Use of Geyser pulse airlift pumps in aquaculture...
APPLICATION OF THE GEYSER PUMP IN AQUACULTURE AND AQUAPONICS Sam Kondo, Ph.D., Geyser Pump Tech, LLC Seattle, Washington Phone (614) 398-0960,
[email protected] Gary Rogers, Ph.D., P.E.,Vice President of Engineering, Aquatic Eco-Systems, Inc., 2395 Apopka Blvd, Apopka, FL 32703, 407-886-3939,
[email protected] Abstract The Geyser pump is based on airlift pump technology requiring an air supply, diffuser, and partially submerged lift for pumping liquids. Unlike an airlift pump, the Geyser pump accumulates air at the lower end of the riser and then allows a single, large release of air up the riser pipe. An equal volume of liquid is discharged each time the Geyser pump releases air. The pumping rate may be adjusted by the amount of air supplied to the Geyser pump resulting in a wide range of liquid flow rates. The sudden release of air in the riser pipe ejects water, sludge and wastes without clogging the pump. The Geyser pump was originally designed for use in domestic and industrial wastewater treatment applications to pump activated sludge. It has also been used in Aquaculture and Aquaponics for pumping water and wastes. This presentation provides an overview of Geyser pump design and operation along with specific results of applications to remove fish wastes and uneaten feed in Aquaculture and Aquaponics. The Geyser pump provides an economical low to high-head alternative for pumping liquids and wastes. Introduction The Geyser Pump is based on very old airlift pump technology. However, it was not until December 2000, that the concept was put to its most efficient use and the first U.S. patent for the Geyser Pump was granted. The unique aspect of the Geyser Pump is that injected air to the Geyser Pump is not released to the discharge pipe like the airlift pump. Instead, the air is accumulated to a certain volume, and then a constant amount of air is released to the discharge pipe. The water (with solids) is ejected through the discharge pipe by a large volume of air, which works like an air piston. Due to its stronger suction, the Geyser Pump has been used to replace the airlift to pump return activated sludge (transferring thick sludge from a clarifier to an aeration basin) in wastewater treatment plants. The physical phenomenon in the Geyser Pump air cylinder is now well understood. This has been the result of thorough analyses with laboratory-scale and field experiments using slowmotion video, in conjunction with numerical analyses with finite element methods. The Geyser Pump effect (concept of lifting water with solids by releasing a large volume of air intermittently) is now conceptually understood and several different designs have been developed for high lift and also for pumping heavy materials like grit and gravel. This paper explains the basic concepts of the Geyser Pump family and compares it with ordinary airlift pumps and mechanical pumps. In addition, typical applications are introduced for wastewater treatment, aquaponics, and aquaculture.
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Overview of Airlift Pump It is believed that the airlift pump was probably invented by Carl Loescher about 1797. For many years, the system fell into comparative desuetude and only recently has it been revived and improved. In view of its increasing use, and because of its adaptability to many difficult cases of pumping, it is again worth considering. The airlift pump consists of an open vertical pipe with its lower end submerged in the liquid to be raised and having its upper end arranged to discharge into a reservoir at the required height. Air from a compressor or blower is forced through a smaller air pipe into the submerged opening of the lift pipe or rising main. The air bubbles, rising through the water in the lift tube, reduce the specific gravity of the mixture, and therefore the weight of the column. The excess pressure at the base of the column, due to the external water pressure, becomes sufficient to force the mixture above the supply level and out of the top of the pipe. This excess pressure increases with the depth of submersion of the pipe and can be regulated to affect the height to which water is lifted (Gibson, 1961). The advantages and disadvantages of an airlift pump are listed in Table.1. Table 1. Advantages and Disadvantages of Airlift Pumps Advantages • No moving parts • Simple structure • Possible to pump large volume of water
Disadvantages • Difficult to control flow rate • Big air consumption for high lift • Frequent clogging • Weak suction • High electricity cost
Pump Curve for an Airlift Pump Many researchers have tried to develop a theoretical formula to calculate the required air flow for airlift pumps based on the pipe size, submergence depth, lift and pumping rate. However, there is no simple way to determine the air flow rate required. The proposal by Zenz (see Figure 1) is probably the most accurate method of airlift sizing available today (Zenz, 1993).
Fig. 1 Pump curve of Airlift Pump Figure 1. Non-‐dimensional pump curve of airlift pump 2
Zenz proposed a way to estimate the characteristics of the airlift pump from (1) pipe crosssectional area, (2) lift height, (3) submergence, (4) pumping rate, (5) density of lifted fluid, (6) gas density, (7) liquid density, and (8) gas flow rate. Based on this information, one first calculates the ordinate of the graph in Figure 1. Then, the corresponding value of abscissa is found from the graph and finally, the gas flow rate can be calculated using information available for the specific application. Overview of the Geyser Pump The Geyser pump which is sized according to flow requirements, may be constructed of Fiberglass, PVC, or Steel as specified. The Geyser Pump overcomes common problems characteristic of airlift pumps by allowing air to accumulate at the lower end of the riser, and to allow a large single release of air up the riser pipe. This increases the thrust up the pipe to discharge liquids and sludge. An equal volume of liquid and sludge is discharged up the riser each time the geyser releases. The rate of flow may be adjusted by the rate that air is supplied to the Geyser Pump. Air is supplied from a continuously operated air compressor or blower and airflow is adjusted by a valve in the airline. Figure 2 shows the sequence of the Geyser Pump effect.
(1) Pump
Ai r
Ai r
is filled with water.
(2) Air is
Ai r
accumul ated in an air pocket of the bell housing. (4) Air accumulate d in the air pocket is ejected to All air (5) the riser is ejected. with the Water is static sucked water pressure. from the bottom of the riser.
(3) Water seal is about to be broken.
Ai r
Ai r
(6) The bell casing is filled with water for the next step.
Figure 2. Sequence of the Geyser Pulse Effect The Geyser Pulse Pump significantly improves the characteristic advantages of the airlift pump, while allowing it to operate over a much larger range of flow rates. In the Geyser Pump, the bubble is produced in the air cylinder and U-tube assembly. Air is supplied to the 3
air cylinder with the pump riser extending through its center. In the case of an airlift pump, air is released to the riser and the series of air bubbles create an upward flow. The Geyser Pump has an air cylinder with a U-tube. Air is supplied to the U-tube assembly and is not released to the riser, but is trapped in the air cylinder. The air cylinder holds the supplied air to a certain volume. Then, suddenly, all of the air is released through the riser within a second with the help of hydrostatic pressure. This big bubble becomes a kind of air piston that ejects the water and sludge in the riser. The intermittent discharge of a large volume of air makes the Geyser Pump unique. The driving force of an airlift pump is the density difference inside and outside the riser pipe. Considering a 4 inch airlift pump installed in a tank of 10 feet water depth, and air supplied at 2 cfm, the pressure created from the weight of the water in the riser is approximately 3.5 psi downward at the bottom of the riser. The water static pressure is applied upward at the bottom of the riser, which is 4.3 psi for the 10 feet water depth. This pressure difference, 0.8 psi (= 4.3 psi – 3.5 psi), is the force available to pump water and sludge by the airlift pump. The Geyser Pump has two driving forces to transfer the liquid and solids; (1) Ejection and (2) Suction. (1) Ejection When a big bubble is released from the air cylinder within a second, it occupies the whole cross-sectional area of the riser pipe and rises with a large buoyant force. The big bubble acts like an air piston in the riser ejecting water and solids that have accumulated in the riser. The water velocity in the riser above the big bubble, called “ejection velocity”, reaches 13 ft/sec in this geometry. (2) Suction After all of the air is released from the air cylinder, the riser is filled with air. At this moment, water flows to the suction port of the Geyser Pump as the pressure difference is 4.3 psi (= 4.3 psi – 0 psi). Water rises in the riser pipe and this velocity, or “suction velocity”, reaches 20 ft/sec in this geometry. This momentum results in moving the water above the water level outside the riser. Thanks to these two forces, the Geyser Pump can transfer water with heavy sludge, solids or viscous fluids. The Geyser Pump is operated with air much like an airlift pump. However, due to these differences, it is not an improved airlift pump, but rather, a “modified siphon ejection pump”. The water velocity of an airlift pump depends upon the supplied air flow. On the other hand, the “ejection velocity” and “suction velocity” are independent of the air flow and it is far greater than that of an airlift pump. The maximum pumping rate of the airlift pump is 200,000 GPD at an air flow of 35 cfm in this geometry. The water velocity increases to 3 ft/sec at the air flow of 15 cfm and does not increase much even if air flowrate is increased above 35 cfm. The maximum velocity of the water does not exceed 3.5 ft/sec in a 4” airlift pump under these conditions. On the other hand, velocities of water above and below the big bubble in a Geyser Pump are almost independent of the supplied air flow. “Suction velocity” is greater than the “ejection velocity” in general. Both velocities generated by “ejection” and “suction” gradually increase as the bubble rises. “Suction velocity” reaches 18 ft/sec when submergence is 6 feet. 4
This is almost 6 times the airlift pump velocity (3.5 ft/sec) and results in the suction of grit or sludge below the Geyser Pump. This velocity difference is another way to distinguish the Geyser Pump from an airlift pump. Pump Curve of Geyser Pump Pump curves of the Geyser Pump were derived from the actual experiments and the analyses of hydrodynamics. The pump curve data are based on the assumption that pure water is pumped with various Geyser Pumps. If solids are present in the water, or the viscosity of the fluid is higher than that of water, then the pumping rate will be reduced for the same air flowrate or more air flow will be required for the same flow rate. The pumping rate of the Geyser Pump is a function of the size of discharge pipe, air flowrate, submergence and lift. The pump curves are given for various submergence ratios, which are calculated from the following formula: Submergence ratio = Submergence / (Submergence + Lift) Where submergence is the distance from Geyser Pump to water level and lift is the distance from water level to the highest lifting level. Figure 3 shows a typical pump curve for the Geyser Pump.
Figure 3. Pump Curve of 4” Geyser Pump Overview of Geyser Ejection Pump for High Lift Based on the Geyser Pump Effect, the Geyser Ejection Pump was developed to supply a constant flow pumping rate for very high lift. The Geyser Ejection Pump has one or two check valves. If needed, aerated fine screens are equipped to the pump to protect the check valve, so that no large solids flow into the pump to assure a steady performance for a long period and for easier maintenance. Figure 4 shows the conceptual design of the Geyser Ejection Pump and how the pump transfers liquid. 5
Step 1 (Fill period) When the Geyser Ejection Pump is installed in a tank with water, water flows into the air cylinder through the check valve. Depending upon the size of the pump, one or two check valves are installed at the bottom of the tank. The length of the fill period is dependent upon the water depth outside the pump, the size of the check valve, and the volume of the air cylinder. This is typically 0.5 sec to 1.5 sec. The check valve is normally closed.
Ai r
Step 1
Ai r
Step 2
Ai r
Step 3
Ai r
Step 4
Step 5
Figure 4. What’s happening in the Geyser Ejection Pump Step 2 (Ejection period) Air is injected through the air intake. As the U-tube creates an air pocket, air cannot be released to the discharge pipe, but it is stored in the air cylinder. As the pressure inside the air cylinder builds up, water in the air cylinder is ejected gradually through the discharge pipe. The length of the period is controlled by adjusting the air flow with a valve. It varies from 1 second to 2 minutes, depending upon the air cylinder volume and air flow. Step 3 (End of ejection period) Air is accumulated to the air cylinder until water level reaches the bottom of the U-tube. Until this moment, the check valve has been closed from Step 1. Step 4 (Air Release + fill period) All air accumulated in the air cylinder suddenly escapes to the discharge pipe through Utube. The pressure in the air cylinder drops and the check valve is opened. Static water pressure outside the Geyser Ejection Pump enhances the air release. This period lasts typically 1 to 3 seconds, depending upon the air cylinder volume and the size of the check valve. Step 5 (Fill period) 6
After air release, water flows into the air cylinder continuously until the inside pressure reaches the static water pressure outside the pump, then check valve is closed.
The Geyser Ejection Pump has the following advantages: (1) (2) (3) (4) (5) (6) (7) (8)
High lift Running dry does not hurt the pump Pumping rate is independent of the water depth Easy to control flow rate Easy to estimate the pumping rate Easy to maintain Less energy required No control panel needed
Applications – Improvement of Wastewater Treatment Plant Performance Extended aeration package plants are frequently used by small communities that are required to achieve high quality effluent standards. These systems perform well in treating various waste loads and flow rates. One of the main limitations of the package treatment system is that there is very little operational control designed into the system. With the limited ability for operational modifications, the system will typically release excessive solids from time to time. This results in a labor-intensive response of cleaning slow sand filters or degradation of water quality in the receiving stream. Operators rely on four basic control mechanisms for activated sludge systems: (1) How much aeration to apply. (2) How much excess solids to waste from the system. (3) The ability to switch from plug flow to contact stabilization. (4) Adjustment of the return activated sludge (RAS) flow rates. Package plants usually provide the operator with limited aeration and wasting capabilities. The Geyser Pump can provide the operator a wide range of control over the RAS flow rates and reduce the clogging events of standard airlift designs. The ability to reduce the RAS flow rate with strong suction will create following advantages: (A) The oxidative treatment capability of the system is increased by increasing hydraulic detention time in the aeration basin. (B) The performance of the clarifiers is increased by increasing the hydraulic and solid detention time in the clarifier. The layer of sludge blanket is reduced and the supernatant layer is increased with improved clarity. (C) The RAS line connected to the Geyser Pump seldom clogs, based on the experience of over 2,000 applications of Geyser Pumps. After the power failure, an airlift pump requires a valve operation to start up the operation again. However, even after the power failure, the Geyser Pump regains the performance automatically. (D) Bridge formation of sludge in the clarifier does not occur as the Geyser Pump gives frequent pulsated suction to accumulated sludge. (E) The thickest sludge pumped by the Geyser Pump is 6% solids (60,000 mg/l) in the sewage treatment plant. When the Geyser Pump is properly installed, no pop-up sludge is observed in the clarifier. 7
(F) Increasing the solids concentration of the solids. This is very significant when a slow settling, filamentous sludge condition exists. Filamentous sludge conditions are common with package plants and often are the reason for excessive loss of solids from the treatment system. (G) Reduction in the amount of extraneous water pumped to the digester. This will reduce the amount of staff time required for decanting of holding tanks, thus increasing holding capacity. Applications – Solids Removal for Aquaponics and Aquaculture The accumulation of waste solids in an aquaculture system results from uneaten feed, feed fines, fish fecal matter, algae, and biofilm cell mass sloughed from biological filters. These wastes are major sources of carbonaceous oxygen demand and nutrient input in the water. They can directly affect fish health within recirculation systems by damaging fish gills and harboring pathogens. If this organic matter accumulates in the system, it will depress dissolved oxygen (DO) levels as the waste decays and produce carbon dioxide and ammonia. If deep deposits of sludge form, they can decompose anaerobically (without oxygen) producing methane and hydrogen sulfide, which are very toxic to fish. Waste solids influence the efficiency of all other unit processes in a recirculating system. Therefore, solids removal can be one of the most critical processes in an aquaculture system. Studies indicate that fish produce between 0.3 to 0.4 kg of total suspended solids (TSS) for every 1 kg of feed fed. Optimally, solids need to be removed from the fish culture tank as soon as possible, while creating as little turbulence and mechanical shearing as possible. Circular fish tanks are operated by injecting water flow tangentially to the tank wall at the tank outer radius so that the water spins around the tank center, creating a primary rotating flow. This inward radial flow along the bottom of the tank carries settleable solids to the center drain and thus creates the self cleaning property desired in circular tanks. Unfortunately, in a circular tank with such flow, the area near the center drain will be a zone of lower velocities and poor mixing, resulting in solids settling onto the tank bottom. Because aquaculture solids have specific gravities that are relatively close to that of water (typically 1.05 - 1.20 vs. 1.00 for water; Chen et al, 1993; Potter, 1997), sloping the floor towards the center drain does not improve the self-cleaning attributes of the circular tank when velocities are low. The existing technology will not allow the effective elimination of solids from the fish tank. Therefore, the solids loading to the clarifier increases and scum is generated due to a long solids detention time. To prevent scum formation in aquaponics, researchers at the University of Virgin Islands let approximately 30 male tilapia fingerlings swim in the clarifier to graze on the clarifier walls and consolidate solids at the base of the cone. The fingerings swim into and through the drain lines to keep them clean. Although fingerlings are needed for effective clarifier performance, their grazing and swimming activities are also counterproductive as they resuspend solids which exit through the clarifier outlet. As fingerlings become larger (>200 g), clarifier performance diminishes. Therefore, periodic replacement (once every 4 months) of clarifier fish with small fingerlings (50 g) was necessary. Another problem caused by the wastes in granular hydroponic media such as gravel and sand is the tendency to clog. In the case of serious clogging due to organic matter the gravel and 8
sand filters may be overloaded resulting in the production of ammonia as the organic matter decays which may kill the plants or greatly reduce the growth. Significant improvement in solids control was observed in an aquaponics system set up in San Diego. In September of 2007, a 150 gallon stock tank (in a 6' x 8' Portable Farms greenhouse) was installed to hold the 80+ Tilapia in an aquaponics system. An air lift pump made from a ½'' PVC pipe was used to move the water to a settling tank. The air lift was mounted a modified 3'' PVC end cap with four arches cut to function as legs. This old style air lift pump was capable of lifting water approximately 5 inches above the surface of the water. The air lift pipe assembly was on the bottom of the stock tank in order to draw fish waste and debris from the bottom of the tank. The water in the tank was never clear. Even the lightest feeding schedule produced turbid water and feeding the Tilapia at the recommend level produced very dirty water. The discharge pipe required cleaning every two weeks as it became clogged with a slimy growth and fish waste. The settling tank required cleaning every two weeks. In March a ½'' outlet Geyser Pump was installed and a noticeable difference in water quality was observed. Within four hours the water was so clear that the bottom of the stock tank can be seen. The fish were visible and so was the Geyser Pump. Even the pebbles that had fallen from the Growth Trays were visible. The best part was the fact that the Geyser Pump could easily lift the water over 16” above the stock tank water surface. The secondary effects were just as pronounced. The discharge pipe never requires cleaning. It remains clean and clear with no growth or accumulation of any kind. The other effects noted include the fact that the settling tank now needed to be cleaned on a weekly basis because the feeding of the fish was back up to the recommended level. The water would occasionally become cloudy after the fish are fed. After about an hour, the water becomes clear again and the pebbles may be seen on the bottom. Duckweed is also fed to the fish. When the air lift pump was used there was never any duckweed in the settling tank. With the Geyser Ejection Pump the duckweed would occasionally show up in the settling tank because of the effectiveness of moving water from the fish tank. Applications – Water Pumping for Aquaponics and Aquaculture Typical aquaponics for hobby use requires a pump that has a capacity of between 60 and 150 gallon per hour with 5 to 6 feet of lift. Mechanical pumps like centrifugal pumps, submersible pumps and magnetic drive pumps requires frequent maintenance due to problems associated with debris in the water. For this application, a 1” Geyser Ejection Pump was developed and used in Portable Farms in San Diego. Figure 5 shows a pump curve of this special model. The required air flow is less than 0.2 cfm at 2 psi (for 5 feet lift). Wastes can be eliminated effectively using the Geyser Ejection Pump since the suction force is the same as that of the Geyser Pump. 9
Figure 5. Pump Curve of 1” Geyser Ejection Pump for Aquaponics References Bullock, C.L., Herman, J., Noble, A., Weber, A., Hankins, J.A. 1994. Observation on the occurrence of bacterial gill disease and amoeba gill infestation in rainbow trout cultures in a water recirculation system, J. Aquatic. Animal. Health. 6:310-317. Chapman, P.E., Popham, J.D. Griffin, J., Michaelson, J. 1987. Differentiation of physical from chemical toxicity in solid waste fish bioassay. Water, Air, and Solid Pollution. 33:295-308. Chen, S., Timmons, M.B., Aneshansley, D.J., Bisogni, J.J. 1993. Suspended solids characteristics from recirculating aquacultural systems and design implications. Aquaculture. 112:143-155. Colt, J. 2006. Water quality requirement for reuse systems. Aquacultural Engineering. 34: 143156. Gibson, A.H. 1961. Hydraulics and its Applications. London, England. Potter, A. 1997. Alteration of the mechanical properties of fish waste via dietary components. Master Science Thesis. Cornell University. Ithaca, NY. Serfling, S.A. 2006. Microbial flocs--Natural treatment method supports freshwater, marine species in recirculating systems. Global Aquaculture Advocate. 9(3), 34-36. Timmons, M.B., Ebeling, J.M., and Rakocy, J.E. 2007. Recirculating Aquaculture. 19: 767-822. F.A. Zenz. 1993. Explore the Potential of Air-Lift Pumps and Multiphase Flow. Chemical Engineering Progress. August 1993, pgs 51-56.
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