W-3 Water Production Plant
The Lakeway Municipal Utility District’s water treatment is a conventional surface water treatment with a capacity of 6 million gallons per day (MGD). The plant was originally constructed in 1988 and expanded in 1997, 2000 and 2002.
The water source is Lake Travis, where two raw water pump systems are located. To accommodate the lake level variability, the raw water pumps reside on floating barges. The original raw water line to the water treatment plant is a 12-inch pipeline, approximately 5,700 feet long. The newer barge, constructed by the LCRA, serves both the Lakeway MUD and the Hurst Creek MUD, with other future capacity built in. This transmission line is 24" to the Lakeway plant then (currently) is 12" to Hurst Creek. The two raw water delivery systems are completely independent but interconnected at several points for redundancy.
As the raw water enters the plant site, venturi meters measure the flow. When appropriate, the backwash recycle is returned to the head of the plant and the resultant flow is split between Plant A and B/C through the use of set point controllers, flow meters and flow control valves. Next is chemical injection. Alum, the primary coagulant; a cationic polymer, as a coagulant aid; chlorine, as the primary disinfectant; (and atypically, caustic soda, if required for pH adjustment); are injected into static in-line mixers downstream of the flow split. The nominal flow rating for each Plant A, B and C is 2 MGD. The operator selects the desired treatment flow rate set point to each plant and the Supervisory Control and Data Acquisition (SCADA) system will vary the raw water pumping accordingly, automatically taking into account the return of recycled water.
Plant A: Plant A consists of a reactor (solids contact) clarifier and three dual media filters that are operated in parallel. The nominal capacity of Plant A is 2 MGD, but is reduced by 1/3 during backwash of one of the three filters. This keeps the flow rate to the filters constant to improve performance. During peak summer demand, this plant is operated continuously, while plants B and C cycle on and off as required. This also improves performance. The clarifier has a recirculating pump within the centerwell, reaction chamber, and sludge collection mechanism. The effluent from the clarifier is collected in V-notch radial launderers. Sludge is withdrawn on a periodic basis to the lift station, which pumps the sludge to the S-5 wastewater plant.
The settled water from the clarifier flows through a stilling well before the filters. The three filters are operated in parallel as down flow, constant rate, variable head design. Each filter consists of 103.8 square feet of area, dual media (30 inches of anthracite and sand), and perforated stainless steel plate underdrain. The flow into the filters is limited to 5 gpm per square foot by orifices, and by the SCADA system. Each filter is equipped with loss-of-head instrumentation and an on-line turbidimeter. The filtered water is metered in route to the clearwell. Fluoridation occurs just before the clearwell, as does post chlorination, as a secondary disinfectant.
The backwash of the filters is normally done automatically based on filter run length in hours. Six feet filter loss of head can also initiate backwash. The backwash sequence starts with surface scour, concurrent scour and filter backwash, and ends with backwash only. After a backwash, water is filtered to waste for 30 minutes to allow the filter to "ripen" before placing it into production service.
Plant B/C: Plant B/C consists of two "Microfloc" modular treatment units having a nominal capacity of 4 MGD. The unit includes an adsorption clarifier, tri-media filter, and controls. The coagulated flow from the static mixers flows up through the adsorption clarifier media. In the adsorption clarifier, the rough filtering effect removes most turbidity. The clarified water flows from the adsorption clarifier to the filter through a distribution trough. The filter is a constant head, variable flow rate design with 280 square feet filter area, mixed media, and well screen underdrains. The mixed media consists of anthracite, sand, and garnet with no support gravel. From the treatment unit, the filtered water flows to the clearwell, where it is metered and fluoride and chlorine are added.
The backwash of the filters is normally done automatically based on operating time, although filter loss of head will also initiate. The backwash sequence starts with air scour, concurrent scour and filter backwash, and ends with backwash only. After a backwash, the filter effluent is wasted to drain for 30 minutes to allow the filter to "ripen" before placing it into service.
Filtered water flows into the 275,000-gallon clearwell. Clearwell levels control the raw water pump operation, based on lead, lag and 2nd lag levels selected by the operator. A venturi meter totalizes the finished water flow while an on-line chlorine analyzer and turbidity meter continuously monitor finished water quality.
The high service pump station delivers water to the Lakeway lower pressure plane using one 500, two 1400, and/or two 2800 gpm pumps. The pumps all have smart motor controllers for soft starts and stops to eliminate pressure surges. The station also has two backwash and one surface wash water pumps.
Operators do not continuously man this plant, but there is always an "On Call" operator available. The very sophisticated Supervisory Control and Data Acquisition system (SCADA) enables this. Through a graphic Human Machine Interface (HMI), the system allows the operator to observe and control every aspect of the plant operation. The control network includes raw water supply systems, the plant site, as well as the complete Lakeway water distribution system of booster pump stations and elevated storage. The network allows operator access remotely by laptop from anywhere. There are 32 separate water quality, tank level, and equipment alarms that call an operator via cell phone. Often, the operator can respond via remote connection and resolve the alarm. If not, the operator has the information to mobilize the proper response.
The District is conscious of its responsibility to promote conservation of all our natural resources. In cooperation with the Lakeway Golf Clubs and the City, the District’s recycled water produced by the plants is used as irrigation water for Live Oak and Yaupon Golf Courses as well as the medians of Lakeway Boulevard and Lohmans Crossing, thus eliminating the need to use potable water. The District also has a program for residential association and commercial landscape irrigation. In addition, water is applied to a cedar forest. This effort goes hand in hand with other ongoing conservation efforts such as those aimed at minimizing the consumption of potable water.
Lakeway Water Production Plant: How Does it Work?
The Lakeway Municipal Utility District’s production plant supplies water to about 10,000 people living in the Lakeway area. To supply all the water that is needed in the hot summer months, we treat about four million gallons per day. The water comes from Lake Travis and is pumped to the treatment plant to make it safe to drink.
There are several treatment steps for lake water:
Coagulation: Small amounts, in the low parts per million, of alum and polymer are added to water and mixed to coagulate tiny algae, dirt and silt particles. These particles make water cloudy, and this cloudiness is called turbidity.
Flocculation: The coagulated particles are gently mixed in a large tank and they become visible as something called floc. They are now large enough to settle out of the water using gravity.
Sedimentation: The heavy floc particles settle to the bottom of the clarifier, and the clean water from the top is carried to the filters. Alternately, in the Microfloc unit, the floc is removed while it is still very small by passing it through a bed of small plastic beads in something known as the absorption clarifier.
Filtration: The water is passed through filters containing very fine sand and anthracite, which is called filter media. The floc particles that did not settle to the bottom during sedimentation are caught in the sand filter media.
Disinfection: Small amounts of chlorine and ammonium sulfate are added to the filtered water to kill any harmful bacteria or microorganisms that may be in the water. Disinfection is the most important step of water treatment because it protects water drinkers from getting sick.
Fluoridation: Fluoride is present in the raw water, but not enough to be effective in improving dental health. We add fluoride to reach the recommended amount – 0.7 parts per million.
Sludge Processing: The particles that are removed from the water are not returned to Lake Travis. They are concentrated into a thick soup, called sludge. The liquid sludge is transported to our Water Recycling Plant, where it is converted into a substance that has the consistency of soil and then is taken to a landfill.
After the water is treated, it is pumped to water towers, like the one that looks like a giant golf ball and stands at Lakeway Boulevard and Highway 620. These tanks are called elevated storage. By having the water in a tank high in the air, gravity pulls the water downward, providing pressure so the water will flow when you turn on the faucet.
