oil-water design installation & operation

Oldcastle®

OOii ll --WWaatt eerr SSeeppaarraatt oorrss

Guidelinesfor

Design, Installation andOper ation

for surf ace r unoff treatment

The tab les below may be used for guidance in choosing appropr iate separ ator sizes. Models similar to those shown here, as w ell as custom models are av ailable from allOldcastle Precast manu facturing locations. Because of diffe ring requirements of customers in different regions, some variations may e xist betw een the standard f eatures ofmodels indicated here and those provided at each Oldcastle manu factur ing location. Additional inter mediate, larger and smaller sizes may also be av ailabl e.Contact y our localOldcastle representative for guidance. Oldcastle Precast Inc. reserves the r ight to make modifications without notice in the course of technological prog ress and in responseto customers’ needs.

† Flow ratings are calculated on the basis of the pro vision of hori zontal separ ation area according Hazen’s surf ace-loading theory and are in accordance with theAmerican Petroleum Institute’s pr inciples for separ ator sizing —API Pub lication 421, February 1990.

‡ Flow-rates in e xcess of the maximum surge v alue given above can result in str ipping of captured oil from coalescing plates.The pipe sizes given above are suitabl e for flow-rates up to the Standard T reatment flow-ra te for this model. Flow-r ates in excess of this level may require larger pipes and hydr aulic analysis of do wnstream conditions toensure that the outlet pipe can carry w ater at the maximum flow-r ate required without e xcessive head b uilding up inside the separ ator chamber.

FFll ooww--RRaatt ii nnggss aanndd GGeeoommeett rr ii ccaall DDaatt aa ff oorr SStt aannddaarrdd CCoonnff ii gguurraatt ii oonnss

© Oldcastle Precast Inc. 1996

FFll ooww--RRaatt ii nnggss aanndd GGeeoommeett rr ii ccaall DDaatt aa ff oorr SStt aannddaarrdd CCoonnff ii gguurraatt ii oonnss

UUnnddeerr sstt aannddii nngg SSeeppaarr aatt oorr PPeerr ff oorr mmaanncceeHow oil-remova l effectiv eness va ri es

Droplets in the 10-300 micron r ange

Basic measures of separ ator perfo rmance

Effluent water quality standards

HHoorr ii zzoonntt aall SSeeppaarraatt ii oonn AArreeaaHazen’s pr inciple of Surf ace Loading

W hy “hori zontal area” (and not depth)

Using Hazen’s pr inciple to size a separ ator

RRaatt ii nngg aanndd SSii zzii nngg SSeeppaarraatt oorrssSelecting an appropr iate perfo rmance

Deter mining the design flow-r ate

Deter mining the eff ective hori zontal separation area required

Horizontal separation area of simple retention tanks and ponds

Horizontal separation area of a coalescing-plate separ ator

SStt ookkeess’’ LLaawwDeter mining cri tical ri se-rates

The Stokes’ Law equation

How accur ate is Stok es’ law?

Using Stok es’ law

EExxaammppll ee CCaall ccuull aatt ii oonnss

IInnssttaall ll aatt ii oonnPlanning for site-dr ainage and location of oil-water separ ator

Planning Installation

Connection of pipe fittings

OOppeerr aatt ii oonn aanndd MMaaii nntt eennaanncceeNote on Safety

Role and Function of Separ ator

Inspection

Servicing and Maintenance

10 Essentials for a successful and cost-eff ective

oil-pollution prev ention plan for surf ace runoff

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44

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inside frontcover

inside rearcover


UUnnddeerr sstt aannddii nngg SSeeppaarr aatt oorr PPeerr ff oorr mmaannccee

HHooww ooii ll -- rreemmoovvaall eeff ff eecctt ii vveenneessss vvaarr ii eess

Many factors aff ect separ ator perfo rmance. Large perfo rmance dif-

ferences not only e xist betw een diff erent separator designs and

models, but also between identical separ ators that are used in dif-

ferent en vironments and separ ators that are subjected to diff erent

incoming oil-water mixtures. The quantity of oil in the influent, its

density (specific gr avity) and w ater temper ature all influence the

perfo rmance of gr avity separators.Howeve r, though significant,

these factors are still not nearly as important as the ph ysical

nature of the oil-w ater mixture itself: the deg ree to which the oil is

dispersed in the w ater: the size of the oil-droplets .

It is convenient to divide the diff erent fo rms of oil-w ater mixtures

into the fo l lowing f our main categori es:

. The oil is a slick or film on the surf ace —In this case it has

already separated from the w ater

. The oil is made up of relativ ely large drops —greater than 300

microns † in diameter (approximately) and globules dispersed

throughout the w ater

. The oil exists as small droplets (g reater than 10 microns in

diameter, less than ~300 microns in diameter)

. The oil exists as e xtremely small par ticles (less than 10 microns)

and em ulsions

Most oil-w ater mixtures in r unoff will tend to be a combination of

these fo rms.The first two can be removed from w ater using the

most r udimentary form of retention-vessel type oil-w ater separ ator

(sometimes ref erred to as a "spill-trap"). Although the quantity of

oi l in this form can be substantial, its removal alone is seldom suf-

ficient for safeguarding today’s more str ingent effluent w ater-

quality standards for grease and oil.

The third fo rm —droplets betw een 10 and 300 microns— can make

up a significant quantity of the oil in r unoff and is more difficult to

remove.It is in their ability to remove this oil that individual sepa-

rators differ most from each other. See next section below.

The fourt h fo rm of oil-water mixture usually occurs significantly

after intense mechanical mixing or when surfactants, s olvents or

detergents are present. Significant quantities of this fo rm of oil-

water mixture can be avoided by preventing the use of detergents

and by not using devices such as centr ifugal pumps upstream of

the separ ator. Otherwise more costly treatments may be neces-

sary —such as biofiltration or ph ysio-chemical methods.

µ

µ

µ

FFii gguurree 11:Compar ison of oil-remova l effectiv eness of three separa-

tors for three diff erent dispersions of oil.

Note: the above data assumes influent oil with a specific

gravity of 0.88 at a concentration of 250 mg/liter ‡ and

water at a temperature of 50°F.

DDrrooppll eett ss ii nn tt hhee 1100 -- 330000 mmii ccrroonn rraannggee

As mentioned above, it is in their ability to remove droplets within

this range that oil-w ater separ ators differ most from each other.

Figure 1 (above) shows the concentr ation of oil to be e xpected in

the effluent from three diff erent separ ators subjected to three dif-

ferent influent oil dispersions.As the same oil v olume is dispersed

as smaller and smaller droplets, t he effectiv eness of each separa-

tor decreases.The oil remova l rates illustr ated are typical of sepa-

rators av ailable today. Separ ator perfo rmance is char acteri zed by

its critical oil-droplet rise-rate (defined below). The ‘High

Performance’ separ ator above clearl y offers the best perfo rm -

ance and w ould theref ore pro vide the greatest saf eguard for efflu-

ent w ater quality standards.Howeve r, when flow-r ates are ve ry

large, t his l evel of perfo rmance may not be the most economical

solution. See choosing the section entitled, ‘Choosing a suitabl e

“cri tical ri se-rate” value’.

† A micron (or ‘micrometer’) is a unit of length equal to one thousandth of a mil-limeter. (There are 25400 microns in one inch. 50 microns is the appro ximate limitof detection for the human eye.

‡ Oil concentr ation in w ater is best measured and quoted in mg/liter (milligr ams perliter). The less precise term, “parts per million”, is frequently intended to meanthe same thing. En vironmental regulations use mg/liter.

EEff ff ll uueenntt wwaatt eerr qquuaall ii tt yy sstt aannddaarrddss

Effluent standards are quantitative limits placed on the amount of

polluting substances allowed in w ater. Alone, they cannot be used

to define separ ator perfo rmance. This is because the perfo rmance

of all separ ators is dependent upon ph ysical character istics of the

oil-w ater mixture going into a separ ator and not just their relative

quantities.

Most separ ators can produce an effluent that meets a w ater qual-

ity standard —if the conditions are ri ght. Unfo rtunately, i t i s also

true that no separator can guar antee that the effluent will neve r

exceed str ingent standards —unless ve ry s trict control over what

goes into it can also be guar anteed. This is because it is impossi-

ble to predict what the nature of the influent oil-w ater mixture char-

acter istics will always be. What a good separ ator can do, howeve r,

is reduce the probability of the effluent being out of compliance

significantly.In order to judge how w ell a separa tor i s l ikely to do

this, basic perfo rmance measures such as its critical rise-rate (as

described above ) m ust be deter mined. Only these types of per-

formance measures are independent of the en vironmental condi-

tions of any par ticular application.

Choosing a suitable “ cri tical ri se-rate” value

Cri tical ri se-rates for separ ators can be chosen with v alues rang-

ing from as little as 0.01 f eet per minute to as m uch as half a foot

per minute. The v alue you choose will depend on the intended

function of the separ ator (as part of a site surf ace-w ater pollution

prevention str ategy), lik ely influent character istics (oil quality and

dispersion char acter istics), effluent w ater quality standards and

the sensitivity of the receiving en vironment.

Values at the lower end of the scale (0.01 f eet per min ute) are best

chosen when design flow-r ates can be k ept reasonabl y l ow and

where a ve ry high perfo rmance is desirabl e.The upper end of the

scale (0.5 f eet per min ute) w ould only be appropr iate where the

emphasis is on pro viding a lo w-cost unit for occasional spill inter-

ception where the spilled oil is unlik ely to be dispersed significant-

ly.

More frequently, howeve r, the best approach has been to choose

an inter mediate v alue to balance costs and benefits. Large reten-

tion-tank separators in the past used to be designed with cri ticalrise-rates of the order of 0.2 feet per minute.But the concentr ation

of oil in the effluent from these de vices has frequently been f ound

not to meet toda y’s more str ingent standards.In applications that

have to meet standards of the order of 10 mg/l, a value of 0.033

feet per minute ( the ri se-rate of a 60 Micron, 0.88 S .G., oil droplet

in 50 o F water) has been f ound to be a suitable choice †:remo ving

a substantial por tion of oil and saf eguarding effluent quality to

meet these standards in almost eve ry case pro vided that proper

control is ex ercised over the use of detergents and other problem

substances.

Al l gr avity separators rely on the tendency of droplets of oil to ri se

in water (because of their natura l buoyancy). They are "caught"

either when they make contact with and adhere to the surface of a

solid object (such as a coalescing plate) or when they enter a laye r

of stationary w ater at the top of the separ ator chamber.

The Critical (Oil-Droplet) Rise-Rate

The ri se-rate of an oil-droplet is the natural speed of ascent it has

as i t rises —the droplet’s “ter minal v elocity”. A separ ator, at a

cert ain f low-rate, will capture all droplets that have ri se-rates

above a certain v alue. This v alue is the critical rise-rate of the sep-

arator at that flow-rate. The cri tical ri se-rate is a conv enient per-

formance measure for any gravity separator - the way you describe

the separ ator’s eff ectiv eness at remov ing o i l . It is usually meas-

ured in f eet per minute.The lowe r it is, the more eff ective the sep-

arator will be at removing oil and saf eguarding a cer tain w ater

quality standard.

Note: The term “surf ace loading” is sometimes used to mean the

same thing as the term “cri tical ri se-rate” (or “cri ti cal s ettl i ng

rate” in the case of sedimentation). Howeve r, t he t erm “surface

loading” is more usually a measure of the flow-rate through a

gravity separ ation chamber divided by the chamber’s area (on

plan). Though sometimes the same, these v alues are not neces-

sarily identical as will be seen in the section entitled “Haz en’s

Principle of Surf ace Loading”.

The Design Flow-rate

The cri tical ri se-rate changes as the flow-r ate of w ater though a

separ ator is increased or decreased. Therefore, to be meaningful-

ly applied to a particular separator design, a cri tical ri se-rate v alue

m ust also be accompanied by a design flow-r ate ("maximum oper-

ating flow-r ate") value for which it applies. When the flow through

the separ ator is increased, the cri tical ri se-rate of the unit increas-

es (i.e. the perfo rmance decreases). Conv ersely: when the flow-

rate is reduced, the cri tical ri se-rate is reduced (i.e.the perfo rm -

ance increases).

The Effective Horizontal Separation Area

If you use a compatible set of units and divide the design flow-rate

of a separ ator by its corresponding cri tical ri se-rate you get an

answer that has units of area. This n umber represents the effec-

tive hori zontal separation area of the separ ator.

This n umber is conv enient because it changes little with flow-rate.

So it is essentially a single n umber that can be used to quantify the

effectiv eness of any separ ator. Separ ators that provide the g reat-

est amount of eff ective hori zontal separation area gener ally have

the highest perfo rmance. Another adv antage of the concept of

effective hori zontal separ ation area is that it may be estimated

from the basic geometry of the oil-w ater separ ation chamber and

its components.

—Horizontal separ ation area is covered later in more detail.

UUnnddeerr sstt aannddii nngg SSeeppaarr aatt oorr PPeerr ff oorr mmaannccee (( ccoonntt ii nnuueedd))

† W ashington State Department of Ecology recommends this v alue for coalescing-plate separators used in Storm water applications — Stormwater ManagementManual for the Puget Sound Basin, February 1992. The Amer ican P etroleumInstitute also suggests 60 Microns as a typical design oil-droplet size in thetreatment of oil-refinery w aste-w aters — Design and Operation of Oil-WaterSeparators, API Pub lication 421, 1990.

BBaassii cc mmeeaassuurr eess ooff sseeppaarr aatt oorr ppeerr ff oorr mmaannccee

HHoorr ii zzoonntt aall SSeeppaarraatt ii oonn AArreeaaHHaazzeenn’’ ss PPrr ii nnccii ppll ee ooff SSuurr ff aaccee LLooaaddii nngg

—where A H is the hori zontal separ ation area (in square feet) as

described above, Q is the flow-r ate through the separ ator (in cubic

feet per min ute) and V T is the critical rise-rate (in feet per min ute).

Hence, the cri tical ri se-rate (or settling-ra te) for many separ ation

devices is frequently taken as being equal to the surface loading .

The “surf ace loading” on a gr avity separ ation chamber is (by def-

inition) equal to the flow-r ate through the chamber divided by i ts

area (on plan).

Other flow regimes

Flow i s rarely perf ectly unifo rm —although it is reasonabl e t o

assume it is in some instances.In other cases, howeve r, eddies

and turb ulence are significant, especially at higher oper ating flow-

rates. Such de viations from unifo rm, laminar flow serve t o reducethe efficiency of gr avity separ ation processes substantially. I n

order to account for this, a design f actor, F, i s i ncorporated into

Hazen’s equation:

—where A H, Q and V T are the same as in the equation used above

and F is a dimensionless f actor (always greater than or equal to 1)

to account for inefficiencies due to non-unifom flow.

F cannot be less than 1 because the perfo rmance of a gr avity sep-

arator cannot be g reater than that predicted by Haz en’s pr inciple

(which assumes ideal conditions). The Amer ican P etroleum

Institute recommends diff erent v alues betw een 1.2 and 1.75 fo r

traditional retention-tank (baffle-type) separators ‡. Many coalesc-

ing-plate separ ators and separators designed to ensure optimal

flow distri bution have near-ideal flo w-conditions in the separ ator

chamber:so that F is taken as being equal to 1 (or is omitted

entirely). In the design of circular clari fiers ( l ike those used in

m unicipal water-treatment projects), F can also be taken as being

equal to 1, because the flow-regime is essentially unifo rm-radial.

FFii gguurree 22: I l lustrations of unifo rm-laminar, non-unifo rm laminar

and turb ulent flow.

Hazen’s pr inciple has been exper imentally v alidated. I t i s a lso

simple to deri ve analytically using basic hydraulic equations of

continu i ty.The fo l lowing is a deri vation of Haz en’s pr inciple using

a simple gr avity separ ation flo w-model.

Consider the illustration in Figure 3 of a simple rectangular oil-

water separ ation chamber.The separ ation pool v olume is com-

posed of two z ones or lay ers: a stationary liquid-layer and a mov-

ing liquid laye r.Any water passing through the separa tor fo rms

part of the mo ving liquid laye r.The top laye r i s kept stationary by

the presence of an oil-dam (or “scum board”) bef ore the outlet of

the separ ator. The depth, d, is the maximum distance an oil-droplet

will have t o rise in order to reach the boundary betw een the sta-

tionary and the mo ving liquid laye r.W e will see, short ly, t hat i t i s

not necessary to know e xactly what “d” is.

Now, consider an oil-droplet moving through this separ ation cham-

ber.This droplet has two velocity components as illustr ated in

Figure 3: a ve rtical componet and a hori zontal component. The

vert ical velocity component is its natura l ri se-rate or “ter minal

velocity”. An oil-droplet natura lly rises in w ater because of its

buoyancy. Equations, such as Stoke s' l aw, can be used to calcu-

late the ri se-rate of any oil-droplet on the basis of its size and den-

sity, as well as other proper ties of the w ater. The symbol, V T, is

used here to represent this v elocity component. The hori zontal

velocity component of this droplet (represented by the symbol V H),

is the same as the hori zontal v elocity of the surrounding w ater that

carries it along.

If the oil-droplet can r ise as far as the boundary betw een the mov-

ing liquid layer and the stationary layer, it will be captured, because

its hori zontal velocity will drop to zero to match the surrounding still

water in this laye r. But if the oil-droplet is not given enough time,

it will pass out of the separ ation chamber bef ore it has a chance to

reach this boundary and will not be caught. Another way of stating

this i s: if the time required for separ ation is g reater than the resi-dence time of the water in the separ ator, the droplet will not be

“caught”.

WWhhyy ““ hhoorr ii zzoonntt aall aarr eeaa”” ((aanndd nnoott ddeepptt hh))

† A common error in sizing gr avity separ ation devices is to assume that perfo rmanceis directly propor tional to the v essel volume or the residence time of liquid in thechamber. While true for some reaction vessels, it is not in this case.

‡ API Pub lication 421 — Design and Operation of Oil-Water Separators , 1990

In 1904 Allen Haze n f i rmly estab lished the principle of how the

effectiv eness of a sedimentation tank va ries directly with the rate

of f low through it, divided by its plan area †. This pr inciple is not

only va lid for sedimentation processes, but applies to all liquid

gravity-separ ation processes, including oil-w ater separ ation.

Uniformly distributed, laminar flow

When the flow is laminar and unifo rmly distri buted throughout the

separ ation chamber cross-section (See figure 2), the cri ti cal r i se-rate is equal to the flow-r ate divided by the area of the separ ation

pool.


—This was an analytical proof of Haz en’s pr inciple.

W e know the speeds in each direction and the maximum dis-

tances, d and L (See figure 3 below). W e also know that, for motion

in a s traight line †at a constant speed, the time taken is simply the

distance divided by the speed. So,

W e can then rearrange this e xpression and wri te

Now, the basic pr inciple of conservation of matter (known in fluid

mechanics as “the contin uity pr inciple”) tells us that the superficialhori zontal velocity of the w ater (the actual hori zontal v elocity in uni-

formly distri buted flow) is equal to the v olumetri c flow-rate (Q) divid-

ed by the area of the ve rt ical flow cross-section:

Now we can wri te:

Notice how the depth, ‘d’, now appears in both the n umerator

and the denominator of the left-hand-side expression. This means

it cancels out of the e xpression — sho wing that the depth of theseparation chamber is not critical to separator performance .

Now, it is easy to rearr ange the e xpression that remains to get

Hazen’s pr inciple. The hori zontal area of the separ ation chamber

is equal to its length m ultiplied by its width:

So, we can use “A H” in our “condition” e xpression and get:

A separ ator must be designed so that even if the droplet comes

into the chamber at the ve ry worst location (i.e.at the bottom of the

separ ator), there will still be enough time for i t to rise up the full dis-

tance, d, to the boundary betw een the stationary and mo ving

water laye r.W e will call this the required time to ensure separ ation

or simply “the separation time” (represented by “t s”).

The amount of time av ailabl e for the droplet to do this is called the

residence time —the time the w ater spends in the separ ator cham-

ber (represented by “t r”). In other w ords, to ensure remova l of

this droplet, the separ ation time must be less than the residence

time.W e can theref ore call the fo l lowing e xpression our first basic

condition for ensuring separ ation of this droplet:

FFii gguurree 33 Model of a flo w-through gr avity separ ation process

So sizing a separ ator requires that we first select a flow-r ate to be

processed and, then, a cri tical rise-rate based on our expectations

for the oil w ater mixture to be separ ated. Dividing the first by the

second gives us the effective separation area needed:

UUssii nngg HHaazzeenn’’ ss pprr ii nnccii ppll ee tt oo ssii zzee aa sseeppaarraatt oorr

A separ ator can, therefore, be sized to provide this separ ation

area either as the plan area of its water surf ace or —much more

efficiently— as the sum of the plan-areas provided by stacks of

hori zontally e xtending coalescing plates. This is outlined in more

detail in the next section.

HHoorr ii zzoonntt aall SSeeppaarraatt ii oonn AArreeaa ((ccoonntt ii nnuueedd))

† In reality there will be some v elocity va riations with w ater depth in a separ atorchamber and hence the tr ajectory of a rising droplet will not be a str aight line, buta curve. Accounting fo r this fact, howeve r, significantly increases the complex i ty ofthe algebraic analysis needed, without alter ing the conclusions. The assumptionthat the hori zontal v elocity does not change with depth was theref ore considereda reasonable simplification.

So now we can wri te:



RRaatt ii nngg aanndd SSii zzii nngg SSeeppaarraatt oorrss

SSeell eecctt ii nngg aann aapppprroopprr ii aatt ee ppeerr ff oorrmmaannccee

DDeett eerrmmii nnii nngg tt hhee ddeessii ggnn ff ll ooww--rraatt ee

In order to size a separ ator it is first necessary to know what kind

of perfo rmance is necessary. The perfo rmance of a separ ator is

measured in terms of the ri se-rate of the slowe st rising droplet the

separ ator is cer tain to remove from w ater flo wing at the separa-

tor’s maximum oper ating flow-r ate (or design flow-r ate). This

value is known as the critical rise-rate of the separ ator. See the

section entitled, ‘Understanding Separ ator Pe rformance’, fo r

more info rmation on this.

The design flow-rate is the maximum operating flow-ra te for the

separ ator.I n runoff applications, t he f low-rate through a separ ator

can be e xpected to va ry over time. It is common theref ore to

design the separa tor for the maximum possibl e f low-rate.But this

is not necessar ily appropri ate.If the main source of w ater is from

washdown processes, the maximum flow-rate can be equated to

the maximum w ater supply rate.If the main source of w ater is from

rainfall, this cannot be done directly.

Stormwater Runoff

The most conv enient method (known as the rational method), for

calculating flow-rates of storm r unoff from small areas uses the fo l-

lowing simple equation.

Q = C i A

Q is the flow-rate

C is a dimensionless factor that accounts for the per meability and

roughness of the surfaces (taken as 0.9 for paved areas)

i i s t he design rainfall intensity (usually e xpressed as inches per

hour

A is the area of the site e xposed to ra infall.

Note:

The design ra infall intensity value is carefully selected on the basis

of local rainfall data (including times of concentration etc.) and also

on the basis of pro viding the optimal economic design. A common

method is to choose a ra infall intensity that would allow treatment

of 90% or 95% of all the w ater leaving the site.

Because it is costly to treat large flows of w ater, it is recommend-

ed that only those areas that are at risk of contamination from oil

and g rease should be allowed to drain through the separ ator.

W ater quality de vices are often designed fo r f low-rates substan-

tially less than the flow-r ates assumed for rest of the convey ancing

system. It is seldom necessary (or feasib le) to treat all of the water

from ve ry high intensity rainstorms. This is especially va l id i n

areas where the major ity of pollutants are tr anspor ted dur ing the

fi rst f lush. Large w ater flows may be either detained upstream of

the oil-w ater separ ator or allowed to b y-pass it using suitabl e flow-

splitting and recombination str uctures. Local guidelines for sizing

should be consulted in any case.

Example:

Deter mine the design flow-ra te for a separ ator on the basis of a

one-hour, half-inch design ra infall intensity on a half-acre paved

area:

DDeett eerrmmii nnii nngg tt hhee eeff ff eecctt ii vvee hhoorr ii zzoonntt aall sseeppaarraatt ii oonnaarreeaa rreeqquuii rreedd

Once suitabl e values for design flow-r ate and cri tical ri se-rate

have been selected, it is easy to calculate the eff ective hori zontal

separ ation required. J ust divide the first by the second (making

sure that their units are compatibl e f i rst) as fo l lows:

Example:

What is the eff ective separ ation area required to ensure the

removal of droplets with ri se-rates of 0.033 f eet/minute and g reater

at a f low-rate of 100 gpm?

In this e xample over 400 square f eet of effective separ ation area

is required to pro vide a separ ator with a design ri se-rate of 0.033

feet / minute †.This implies that if a tr aditional retention vessel or

pond is to be used, its area on plan will have to be of the order of

400 square f eet. If it is not possible to properly control circulation

(non-unifo rm distri bution of flow through the unit), its area may

have to be increased to more than five hundred square f eet to

make up for resulting inefficiencies. The other option is to use an

efficient coalescing-plate oil-w ater separ ator.

HHoorr ii zzoonntt aall sseeppaarraatt ii oonn aarreeaa ooff ssii mmppll ee rreett eenntt ii oonntt aannkkss aanndd ppoonnddss

In traditional retention-tank systems, ponds and clari fiers, the hor-

izontal separ ation area is closely related to the area of the sepa-

ration chamber measured on plan as shown in Figure 4. The flow

through a retention tank is often not unifo rm and a design f actor

is usually necessary (See Hazen’s Principle of surface loading —

page 5) . Baffles and ve rt ical f low distri butors may be included in

the design of such units to limit turb ulence and non-unifo rm flow

distri bution.

Note: vertical plates used for this purpose do not add to theamount of separation area in a separator chamber. Separationarea can only be provided by continuous boundaries that projecthorizontally across the separator chamber.

HHoorr ii zzoonntt aall sseeppaarraatt ii oonn aarreeaa ooff aa ccooaall eessccii nngg--ppll aatt eesseeppaarraatt oorr

In a coalescing plate separ ator the entire cross-section of flow

through the separation chamber is divided into many thin w ater

layers by hori zontally e xtending plates.Each one of these layers

acts like an individual separ ation chamber.The separ ation area of

each of these layers is the same as the plan-area †of each plate as

shown in Figure 5.

The easiest way to calculate the total area is to deter mine the hor-izontal separation-area density of the plate-stack first. This is the

amount of hori zontal area (square fe et) found in one cubic f oot of

stack . If the ave rage ve rtical spacing of the plates, sp is known in

inches —see figure 6— the hori zontal separation-area density, aH,

can be f ound using the fo l lowing ru le:

FFii gguurree 44 The hori zontal separ ationarea provided by a simpleretention tank (baffle-type)separ ator.

The ove rall hori zontal area, A H is then f ound by measur ing the

total volume (based on its gross dimensions —see Figure 7) of the

submerged plate-stack and m ultiplying it by a H :FFii gguurree 66 The ve rtical spacing of

an inclined coalescing-plates

FFii gguurree 55 The hori zontal separ ationarea provided by an inclinedand corrugated plate

FFii gguurree 77 Gross dimensions of a stack of coa-lescing plates

In addition to providing a ve ry large amount of hori zontal separa-

tion area in a small v olume, coalescing plate-stacks, when prop-

erly positioned, promote more ev enly distri buted flow throughout

a f low chamber.It is usually assumed that the eff ective hori zontal

area provided by coalescing plates is simply the sum of their plan-

areas (or projected areas). In other w ords, a value of 1.0 is

assumed for the design factor, “F” (as e xplained on page 4).

RRaatt ii nngg aanndd SSii zzii nngg SSeeppaarraatt oorrss ((ccoonntt ii nnuueedd))

† Inclinations and corr ugations give strength and r igidity to coalescing plates andmake them easier to maintain. Howeve r, while the plates themselves are neitherflat nor hori zontal, fluid contin uity and the incompressibility of w ater means thateach one still creates a hori zontal separation area equiv alent to the par allel- pro-jection of its undersurf ace area onto a hori zontal plane (its area “on plan”).

The symbols used in the equation represent v alues as fo l lows:

VT = the ri se-rate (or ‘ter minal v elocity’) of the oil-droplet

(cm/s or ft/sec)

g = the acceler ation due to gr avity (in cm/s 2 or ft/sec 2)

ro = the density of oil (g/cm 3 or lbm/ft 3)

rw = the density of the w ater (g/cm 3 or lbm/ft 3)

d = the droplet diameter (in cm or ft)

m is the absolute viscosity of the water (g/cm.sec or lbm/ft-sec).

Note: Metric units have also been given here because they arecommonly used for Stokes’ law calculations. You can use eithermetric (CGS) units or customary (FPS) units (but not both at thesame time!).

UUnnii tt CCoonnvveerrssii oonn::

1 cm = 10,000 Microns = 0.3937 in.

Density of w ater (appro ximately) = 1 g/cm 3 = 62.4 lbm/ft 3

Density of oil = Specific Grav i ty of oi l x Density of w ater

Viscosity units: 1 g/cm.sec = 0.1 P a.sec = 1 P oise = 100 cP

1 lbm/ft-sec = 14.88164 P oise = 1488.164 cP

Acceler ation due to gr avity = 981 cm/s 2 or 32.2 f eet per second per

second.

SStt ookkeess'' LLaaww

DDeett eerrmmii nnii nngg CCrr ii tt ii ccaall RRii ssee--rraatt eess

FFii gguurree 88:: The curves represent the boundari es (for water 50 oF

and 70 oF) below which Stoke s' l aw is known to predict

droplet ri se-rate with negligible error.

ρ ρµ

It is not always necessary to do a Stoke s' l aw calculation in order

to si ze a gr avity separ ator or rate its perfo rmance. Appropri ate crit-ical rise-rate values may be obtained directly by e xper iment.

Alter native ly, if specific data about the likely oil-w ater mixtures

enter ing the separator are unav ailabl e, recommended v alues may

be used. Exper ience and analysis can also determine the v alue

chosen. Remember that the final choice of critical rise-rateassumed for any design will not only determine the ultimate effec-

tiveness of the separ ator, but also its size and cost —especially

when flow-rates through the separ ator are e xpected to be sub-

stantial.

It is common to actually list oil character istics (such as density,

droplet size etc.) when defining the perfo rmance of a separ ator.

This would seem a pr actical approach e xcept that many more va ri -

ables controlling separ ator perfo rmance now need to be quoted

with them (e. g. water viscosity and cr itical droplet size ). Unless allof these v alues are provided, the separ ator cannot be assessed

for perfo rmance. When they are pro vided, the equation known as

Stoke s' law can used to conve rt these va riables into a cri ti cal ri se-rate value. This v alue is the same regardless of the oil-water mix-

ture char acteri stics.I t i s the value you can use to compare any two

gravity separators.

The fo l lowing equation, often referred to as Stok es’ law, can be

used to accur ately calculate the ri se-rate (‘ter minal r ise veloci-

ty’) of a droplet of oil of known density in w ater of known viscos-

ity:

Stoke s' l aw is deri ved from Newton’s laws of par ticle and fluid

mechanics and it applies only to objects r ising or falling in a fluid

under cer tain conditions known as Stokes’ flow conditions .These

conditions are that the object m ust be spher ical in shape, and suf-

ficiently small and slo w-moving so that microscopic turb ulence and

“boundary l ayer” eff ects do not come into play in the vicinity of

the rising or f alling object.

Fortunately, Stoke s' l aw is almost perf ectly suited to the design of

most gr avity oil-w ater separ ators. This is true for two reasons.F irst,

because of oil’s natura l hydrophobicity and the phenomenon of

surf ace tension, small droplets of oil are spher ical in shape.

Second, although there may e xist a significant portion of larger

droplets for which Stokes’ flow conditions will not be satisfied as

they ri se, the smaller oil droplets that are most cr itical to separ ator

perf or mance nearly always lie within that r ange for which

Stokes’ law is known to be accur ate (as illustr ated in Figure 8

above).

Stokes’ law will only be useful in predicting the ri se-rates of

droplets ri sing i n s ti l l water or where the flow regime is laminar .

Laminar flow is where the w ater moves as ‘lay ers’ with no ve rti-

cal mixing, circulation or turbulence. The idea is simple. The

droplet must be abl e t o rise ve rtically —from ‘lay er’ to ‘layer’

without interf erence. Turb ulence and eddying simply undo the

process of separ ation. Laminar flow is a prerequisite for any gr av-

ity separ ation process.

In summary, if the oil-droplet in question is within the r ange of

sizes and densities shown in the shaded por tion of the gr aph in

Figure 8, and the flow regime is laminar , Stoke s' l aw is accurate

and reliabl e.

HHooww aaccccuurr aatt ee ii ss SStt ookkeess’’ LLaaww??

SStt ookkeess’’ LLaaww



UUssii nngg SStt ookkeess’’ LLaaww

SStt ookkeess'' LLaaww -- ccoonntt ii nnuueedd

Stokes’ law is an equation of four va riabl es.To calculate the ri se-

rate of an oil-droplet, all you need to do is plug in appropr iate va l-

ues for each of the four va riables on the r ight-hand side of the

equation: the density of the oil, the density of the w ater, the diam-

eter of the oil droplet (which we know to be a sphere) and the

absolute viscosity of the w ater. Care needs to be taken to ensure

correct v alues are chosen for each of these using the correct units.

Viscosity:

The v alue for viscosity used in this v ersion of Stoke s' l aw is the

absolute viscosity of the w ater. This is measured in poise, cen-

tipoise, g/cm.s (gr ams per centimeter-second), P a.s (P ascal sec-

onds) or lbm/ft-sec. Be careful with values given which are

described as “kinematic viscosities” (measured in Stokes,

centiStokes or slugs). These m ust be conve rted to the appropri -

ate “absolute viscosity ” value bef ore this v ersion of the Stokes’

law equation can be used.

Values for the viscosity of w ater may be taken from Tabl e 1 fo r

most conditions. Notice how the viscosity of w ater changes dra-

matically with temper ature. At higher temper atures, w ater

becomes less viscous and theref ore provides less resistance to

the motion of par ticles - so that they separate more easily.

TTaabbll ee 11:: Relationship between viscosity and temper ature of

water

FFii gguurree 99: Rise-r ate v ersus droplet size for oil-droplets of va rious specific gr avities in water at 50 o F

Shor tcut method:

Figure 9 below was deri ved using Stokes’ law.It may be used to

estimate the ri se-rate of a droplet of oil of a given size and densi-

ty when the w ater is at a temperature of 50 o F. For temper atures

other than 50 o F, the correct ri se-rate v alue can be obtained by

m ultiplying v alues taken from Figure 9 by the appropr iate correc-

tion factor given in Table 1 (above ).

EExxaammppll ee CCaall ccuull aatt ii oonnss

EExxaammppll ee 11

Calculate the ri se-rate of a 60 micron droplet of oil that has a spe-ci fic gr avity of 0.888 in w ater at a temper ature of 50° F.

Solution:

ρ

ρ

ρ ρµ

EExxaammppll ee 22

Calculate the eff ective hori zontal separ ation area required tocapture all droplets with ter minal v elocities g reater than or equalto that of a 60 micron droplet of 0.888 specific gr avity oil in w aterat a temper ature of 50 ° F, flowing at 100 gallons per minute.

Solution:

EExxaammppll ee 33

Size a suitable rectangular cross-sectioned retention tank (APIoil-w ater separa tor) for a critical rise-rate of 0.033 feet/min at itsoperating flow-rate of 100 gallons per minute. What size circularunit would be required to provide the same separation effective-ness?

π

π

π

π

Clearly both of these approaches invo lve the creation of ve rylarge str uctures:

† This is similar to the design approach for this type of separ ator recommended bythe Amer ican Petroleum Institute (Pub lication 421, 1990)

Page 10


EExxaammppll ee CCaall ccuull aatt ii oonnss (( ccoonntt ii nnuueedd))

EExxaammppll ee 44Check i f t he fo l lowing Oldcastle separ ator is adequate for thesame perfo rmance requirements of example 3 (Ve rtical platespacing = 18.5 per ve rt ical foot).

Solution:

EExxaammppll ee 55

What is the design ri se-rate of the Oldcastle separ ator in Example4 at 100 gallons per min ute? What size oil-droplet does this corre-spond to (oil specific gr avity = 0.888, w ater temper ature = 50°F)? What is the maximum flow-r ate that can be put through thissepara tor for a design ri se-rate of 0.033 ft/min.?

Solution:

ρ ρµ

EExxaammppll ee 55 (( ccoonntt ii nnuueedd))

T

T

EExxaammppll ee 66

If the concentr ation of oil enter ing the separator was known to be250 mg / liter and info rmation about the droplet size distri bution ofthe oil enter ing the separ ator rev ealed that typically 2% of the oilby volume in Example 1 is less than 60 microns, what would be theconcentr ation of oil in the effluent from a separ ator oper ating toprovide a design ri se-rate of 0.033 ft / min.?

Solution:

Page 11


IInnssttaall ll aatt ii oonnPage 12

Note : The information presented here is for guidance purposes only and is not intended as a detailed instruction manual. It is the r esponsibility of the owner or contrac-tor to ensure compliance with all applicable federal, state and local codes and regulations.

PPll aannnnii nngg ff oorr ssii tt ee--ddrraaii nnaaggee aanndd ooii ll --wwaatt eerr sseeppaarraa--ttoorr ll ooccaatt ii oonn

The Drainage Plan

Careful planning of paved areas and their dr ainage can signifi-

cantly contri bute to successful control of pollutants. See figure 10.

Consider the fo l lowing:

. Vehicles, equipment and storage vessels and activities thatpose a risk of oil-discharge should be limited to a single locationif possible

. Drainage from roofs and green areas that are unlikely to havethe same pollution problems should be routed separately —This

reduces flow-rates and treatment costs for contaminated w ater.

. If occasional excessive flows of water are unavoidable, allow forby-passing of excess water to prevent flows from exceeding themaximum surge capacity of the separator unit and other treat-ment system components — By-passing can be achieved with

the help of detention structures and limited surf ace flooding as

illustrated in Figure 13. Alter nativ ely a flow-div ersion and recom-

bination str ucture can be constr ucted. Your local Oldcastle

manufacturer may be able to pro vide a custom solution for you,

such as a separ ator with integr ated b y-passing f eatures, as

illustrated in Figure 11.

FFii gguurree1122:: Locate the separ ator at a crown in the pavement.

Location of separator

When planning a suitable location for the oil-w ater separ ator, keep

the fo l lowing in mind

. The top of the separator should be located at a ridge or crownin the pavement — Risers can be pro vided to create tops to any

grade level desired (See Figure 12).

. When planning for on-site detention (controlled, limited flood-ing), the flow constriction must be placed upstream of the oil-water separator —Do not position flo w-constr iction de vices

downstream of the separ ator or plan for any backup of flow in

the separ ator chamber (See Figure 13.)

. Make sure that there is room for access by maintenance vehi-cles to the separator —To enable conv enient, regular mainte-

nance as necessary

. Locate the separator where it may be conveniently accessed bypersonnel — Where it is unlik ely to become permanently cov-

ered by stored materi als, vehicles or machinery

FFii gguurree 1100:: Example of site-dr ainage plan fo r runoff treat-ment (The area shaded in gr ey is identified where runofffrom the pavement has the potential to be contaminated).

FFii gguurree 1111:: Oldcastle Separ ator with integr ated b y-pass.

PPll aannnnii nngg tt hhee II nnsstt aall ll aatt ii oonn ooff sseeppaarraatt oorr

Oldcastle oil w ater separators require the same care with installa-

tion as any similar reinf orced-concrete en vironmental str ucture.

Contact y our local Oldcastle representative for specific guidelines

and equipment required to install a par ticular model. In any case,

you should comply with applicable codes and regulations. Things

to plan ahead for are:

. Excavation— In deep excav ations, it is impor tant that the excav ation is kept

properly shored. Clear ance is required to allow for projecting

pipe fittings as w ell as the concrete vault (See figure14).

. Proper bedding preparation —The installed separ ator unit m ust ultimately be suppor ted by

undisturbed or w ell-compacted soil that is unlik ely to settle sig-

nificantly. Care needs to be taken to ensure that the bedding

mater ial is properly screeded and compacted to pro vide a firm,

level foundation for separ ator va ul t.

. Use of proper lifting equipment —Smaller Oldcastle units are installed by boom truck oper ators

from y our local Oldcastle Precast manu facturi ng fa ci l i ty.Larger

sizes will require a cr ane to lower the unit in place.

. Correct positioning of vault with respect to incoming and outgo-ing pipes— Your Oldcastle Precast representative can advise you on suit-

able pipe-sizes and arr angements

. Correct placement of gaskets between vault base, top-sectionand riser-sections— Your local Oldcastle Precast manu facturer can recommend a

durabl e, water- and oil-tight joint if necessary for your applica-

tion. Special care needs to be taken in areas that are prone to

high w ater-tab les to preve nt i nfi l tration of w ater into the sepa-

rator chamber. The joint betw een riser sections in a concrete

vault must be clean bef ore application of any gasket. Gaskets

m ust be resistant to oil and h ydrocarbon compounds. Check

with the manu facturer to ensure that the gasket is suitabl e for

the application. Ask y our Oldcastle representative for advice in

mounting gask ets between concrete sections.

. Even backfilling before filling with water

. Protection of internal components during construction work onthe surface —During site constr uction wo rk, sediments and debris should be

kept out of the separ ator

. Sealing of pipe connections at all points in the drainage systemDurabl e, non-corrosive w ater-tight seals are essential to the

functioning of separ ators. Some special guidelines are given

overl eaf.

FFii gguurree 1144: Planning separ ator installation

FFii gguurree 1133:: Surface detention to control flow-r ates

II nnssttaall ll aatt ii oonn ((ccoonntt ii nnuueedd))


NNoott ee::

PPrrooppeerr ii nnssttaall ll aatt ii oonn ii ss ccrrii tt ii ccaall ttoo tthhee ll oonngg--tteerrmm ssaaffeettyy,, ii nntteeggrrii tt yy aanndd

dduurraabbii ll ii ttyy ooff aann ooii ll --wwaatteerr sseeppaarraattoorr.. II tt ii ss nnoott ddii ff ff ii ccuull tt ttoo ddoo wwii tthh tthhee

rr ii gghhtt tt ooooll ss aanndd eeqquuii ppmmeenntt .. EEnnssuurree tt hhaatt yyoouu ccoommppll yy wwii tt hh aapppprroo--

pprr ii aatt ee eennggii nneeeerr ii nngg ssppeeccii ff ii ccaatt ii oonnss aass wweell ll aass tt hhee rreeccoommmmeennddaa--

tt ii oonnss ooff yyoouurr OOll ddccaasstt ll ee PPrreeccaasstt rreepprreesseenntt aatt ii vvee..

FFii gguurree 1166:: Pipe to pipe seals outside separ ator and for connect-ing inter nal “tees” and e xtensions: (a) using propri etary‘bell’ ends or pipes and (b) a typical sleeve couplingarrangement.

FFii gguurree 1177:: Tee-sections required at inlet and outlet

FFii gguurree 1155:: Typical pipe-w all seals for PVC pipe: (a) usinga non-shr ink g rout and (b) using a propri etary “boot” sys-tem. In both cases careful prepar ation of surf aces of pipesand concrete and the choice of suitable adhesives andprimers for each mater ial is essential to the creation of agood seal.

II nnssttaall ll aatt ii oonn ((ccoonntt ii nnuueedd))

CCoonnnneecctt ii oonn ooff PPii ppee ff ii tt tt ii nnggss::

Pipe fittings for oil-w ater separ ators m ust be oil- and w ater- tight.

Where possibl e, Oldcastle Precast prefers to ship separ ators with

tee-sections and pipes already installed and sealed, so that all the

owner or contr actor has to do is provide the e xter nal coupling to

the outside dr ainage system. Occasions do ari se, howeve r, when

it is necessary to leave the installation of pipes until after the unit

has been installed. In this case the separ ator is shipped with

cored or cast-in holes.In this case, it is the responsibility of the

customer or contr actor to pro vide an adequate seal that is both

durable and flex ible enough to function properl y for t he f ul l range

of a l lowed constr uction toler ances. Ask y our local Oldcastle

Precast representative for advice on this issue.

Pipe to Wall Seals

When installing pipes and tees into the separ ator v ault, it is impor-

tant to pro vide a good seal to reduce r isks of liquid in-flow or out-

flow at the pipe-wa l l seal . There are va riety of methods for insuring

a durabl e, well-sealed connection. Figure 15 shows two suitabl e

ways to do it. Whatever method used, the integr ity and dura bi l i ty of

the seal depends on the quality of prepar ation, wo rkmanship, t he

durability of the mater ials used as w ell as the pro vision of a ri gid

foundation under the v ault-str ucture and piping.

Pipe-to-Pipe Seals

See Figure 16. Most pipe manu facturers have their own recom-

mendations for how best to achieve pipe-to-pipe seals.Again, the

key to achie ving a good durable seal lies with the quality of surface

prepar ation, wo rkmanship, the durability of mater ials used and the

provision of proper bedding materi al for the v ault-str ucture and

pipes. When you use propri etary methods, check with the manu-

facturer to make sure that the seal is durable and can withstand

hydrocarbons and w ater.Fo l low the pipe-manu facturers instruc-

tions when using propri etary products.

Tees and Pipe-extensions inside vault

The tee-sections and pipe e xtensions play impor tant roles in the

functioning of the separ ator.They al low proper venting of incoming

flow and controlled turbulence dissipation in the inlet chamber as

well as pro viding additional containment for oil that becomes

trapped in the separ ator.It is important, therefore, that they are

attached using properly sealed, bonded (or mechanically joined)

connections just like the e xter nal pipe fitting connections.They

m ust be installed so that they extend to the correct elevations, pro-

viding sufficient freeboard and baffling of w ater flo wing out of the

separ ator. See Figure 17

OOppeerr aatt ii oonn aanndd MMaaii nntt eennaannccee

RRooll ee aanndd ff uunncctt ii oonn ooff aa sseeppaarraatt oorr

NNoott ee oonn SSaaff eett yy

Oldcastle’s oil-w ater separ ator may be thought of simply as a

highly efficient gr avity-separation de vice, that is capable of remov-

ing e xtremely small droplets of oil that w ould otherwise be carried

on through the dr ainage system. The separ ator does not destroy

the oil or other pollutants it captures.I t traps and pro vides tempo-

rary stor age for them.

The use of an oil-water separ ator is recognized by the EPA, state

and local en vironmental agencies as a Best Management Pr actice

(BMP). The separ ator will serve its pur pose best when seen as a

component of a larger str ategy to protect r unoff w ater quality.An

oil-w ater separ ator is a part of the site dr ainage system and will

only do its job w ell if the other part s ( i .e.catchbasins, grit-cham-

bers, pipes etc.) are perfo rming their functions properly too.

Though a separa tor for the most part requires little human inter-

vention to wo rk, it does need periodic inspection, cleaning and

preventive maintenance. The amount of maintenance required will

vary with each application. It is the responsibility of the separ ator

owner to ensure that his separ ator inspection and maintenance

plan suits the application requirements and that this plan is prop-

erly carr ied through. The o wner can reduce maintenance costs by

making changes to wo rk practices that gener ate e xcessive quan-

tities of sediments and oil releases.

Regular inspection is the key to ensur ing that an oil-w ater separa-

tor does its job we ll. The inter nals of Oldcastle’s separ ators are

easily viewed simply by opening the large access doors and look-

ing inside. Doing so, in m ost cases, t akes only a minute.

The inspection frequency required for separ ators va ries from

application to application. It depends on the quantities of oil

released at the site.Duri ng t he f i rst few months of opera tion, i t i s

advisable to inspect the separ ator once a w eek to deter mine the

rate of accum ulation of solid mater ial and oil in the unit. If the activ-

ity on the paved area which drains through the separa tor i s fa irl y

consistent, then the frequency of inspection can be reduced to as

little as once eve ry three months.

1. Oil Buildup

Measure the thic kness of the layer of oil that has built up on the

surf ace of the separ ator.It should be removed bef ore it reaches

a depth of two inches.

2. Solids Accum ulation

Use a long pole to deter mine sludge b uild-up on the bottom —

judged by the resistance f elt when you attempt to push the pole

to the bottom of the separ ator chamber.If more than six inches

of sludge has accum ulated at the bottom of the unit, it requires

cleaning out.

3. Presence of debris and floatable mater ials in the inlet chamber

The inlet chamber m ust be k ept clear. This chamber and its

openings are import ant for dissipating turb ulence and distri but-

ing the flow of w ater through the separ ator. The inlet chamber

also acts as a last line of def ense for the separ ator against

heavy gr it, floating and settling debri s.Excessive amounts of

these in the separ ator is an indication of prob lems with the sys-

tem upstream that should screen out these materi als.

4. W ater Level in the unit

Check that the w ater level has not r isen e xcessiv ely inside the

uni t. The w ater level inside the separ ator should neve r rise more

than twe lve inches above its standing level (unless the separa-

tor has been designed to allow for additional freeboard ). The

static w ater level should be the same as the level of the inve rt of

the outlet pipe (or outlet weir —if one is present). An e xcessive

rise in w ater level dur ing oper ation is an indication of bl ockage

either do wnstream of the separ ator or within the coalescing

plates themselves.

5. The Whole System

Inspect catch-basins, other units and dr ained areas upstream of

separ ator. The level of the top of solid material in the base of gr it

chambers and catchbasins should be w ell below the level of the

invert of the pipe lea ving the catch basin. Make sure that paved

II nnssppeecctt ii oonn

Page 15

Always exercise caution when dealing with underground installa-tions, oil and other hazardous substances. The details of all nec-essary safety precautions cannot be covered in detail here.Remember, however, that the following may be necessary:

. Precautions when handling oils and other substances —Some

oils are potentially hazardous substances

. Fire-prevention measures around oil

. Measures to avoid accidents when inspecting or entering under-ground installations —Seek advice from a health and safety

expert if you are in doubt about correct saf ety procedures

. Compliance with local, state and federal safety regulations

FFii gg 1199:Gravity (or buoy ancy) causes droplets of oil dispersed inthe w ater to r ise up and separate from w ater. The film of oilthat dev elops on the surf ace m ust be removed per iodically.

Solids Removal from separator chambersBulk pumping of sludge that collects at the bottom of the oil w ater

separ ator is recommended if the level of solids b uildup inside the

separator chamber e xceeds six inches. This is best achieved using

the services of a prof essional tank cleaning company. This type of

cleaning should not be e xpected to be necessary more than once

a year.I f excessive sludge b uildup is a problem, it may be due to

problems with catchbasins and gr it-chambers upstream of the

separ ator

OOppeerraatt ii oonn aanndd mmaaii nntt eennaannccee (( ccoonntt ii nnuueedd))

SSeerr vvii ccii nngg aanndd MMaaii nntt eennaannccee

Periodic cleaning and prev entive maintenance is essential to the

proper functioning of oil-w ater separ ators.

Oil Removal from the surface

Oil that is removed from a separ ator should be stored separ ately

as a potentially hazardous materi al. If possibl e, store it saf ely with

other used oils and recycle it. Oil can be removed from the surface

of the w ater in oil-w ater separ ators by a n umber of means:

. Wet vacuumingThis is the quick est and most conv enient method for frequent oil-

removal from a separ ator. Most industr ial wet/dry vacuum clean-

ers are suitabl e for this. Entry into the separ ator is not necessary

and special e xtensions and skimming attachments are av ailabl e

for doing this job.

. Using skimming devicesMany skimmer de vices are av ailable that have l ow energy

requirements. The simplest is a rope or belt skimmer where an

adsorbent belt (or looped rope) is fed contin uously through the

oil/w ater surf ace in the separ ator. Skimmers are useful only in

situations where there is constant oil-buildup in the separ ator.

They are slow and require frequent inspection and mainte-

nance.

. Bulk pumping of the entire separator contentsIn some cases (especially after an accidental bu lk s pi l l of oi l ), i t

is best to obtain the services of a prof essional tank cleaning

company, who usually removes a separ ator’s entire contents

(oi l , water and solids) using special v acuum trucks. This materi -

al is then deliv ered to a licensed treatment f acility where the oils,

solids and classifiable mater ials can be ex tracted and recycled

or safely stored.

Maintaining the coalescing plates

The coalescing plates in an Oldcastle separator are designed to

oper ate for long periods without requir ing maintenance.

Inclinations and channels in the plate stacks enable solid part icles

to settle out of the system and oil to gr adually flow to the w ater sur-

face.

In the ev ent of the separator receiving a heavy silt load, it may be

necessary to clear the plate-stacks of this material with a hose

(high-pressure if necessary ). This can be accomplished without

m oving the plate-stacks.It is not necessary to completely clean

the plates in order for them to wo rk properl y.

Removing and Installing Coalescing-PlatesSometimes it is desirable to completely remove the plate-stacks

for closer inspection of the chamber or intensive maintenance if

nuisance substances w ere introduced into the separ ator or if the

plates become damaged. The plate-stacks Oldcastle uses are

modular and may be lifted easily through the separ ator access

doors.Take care to ensure that they are replaced correctly with the

proper components secur ing them in place in their correct posi-

tions l i ke those shown in Figure 19 below.

FFii gguurree 1199:: Inter nal components need to be properl y positioned in the separ ation chamber

areas dr aining to the catchbasins are free of large quantities of

sand and dirt and other mater ials that could interf ere with the

system such as detergents, s olvents, and antifreeze agents.

These substances cause oils to become more thoroughly mixed

with w ater so that g reater quantities of the oil are dispersed as

extremely small droplets (< 10 Microns), emulsions and even

solutions. While in this state, oils have little tendency to sepa-

rate.

II nnssppeecctt ii oonn ((ccoonntt ii nnuueedd))


1100 eesssseenntt ii aall ssff oorr aa ssuucccceessssff uull aanndd ccoosstt --eeff ff eecctt ii vvee

ooii ll --ppooll ll uutt ii oonn pprreevveenntt ii oonn ppll aannff oorr ssuurrff aaccee rruunnooff ff ff rroomm yyoouurr ssii tt ee

11.. SStt aarr tt nnooww —Don’t w ait until a neighbor or environmental official r aises their concerns with you. I nvestigate yourwork practices.As time goes on people are becoming less and less toler ant of av oidabl e runoff pollution andpenalties for non-compliance are becoming more and more severe.

22.. II nnsstt aall ll aann ooii ll --wwaatteerr sseeppaarraattoorr—which pro vides reliabl e, measurable perfo rmance, maintainability and dura bi l i ty

33.. CCrreeaatt ee aa ccoonntt ii nnggeennccyy ppll aann ff oorr ddeeaall ii nngg wwii tt hh ssmmaall ll aanndd ll aarrggee rreell eeaasseess ooff ooii ll ss aanndd ppooll ll uutt ii nngg ssuubbsstt aanncceess— You can start by looking up local tank-cleaning companies and enquir ing about their ser vices and how quick-ly they can respond. When a spill or accident occurs that poses a potentially hazardous situation, yyoouurr pprrii oorrii ttyyii ss tt oo mmii nnii mmii zzee ddaammaaggee.Implement a suitable cleanup procedure promptly and document the incident. Chancesare you will have completely ave rted the risk of any ser ious en vironmental damage. Instead of seeing the inci-dent as a “disaster”, take pr ide in the success of y our contingency clean-up plan.

44.. GGeett tt oouugghh oonn aall ll ll ii qquuii ddss aanndd mmaatt eerr ii aall ss tt hhaatt ccoouull dd bbee ccaarrrr ii eedd ii nntt oo yyoouurr ddrraaii nnaaggee ssyysstt eemm—Maintain a clean shop.Keep litter, sand, soil, etc. off paved areas. Also, keep a close w atch for the fo l lowing liquids that can damagethe functioning of an oil-w ater separ ator: aanntt ii ff rreeeezzee aaggeenntt ss,, ddeeggrreeaasseerrss,, ddeett eerrggeenntt ss,, aall ccoohhooll ss aanndd ssooll vveenntt ss..

55.. PPaayy aatt tt eenntt ii oonn tt oo yyoouurr ooii ll --wwaatt eerr sseeppaarraatt oorr aanndd tt hhee rreesstt ooff tt hhee ddrraaii nnaaggee ssyysstt eemmaanndd ccll eeaann aanndd mmaaii nntt aaii nn ii tt pprroommpptt --ll yy wwhheenn nneecceessssaarr yy —Inspect y our oil-w ater separ ator regularly (it should only take a min ute). Also check yourcatchbasins and other str uctures. Find the saf est, most conv enient method you can use to clean them out asneeded.

66.. SSeerrvvii ccee yyoouurr ddrraaii nnaaggee ssyysstt eemm pprroommpptt ll yy —Do not w ait until the system is full of oil or solids. Oil should beremoved bef ore it reaches a level of 2 inches.Do not use a separ ator or any other part of the dr ainage systemas a receptacle for used oil. Do not leave the separ ator or other parts of the dr ainage system full of large quan-ti ties of oi l for long periods of time †.

77.. SStt oorree uusseedd ooii ll aanndd ooii ll rreemmoovveedd ff rroomm sseeppaarraatt oorr tt ooggeett hheerr ii nn aa ssaaff ee,, wweell ll ccoonntt aaii nneedd ll ooccaatt ii oonn ff oorr hhaazzaarrddoouuss wwaasstt eessaanndd hhaavvee ii tt sseenntt ttoo aa ll ii cceennsseedd rreeccyyccll ii nngg ffaaccii ll ii tt yy —In parts of the country, people are profiting from the sale ofwaste oil. There may be similar oppor tunities in y our area.

88.. II nnssppeecctt yyoouurr mmaacchhii nneerryy aanndd ll ii qquuii dd sstt oorraaggee rreegguull aarr ll yy aanndd uunnddeerr tt aakkee pprreevveenntt ii vvee mmeeaassuurreess ff oorr eeaarr ll yy ddeett eecctt ii oonn aannddpprreevveenntt ii oonn ooff ll eeaakkss ff rroomm ccoorrrrooddeedd oorr wwoorrnn ppaarr tt ss —Preventive maintenance of y our v ehicles’ equipment savesyou money by reducing costly repairs and do wntime.It also reduces the cost of maintaining y our r unoff treat-ment system and enhances its reliability.

99.. RReedduuccee rr ii sskkss tt hhaatt hheell pp yyoouu rreedduuccee ccoosstt ss —When more en vironmentally sound wo rk-pr actices and source con-trol measures are adopted, the separator and other components of y our treatment system require less and lessmaintenance. Regular separ ator ser vicing should not be a time-consuming or costly activity.Maintaining yoursepar ator this way reduces the need for more costly and intensive ov erhaul at a later stage.

1100.. TTaakkee rreessppoonnssii bbii ll ii tt yy ff oorr yyoouurr oowwnn eeff ff ll uueenntt —No-one else has the power you have to control r unoff pollution fromyour site. So it is y our responsibility. Oldcastle’s oil-water separ ator represents the state of the art in gravity-separ ator design, bu t i t is sti l l only a tool to help you control y our w ater quality. There is no technology that canautomatically take care of all of y our w ater quality wo rri es.Avoid over-reliance on oil-level sensors and alarms,leak detection equipment etc. Such de vices may be unreliable in the long run and may pro vide you with a f alsesense of securi ty.There is no substitute for visually inspecting y our dr ainage system on a regular basis (whichshould only take a minute anyway ).

† Storing large quantities of oil and other potentially hazardous liquids in an open hydr aulic system is strongly discour aged and illegal in many parts of the country.Aswell as creating a fire and saf ety hazard, environmental r isks remain. Double-w alled containment of oil-water separ ator chambers does not constitute secondary con-tainment of liquids because the system is open. Oil and other mater ials can be re-entrained into the effluent as a result of bl ockage-induced surge or turb ulence, t heaccidental release of interfe ring substances (e. g. detergents) into the separ ator and the occurence of other unpredictable ph ysical, chemical or microbiological reac-tions at the oil-w ater interface.

Applications

Vehicle maintenance facilitiesTruck stops

Petroleum marketing facilitiesAuto recycling and repair yards

Railway maintenance yardsIndustrial plants

RefineriesWaste disposal and transfer depotsVehicle / equipment washdown sites

Airfields and aircraft maintenanceMarine repair yards

Vehicle and equipment storage pools

Manufacturing plants are located throughout the country


Some models supplied may differ from illustrations shown in this manual.Oldcastle reserves the right to make changes without notice in the courseof technical progress and in response to customers’ requests.

[email protected] 888-232-6274 oldcastle-precast.com

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The tables below may be used for guidance in choosing appropriate separator sizes. Models similar to those shown here, as well as custom models are available from allOldcastle Precast manufacturing locations. Because of differing requirements of customers in different regions, some variations may exist between the standard features ofmodels indicated here and those provided at each Oldcastle manufacturing location. Additional intermediate, larger and smaller sizes may also be available. Contact your localOldcastle representative for guidance. Oldcastle Precast Inc. reserves the right to make modifications without notice in the course of technological progress and in responseto customers’ needs.

† Flow ratings are calculated on the basis of the provision of horizontal separation area according Hazen’s surface-loading theory and are in accordance with theAmerican Petroleum Institute’s principles for separator sizing —API Publication 421, February 1990.

‡ Flow-rates in excess of the maximum surge value given above can result in stripping of captured oil from coalescing plates. The pipe sizes given above are suitable for flow-rates up to the Standard Treatment flow-rate for this model. Flow-rates in excess of this level may require larger pipes and hydraulic analysis of downstream conditions toensure that the outlet pipe can carry water at the maximum flow-rate required without excessive head building up inside the separator chamber.

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Oil-Water Separators

oil-water design installation & operation

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