Petroleum refining water/wastewater management...Overall refinery water balance 3 Sources of water 3 Water leaving the refinery 4 Raw water treatment: 5 Use of raw water in refineries

Petroleum refiningwater/wastewateruse and management

OperationsBest PracticeSeries2010

www.ipieca.org

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Petroleum refiningwater/wastewateruse and managementIPIECA Operations Best Practice Series

This document was prepared by AECOM, Inc. on behalf of the IPIECA Refinery Water Management Task Force. The assistance of M. Venkatesh of ENSR–AECOM is gratefully acknowledged.

Cover photographs reproduced courtesy of the following (clockwise from top left): ExxonMobil; Nexen; Photodisc Inc.;iStockphoto; Corbis; Shutterstock.com.

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Contents

Introduction 2

Refinery water overview 3

Overall refinery water balance 3

Sources of water 3

Water leaving the refinery 4

Raw water treatment: 5

Use of raw water in refineries 5

Wastewater 8

Process water 8

Desalter effluent 8

Sour water 11

Tank bottom draws 13

Spent caustic 14

Cooling water 16

Cooling tower blowdown—best practices 18

Condensate blowdown 18

Boiler blowdown 18

Steam generator blowdown 18

Unrecovered condensate 18

Condensate blowdown—best practices 20

Raw water treatment 20

Raw water treatment—best practices 20

Miscellaneous discharges—best practices 20

Miscellaneous discharges—minimization 21

Laboratory wastewater 21

Spent/unused hydrocarbons samples—best practices 21

Spent/unused wastewater samples—best practices 21

Discharges from laboratory sinks—best practices 21

Discharges from bottle washing systems—best practices 21

Stormwater and sewerage 22

Stormwater segregation and management 22

Contaminated stormwater 22

Non-contaminated stormwater 23

Sewerage management 24

Effluent treatment 25

Process wastewater pretreatment 25

Desalter effluent treatment 25

Wastewater segregation 27

Primary treatment 28

First stage: separation (oil/water separators, API separators) 28

Secondary oil/water separation 29

Equalization system 30

Location of the equalization system 31

Secondary treatment 31

Suspended growth processes 31

Attached growth processes 35

Tertiary treatment 38

Sand filtration 38

Activated carbon 38

Chemical oxidation 39

Treatment of sludges 40

API separator bottom sludge 40

DGF/IGF float and sludge 40

Waste biological sludge 41

Recycle and reuse issues 42

Re-use of non-contaminated stormwater 43

Fire water 43

Cooling tower makeup water 43

Utility water 43

Boiler feedwater makeup 43

Technologies for upgrade of refinery wastewater 43

Basic media/sand filtration 44

Microfiltration or ultrafiltration 45

Microfiltration or ultrafiltration, with reverse osmosis 47

Microfiltration or ultrafiltration, with nanofiltration 48

Ion exchange 48

Technology summary—refinery wastewater reuse 50

Reuse of municipal wastewater 50

Media filtration 51

Microfiltration or ultrafiltration 51

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PETROLEUM REFINING WATER/WASTEWATER USE AND MANAGEMENT

Microfiltration or ultrafiltration, plus reverse osmosis 52

Microfiltration or ultrafiltration, plusnanofiltration 53

Ion exchange 53

Zero liquid discharge 53

References 55

List of Tables

Table 1 Typical sources of water 4

Table 2 Contaminants in raw water 7

Table 3 Sources of wash water 9

Table 4 Desalter effluent contaminants 10

Table 5 Sour water producers 12

Table 6 Stripped sour water contaminants 12

Table 7 Crude tank bottom sediment andwater contaminants 14

Table 8 Intermediate product stream—caustic treated 14

Table 9 Cooling tower blowdown—contaminants 18

Table 10 Contaminant specification for reuse water 42

Table 11 Refinery wastewater reuse—summary 50

List of Figures

Figure 1 Refinery water balance 3

Figure 2 Typical desalter configuration 8

Figure 3 Sour water stripper configuration 11

Figure 4 Crude tank water draw 13

Figure 5 Typical distillation system 16

Figure 6 Once-through cooling water system 17

Figure 7 Closed loop cooling system 18

Figure 8 Evaporative cooling water system 19

Figure 9 Boiler blowdown—typical 19

Figure 10 Steam generator blowdown—typical 19

Figure 11 Typical refinery wastewater treatment 25

Figure 12 Desalter oil/water separation 26

Figure 13 Desalter effluent stripper 26

Figure 14 Segregated wastewater treatment 27

Figure 15 API separator 28

Figure 16 Dissolved air flotation—a typical DAF unit 29

Figure 17 Induced air flotation (IAF) unit 30

Figure 18 Activated sludge system 32

Figure 19 The PACT® (Powdered ActivatedCarbon Treatment) system 33

Figure 20 Sequencing batch reactor system 34

Figure 21 Membrane bioreactor system 34

Figure 22 Aerated lagoon system 35

Figure 23 Trickling filters 36

Figure 24 Rotating biological contractor system 36

Figure 25 Nitrification/denitrification system 37

Figure 26 Sand filtration 38

Figure 27 Activated carbon system 39

Figure 28 Chemical oxidation system 39

Figure 29 API sludge treatment system 40

Figure 30 DGF/IGF float treatment 41

Figure 31 Biological sludge treatment 41

Figure 32 Contaminant removal for different types of filtration processes 44

Figure 33 Media filtration 45

Figure 34 Microfiltration or ultrafiltration 46

Figure 35 Microfiltration or ultrafiltration, with reverse osmosis 47

Figure 36 Microfiltration or ultrafiltration, with nanofiltration 48

Figure 37 Ion exchange treatment 49

Figure 38 Media filtration 51

Figure 39 Microfiltration/ultrafiltration 52

Figure 40 Microfiltration/ultrafiltration, plus reverse osmosis 52

Figure 41 Microfiltration/ultrafiltration, plusnanofiltration 53

Figure 42 Zero liquid discharge 53

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This manual describes typical ‘best practices’ andstrategies used in petroleum refineries to managewater, including ways to reduce water usage.Improved water management in a petroleumrefinery can potentially reduce the volume and costof raw water used in refinery operations.Furthermore, improved water management mayresult in reductions in wastewater flow orcontaminant load or both. Lower flow andcontaminant load may result in lower wastewatertreatment operating and maintenance costs.Optimized water management may also reduce themass of contaminants in the treated effluent, thusimproving the quality of a wastewater dischargeand ultimately the environmental impact of arefinery’s discharge.

These practices are a collection of operational,equipment and procedural actions related to water

management in a refinery. Since each refinery isuniquely configured, some of these practices may ormay not be applicable based on the complexity ofthe refining operations, type of wastewatertreatment operations available at a particular site,availability of raw water sources, dischargeconfiguration and type of receiving water body. Thismanual will enable a refiner to compare theiroperations with typical industry practices anddevelop a plan for optimizing water management inthe refinery.

The manual is organized as follows:● Introduction ● Refinery water overview● Wastewater● Stormwater and sewerage ● Effluent treatment● Recycle and reuse issues

Introduction

Petroleum refineries are complex systems of multipleoperations that depend on the type of crude refinedand the desired products. For these reasons, no tworefineries are alike. Depending on the size, crude,products and complexity of operations, a petroleumrefinery can be a large consumer of water, relativeto other industries and users in a given region.Within a refinery, the water network is as unique tothe refinery as its processes. This section describesthe typical sources of water supplied to a refineryand the typical discharges of water from a refinery.It also provides an overview of the types ofcontaminants contained in the raw water and themethods used to remove these contaminants.

Overall refinery water balance

Many of the processes in a petroleum refinery usewater, however, not each process needs raw ortreated water, and water can be cascaded or reusedin many places. A large portion of the water usedin a petroleum refinery can be continually recycledwith in a refinery. There are losses to theatmosphere, including steam losses and cooling

tower evaporation and drift. A smaller amount ofwater can also leave with the products. Certainprocesses require a continuous make-up of water tothe operation such as steam generating systems orcooling water systems. Understanding waterbalance for a refinery is a key step towardsoptimizing water usage, recycle and reuse as wellas optimizing performance of water and wastewatertreatment systems.

Figure 1 shows a typical example of the waterbalance in a refinery.

Sources of water

Surface waterWater to the refinery can be supplied from varioussurface-water sources such as rivers or lakes. In somecases it may also be supplied from the sea or fromother brackish water sources. Additional supply ofwater can come from groundwater located inaquifers, if the subsurface water is available andaccessible. Typical characteristics of raw water caninclude varying amounts of solids and/or salts, alsoreferred to as total suspended solids (TSS), totaldissolved solids (TDS) and turbidity. Each water body

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Refinery water overview

A schematicexample of thetypical waterbalance in arefinery

Figure 1 Refinery water balance

and aquifer has a unique quality associated with itand may require treatment before use in a refinery.The level of pretreatment required for source waterbefore it is used in the refinery is dependent on theuses of the water in the refinery and what level ofsolids and salts is compatible with the process.

Table 1 shows the types of water sources and typicalcharacteristics of the water from each source.

Purchased waterWater can also be supplied from a municipality.Municipalities generally can offer potable water(drinking water) but may also be able to offer atreated effluent for industrial use or reuse. Potablewater (drinking water and sanitary water) requiredby a refinery is frequently purchased from a localmunicipality. If available, potable water may alsocome from groundwater aquifers or alternativesources.

Water in crudeWhen crude arrives at a refinery, it often carriesentrained water that remains from the oil wellextraction process and/or pickup duringtransshipment. The water is typically removed asstorage tank bottom sediment and water (BS&W) orin the desalter which is part of the crude unit in therefinery, and is typically sent to wastewater treatment.

RainAnother source of water for a refinery is rain. Rainthat falls within the refinery battery limits is typicallytreated before discharge. Rain that falls in non-industrial areas of a refinery, e.g. parking lots, green

areas or administrative housing, may be dischargedwithout treatment depending on local regulations.Stormwater harvesting can be a technique that isemployed to capture uncontaminated stormwater.With proper storage and or treatment (if needed)this stormwater can be used for certain processessuch as equipment washing.

Water leaving the refinery

The water that leaves refineries is indicated inFigure 1 and described briefly below.

WastewaterRefineries can generate a significant amount ofwastewater that has been in contact withhydrocarbons. Wastewater can also include waterrejected from boiler feedwater pretreatment processes(or generated during regenerations). Wastewater canalso refer to cooling tower blowdown stream, or evenonce-through cooling water that leaves the refinery.Once-through cooling water typically does notreceive any treatment before discharge. Coolingtower blowdown water and wastewater from rawwater treating may or may not receive treatment atthe wastewater treatment plant (WWTP) beforedischarge. Contaminated wastewater is typicallysent to either a wastewater treatment plant that islocated at the facility, or it can be pretreated andsent to the local publicly owned treatment works orthird-party treatment facility for further treatment.Water that has not been in direct contact withhydrocarbons or which has only minimalcontamination can be a source for reuse and isdiscussed in the section on ‘Recycle and reuse

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Table 1 Typical sources of water

Source Typical characteristics

Suspended solids, dissolved solids (metals), turbidity

Suspended solids (with seasonal variation), dissolved solids (metals), turbidity

Suspended solids, dissolved solids (metals), dissolved organics

Suspended solids, dissolved solids (metals, chlorides)

Lake

River

Groundwater (wells)

Sea water

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issues’, beginning on page 42 of this document.Wastewater can sometimes also be reused afterpassing through the wastewater treatment plant,sometimes requiring additional treatment to removesuspended solids and other contaminants.

Steam lossesLow pressure steam that is produced in the refineryis vented to the atmosphere when it is in excess.Other sources include tracing steam that is vented atsome locations in the refinery. Proper monitoring ofthe steam system in the refinery will help minimizethe production of excess steam and minimize/eliminate the need for venting. Any expected lossesshould be considered when reviewing the waterbalance in a refinery.

Cooling tower lossesAs water is cooled in the tower by evaporation, thisresults in a loss of water in the refinery. Some of thewater in the cooling tower is entrained by the largequantities of air passing through the tower and arelost to the atmosphere. These entrainment losses arealso referred to as cooling tower drift. Any expectedlosses from cooling towers should be consideredwhen reviewing the water balance in a refinery. Insome cases once-through cooling water is used inthe refinery (see overleaf, and page 17).

Water in productThere is some water that leaves with some of theproducts in the refinery although this is a very smallamount because it is limited by product qualityspecifications.

Raw water treatment

Source water for a refinery typically needs to betreated before being used in different processes. Thetype of treatment depends on the quality of thesource water and its ultimate use in the refinery.Turbidity, sediments and hardness are examples ofsource water constituents that may require treatment.

Water having a high mineral content is generallyreferred to as ‘hard water’ and has a tendency toform scale. Calcium salts are deposited as scale whenwater is heated causing a decrease in heat transferrates in heat exchangers (heaters and coolers). Bothcalcium and magnesium salts form scale uponevaporation of water in steam-generating equipment.These deposits not only reduce heat transfer rates butalso restrict fluid flow. Removal of calcium andmagnesium from water is referred to as softening,and the treatments commonly used include lime-soda,phosphates, ion exchange and reverse osmosis. Othercontaminants that could be present in raw waterand their removal methods are shown in Table 2.

Use of raw water in refineries

The required degree of water purity depends on theparticular use. Preliminary treatment of all rawwater entering a plant may include screening andsedimentation to remove suspended solids, butsubsequent treatment will depend on the ultimateuse for each water system. A typical plant watersupply might be separated into process, boiler feed,cooling, potable, fire water and utility water systems.

Brief descriptions of the different water uses inrefineries are given below.

Process waterIn refineries, water is typically used for variouspurposes where the water is closely contacted withthe hydrocarbons. Softened water is usually used forthese purposes.

Boiler feedwaterThe boiler feedwater (BFW) required for thegeneration of steam in a refinery needs to betreated prior to use. The higher the steam pressurebeing generated, the higher the purity of the BFWrequired. Ordinarily water is treated by the lime-soda process and further purified by ion exchangeor by hot phosphate treatment in order to produceboiler feedwater. Reverse osmosis can also be usedto soften the water.

Typically, a purge stream is removed from the waterpurification systems in order to prevent the buildupof contaminants. This purge stream is sent towastewater treatment and is replaced by freshmakeup water.

Cooling waterWater-cooled condensers, product coolers (heatexchangers) and other heat exchangers can use alarge amount of water in a refinery. Some refineriesuse air coolers, where the process stream isexchanged with air prior the being sent to a coolingwater heat exchanger. This will minimize the use ofcooling water in the refinery.

Some refineries use a once-through system wherethe incoming water is exchanged against theprocess fluid and the warmer cooling water is thenreturned to the source of the water. However, ifwater is a scarce commodity at a particular locationit may be preferable to recirculate the water througha cooling tower and then back to the process. Inthese circulating systems water is supplied at about90° F and returned to the cooling tower at amaximum of about 120°F.

Some water treatment is necessary even for once-through cooling systems to prevent scale formation,corrosion, and slime and algae formation. Theextent of treatment required for circulating systems ismuch greater since impurities are concentrated inthe system as evaporation losses occur.

In cooling tower systems, a build-up of saltconcentration is unavoidable since water isevaporated in the cooling tower. Make-up water isrequired to replace these and other losses.

Sea water has been used successfully as coolingwater especially in coastal areas with fresh watershortages. Cathodic protection systems employingmagnesium anodes located in the floating head andchannel of exchangers prevent excessive corrosion.Deposits are minimized by restricting cooling water

temperature increase below the point where thecalcium salts begin to precipitate.

Potable waterPotable water is required for use in kitchens, washareas and bathrooms in refineries as well as insafety showers/eyewash stations. City water ortreated groundwater can be used for this purpose.In remote locations or in small towns a portion ofthe treated water from the plant softening unit maybe diverted for potable water use. The treated watermust be chlorinated to destroy bacteria, and thenpumped in an independent system to preventpotential cross-contamination. Potable quality watermay also be required in some specialist chemicaloperations (e.g. as a diluent).

Fire waterThe requirements for fire water in refineries areintermittent, but can constitute a very large flow.Often, refineries collect stormwater from non-process areas and store it in a reservoir dedicatedto the fire water system in the plant.

Provisions are typically made for a connection (foruse in emergency situations) of the fire water systeminto the largest available reservoir of water. Usuallythis is the raw water supply since fire water requiresno treatment. Sea water or brackish water is oftenused as fire water by plants located along coastalareas.

Utility water Utility water is used for miscellaneous washingoperations, such as cleaning an operating area. Itshould be free from sediment but does not requireany other treatment.

Table 2 shows the typical impurities in various typesof water and the processes generally used toremove them.

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Table 2 Contaminants in raw water

Contaminant Problem Removal methods

Makes water cloudy and deposits in water lines andprocess equipment

Primary source of scale formation in heat exchangersand pipe lines

Causes foaming in steam systems and attacks boilersteel. Bicarbonate and carbonate produce carbondioxide in steam which is highly corrosive

Adds to the solids content of water and combines withcalcium to form calcium sulfate scale

Adds to solids content and increases the corrosiveproperties of water

Scaling on heating and cooling equipment andpipelines

Discolors the water and precipitates in water linesand process equipment

Source of scale, sludge and foaming in boilers.Impedes heat exchange. Undesirable in mostprocesses

Corrosion of water lines heat exchange equipment,boilers, return lines, etc.

Cause of ‘rotten egg’ odor. Corrosion, toxicity

Conductivity is the result of ionizable solids insolution. High conductivity can increase the corrosivecharacteristics of a water

‘Dissolved solids’ is the measure of total amount ofdissolved material. High concentrations of dissolvedsolids are objectionable because of processinterference and as a cause of foaming in boilers

‘Suspended solids’ is the measure of undissolvedmatter. Suspended solid plug lines, cause deposits inheat exchange equipment, boilers, etc.

Coagulation, settling and filtration

Softening, distillation, surfactants

Lime and lime-soda softening, Zeolite softening,Dealkalization by anion exchange

Demineralization, distillation

Demineralization, distillation, desalination (if seawater is being used)

Anion exchange resins, distillation

Aeration, coagulation and filtration, lime softening,cation exchange

Oil/water separators strainers. coagulation andfiltration. Diatomaceous earth filtration

Deaeration, sodium sulphite, corrosion inhibitors

Aeration, chlorination, highly basic anion exchange

Processes which decrease dissolved solids content willdecrease conductivity. Examples are demineralization,lime softening

Various softening process, such as lime softening andcation exchange by zeolite, will reduce dissolvedsolids. Demineralization, distillation

Sedimentation. Filtration, usually preceded bycoagulation and settling

Turbidity

Hardness

Alkalinity

Sulphate

Chloride

Silica

Iron and magnesium

Oil

Oxygen

Hydrogen sulphide

Conductivity

Dissolved solids

Suspended solids

This section describes the various sources ofprocess water in refineries and discusses bestpractices with respect to how they are managed.Also included is a discussion of the wastewatergenerated from the different utility systems in therefinery and how these systems are managed.Finally, concepts for pretreatment of wastewatergenerated in the process units are discussed.

Process water

Process water is defined as water that has been inintimate contact with hydrocarbons in the refinery.Water that is generated in the process units isrepresented by the following categories:● desalter effluent;● sour water; ● tank bottom draws; and● spent caustic.

Desalter effluent

Inorganic salts are present in crude oil as anemulsified solution of salt (predominantly sodiumchloride). The source of the aqueous phase is thenaturally occurring brine that is associated with the

oil field from where the crude is extracted. Theamount of water received at the refinery with thecrude varies widely but an approximate rangewould be 0.1–2.0% volume.

The salts contained in the aqueous phase arevariable and range from 10 to 250 pounds perthousand barrels (p.t.b.) of crude. The salts arepresent mostly in the form of chlorides of sodium,magnesium and calcium. Typically, the firstoperation in a refinery crude unit is desalting, whichis used to wash out the salt present in the crude. Themost important reasons for removing the salts fromthe crude are to:● prevent plugging and fouling of process

equipment by salt deposition; and● reduce corrosion caused by the formation of HCl

from the chloride salts during the processing ofthe crude.

There are two basic types of desalters: chemical andelectrical. Refineries that use two-stage electricaldesalters can achieve a desalted crude specificationof 0.1 p.t.b. of salt in the crude. The wash waterthat is used in the desalter is discharged from theunit. Figure 2 shows the typical configuration of atwo-stage desalter.

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Wastewater

Figure 2 Typical desalter configuration

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Table 3 Sources of wash water

Source Advantages Disadvantages

1. The water requires no or minimal pretreatment

1. Results in lower sour water stripping requirements

2. Avoids the piping required to send the crudetower overhead to the sour water strippers

3. Minimizes fresh water use in the refinery

1. Results in lower sour water stripping requirements

2. Avoids the piping required to send the crudetower overhead to the sour water strippers

3. Minimizes fresh water use in the refinery

1. The phenol contained in the stripped sour wateris extracted into the crude resulting insignificantly lowering the phenol content of theeffluent.

2. The lower phenol content results in lower capitaland operating costs for wastewater treatment.

3. Allows better pH control in the desalter

1. Increases overall water usage in the refinery

2. Increases generation of wastewater in the refinery

3. Increases capital and operating costs forwastewater treatment

1. More challenging to control the pH in thedesalters due to the ammonia content of the crudetower overhead

2. Results in more emulsion formation in thedesalters leading to the inadvertent discharge ofhydrocarbons to wastewater treatment

3. Results in the discharge of H2S from the desaltereffluent to the atmosphere in the sewers as well aswastewater treatment if effluent does not go forpretreatment before discharge to sewers orwastewater treatment

1. More challenging to control the pH in thedesalters due to potential ammonia content of thevacuum tower overhead

2. Results in more emulsion formation in thedesalters leading to the inadvertent discharge ofhydrocarbons to wastewater treatment

3. Results in the discharge of H2S from the desaltereffluent to the atmosphere in the sewers as well aswastewater treatment if effluent does not go forpretreatment before discharge to sewers orwastewater treatment

1. Routing all sour water generated in the plantresults in requiring a large sour water stripper

2. Increase the piping required to convey the crudeand vacuum tower overhead to the sour waterstripper

Fresh water

Recycled crudetower overhead

Recycled vacuumtower overheadsupplemented bysources of water

Recycled strippedsour water

The wash water is usually injected into the secondstage of the desalter after being heated byexchange with the hot effluent. The water from thesecond stage is sent to the first stage where it iscontacted with the incoming crude. The hot (about300°F) brine is then discharged to the wastewatertreatment plant after being cooled. The optimum

operating pH in the desalter is 6 to 7 because theemulsion formation is minimized and the oil/waterseparation is most effective at this pH. The pH issignificantly impacted by source of the wash waterthat is used as well as corrosion considerations inthe crude tower system.

Some of the drilling muds that come in with thecrude tend to accumulate in the desalter and needto be removed. This can be done either continuouslyor periodically. Some desalters have a continuousmud washing system in which the muds are notallowed to accumulate in the vessel. Most refineriesdo the mud washing on an intermittent basis(typically once a shift) by temporarily increasing thewash water flow to the mud washing nozzleslocated at the bottom of the desalter. However, whenthis operation is carried out it can result inincreased discharges of hydrocarbons to thewastewater treatment system.

Oil/water interface control is an important aspect ofthe design and operation of desalters. There are anumber of interface controllers that are available inthe marketplace and one such system uses highfrequency electromagnetic measurement to detect theinterface. Control of the oil/water interface will helpminimize/eliminate the inadvertent discharges ofhydrocarbons to the wastewater treatment systems.

The wash water used in desalters is typically 5 to8% of the crude throughput. The source of washwater that is used in the desalters varies widely indifferent refineries. Table 3 describes the varioussources that are used and discusses the advantagesand disadvantages of each source.

The level of contaminants contained in the desalteris highly variable depending on factors such as:● operating pH of the desalter (higher pH results in

more emulsions);● effectiveness of the interface control device in the

desalter; and● frequency and effectiveness of the mudwash.

Table 4 shows the expected concentration ofcontainments in desalter effluent.

Desalter—summary of best practices● Avoid using fresh water as wash water in the

desalter.● Preferentially use stripped phenolic sour water as

wash water.● Operating pH in the desalter should be

optimized to about 6–7.● Use proper interface probes in the desalter for

effective oil/water separation.● Consider diverting desalter brine to a separate

tank where solids can drop out during mudwashing operations.

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Table 4 Desalter effluent contaminants

Contaminant Expected concentration (mg/l)

400 to 1000

Up to 1000

Up to 500

10 to 100

5 to 15

Up to 100

Up to 100

Chemical oxygen demand (COD)

Free hydrocarbons

Suspended solids

Phenol

Benzene

Sulphides

Ammonia

Sour water

Steam is used in many processes in refineries as astripping medium in distillation and as a diluent toreduce the hydrocarbon partial pressure in catalyticcracking and other applications. The steam iscondensed as an aqueous phase and is removed assour water. Since this steam condenses in thepresence of hydrocarbons, which contain hydrogensulphide (H2S) and ammonia (NH3), thesecompounds are absorbed into the water at levelsthat typically require treatment.

The typical treatment for sour water is to send it to astripper for removal of H2S and NH3. Steam is usedto inject heat into the strippers. High performancestrippers are able to achieve < 1 ppm H2S and< 30 ppm NH3 in the stripped sour water. Withthese levels, the stripped sour water is an idealcandidate for recycle/reuse in the refinery. Strippersthat use direct steam injection as the strippingmedium create more wastewater in the refinerycompared to strippers that use reboilers to injectheat into the strippers. Figure 3 shows theconfiguration of a typical sour water stripper.

In this system, all the sour water produced in therefinery is flashed in a drum and any separated oil

is sent to refinery slops. The vapours from this drumare sent to the flare. The sour water from the drumis then sent to a storage tank which provides therequired surge in the system. The sour water is thenpassed through a feed/bottoms exchanger where itis heated up and then sent to the stripper. Steam isused in the reboiler to heat up the bottoms andprovide the vapour traffic in the tower. The separatedvapors containing H2S and NH3 are typically sentto a sulphur plant. The stripped water is routed via thefeed/bottoms exchanger and a trim cooler for reusein the refinery. Any excess water that cannot bereused would be sent to a wastewater treatment plant.

Refineries that include process units such as catalyticcrackers and delayed cokers produce more sour waterthan a less complex refinery. The sour water fromthese sources also contains phenols and cyanides,and should be segregated from the remaining sourwater produced in the refinery. Dedicated sour waterstrippers may be used to process this water, and thestripped sour water from this stripper should bepreferentially reused as wash water for the desalters.This will result in the extraction of a substantialportion (up to 90%) of the phenol contained in thissour water and result in a lowering of the load ofphenol to the wastewater treatment system.

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Figure 3 Sour water stripper configuration

Table 5 (above) shows the various process units thatproduce sour water and best practices with respectto reuse of stripped sour water.

The composition of the stripped sour water is highlydependent on the design and operation of the sourwater stripper. Table 6 (left) shows the expected levelof contaminants in stripped sour water.

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Table 5 Sour water producers

Unit Producer Typical destination Comments

Atmospheric toweroverhead drum

Tower hotwell

Fractionator overheaddrum

Fractionator overheaddrum and blowdown drum

Fractionator overheaddrum and blowdown drum

Wash water separator

Wash water separator

Tail gas treater

Sour water stripper ordesalter

Sour water stripper ordesalter

Sour water stripper

Sour water stripper

Sour water stripper

Sour water stripper

Dedicated sour waterstripper

Sour water stripper

Some refineries use this stream as desalterwash water without stripping. This practice canlead to emulsion formation resulting in reducedoil/water separation in the desalter

Some refineries use this stream as desalterwash water without stripping. This practice canlead to emulsion formation resulting in reducedoil/water separation in the desalter

This sour water contains elevated levels ofphenol and cyanides which do not getremoved in the sour water strippers



Stripped water is generally used as washwater and therefore hydrotreaters are typicallynot net producers of sour water

Hydrocrackers generally require very cleanwash water and one strategy is to send sourwater to a dedicated sour water strippers toavoid impurities that might be present in sourwater produced in other units in the refinery

none

Crude

Vacuum

Catalytic cracker

Delayed coker

Visbreaker

Hydrotreaters

Hydrocracker

Sulphur plant

Table 6 Stripped sour water contaminants


600 to 1200

< 10

< 10

Up to 200

0

< 10

< 100


Free hydrocarbons

Suspended solids

Phenol

Benzene

Sulphides

Ammonia

Sour water stripper—summary of best practices● Route sour water produced in the refinery to the

sour water stripper except where it can be useddirectly, e.g. as desalter wash water (see below).

● If the refinery has a catalytic cracker or a coker,the sour water produced in these units should besegregated and processed in a dedicatedphenolic sour water stripper.

● The stripped water from the phenolic strippershould be preferentially used as wash water inthe desalter.

● Where necessary, the stripped sour water shouldbe cooled prior to discharge to wastewatertreatment, to avoid subjecting the biologicaltreatment system to excessive temperatures.

Tank bottom draws

Typically, the categories of tanks that may requirewater draws in refineries include:● crude tanks;● gasoline tanks; and● slop tanks.

The incoming crude to refineries normally containswater and sediments (mud) that are picked up whenthe oil is extracted from the wells—this is referred toas bottom sediment and water (BS&W). When thecrude is stored in large tanks, the BS&W settles tothe bottom and must be periodically removed toprevent a buildup of this material which would

otherwise result in a loss of storage capacity. Waterdraws are normally sent to either the wastewatertreatment or to a separate tank where the solids areseparated from the oil and water. Figure 4 shows atypical arrangement of a crude tank draw.

In this system, the crude tank, which is locatedinside a berm for secondary containment, isequipped with a valved drain line that is sent to asump located outside the berm. The operator usesthe valve to drain the BS&W periodically using theinterface level indicator to ensure that hydrocarbonsdo not get inadvertently drained out. Manyrefineries make it an operating practice that theoperator be present to monitor the drainingoperation during the entire draining period toensure that free hydrocarbons are not inadvertentlydrained. There can be many variations of the systemshown in this example but the principle of operationwould be similar. The type of interface indicatorused is also an important consideration. Some crudetanks use probes, which use high frequencyelectromagnetic measurement to detect the interface.

Tanks that store gasoline also tend to collect water.These tanks should be equipped with drainagesystems similar to that of crude tanks to ensure that thehydrocarbon product is not inadvertently drained fromthe tanks. It should be noted that the amount of waterthat is drained from gasoline tanks is relatively smallcompared to the amount of water from crude tanks.

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Figure 4 Crude tank water draw

Table 7 shows the expected level of contaminants intypical crude tank BS&W.

Tank bottom draws—summary of best practices● Adequate piping and valves should be provided

to allow proper draining of the tank.● Proper instrumentation should be provided so

that the oil/water interface in the tank can bemonitored properly.

● Operating procedures that require the presenceof an operator at the tank during the entiredraining period should be implemented ifpractices and automation still results in excessiveoil to the sewer.

Spent caustic

Spent caustic is formed due to the extraction ofacidic components from hydrocarbon streams. Thisincludes residual H2S, phenols, organic acids,hydrogen cyanide and carbon dioxide. These acidic

compounds are absorbed into the reagent, and theresulting spent caustic solution cannot beregenerated. As a result, these absorbed acidiccompounds contained in the caustic solution must bepurged intermittently or continuously from thecaustic treating system, and replaced by freshcaustic. The caustic solution will drop out asseparate aqueous phase in intermediate or productstorage tanks. Subsequent drawdown and dischargefrom the tanks will be required. This dischargeusually occurs to the sewer, frequently on a batchbasis and can cause problems in the wastewatertreatment plant.

The intermediate/product streams most frequentlytreated with caustic in a refinery are shown inTable 8.

If a refinery is running a particularly corrosivecrude, e.g. one with a high TAN (total acidnumber), the naphthenic acid that is contained insuch a crude tends to concentrate in thekerosene/jet fuel cut in the refinery. When thisstream is caustic treated the acids are converted tonaphthenates which are especially refractory tobiological treatment.

Traditionally, spent caustic has been disposed of ina number of different ways. Discharge to the sewersystem is common but not necessarily the bestpractice. An alternative option is off-site disposal of

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Table 7 Crude tank bottom sediment and water contaminants


400 to 1000

Up to 1000

Up to 500

Up to 100


Free hydrocarbons

Suspended solids

Sulphides

Table 8 Intermediate product stream—caustic treated

Intermediate productUsual contaminants/impurities

H2S RSH Phenol HCN Other

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

Straight run LPG

Light straight-run naphtha

FCC* C3 + C4 LPG (produces phenolic spent caustic)

FCC* gasoline (produces phenolic spent caustic)

Coker C3 + C4 LPG (produces phenolic spent caustic

Kerosene/jet fuel)

* Fluid catalytic cracker

phenolic spent caustics where recovery of containedorganic components can occur. Off-site disposal ofsulphidic spent caustics (often the largest portion ofrefinery spent caustic) is more difficult because thereare few reprocessing options for this stream.

There are two strategies for dealing with spentcaustic in refineries: in-process abatement and end-of-pipe treatment.

In-process abatement/minimization—best practicesThe following in-process options have the commonobjective of minimizing quantities of spent causticrequiring disposal. ● As discussed above, there are two types of

spent caustic that are generated in refineriesdepending on the types of process units present.Some refineries are able to treat the sulphidicspent caustic in the refinery wastewatertreatment plant. Phenolic spent caustic is veryodorous and therefore cannot be treated in thewastewater treatment plant. Phenolic spentcaustic (from catalytic cracker, coker andkerosene/jet fuel treater) should be segregatedfrom the sulphidic spent caustic and storedseparately. This will allow the refinery toproperly evaluate appropriate strategies forspent caustic disposal.

● The amount of spent caustic generated isdependent on operating procedures in thetreating units. These procedures often call forthe caustic to be purged when the sodiumhydroxide concentration in the solution reachesa certain value. The production of spent causticcan be minimized by exercising tighter controlof caustic treating operations by ensuring thatthe caustic solution is not purged prematurely.

● Hydrocarbons are normally treated in an aminesystem to absorb the hydrogen sulphide prior tobeing sent to a caustic treater. The operation ofthe absorber should be reviewed to maximizeits efficiency of absorption so that the amount ofhydrogen sulphide reaching the caustic treatingsystem is minimized.

● Consider prewashing (absorbing) hydrocarbonswith stripped sour water to reduce the quantity ofacidic compounds in these streams prior to thembeing sent to the caustic treater. This willminimize the amount of acidic compoundsrequiring removal in the caustic treater, and thusminimize the discharge of spent caustic.

● The strength, purity and composition of causticrequired for a given treatment, or generated bya treatment process, vary widely. The quality ofcaustic will depend on both the product beingtreated and the type of treatment system beingemployed. An effective strategy to reduce the useof fresh caustic and minimize the generation of‘end-of-pipe’ spent caustics is to carefully matchcaustic treatment needs with available spentcaustics being generated.

End-of-pipe treatment—best practicesThe following treatment systems are used inrefineries for treating spent caustic.● Sulphidic spent caustic can be treated in the

wastewater treatment plant as long as it is addedin a controlled manner. This will prevent shockingthe system and will minimize the generation ofodours from the system.

● Off-site disposal of phenolic caustic is practicedin many refineries. The cost of disposal togetherwith transportation and potential liability costsneed to be taken into account before choosingthis option.

End-of-pipe treatment—other optionsOther options available for the treatment/disposalof spent caustic are listed below:● Deep neutralization (lowering the pH to less than

4) which results in the stripping of the H2S andthe separation of phenols is an option fortreating phenolic spent caustics. This optionrequires relatively high capital and operatingcosts.

● If the spent caustic includes a significant amountof naphthenates (such as spent caustic fromkerosene/jet fuel treaters), wet air oxidation ofthe spent caustic should be considered. In this

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system, the spent caustic is oxidized with air atvery high temperature and pressure (~700 psi,~500°F). This type of system is also very high incapital and operating costs.

● The potential for sale of spent caustic to anadjacent industry such as a pulp and paper millor cement plant should also be explored.

Cooling water

In refineries, crude oil is separated in variousfractions based on boiling point. This isaccomplished by fractional distillation of the crudeoil. The distillation is carried out in distillationcolumns where the crude is heated up andvapourized in a fuel (fuel oil, natural gas or refineryfuel gas) fired heater. Various fractions areseparated by condensing and cooling products thatare withdrawn from the tower. From an overall heatbalance point of view, the heat that is put into thesystem by burning fuel and/or the introduction ofsteam has to be removed or ‘rejected’. This isaccomplished in various ways, including:

● heat exchange with boiler feedwater to generatesteam;

● heat exchange with other process streams;● rejection of heat using air coolers; and● rejection of heat to cooling water.

Figure 5 shows a typical distillation system in arefinery. In this system, three types of heat rejectionsystems are shown. The crude oil is preheated byexchanging with another process stream and fed toa fired heater. The partially vapourized products aresent to the distillation tower where different sidestreams are withdrawn based on the boiling pointrange of the product. The side streams are sent tostrippers which are also distillation columns wherethe boiling point range of the product is adjustedfurther by the addition of steam. The bottomsproduct from these strippers is cooled and sent tostorage. The vapours from these side strippers aresent back to the main tower. The overhead vapoursfrom the main tower are condensed using an air-cooled exchanger, and then further cooled using acooling water heat exchanger. Three types of heatexchangers are shown in this system:

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Figure 5 Typical distillation system

● Type 1 heat exchangers such as steamgenerators and process stream heat exchangers;

● Type 2 heat exchangers which use cooling water;● air coolers.

There are three types of cooling water systems:

1. Once-through cooling water system: In this typeof system the cooling capacity of the water is usedonly once without contacting the fluid or vapourbeing cooled. These systems use water withdrawnfrom a surface water source such as a lake, river orestuary and typically return the water to the samesource. Figure 6 shows a typical once-throughcooling water system.

2. Closed-loop cooling water system: In this systemwater is circulated in a closed-loop piping systemand is subject to cooling and heating withoutevaporation or air contact. Heat that is absorbed bythe water in a closed-loop system is normallyrejected using a heat exchanger to a once-throughcooling system. Figure 7 shows and example of aclosed-loop system.

3. Evaporative cooling water system: In this type ofsystem, the heat that is picked up by the recirculatingcooling water is rejected in a cooling tower byevaporation. In the cooling tower the hot water issprayed against a rising stream of atmospheric air.The heat in the cooling water is removed by heatingthe air as well as evaporation of the cooling water.An example of an evaporative recirculating coolingtower system is shown in Figure 8.

In a cooling tower system, part of the circulatingwater is removed as blowdown to prevent the build-up of dissolved solids in the system. The quantity ofblowdown required depends on the quality of themake-up water, and the number of cycles ofconcentration that the cooling tower is operated at(typically 4 to 7).

Cooling tower blowdown is typically sent towastewater treatment in refineries via the sewer. This

is because in many cases the pressure on the processside of heat exchangers is higher than the coolingwater pressure, and any leaks in a heat exchangerwould result in the contamination of the coolingwater with hydrocarbons. This practice imposes ahydraulic load on the wastewater treatment system.The full impact on wastewater treatment needs to beevaluated on a case-by-case basis.

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Figure 6 Once-through cooling water system

Figure 8 Evaporative cooling water system

Figure 7 Closed-loop cooling system

Table 9 shows the expected level of contaminants inthe cooling tower blowdown stream.

Cooling tower blowdown—best practices● Monitor the cooling tower circulation loop for

hydrocarbons.● If hydrocarbon is detected the source of the leak

needs to be determined and isolated quickly.● Route the cooling tower blowdown to the

wastewater treatment plant by a separate lineand not through the sewer. This will prevent theblowdown from getting further contaminated withhydrocarbons that may be present in the sewers.However, this is costly, and may not bepracticable in all cases. Reduction of oil to thesewer should be regarded as a primary strategyand can accomplish similar results.

● The cooling tower blowdown can be routeddirectly to the secondary oil/water separationequipment in wastewater treatment (bypassingthe primary oil/water separation system).

Condensate blowdown

In a refinery condensate losses are from:● blowdown from the plant boiler system;● blowdown from the various steam generators that

are located in the process units; and● unrecovered condensate from steam traps, steam

tracing etc.

Boiler blowdown

A portion (usually up to 5%) of the boiler feedwater(BFW) and condensate that is fed to the boilers inthe refinery is purged from the system to maintainthe dissolved solids level in the system at anacceptable level. This level could be differentdepending on the pressure level of the steam beingproduced (150 psi, 600 psi or 1500 psi). Figure 9shows a typical system.

Steam generator blowdown

A steam generator system is similar to the systemshown above but the heat source is a process heatexchanger that needs to reject heat. Figure 10shows the typical configuration of this system.

Unrecovered condensate

The drivers for condensate recovery in refineriesinclude:● more energy savings if more condensate is

recovered;● quantity of boiler feedwater makeup required is

directly proportional to the quantity ofcondensate lost; this results in increasedoperating costs for treating the BFW; and

● any condensate lost to the sewer increases thetemperature of the wastewater and thus imposesa heat load at wastewater treatment.

The percentage of condensate recovered can below in some refineries depending on design andlayout of the refinery. Additionally, some of thecondensate from steam traps and heat tracing isalso lost to the atmosphere and/or sewer. Oftenthese traps are discharged directly to the sewer andthe hot discharge can ultimately cause deteriorationof the sewers.

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Table 9 Cooling tower blowdown—contaminants


150

< 5

Up to 200

Up to 700


Free hydrocarbons

Suspended solids

Dissolved solids

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Figure 9 Boiler blowdown—typical

Figure 10 Steam generator blowdown—typical

Condensate blowdown—bestpractices

● The refinery should monitor the condensatebalance in the refinery on an ongoing basis andefforts should be made to maximize recovery.

● The total volume of condensate blowdown (boiler,steam generator blowdown and others) shouldbe less than 10% of the total flow of wastewaterfrom the refinery.

● The quantity of blowdown taken at each boiler orsteam generator in the refinery should bemonitored and minimized.

● The blowdown from each location or a group oflocations should be collected and sent to a flashdrum (as shown in Figure 10) where the pressureis let down to atmospheric pressure before beingdischarged. The flashed blowdown should thenbe cooled with a heat exchanger. This willprevent deterioration of the sewers and alsoavoid heating and vaporizing of anyhydrocarbons that might be present in the sewer.The discharge should not be cooled by directlyadding water (such as utility water) because thiscould require the addition of a substantialquantity of water to adequately cool the stream.This will also result in an increase of the totalflow of wastewater to the treatment plant.

Raw water treatment

The raw water treatment in a refinery createswastewater and sludges that require disposal. Thefollowing section describes the best practices withrespect to these discharges.

Raw water treatment—best practices

● When lime softening is used for raw watertreatment, the sludge generated in this processshould be thickened, and optionally dewatered.The thickener overflow water can be dischargeddirectly without any further treatment, when localregulations allow. The sludge that is generated

should be disposed off-site. Not discharging it tothe sewer in the refinery will prevent theintroduction of inert solids into the sewer in therefinery which in turn will avoid creation of moreoil sludge that requires disposal.

● The use of ion exchange for treatment of rawwater creates an alkaline wastewater stream andan acidic wastewater stream as a result of theregeneration of the ion exchange beds. Thesestreams should be collected in a tank and the pHneutralized prior to being discharged directly toan outfall (bypassing wastewater treatment) ifallowed by local regulation.

● The use of reverse osmosis for raw watertreatment results in the creation of a reject streamthat is very high in dissolved solids. This rejectstream should be discharged directly to an outfall(bypassing wastewater treatment) if allowed bylocal regulation.

Miscellaneous discharges—best practices

There are a variety of additional activities that, ifimplemented routinely at a facility, could reducewater use. Some of these activities include thefollowing:● Housekeeping and washdowns: If facilities use

utility water hoses to washdown the process areaand small inadvertent spills of hydrocarbons andother materials, operating procedures and trainingmust be implemented to ensure that hoses areturned off after their use, and that other non-watermeans (for example, adsorbent pads or booms,brooms) be used to clean up area as appropriate.

● Closed-loop sample systems: For samplinghydrocarbons, closed-loop samplers should beinstalled and used. This will prevent thedischarge of hydrocarbons to the sewer duringthe purging of sample lines.

● Leak identification programme: Firewater, orother water leaks in raw water piping or cooling-water piping can add to WWTP flows. These

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systems should be periodically monitored forleaks.

Miscellaneous discharges—minimization

Some additional ideas that can contribute tominimization of wastewater discharges to the sewer:● External cooling of heat exchangers: At some

locations due to the lack of adequate heatexchanger area or the high cooling water inlettemperature during the summer months, utilitywater hoses are used to cool these exchangersexternally. This results in the discharge ofsubstantial quantities of clean water to the sewer.This practice should be discontinued and the lackof adequate heat exchanger area should beaddressed as soon as practical.

● Fire water system: Some refineries use treatedwastewater in their firewater system.Implementation of this practice should beexplored since it will not only minimize thedischarge to the sewer but also result in savingsof raw water.

Laboratory wastewater

Typical refinery laboratories analyse bothhydrocarbon and water samples. The wastewaterthat is generated in these laboratories can becategorized as follows:● spent/unused hydrocarbon samples;● spent/unused wastewater samples;● discharges from sinks in the laboratory; and● discharges from bottle washing systems in the

laboratory.

Spent/unused hydrocarbonssamples—best practices

The spent/unused hydrocarbon samples should bedisposed of in segregated drums located atconvenient locations inside the laboratory. These

drums should drain to a slop drum located outsidethe laboratory building where it will be collectedand picked up periodically by a vacuum truck in therefinery and sent to the refinery slop system.

Spent/unused wastewater samples—best practices

The wastewater samples should be discharged to alocal sewer and, if necessary, routed through a localoil/water separator, prior to discharge to thewastewater treatment plant.

Discharges from laboratory sinks—best practices

Discharges from the sinks in the laboratory shouldbe routed to the wastewater treatment plant via alocal oil/water separator, in cases where practicesto ensure the discharges are oil free areunsuccessful. Care must be taken not to dischargevarious chemicals or reagents (such as nitrobenzene) that could cause problems in thewastewater treatment plant. Chemicals or reagentsthat could upset a wastewater treatment plantshould be managed separately, for example,disposed of in a separate drum and sent off-site fordisposal.

Discharges from bottle washingsystems—best practices

It must be ensured that sample bottles are emptiedto their respective systems (hydrocarbons to slopsand wastewater to the sewer) prior to being washedin the bottle washing machines. This will minimizethe formation of emulsions in the discharges fromthese machines. The discharges from the machinesshould be sent to the local sewer.

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In this document, stormwater refers to precipitationfrom rainfall or snowfall. Stormwater from within therefinery process areas is potentially contaminatedand typically needs to be treated prior to discharge.Non-process area stormwater may be dischargedwithout prior treatment if allowed by local regulation.

Sewerage refers to wastewater that is dischargedfrom kitchens, employee locker rooms andwashrooms.

Stormwater segregation andmanagement

A variety of stormwater management practices areemployed at refineries across the world. Theparticular approach a refinery adopts towardsstormwater management is influenced by the age andcondition of the sewer system, the frequency andintensity of precipitation, the water quality of processarea runoff, the area contained within the processzones and local regulations and requirements.

Segregating non-process area stormwater fromprocess area stormwater allows a refinery topotentially discharge the stormwater from non-process areas without treatment provided it isallowed by local regulation. Effective segregationcan be achieved by means of curbing, grading, andproper selection of collection points. Non-processarea stormwater is generally clean, and can bereused if it is segregated. A separate sewer ordrainage system must be in place to implement thisoption. Contaminated stormwater must be collectedseparately and stored until it can be treated in thewastewater treatment plant.

Contaminated stormwater

Process area runoff (contaminated stormwater) canbe collected in storage tanks or impoundment basinsand discharged for wastewater treatment at acontrolled rate. This lowers the hydraulic loading on

the treatment plant, and consequently decreases thesizing requirements for individual units in thetreatment plant. Equipment sizing of manywastewater treatment units depends on hydraulicload. Biological treatment systems in particular donot respond effectively to sudden changes in flow orcontaminant loadings.

The quantity of precipitation depends on rainfallintensity and the drainage area (process area). Thedrainage area can be directly measured throughtopological surveys or satellite imagery of therefinery property, taking into consideration thelocation of drainage points. Rainfall intensity datacan be obtained from the local weather service orairport. The required storage volume required forstormwater is governed by local regulations.

Contaminated stormwater—best practicesTechniques that can be used to minimize thequantity of contaminated stormwater include:● Minimize process collection area: Stormwater

runoff from process units where they wash outhydrocarbons that have been inadvertentlyspilled on the pads should be directed to thecontaminated stormwater collection system.Process areas where the stormwater cannotpotentially come into contact with hydrocarbonsor other chemicals can be routed to the non-contaminated stormwater collection system, ordischarge it directly if it is allowed by regulation.Curbing or other modifications can be made toreduce the area draining to the contaminatedstormwater sewer.

● Treatment of ‘first flush’: ‘first flush’ ofstormwater refers to the stormwater that initiallyruns off the process area. First flush treatment isbased on the assumption that the initial runoff ismore contaminated because the hydrocarbonsand other pollutants deposited on process areasget washed off by the first flush. The first flushwill also contain any oil or solids that weretrapped in a catchment area or sewer system.After the first flush is captured, subsequent runoffcan then be diverted to the non-contaminated

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Stormwater and sewerage

stormwater system if allowed by local regulation.Even if the stormwater is allowed to be divertedto the non-contaminated system, local regulationstypically require that the diverted stormwater besubjected to one stage of oil/water separationjust as a precaution. The amount of first flushstormwater collected is the first inch or the firsttwo inches of rain, and is usually governed bylocal regulations. The first flush is collected in atank or basin and discharged to wastewatertreatment at a controlled rate to avoidoverloading the system hydraulically.

● Minimize solids in stormwater: Any sand or gritthat collects in the process areas gets washedinto the sewer with stormwater. These solids willmix with any hydrocarbons present in the sewerand create oily sludge. Typically, one pound ofdry solids creates ten pounds of oily sludge andincreases the load on the API separator1 and thesludge treatment system. One of the sources ofsolids could be the erosion from unpaved areasthat make their way into the process units. Pavingadjacent areas or covering them with gravel willminimize the migration of sand and grit. Plantareas should be periodically swept and the solidsshould be collected and disposed of in anappropriate manner. Vegetation can be plantedin strategic areas to minimize soil erosion duringstorm events.

● Cover process areas: Covering processequipment (where feasible) reduces the amountof stormwater that comes into contact withpotentially contaminated areas. Water flows overthe covers and can be directed to the non-contaminated stormwater collection system. Someexamples of process areas where covers wouldbe beneficial and practical are pump stations,heat exchangers and separation drums. Areasadjacent to non-contaminated drainage areasare the most logical candidates for covers, as thestormwater from these locations can be diverted

directly into the non-contaminated drainagearea. This technique would not be practical forlarge process units, where elevations of variouspieces of equipment can vary significantly. Alarger area of low-lying process equipment,however, could be covered by a single roofsloped towards a non-contaminated drainagearea. In climates where significant snowfalls canbe expected, covering for process areas must bedesigned to account for snow loads and/or mustbe equipped with measures to prevent freezingof drain lines.

Non-contaminated stormwater

Segregating non-process area stormwater fromprocess area stormwater requires a separatedrainage system. This may consist of a burieddrainage system, or a system based on grading,trenches and culverts. Curbing may also benecessary to separate process area stormwater fromnon-process area stormwater. Non-contaminatedstormwater can be sent to a pond or lagoon for useas raw water for the refinery.

Non-contaminated stormwater—best practices● Re-use: There are several potential re-use

opportunities for non-contaminated stormwaterincluding fire water, cooling tower makeup, utilitywater and boiler feedwater makeup. These issuesare discussed in more detail in the section on‘Re-use of non-contaminated stormwater’ onpage 43 of this document.

● Retention: Local regulation will dictate the typeand frequency of testing that will be requiredprior to the discharge of non-contaminatedstormwater. Many refineries choose to hold thestormwater in a pond or basin prior todischarge. This will also allow time for therefinery to evaluate whether to reuse this wateror not.

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1 See page 28 for more information on API separators.

Sewerage management

Sewerage refers to wastewater that is generated inkitchens, locker rooms and washrooms in therefinery. At many locations the sewerage iscombined with the wastewater generated in therefinery and sent to the wastewater treatment plant.Other refineries segregate the sewerage and treat itseparately from the refinery wastewater.

The strategy for treatment should be dictated by therequirements of the local regulation. Typically theflow of sewerage in a refinery is relatively smallwhen compared to the other wastewater generatedin the refinery. If local regulations require that thecombined treated wastewater needs to bechlorinated prior to discharge then segregation andseparate treatment will result in substantial savingsin chlorination costs. Local regulations may dictateadditional certification and training for refineryWWTP operators when sanitary waste is comingledwith refinery wastewater.

Treatment of sewerage can be effectively carried outin small self-contained packaged treatment systemsat relatively small capital and operating costs.

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This section discusses the various types of treatmentthat are usually practiced by refineries for treatingwastewater. It should be noted that best practices forthe various types of treatment are not included inthis section. This is because the technology used forrefinery wastewater systems is site-specific anddepends largely on influent conditions and the levelof treatment required which is governed by localregulations.

Typical refinery wastewater treatment plants consistof primary and secondary oil/water separation,followed by biological treatment, and tertiarytreatment (if necessary).

A typical refinery wastewater treatment system isshown in Figure 11.

In a refinery wastewater treatment system, two stepsof oil removal are typically required to achieve thenecessary removal of free oil from the collectedwastewater prior to feeding it to a biologicalsystem. This oil removal is achieved by using an APIseparator followed by a dissolved air flotation (DAF)or induced air flotation (IAF) unit.

The wastewater from the secondary oil/waterseparation unit is sent to the equalization system (thechoices for location of the equalization system arediscussed on page 31) that is used to dampen outvariations in flow and concentration in the refinerywastewater. The wastewater is then routed to the

aeration tank/clarifier which constitutes the biologicalsystem. The effluent from the clarifier is then sent totertiary treatment (if necessary) prior to discharge.

Process wastewater pretreatment

In some refineries the wastewater generated fromsome of the units can be pretreated prior todischarge to wastewater treatment. Some of thepractices that are used in refineries are summarisedbelow.

Desalter effluent treatment

The effluent from the desalter can be the cause ofoperating problems in wastewater treatment. Often,this is the result of changes to crude slates or otherdesalter upsets that affect the operation of thedesalter resulting in inadvertent discharges of oil,emulsion and solids to wastewater treatment. Thedesalter effluent can also contain significantconcentrations of benzene and other volatileorganic compounds (VOCs) that tend to vapourizein the sewers leading to excessive emissions as wellas odour problems in the refinery if the desaltereffluent is not managed properly.

Desalter oil/water separationSome refineries choose to subject the desaltereffluent to an oil/water separation step (possiblyusing a separation tank) prior to discharge to the

25


Effluent treatment

Figure 11 Typical refinery wastewater treatment

wastewater treatment plant. This approach is usedespecially when the capacity of a primary oil/waterseparator in the wastewater treatment is limited,and an analysis indicates that it is more cost-effective to install a separation step on the desalterstream rather than change or upgrade the existingwastewater treatment plant configuration. This isalso a way of handling the increased load of solidsthat get discharged during mud washing of thedesalter. Some refineries also use such a tank to

divert the brine during upsets in the desalter. Figure12 shows the configuration of a typical desaltereffluent pretreatment system.

The desalter effluent is sent to a floating roof tank(floating roof in order to control VOC emissions)which typically has a residence time of a day or so inorder to provide equalization, upset buffering etc.The brine is allowed to settle and separate. The oil isskimmed off and sent to refinery slops and the water

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Figure 12 Desalter oil/water separation

Figure 13 Desalter effluent stripper

phase is sent to the wastewater plant. The bottomsolids from the tank is sent to the sludge treatmentplant or the coker unit if the refinery has such a suit.

Desalter effluent VOC controlIn some countries emissions of benzene and othervolatiles are required to be controlled, by regulation.Since the desalter effluent can contain significantconcentrations of these compounds, attempts havebeen made to control the emissions using strippers(steam/natural gas). The oil and solids content of thedesalter effluent are high and this can foul/plug theinternals of the stripper if proper pretreatment andequalization are not utilized. Figure 13 shows theconfiguration of a desalter effluent stripper.

Wastewater segregation

Given that there is a shortage of available rawwater in many locations, and the fact that a typicalrefinery produces anywhere from 10 to 50 gallons

of wastewater per barrel of crude processed, thereuse of treated refinery wastewater is increasinglycoming into focus. An effective strategy forsegregation of refinery wastewater is by the TDScontent of the wastewater. As previously discussedthe sources of wastewater in a refinery can becategorized as follows:● desalter effluent (high TDS);● tank BS&W (high TDS);● spent caustic (high TDS);● stripped sour water (low TDS);● stormwater (low TDS); and● miscellaneous wastewater (low TDS).

In a segregated system the refinery wastewatersystem would consist of two parallel trains with thesame unit operations, except that the low TDS trainwould not include an API separator because thesuspended solids loading of the inlet wastewatertends to be quite low. Figure 14 describes the twoparallel trains.

27


Figure 14 Segregated wastewater treatment

It should be noted that this level of segregation andtreatment is not common practice in refineries but issometimes considered in water-scarce areas. Theissues associated with recycle and reuse of treatedwastewater are discussed in greater detail in thesection on ‘Recycle and reuse issues’, beginning onpage 42 of this document.

Primary treatment

The primary treatment for refinery wastewater is aphysical operation, usually gravity separation, toremove the floating and the settleable materials inthe wastewater. In a typical refinery wastewatertreatment system, the primary treatment step consistsof an oil/water separator where oil, water andsolids are separated. This is followed by asecondary oil/water/solids separation step in whicha DAF or an IAF unit is used. The primary treatmentsteps are discussed in detail below.

First stage: separation (oil/waterseparators, API separators)

API separators are frequently used in the treatmentof refinery wastewater which usually contains oiland oil-bearing sludge. Separators use thedifference in specific gravity to allow heaviermaterial to settle below lighter liquids.Hydrocarbons that float on the surface are skimmedoff, while the sludge that settles to the bottom isremoved periodically.

In a typical API separator, wastewater is firstcollected in a pretreatment section that allowssludge removal. A diffusion barrier slowly allowsthe wastewater to flow down the separator towardsthe outlet while the lighter oil fractions can beskimmed off. Flights and scrapers are sometimesused to remove heavier solids. Underflow baffleplates are usually used to prevent oil from escapinginto the outlet section. Figure 15 shows a typicalAPI separator.

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(Reproduced courtesy of PSENCO)

Figure 15 API separator

1. Trash trap (inclined rods)2. Oil retention baffles3. Flow distributors (vertical rods)4. Oil layer5. Slotted pipe skimmer6. Adjustable overflow weir7. Sludge sump8. Chain and flight scraper

Some of the performance-limiting factors relating tothe API separators are listed and discussed below:● Emulsified or dissolved oil that is usually present

cannot be removed by an API Separator.● High pH at the API separators can stabilize

emulsions. Spent caustic streams should be eitherneutralized or routed directly to equalization inorder to reduce pH at the API separators.

An API separator is an effective device forseparating three phases (oil, solids and water) thatare usually present in refinery wastewater. There aresome refineries that use corrugated plateinterceptors (CPI) or parallel plate separators (PPI).Both CPI and PPI separators tend to be smaller thana comparable API and require less plot space.However, while these devices are very effective astwo-phase separators (oil and water), they are lesseffective when a third phase (solids) are present.The solids that are present in refinery wastewatertend to foul and plug the parallel plates resulting inthe need for frequent maintenance.

Secondary oil/water separation

The effluent from the primary oil/water separationstep is sent for further oil and fine solids removal toeither a DAF unit or an IAF unit. The choice ofwhether to use a DAF versus an IAF unit is refinery-specific, and needs to be evaluated based on theinfluent conditions and the required outlet conditions.

Dissolved air flotation (DAF)The first step in a DAF system is coagulation/flocculation. Dispersed particles (oil/solids) arestabilized by negative electric charges on theirsurfaces, causing them to repel each other. Sincethis prevents these charged particles from collidingto form larger masses, called flocs, they do not settle.To assist in the removal of colloidal particles fromsuspension, chemical coagulation and flocculation arerequired. These processes, usually done in sequence,are a combination of physical and chemicalprocedures. Chemicals are mixed with wastewaterto promote the aggregation of the suspended solidsinto particles large enough to settle or be removed.

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Figure 16 Dissolved air flotation—a typical DAF unit

(Reproduced courtesy of AJM Environmental)

In a DAF system, part of the effluent is recycled,pressurized, saturated with air and mixed with theincoming feed. When the recycle stream isdepressurized it releases the air bubbles whichattach themselves to any free oil/solids contained inthe feed and float them to the surface of the vessel.The floated material is skimmed off and sent torefinery slops after further dewatering. Some solidsalso settle to the bottom of the DAF where they arescraped off and removed periodically. Figure 16shows a picture of a typical DAF unit.

Induced air flotation (IAF)In an IAF unit, air is induced by a rotor-dispersermechanism, the spinning rotor acts as a pump andforces the fluid through the disperser openings andcreates a vacuum in the stand pipe. The vacuum inthe standpipe pulls the air and mixes it with theliquid. The liquid moves through a series of cellsbefore leaving the unit and the float passes over theweir on one or both sides of the unit. The

advantages of the IAF technique are compact size,low capital cost and the effective removal of free oiland suspended materials. The configuration of atypical IAF unit is shown in the Figure 17.

Other types of dispersed gas flotation units exist,such as the hydraulic type, where effluent is pumpedand educts vapour from the top, before beingdistributed to each cell via a striker plate to createsmaller bubbles which again attract and pull oil outof suspension.

Equalization system

The objective of the equalization system is tominimize or reduce the fluctuations caused due toeither sudden change of flow or composition in thewastewater treatment plant.

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(Reproduced courtesy of WEMCO)

Figure 17 Induced air flotation (IAF) unit

● Flow equalization: Flow equalization providesdampening of the flow variations, therebyreducing potential spikes in flow and loads to thedownstream units; it also reduces the size of thedownstream units and the cost of the overallrefinery wastewater system.

● Concentration equalization: This system providesdampening of contaminants, thereby preventingthe shock loading of the downstream units suchas biological systems. In a biologically-basedsystem, performance is limited by the capacity ofthe microorganisms to adapt to the changingconditions of variation in flow and composition.

Location of the equalization system

In Figure 11, the equalization system is shown afterthe secondary oil/water separation step. Otherpotential locations for the equalization system arediscussed below.

Upstream of the API separatorSome refineries choose to locate the equalizationtank upstream of the API separator in order todampen the variations in flow to the separator. Ifthis location is chosen, it must be recognized that allthe oil and solids contained in the refinerywastewater will pass through this tank and some ofthem will separate. Hardware (piping/pumps andcontrols) must be provided to allow removal of freeoil and solids from the tank in order to avoidaccumulation of these materials. Frequent cleaningof this tank (once or twice a year) may also berequired depending on the loading of solids and oilthat are contained in the refinery wastewater.

Upstream of the DAF/IAFThe equalization tank is installed at this location inorder to dampen the flow variations to the DAF/IAFand downstream equipment. While this will tend tomake all the downstream equipment smaller, any oilthat is present in the effluent from the API separatorwill accumulate in this tank if it is not removedperiodically.

Downstream of the DAF/IAFThe primary goal of installing the equalization tankat this location is to protect the downstreamequipment (biological system) from wide variationsin flow and concentration.

Secondary treatment

Biological treatment is the most widely used wastewatertreatment technology for removal of dissolvedorganic compounds in the oil refining industry.

In general, biological treatment can be classifiedinto two categories: ● suspended growth processes; and● attached growth processes.

Suspended growth processes

Suspended growth processes are biological treatmentprocesses in which the microorganisms are thoroughlymixed with the organics in the liquid, and maintainedas a suspension in the liquid. Micro organisms useorganic constituents as food for their growth andclump together to form the active biomass. The mostcommonly practiced suspended growth process usedin the treatment of refinery wastewater is the‘activated sludge process’ (discussed below).

Activated sludge An activated sludge process is the most effective ofall the biological systems available. It is used inmany refineries around the world and offers areliable method of biological treatment.

Activated sludge is a continuous suspension ofaerobic biological growths in a wastewatercontaining entrapped suspended colloidal, dissolvedorganic and inorganic materials. Themicroorganisms use the organic material as acarbon source and energy for the microbial growth,and convert the food into cell tissue, water andoxidized products (mainly CO2).

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In an activated sludge process, the wastewaterenters an aeration tank where the microorganismsare brought in contact with organic contaminants ofthe wastewater. Air is continuously injected into thesystem to keep the sludge aerobic and to maintainthe solids in suspension. The mixture of wastewaterand sludge in the aeration basin/tank is referred toas the ‘mixed liquor’, and the biomass in the mixedliquor is referred to as ‘mixed liquor suspendedsolids’ (MLSS). The organic portion of the biomass isgenerally referred to as the ‘mixed liquor volatilesuspended solids’ (MLVSS). In a typical refinerywastewater treatment system, the MLSS arecomposed of 70–90% active MLVSS and 10–30%inert solids.

A schematic of a typical activated sludge system isshown in Figure 18. The incoming wastewaterenters the aeration tank where it is contacted withmicroorganisms and air. The effluent from theaeration tank is sent to the clarifier. The organiccontaminant in the wastewater gets converted intothe biomass and gets separated later in theclarifier. A portion of the concentrated sludge,referred to as ‘return activated sludge’ (RAS), fromthe clarifier is recycled back and mixed with

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Figure 18 Activated sludge system

incoming wastewater, and the remainder of thesludge is discharged as ‘waste activated sludge’(WAS).

Activated sludge treatment with powderedactivated carbon (PACT®)The PACT® (Powdered Activated Carbon Treatment)system is similar to the conventional activated sludgesystem described above. In this treatment systemboth biological oxidation and carbon absorptionoccur simultaneously, thus enhancing the removal ofcontaminants in the wastewater. Most of thepowdered activated carbon is recycled with theactivated sludge, but the system requires acontinuous makeup of fresh carbon. PACT® systemsare generally used for refinery wastewater in thosecases where stringent standards need to be met forcertain contaminants.

A schematic of a typical PACT® system is shown inFigure 19.

Sequencing batch reactorA sequencing batch reactor (SBR) is a fill-and-drawsemi-batch biological treatment alternative thatemploys aeration, sedimentation and clarification ina single reactor. The unit processes of aeration andsedimentation are common to both the SBR andactivated sludge systems. In activated sludge systemsthe unit operations take place in different basins,while in the SBR the operations take place in asequential order in a common basin.

Although still practiced in some refineries, SBRtechnology is increasingly uncommon and haslimited application in refinery wastewater treatment.Figure 20 (overleaf) shows a typical SBR system.

The various steps of operation are described below:● Fill: During the fill operation, wastewater with the

substrate is added to the reactor. The aerationsystem is not operated as the reactor is chargedwith wastewater from the equalization tank.

● React: During this step, wastewater is aerated inthe same way as in the activated sludge system.Biological activity is initiated in this cycle ofoperation.

● Settle: In this step, aeration is terminated andMLSS is allowed to settle. The settling isaccomplished under quiescent conditions; noflow enters, or is withdrawn from the reactorduring the settle period.

● Decant: During the decant period, clarified ortreated supernatant effluent is withdrawn fromthe upper portion of the reactor. The sludgeblanket at the bottom of the reactor is maintainedso that it is available as seed sludge for the nextcycle.

● Idle: This is not a necessary step and is usuallyomitted for the refinery wastewater treatmentsystem. The idle period is the time between thedraw and the fill; it could be zero or could bedays. Generally, it is used in multi-tank systems,thereby providing time to one reactor to completeits fill phase before switching to another unit.

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Figure 19 The PACT® (Powdered Activated Carbon Treatment) system

Membrane bioreactor technologyMembrane bioreactors (MBRs) are suspended-growth biological treatment processes and are avariation on the activated sludge system. Amembrane bioreactor combines a membrane

process (e.g. microfiltration) with a suspendedgrowth bioreactor, thereby eliminating thesecondary clarification used in an activated sludgesystem. A schematic of a typical MBR system isshown in Figure 21.

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Figure 20 Sequencing batch reactor system

Figure 21 Membrane bioreactor system

The micro-filtration membranes are located in a steelmembrane tank and are subjected to a low-pressurevacuum that pulls water through the membranes andpumps the filtered water to the next process stepwhile retaining solids in the reactor. Compressed airis injected into the system to scour the exterior of themembranes. The MBR system usually operates athigher MLSS concentrations (15,000–20,000 mg/l)than conventional activated sludge systems. MBRsystems are not used in refining due to increasedcost compared to conventional activated sludge,however for activated sludge systems that requiretertiary filtration, MBR is more cost competitive, sinceit is equivalent to having an effluent filter. Forapplications where

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