by Austin Hill Shaw (firstname.lastname@example.org)
Behind the potable, palatable water that all of Virginia Tech and the surrounding communities depends upon daily, is the Blacksburg, Christiansburg, V.P.I., Water Authority. Established in 1954 and constructed in 1957, the Water Authority has been treating an average of six to seven million gallons of water a day. The Water Authority deals solely with drinking water and is not involved in waste water treatment. Operating 24 hours a day, 7 days a week, the Water Authority runs a lean operation, staffing only 12 people to run the entire facility. Despite this, using automated and manual methods, the water is tested on average 249 times a day in and around the plant to ensure it is fit for consumption. These test can be divided into two broad categories: test for contaminants and tests for physical parameters. The contaminants include such things as pesticides, metals, and parasites; substances that can adversely affect the health of the consumers. The physical parameters, such as temperature, turbidity, pH, salinity, and odor tests are used to determine the amount of certain chemicals that will need to be added to successfully treat the water for the lowest cost possible.
As a water treatment plant, The Blacksburg, Christiansburg, V.P.I. Water Authority is unique in that it is neither a privately owned treatment system, nor is it a public municipality. Around the United States, most water treatment plants fall into one of these two broad categories. Privately owned systems are required to pay taxes on the water they sell and sometimes must compete with other privately owned water treatment operations. On the other hand, public municipalities are sometimes neglected due to their control by elected officials, who sometimes fail to maintain facilities in face of more politically charged projects. Walking the line between a private facility and a municipality, the Water Authority is a tax exempt organization governed by a representative from each of the three served areas, plus two additional representatives the three districts agree upon. The Water Authority pays for the cost of operation by wholesaling their water for $1.06 per thousand gallons to the three districts and not through tax collection. Thus, the Water Authority doesn't have to rely on tax revenues because they are wholesaling the water to cover their operating expenses; they don't have to compete with privatized treatment plants; they are immune from the deleterious affects of political cycles and, in addition, the Water Authority maintains the right to condemn property as needed. As Superintendent Jerry Higgens puts it, "our situation is ideal."
The process begins with raw water being pumped out of the New River. The water is filtered though large screens which prevent large objects from entering the treatment system and damaging the equipment therein. The water is analyzed for turbidity and pH automatically so appropriate levels of chemicals may be added further along in the treatment process.The raw water is pumped into the flash mix basin, a turbulent well mixed environment. Before the raw water enters this area it is injected with chlorine gas, sodium hydroxide, and polyaluminum chloride (PAX). These three chemicals are used for the sterilization of the water, the adjustment of pH to hinder corrosion in pipes, and to begin the coagulation process respectively. PAX promotes the process of coagulation by creating chemical attractions between particles due to the presence of the aluminum ion in the water. As the name implies, the primary purpose of the flash mix basin is to distribute the added chemicals evenly throughout the water.
Flash Mix Basin Chemical Feeder(Image)
Next the water flows into two separate flocculation basins and mixers. The conditions are far less turbulent but the water remains well stirred. The water remains in these two separate, but identical treatment paths, until the they converge again at the clear wells which mark the final stage in the treatment process. In the flocculation basins and mixers, large low speed paddles stirs the chemically treated water. Suspended solids in the presence of PAX form larger and larger clusters of particles, known as flocs, as the water is slowly stirred. As the flocs grow in size as additional particles latch, they also become heavier, which is crucial for the next step in the treatment process to be successful.
The water with its well formed flocks flows from the flocculation basins into the settling basins, an environment almost devoid of turbulence which is designed as a plug flow reactor. The water takes a minimum of 3 hours to cross the settling basin and during that time, the flocs settle out of the water and collect at the bottom. Twice a year, the basins are drained and using pressurized water, the flocs at the bottom are pushed out to the settling pond below the plant. In the settling ponds, water from the settled solids and from the backwash (described later) is allowed to discharge into the steam. After the water has left the settling basins, most solids have settled out.
The final step in removing particles takes place in the dual media filters. The water that has left the settling basins filters through a matrix of anthracite coal, sand, and gravel. The fine particles that did not settle out in the previous treatment steps, become trapped in the filter, the smallest particles being trapped at the interface of the coal and the sand where porosity is at a minimum. The clean water proceed through the filter to the clear well below. According to regulations, whenever readings show a turbidity of 0.1 in the water exiting the dual media filters, or there is a head loss of 6.5 feet, or 100 hours elapses, which ever comes first, the dual media filters must be cleaned. By forcing clean water from the clear well back up through the filter, the matrix is disturbed and fine particles are flushed out of the system. This process is called backwashing.
Backwashing in progress
The backwash water with the fine particles in suspension, exits over a baffle and out to the settling ponds. The matrix is then reestablished simply by allowing the different sized particles within the matrix to settle back out in order of there respective densities. In the settling ponds, the solids from both the settling basins and the backwash are allowed to settle out while the excess water drains off to the New River.
The final step in the treatment process occurs in the clearwell were the filtered water is collected and disinfected below the dual media filters. As was said earlier, the disinfection process really begins at the flash mix basin, where the entering water is injected with chlorine gas. After settling and filtration, the water is now almost devoid of suspended particles. But pathogenic organisms and other organic substances may still remain in the water. While organics can make the taste and odor of the water undesirable, the presence of certain pathogens can cause sickness and even death to consumers. In the clearwells, additional chlorine gas is dissolved into the water. This dissolved chlorine oxidizes both the pathogens and organics and leaves the waters safe and desirable to drink. The Water Authority also adds additional chlorine to protect the water from contamination on its path from the plant to the consumer.
Chemical Storage Tank
Once the water has been disinfected in the clearwell, it is suitable for consumption. However, to inhibit the corrosion process in the pipes of the communities, pH is adjusted if need be using sodium hydroxide to make the water slightly basic. In addition, as water leaves the clearwell, zinc orthophosphate is added to further retard corrosion. The finished water then makes its way through a series of pumps, pipes and reservoirs to storage tanks in the three surrounding communities. The water in the distribution system is under pressure so that opening any tap in the community will yield safe, pleasant water for consumption.
The majority of the information for this web page was gathered during an interview with the Water Authority's superintendent, Jerry Higgens. The interview and tour of the plant took place on Thursday, October 16, 1997. Additional information was gathered via telephone conversations with Jerry Higgens and various employees of the plant. The authors of this web page would like thank Jerry Higgens and all those at the Water Authority who helped to make this web page possible.
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Student Authors: Austin Hill Shaw and Hee-Chae Lee
Faculty Advisor: Daniel Gallagher, email@example.com
Copyright © 1997 Daniel Gallagher
Last Modified: 2-14-1998