
DATE: Sunday, October 5, 1997               TAG: 9710050067

SECTION: LOCAL                   PAGE: B1   EDITION: FINAL 

SOURCE: BY LIZ SZABO, STAFF WRITER 
DATELINE: CHESAPEAKE                        LENGTH:  195 lines

CHESAPEAKE SOON TO GET PRICEY WATER

Tired of tap water that tastes like Alka-Seltzer?

Relief is in sight.

A long-awaited upgrade to Chesapeake's Northwest River Water Treatment Plant, which will filter out the chloride and salty taste from Chesapeake water, is 70 percent complete.

The new reverse-osmosis water treatment system at the plant will be the first of its kind in Virginia, according to J.K. Walski, assistant director of public utilities.

The $64 million water plant is one of the largest public works projects Chesapeake has ever undertaken. Its cost is nearly one-third the $150 million cost for the entire Lake Gaston pipeline, and projects like the $16.5 million City Hall and the $34 million Hickory High School seem small by comparison.

The purpose of the plant is to ensure that the city's water meets federal Safe Drinking Water Act requirements, Walski said. But the changes will have the added benefit of eliminating the periodic bouts with high chloride and sodium levels that have plagued users of Northwest River water for years during droughts.

The city is in the throes of such a problem. On Friday, the water from the Northwest River plant measured 382 parts per million of chlorides and 191 parts per million of sodium. The federal taste threshold for chloride is 250 parts per million, and people with sodium-restricted diets should be be concerned about levels of sodium higher than 20 parts per million.

So the new plant will be open none too soon to suit Chesapeake residents, who have had to buy bottled water for their tea and coffee to avoid the salty taste.

How it will work

Scientists define osmosis as the movement of liquid from one solution through a membrane into a more concentrated solution.

Normally, in separating fresh water from sea water by a semi-permeable membrane - one that allows some but not all molecules to pass through - the fresher water will flow through the membrane until the concentrations on each side are equal.

Chesapeake's water treatment plant will reverse this process - forcing fresh water out of a salt-water solution and into a collection area, leaving salt, bacteria and other unwanted particles behind.

Pumps will force the Northwest River's brackish water through special filters at extremely high pressure: 500 pounds per square inch (psi) for ground water; 600 psi for river water. The plant uses five 800-horsepower pumps and three 700-horsepower pumps to create that kind of water pressure, Walski said. The pressure is strong enough to overcome osmotic pressure, sending fresh water through the membrane and leaving a dense, briny solution on the other side.

Reverse osmosis is only one of five treatment processes that will be used at the Northwest River Treatment plant. River water will undergo three stages before it even gets to the reverse-osmosis membranes. Removing some of the largest particles from the water prevents them from clogging the reverse osmosis membrane's tiny pores.

First, the water is simply allowed to settle in a large tank. With the assistance of some chemicals, gravity will cause the largest particles to settle at the bottom of the tank.

Second, the somewhat purer water is run through a traditional sand filter. Water flows through tiny spaces between grains of sand. The sand catches particles that are too light to settle out on their own from the force of gravity - such as dirt, pollen and even some large bacteria, Walski said.

Third, the water is run through a cartridge filter, contained in a large tank that looks something like a water heater. These filters are made of thin, tightly wound polypropylene fibers that are knit together somewhat like cheese cloth. This stage removes smaller bacteria, colloids and viruses.

Fourth, the water enters the reverse-osmosis membranes, which are housed in long, tubular fiberglass pressure vessels that are capped at the end. These pressure vessels are stacked row upon row at the water-treatment plant, in a building the size of a football field.

Last, the purified water is disinfected with a chlorine solution, Walski said. It's then sent to storage, from where it can be pumped out and distributed when needed.

The reverse-osmosis membrane, of course, is no ordinary filter. The filter membrane itself is built like a jelly roll: On the outside, there's a fabric coated with cellulose acetate; in the middle, a plasticized knit fabric; on the inside, another sheet of cellulose-acetate coated fabric.

The three layers are then rolled into a cylinder. The membranes are wound around a perforated, hollow tube with a loose mesh called a brine spacer inserted between envelopes.

The pressurized river water flows across the membrane surface of the cellulose acetate, forcing water to diffuse through the pores in the membrane. The process leaves larger salt ions, as well as salt-laden water molecules, behind.

The membrane's pores are microscopically small - only 1 to 10 angstroms wide. An angstrom is a unit of length equal to one hundred millionth of a centimeter.

The pores are even small enough to filter out germs and individual chloride ions, electrically charged atoms or groups of atoms.

Clean water then flows into the hollow center of the tube, where it is trapped within the cellulose acetate envelope. It travels along miniscule grooves in the plasticized fabric toward the hollow tube at the center. There, the filtered water empties into the clean water tube through pores.

The filter doesn't extract all the fresh water rushing through the system. About 30 percent of the raw river water remains behind, with the salt and other impurities in a concentrated solution.

This briny solution is then routed through a 14-mile pipeline and discharged into the Southern Branch of the Elizabeth River near the Steel Bridge.

The discharged water actually has fewer chloride particles than the water in the river, Walski said. To prevent pollution, treatment plant managers also must make sure that any water dumped into the Elizabeth River meets federal environmental guidelines.

The reverse osmosis process thus only captures 70 percent of the 10 million gallons per day of raw water that will be treated at the plant, Walski said.

That comes to about 7 million gallons of water a day. Chesapeake water customers need at least 8 million gallons of water a day, he said.

To meet that demand, the treatment plant will draw 5 million gallons a day of groundwater from four deep wells built last year, Walski said. After treatment, that groundwater will give the city another 3 million gallons - a total of 10 million gallons of water a day.

This kind of technology doesn't come cheap.

Powering the reverse-osmosis plant will quadruple the water plant's electric bills, increasing the yearly cost by well more than $1 million, Walski said. The water treatment plant now spends $300,000 to $400,000 on electricity, said Karen Harr, water resources management administrator.

The price of construction of the reverse-osmosis facility: $64 million. It will cost an estimated additional $4.5 million a year to operate - 50 percent more than the plant's current operating and maintenance costs, Walski said.

Those expenses may raise water bills for customers, Walski said. How much will bills increase? That hasn't been determined yet, he said.

To save money, the water treatment plant will purify some of its water with a manganese removal contactor - a more traditional system using oxidation and high-pressure filter technology - when chloride levels are low, Walski said.

During the winter, river water may need to be heated before it can be sent through the reverse-osmosis membrane. Cold water is denser than warm water, so less of it would flow through the membrane into the freshwater pipeline. Ground water remains at a constant temperature and will not need to be heated.

``During production, we'd lose more water (to the brine solution) because it's `thicker' when it's cold,'' Walski said. ``We'll use a heat exchange to prevent that.''

What makes Chesapeake water so salty in the first place?

Blame it on the rain - or lack thereof.

The Northwest River flows southeast into the salty waters of North Carolina's Currituck Sound and into the Atlantic Ocean. Those salty water bodies occasionally drift up river into our water area, said Amar Dwarkanath, director of public utilities.

Everything's fine as long as Chesapeake gets enough rain. Then, rain and groundwater runoff fill the Elizabeth River with enough fresh water to keep the salty waters at bay, he said.

Strong winds from the southeast, however, can blow briny water into the Elizabeth River, Dwarkanath said. Lack of rain also depletes the force of the Elizabeth River's flow, allowing salty water to creep in.

Not everyone in Chesapeake receives water from the Northwest River.

Approximately 71,000 Chesapeake residents, including all in Greenbrier and Great Bridge, receive the salty Northwest River water, according to the department of public utilities.

More than 10 percent of the city receives a mixture of that water and reserves that aren't salty, and half of Chesapeake's 193,000 residents are on Norfolk or Portsmouth water, which has no salt problems. ILLUSTRATION: Color photo

MORT FRYMAN/The Virginian-Pilot

Big pipes carry water through stages of purification.

Graphic

Why relief is needed:

High chloride and sodium levels have plagued users of the Northwest

River water for years during periods of drought.

The solution:

New reverse osmosis water treatment system will help ensure the city

meets federal Safe Drinking Water Act requirements.

How much it will cost:

Construction will cost $64 million. Annual operation costs will be

$4.5 million, 50 percent more than the current yearly budget.