Abstract:
A liquid purification and separation apparatus for separation of pollutants in stormwater runoff is disclosed. This apparatus utilizes gravitational separation and tortuosity, resulting from a plurality of baffles both perpendicular to and oblique to the primary water flow direction, to trap substances less-dense and more-dense than water. The apparatus features improved resistance to pollutant remobilization through an interactive hydraulic design process resulting in greater pollutant retention.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Apparatus for treatment of stormwater runoff through volume-control-based detention and minimization of pollutant remobilization. 
     2. Description of the Related Art 
     This invention relates generally to liquid purification and separation and, more specifically, to an apparatus for separation of pollutants in urban stormwater runoff from the runoff water. This apparatus utilizes gravitational separation and tortuosity, resulting from a plurality of baffles both perpendicular to and oblique to the primary water flow direction, to trap substances less-dense and more-dense than water. This invention is differentiated from prior art by improved resistance to pollutant remobilization, resulting from an iterative experimental hydraulic design process. In addition, this invention provides a degree of retention through volume-control that exceeds that provided by existing gravitational, sub-surface, stormwater treatment systems. 
     Impacts of stormwater runoff on receiving environments have been documented extensively in engineering and scientific literature. Section 402 of the Federal Clean Water Act (CWA) regulates stormwater discharges through the National Pollutant Discharge Elimination System (NPDES). Treatment of stormnwater runoff using best management practices (BMPs) is a typical requirement of state and local regulations, as well. In the 1990s, there has been growing interest in ‘ultra-urban/space limited’ BMP&#39;s, such as sand filters, water quality inlets, and, reservoir/vault type of structures. Space constraints, high property values, soil conditions, and the proximity of other building foundations often preclude the use of conventional, space-intensive stormwater BMP&#39;s such as detention ponds. For in-fill construction or redevelopment in built-up urban areas, where pollutant loads from urban runoff are usually the greatest, unconventional stormwater treatment technologies may be necessary. 
     Vault-type treatment technologies have been widely used for stormwater treatment in urban areas; however, the effectiveness of these devices for removal of suspended solids and oil and grease has been only marginal. A great weakness of these types of devices has been that large storm events tend to flush out the system, thereby releasing pollutants that were previously removed. 
     Prior art in the field of this invention of which the applicant is aware includes the following: 
     U.S. Pat. No. 4,127,488, Bell, J. A. et al., November 1978, Method and apparatus for separating solids from liquids. 
     U.S. Pat. No. 4,136,010, Pilie, R. J. et al., January 1979, Catch basin interceptor. 
     U.S. Pat. No. 4,328,101, Broden, C. V., May 1982, Device for separating particulate matter from a fluid. 
     U.S. Pat. No. 4,363,731, Filippi, R., December 1982, Device for regulating the flow of waste waters. 
     U.S. Pat. No. 4,383,922, Beard, H. J., May 1983, Waste water clarifier. 
     U.S. Pat. No. 4,983,295, Lamb, T. J. et al., Jan. 1991, Separator. 
     U.S. Pat. No. 4,985,148, Monteith, J. G., January 1991, Improved separator tank construction. 
     U.S. Pat. No. 5,004,534, Buzzelli, V., April 1991, Catch basin. 
     U.S. Pat. No. 5,186,821, Murphy, D. T., February 1993, Wastewater treatment process with cooperating velocity equalization, aeration, and decanting means. 
     U.S. Pat. No. 5,342,144, McCarthy, E. J., August 1994, Stormwater control system. 
     U.S. Pat. No. 5,520,825, Rice, W. M., May 1996, Oil-water separator. 
     U.S. Pat. No. 5,536,409, Dunkers, K. R., July 1996, Water treatment system. 
     U.S. Pat. No. 5,637,233, Earrusso, P. J., June 1997, Method and apparatus for separating grease from water. 
     U.S. Pat. No. 5,679,258, Petersen, R. N., October 1997, Mixed immiscible liquids collection, separation, and disposal method and system. 
     U.S. Pat. No. 5,759,415, Adams, T., June 1998, Method and apparatus for separating floating and non-floating particulate from rainwater drainage. 
     U.S. Pat. No. 5,788,848, Blanche, P. et al., August 1998, Apparatus and methods for separating solids from flowing liquids or gases. 
     U.S. Pat. No. RE30,793, Dunkers, K. R., November 1981, Apparatus for water treatment. 
     In addition to the patents listed above, a number of inventions in the general field of stormwater treatment methods and devices were discovered during the patent search. The inventions listed below have an element or elements similar to the invention disclosed herein; however, additional elements, details of elements, and/or applications of the inventions differ significantly from the forms and functions of the present invention. While the inventions listed below are intended to provide stormwater treatment, the principle of operation for many of these devices is filtration rather than sedimentation. 
     U.S. Pat. No. 4,298,471, Dunkers, K. R., November 1981, Apparatus for equalization of overflow water and urban runoff in receiving bodies of water. 
     U.S. Pat. No. 4,377,477, Dunkers, K. R., March 1983, Apparatus for equalization of overflow water and urban runoff in receiving bodies of water. 
     U.S. Pat. No. 4,664,795, Stegall, W. A. et al., May 1987, Two-stage waste water treatment system for single family residences and the like. 
     U.S. Pat. No. 4,747,962, Smissom, B., May 1988, Separation of components of a fluid mixture. 
     U.S. Pat. No. 4,865,751, Smissom, B., September 1989, Separation of components of a fluid mixture. 
     U.S. Pat. No. 5,080,137, Adams, T. R., January 1992, Vortex flow regulators for storm sewer catch basins. 
     U.S. Pat. No. 5,232,587, Hegemier, T. E. et al., August 1993, Stormwater inlet filters. 
     U.S. Pat. No. 5,322,629, Stewart, W.C., June 1994, Method and apparatus for treating stormwater. 
     U.S. Pat. No. 5,403,474, Emery, G. R., April 1995, Curb inlet gravel sediment filter. 
     U.S. Pat. No. 5,437,786, Horsley, S. W. et al., August 1995, Stormwater treatment system/apparatus. 
     U.S. Pat. No. 5,480,254, Autry, J. L. et al., January 1996, Storm drain box filter and method of use. 
     U.S. Pat. No. 5,549,817, Horsley, S. W. et al., August 1996, Stormwater treatment system/apparatus. 
     U.S. Pat. No. 5,702,593, Horsley, S. W. et al., December 1997, Stormwater treatment system/apparatus. 
     U.S. Pat. No. 5,707,527, Knutson, J. H. et al., January 1998, Apparatus and method for treating stormwater runoff. 
     U.S. Pat. No. 5,730,878, Rice, T., March 1998, Contaminated waste water treatment method and device. 
     U.S. Pat. No. 5,744,048, Stetler, C. C., April 1998, Clog resistant storm drain filter. 
     U.S. Pat. No. 5,770,057, Filion, G., June 1998, Overflow water screening apparatus. 
     U.S. Pat. No. 5,779,888, Bennett, P. J., July 1998, Filtering apparatus. 
     U.S. Pat. No. 5,810,510, Urriola, H., September 1998, Underground drainage system. 
     U.S. Pat. No. 5,840,180, Filion, G., November 1998, Water flow segregating unit with endless screw. 
     U.S. Pat. No. 5,890,838, Moore, Jr. Et al., April 1999, Stormwater dispensing system having multiple arches. 
     U.S. Pat. No. 5,972,216, Acemese, P. L. et al., October 1999, Portable multi-functional modular water filtration unit. 
     U.S. Pat. No. 5,985,157, Leckner, J. P. et al., November 1999, Filter device. 
     SUMMARY OF THE INVENTION 
     An aspect of this invention is to provide an apparatus for removal of pollutants with densities greater than and less than water from stormwater runoff. 
     Another aspect of this invention is to provide an apparatus that retains and immobilizes trapped pollutants, even during periods when flows are high. 
     Another aspect of this invention is to accumulate pollutants that are less and more dense than water until a time when the apparatus is cleaned out. 
     Another aspect of this invention is to minimize velocity in the vicinity of the bottom of the apparatus to minimize resuspension of deposited sediments and associated pollutants. 
     Another aspect of this invention is to provide an apparatus that can provide treatment of stormwater for larger tributary drainage areas by addition of modular sections. 
     Another aspect of this invention is to collect stormwater runoff and release it at a controlled rate over a specified period of time via an outflow opening. 
     Other aspects and advantages will become apparent hereinafter. 
     One aspect of the invention is a rectangular chamber of variable length and height assembled in a modular fashion. The rectangular chamber contains a system of overflow and underflow baffles, both perpendicular to and oblique to the primary direction of flow from the inlet to the chamber to the outlet from the chamber, which are located at opposite ends of the rectangular chamber. The baffles in the chamber serve several purposes including: flow momentum and energy dissipation, creation of a tortuous flow path, retention and immobilization of pollutants less and more dense than water, minimization of resuspension of sediments, and minimization of remobilization of floatable pollutants into the water column. The primary process for pollutant removal is gravitational separation, which occurs while water is detained in the chamber. 
     A baffle configuration for minimization of resuspension of trapped sediments and associated pollutants was first conceptualized by the inventors and then optimized by iterative experimentation involving three dimensional velocity measurements and dye visualization for a plurality of baffle configurations using a geometrically and hydraulically scaled physical model. Baffle configurations were evaluated for both dynamic (chamber filling and draining) and steady-state (chamber full with inflow rate equal to outflow rate) conditions. This exhaustive experimentation indicates that the baffle configuration of the invention disclosed minimizes resuspension of fine and coarse sediments and associated pollutants to a degree that exceeds the capabilities of prior art. In addition, a trapezoidal underflow baffle, the shape of which was optimized during hydraulic experimentation, impedes material less dense than water from entering the outflow section and exiting the vault. The trapezoidal configuration has the advantage of decreasing the downward velocity of water approaching and then moving under the baffle and into the outlet section and, thereby, decreases the risk of entraining floatable pollutants trapped behind the trapezoidal baffle into the flow passing into the outlet section. 
     In one aspect, the apparatus has an inlet that delivers water to the chamber from a tributary surface land area, either directly or via storm sewer system piping. Water entering the chamber passes through a system of underflow and overflow baffles both perpendicular to and oblique to the primary direction of flow from the inlet to the outlet, which is located at the end of the rectangular chamber opposite the inflow. As water enters the chamber, the water level in the chamber rises above the permanent pool water surface elevation, which normally is less than or equal to the elevation of the invert of the outflow opening. Outflow from the chamber is controlled by an opening that is sized to provide a specified time for the water in the chamber to drain from the elevation at which the chamber is full to the elevation of the permanent pool. When the rate of inflow is greater than the rate of outflow, the water level in the chamber will rise to the elevation at which the chamber is full. Once the chamber is full, any flow in excess of the outflow rate under full conditions will bypass the chamber via an overflow structure  294 . When the rate of outflow is greater than the rate of inflow, the water surface elevation in the chamber will decrease at a rate controlled by the size of the outflow opening and the water surface elevation in the chamber to the elevation of the outflow opening invert, at which time outflow will cease. 
     Another aspect of the invention is a stormwater treatment apparatus, including a receptacle adapted to receive water flowing from a surface drainage area, the receptacle having a bottom and a top, the receptacle having an inlet and an outlet, the inlet and the outlet being in fluid communication with one another; and at least one baffle positioned within the receptacle between the inlet and the outlet, the baffle extending from the bottom of the receptacle, a first portion of the baffle and the bottom of the receptacle forming an angle therebetween. 
     A stormwater treatment apparatus varies from other types of treatment apparatus, such as septic tanks, in that stormwater treatment apparatus must capture a wide variety of particles of different sizes and compositions in a pulsed hydraulics environment, as opposed to the more constant flow environment of a septic tank. A stormwater treatment apparatus also differs from septic tanks in that the goal is to permanently trap sediments and other pollutants less or more dense than water, rather than to degrade organic matter and other biodegradable substances and in that a stormwater treatment apparatus is much larger than septic tanks, desirably having a volume of at least 500 cubic feet, more desirably at least 600 cubic feet and, preferably, at least 750 cubic feet. 
     The apparatus advantageously substantially reduces bottom velocities, thereby greatly reducing resuspension of sediments. In particular, the angle formed between the first portion of the baffle and the bottom of the receptacle is desirably between 30 and 60 degrees, at is desirably inclined in a downstream direction. Further, the height of the baffle is desirably at least two feet to limit the washing out of sediment. To facilitate manufacture and cleaning the baffle desirably includes a second portion, the second portion of the baffle extending from the bottom of the receptacle and forming an angle with the bottom of the receptacle, the angle being roughly 90 degrees. 
     The apparatus desirably includes an inlet baffle positioned between the inlet and the outlet, the inlet baffle spaced from said bottom and extending between generally opposing walls and an outlet baffle positioned between the inlet and the outlet, the outlet baffle spaced from said bottom and extending between generally opposing walls of the receptacle. The lower end of the outlet baffle is desirably positioned below said outlet. The outlet baffle advantageously may define a horizontal cross-section between a first baffle extending from said bottom and said outlet baffle larger than the horizontal cross-section between said first baffle and a vertical plane tangent to an upstream side of said outlet baffle. This has the effect of reducing the velocity of fluid. In this regard, it is desirable that outlet baffle defines a center section and at least one outer section which extends toward said outlet from said center section. Advantageously, however, the spaces between the outlet baffle and the opposing walls are sufficiently large to permit cleaning and to facilitate manufacture. 
     Yet another aspect of the invention is an apparatus for cleaning stormwater run-off, the apparatus including a vault having a top, a bottom, two sides, a front and a back, the vault comprising a first baffle extending from the bottom of the vault; a second baffle extending from the bottom of the vault, an inlet section having an opening and an outlet section having an outlet opening. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     One embodiment of this invention, the best mode, is illustrated in the attached drawings, in which like numerals indicate like components throughout the several views. Views include: 
     FIG.  1 —a plan (from a perspective above the apparatus) view of the apparatus that is the subject of this invention; 
     FIG.  2 —a profile (side elevation) view of the apparatus; 
     FIG.  3 —a cross-sectional view of the inlet section of the apparatus (crosssection  1 — 1  shown on FIG.  1  and FIG.  2 ); 
     FIG.  4 —a cross-sectional view of the outlet section of the apparatus (crosssection  2 — 2  shown on FIG.  1  and FIG.  2 ); 
     FIG.  5 —a detailed (enlarged) profile view of the inlet section baffle configuration; 
     FIG.  6 —a detailed plan view of the outlet section 
     FIG.  7 —a detailed view of the outflow opening configuration; 
     FIG.  8 —an illustration of baffle spacing for this invention for even and odd numbers of chambers for a multi-chambered apparatus (the number of midsections depicted in this view, four for the even illustration and five for the odd illustration, are specific examples of the generalized odd and even baffle spacing rules and are not intended to be restrictive). 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The drawings illustrate one embodiment of an apparatus  100  for separation of pollutants that are less and more dense than water from stormwater runoff. Referring to FIG..  1  and FIG. 2, the apparatus  100  consists of a top  140 , a bottom  160 , an inlet end  170 , an outlet end  150 , a left side  300 , and a right side  310  (left and right are relative to the view from the inlet end  170  to the outlet end  150 ). These sides define a rectangular chamber with an inlet section  110 , an outlet section  120 , and one or more midsections  130 . The length of the most basic configuration of the apparatus  100  is desirably at most 20-ft, more desirably at most 18-ft 6-in, and, most preferably, 17-ft 6-in (inside dimension); the width of the apparatus is desirably at most 10-ft, more desirably at most 8-ft 6-in, and, most preferably, 7-ft 6-in (inside dimension); the height of the apparatus is 6-ft or 8-ft (inside dimensions). Outside dimensions may vary due to structural strength requirements of the apparatus  100 . Desirably, the length of the apparatus  100  increases in 4-ft, 8-ft, or 16-ft increments as additional midsections  130  are employed. The top  140  and bottom  160  are desirably parallel to each other and are separated by a distance of 6-ft or 8-ft (inside dimensions). The left side  300  and right side  310  are desirably parallel to each other and are separated by a distance of at most 10-ft, more desirably at most 8-ft 6-in and, preferably, 7-ft 6-in (inside dimensions). The inlet end  170  and the outlet end  150  are desirably parallel to each other and, for the most basic configuration, are desirably separated by a distance of at most 20-ft and, more desirably, 17-ft 6-in (inner dimension). The distance between the inlet end  170  and the outlet end  150  desirably increases by 4-ft, 8-ft, or 16-ft increments as additional midsections  130  are employed. The thickness of the inlet end wall  170 , the outlet end wall  150 , the left side  300 , the right side  310 , and the bottom  160  is desirably at least 3-in and, preferably, 6-in or more. The thickness of these walls may increase or decrease as structural needs of an installation dictate. The thickness of the top  140  of the apparatus  100  is at least 3-in and, desirably, 6-in or more but may increase or decrease as structural needs of an installation dictate. 
     The ability to increase the size and treatment capacity of the apparatus  100  by addition of modular midsections  130  is advantageous for manufacturing since the apparatus  100  can be manufactured in a wide range of incremental sizes using the same set of forms for precasting. In addition, the modular construction is favorable for applications requiring a large apparatus  100  as the modular sections  110 ,  120 , and  130  can be transported on one or more trucks and then assembled on-site. The incremental sizing may be advantageous for performance at improving water quality as well when the apparatus  100  is sized according to manufacturer&#39;s recommendation. For example, if a user, based on sizing calculations, determines that the required capacity of the apparatus  100  necessary to achieve a desired performance is equivalent to the capacity of a midsection with a length of 11-ft, then the user would specify that 2 midsections  130  are needed, one 8-ft long and the other 4-ft long (or two 8-ft long sections), since midsections  130  are discrete components and 1 mid-section  130  would not provide the required capacity. As a result of this modular, incremental sizing, the apparatus  100  specified by the user would always have a capacity equal to or in excess of that required and would, therefore, provide a minimum degree of desired treatment. 
     A plurality of baffles  220  and  250  are positioned between the inlet end  170  and the outlet end  150 . The primary direction of flow is defined as the direction from the inlet end  170  toward the outlet end  150  in the horizontal plane. In the disclosed embodiment, the primary direction of flow is perpendicular to the inlet end  170  and the outlet end  150  and parallel to the top  140 , bottom  160 , left  300 , and right  310  sides. There are two types of overflow baffles employed in this invention. These baffles are referenced as components  220  and  250 . Component  220  is a hybrid baffle, and component  250  is an angled baffle. The results of extensive hydraulic testing indicate that the baffle configuration illustrated, as well as the claimed baffle configurations using various combinations of hybrid  220 , vertical, and angled  250  baffles, is highly effective at minimizing resuspension of trapped sediments and associated pollutants. Velocity measurements and dye visualization experiments indicate that the apparatus  100  disclosed herein provides a degree of reduction of resuspension that significantly surpasses that of existing art. 
     Referring to FIG. 1, FIG. 2, and FIG. 5, the hybrid baffle  220  consists of a vertical section  240  that is perpendicular to the primary flow direction and an angled section  230  that is oblique to the primary direction of flow, forming an angle, α, with the horizontal plane (angle α is depicted in FIG.  5 ). Preferably, the vertical baffle section  240  has a length of 1-ft and the angled section of the baffle  230  rises from the top of the vertical section  240  at a 45° angle for a distance of 1-ft in the horizontal plane and a distance of 1-ft in the vertical plane. Preferably, the total vertical rise for a hybrid baffle  220  is 2-ft from the chamber bottom  160 , and the horizontal projection is 1-ft 3-in. in the downstream direction (including thickness of the vertical section  240 ). An angle other than 45° may be used for the hybrid baffle  220  as long as the lengths of components  230  and  240  are adjusted to provide a total rise of 2-ft and the downstream end of component  230  does not extend beyond the dimensions of the top  140 , bottom  160 , and walls  300 ,  310 , and  170  of the precast unit containing the baffle. Desirably, the angle α is between 0° and 90°, and, more desirably, between 30° and 60° degrees The angled baffle  250  rises 2-ft from the bottom of the chamber  160 . An angled baffle  250  is illustrated in FIG.  1  and FIG. 2 in plan and profile views, respectively. For the best mode, the baffle  250  forms an angle, α, of 45° with the chamber bottom  160 . An angle other than 45° may be used, provided that a vertical rise of 2-ft is maintained and that the downstream end of the angled baffle  250  does not project beyond the end of the associated 8-ft midsection  130 . Hybrid baffles  220  and angled baffles  250  may be interchanged to create numerous embodiments; however, the best mode utilizes a single hybrid baffle  220  in the inlet section  110  and angled baffles  250  in midsections  130 , the spacing of which is described below. Other shapes and heights of baffles, up to the full depth of the permanent pool have been tested and are viable alternates to the “best design” shown herein and are part of the design claims of this apparatus  100 . 
     Extensive hydraulic experimentation and testing of baffle configurations and types was conducted to determine baffle geometry that effectively reduced velocities in the lower section of the apparatus  100  where sediments accumulate after settling out of the water. Initial testing indicated that angled baffles  250  were more effective than vertical baffles at decreasing bottom velocities in the apparatus&#39; midsections  130 . The inventors initially tested angled baffles  250  for the purpose of examining the effect of the angled baffles  250  on flow passing over the crest of the angled baffles  250 . In the process of this experimentation, the inventors discovered that the angled baffles  250  had a favorable effect on bottom velocities between two angled baffles  250  separated by a distance of 16-ft or less. A hybrid baffle  220  was developed and tested for the purpose of achieving a reduction in bottom velocities in the midsections  130  comparable to that found using an angled baffle  250 , while at the same time decreasing the length in the horizontal plane consumed by the angled baffle  250  by a distance equivalent to the product of the height of the vertical portion of the baffle  240  and the tangent of the angle 90°-α. This reduction in the horizontal distance required to accommodate the hybrid baffle  220  allows the inlet section  110  to be shortened, resulting in a reduction in the amount of material necessary to fabricate the inlet section  110 . In addition, the vertical portion  240  of the hybrid baffle  220  has the advantage of improved access for a hose or vacuum to clean out the area beneath the baffle  220 . An angled baffle  250  permits access beneath the baffle  250  for cleaning only where the distance between the under-surface of the baffle  250  and the bottom of the chamber  160  (inside dimension) is greater than the diameter or height of the intake component of the vacuum or pumping cleaning system. For both angled  250  and hybrid  220  baffles, the experimentation conducted indicated that both types of baffles  250  and  220 , performed very well at evenly distributing flow across the width of the apparatus  100 . 
     Water is supplied to the apparatus inlet section  110  via an inlet pipe or other conveyance  180  carrying water from the tributary drainage area to the inlet of the apparatus  190 . The invert of the inlet aperture  190  is desirably at least 3-ft above the chamber bottom  160  (inside dimension). The apparatus  100  may also receive water from the tributary drainage area directly rather than via an up-gradient, piped storm sewer system. An example of this configuration would be an apparatus  100  installed to receive water from a manhole chamber below a curb-side drop inlet. 
     The inlet section  110  consists of several distinct components that are shown in FIG. 1, FIG. 2 in plan and profile views, respectively. FIG. 3 shows a cross-section ( 1 — 1 ) of the inlet section  110 , and FIG. 5 shows details of the baffle configuration for the inlet section  110 . The dimensions of the inlet section  110  are defined by the inlet end wall  170 ; the top  140 , bottom  160 , left  300 , and right  310  sides; and a plane perpendicular to the primary direction of flow located a prescribed distance from the inside dimension of the inlet end wall  170  in the downstream direction. This prescribed distance is defined by the length dimension of the precast segment containing the energy dissipation baffle  200  and the most upstream hybrid  220  or angled  250  baffle and, most preferably is 4-ft 9-in. The dimensions of the inlet section  110 , exclusive of baffles, desirably are equivalent to the dimensions of the outlet section  120 , providing the advantage of having the capability to cast inlet  110  and outlet  120  sections using the same form. The inlet section  110  desirably includes a manhole  135  for access to the inlet section  110  for maintenance. The cover of the manhole  135  is desirably vented to allow exchange of air between the inside of the apparatus  100  and the surface atmosphere to prevent anoxic conditions from developing in the permanent pool. The permanent pool is defined as the volume of water and trapped pollutants in the apparatus  100  above the bottom of the chamber  160  and below the invert of the outflow opening  280 . 
     A component of the inlet section  110  is a flow energy dissipation baffle  200  that is perpendicular to the primary direction of flow. The energy dissipation baffle  200  is parallel to the inlet end wall  170  and is positioned so that the side of the energy dissipation baffle  200  facing the inlet wall  170  is desirably at most 1-ft 6-in and preferably 1-ft from the inner side of the inlet end wall  170  in the primary direction of flow. The energy dissipation baffle  200  desirably is connected to the left side  300  and right side  310  from a distance of desirably at most 2-ft and preferably 1-ft 6-in above the chamber bottom  160  (inside dimension) to a distance of desirably at most 1-ft, and preferably 6-in from the chamber top  140  (inside dimension). The energy dissipation baffle  200  desirably has a thickness of 3-in. The purpose of the flow energy dissipation baffle  200  is to decrease the energy of the incoming flow. For the apparatus  100  described herein, the decrease in flow energy translates to a decrease in the velocity of the incoming water. The space  210  is provided between the top of the energy dissipation baffle  200  and the top  140  of the apparatus  100  for the purpose of allowing overflow for high flows and for the purpose of providing access for maintenance. Hydraulic testing indicates that the energy dissipation baffle  200  is effective at decreasing flow energy. The inventors examined several options for spacing between the inlet end wall  170  and the flow energy dissipation baffle  200  and found that the above-described spacing provided a good balance between the effectiveness of energy dissipation and the space necessary to access the area between the inlet end wall  170  and the baffle  200  for maintenance. 
     Another element of the inlet section is the inlet overflow baffle  220 . The inlet overflow baffle  220  is a hybrid baffle (described above). The inlet overflow baffle  220  desirably is connected to the chamber bottom  160  and the left  300  and right  310  sides of the chamber so that water can only pass over the top of the baffle, defined by component  230 . The vertical portion  240  of the inlet overflow baffle  220  desirably is located a distance of at least 2-ft 6-in, more desirably at least 3-ft, and preferably 3-ft 6-in from the inlet end wall  170  (inside dimensions). The thickness of the inlet overflow baffle  220  is desirably 3-in. The vertical rise for the inlet overflow baffle  220  is desirably at most 3-ft, more desirably at most 2-ft 6-in, and, preferably, 2-ft, and the horizontal distance in the direction of flow is desirably at most 2-ft, more desirably at most 1-ft 6-in, and, preferably, 1-ft 3-in (including the baffle thickness of 3-in) for the best mode. 
     A midsection  130  of the apparatus  100  is defined by a top  140 , a bottom  160 , a left  300 , and a right  310  side that desirably are connected at 90° angles to form an openended rectangular section. FIG.  1  and FIG. 2 depict an apparatus  100  with two, 8-ft midsections  130 . The apparatus  100  desirably has at least one midsection  130  but may have additional midsections  130 . Desirably, the midsections  130  have a length of 16-ft, more desirably 4-ft, and, preferably, 8-ft. Angled baffles  250  desirably are spaced at 4-ft increments, more desirably at 8-ft increments, and, preferably, at 16-ft increments in midsections  130 . For midsections  130  requiring angled baffles  250  to achieve this spacing, an angled baffle  250  (described above) is attached to the bottom of the midsection  130  so that the downstream tip of the angled baffle  250  coincides with the end of the midsection  130 . Such an angled baffle  250  in a midsection  130  is shown in FIG.  1  and FIG. 2 in plan and profile views, respectively. While an angled baffle  250  desirably is used in the midsections  130 , vertical, hybrid, or other baffle shapes  220  may be used. Since baffle  220  and  250  spacing is preferably 16-ft and midsections  130  are added in 4-ft, 16-ft, or 8-ft increments, not all midsection segments  130  will need baffles  220  and  250 . FIG. 8 illustrates baffle  220  and  250  spacing. As FIG. 8 indicates, baffles  220  and  250  preferably are spaced every 16-feet, and a baffle  220  and  250  is desirable at the end of the most downstream midsection  130 . Therefore, for an even number of midsections  130 , desirably with a length of 8-ft (four as an example in FIG.  8 ), all overflow baffles  220  and  250  are preferably spaced 16-feet apart. For an odd number of midsections  130 , desirably with a length of 8-ft, (five as an example in FIG.  8 ), however, spacing is preferably 16-feet between all overflow baffles  220  and  250  with the exception of the spacing between the penultimate and ultimate downstream baffles  220  and  250  at the end of the most downstream midsection  130 . The number of midsections  130  depicted in FIG. 8 are shown as examples of even and odd numbers of midsections  130  and should not be interpreted as restrictive specifications. Each midsection  130  desirably will have a manhole  135 , allowing access through the top of the chamber  140  for maintenance. Desirably, all manholes  135  will be vented to prevent development of anoxic conditions in the permanent pool and will be of sufficient size to allow the contents of the apparatus  100  to be pumped out as a part of regular maintenance. Manholes  135  positioned above midsections of the apparatus  100  desirably will have a collar  145  with approximately the same inner diameter as the manhole that extends into the chamber 3-in below the top  140 . The purpose of the collar  145  is to limit the surface area of the water and associated floatable pollutants in the chamber that could potentially be forced out of the apparatus  100  via vents in manhole access areas  135  when the apparatus  100  fills completely. 
     The midsection  130  components of the apparatus  100  are the primary treatment and pollutant collection chambers. During the time that water is detained in the apparatus  100 , sedimentation occurs, resulting in deposition of sediments and associated pollutants with densities greater than water on the bottom  160  of the midsections  130 . The configuration of baffling  220  and  250  is such that sediments deposited on the bottom  160  of the midsections  130  resist resuspension during subsequent runoff events. Once the thickness of the sediment layer on the bottom  160  of the midsections  130  increases to a prescribed depth, the apparatus  100  is cleaned via a pump-out or other method to remove the permanent pool and trapped pollutants from the apparatus  100  for disposal. 
     In addition to sediment removal, the midsections  130  of the apparatus  100  collect and retain materials less dense than water. During the time that water is detained in the apparatus  100 , materials that are less dense than water rise toward the water surface. Since flow from the midsections  130  passes to the outlet section  120  by flowing beneath the trapezoidal baffle  260 , pollutants on the water surface in the midsections  130  are retained on the upstream side of the trapezoidal baffle  260 . Due to the elevation of the invert of the outlet opening  280 , the surface of the permanent pool in the apparatus  100  desirably remains at least 1-ft above, and, preferably, 1-ft 5-in above the highest elevation at which water can pass below the trapezoidal underflow baffle  260 . As described below, the trapezoidal geometry of the underflow baffle  260  is advantageous for prevention of entrainment of pollutants collected on the surface of the mid-sections  130  into the flow beneath the trapezoidal baffle  260  entering the outlet section  120 . Desirably, a mat or mats composed of material capable of absorbing petroleum-based hydrocarbons with densities less than that of water will be placed in the midsections  130  of the apparatus  100  for the purpose of immobilizing these pollutants. Manholes  135  will be large enough to permit removal of the absorbent mats. 
     A detailed plan view of the outlet section  120  is shown in FIG. 6, and a detail of the outflow opening configuration  280  is shown in FIG.  7 . The dimensions of the outlet section  120  are defined by the outlet end wall  150 ; the top  140 , bottom  160 , left  300 , and right  310  sides; and a plane perpendicular to the primary direction of flow located 4 ft 9in from the inside dimension of the outlet end wall  150  in the upstream direction. The dimensions of the outlet section  120 , exclusive of baffles, are equivalent to the dimensions of the inlet section  110 , providing the advantage of having the capability to cast inlet  110  and outlet  120  sections using the same form. 
     One component of the outlet section  120 , is a trapezoidal underflow baffle  260 . In the plan view (FIG.  1  and FIG.  6 ), the trapezoidal underflow baffle  260  desirably consists of a center segment parallel to the outlet end wall  150  and a pair of outer segments. The center segment is located desirably at least 2-ft, more desirably 3-ft, and, preferably 4-ft from the outlet end wall  150  (inside dimension of end wall to upstream side of trapezoidal baffle  260 ). The center segment of the baffle  260  is centered with respect to the left  300  and right  310  sides of the chamber. Preferably, the length of the center segment  260  is 1-ft and, as a result, the distance between the ends of the center segment of the baffle  260  and each wall  300  and  310  is 3-ft 3-in. In the plan view, the trapezoidal baffle extends from the ends of the center segment to the comers defined by the intersection of the left side wall  300  and the outlet end wall  150  and the right side wall  310  and the outlet end wall  150 . In the profile view (FIG.  2 ), the trapezoidal baffle  260  is located so that the bottom of the baffle  260  desirably is at most 1-ft 11-in and, preferably, 1-ft 6-in above the bottom of the chamber  160  (inside dimension). The baffle  260  extends to the top of the chamber  140  and is joined to the top of the chamber  140  along the trapezoidal-shaped top edge of the baffle  260  displayed in the plan view (FIG.  1  and FIG.  6 ). The trapezoidal underflow baffle  260  desirably is also attached to the sides of the apparatus  100  where the left and right sides  300  and  310 , respectively, form corners with the outlet end  150  from a distance, preferably, 1-ft 6-in above the bottom of the chamber  160  (inside dimension) to the top of the chamber  140 . 
     Initially, the inventors tested a simple, vertical underflow baffle with a thickness of 3-in that was positioned in a plane entirely perpendicular to the outlet end wall  150 . This incarnation of the underflow baffle was located a distance of 4-ft from the outlet end wall  150  (inside dimension of end wall to upstream side of underflow baffle) and resulted in an area of 5.625 ft 2  between the downstream end of the angled baffle  250  and the upstream side of the underflow baffle in the plan view (see FIG.  1 ). The inventors investigated the trapezoidal underflow baffle  260  of the present invention for the purpose of decreasing the velocity of the flow passing through the plane between the downstream end of the angled baffle  250  and the upstream side of the underflow baffle  260  in the plan view. The area in the plan view between the downstream end of the angled baffle  250  and the upstream side of the underflow baffle  260  is preferably 18.625ft 2 . Comparison of the areas between the underflow baffle and the upstream angled baffle  250  for the vertical underflow baffle configuration and the trapezoidal underflow baffle  260  configuration indicates that for equivalent rates of flow passing between the upstream angled baffle  250  and the underflow baffle, the velocity for the vertical baffle configuration preferably would be 3.3 times greater than the velocity for the trapezoidal baffle  260  configuration. The lower velocity attained using the trapezoidal baffle  260  configuration of the present invention is advantageous for protection from entrainment of pollutants residing on the surface layer of the midsections  130  into the flow from the midsection  130  to the outlet section  120 . Desirably, the angle between the center segment of the baffle and the outer segments of the baffle is between 90° and 180, more desirably between 120° and 160°, and, preferably 130°. 
     Another component of the outlet section  120  is outlet screening  270  which is designed to keep trash and/or debris from clogging the outlet opening  280 . The outlet screening  270  consists of fine screening or a fine mesh configured as a semi-circle, arch, rectangle, or straight screen in front of the outflow opening  280 . The screening is attached to the outlet end wall  150  a horizontal distance in front of the outlet opening that is proportionate to the outlet opening size, but no less than 2 times the diameter of the outlet opening and to the bottom  160  and top  140  of the chamber so that all water passing through the outflow opening  280  will have first passed through the screening  270 . The screening  270  will be attached in a manner that will permit removal and cleaning of the screening via an access manhole  135  located in the top of the outlet section  120 . The cover for the manhole  135  will be vented to allow exchange of air between the inside of the apparatus  100  and the surface atmosphere to abate development of anoxic conditions in the permanent pool and to relieve air pressure as the apparatus fills and drains with water. 
     The outflow opening  280 , shown in FIG. 1, FIG. 2, and FIG. 4 is the device controlling the release of water from the apparatus  100 . A detail of the outflow opening  280  components is shown in FIG.  7 . The outlet desirably consists of an 8-in diameter pipe  290 , desirably extending from 3-in upstream of the outlet end wall  150  (inside dimension), through the outlet end wall  150 . The end of the pipe  290  that is inside the apparatus  100  desirably is covered with an 8-in cap  282 . An opening  284  that is sized to provide a predetermined time for the water in the chamber to drain from the elevation at which the apparatus  100  is full to the elevation of the permanent pool is machined into the 8-in cap  282 . The opening  284  is manufactured so that the lowest point of the opening is preferably at least ½-in above the lowest point of the 8-in pipe  290  at the end where the cap  282  is attached. 
     An advantage of creating the outflow opening aperture  284  in a cap  282  that is placed over the end of the outflow pipe  290  that is inside the outlet chamber is that the opening size can be changed as desired during maintenance by replacing the cap  282  with another cap  282  with a different sized opening  284 . This flexibility in opening  284  sizing is advantageous for providing an apparatus  100  that can provide an array of treatment levels. The opening  284  size dictates the time that water is detained in the apparatus  100 . A smaller opening  284  size would result in detention of water for a longer period of time than that afforded by a larger opening  284  size. The treatment efficiency of an apparatus  100  will increase as the time that water is detained increases. Therefore, the level of treatment can be adjusted by increasing the opening  284  size (decreasing the level of treatment) or decreasing the opening  284  size (increasing the level of treatment). Another advantage of the outflow opening configuration  280 , is that the positioning of the opening  284  invert, preferably, a distance of 2-ft 11-in above the bottom  160  and downstream of all baffling  200 ,  220 ,  250 , and  260  results in release of water with the lowest sediment concentrations through the opening  284 . An outflow opening  280  positioned lower than that in the illustrated embodiment would draw more water from the lower part of the outlet section  120 , which would contain more suspended sediments. An outflow opening  280  positioned higher than that in the illustrated embodiment would result in a greater permanent pool volume that would need to be pumped out during maintenance.