Pilot Tests Demonstrate Filter Can Address New EPA Standard

Pulp & Paper, Feb 2005 by Hanson, Jeffrey H, Murphy, John T

Pilot test shows filter can protect marine life to comply with Clean Water Act Section 316 (b) standards, while also reducing suspended solids and organic carbon

In the design of intake systems for withdrawing large quantities of water from surface water sources, the entrainment and/or impingement of fish, larvae, fish eggs, and other biota have been a longstanding problem. Various investigations have concluded that entrainment and impingement of small marine organisms have impacted the population of some fish species. As a result, close attention is now given to the design of new and existing intake structures to minimize the occurrence of such situations.

As a part of the Clean Water Act section 316(b) rules, the EPA monitors and regulates "the location, design, construction and capacity standards for cooling water intake structures"1 to require that they reflect "the best technology available for minimizing adverse environmental impact(s)." The EPA further notes, "Cooling water intake structures cause adverse environmental impact by pulling large numbers of fish and shellfish or their eggs into a power plant's or factory's cooling system. There, the organisms may be killed or injured by heat, physical stress, or by chemicals used to clean the cooling system. Larger organisms may be killed or injured when they are trapped against screens at the front of an intake structure."

Under the recently issued "316(b) Phase II & III" rules, cooling water intake velocities must be reduced to 0.5 fps, and it is not uncommon to see maximum intake velocities of 0.3 fps at some other new intake locations. In some locations, velocities as low as 0.1 fps are desirable to avoid the entrainment offish eggs and other passive organisms.

To address the new issues raised by implementation of the Clean Water Act Section 316 (b) Phase II & III, an investigation and field test were conducted to analyze the effectiveness of a small particle filtration element. As a result of this filter's geometric configuration, used with a three-dimensional approach to intake screening, the necessary large-scale intake structures can now be configured to fit within a relatively small footprint while meeting the new velocity, entrainment, and impingement standards.

Issues with Water Intake Systems

Because of the small size of passive fish eggs (about 0.5 mm) and larvae (about 0.34 mm), it is important to have very low approach velocities to avoid capture of organisms that are near intake screening devices. Thus, the new EPA rules require the incorporation of much larger surface areas at the filtration interface than have traditionally been required. Although the new rules were primarily established to minimize the mortality rates of various marine organisms, once implemented, the new requirements will also result in inherent operating benefits, since raw water quality will be improved as compared with current screening designs and operations.

Historically, intake systems for withdrawing large quantities of water are typically comprised of either an intake canal or bulkhead wall leading from the water body to a screening device typically containing stationary or moving screens with openings sized at /- 3/8 in. These screens prevent fish from entering the system but do little to prevent eggs and larvae from being drawn in.

With the large size openings, sufficient "free area" is provided to result in reasonable velocities (0.5 fps or less) without excessively large footprint requirements. However, when the maximum screen velocity is set at 0.5 fps 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.

Existing Filter Shows Potential

As a part of its ongoing investigation of available technologies to address the new standards established by Section 316 (b), Hanson, Murphy & Associates, independent consultants for power plants and municipalities, used filter technology developed by Filtrex Corp. to conduct investigations and field tests regarding the modified use of a Filtrex filter to address the new standards.

In particular, the new standards set a maximum intake velocity of 0.5 fps and require that existing systems be improved to achieve major reductions in the "mortality rate" (eg., 80% reduction) in fish eggs and larvae. The difficulties encountered in designing and operating these systems are categorized by three main issues:

* Because of the small opening sizes, the physical dimensions of the intake barrier necessary to achieve the desired low velocities can be very large, resulting in undesirable impacts to waterfront areas and the waterways where they are located.

* Once blocked, effective cleaning and removal of the "captured" organisms from the filtering media should be accomplished without damage to the organism.

* The system must incorporate design features to prevent "recapture" of organisms once removed from the filter surface after backwashing. Traditional intake canals and bulkhead walls typically result in "dead-end" situations that provide no means of escape for the captured organisms.