Removal of hydrocarbons, mercury and arsenic from oil-field produced water

A process is disclosed which involves removing hydrocarbons, arsenic and mercury from wastewater produced in oil and gas fields. An oxidant, ferric ions, and flocculent are sequentially added to the wastewater to form a removable sludge containing the arsenic, hydrocarbon, and mercury contaminants. The Oxidation-Reduction Potential of the wastewater is controlled by oxidant addition to allow the required arsenic oxidation to occur while maintaining the mercury in elemental form. The process requires relatively short residence times between chemical additions and provides for large wastewater throughputs. The cleaned wastewater is suitable for discharge to the environment.

FIELD OF THE INVENTION 
The invention relates to a process for reducing contaminants in wastewater 
such as water produced during the oil field production of hydrocarbon 
materials including petroleum oils, natural gas and condensate. 
BACKGROUND 
Water is often co-produced along with the production of petroleum oils, 
natural gas and condensate. In the case of off-shore production, such 
produced water, i.e., wastewater, is generally discharged to the body of 
natural water surrounding the producing platform structure. The wastewater 
may contain relatively high levels of environmentally contaminating 
materials that make the water unacceptable for discharge. Such materials 
include mercury and arsenic, along with varying concentrations of 
dissolved and dispersed hydrocarbons. The contaminant materials can be 
found in the water in the elemental or ionic forms. The dispersed 
hydrocarbons can be present as fine droplets contained in water in the 
form of an emulsion, i.e., emulsified hydrocarbons, or in the form of 
undissolved, yet non-emulsified hydrocarbons. Recent governmental 
regulations of such contaminating materials in the discharged wastewater 
set permissible concentrations and seek to reduce the contaminant levels 
to those that are environmentally innocuous. If the concentration of a 
contaminating material in the wastewater exceeds that specified in the 
regulations, the concentration of that contaminant must be reduced prior 
to discharge. 
Accordingly, wastewater treatment processes are being developed to reduce 
the amount of contaminants in the wastewater to acceptable discharge 
levels, particularly processes feasible for use on off-shore hydrocarbon 
production platforms. 
SUMMARY OF THE INVENTION 
The present invention relates to a process for reducing the concentrations 
of contaminants such as hydrocarbons (emulsified or non-emulsified), 
mercury (primarily elemental) and arsenic components from wastewater, such 
as that co-produced during the production of petroleum oils, natural gas, 
condensate and other hydrocarbon-containing materials from oil fields. 
More particularly, the process of the invention results in a cleaned 
wastewater stream having relatively low contaminant weight levels, e.g., 
less than about 10 parts per billion (ppbw) of mercury (calculated as Hg), 
less than about 250 ppb and preferably less than 100 ppb of arsenic 
(calculated as As), and less than about 40 parts per million (ppmw) of 
total petroleam hydrocarbons (TPH) as defined by the EPA analytical method 
413.1 (Freon extraction, infrared detection). 
In the process of the invention, non-emulsified hydrocarbon droplets, 
emulsified hydrocarbon droplets, elemental mercury, arsenic and dissolved 
hydrocarbons are expeditiously removed from large throughputs of 
wastewater streams. If present, relatively large, non-emulsified 
hydrocarbon-containing droplets (i.e., minimum cross-sectional diameters 
above about 25 microns) are initially separated from the wastewater to 
form a pre-treated wastewater containing primarily emulsified 
hydrocarbons, and mercury and arsenic contaminants, but generally depleted 
of non-emulsified hydrocarbons. 
Suitable water clarifiers may be added to the pre-treated wastewater stream 
to destabilize residual, emulsified hydrocarbon droplets. Such water 
clarifiers also act to enhance the attraction of the wastewater 
contaminants, such as elemental mercury-containing particulates, with the 
destabilized, emulsified hydrocarbon droplets for convenient separation 
from the water, i.e., the hydrophobicity of the elemental 
mercury-containing entities is maintained or enhanced. A substantial 
proportion of the destabilized, emulsified hydrocarbon droplets and 
hydrophobic elemental mercury-containing entities can then be separated 
from the pre-treated wastewater, with, if necessary, the aid of a 
sludge-contracting material, such as a polymer-containing flocculent. A 
resulting, partially-cleaned wastewater stream, containing arsenic, 
remaining mercury and hydrocarbon contaminants, is then recovered for 
further processing 
A key aspect of the process of the current invention involves the removal 
of substantial amounts of the targeted remaining contaminants from the 
partially-cleaned wastewater stream. Controlled amounts of (1) oxidizing 
agent, such as a sodium oxychlorite, (2) ferric ions, usually from a 
ferric salt such as ferric chloride, and (3) sludge-contracting material 
(such as a polymer flocculent), are sequentially added, respectively, to 
the partially-cleaned wastewater stream, with relatively short residence 
times allowed between additions, in order to recover a product wastewater 
having acceptable concentrations of contaminants. First, the added 
oxidizing agent, after sufficient residence time, oxidizes reduced forms 
of arsenic to the +5 oxidation state, e.g., arsine or other organic 
arsenic forms (soluble in hydrocarbons) or arsenite (soluble in water) are 
converted to arsenate (soluble in water), to enhance separation of the 
arsenic by the subsequently added ferric ions and the sludge-contractor. 
Second, iron-containing precipitates encompassing arsenic, e.g., ferric 
arsenate and ferric hydroxide, are formed after sufficient reaction time 
between ferric ions, the wastewater and such oxidized arsenic components. 
Third, the addition of sludge-contracting material enhances the 
consolidation (i.e., volume reduction) of the iron/arsenic-containing 
precipitates, which are then separated from the partially-cleaned 
wastewater stream along with the residual hydrocarbons and residual 
mercury, usually as a floating sludge, to form a cleaned wastewater stream 
capable of being discharged into the surrounding environment in the form 
of an environmentally safe or benign liquid. 
In a preferred embodiment, arsenic and mercury may be simultaneously 
removed in a single processing step. After removal of any non-emulsified 
hydrocarbon particulates from the wastewater, the Oxidation Reduction 
Potential (ORP) of the pre-treated wastewater stream (primarily due to the 
addition of the oxidizing agent) is controlled within the range from about 
+75 mv to about +400 mv to (a) maintain mercury in the elemental form and 
(b) still oxidize arsenic components from reduced oxidation states, such 
as -3 or +3, to an oxidation state of +5. Accordingly, the elemental 
mercury and iron/arsenic-containing precipitates formed as a result of the 
presence and/or addition of ferric ions can then be simultaneously 
consolidated with the aid of a sludge-contracting material and separated 
from the wastewater stream. 
Common oil field equipment can be utilized in the process of the invention. 
For example, a surge tank, a skimmer vessel and/or hydrocyclones can be 
utilized for removal of hydrocarbon and/or mercury-containing sludge 
during the pre-treatment of wastewater streams, while an induced gas 
flotation (IGF) vessel can be employed effectively for hydrocarbon, 
mercury and/or arsenic removal during treatment of pre-treated or 
partially-cleaned wastewater streams.

DETAILED DESCRIPTION OF THE INVENTION 
The wastewater is usually treated by the process of the current invention 
in a continuous manner at a flow rate of at least about 100 barrels per 
day (bpd), but in typical commercial embodiments at a rate of about 1,000 
to 200,000 bpd, or even more. The wastewater generally contains at least 
about 60 ppbw of arsenic (calculated as As), at least about 10 ppbw of 
mercury (calculated as Hg) as well as total petroleum hydrocarbons (TPH) 
of above about 40 ppmv to about 15 volume percent. Usually the wastewater 
treated by the invention contains from above about 60 to about 10,000 
ppbw, normally from above about 75 to about 2,500 ppbw, and more often 
about 100 to about 500 ppbw of As. With respect to mercury, the wastewater 
ordinarily contains more than about 25 ppbw, and typically in the range 
from about 25 to about 2,000 ppbw of Hg. In the case of the hydrocarbon 
concentration in the wastewater, the concentration can be as high as 
300,000 ppmv, but ordinarily is less than about 500 ppmv, and often less 
than 200 ppmv. 
In FIG. 1, wastewater derived from oil field production methods is first 
introduced through line 2 from an inlet separator into a surge tank 4 
wherein the water is at least partially degassed (with gas released 
through conduit 5) and the bulk free hydrocarbons, i.e., non-emulsified 
hydrocarbons, are collected and removed through conduit 5A. Such 
pre-treated wastewater is then passed from the surge tank 4 via lines 6 
and 10 serially through two retention vessels 8 and 12, respectively 
(i.e., RV1 and RV2), that function to provide sufficient residence time 
for treatment of the water with added chemicals prior to collection of 
hydrocarbons and the contaminant-containing reaction products in the 
skimmer vessel 14. 
A water clarifier is added to the pre-treated wastewater immediately 
upstream of or within RV1. Suitable water clarifiers, such as those 
containing aluminum chloride (AlCl.sub.3), ferric chloride (FeCl.sub.3), 
anionic or cationic polymers or other water clarifiers known to those 
skilled in the art, are normally added (through conduit 7) to the 
pre-treated wastewater stream depleted of non-emulsified hydrocarbons (in 
line 6) to destabilize the residual, emulsified hydrocarbon droplets to 
form non-emulsified hydrocarbons. An economical and convenient water 
clarifier contains a source of ferric ions, such as ferric chloride, since 
ferric ions are required in other downstream steps of the invention to aid 
in the removal of arsenic contaminant components. The practice of the 
invention requires that one or more water clarifiers be selected and 
applied in such a manner that they enhance the association of wastewater 
contaminants such as mercury-containing particulates, particularly 
elemental mercury, with the residual, emulsified hydrocarbon droplets for 
eventual separation from water, i.e., that the hydrophobicity of the 
mercury-containing entities should be maintained or enhanced. Note that 
any organic or organically soluble mercury forms will be automatically 
removed from the wastewater stream as the hydrocarbon content of the 
produced water is reduced. 
The dimensions of RV1 are sufficient to provide about 10 to about 900 
seconds of residence time to the pre-treated wastewater after chemical 
addition at the designed flow rate for the process. Water containing the 
destabilized, previously emulsified hydrocarbons and hydrophobic elemental 
mercury contaminants is removed from RV1 via conduit 10 and then treated 
with a suitable sludge-contracting material, such as a flocculent, more 
particularly, a polymer-containing flocculent, introduced through conduit 
9 before or during entry of the water into RV2. 
Any suitable technology known to those skilled in the art of wastewater and 
oilfield produced water cleaning may be used for consolidating contaminant 
phases in RV2 into a material which is easily retained such as in the 
disclosed skimmer vessel 14. In some cases, the contaminants will be 
retained within the skimmer vessel 14 in the form of a sludge which 
requires further processing and consolidation prior to disposal. In other 
cases, the contaminants will be held within a floating oil layer in the 
skimmer vessel which can be removed and combined with other produced 
liquid hydrocarbons for shipment from the oil production facility. 
Suitable sludge processing equipment may include centrifuges, 
hydrocyclones, CPI separators, skim tanks, filters, filter presses, and 
the like. Suitable sludge-contracting materials include high molecular 
weight polymer flocculents, commercial compositions of which are known to 
those skilled in the art. Examples include cationic and anionic polymers 
such as acrylamide/acrylate polymers, acrylamide modified terpolymers and 
acrylic polymers. A resulting, partially-cleaned wastewater stream, 
containing arsenic, mercury, and residual hydrocarbon contaminants, but 
devoid of a substantial amount of both the original wastewater 
hydrocarbons and elemental mercury, is separated from the contaminants 
retained in the skimmer vessel for further processing. 
For instance, after exiting RV2, the wastewater containing the 
mercury/hydrocarbon laden phases is introduced through conduit 13 into a 
separation means such as a three phase skimmer vessel 14, wherein excess 
dissolved gas, hydrocarbon contaminants, and mercury contaminants are 
recovered as either a skimmable hydrocarbon layer or as a floating sludge 
layer. The excess gas exits the skimmer vessel through conduit 15, while 
the skimmed hydrocarbon and elemental mercury contaminants exit the 
skimmer vessel by overflowing into a recovery system represented herein by 
an oil bucket 18 and are collected via conduit 16. At the same time the 
partially-cleaned wastewater underflows the bucket, exits the skimmer 
vessel through conduit 20, and passes to a third retention vessel 22 ( 
i.e., RV3). 
Immediately prior to or within RV3, an oxidant (i.e., oxidizing agent) is 
added to the partially-cleaned wastewater through conduit 24. The 
partially-cleaned water stream, devoid of free hydrocarbons, is normally 
treated with a suitable oxidizing agent in order to oxidize reduced forms 
of arsenic having oxidation states less than +5, such as oil-soluble 
arsine (oxidation state of -3) or its water-soluble hydrolyzed form, 
arsenite (oxidation state of +3), to arsenic compounds having an oxidation 
state of +5, e.g., arsenates (normally water-soluble). Although a portion 
of the arsenic contaminants may be in an inorganic form that is highly 
water soluble, efficient arsenic removal can be accomplished by converting 
at least some organically-bound arsenic components into a form which is 
more soluble in aqueous media than in the oil-derived hydrocarbons. 
Such conversion of these arsenic forms results from the treatment of the 
wastewater with any oxidizing agent that reacts with the reduced arsenic 
compounds to form oxidized arsenic compounds. These oxidized arsenic 
compounds are more easily precipitated from water, particularly those 
oxidized arsenic compounds that precipitate in the presence of ferric ions 
and water. Oxidizing agents contemplated for use herein include 
oxygen-containing inorganic compounds of Group IA, Group IIA, Group IVA, 
Group IVB, Group VA, Group VB, Group VIA, Group VIB, Group VIIA and Group 
VIIB of the Periodic Table. Such oxygen-containing compounds include 
oxides, peroxides and mixed oxides, including oxyhalites. Examples of such 
oxidizing agents include vanadium oxytrichloride, chromium oxide, 
potassium chromate, potassium dichromate, magnesium perchlorate, potassium 
peroxysulfate, potassium peroxydisulfate, potassium oxychlorite, elemental 
halogens such as chlorine, bromine, iodine, chlorine dioxide, sodium 
hypochlorite, calcium permanganate, potassium permanganate, sodium 
permanganate, ammonium persulfate, sodium persulfate, potassium 
percarbonate, sodium perborate, potassium periodate, ozone, sodium 
peroxide, calcium peroxide, and hydrogen peroxide. Also contemplated are 
organic oxidizing agents such as benzoyl peroxide. Typical oxidizing 
agents for use herein are contained in compounds providing 
oxidation-reduction couples (1 molal solution at 25.degree. C. and 1 
atmosphere) in acidic aqueous solutions having an E.degree. value greater 
than +0.56 volts and in basic aqueous solutions having an E.degree. value 
greater than -0.67 volts. Examples of couples are disclosed on pages 
342-345 and 347-348 of The Oxidation States of the Elements and Their 
Potentials in Aqueous Solutions, second edition, authored by Wendell A. 
Latimer, and published by Prentice Hall, Inc. (1952), the disclosure of 
which is incorporated by reference in its entirety herein. More highly 
preferred oxidizing agents are oxyhalites such as the alkali, ammonium and 
alkaline-earth hypochlorites including KOCl , NH.sub.4 OCl and NaOCl, 
peroxides, ClO.sub.2, and ozone--O.sub.3. 
A sufficient amount of oxidizing agent is added to increase the Oxidation 
Reduction Potential of the partially-cleaned wastewater introduced into 
RV3 to at least +75 mv, and preferably from +100 mv to +385 mv, with a 
highly preferred range from +150 to +300 mv. Increasing the ORP above +440 
mv, and even above +400 mv, is usually unnecessary, and can hinder the 
subsequent removal of residual mercury and/or arsenic from the wastewater. 
Although the added amount of oxidizing agent is dependent upon the 
compositions of the wastewater and oxidizing agent, normally about 2 to 
about 50 ppmw of oxidizing agent is introduced into the wastewater, the 
pre-treated wastewater or the partially-cleaned wastewater. 
After permitting sufficient time for the oxidant to be in contact and react 
with the components of the wastewater to increase the value of the ORP, 
typically from about 5 to about 300 seconds, and preferably less than 60 
seconds, e.g., about 30 to about 60 seconds, the wastewater is passed to a 
retention vessel wherein ferric ions can contact and react with the 
oxidized arsenic components. A residence time of less than about 5 minutes 
(preferably about 30 to about 60 seconds) after addition of oxidant and/or 
after the subsequent contact of femecions with the wastewater, allows the 
overall process flow rate to be expedited, thus achieving relatively high 
wastewater throughputs for the inventive process. 
Ferric ions, from a source such as ferric chloride, are usually added 
through conduit 27 to the wastewater in conduit 26 before or during the 
subsequent introduction of the oxidantcontaining wastewater exiting RV3 
into a fourth retention vessel 28, (i.e., RV4) which allows the 
interaction of ferric ions and arsenic in the +5 oxidation state. Any 
suitable source of ferric ions is acceptable for introduction into RV4 or 
for contact with the wastewater in RV4, and particularly water-soluble 
ferric ion sources. Examples of ferric ion sources include ferric 
chloride, ferric sulfate, or even the wastewater itself, wherein the added 
oxidant can oxidize indigenous dissolved ferrous ion (Fe.sup.++) to the 
ferric (Fe.sup.+++) oxidation state. As in the case of added oxidant, a 
sufficient residence time from about 5 to about 300 seconds is allowed for 
contact and reaction between the ferric ions and the oxidized arsenic 
components in order to form iron-containing precipitates (including 
floccules), such as ferric hydroxide (preferably as a floe) and ferric 
arsenate components. The formation of significant amounts of ferric 
hydroxide is necessary due to its role of entrapping the 
contaminant-containing ferric arsenate. The ferric ions are normally 
introduced into or contained in the wastewater in RV4, and usually in RV1, 
in a concentration of at least about 2 to about 100 ppmw, preferably from 
about 3 to about 25 ppmw, and most preferably from about 5 to about 15 
ppmw. It has been discovered that more effective arsenic removal from the 
wastewater results when the concentration of ferric ions (as detected in 
the cleaned water for disposal from, for instance, an IGF vessel) 
decreases during arsenic removal by at least a minimum amount, usually at 
least about 3 ppmw, and preferably at least about 5 ppmw (calculated as 
Fe). However, a decrease of more than 20 ppmw, and usually more than about 
15 ppmw of ferric ions does not result in increased arsenic reduction in 
the process of the invention. 
The water containing the ferric ions, oxidant and such precipitates is 
passed from RV4 through conduit 30 and can be treated with a suitable 
sludge-contracting material, such as a polymer-containing flocculent 
introduced through conduit 32 either immediately before or within a fifth 
retention vessel 34 (i.e., RV5) wherein growth and maturation of sludge 
particles (including floc, floccule, precipitate, and the like) occurs. 
After the ferric hydroxide/ferric arsenate precipitates have initially 
formed in RV4 and matured to a suitable size and density in RV5, a 
properly selected sludge-contracting material may again be added to the 
wastewater in order to further collect, consolidate, partially dewater and 
render a hydrophobic character to the sludge. Polymeric flocculents known 
to those skilled in the art can be selected from polymers, particularly 
anionic and/or cationic polymers, including the sludge-contracting 
materials disclosed hereinbefore. The concentrations of sludge-contracting 
materials added to the wastewater in RV2 and/or RV5 for effective 
contaminant removal is usually about 1 to about 50 ppmw, preferably about 
2 to about 13 ppmw, and more preferably about 2 to about 8 ppmw. The 
properties imparted to the ferric hydroxide/ferric arsenate-containing 
precipitates or flocs due to the addition of the sludge-contracting 
material normally beneficially reduce the volume of the sludge and allow 
the efficient disposal thereof. In a similar manner as in RV2 relative to 
the enhancement of the hydrophobicity of the mercury-containing entities, 
the sludge-contracting material in RV5 is capable of coagulating the 
ferric hydroxide precipitate (which entraps the ferric arsenate 
precipitate) to enhance the hydrophobicity of the precipitates. 
In a subsequent step, the wastewater containing sludge is removed from RV5 
through conduit 36 and introduced into, for example, an induced gas 
flotation vessel 38 (i.e., IGF) where the floc or sludge containing ferric 
hydroxide, ferric arsenate, previously unremoved hydrocarbons and residual 
mercury (normally elemental mercury) is collected through line 39 for 
suitable disposal. The cleaned wastewater exiting the IGF via conduit 37 
is environmentally innocuous and may be disposed by overboard discharge, 
deep well injection or other acceptable disposal methods. It should be 
noted that once the collected, consolidated, partially dewatered 
hydrophobic precipitate or floc is well formed, it can be separated from 
the wastewater by any suitable process known to those skilled in the art. 
For example, froth flotation, hydrocyclones, filtration, centrifuging, and 
the like, are acceptable for purposes of practicing the invention. 
A preferred embodiment of the process of the invention involves the 
simultaneous recovery of arsenic and mercury in a single process step. In 
this embodiment, the pre-removal of free (non-emulsified) hydrocarbons is 
still necessary. However, the initial addition of water clarifiers and/or 
sludge-contracting materials or flocculents to recover mercury 
(particularly elemental mercury) from the pre-treated wastewater is not 
necessarily practiced. Following the removal of free (non-emulsified) 
hydrocarbons, the oxidant is added to the pre-treated water. In this 
embodiment of the invention, the ORP of the water is carefully controlled 
to maintain the mercury contaminants in the wastewater in the elemental 
mercury form. This usually requires an ORP between about +100 mv and +385 
mv. If the ORP exceeds about +385 mv, elemental mercury is often converted 
to cationic mercury forms, such as Hg + and Hg++, and the extraction of 
contaminant mercury from the wastewater will be seriously degraded in the 
overall process. 
In FIG. 2, the wastewater contaminated with hydrocarbons, mercury and 
arsenic is passed from an oil/water/gas separator in the production 
process through line 41 to a hydrocyclone 40 in order to remove free or 
non-emulsified hydrocarbons and any hydrophobic mercury either soluble or 
entrapped in the free hydrocarbons. A reject stream, i.e., a 
hydrocarbon-rich wastewater stream exiting the top of the hydrocyclone, is 
routed via conduit 42 to a three phase skimmer vessel 44 where free 
hydrocarbons and any entrapped or organically soluble contaminants are 
separated and recovered (usually skimmed) through conduit 45 (and gases 
through 45A). Optionally, chemical treatment of the reject stream 42 in 
the manner previously described for mercury and emulsified hydrocarbon 
removal in RV1 and RV2 in FIG. 1 may be practiced upstream of the three 
phase skim vessel 44. 
Underflow pre-treated wastewater from the bottom of hydrocyclone 40, devoid 
of free or non-emulsified hydrocarbons, is passed through conduit 50 and 
thereafter treated with oxidant, ferric ions, and flocculent or other 
sludge-contracting material, in a process similar or identical to the 
process employed within RV3, RV4, and RV5 in FIG. 1. (Also, in the 
optional treatment of the hydrocyclone reject stream in conduit 42, the 
retention vessel(s) RV1 and RV2 of FIG. 1 provide the residence times 
required between the serial addition of the water clarifier and 
sludge-contracting material, and the product wastewater may be passed 
directly to an IGF vessel through conduit 47 or combined with the 
pre-treated wastewater in conduit 50.) 
The wastewater removed from the three-phase skimmer vessel 44 through 
conduit 46 is combined with the underflow wastewater stream exiting 
hydrocyclone 40, and the combined stream is in conduit 50 treated for the 
removal of residual hydrocarbons, residual mercury, and arsenic upstream 
and/or within RV3, RV4 and RV5 in the manner previously described. The ORP 
of the combined pre-treated wastewater streams is normally maintained 
below +400 mv, and preferably below +385 mv, by the addition of the 
oxidizing agent prior to or within RV3. After formation and contraction of 
the contaminant-laden sludge obtained from RV5 through conduit 52, the 
wastewater stream is introduced into an IGF vessel 54 where final 
contaminant recovery is effected, with hydrocarbons, arsenic and mercury 
being removed through conduit 57. A cleaned, environmentally innocuous 
water is then discharged from the process through conduit 56 and is 
removed from the oil production area, as for instance, from an off-shore 
oil-producing platform by overboard discharge, deep well injection, or 
other suitable means. 
Particularly in the embodiment of the scheme of FIG. 2, but also 
appropriate for the scheme of FIG. 1, the ratio of ferric ions to 
sludge-contracting material may be controlled to achieve more effective 
arsenic removal. Although the chemistries of the sludge-contracting 
materials and the wastewater compositions may vary from operation to 
operation, the ferric ions/sludge-contracting material weight ratio is 
ordinarily in the range from about 1 to about 10, and preferably at least 
1.5, and most preferably above about 2 to less than about 8, or even less 
than about 7.5. Due to solubility limitations of the oxidized arsenic 
components contained in the wastewater (e.g., arsenates), it is preferred 
that at least 5 ppmw of ferric ion be lost in the overall process to 
achieve maximum arsenic removal. 
Common oil field equipment can be utilized in the process of the invention. 
For example, separator, surge, and skimmer vessels as well as 
hydrocyclones or other well known oil field equipment suitable for this 
process can be utilized for the treatment of the originally produced 
wastewater and pre-treated wastewater streams to recover free hydrocarbons 
or the combination of hydrocarbon and mercury contaminants. An induced gas 
flotation (IGF) vessel and other functionally equivalent vessels are 
examples of processing equipment that can be employed effectively for 
removal of sludge containing arsenic, mercury and hydrocarbons from the 
pre-treated and/or partially-cleaned wastewater stream. Furthermore, 
although the retention vessels and sludge separating equipment have been 
disclosed for use herein in separate processing steps, it is contemplated 
that steps be added or combined to the process in other modes of operation 
known to those skilled in the art--depending upon the particular 
wastewater compositions, chemical additives available, waste disposal 
environments, target contaminant levels, and the like. 
The invention is further described by the following examples which are 
illustrative of specific modes of practicing the invention and are not 
intended as limiting the scope of the invention as defined by the appended 
claims. 
EXAMPLE 1 
A wastewater having an initial ORP of -295 mv, an initial inlet contaminant 
concentration including 748 parts per million by weight (ppmw) of total 
petroleum hydrocarbons (TPH), 40 parts per billion by weight (ppbw) of 
mercury, calculated as Hg, and 105 to 134 ppbw of arsenic, calculated as 
As, is processed through the scheme of FIG. 1 herein at a flow rate of 
about 256 barrels of water per day (i.e. 7.5 gallons per minute) on a 
continuous basis. 
In one pilot run, the water is contacted and mixed with 5.8 ppmw of ferric 
ion prior to RV1 (via conduit 7), with 2.8 ppmv of a sludge-contracting 
material containing flocculent prior to RV2 (via conduit 9), with 5.3 ppmw 
of an oxidizing agent containing sodium hypochlorite prior to RV3 (via 
conduit 24) providing an ORP of +125 mv, with 21 ppmw of ferric ion prior 
to RV4 (via conduit 27), and with 4.8 ppmv of additional 
sludge-contracting material, including flocculent prior to RV5 (via 
conduit 32). 
The partially-cleaned wastewater exiting the skim tank in conduit 20 
contains 22 ppmw of hydrocarbons, 4.8 ppbw of Hg and 112 ppbw of As. The 
concentration of remaining contaminants in the partially-cleaned 
wastewater stream indicates that arsenic is not substantially removed from 
the wastewater stream by the addition of ferric ion alone. 
Water exiting the IGF vessel 38 contains 19 ppmw of hydrocarbons, 1.0 ppbw 
of Hg, and 48 ppbw of As. This cleaned wastewater, treated with both the 
oxidizing agent and ferric ion prior to entry into the IGF vessel, has an 
arsenic concentration substantially lower than the arsenic concentration 
in the original wastewater or the pre-treated wastewater streams. This 
cleaned wastewater meets the criteria for disposal for the geographical 
area in which it was produced, i.e., the Gulf of Thailand, during 
off-shore production of oil, natural gas and condensate. (An acceptable 
criteria for disposal in the Gulf of Thailand is: 
EQU TPH&lt;40 ppmw, Hg&lt;10 ppbw, and As&lt;100 ppbw.) 
In another pilot run under the same conditions as the first, the 
concentration of the oxidizing agent, NaOCl, (in RV3) is decreased to 3.8 
ppmw providing an ORP of -20 mv, the concentration of ferric ion (in RV4) 
is decreased to 5.9 ppmw, and the flocculent.sub.-- (in RV5) is increased 
to 6.6 ppmv. During this run, the cleaned water exiting the IGF vessel 
contained 11 ppmw of hydrocarbons, 2.2 ppbw of Hg, and 114 ppbw of As. In 
this run, the concentration of hydrocarbons and Hg in the cleaned 
wastewater reflects essentially the same reduction compared to that in the 
first run. However, the arsenic concentration reflects little arsenic 
reduction by the process, thus illustrating that sufficient amounts of the 
oxidant must be added to achieve the desired arsenic reduction. 
Runs 3 through 6 are conducted in a similar manner as the first run, except 
ferric ion is injected at a concentration of 14.2 ppmw and 16 ppmw in RV1 
and RV4, respectively, and the flocculent injected in RV5 is 5 ppmv. The 
concentrations of the oxidizing agent in RV3 are varied from zero to 18 
ppmw in runs 3 through 6 and changes in the ORP of the wastewater and the 
reductions of Hg and As are summarized in Table 1. 
TABLE 1 
__________________________________________________________________________ 
Mercury Arsenic 
IGF IGF IGF IGF 
Run 
NaOCl 
ORP 
Inlet 
Outlet Inlet 
Outlet 
No. 
(ppmw Cl) 
(mv) 
(ppbw) 
(ppbw) 
Reduction 
(ppbw) 
(ppmw) 
Reduction 
__________________________________________________________________________ 
3 0 -295 
3.0 0.2 93% 104 101 3% 
4 1.8 -135 
4.7 1.3 72% 102 101 1% 
5 6.4 270 
2.7 0.6 78% 110 63 43% 
6 18 449 
4.3 5.1 0% 112 66 41% 
__________________________________________________________________________ 
The data in Table 1 indicate, for instance, that in run 4, an ORP of -135 
mv (versus an ORP for untreated water of-295 mv in run 3) is below the 
value required to significantly reduce the arsenic concentration, whereas 
the addition of sufficient oxidant to the wastewater to generate an ORP of 
+270 mv in run 5 allows the inventive process to reduce As in the water to 
63 ppbw, i.e., a reduction of As of over 40 percent, and still reduce the 
Hg by over 75 percent. Runs 3 through 5 all exhibit Hg reduction well over 
50 percent when little or moderate amounts of oxidant are added to the 
wastewater and the ORP is kept below about 400 mv. However, in run 6, 
where the ORP of the wastewater is increased by oxidant addition to +449 
mv, the water exiting the IGF vessel contains approximately the same 
concentration of Hg as the water entering the IGF. Such data illustrate a 
requirement of the inventive process to restrict oxidant addition to the 
wastewater so as to maintain the ORP level below that necessary to remove 
both As and Hg simultaneously in the IGF. Ordinarily the ORP level must be 
kept below about 400 mv to prevent substantial oxidation of elemental Hg 
to Hg+ or Hg++ cations--which are not significantly removed by the 
process. 
EXAMPLE 2 
A wastewater is processed through a hydrocyclone in the scheme illustrated 
in FIG. 2 (without the optional processing through RV1 and RV2), at a 
continuous rate of 250 barrels per day. The wastewater contains 309 ppmw 
of hydrocarbons, 22 ppbw of Hg, and 74 to 125 ppbw of As. The Hg and As 
contaminants are simultaneously removed from the underflow wastewater 
which exits the hydrocyclone through conduit 50. Data are summarized in 
Table 2. 
TABLE 2 
__________________________________________________________________________ 
Contaminant 
Concentrations 
RV3 RV4 RV5 Hydrocyclone 
NaOCl, 
ORP, 
Ferric Ion, 
Polymer, Outlet IGF 
Run No. 
ppmw 
mv ppmw ppmw (Prior to RV3) 
Discharge 
__________________________________________________________________________ 
7 0 -295 
15 6.3 TPH (ppmw) 
62 13 
Hg (ppbw) 
8.4 6.3 
As(ppbw) 
118 125 
8 0 -295 
19 3.3 TPH(ppmw) 16 
Hg(ppbw) 2.6 
As(ppbw) 94 
9 0 -295 
9.3 6.3 TPH (ppmw) 13 
Hg(ppbw) 1.6 
As(ppbw) 79 
10 0 -295 
6.8 3.3 TPH(ppmw) 10 
Hg(ppbw) 0.6 
As(ppbw) 74 
11 15.3 
+270 
15 6.3 TPH(ppmw) 
76 11 
Hg(ppbw) 
9.2 0.3 
As(ppbw) 
116 48 
12 15.3 
+270 
15 3.3 TPH(ppmw) 14 
Hg(ppbw) 2.8 
As(ppbw) 52 
13 15.3 
+270 
9.3 6.3 TPH(ppmw) 15 
Hg(ppbw) 4.3 
As(ppbw) 50 
14 15.3 
+270 
6.8 3.3 TPH(ppmw) 14 
Hg(ppbw) 2.8 
As(ppbw) 52 
__________________________________________________________________________ 
Oxidant, NaOCl, is added in runs 11 through 14 prior to the introduction of 
the wastewater into RV3 to generate an ORP of +270 mv and ferric ion is 
added in runs 7 through 14 prior to RV4 while the flocculent is added 
prior to RV5. 
Although the hydrocarbon and Hg concentrations exiting the hydrocyclone are 
substantially reduced compared to the original wastewater, As 
concentrations are not substantially reduced until after discharge of the 
cleaned water from the IGF vessel and only in runs 11 through 14 where the 
ORP is increased to +270 mv due to oxidant addition (i.e., As is reduced 
to 48-52 ppbw in runs 11-14). The ORP is sufficiently high in value in 
runs 11 through 14 to allow the simultaneous substantial reduction in the 
Hg concentration (i.e., at least 50 percent and as high as 96 percent 
reduction). Furthermore, the hydrocarbon concentrations are also reduced 
in inventive runs 11 through 14 by at least about 80 percent). 
EXAMPLE 3 
A wastewater having an ORP of -295 mv and an initial inlet concentration 
including 297 parts per million by weight (ppmw) of total petroleum 
hydrocarbons (TPH), 36 parts per billion by weight (ppbw) of mercury, 
calculated as Hg, and 429 ppbw of arsenic, calculated as As, is processed 
through the scheme of FIG. 1 herein at a flow rate of about 256 barrels of 
water per day (i.e. 7.5 gallons per minute) on a continuous basis. 
In a fifteenth pilot run, the waste water is contacted and mixed with 15 
ppmw of ferric ion prior to RV1 (via conduit 7), with 5 ppmv of a 
sludge-contracting material containing flocculent prior to RV2 (via 
conduit 9), with sufficient oxidizing agent containing sodium oxychloride 
prior to RV3 (via conduit 24) to provide an ORP of +300 mv, with 15 ppmw 
of ferric ion prior to RV4 (via conduit 27), and with 5 ppmv of additional 
sludge-contracting material, including flocculent, prior to RV5 (via 
conduit 32). 
Water exiting the IGF vessel 38 contains 16 ppmw of hydrocarbons, 3.8 ppbw 
of Hg, and 57 ppbw of As. This cleaned wastewater has an arsenic, mercury 
and hydrocarbon concentration substantially lower than the arsenic 
concentration in the original wastewater stream, i.e., at least an 85 
percent reduction. This cleaned wastewater meets the criteria for disposal 
for the Gulf of Thailand, during off-shore production of oil, natural gas 
and condensate. 
While the preferred embodiment of the invention has been shown and 
described, and some alternative embodiments and examples also shown and/or 
described, changes and modifications may be made thereto without departing 
from the invention. Accordingly, it is intended to embrace within the 
invention all such changes, modifications and alternative embodiments as 
fall within the spirit and scope of the appended claims.