BUBBLE WASHED BEAD FILTERS
Bubble washed bead filters have an "hourglass" shaped internal geometry with a constricted washing throat, although some models may appear to be cylindrical. These filters contain no moving parts for agitation of the bead bed. During continuous filtration, water from the rearing tank enters the unit at the bottom through a slotted inlet pipe, passes upward through the bead bed which floats in the upper filtration chamber, and exits through a slotted discharge pipe at the top. The inlet pipe also serves as the sludge discharge line during backwashing. The filter is equipped with an air inlet below the center of the washing throat. Four valves (inlet, sludge, air, and discharge) control the filter's operation. Check valves are sometimes used on the air inlet and water outlet lines to simplify backwashing.
Backwashing (illustration) is accomplished by completely draining all fluid from the unit. This causes the bead bed to collapse, and individual beads to be vigorously scrubbed by cavitation and bubbles as they are sucked downward through the washing throat. The backwashing process removes solids captured by the bead bed, but retains the beads with their delicate films of nitrifying bacteria.
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BEAD FILTER SIZING CRITERIA
Bead filter sizing criteria are dependent on:
- Filter Application
- Density
- Feed Rate
Generally, bead filters are sized according to the maximum amount of feed (dry pellets - 35% protein) that will be put into the system per day and/or the maximum anticipated load of animals in the system.
One cubic foot of beads (400 ft 2 of surface area) can provide complete solids capture and nitrification for up to one pound of feed per day, corresponding to 100 pounds of fish fed 1% body weight per day. At this feed rate and/or density, one can generally expect total ammonia nitrogen (TAN) levels below 1.0 mg/l, nitrite levels below 1.0 mg/l, and turbidity levels below 5 NTU's. However, these levels may vary through synergistic impacts of other water quality parameters and/or management practices.
Water quality standards may vary with different applications. For instance, higher standards of water quality and clarity may be desired for maintenance of brood stock and display of ornamental fish than for a commercial food fish grow-out system. A range of water quality and clarity standards can be achieved by adjusting feeding rates and/or stocking densities. Table 2 presents the filtration capacities for several applications, feed rates and animal densities.
FLOW RATE
The general rule of thumb for estimating optimal flow rates is 5 gallons per minute per 100 pounds of fish fed 1% body weight per day. This corresponds to 5 gallons per minute per pound of feed per day, or 5 gallons per minute per cubic foot of beads. This flow rate assures that the mass transport criteria for TAN and oxygen are met, thus avoiding ammonia buildups in the system and assuring that sufficient oxygen is transported to the bacteria at peak loads. Flow rates below this level are acceptable if the feed rate is reduced proportionally. Flow rates above the optimum do not adversely affect performance but are undesirable because they waste energy and can lead to critical pressure buildups.
PUMP SELECTION
The maximum operating pressure of the fiberglass propeller-washed bead filters is 20 psi, while the maximum operating pressure of the fiberglass bubble-washed bead filters is only 15 psi, the maximum operating pressure of the plastic bead filters (BBF-1, BBF-2 and PBF-3) is only 10 psi. A continuous duty pump that is rated to deliver the desired flow at a pressure of about one-half the maximum operating pressure should be selected if the filter is to be heavily loaded. The pump must also be able to accommodate pressures as low as 2 to 5 psi. Pumps with high shut-off pressures (> the maximum operating pressure) or high minimum pressures (> 5 psi) should be avoided.
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BBF Pumps & PBF Pumps
DISPOSAL OF BACKWASH WATERS
The discharged effluent will display the same water quality parameters as the system water; however, it will be burdened with a suspended solids load of 500-1500 mg/l, reflecting the scouring of the solids accumulated in the filter. These solids are generally well decomposed by the bacteria in the bead filter. The backwash waters are enriched with phosphorus and nitrogen (principally nitrates), and can be used as a low grade fertilizer. The solids show little tendency to generate odors and are quickly assimilated by the receiving soils.
Alternatively, the backwash waters can be discharged directly into a sewer line. The wash waters are entirely compatible with centralized or household treatment systems, which are based in large part on the same bacterial communities that effectuate treatment in the bead filters.
Direct discharge to natural ponds and streams should be avoided. The quantity of water produced is very small and impacts from direct discharges would probably be minimal; even so, the quality of the water is poor when its solids content is considered. A small retention pool or tiny artificial wetland would quickly capture the solids and allow fluids to seep into the ground without impact. Some of the water's fertilization potential will remain, however, so discharges that could seep into pristine waters such as clear ponds or trout streams should be undertaken with extreme caution.
TYPICAL TREATMENT CONFIGURATIONS
Bead filters are very versatile and can be used with a variety of other treatment processes. The pressure limits on the filter housing preclude the connection of bead filters ahead of devices such as heat exchangers that must be driven at high pressures (Table 3). However, serial treatment is compatible with low head devices, e.g., biofilters, packed columns, and UV lights, which can tolerate the minor flow declines that occur near the end of the filtration cycle. Parallel treatment is used to alleviate concerns about pressure build-up, flow variation, or oxygen depletion. As a general observation, systems configured with the major treatment processes in parallel are easier to manage. Each process can be optimized on its own flow loop without interaction from serially connected devices. Traditional wastewater treatment philosophies which dictate sequentialoperations have little validity in a recirculating system where the animals live on the influent (intake) side of the filter. Recirculation rates dictated by TAN (total ammonia nitrogen) and oxygen mixing constraints are generally so high that the order of treatment has little relevance. The principal disadvantage of parallel treatment is the higher flow rates required. This problem can be addressed by selecting a pump that operates efficiently at the lower pressures characteristic of parallel treatment loops.
Figure 5 illustrates four common treat configurations which utilize bead filters. Bead filters are often used to treat lightly loaded ornamental fish systems (below about 0.50 lbs. of feed per cubic foot of beads per day). Ultraviolet light treatment units are often placed after the bead filter to provide for disease control. The addition of UV lights to pond treatment systems will also effectively control blooms of single-celled algae. For moderate loading regimes (up to about .75 lbs. feed per cubic foot of beads per day), the bead filter will provide excellent clarification and biofiltration by itself. If aeration provided by the return flow from the bead filter is insufficient, aeration and carbon dioxide stripping by blown air are necessary. In cases where the feed causes foaming, air-driven foam fractionators are added to the system.
As the loading increases above 1.0 lb. feed per cubic foot of beads per day, supplemental nitrification must be provided. Two commonly used biofilters for supplemental nitrification are the fluidized bed and the rotating biological contactor (RBC). Fluidized beds are best operated in parallel with the bead filters to maintain proper flow rates and supply of oxygen. However, they can also be installed in series. This powerful combination can produce a large amount of carbon dioxide that drives down the pH and reduces nitrification, so a packed column degasification unit is often installed after the bead filter. The packed column unit may not be required when the bead filter is placed in series with an RBC. The RBCs possess excellent gas exchange characteristics needed for efficient removal of carbon dioxide.