Flow control bypass basin apparatus

In combination, a flow control bypass basin for receiving rainfall runoff water from a surface, an immiscible liquids separator downstream of the bypass basin, and a drain. The bypass basin passes the initial, dirty and oily, portion of the rainfall runoff to the separator and bypasses subsequent runoff water instead to the drain. The bypass basin includes a tank, an inlet in communication with the tank's interior, a primary outlet from the bypass basin to the separator, and a bypass outlet from the bypass basin to the drain, with the bypass outlet being located a substantial height above the primary outlet. Interposed between the bypass outlet and the interior of the tank is a riser tube having a mouth disposed in the lower portion of the tank and an upper end closed to the interior of the tank but open to the bypass outlet. The bypass outlet is located a certain hydraulic head height above the liquid level in the separator such that all water within the bypass basin above that height is bypassed into the drain. The primary outlet is substantially smaller than the inlet to the bypass basin so that the initial dirty portion of the rainfall runoff is accumulated within the bypass basin's tank. Vent pipes prevent pressure buildup within the bypass basin and siphoning by the riser tube, and a baffle having a upwardly-extending screen prevents trash from flowing from the inlet of the bypass basin to the primary and bypass outlets.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates, in general, to separating two immiscible 
fluids, such as oil and water, front each other, and in particular, to a 
device for accumulating oily runoff water from a surface such as a parking 
lot and passing a portion of that water to an immiscible fluids separator. 
2. Information Disclosure Statement 
Separators for separating immiscible fluids, such as oil and water, from 
each other are well-known. An example of such a separator is given in 
Greene et al., U.S. Pat. No. 5,266,191, issued Nov. 30, 1993, and assigned 
to the assignee of the present invention. 
Some have attempted to use immiscible liquid separators to process runoff 
water from a surface, such as a parking lot or the roof of a building, in 
order to prevent the oils, etc., that have accumulated on that surface 
from passing into a municipality's sewage system or a nearby river and 
causing environmental damage. Such accumulation can happen as oils drip 
front cars onto the parking lot, or as oils drip from machinery on the 
roof of the building. When rainfall occurs, the initial runoff water from 
the surface washes the oil from the surface, and subsequent runoff water 
during the same storm is relatively clean. It is impractical to design and 
dedicate an immiscible liquids separator for the task of processing all of 
the vast quantity of runoff water that might flow from the surface during 
any expected possible rainfall, because such a separator would be huge and 
the enormous capacity that would be required of such a separator would 
only rarely be required during peak rainfall activity, whereas the volume 
of oily runoff water that truly would need to be processed is relatively 
small, appearing during the initial moments of the rainstorm. 
It is therefore desirable to have an apparatus that allows the use of a 
smaller immiscible liquids separator than would be otherwise possible, 
that will direct the dirty initial runoff water to the separator, and that 
will bypass the separator and direct subsequent and relatively clean 
runoff water into a drain. Such a bypass should only occur when the rated 
flow capacity of the separator might be exceeded, and means should also be 
provided to ensure that the rated flow capacity of the separator is not, 
in fact, exceeded. Such a bypass should also only be of the subsequent and 
relatively clean runoff water, and not of the dirty initial runoff water. 
A preliminary patentability search in Class 210, subclasses S 13, 519, 521, 
532.1, 538, 539, 540, and 799, produced the following patents, some of 
which may be relevant to the present invention: Pike, U.S. Pat. No. 
1,734,777, issued Nov. 5, 1929; Boosey, U.S. Pat. No. 2,071,160, issued 
Feb. 16, 1937; Marsh, U.S. Pat. No. 2,076,380, issued Apr. 6, 1937; 
Hirshstein, U.S. Pat. No. 2,284,737, issued Jun. 2, 1942; Boosey, U.S. 
Pat. No. 2,288,989, issued Jun. 26, 1941; Mathels, U.S. Pat. No. 
2,479,386, issued Aug. 16, 1949; Johnson, U.S. Pat. No. 2,644,584, issued 
Jul. 7, 1953; LaLonde et al., U.S. Pat. No. 3,527,348, issued Sep. 8, 
1970; Preus et al., U.S. Pat. No. 3,862,040, issued Jan. 21, 1975; 
Wolde-Michael, U.S. Pat. No. 4,422,931, issued Dec. 27, 1983; Cloud, U.S. 
Pat. No. 4,684,467, issued Aug. 4, 1987; Hall, U.S. Pat. No. 4,915,823, 
issued Apr. 10, 1990; Keep et al., U.S. Pat. No. 5,229,015, issued Jul. 
20, 1993; Fink, U.S. Pat. No. 5,236,585, issued Aug. 17, 1993; and 
Steadman et al., U.S. Pat. No. 5,204,000, issued Apr. 20, 1993. 
SUMMARY OF THE INVENTION 
The present invention is a flow control bypass basin apparatus for 
receiving runoff water during a rainstorm from a surface, such as a 
parking lot, and for accumulating and passing the initial and dirty first 
portion of the runoff water to an inmiscible liquids separator, and for 
bypassing a cleaner second portion of the runoff water not to the 
separator but instead to a drain. The bypass basin comprises a tank having 
an inlet that empties the runoff water into a lower portion of the 
interior of the tank, a primary outlet from the bypass basin to the 
downstream separator, and a bypass outlet to a drain for bypassing clean 
water to the drain. The primary outlet has a smaller transverse 
cross-sectional area than the inlet to the tank, so that runoff water 
accumulates in the tank because of the greater flow into the tank through 
the inlet than out through the primary outlet. 
The bypass outlet is located a certain height above the primary outlet, and 
receives water from a lower portion of the tank through a riser tube 
interposed between the bypass outlet and the lower portion of the tank, 
with the riser tube having an open end disposed in the lower portion of 
the tank. The bypass outlet has a transverse cross-sectional area such 
that the sum of the transverse cross-sectional area of the bypass outlet 
plus the transverse cross-sectional area of the primary outlet is at least 
as large as the transverse cross-sectional area of the inlet. 
Such a construction causes the dirty initial water to pass into the 
separator or to be accumulated within the bypass basin for subsequent 
metering to the separator, and bypasses the cleaner runoff water, which 
appears after the surface has been washed by the initial runoff water of 
the rainstorm, into the drain during peak periods of the rainstorm, 
thereby allowing a much smaller separator to be used than would otherwise 
be possible. 
It is an object of the present invention to provide means for accumulating 
and passing the initial dirty runoff water from a surface, such as a 
parking lot, into an immiscible liquids separator, and also to provide 
means for bypassing the cleaner runoff water, subsequent to the initial 
dirty runoff water, into a drain during the peak periods of a rainstorm. 
Another object of the present invention is to provide means for ensuring 
that the maximum rated flow capacity of the downstream separator is not 
exceeded during peak periods of rainfall intensity. Still a further object 
of the present invention is to allow the use of a smaller immiscible 
liquids separator to treat rainfall water than heretofore possible, with 
the separator being sized only large enough to handle the expected initial 
volume of dirty runoff water rather than being sized for a much larger 
flow capacity sufficient for the entire volume of runoff water.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 is a side sectional view of a prior art immiscible liquids separator 
20 such as might be used in combination with the present invention. It 
shall be understood that, while this disclosure uses oil and water as an 
example of two immiscible liquids of differing density, the problems faced 
by the present invention and its novel solution are equally applicable to 
other immiscible liquids of differing densities (buoyancies), in a manner 
that those skilled in the art will readily recognize. The separator shown 
in FIG. 1 is that described in U.S. Pat. No. 5,266,191, issued on Nov. 30, 
1993, to Greene et al., now hereby fully incorporated by reference herein. 
Other separators than the one shown, e.g., retention ponds, etc., may be 
used as well with the present invention, although the one shown is to be 
preferred, and the term "separator", as used herein, shall be understood 
to encompass all such liquids separators. 
Separator 20 receives runoff water from a surface, such as a parking lot P 
as shown, through a drain pipe 22 into the inlet 24 of separator 20, and, 
after processing by the separator, clean water emerges from the outlet 26 
of separator 20 and flows into a drain, not shown in FIG. 1. Separators 
such as separator 20 have a certain defined maximum rated flow capacity 
for processing fluids passing therethrough from inlet 24 to outlet 26 that 
is determined by the various physical dimensions and design parameters of 
the separator, and, if this rated flow capacity is exceeded, the separator 
will allow excessive amounts of oil to emerge from outlet 26 because of 
insufficient processing, instead of the emerging water being relatively 
pure, as it should be. At such a maximum rated flow capacity, separator 20 
will have a maximum permitted internal water level therewithin. 
The obstacle to using a separator such as separator 20 to process runoff 
water from a large parking lot P having relatively low levels of surface 
contamination is that, if properly sized to handle all of the runoff flow 
during the most intense rainfall that might occur, the separator would be 
enormous and prohibitively expensive. To overcome this obstacle, the 
present invention takes advantage of the unique characteristics of runoff 
water during a rainfall, as well as the fact that rain comes and goes. 
As shown in the top plan view of FIG. 2, the present invention is a flow 
control bypass basin apparatus 30 that is typically buried alongside 
parking lot P and interconnected with similarly-buried separator 20. For 
the sake of brevity, the present invention will hereinafter be referred to 
simply as a "bypass basin". Bypass basin 30 receives runoff water from 
well-known storm grating G of parking lot P through drain pipe 22 to inlet 
32 of bypass basin 30, and bypass basin 30 has a primary outlet 34, 
hereinafter described, leading to the inlet 24 of separator 20 as by 
through pipe 25. Bypass basin 30 also has a bypass outlet 36, hereinafter 
described, that leads as through pipe 39 not to separator 20 but instead 
to a drain or collection cistern 38, schematically represented as shown, 
which, in turn, leads to a river or a sewage system (not shown) for a 
municipality. Separator 20 may also empty its processed and cleaned water 
from its outlet 26 into the same drain 38 as by through pipe 98, or it may 
empty its cleaned water into another drain. 
The theory and insight behind the present invention can be best explained 
by reference to FIGS. 3 and 4, which show graphs describing rainfall 
behavior. 
FIG. 3 is a graph showing rainfall intensities graphed versus the duration 
of the rain event, for various expected recurrence periods in years. It 
should be understood that the precise nature of the well-known family of 
curves shown in FIG. 3 will be different for each particular locality 
because, for example, the rain fails harder and faster in Seattle, 
Washington, than it does in the middle of the desert. As is well known and 
shown in FIG. 3, once every ten years, rainfall can be expected that is of 
a certain high intensity (in inches per hour) and that lasts for a short 
duration, and rainfall can ,also be expected that is of a lesser intensity 
but that lasts for a longer duration. Similarly, once every one hundred 
years, rainfall can be expected that is of a much higher intensity than 
every ten years and that lasts for a short duration, and the expected 
rainfall intensity versus duration may have a shape as shown. Multiplying 
these expected intensities times the area of the collecting surface, such 
as parking lot P, gives the expected rainfall in gallons (or liters) per 
minute. 
FIG. 4 is a typical well-known graph of rainfall runoff flow rates (in 
gallons per minute) for rainstorms of given intensity; and duration 
graphed versus time for two typical rainfalls, that corresponding to the 
rainfall of lesser intensity being denoted by curve 40 and that 
corresponding to the rainfall of greater intensity being denoted by the 
similarly-shaped curve 42. As is well-known to those skilled in the art, 
surface water flowing from the grating G will rise from zero as the rain 
begins, reach some maximum value as the rainfall intensifies, then taper 
off as the rainfall subsides. The precise shape of any such rainfall curve 
will, of course, depend on the local weather conditions where the rainfall 
occurs. 
Before the rain occurs, oils will have accumulated over time on the parking 
lot from parked cars. Similar oils will accumulate on the roofs of 
buildings from air conditioning equipment and other machinery thereatop. 
As the rainfall begins, an oily film is washed from the surface of the 
parking lot (or building roof) and enters runoff drain pipe 22, and is 
then passed by bypass basin 30 out its primary outlet 34 and into inlet 24 
of separator 20, in a manner hereinafter described. After a few minutes, 
the runoff water from the parking lot becomes substantially clean because 
most of the oils have been washed away by the initial downpour. There is 
little need to process this now substantially-clean water, and, if the 
processing of that water would exceed the rated flow capacity of separator 
20, represented by dotted line 44 in FIG. 4, that water will then be 
passed out bypass outlet 36 of bypass basin 30 and into drain 38 as 
through pipe 39, in a manner also hereinafter described. If the downpour 
later increases in intensity and rises to another peak, the parking lot is 
now clean, and the subsequent inrush of clean water can be bypassed into 
drain 238 as well. 
It is generally accepted that approximately five minutes of rainfall is 
required before a surface becomes substantially cleaned of oils and other 
contamination. On days when very light rainfall occurs, it may take longer 
for the surface to become cleaned, if at all, but the flow rate into 
runoff drain pipe will be so small on those days that separator 20 can 
easily handle the entire flow. It is only in the case of very heavy 
rainfalls that the need for the present invention becomes evident, because 
the expected flow rates during such rainfalls will be very large, and 
would otherwise require an enormous separator 20 if the present invention 
were not used. 
Referring now to FIGS. 5-11, the detailed structure of bypass basin 30 can 
now be explained. 
Bypass basin 30 comprises a tank 50 having an interior 52, an inlet 32, 
primary outlet means 34 for passing a first portion of the runoff water to 
separator 20, bypass outlet means 36 for bypassing a second portion of the 
runoff water to drain 38, and riser tube means 53 for placing bypass 
outlet means 36 in communication with interior 52 of tank 50, with riser 
tube means 53 being interposed between bypass outlet means 36 and interior 
52 of tank 50. 
Tank 50 may have various reinforcing ribs, not shown, for structural 
reinforcement of tank 50 in a manner well-known to those skilled in the 
art, and tank 50 may rest on a concrete slab 54 and may be buried beneath 
the ground, as shown. Tank 50 has a bottom 56 and a ceiling 58, and 
interior 52 has a lower portion 60 adjacent to bottom 56. Tank 50 
preferably has a baffle 62 extending upwardly from bottom 56 of tank 50 
toward ceiling 58, with baffle 62 having a top 64 that is below primary 
outlet means 34, and tank 50 also preferably has a coarse mesh screen 66 
extending upwardly from top 64 of baffle 62 to ceiling 58, with inlet 32 
being segregated from primary outlet means 34 and riser tube means 53 by 
baffle 62 and screen 66. Together, baffle 62 and screen 66 form a 
separation wall, preferably along a diameter of tank 50, that collects 
heavy solids and trash 68, as well as soil, sand, and grit, and also 
floatable trash (not shown) such as cigarette butts, ink pens, etc., and 
prevents this trash from passing into and thereby clogging separator 20. 
It should be understood that baffle 62 and screen 66 have little or 
nothing to do with the hydraulic operation of bypass basin 30, and serve 
only to collect trash, etc. The top 64 of baffle 62 is preferably below 
primary outlet means 34 so that oil at the surface of the liquid within 
tank 50 can easily flow to primary outlet means 34. 
Tank 50 also preferably has pressure equalization means 70 for placing the 
interior 52 of tank 50 in communication with the atmosphere and thereby 
equalizing the pressure within tank 50, with pressure equalization means 
70 preferably including a vent pipe 72 having one end 74 open to the 
interior 52 of tank 50 and having another end 76 extending above ground 
and open to the atmosphere as shown. 
Tank 50 may also have a ladder 78 extending downwardly into the interior 52 
of tank 50 from access hatch 80 in the ceiling 58 of tank 50, and access 
hatch 80, leading upwardly to the ground's surface, preferably has a cover 
82 for sealing tank 50 in a manner well-known to those skilled in the art. 
Periodically, a maintenance worker may climb down ladder 78 to clean tank 
50 and remove any collected trash therewithin, in a manner also well-known 
to those skilled in the art. 
Inlet 32 has a certain transverse cross-sectional area, and the sizing of 
the various cross-sectional areas of inlet 32 and the two outlets, 34 and 
36, will be described hereinafter in detail. Preferably, inlet 32 empties 
the incoming water from the parking lot into the lower portion 60 of tank 
550 so that the incoming water doesn't emulsify oil that floats on the 
surface of the water within tank 50. Were the incoming water to enter tank 
50 above the surface of the water therewithin, the clean water that enters 
tank 50 during the later portions of a rainstorm would have to pass 
through the oil floating on the surface of the water within tank 50, 
thereby tending to drag the surface oil down into the tank and also 
tending to emulsify the surface oil. Also, on cold days, incoming water 
might not pass completely through the very viscous surface oil, and the 
water would become emulsified in that oil, rather than vice versa, at 
which time it would become very difficult to remove that water from the 
thick oil. With the preferred emptying of inlet 32 into the lower portion 
60 of tank 50, oil within the incoming water will rise to the surface of 
the water within the tank, for subsequent exit through primary outlet 
means 34, in a manner hereinafter explained. 
Primary outlet means 34 has a transverse cross-sectional area substantially 
smaller than the transverse cross-sectional area of inlet 32, so that, 
when the rain begins falling, water will accumulate within tank 50 because 
all of the water that is entering tank 50 cannot flow out of primary 
outlet means 34 due to its smaller diameter. Preferably, the transverse 
cross-sectional area of inlet 32 will be approximately four times the 
transverse cross-sectional area of primary outlet means 34. As water 
accumulates within tank 50, the water level will rise therewithin as can 
be seen by comparing FIGS. 7 and 10. Preferably, the mouth 84 of primary 
outlet means 34 is angled upwardly slightly as shown so as to encourage 
the flow of fluid into primary outlet means 34 from the surface of the 
liquid within tank 50, rather than from the lower portion 60 of the 
interior 52, thereby causing the dirtier, i.e., oilier, water towards the 
surface to pass into primary outlet means 34. 
Bypass outlet means 36 is located a certain height substantially above 
primary outlet means 34, with that certain height, multiplied by the 
transverse cross-sectional area of tank 50, determining the volume of 
liquid that will be accumulated within tank 50 before water begins being 
bypassed to drain 38. It should be understood that primary outlet means 34 
will still be passing water to separator 20 while bypass is occurring 
through bypass outlet means 36, and that such flow to separator 20 will be 
at full rated flow through primary outlet means 34 during such bypass. 
Also, the height of bypass outlet means 36 above the maximum permissible 
internal water level within separator 20 determines the maximum hydraulic 
"head pressure" that will be seen at separator 20, and this head pressure, 
together with frictional losses within pipe 25 and the transverse 
cross-sectional area of primary outlet means 34, ensures that the rated 
flow capacity of separator 20 will not be exceeded, in a manner 
hereinafter described. The transverse cross-sectional area of bypass 
outlet means 36 is such that the sum of the transverse cross-sectional 
area of bypass outlet means 36 plus the transverse cross-sectional area of 
primary outlet means 34 is at least as large as the transverse 
cross-sectional area of inlet 32 of bypass basin 30, thereby ensuring that 
once the rising water level reaches the height of bypass outlet means 36, 
the water level will rise no further because all subsequent water will be 
bypassed through bypass outlet means 36 into drain 38. In this manner, the 
maximum head pressure that will be seen at separator 20 may be controlled. 
For a conservative design, the transverse cross-sectional area of bypass 
outlet means 36 will preferably be the same as that of inlet 32, thereby 
ensuring effective bypass of all incoming water once the internal water 
level reaches the height of bypass outlet means 36. 
Riser tube means 53 has an open end 86 disposed in the lower portion 60 of 
tank 52, with open end 86 being substantially below primary outlet means 
34. Riser tube means 53 further has an upper end 88 that is closed to the 
interior of tank 50. Preferably, riser tube means 53 includes venting 
means 90 for preventing siphoning by riser tube means 53, with venting 
means 90 preferably including a vent pipe 92 having one end 94 open to the 
upper interior of riser tube means 53 and having another end 96 open to 
the atmosphere as shown, thereby ensuring that the pressure within riser 
tube means 53, and consequently, at bypass outlet means 36, is equalized 
to atmospheric pressure, thereby preventing siphoning. Preferably, the 
transverse cross-sectional area of riser tube means 53 is equal to that of 
bypass outlet means 36, and therefore, as previously described, preferably 
equal to that of inlet 32. 
It should be noted that, while riser tube means 53 and bypass outlet means 
36 are shown in the preferred embodiment within tank 50 with bypass outlet 
means 36 extending through the wall of tank 50 as shown, an equivalent 
structure would be to have riser tube means 53 and bypass outlet means 36 
located substantially on the exterior of tank 50, as long as the mouth 86 
of riser tube means 53 were located in the lower portion 60 of tank 50, 
substantially below primary outlet means 34, and as long as the height of 
bypass outlet means 36 substantially above primary outlet means 34 is 
preserved so as to establish the volume of liquid that will be accumulated 
within tank 50 before bypassing begins, and further as long as the height 
of bypass outlet means 36 above the maximum permissible internal water 
level within separator 20 is maintained so as to control the maximum 
hydraulic head pressure that will be seen by separator 20. 
The operation of the present invention can now be explained. Initially, 
before the rainstorm, the liquid level within tank 50 is as shown in FIG. 
7, at the level of primary outlet means 34, it being understood that all 
liquid above that level will have passed through primary outlet means 34 
to separator 20. Because some time has passed since the last rainfall, 
substantially all of the oil will have risen to the surface within tank 
50, although water within tank 50 below this surface oil will be 
substantially clean. As the rain begins, it washes oil off the surface of 
the parking lot, into grating G, and into tank 50 through inlet 32. The 
oil at the surface of the liquid within tank 50 begins to flow out of 
primary outlet means 34 and into separator 20, together with the initial 
inrush of oily water presented by the new rain event. 
As the rainfall intensity increases, the flow of liquid into tank 50 
increases, with dirty water continuing to flow into tank 50 at an 
increasing rate as shown in FIG. 4, and the liquid level within tank 50 
rises because the transverse cross-sectional area of primary outlet means 
34 is substantially smaller than that of inlet 32, and the initial inrush 
of dirty water is thus accumulated within tank 50, with surface oil 
remaining on the rising surface. As the water level rises, the flow into 
separator 20 continues, controlled by the transverse cross-sectional area 
of primary outlet means 34. Once the height of the liquid within tank 50, 
and thereby also within riser tube means 53, reaches the height of bypass 
outlet means 36, the cleaner water from the bottom of the tank will be 
bypassed into drain 38, and any remaining surface oil within tank 50 will 
not be bypassed into drain 38. As heretofore explained, the water entering 
tank 50 after the first few minutes of the rainstorm is much cleaner than 
the water entering the tank at the very beginning of the rainstorm, and 
this cleaner water can be safely bypassed into the drain. It shall be 
understood that, when bypassing begins, the water within the lower portion 
of tank 50 will be substantially clean because most dirty water will have 
already passed out primary outlet means 34 into separator 20. After the 
rainfall subsides, bypassing through bypass outlet means 36 will cease as 
the liquid level drops within tank 50, and the accumulated dirty and oily 
liquid will exit through primary outlet means 34 and into separator 20 for 
subsequent processing. 
It should be understood that bypassing will only occasionally occur during 
rainfall because the accumulation within bypass basin 30, combined with 
the flow rate out of primary outlet means 34, will, in most cases, be 
sufficient to pass all water from the rain event to separator 20. It is 
only when a huge downpour occurs that bypassing will occur. The net effect 
of bypass basin 30 will be to smooth the flow rate seen by separator 20 
from that shown in FIG. 4 by curves 40 and 42 to that shown by curve 45. 
Referring to FIG. 11, various equations can now be explained that can be 
used to design and size bypass basin 30 for any particular requirements. 
For the following discussion, variables with the subscript "1" will be 
understood to refer to primary outlet means 34, variables with the 
subscript "2" will be understood to refer to bypass outlet means 36, and 
variables with the subscript "3" will be understood to refer to inlet 32. 
Letting m.sub.1, m.sub.2, and m.sub.3 respectively designate the liquid 
masses moving through primary outlet means 34, bypass outlet means 36, and 
inlet 32, and letting m.sub.ACC represent the liquid mass accumulating 
within tank 50, conservation of mass requires: 
##EQU1## 
Because the fluids under consideration here are essentially 
incompressible, this equation can be readily changed into one dealing with 
flow rates of the fluids. Letting Q.sub.1, Q.sub.2, and Q.sub.3 
respectively designate the flow rates (gallons per minute or liters per 
minute) through primary outlet means 34, bypass outlet means 36, and inlet 
32, and letting Q.sub.ACC represent the liquid accumulating within tank 
50, the corresponding equations become: 
EQU Q.sub.3 =Q.sub.1 +Q.sub.2 +Q.sub.ACC 
where 
##EQU2## 
with .DELTA.h.sub.1 being defined as the height of the surface of the 
liquid within tank 50 above the surface of the liquid in separator 20, and 
with A.sub.T being defined as the transverse cross-sectional area of tank 
50. 
For any accumulation of liquid within tank 50 to take place, primary outlet 
means 34 must be designed and sized such that it provides more resistance 
to the liquid flow therethrough than does inlet 32, and bypass basin 30 
has the further constraint that, when operating at full capacity, i.e., 
when the liquid level within tank 50 is at its maximum height of bypass 
outlet means 36, the flow rate through primary outlet means 34 must not 
exceed the maximum permissible flow rate through separator 20. The flow 
rate through primary outlet means 34 and pipe 25 when pipe 25 is operating 
under so-called "full-flow" conditions, is determined by the following 
equation: 
##EQU3## 
where A.sub.1 is defined to be the transverse cross-sectional area of 
primary outlet means 34 and pipe 25; g is the well-known gravitational 
constant; and F.sub.1 represents the total frictional losses within pipe 
25 between bypass basin 30 and separator 20. Those skilled in the art will 
recognize that F.sub.1 is a function of the size of pipe 25, the length 
L.sub.1 of pipe 25 from bypass basin 30 to separator 20, the flow rate 
Q.sub.1, the density and viscosity of the flowing liquid, etc. 
Initially, before bypassing begins, there is no How through bypass outlet 
means 36, thereby causing Q.sub.2 to be equal to zero so that the flow 
rate equation becomes: 
EQU Q.sub.3 =Q.sub.1 +Q.sub.ACC 
or, by substitution, 
##EQU4## 
or, equivalently, 
##EQU5## 
In practice, the solution of this equation can be somewhat difficult, and 
so several simplifications may be used to make the design of bypass basin 
30 somewhat more tractable, and thereby also yielding a very conservative 
design. 
The first step in this simplification is to determine what liquid height 
.DELTA.h.sub.1 within bypass basin 30 would produce the maximum rated flow 
through separator 20 because of the resulting hydraulic head pressure at 
inlet 24 of separator 20. Once this height is determined, the height of 
bypass outlet means 36 is then known, given the depth at which tank 50 
will be buried. Those skilled in the art will readily be able to determine 
the height of the liquid level within any given separator 20 at its 
maximum rated flow. Also, the frictional losses within pipe 25 can be 
readily calculated in a manner well-known to those skilled in the art. 
When designing bypass basin 30 so that all of the initial oily and dirty 
water is accumulated within bypass basin 30 for later passing to separator 
20, the size of the surface, such as parking lot P, that will drain into 
bypass basin 30 is known. Those skilled in the art can readily calculate 
the time for water to flow to bypass basin 30 from the most remote portion 
of the surface. An assumption that can be made to simplify calculations is 
that the shape of the rainfall curve shown in FIG. 4 does not rise 
smoothly as shown, but that the shape is instead that of a well-known 
"step function", with the maximum intensity of the rainfall occurring 
immediately. Thus, knowing this maximum time during which the initial 
dirty and oily water will flow into bypass basin 30 and thereby the 
maximum volume of rainwater that is expected to occur during the initial 
moments of the rainstorm until the parking lot has been washed clean, and 
also knowing the maximum liquid height .DELTA.h.sub.1 within bypass basin 
30 that can be permitted, the transverse cross-sectional area A.sub.T of 
tank 50 can now be calculated that will allow accumulation of this initial 
volume of dirty water within tank 50 for the given time prior to bypass. 
When bypass begins, a certain minimum amount of water must have already 
passed into separator 30, sufficient to have washed parking lot P 
substantially clean, as measured by some local environmental standard. It 
will be assumed permissible to bypass all subsequent water into drain 38, 
with the understanding that flow will still be occurring through primary 
outlet means 34 at full rate during bypass. The specific parameters of any 
particular such design will be based on the frequency with which the rated 
capacity of the separator will be exceeded, i.e., whether a so-called "100 
year rainfall" or a so-called "25 year rainfall", etc., is to be 
successfully accommodated. The volume of water that must be accumulated 
will now be understood to be a function of the weather patterns at the 
site where separator 20 and bypass basin 30 are located, as well as of the 
time required, at a given rainfall intensity and separator flow rate, for 
parking lot P to have been washed relatively clean. 
In practice, such a design is very conservative. In the first place, the 
rainfall will not reach its maximum intensity immediately, but instead 
will rise as shown in FIG. 4. Also, pipe 22 from grate G will also act as 
an accumulator, as will the parking lot itself, to a certain degree, 
thereby easing the load imposed upon bypass basin 30. Also, the above 
simplified calculations have ignored the fact that, while water is 
accumulating within bypass basin 230, flow will simultaneously be 
occurring through primary outlet means 34, thereby causing less 
accumulation to occur within tank 50. 
For a typical design, it is preferred that: 
EQU D.sub.3 =D.sub.2 =2D.sub.1 
where D.sub.1, D.sub.2, and D.sub.3 respectively designate the pipe 
diameters of primary outlet means 34 and pipe 25, bypass outlet means 36 
and pipe 39, and inlet 32 and pipe 22. The transverse cross-sectional 
areas of these pipes therefore obey the relationship: 
EQU A.sub.3 =A.sub.2 =4A.sub.1 
By definition, the flows through each of these three pipes is related to 
its transverse cross-sectional area and average liquid velocities by: 
EQU Q.sub.1 =A.sub.1 V.sub.1 
EQU Q.sub.2 =A.sub.2 V.sub.2 
EQU Q.sub.3 =A.sub.3 V.sub.3 
At maximum rated flow, i.e., when the height of the liquid within bypass 
basin 30 is at bypass outlet means 36, there is no accumulation within 
tank 50 because all excess water is being bypassed through bypass outlet 
means 36, and therefore: 
EQU Q.sub.3 =Q.sub.1 +Q.sub.2 
When bypass pipe 39 is running at maximum design capacity, the average 
velocity in pipe 25 to separator 20 follows the equation: 
##EQU6## 
Because the pressure within tank 50 and riser pipe 53 is equalized to be 
atmospheric pressure by vent pipes 72 and 92, the well-known Manning 
equation can now be used to determine the flow rate through bypass basin 
30, such that: 
##EQU7## 
where n is the well-known roughness factor for the pipe, r is the 
hydraulic radius of the pipe, and S is the slope of the pipe in vertical 
rise/fall per horizontal unit length of pipe. Multiplying both sides of 
this equation by the transverse cross-sectional area A of the pipe gives: 
##EQU8## 
Typical and well-known values of n are 0.01 for concrete pipe and 0.013 
for steel pipe. 
For a typical installation, L.sub.1, the length of pipe 25 from bypass 
basin 30 to separator 20, will be twenty to eighty times the diameter of 
pipe 25; L.sub.4, the length of pipe 98 from separator 20 to drain 38, 
will be ten to eighty times the diameter of pipe 98; h.sub.4, the height 
of outlet 26 of separator 20 above drain 38, will be four times the 
diameter of pipe 98; the slope of pipe 39 will be such that, at maximum 
design flow rate, it is steep enough not to allow substantial additional 
accumulation during bypass; and L.sub.2, the length of pipe 39 from bypass 
basin 30 to drain 38, will be greater than or equal to the sum of lengths 
L.sub.1 and L.sub.4. 
Using such an approach, the following table shows various preliminary 
calculations and values expected for various rainfall occurrences at a 
particular location, with the rainfall assumed to occur on a surface that 
is twenty per cent grass and eighty per cent paved and with a runoff 
factor of 0.85, with flow rates given in cubic feet per second ("C.F.S."), 
and with time until bypass being given in minutes as measured from the 
beginning of an intense rainfall of the given recurrence frequency: 
TABLE 1 
______________________________________ 
Process 
Runoff 
Separ. Time 
Freq. Freq. Slope Water in Flow Until 
(years) 
Factor (avg.) (C.F.S.) 
C.F.S. 
(C.F.S.) 
Bypass 
______________________________________ 
100 1.5 1% 0.89 28.95 
4.86 0.75 
50 1.2 1% 0.89 23.16 
4.5 1.5 
25 1.0 1% 0.89 19.3 4.1 3 
10 0.8 1% 0.89 15.4 3.92 5 
5 0.6 1% 0.89 11.6 3.74 7 
2 0.4 1% 0.89 7.7 3.74 10 
______________________________________ 
The above column entitled "Process Water" should be understood to be an 
additional constant load imposed on the separator and bypass basin, 
unrelated to rainfall runoff, consisting of additional effluent that must 
be constantly processed by the separator and bypass basin combination. 
This additional load merely adds a constant flow to the inlet of bypass 
basin 30 that accordingly increases the necessary capacity of the bypass 
basin and downstream separator. 
Although the present invention has been described and illustrated with 
respect to a preferred embodiment and a preferred use therefor, it is not 
to be so limited since modifications and changes can be made therein which 
are within the full intended scope of the invention.