The Wetland Ferrisdale Built: Polishing Effluent With Plants

The Wetland Ferrisdale Built

The last stage of treatment at the fictional Ferrisdale Water Reclamation Facility does not look like treatment. It looks like eleven acres of cattails.

Ferrisdale's plant does what a conventional secondary plant does. Screens take out the solids that arrive as solids. Aerated tanks give bacteria oxygen and time, and those bacteria consume the dissolved organic material that would otherwise consume oxygen in a stream. That consumption is measured as biochemical oxygen demand, usually shortened to BOD. Raw domestic sewage arriving at a plant typically carries a BOD somewhere in the range of about 100 to 300 milligrams per liter, with medium-strength sewage near 200. Federal secondary treatment standards require a plant to discharge water whose monthly average BOD and suspended solids sit at or below 30 milligrams per liter, and to remove at least 85 percent of each.

Ferrisdale met that standard for thirty years and still had a problem. The plant discharges into Penmarch Brook, a small stream with little flow in late summer. Water that satisfies a federal minimum can still be more than a small brook can absorb, and the town's own monitoring showed dissolved oxygen in the brook dropping in August toward the level below which most fish struggle, which is about 5 milligrams per liter. Nitrogen was the other concern. Secondary treatment converts much of the nitrogen in sewage into nitrate, and nitrate leaving a plant is fertilizer arriving in a stream.

The town could have built a tertiary treatment building. Instead it built a wetland.

Water leaving the secondary clarifiers now enters four shallow cells planted with cattail, bulrush, and sedge, and it takes about six days to cross them. Almost nothing in that sentence describes the actual treatment. The plants are not eating the pollution. What the plants provide is surface area and structure: stems, roots, and decaying litter, all of it colonized by films of bacteria. Those films do the work.

In the shallow, oxygen-rich water near the surface of the first two cells, one group of bacteria converts ammonia to nitrate, a process called nitrification. Deeper in the cells, in the sediments where oxygen runs out, a different group strips the oxygen atoms off that nitrate for their own respiration and releases nitrogen gas to the air. That second process is denitrification, and it is the only step in the sequence that removes nitrogen from the water rather than moving it from one form to another. A wetland is useful precisely because it holds both conditions a few inches apart.

The remaining organic material follows a simpler route. Slow, shallow flow lets fine particles settle where the stems break up the current, and the bacterial films consume what stays dissolved. Water leaving the fourth cell reaches Penmarch Brook with a BOD well below the federal ceiling and with a good deal of its nitrate gone.

The wetland is not free of trouble. It occupies eleven acres that the town cannot use for anything else, it takes six days rather than six hours, and its performance moves with the season, because the bacteria that do the work slow down when the water gets cold. The table below shows one year of monitoring at the inlet and outlet of the fictional Ferrisdale wetland.

All BOD and nitrate values are milligrams per liter.

Ferrisdale's operators plan around the winter column rather than apologize for it. Penmarch Brook runs high and cold in January, so the stream absorbs a weaker effluent more easily in exactly the season when the wetland works least well. The design does not perform evenly through the year, yet its strongest performance falls in the season when the brook can least absorb what the plant discharges.

1. According to the passage, what does biochemical oxygen demand measure, and why does it matter to a stream?

2. The passage states that the plant met the federal secondary standard for thirty years and still had a problem. Explain what that problem was, using two specific details from the passage.

3. The passage says the plants in the wetland "are not eating the pollution." Explain what the plants actually contribute, and identify what does the treating.

4. Using the table, describe what happens to nitrate removal between summer and winter, and explain the reason the passage gives for that change.

5. The passage argues that the wetland's uneven seasonal performance is acceptable. Using the table and the final paragraph together, state the reasoning and judge whether the evidence supports it.

6. Nitrification and denitrification both act on nitrogen. Using the passage, explain the difference between them and why only one of the two actually removes nitrogen from the water.

7. The passage opens with a short paragraph about eleven acres of cattails and returns to those eleven acres near the end. Explain what the writer accomplishes by repeating that detail in the two positions.

Writing task. In one paragraph of six to nine sentences, explain how water moves through the fictional Ferrisdale wetland and what happens to it at each stage. Name at least two values from the table, and make clear which stage removes nitrogen from the water rather than changing its form.