Those nutrients encourage the growth of algae, which can deplete oxygen in the water needed by crabs, oysters, rockfish and other aquatic life, officials said.
The presence of phosphorous in bay tributaries along the lower Eastern Shore also contributed to the Pfiesteria problem that occurred this summer, said Maryland Department of Environment spokesman Quentin Banks.
The source of that phosphorous is still being debated, but some theories point to chicken manure runoff and manure from menhaden, a type of fish, he said.
Thomas said some people might question the need for the nutrient removal improvements since nutrients also get into the bay from runoff and storm water drains.
Now is the time to make these improvements because state grants are available if the city voluntarily does the work, Thomas said.
All publicly operated waste water treatment plants in Maryland that can treat at least 500,000 gallons a day are, at the very least, planning nutrient removal improvements, Banks said.
The improvements will help the state reach its goal of reducing the level of nutrients by 40 percent by 2000, Banks said.
The State of Maryland will contribute an estimated $5 million in grants for the city's improvements, although the city expects eventually to receive $6 million from the state, officials said.
Besides helping the bay and its tributaries, the nutrient removal project will help improve the city's treatment process.
A bypass will be built for ground water that leaks into collection pipes and enters the treatment plant. As a result of the bypass, the ground water will be only partially treated rather than going through unnecessary portions of the treatment process, Thomas said.
The work also will include adding some odor controls, he said.
Sewer customers will pay off the $7.6 million bond for the city's share of the nutrient removal improvements, Thomas said. Rates went up in January 1997 to help offset costs of general plant improvements and could go up again as soon as July 1998, he said.
One option is to increase sewer rates 5 percent a year for three years, Thomas said. City officials will discuss whether to raise rates and by how much during upcoming budget talks.
Service for the city sewer department's 15,035 customers will not be interrupted during the work, Thomas said. City sewer serves Hagerstown, Maugansville, Fountain Head, the Robinwood area and along Sharpsburg Pike.
The city made some cost-efficient improvements earlier this year that are already paying off, Thomas said.
In April, the city started using a new $419,000 oxygen ventilation system that will pay for itself in about two years, rather than the anticipated four years, he said.
The ventilation system reduced the amount of caustic soda needed to maintain the proper pH level, Thomas said. The city's caustic soda costs dropped 50 percent in 1997 to $196,087 and is expected to drop again in 1998 to $111,204, he said.
Two other rehabilitation projects costing $5.3 million could start in the next five years, Thomas said. Those projects involve replacing equipment that is about 18 years old.
Thomas said a plant expansion also is being considered, but could be delayed until there are enough customers to warrant it.
The plant, which now can treat 8 million gallons a day, could be expanded to treat 10.5 million gallons a day for $3.6 million, Thomas said. The city's average daily usage in 1997 was 6.8 million gallons a day.
If a business came to the area and needed 1 million gallons per day of sewage treated, the expansion could be speeded up, he said.
Another factor in deciding whether to expand is whether some of the sewage the city is treating will be diverted to Washington County's underused Conococheague Wastewater Treatment Plant west of Hagerstown, Thomas said.