Why Hollantree's Phosphorus Numbers Rise Every Spring

The Kestermill Water Reclamation Facility sits at the low end of Hollantree, a fictional town of about eleven thousand people, and sends its treated water to the Threlkeld River. Its permit is ordinary in most respects. One line in it is not. The permit allows a monthly average of 1.0 mg/L of total phosphorus in the water the plant discharges, and Kestermill meets that limit without adding a chemical to do it. Most of the year, bacteria do the work.

The process has a name that sounds more complicated than the idea behind it. Enhanced biological phosphorus removal relies on a group of microorganisms that store phosphorus inside their cells far beyond what they need to live. Operators call them phosphorus accumulators. Left alone in an ordinary tank, they have no reason to overstock. The plant has to give them one.

Ines Vardy, who has run Kestermill's process side for nine years, describes the trick as a controlled hardship followed by a controlled reward. The incoming wastewater first enters a tank with no oxygen in it. In that tank the accumulators cannot breathe in the ordinary sense, so they fall back on stored energy. They take in small carbon compounds called volatile fatty acids, which are what raw sewage produces as it sits and sours in the collection system. Paying for that uptake costs them phosphorus, which they release into the water. Anyone sampling that first tank finds phosphorus levels going up rather than down.

The reward comes next. The mixture moves into an aerated tank where blowers push air through the water. The accumulators now have oxygen and a load of stored carbon, and they use both to take phosphorus back out of the water. They take back more than they released. The phosphorus ends up locked inside the cells, and the cells settle out as sludge. Every day the plant wastes a portion of that sludge, hauling it away, and the phosphorus leaves the plant inside it. Vardy makes the point that a plant cannot remove phosphorus from water and keep all of its sludge. The phosphorus has to leave in something.

That system ran well at Kestermill for six years. Then came a spring the crew still refers to by its rainfall. Snow melted early, rain followed for three weeks, and groundwater found every crack in the town's old sewer pipes. Flow at the plant rose from about three million gallons a day to more than five. The extra water was not sewage. It was clean water leaking into a pipe built to carry sewage, and it diluted everything.

Toby Lenane, the facility's laboratory technician, saw the effect first in the influent numbers. Strength fell steadily while volume rose. By April the incoming wastewater was little more than half as strong as it had been in February. There were still plenty of accumulators in the tanks. What there was not was enough volatile fatty acid to feed them. Bacteria that cannot store carbon in the first tank have nothing to spend on phosphorus uptake in the second.

The effluent numbers followed. Total phosphorus in the discharge climbed month by month, and in March the monthly average went past the permit line, and in April it climbed higher still. Lenane put it plainly at the staff meeting. The plant did not have a bacteria problem; it had a groceries problem.

Two responses were on the table. The first was chemical. Adding a metal salt such as alum makes phosphorus fall out of the water as a solid, and the dose can be raised or lowered within an hour. It works in any weather. It also costs money every day it runs, and it produces additional sludge that the plant has to dewater and haul, which raises the disposal bill in the same month the chemical bill arrives.

The second response was biological. A fermenter holds a portion of the plant's own primary sludge in a warm tank without oxygen and lets it sour on purpose, producing volatile fatty acids that can be pumped forward to the first tank. It makes the plant's food supply out of the plant's own waste, and once built it costs little to run. It is also slow to respond, takes weeks to establish, and depends on having enough primary sludge to ferment, which a diluted influent does not always provide.

Hollantree's board approved both. The plant now keeps an alum feed available for the weeks when the river is high and the sewers are full, and it built the fermenter to carry the ordinary year. Vardy is careful about how she describes the result, since the alum feed is the part that never fails, while the fermenter is the part that removes most of the phosphorus in an ordinary year.

1. According to the passage, what do the phosphorus accumulators do in the tank without oxygen, and what do they do in the aerated tank?
2. Using the table, name the month with the highest plant flow and give that month's effluent total phosphorus.
3. Lenane says the plant had a groceries problem rather than a bacteria problem. Explain how the table supports that statement.
4. Explain why a plant cannot remove phosphorus from its water without also removing sludge.
5. Give one advantage and one disadvantage of the chemical response, and one advantage and one disadvantage of the fermenter.
6. Use context to explain what volatile fatty acids do in this process, and why the spring problem is described as a shortage rather than a poisoning.
7. Suppose a June arrives with flow near 3.0 MGD and influent BOD near 200 mg/L. Predict what the effluent total phosphorus would do, and support the prediction with two details.