Reverse Osmosis Treatment Facility

Background

Testing of Clara City’s drinking water during the early 1990’s began to show elevated levels of nitrite. These levels eventually exceeded the Environmental Protection Agency’s (EPA) Maximum Contaminant Level (MCL) of 1.0 mg/L for nitrite and the City was given an official Notice of Violation in 1994.

Elevated levels of nitrites are known to cause methemoglobinemia, otherwise known as "Blue Baby" syndrome. The nitrite causes a reaction which reduces the oxygen carrying capacity of blood in infants, creating oxygen deficiency indicated by blue lips and fingers, thus the "Blue baby" syndrome.

The City and its Consultants, Rodeberg & Berryman, Inc., did extensive research in trying to locate an alternative water source that would have lower concentrations of nitrite. Ten separate wells were drilled at various locations, none of which had sufficient quality or quantity for use, by the City.

The City also tried various methods of treating the existing water supply with chlorine in an effort to reduce the nitrite. None of these methods produced significant improvements. After exhausting all alternatives of treating the water with the existing treatment facility, and facing an upcoming mandate by the Minnesota Department of Health to submit a plan to remove the nitrite, the City began reviewing construction of a new treatment

facility.

Bolton & Menk, Inc. joined Rodeberg & Berryman, Inc. and reviewed treatment alternatives, finding that the best alternative was a new filtration and reverse osmosis (RO) facility. A new well, 200,000-gallon water tower and 20 blocks of water main replacement were also included with the project.

Construction began in the fall of 2001 with the treatment facility and water main improvements completed in the fall of 2002, and the tower was completed in the spring of 2003.

Description and Process

The Clara City Water Treatment Facility has a design capacity of approximately 350,000 gallons per day. The treatment process consists of two major processes, sand filtration and reverse osmosis treatment. The water is pumped from two wells located approximately three (3) miles west of the facility, and line backup well located at the facility.

The filtration process consists of a pressure aerator followed by pressure filters. The pressure aerator introduces air and oxygen to the water. Then when exposed to oxygen, the iron in the water is oxidized into essentially a rust particle, which can be captured in the sand filter. The aerator is followed by chemical addition of chlorine and potassium permanganate, which completes the iron oxidation, and also oxidizes the manganese in the water.

RO Plant 1.jpg

align=justify>The pressure filters consists of three, 8-foot diameter filter vessels that are approximately ten feet tall. Within the filters is filtration media that captures the oxidized iron or "rust" particles and the oxidized manganese particles.

The media consists of 12-inches of anthracite and 18-inches of green sand. Unoxidized iron and manganese attach to charges present on the green sand surface. The green sand is recharged through a combination of backwashing and chemical treatment with potassium permanganate. Polymer is also added prior to the filter to catch the oxidized iron and manganese and develop large particles called "floc" which are more easily filterable.

Removing the trapped iron and manganese particles from the filter media is accomplished through a process called backwashing. Backwashing is accomplished by pumping water through the media from the bottom up. The lighter iron and manganese particles are carried out of the filter in the backwash water while the heavier sand filter media settles back down once the process ceases.

The filtered water flows to the reverse osmosis system and booster pumps. Approximately 75 percent of this water is pumped to the reverse osmosis system. Chemicals added before the reverse osmosis are antiscalant, and Sodium bisulfate. The antiscalant is added to prevent the calcium carbonate from forming a precipitate. Sodium bisulfate is added to remove any remaining oxidants that were not exhausted in the filter.

The osmosis process occurs

in nature when clean water passes through plant membranes into the roots. This process can also be reversed, resulting in clean water passing through a membrane. In simpler terms, the reverse osmosis process is essentially a high-pressure filter that removes contaminants as small as molecules.

The reverse osmosis process begins with a pump, which increases the water pressure to approximately 160 pounds per square inch (psi). The pressure forces the water molecules through the membrane. Dissolved solids in the water are not able to pass through the membrane and are discharged in a reject stream. The reject stream, (approximately 30 percent of the total reverse osmosis flow) is sent to the wastewater facility. The membranes remove 95 to 99 percent of the dissolved solids, including nitrite, nitrate, hardness and sulfates.

The product water which passes through the membranes is then blended with the filtered water that was not in the reverse osmosis system to decrease the corrosiveness of the water. The blended reverse osmosis treated water is pumped to the distribution system using two, 30-horsepower pumps. The water is chlorinated before leaving the plant to provide a disinfecting residual. Fluoride is also added to enhance public dental health.

The facility is designed to operate automatically when the operators are not present. A computer operating system is used to control the various motors and equipment, and can also be assessed by computers at remote sites. An alarm dialer is also provided to call the operators should anything fail with the plant.