The Students: Todd Freeman, Wade Heath, and Harmony Hudson
The Teacher: Clinton A. Kennedy
Designing, Constructing, and Testing the use of a Photosynthetic Bioreactor to Remove Nutrients From Waste Water
The small town of Cascade is located in central Idaho at an elevation of 4800 feet, on the shores of Cascade Reservoir, an eutrophic body of water. Cascade Reservoir is a 17 mile long, 4 mile wide, man made, hydroelectric, irrigation water impoundment that is used for boating, fishing, and other forms of recreation. The reservoir brings many tourists and recreationalists to the area. Cascade's economy greatly depends on this tourism that the reservoir brings. Unfortunately, Cascade Reservoir is being polluted with excess nutrients that are causing toxic algae blooms which adversely affects its value to the community. In the summer of 1993, twenty-two cattle that came into contact with the reservoir died because of these blooms. The Environmental Protection Agency temporarily banned all human contact with the reservoir after the toxic blooms and cattle deaths, and it became increasingly clear that something had to be done to reduce the amount of nutrients flowing into the reservoir. One major source of excess nutrients was from the discharge of secondary treated sewage into the river that fills the reservoir. This sewage came from the city of McCall, a town 30 miles north of Cascade. The Clean Water Act forced McCall to investigate and install a tertiary sewage treatment system that would reduce the amount of excess nutrients being discharged from the city's sewage into the river that fills the reservoir.
The Advanced Biology class of Cascade High School studies limnology so that the students can better understand the eutrophication of the reservoir. With this knowledge of eutrophication, each year they design multiple projects to study, control, and reduce this eutrophication. Four of these students, who called themselves the "Sewage Sisters", found information on a British based company called Biotechna in a small commentary in a "National Geographic" magazine. This article discussed Biotechna's new tertiary sewage treatment system, called the Biocoil.
A Biocoil is a biological, tertiary sewage treatment system that uses Chlorella sp. algae grown in a controlled environment to remove excess nutrients from waste water. The nutrients are removed through the photosynthetic process of the Chlorella sp. algae. The algae uses the nitrate and phosphate from the sewage water for development , energy, and reproduction. The algae is extracted from the system to remove the nutrients, and can then be used for cattle feed, organic fertilizer, or dosed back into the Biocoil system. The Biocoil is designed to produce an environment that provides every necessary ingredient for the algae to continue its life cycle. The original engineering company, Biotechna, claimed that the Biocoil can remove 92% of the nitrates, and 98% of the phosphates from waste water. The Biocoil was proposed by the Sewage Sisters for a tertiary sewage treatment system at McCall?s sewage treatment facility. The added tertiary stage would reduce the amount of nutrients being discharged into the river, and therefore reduce the eutrophication of Cascade Reservoir.
Through the internet and E-mail, the Sewage Sisters received information and plans on the Biocoil. This information, along with the claims proposed by Biotechna, were proposed for McCall's tertiary system of sewage treatment. McCall refused to spend the two to five million dollars required to construct and run this system, mainly due to a lack evidence and tests proving that the Biocoil could treat two million gallons of discharge water. The new system had to reduce the phosphate content of the treated sewage to 0.01 part per million and the nitrate to 1.0 part per million, and do so without freezing. The Biocoil simply had not been tested or proven efficient under such conditions. Yet, the Sewage Sisters believed in this new technology, and wanted to prove that it would work. They proposed that McCall set up and test a pilot project at the McCall sewer lagoons. McCall turned down this proposal also. With their faith unbroken, the Sewage Sisters decided to build their own Biocoil. Many people, who were convinced that the Sewage Sisters had something to offer, helped the group write grants in order to receive the money it would take to build the pilot Biocoil in Cascade. As a result of much hard work and perseverance, the Sewage Sisters received grants from the following: Cascade Reservoir Association, Phillips Environmental Partnership, the Division of Environmental Quality, Valley Soil and Water Conservation District, Lightfoot Foundation, Toyota Tapestry/National Science Teacher's Association, and other small contributors. These grants and donations totaled $23,636, and the money was used to buy materials to build the pilot Biocoil in Cascade. When construction had barely started, the Sewage Sisters graduated and the Biocoil project was handed down to Cascade High School?s Advanced Biology class. We inherited the project at this point. We had to test and redesign the Biocoil to withstand Cascade's harsh winter climate, be cost effective to run, require little maintenance, and remove the nutrients of McCall?s maximum discharge to the required levels.
Sewage treatment is generally a two stage process. The first stage, or primary sewage treatment, basically constitutes the settling of solids. Secondary sewage treatment is a bacterial decomposition of the suspended solids and all other organic matter and biological compounds. This organic matter is broken down into nitrate, phosphate, ammonia, and other nutrients. These nutrients are then discharged with the secondary treated waste water. Secondary sewage treatment usually meets government standards which require that all solids and biological matter be removed from the water before it is discharged. A third, or tertiary stage, can be added for further waste water treatment, usually in order to remove the phosphate and nitrate from the secondary treated water before it is discharged. A tertiary system is sometimes composed of a chemical precipitation process which is generally greater than 90% efficient. In this process, the nutrient-loaded water is exposed to certain chemicals that react with the nutrients and lock to them, or cause them to settle. The chemicals and nutrients then have to be disposed of. However, large amounts of chemicals are needed if used on a large scale sewer treatment facility. As a result, the chemical processes are expensive to build and run. A biological rather than a chemical system of tertiary nutrient removal can be used to keep costs down. With the Biocoil, the algae naturally reproduces, so that the need to buy more materials is nonexistent. The biological system uses a natural process to remove the nutrients, and therefore can be a less complicated system than the chemical precipitation process.
The Biocoil is basically a greenhouse for algae. It consists of clear PVC tubing wrapped around a circular frame. Water is pumped from an effluent source into a series of tanks, and then is cycled through the tubing and back to the main tank. Then the algae settles and the treated water is discharged. Biotechna's Biocoil had a simple gravity fed, overflow system (see Figure 1). This system ran off of one main pump that would fill the holding tank to a certain level. When the water would reach the level of an outlet in the tank, the water would overflow into a tube that ran to the settling tank. The settling tank would fill to a certain level and then discharge the treated water from an outlet in the tank. In order achieve the proper tank locations so that the overflow system would operate as effectively as possible, Biotechna mounted the settling tank outside of the Biocoil. With no heat to the settling tank and the cold winter weather common in Cascade, the settling tank and the hoses that filled it would freeze if this system was used. Upon examination of Biotechna's design plans for their Biocoil, we decided some obvious changes had to be made. The first thing to be changed was their system of filling the tanks and discharging water. This system seemed inefficient, as untreated water and suspended algae could be discharged with the treated water. We devised a system of filling and emptying the tanks with pumps (see Figure 2). The system first discharges the treated water from the settling tank using a pump suspended in the setting tank at the water level. The pump is suspended so that the discharged water is clean, since the algae has settled to the bottom of the conical shaped settling tank. The holding tank with nutrient-deficient but algae-loaded water is emptied; and the water is pumped into the settling tank. The holding tank is then refilled with nutrient-loaded water by a pump that is mounted on a buoy located in the middle of the sewer pond. Finally, a small "dosing" pump at the bottom of the settling tank forces the settled algae from the settling tank into the holding tank; where the untreated sewer water now resides. The main pump continues to circulate the water through the tubing. This pump system is operated by a Nelson sprinkler pump controller. A local electrician, Terry Dunn, helped us to wire this pump system so that it would be done correctly, safely, and pass electrical inspection. This sophisticated electronic device was programmed to run the pumps in the correct order and for certain periods of time so that the volumes in the tanks remained balanced. The Nelson controller allows us to run a complete cycle every 12, 24, or 48 hours. The various time intervals that the water circulates in the tubing allow us to test for the maximum nutrient removal in the minimum amount of time. Our more efficient pump system allowed us to mount all of the tanks on the inside of our Biocoil (see Figure 3).
To withstand the cold weather, our version of the Biocoil was wrapped top to bottom in transparent greenhouse plastic. This helped to maintain the heat inside the Biocoil, while still allowing sunlight to penetrate the tubing. The vents in the eves were plugged in the fall and winter with foam rubber, and a wall-mounted thermostatically-controlled electric heater was installed. The intake hose from the City of Cascade sewer pond was outfitted with a heated electrical cord, and wrapped in insulation. Halogen lights were used rather than the fluorescent ones in Biotechna's design, mainly because of their brighter light and greater heat output. We now had an efficient, nearly maintenance free system to cycle the water, and did not have to worry about the Biocoil tanks or tubing freezing, as they were contained within the Biocoil itself.
A manifold was designed so that the direction of the flow of water could be reversed and so that the pressure from the pump was divided equally among the four sections of tubing (see Figure 4). The manifold allowed for sections of tube to be shut off, so that the pressure in the other tubes would increase. It also allowed for two sections of tube to be joined together, increasing the amount of time the algae/sewer water solution remained in the sunlight. The 1600 feet of tubing was divided into four sections, reducing the pressure on the pump, and creating a more constant pressure in all of the tubes. The manifold is built out of PVC plastic pipe, valves and connectors. A local plumber, Ben Wellington, helped us to build the manifold, and also came up with a plan and materials for the frame. Mr. Wellington's talent of gluing and threading pipe so that it would seal and work efficiently was extremely helpful.
The frame for our Biocoil consists of one-inch, galvanized water pipe. Threaded connectors hold the pieces of pipe together. A near octagonal shape was achieved using 45 degree angle bends. This frame is 10 feet tall, 8 feet in diameter, and wrapped with welded wire horse fence. This fence allowed us to zip-tie the one-inch inside-diameter tubing around the frame.
Tube cleaners called "pigs" were installed in the tubes to remove the algae that stuck to the inside of the tubes (see Figures 5 and 6). The water moving through tubes pushes the pigs around the tubing, and they scrape off the algae that has clung to the tubes so that sunlight can still penetrate the tubing. Nails through the tubing at the manifold kept the pigs in the tubing and out of the holding tank. The function of connecting two sections of tubing was removed to allow the installation of a "pig-stop". This pig-stop allowed the pig to go into a larger area where the water could flow around the pig with ease, decreasing the potential for clogging to occur. Various materials were tried for pigs. Stainless steel and plastic pot scrubbers were both attempted, and both became clogged with algae, preventing water from flowing through. Chunks of foam rubber were used and cleaned the tubes exceptionally well, but would allow no water to flow when the foam pig stopped at the end of the section of tube. All of these pigs would clog, and not let water to pass through them. The final pigs we tested were the innards out of hair curlers. We cut them in half to achieve proper length, glued the ends to prevent the pigs from coming apart, and installed them. They work well as cleaners, and pose little to no problems with clogging.
In order to keep the pH of the system down and provide a source of carbon, carbon dioxide was bubbled into the system. An electronic pH probe is in the holding tank that monitors the pH level of the water in the tubing and holding tank. If the pH falls below 6.5, a solenoid valve opens, which allows carbon dioxide from a tank to bubble into the holding tank. Our original design placed the carbon dioxide input in the tubing after the main pump. This system allowed extreme amounts of carbon dioxide into the system in a short period of time, and caused the carbon dioxide tank to empty quickly. When the carbon dioxide tank would empty, the solenoid valve would stay open to adjust the pH of the system, and the water pressure from the main pump would fill the carbon dioxide tank with water. Three complete carbon dioxide tanks were emptied in the first two months of operation. We then moved the input of carbon dioxide into the holding tank, with a stone bubbler on the end of the hose. Excessive amounts of carbon dioxide were still used, but not at the previous rate; and backfill could not occur with the bubbler in the tank. The pH probe was then moved to a place in the holding tank where the inlet water would splash on the end of the probe. This would prevent algae from collecting on the electrode. As a result of these modifications, the same carbon dioxide tank has been used for over four months, and it still contains a fair amount of carbon dioxide. The carbon dioxide is used by the algae, and carbonic acid is produced to balance the pH. Algae also uses carbon dioxide in photosynthesis. The carbon is chemically joined with water to produce sugar. This sugar is used for energy.
The nutrients phosphorous and nitrogen are used by the algae for growth, development, and energy. The phosphorous is used in ATP, an energy storing compound. Phosphate groups and a nitrogenous base are used in the structure and synthesis of DNA. Nitrogen is used to make amino acids, and proteins. These proteins allow the algae cells to grow and divide, and the proteins provide cell structure.
Potassium Nitrate was fed to the Biocoil at various times to keep the algae alive. We had to feed it because of the low amount of nitrates found in the sewer pond that filled the Biocoil. The Cascade sewer lagoons are large for the amount of water treated, so the parts per million in the ponds that feed our Biocoil were much lower than we wanted to test. The average inflow from Cascade's sewer pond was less than one ppm of nitrate, and one ppm of phosphate. McCall,on the other hand, has the opposite problem. Their sewer lagoons are small for the amount of water they treat, and discharge up to 30 ppm of nitrate, and 15 ppm of phosphate. Feeding was also necessary at the beginning of operation of our Biocoil. After inoculating the system with chlorella from our small classroom Biocoil, we fed it 100 grams of potassium nitrate every other day to develop an algae culture of 2-5 grams per liter of dry weight algae. This density was recommended by Biotechna for optimal nutrient removal.
In order to test our Biocoil's efficiency at removing nitrate and phosphate, we set up a testing schedule. We fed 30 ppm of potassium nitrate to our Biocoil at 6:00 in the morning on the test day. The water was allowed to circulate and a sample was taken. Every third hour after the Biocoil was fed, a sample was taken and tests were run. This monitoring continued until total nitrate removal was achieved. After 36 hours, the tests were complete, and all of the nitrates had been removed from the system (see Figure 7).
Tests for nitrate, phosphate, and ammonia were performed using a LaMotte colorimeter. The Cadmium Reduction Method was used for nitrate tests, and the Ascorbic Acid Reduction and Vanadomolybdophosphoric Acid Methods were used for phosphate tests. Our original samples were too clouded with algae to produce accurate test results. It was necessary to centrifuge the water to settle the algae, filter the solids remaining after centrifuging, and then dilute the sample in order to produce accurate readings of the parts per million of the nutrients on the colorimeter. A spectrophotometer (the Spect 20-D) was used on the major test days to achieve an accurate curve of the hourly nitrate removal rate. In a 36 hour period, 35 parts per million of nitrate were completely removed from 300 gallons of water.
Analysis and Conclusion
When the temperatures at our Biocoil would drop below zero degrees Fahrenheit, the air temperature inside the Biocoil never went below 45 degrees Fahrenheit. The inflow never froze, and with the tanks on the inside, they had no opportunity to freeze.
Through the winter's cold spell, water temperatures went below 13 degrees Celsius in the coils of tubing and holding tank. We believe that maximum nutrient removal will not be reached if the water temperatures are too low, or too high. Tests are planned to see if moderate temperatures do produce the maximum nutrient removal. The heater on the wall of the Biocoil kept the coils from freezing, but a new type of heater could be used to maintain a more constant water temperature. A stock tank heater may be tested in the future. The stock tank heater will go directly in the water, and turn on and off when the water temperature varies. This will maintain the actual water temperature, and not the air temperature inside the Biocoil.
Phosphate removal did occur in our Biocoil; however, the rate of phosphate removal was not fast enough to be effective for sewage treatment (see Figure 8). It took 4 days for the biocoil to remove 36 ppm of phosphate down to 0.36 ppm. McCall's requirement for phosphate discharge was 0.01 ppm. Because phosphate removal did not meet this requirement, the Biocoil could not be used for McCall's sewer treatment.
A new funnel style pig-stop could be developed so that the pig would have an easier time getting out of the stop and back into the tubing. An outside connector on the rubber tubing was never found, but would have worked better than the typical inside style. The outside connector would allow the pigs to flow through the tubing with greater ease, and provide the opportunity to use a larger and better cleaning pig.
Tests were run both when the Biocoil had algae suspended in the water and when the algae had adhered to the sides of the tanks and tubing (see Figure 9). Nitrate removal was not slowed by the algae adhering on the tubing; the 50 parts per million we fed it in both algae conditions were completely used in the 48 hour testing period.
The water level in the holding tank is a variable which we never officially tested. The tubing holds 65 gallons of water. If 65 gallons are in the holding tank, this water can be cycled through the tubes and exposed to light for 5-7 minutes; 5-7 minutes after being in the holding tank, it is cycled back into the tubing. If there are 260 gallons in the holding tank, 65 gallons are in the tubing for 5-7 minutes, and then in the holding tank for 20-30 minutes. This difference of time the water is in the dark (in the holding tank) may affect the rate at which nitrates are used. Tests with varied amounts of water have not been performed. We will be testing for optimal light/dark conditions by varying the amount of water in the holding tank.
Complete algal settling in the settling tank did not occur. We believe the diameter of the settling tank we used was too large, and that a tighter, more funneled version would produce better settling results. With no way to change settling tanks (because the Biocoil was built around the tanks), we decided to develop a filter system. Plans for a sand filter were found, and a local cement plant donated small amounts of sand and gravel. A two-and-a-half foot tall, six-inch diameter housing was built out of PVC plastic tubing and caps. This was placed in the discharge line from the settling tank. Water would not flow through the filter at a fast enough rate, and would run out of the cap of the filter due to the built up water pressure in the filter. As a result, we plan to test more filter designs, and are searching for a filter that can reclaim the algae so that it can be used again.
Nitrates were used at a rapid rate. With this information, we believe that the Biocoil could be used in a specialized application where nitrate removal is needed, but phosphate removal to extremely low levels is not mandatory. In a septic system where all of the phosphates are locked in the ground, the Biocoil could be used to shorten the needed drain field.
The practicality of using the Biocoil for sewer treatment doesn't seem promising. The pilot model we built has 1600 feet of tubing that holds 65 gallons of water. Our holding tank on the average holds 235 gallons of water. The pilot model can use 35 ppm of nitrate in 36 hours from the 300 gallons of water cycled through it. A Biocoil 10 meters in diameter and 7 meters tall would have 16,000 feet of tubing holding 650 gallons of water. This scale Biocoil would also need to have a holding tank with a volume of 2500 gallons, and could theoretically treat 3000 gallons of water containing 35 ppm of nitrate in 36 hours. It would take 600 Biocoils of this size to treat the 2 million gallons of water that McCall?s sewage system expels per day at maximum spring discharge. The Biocoil therefore is not a cost efficient method of treating sewer water because of the number of Biocoils it would require. The cost to build these 600 Biocoils would be around $6,000,000, not including the land that would need to be purchased in order to house them (see Figure 10).
The Cascade Biocoil is going to be handed down to some dedicated underclassmen at the end of this school year. Before it is handed over, we must train the newcomers on how our Biocoil works, what tests need to be run, and what problems still need to be solved.
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