The demand for efficient and cost effective wastewater treatment technology in the refining and petrochemical sector is being driven by not only ever-tightening environmental legislation, but also by the sectors own desire to follow a meaningful sustainability agenda and to take its responsibilities around product stewardship seriously. Treatment of wastewater from petrochemical plants can be a challenging and costly matter; particularly when needing to comply with the requirements of operational permits and national environmental legislation governing the discharge of treated wastewater into community treatment plants or natural water bodies such as rivers, lakes and oceans.

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INNOVATIONS IN WASTEWATER TREATMENT
The demand for efficient and cost effective wastewater treatment technology in the
refining and petrochemical sector is being driven by not only ever-tightening
environmental legislation, but also by the sectors own desire to follow a meaningful
sustainability agenda and to take its responsibilities around product stewardship
seriously. However, refining and petrochemical companies are continually confronted
with the challenge of striking a balance between making their activities profitable while
ensuring the industrial processes involved in the production and application of a
chemical product, across its lifecycle, have minimal impact on the environment.
Treatment of wastewater from petrochemical plants can be a challenging and costly
matter; particularly when needing to comply with the requirements of operational
permits and national environmental legislation governing the discharge of treated
wastewater into community treatment plants or natural water bodies such as rivers,
lakes and oceans. The segregation, collection and treatment of wastewater play a vital
part in the protection of public health, water resources and wildlife. Refining and
petrochemical facilities, as part of their permit to operate, must demonstrate that they
are successfully able to treat all their pollution streams to the appropriate regulatory
standards.
One of the most widely used strategies to meet the ever-rising demand for water and
increasingly strict regulations governing water protection is through improved water
management and strengthened investment in the technologies that preserve and
recycle process wastewater.
The refining industry converts crude oil and associated petroleum gas (APG) into
hundreds of refined products, including petroleum, diesel fuel, kerosene, aviation fuel,
fuel oils, lubricating oils, and primary feedstock for the petrochemical industry, and in
doing so it employs a wide variety of physical and chemical treatment processes in
which large volumes of water are utilised, especially for cooling systems, distillation,
filter backwashing, and deionisation techniques. Vessel cleaning, equipment flushing
and surface water runoff also generate additional volumes of wastewater which need to
be treated. In turn, the petrochemical industry produces a multitude of essential
products to modern day living including intermediates for the pharmaceutical industry,
aromatic organics, solvents, alcohols, ketones, polymers and aldehydes, all of which

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are synthesised through various process operations which can produce large quantities
of wastewater that must be treated.
Given the complex and diverse nature of refinery wastewater pollutants, a combination
of physical, chemical and biological process trains and treatment methods are usually
required before it is finally discharged into the aquatic environment.
Wastewater treatment can be improved significantly by harnessing industrial gases
such as pure oxygen, for example, to enhance the biological assimilation and oxidation
processes of wastewater treatment plants or prevent undesired odours in refinery
mains or storage lagoons and tanks. Carbon dioxide (CO2) is a versatile and safe
substitute for corrosive mineral acids to effectively neutralise alkali wastewater.
The technology used for refinery wastewater systems is site-specific and depends on
the nature of influent (incoming wastewater) conditions and the level of treatment
required by local regulatory authorities. However, a typical refinery wastewater
treatment plant usually consists of physio-chemical pre and primary treatment, followed
by secondary biological treatment and tertiary treatment, if necessary.
In a refinery wastewater treatment system, two steps of oil removal are typically
required to achieve the necessary removal of free oil from the collected wastewater
prior to feeding it to a biological system. This oil removal is achieved by using an
American Petroleum Institute (API) or equivalent oil-water separator followed by a
dissolved air flotation (DAF) or induced air flotation (IAF) unit.
The wastewater is then routed to the primary treatment clarifier and then to the aeration
tank and secondary clarifier which constitutes the biological system. The effluent from
the clarifier is then sent to tertiary treatment, if required, prior to discharge. The
activated sludge process is the most widely used wastewater treatment technology for
removal of soluble organic contaminants in the oil refining and petrochemical industry.
Often the pH of the raw wastewater requires reducing before it can be accepted by the
bio-treatment stage, as the high pH could potentially kill off the bacteria doing the
treatment.
CO2 the versatile acid alternative for pH control

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In the United Kingdom, the industrial gases technology company, BOC Ltd, part of The
Linde Group, has seen wastewater treatment success at a major plant operated by one
of the world’s leading petrochemical manufacturers.
The 1,700-acre site is highly integrated, exploiting synergies between the
petrochemicals plant and adjacent refinery. The petrochemicals facility manufactures
over 2 million tonnes of chemicals products per annum and the refinery has an annual
capacity of 10 million tonnes.
At the petrochemicals plant an environmentally friendly CO2 based technology,
SOLVOCARB®
, is being used to control alkali aqueous wastewater pH prior to
discharge. The system uses gaseous CO2 to neutralise alkaline waters through the
production of carbonic acid.
The refinery, in compliance with legislation at the time, had been discharging
wastewater from the plant into the local river estuary after adjusting its pH using
mineral acids, such as sulphuric and hydrochloric. Variability in discharge pH and the
corrosive nature of strong mineral acids led to concerns over potential harm the
discharge may cause to aquatic wildlife resident in estuaries.
“In addition to the plant needing to find a more environmentally friendly wastewater pH
control solution, they also needed to find one that would give them more robust control
over the whole process,” says Darren Gurney, Senior Process Engineer at Linde
Gases Division. “In order to achieve the target pH range through the use of mineral
acids, the company observed periods of pH oscillation from too much acidity dosing,
requiring adjustment with additional alkalinity; inevitably leading to extra cost and
operating complexity arising from operating two pH adjustment processes. The
company ultimately opted for a single process route involving CO2 which preserves the
natural alkalinity of the wastewater and the process pH control is more stable over the
desired pH control range. BOC Ltd, was appointed to design the pH control system for
the newly designed wastewater treatment plant.”
Owing to strict environmental permits, wastewater may only be discharged into the
outlet channels if it is within a narrow pH range – usually between 9 and 6. The
SOLVOCARB method employs gaseous carbon dioxide to neutralise alkaline waters.
When dissolved in water, carbon dioxide forms carbonic acid which reacts with the

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alkalinity to form a salt, the neutralisation reaction controls the pH value to the
appropriate discharge level.
Awarded the contract to design two SOLVOCARB systems to neutralise all plant
wastewater, anywhere between 10,000 and 20,000 cubic metres per day, BOC
engineers designed each system to simultaneously mix and dissolve CO2 into each
10,000 m3
tank, controlling the pH to the appropriate set point. Consent permits
dictated the site could only discharge pH corrected wastewater into the estuary when
the tide was in, allowing the treated wastewater to dilute effectively into the larger body
of available water in the estuary. However, this meant that the whole process had to be
completed within a restricted time period of a six hour window between the two tides.
It was critical for the wastewater to be neutralised in the two tanks within the time
available, which called for challenging process hydrodynamics. Large and variable
volumes of wastewater needed to be brought within the correct pH range within a fixed
timeframe – the wrong pH value could result in the refinery being unable to discharge
the wastewater, causing potential bottleneck and resulting in back-ups further up the
process chain. A significant amount of testing was conducted before the team was
satisfied that the proposed system would operate to the required parameters.
The new wastewater treatment plant was commissioned in February 2000 and was
completed on time and on budget. BOC has continued to supply the plant for nearly 12
years. The continued successful operation of the plant will help safeguard the
environmental status of aquatic areas in which the wastewater is discharged.
Today the main driver for treating effluent high in alkalinity prior to discharging to the
outfall is the strict regulation to protect the sensitive, bio diverse ecosystem within the
estuary. Using CO2 to neutralise an alkali effluent not only avoids large swings in the
discharge pH, a vital component in creating a sustainable and suitable environment for
marine life.
Compared with mineral acids commonly used in previous years, carbon dioxide offers
many advantages, amounting to the best economical and ecological alternative.
Carbon dioxide is not categorized as a substance that is harmful to water and does not
lead to the addition of unwanted anions in the water environment, such as chlorides
and sulphates. There is also no over-acidification of the wastewater, owing to the self
buffering nature of CO2 in water that produces a flat neutralization curve and no

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corrosion of system and equipment components. Carbon dioxide is also far safer than
the acids previously used. Simple to handle, it is delivered as a liquid cryogen that is
stored in tanks onsite and dosed automatically into the process.
The fact that carbon dioxide is sequestered and effectively removed from the global
carbon cycle means that it is not available to take part in global warming - therefore
less carbon dioxide is being released into the atmosphere. This gives the
SOLVOCARB process excellent green credentials.
Pure oxygen wastewater treatment
The application of pure oxygen has been successfully applied in the activated sludge
process to treat a variety of industrial and refining operations wastewaters for over 30
years. The activated sludge process is the most widely used form of biological
treatment for organic contaminants in aqueous wastewater, globally. There have been
numerous industrial wastewater installations built around the world where pure oxygen
is routinely used and the application technology can be designed into the process
during a greenfield build or later as a retrofit application to increase the capacity of an
existing asset.
Linde’s SOLVOX®
range of wastewater applications are an example of a pure oxygen
wastewater technology that can be employed in the original design of an activated
sludge plant or retrofitted as an upgrading technology for existing wastewater treatment
plants. The primary application in most case is to increase treatment capacity in
existing plants that are overloaded or experience wide swings in dissolved oxygen
demand. The philosophy of Linde has been to provide, where practical, performance
improvement of existing assets rather than building new plant capacity to increase
wastewater treatment throughput. This approach allows the operator to harness the
benefits of pure oxygen within the existing plant footprint and improve operational
performance, for example, lower surplus sludge production, reduced volatile emissions,
lower operational power consumption, better settlement of biomass and simple
installation.
Efficient oxygen transfer and adequate process mixing are essential components of all
aerobic wastewater treatment processes and these can be readily achieved using air or
pure oxygen supplied equipment. However, traditional aeration systems, designed as
fixed mass transfer- often cannot match the variability or increased oxygen transfer

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intensity requirements of the activated sludge process, especially when operating
temperatures increase above aeration performance design values, normally around
20O
Celsius. Pure oxygen-based systems, on the other hand, have the potential to
significantly enhance the aeration process by augmenting, or completely replacing,
portions of the installed existing aeration systems, even at elevated temperatures of >
30O
C.
Pure oxygen is able to maintain a higher aeration intensity because air only contains
one-fifth oxygen by volume and the limitation on oxygen transfer is controlled by the
partial pressure of oxygen in air and the oxygen solubility at a given water temperature.
The activated sludge process requires a positive dissolved (DO) oxygen of at least >2
mg/l to be effective, as the temperature increase and the background DO remains
constant, the driving force of air effectively reduces. It is the driving force potential of
pure oxygen ,compared with air at the same temperature, that proportionately
increases the rate of oxygen transfer into activated sludge treatment process.
Combined with effective mixing, a constant supply of dissolved oxygen ensures that the
micro-organisms providing the biological wastewater treatment perform to their
maximum potential, in an environment where oxygen transfer matches their demand.
The SOLVOX concept is to introduce pure oxygen into the wastewater treatment
process via a family of specially designed and developed pure oxygen and air-oxygen
application equipment. SOLVOX equipment is configured to work alongside existing
aeration equipment and meet the oxygen demand of the biomass generated during
synthesis and oxidation of biodegradable contaminants. Within industrial wastewater
treatment plants, pure oxygen can be used in conjunction with air to increase
operational efficiency or overall treatment capacity. Often the existing aeration
equipment has reached its maximum transfer intensity due to limitations brought about
by the nature of the wastewater, plant operating temperature or reduced efficiency due
to equipment wear and tear.
SOLVOX oxygen solutions can also be used to prevent odour nuisance from
wastewater that is transferred in large pressurised pipelines and mains, especially if the
retention times are long. The addition of oxygen prevents the naturally occurring
bacteria on the pipe walls from consuming the chemically bonded oxygen and
producing highly odorous compounds like hydrogen sulphide and mercaptans, for
example. Such compounds can quickly result in the corrosion of pipework networks

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and ancillary equipment. The addition of free oxygen creates ideal conditions for
successful preliminary treatment and aerobic sealing of sludges in tanks and vessels.
Linde SOLVOX application equipment and process knowledge can be applied in a
variety of ways to improve the operational performance efficiency of many industrial
wastewater treatment plants and processes. Adding oxygen using SOLVOX equipment
requires an initial low capital investment and once installed, designed to be operated
flexibly so it can be adjusted to seasonal needs or used to track peak loading during
ramp-ups during production campaigns
Water reuse and recycling
Recycled wastewater makes it possible for companies who use water in their
processes to treat the effluent on-site and reuse or recycle as much of it as possible
back into the industrial operation. Many companies are turning to this alternative
against a background of increasing pressure, in the form of restrictions on allowable
discharge volumes, limitations on abstraction quotas and mains supply, legislative
pressure and the upward spiralling increase in the “turnaround costs” of water. The
latter considers both incoming water supply and trade effluent discharge costs. Linde’s
own wastewater recycling technology, AXENIS™, employs a high rate Membrane
bioreactor treatment stage and uses a combination of pure oxygen and air to ensure
optimal process performance. The membrane replaces the secondary clarifier used in
conventional activated sludge plants.
Against a conventional wastewater treatment solution, the innovative, patented process
offers very substantial cost savings across the industrial water lifecycle. This includes
much reduced capital investment, lower energy costs and superb final effluent quality.
When combined with reverse osmosis as a tertiary treatment step to provide high
quality softened water, AXENIS makes wastewater re-use a possibility for a large
number of organisations. By unlocking the resource they already have available
operators have the potential to increase production capacity in a more sustainable
manner without further depleting a finite natural resource. All this can often be achieved
at much lower cost than paying for supply and disposal of process water.
Advanced oxidation, ozone solutions

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Oxygen can also be applied in other highly effective wastewater treatment applications
such as ozone water treatment, where in combination with either hydrogen peroxide
(H2O2) or ultraviolet radiation (UV) it is effective in partial or total oxidation of many non-
biologically treatable wastewater compounds, colour removal and as a chlorine
replacement for primary disinfectant. Ozone (O3) is a molecule consisting of three
oxygen atoms and is created by passing oxygen through ultraviolet light or a "cold"
electrical discharge. Under ambient conditions ozone is very unstable, readily giving up
one atom of oxygen within its short lifespan of usually less than 10 minutes. However
this process results in a powerful oxidizing agent called a free radical which is toxic to
most waterborne organisms. This property makes ozone a very strong, broad spectrum
disinfectant and biocide that will oxidise many organic and inorganic substances. For
this reason it is used widely throughout the world.
Its strong oxidising properties make ozone an effective chemical for water treatment,
but to use ozone, it must be created on-site and added to the water by bubble contact.
A major advantage of ozone includes the production of fewer dangerous by-products in
comparison to chlorination, for example.
Due to the growing concerns over chlorinated by-product formation, the use of ozone in
water-based evaporative cooling towers is becoming an increasingly attractive option,
particularly within process industries. It can also provide wider environmental benefits
in comparison to the more traditional chemical treatment programmes.
Typically, chemicals such as chlorine and chelating agents are added to cooling tower
water to control microbiological growth and inhibit mineral build-up. However, as the
volume of water in a cooling tower is reduced through evaporation, the concentration of
water treatment chemicals and their by-products contained within the tower increases.
To maintain chemical and contaminant concentrations at prescribed levels, water is
periodically removed from the system through a process called "blowdown." The
blowdown wastewater needs to be subsequently discharged to a local wastewater
treatment facility or treated on-site to permit conditions before it can be discharged. A
key benefit of ozone is that it dissipates quickly and reduces the overall chemical load
found in the discharged water, making it easier to comply with regulations.
A further wastewater treatment process is supercritical water oxidation (SCWO). This
process takes advantage of the unique temperature and pressure properties displayed

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by water when it is used above its thermodynamic critical point – that is 374°c (705°F)
and 220 bar (3210 psi). Under these conditions water develops unique properties that
can be applied to completely oxidise a multitude of complex biologically inert organic
compounds, inorganic complexes and organic sludges.
Virtually all the organic content of the wastewater is effectively converted to carbon
dioxide, water and salts with almost no production of carbon monoxide, NOX or SOx
and has significant environmental compliance credentials.
SCWO is often categorised as “green chemistry” or as a clean technology – and the
elevated pressures and temperatures required for SCWO are routinely and
conveniently produced in industrial applications including petroleum refining and
process industries. It is also possible to incorporate the excess energy into power
generation schemes providing a source of waste to green energy on-site.
At the other end of the scale, low pressure oxidation (LoProx) is a wet air oxidation
application historically used to pre-treat recalcitrant high strength industrial wastewater,
prior to conventional aerobic bio-treatment. The treated wastewater is then blended
with biodegradable wastewater and then treated through an industrial wastewater
treatment plant.
Gurney says there are a myriad of approaches to dealing with industrial wastewaters
that have their own place on the water technology map and there is no single solution
that fits all. The desired level of treatment required, available budget and nature of the
wastewater water, amongst other factors determine the elements that make up the final
wastewater treatment process train.
Ends
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