Filter Approach Keeps Out Organisms
A new approach to surface-water intake design, which uses a small-particle filter element as the primary screening device, prompted a small-scale pilot test over three months in the fall of 2004 in the tidal region of the Taunton River in Dighton, Mass. The program set out to demonstrate the feasibility of the concept and to obtain operating data on the filter backwash system by Filtrex Industrial Filtration, Attleboro, Mass.
Test results indicate that the filter provides an effective tool for eliminating the potential for entrainment of marine organisms as small as 40 micron. The low approach velocities associated with the system design resulted in minimal, if any, impingement.
The Clean Water Act stipulates that the Environmental Protection Agency monitor and regulate "the location, design, construction and capacity standards for cooling-water intake structures," requiring that they reflect "the best technology available for minimizing adverse environmental impact(s)."
Small Marine OrganismsAccording to the EPA, cooling-water intake structures often pull fish, shellfish or their eggs into the cooling system of an industrial or power plant facility. "There, the organisms may be killed or injured by heat, physical stress or chemicals used to clean the cooling system," the EPA notes. "Larger organisms may be killed or injured when they are trapped against screens at the front of an intake structure."
Entrainment and impingement of these small marine organisms can and have impacted the population of some fish species. As a result, close attention now is being given to the design of new and existing intake structures to minimize environmental harm. Under the recently issued Clean Water Act 316 (b) Phase II and III rules, cooling-water intake velocities must be reduced to at least 0.5 ft/sec. However, it is not uncommon to see maximum intake velocities of 0.3 ft/sec at some new intake locations. Velocities as low as 0.1 ft/sec are desirable to avoid the entrainment of fish eggs and other passive organisms.
The small size of the passive fish eggs (as tiny as 0.01969" [0.5 mm]) and larvae (even smaller at 0.01339" [0.34 mm]) makes it important to have very low approach velocities to avoid capturing these organisms when they are near intake-screening devices. Therefore, the new rules require the incorporation of much larger surface areas at the filtration interface than traditionally have been required.
Historically, intake systems that withdraw large quantities of water typically comprise either an intake canal or bulkhead wall leading from the water body to a screening device that usually contains stationary or moving screens with openings sized at +/-0.375" (+/-9.525 mm). These screens prevent fish from entering the system but do little to keep out eggs and larvae. With large openings, sufficient free area is provided that results in reasonable velocities (0.5 ft/sec or less) without excessively large footprint requirements. However, when the maximum screen velocity is set at 0.5 ft/sec and the allowable opening size is reduced to 0.34 mm or less, the percentage of free area is significantly reduced and the footprint for traditional screening devices becomes unacceptably large.
The pilot test investigating a modified use of the Filtrex filter explored whether the technology was suitable to meet the updated standards, in particular the new maximum intake velocity of 0.5 ft/sec and the requirement that existing systems be improved to achieve major reductions in organism mortality rate -- as high as 80 percent -- in fish eggs and larvae. The difficulties encountered in designing and operating these systems highlight three main issues:
Although the Filtrex filter was designed to be used as a pressurized filter for industrial applications, preliminary bench-scale testing of a 40 micron (0.040 mm) element indicated that reasonable flow rates through the filter could be achieved under gravity flow or low-suction head conditions associated with surface-water intake designs. With such arrangements, the filter provided an effective and safe barrier to extremely small organisms (>0.040 mm) and other suspended solids in the raw-water source. The filter's geometry virtually eliminated the entrainment of marine organisms in the size range of concern (300 to 500 micron).
Field TestingUnder actual low-head operating conditions at Taunton River, the Filtrex filter performance was evaluated. The Taunton River provides drainage for a river basin of 458 mi2 and, at the locale of the pilot test, also experiences bidirectional flow with a tidal range of approximately 5'.
Taunton River water quality varies from fresh-water runoff (150 ppm TDS) at low tide to brackish (4,000 to 20,000 ppm TDS) at high tide. During ebb and flood tide conditions, stream-flow velocity is approximately 1.5 to 2.0 ft/sec. After periods of rain and heavy surface-water runoff upstream, the water contains a high level of silt and debris with particle sizes ranging from very large to well below 40 micron.
The test unit contained ports for up to nine elements with separate connections for filtered water and backwash water. A support platform was installed and the unit was mounted in a location approximately 14' deep at high tide. The filter assembly connected to the suction side of an intake pump (170 gpm/10' TDH), and water was drawn through the filter and discharged back to the river.
For various element lengths and flow rates, the head loss across the filter was monitored and the filter was periodically backwashed by a separate pumping system (60 gpm/20' TDH) in accordance with observed increases in filter pressure drop over time. Periodically, the filter was removed and inspected, including inspection of the wafers under microscope.
Using the 40 micron rated element, flow tests with clean water were performed with various length elements to determine the optimum candle length. As might be expected, at a constant flow rate, the longer the element, the lower the pressure drop. However, because the cross-sectional flow area of the internal cavity is fixed, there is a point at which continued increase in length does little to reduce the overall pressure drop across the element. Generally speaking, a typical intake installation provides only a small amount of head; therefore, it is desirable to optimize the element length to provide the maximum flow-to-head-loss ratio, and thereby reduce the system footprint.
Spring 2005 TestingBased on several months of immersion and periodic testing of various filter elements, the conclusion is that the operational characteristics regarding head loss, plugging, backwash capability and marine fouling were consistent with prior bench-scale tests. After a test run of approximately two weeks, the Filtrex unit showed no signs of fouling or plugging. Given the geometric configuration of the basic filter and its use of a three-dimensional approach to intake screening, large-scale intake structures can be configured to fit within a relatively small footprint while meeting the Clean Water Act's new Section 316 (b) velocity, entrainment and impingement standards.
The Filtrex filter element provided a high degree of filtration, and in addition to addressing the issues of entrainment and impingement, the filter provided secondary benefits to water users by reducing the quantity of suspended material that would otherwise be brought into the system. Although the effectiveness of the filter in reducing suspended solids and organic carbon will vary depending upon the characteristics of the water source, it appears that a reduction of 20 percent to 40 percent reasonably can be expected.
Although the field tests did not capture the primary egg and larvae season, some species were known to be present and inspection of the filter after operation showed no evidence of impingement of either organic or inorganic material on the filter surface. For purposes of fully demonstrating the impingement characteristics of the filter, field testing will resume in spring 2005 during the period when populations of various egg and larvae species are significant. At that time, video-monitoring equipment will be installed to monitor real-time operation of the filter. PCE
Sidebar: It's All in the Design DetailsThe primary filtration element of the intake filter from Filtrex Industrial Filters, Attleboro, Mass., consists of a support rod on which polypropylene wafers are stacked to form a filter element of the desired length, typically 6" or less for intake applications.
Both face surfaces of the wafers are etched with small "V" grooves that provide the filtration passage from the outer surface into six longitudinal cavities leading through each wafer and into the common chamber on the head end of the element. The element's filtration rating is determined by the dimensions of the "V" groove. For the gravity-feed surface-water intake application, the 40 micron rating was selected.
The filter provides 30 percent free area. With the filter operating at a design intake rate of 2 gpm/in of element length, a slot velocity of 0.254 ft/sec and approach velocity of 0.1 ft/sec at the wetted perimeter are realized. These velocities are well within the Clean Water Act's Section 316 (b) new standard of 0.5 ft/sec maximum.