Construction And Testing Of A Pilot Scale Drinking Water Treatment Process

This project was a senior design project for a civil engineering student. The project’s goal was to build a cost-effective and energy efficient system to treat surface water on a pilot scale. Due to its relatively inexpensive costs and the safe nature of the project, this type of project can serve as an open-ended design and economic study for a group of students, which they can build what they design. This system would serve as a lab experiment and a continued senior project to improve its removal efficiency. A multi-discipline team could fit various measurement devices such as pH and turbidity and design a feedback control scheme to optimize the water treatment process. This paper demonstrates that such a system is feasible, and it can serve as a great learning experience for a relatively small cost. The student and an adjunct faculty member designed and constructed this system at Christian Brothers University in Memphis, TN with the help of another student and a volunteer retiree with industrial experience. The base system was designed to treat water collected from the Mississippi River at a 4.5 gpm flowrate at a capital cost of approximately $2300. This system effectively treated water pumped from the Mississippi River and met drinking water standards that it was designed for. The system consisted of an equalization basin tank, feed pump, dosing pump with chemical reservoir, a static mixer, settling tank, and a three-layer filter. The system was portable and powered by a portable generator A truck transported the system to the river. The system design was based on the addition of organic polymer/inorganic coagulants added to the feed stream. The use of stateof-the art polymers greatly reduced the system’s size. This paper presents the system design and operating limitations. An improved design is also presented costing less than $9,000. The alternative system consists of items purchased from vendors and could be assembled by students. This system is intended to be for education and demonstration. The senior student considered this project to be her best educational experience in college. This type of project greatly improves a student’s design and critical thinking skills. Introduction This project was intended to be a senior project and to serve as a laboratory experiment for civil/environmental and chemical departments. Initially the project’s goal was to design and built a solar powered water treatment system, but building an effective energy efficient mixing process in the flocculation tank posed a problem not easily solved at that time. The solar energy panel costs for powering the mixers were quite high and the construction of the baffled flocculator was quite tedious. As a result, a more traditional system was designed to be powered by a gasoline fueled generator. The generator powered pumps and gear motors to facilitate the conversion of river water to drinking water. The system was designed for portability using a flat bed truck or a truck with a 12-foot long trailer. The system was transported to the Mississippi river to demonstrate how surface water could be treated. The P ge 939.1 Proceeding of the 2004 American Society for Engineering Education Annual Conferences & Exposition Copyright © 2004, American Society for Engineering Education equalization basin and main flocculation/sedimentation/filter tank was built on casters for mobility. Also, 4” x 4” lumber was used as a support so that a forklift could be used to load it if necessary. Due to a lack of funds it was desired to be inexpensive yet able to meet acceptable drinking water quality standards. The system was designed based on water collected from the Mississippi River at a location in Shelby Forest State Park, North of the City of Memphis. Lab tests determined the proper coagulant dosing and sediment settling time. The Mississippi river supplies drinking water for several cities in the United States including the City of New Orleans and St. Louis, MO. Background Information In Typical surface water treatment systems, there are 5 processes that are used to treat surface water. These processes include: • Coagulant Addition and Mixing • Flocculation • Sedimentation • Filtration • Disinfection Figure 1 shows the typical treatment train for surface water. Surface water flows into the system and it is mixed with a coagulant. After mixing, the water flows into the flocculation basin where it is gently mixed to facilitate the coalescence of the colloidal particles into large particles. The water flows into a sedimentation basin where the large particles drop to the bottom of the tank forming a layer of sludge. Sludge consists of various particles and microorganism suspended in the surface water. It is necessary to remove the sludge from the clarifier. Water overflows from the surface of the clarifier into the filter via the weir. The turbidity is further reduced as particles and organisms migrate from the water to the filter media’s surface due to mass transport gradients. Fiinally, a disinfectant is added to destroy microorganisms. Figure 1Typical Treatment train for Surface Water Alternative Treatments This system consists of technologies commonly found in surface water treatment systems. However, newer technologies could be utilized if determined to be economically viable or the P ge 939.2 Proceeding of the 2004 American Society for Engineering Education Annual Conferences & Exposition Copyright © 2004, American Society for Engineering Education water quality constraints require them. State-of-the-art surface water purification includes several other techniques including reverse osmosis, membrane filtration, cartridge filtration, ion exchange, electrodialysis, aeration, and softening. On the other hands, students could attempt to build a solar powered treatment system. Such a project would certainly produce student enthusiasm. It should be noted that many universities incorporate newer surface water treatment technologies into their laboratory experiments. This paper does not exclude these technologies as an alternative to water treatment. Experimental System Figure 2 shows the system that was designed and built for this project. It consists of two pumps, an equalization basin, static mixer, flocculation tank, sedimentation tank, and a filter. The system’s rough dimensions are 4 feet high, 8 feet long, and 4 feet wide. The equalization basin is not shown. Figure 2 System Designed and Constructed System Design and Construction In general, the system was designed based on a flowrate of 4.5 gpm, and the use of a highly efficient organic polymer to cause rapid particle coalescence in the flocculator and settling in the clarifier. Pumps A pump with a capacity of 20 gpm at 50 feet of head was leased to collect water from the river and deliver it to the equalization basin. This pump was operated intermittently. The leasing fee was $20 a day. A small centrifugal pump manufactured by March Model DX-3 centrifugal P ge 939.3 Proceeding of the 2004 American Society for Engineering Education Annual Conferences & Exposition Copyright © 2004, American Society for Engineering Education pump ($110.00) was used to remove the water from the equalization tank to the flocculation tank. This pump had a rating of 6 gpm at 0 feet of head. This pump had virtually no suction head. As a result, a water level above the pump intake was necessary for the pump to work properly. The pump’s power rating is quite low, only 1/55 of a horsepower. This was the most economical choice for a pump to perform this job. A small feed pump delivered the proper dosage of (LMI Roy Solenoid Diaphragm Pump Model number 74502-00) coagulant to the system. Mixer A static mixer was used to perform the mixing having an ID of 1.029 inches, and a length of 11inches. It became apparent that a static mixer was insufficient and an in-line motorized mixer is necessary to properly mix the coagulant with the inlet feed stream into the system. Coagulant The type and quantity of coagulants are determined by using a jar test. Coagulants can be classified as inorganic and organic. Typically inorganic coagulants depress the pH of the system and lime is added to raise the pH back to acceptable levels. Using a polymer can eliminate the need for this step. The coagulant that was used for this design is an aluminum chloride/organic polymer mixture developed by the Nalco Chemical Company in Napierville, IL. Based on a jar test, the required dosage is 60 ppm for the proper flocculation to occur. For this setup it was determined that 1 part coagulant should be diluted with 19 parts distilled water. This allows the coagulant to easily flow (low viscosity) in tubing with a low-pressure drop. Flocculation Flocculation is the process that brings the colloid particles together so they can settle in the clarifier. The mixing must be very precise: an inadequate amount of mixing energy will cause settling in the flocculation basin; too much energy will cause the floc particles to break (Davis, 1998). As the water moves through the basin, the velocity gradient (G) should decrease to prevent the flocs from breaking. Equation 1 describes the velocity gradient. G is a function of volumetric flowrate (Q), headloss in the flocculator (ΔH), water’s density (ρ) , viscosity (μ), and volume of the tank (V). In this system a variable speed motor/gear reducer drives a shaft fitted with wooden paddles cut to the proper size to deliver a range of velocity gradients in the flocculation tank. The flocculation tank is divided into 3 compartments partially separated by Plexi-glass sheeting allow velocity gradient variations. Each compartment has its own individual gear motor/controller. The gear motors are 1/15 hp Dayton AC gear motors (Grainger Part No. 2Z802, $149.55 each) each controlled by the AC Motor Speed Controllers (Grainger Part No. 4X796, $26.65 each), which can lower the speed to between 30 and 45 rpms. The tank is 2 feet wide