw sewage arriving at Rushmere carries a BOD of roughly 200 to 300 milligrams per liter. Before the treated water may be released to the river, the plant must lower the monthly average to 30 milligrams per liter or less. Bacteria in an aerated tank consume most of that organic load, using oxygen to break it down, much as our own cells use oxygen to release energy from food.

Nitrogen is harder to remove, and it takes two opposite conditions. The first step is nitrification. In an oxygen-rich tank, one group of bacteria converts ammonia, a form of nitrogen that is toxic to fish, into nitrate. Nitrate is less harmful, but it is still a nutrient that can feed algae downstream. So a second step follows. In a tank kept free of oxygen, a different group of bacteria uses the nitrate to breathe. As they do, they strip the oxygen off it and release the nitrogen back to the air as harmless nitrogen gas. This step is called denitrification. Together, nitrification and denitrification can remove most of the nitrogen from the water.

Phosphorus is removed by taking advantage of how some bacteria store it. In a process known as enhanced biological phosphorus removal, these bacteria are first starved of oxygen and then given plenty of it. Under the right cycle of stress and feeding, these bacteria take up far more phosphorus than they need and store it inside their cells. When the plant removes the settled bacteria as sludge, the phosphorus leaves with them. No added chemical is required.

Why go to all this trouble? The answer is what happens when nutrients reach a lake. Nitrogen and phosphorus are fertilizers. In a lake, they fertilize algae, which grow into thick blooms. When the algae die, other bacteria decompose them, and that decomposition consumes oxygen. As the dissolved oxygen falls, fish begin to suffer, because most fish need dissolved oxygen above 5 milligrams per liter to survive. If the oxygen drops far enough, fish die and a dead zone forms. This chain of events is called eutrophication, and it is the main reason nutrient limits keep getting stricter.

Yet the same nutrients that harm a lake are valuable on a farm. Nitrogen and phosphorus are two of the main nutrients crops need, and phosphorus in particular is mined from a limited supply of rock. That is why some plants now recover phosphorus from their sludge and turn it into a slow-release fertilizer. A nutrient that was a pollutant in the river becomes a product on a field. The same is true of the nitrogen and energy in sludge, which can be captured rather than wasted.

Seen this way, the microbes at Rushmere are a kind of technology. They are a living treatment system that reduces the human impact of a town's waste on its river. Like any technology, they have limits. The bacteria are sensitive to temperature, to sudden slugs of industrial chemicals, and to swings in flow. Cold winters slow them down. A spill can kill them. Keeping the process stable takes skilled operators, steady monitoring, and a plant designed with enough tank volume to give the bacteria time to work. When those conditions are met, the microbes do a job that would be far more expensive to do with chemicals alone.

1. What happens to ammonia during nitrification, and what kind of condition does that step require?

2. During denitrification, what do the bacteria release, and is the tank oxygen-rich or oxygen-free?

3. According to the passage, what is the secondary-treatment limit for BOD, and roughly how strong is raw sewage by comparison?

4. Explain how extra nitrogen and phosphorus in a lake can lower dissolved oxygen and harm fish.

5. The passage calls nutrients both a pollutant and a resource. Give one detail that supports each label.

6. Explain why recovering phosphorus from sludge can be seen as turning a pollutant into a resource.

7. The microbes are described as a technological solution. State one impact they reduce and one limitation of relying on them.