Wastewater Treatment Problems in the Oil & Gas Industry
The oil and gas sector is the core of many other branches of industry. However, this industry is listed among the TOP ten largest water consumers and causes significant water pollution.
The oil and gas industry is faced with depleted oil wells, higher standards for environmental protection and due to the current global market, lower prices. The environmental situation is exacerbated by a lack of water at most drilling sites. At many locations, the wastewater is not adequately treated. Wastewater treatment of produced water in oil fields is now considered to be urgent.
- Standards for Oil & Gas
- Requirements for wastewater
- Process Technology
- Comparison of process technologies
API and ISO standard for Oil & Gas
It is a challenge to find economical solutions to protect the environment by conserving sustainable resources. Clearfox (PPU Umwelttechnik GmbH) has taken on this challenge and provides a range of solutions.
On the date of this article, we have broken new ground in terms of approvals, compliance and quality, and we are implementing the following standards:
- Quality standard for products and services
- ISO 29001 Oil and Gas, closely based on the API (American Petroleum Institute), which also belong to the same for this industry. They are designed to maintain the security and quality of claims in the oil and gas industry.
The basis for an accredited consideration of DIN EN ISO 9001, ISO 29001 is an extension for companies that are in the supply chain of the oil and gas industry.
We use well-known technologies that we have been implementing successfully for years and that are proven. However, new directions of thinking must also be taken. The target can be simply identified as high quality standards, but still economically viable solutions, that are qualitatively and ecologically sustainable.
The wastewater on oil well fields can be contaminated in different ways. We use our approved-and-tested module system and select the technology used with an evaluation procedure that is always tailored to the specific requirement.
With our modular system, we have experience worldwide in cleaning a wide variety of wastewater types. We are a members of the leading professional associations in Germany:
- VDE-Association German Technologies Electrical, electronical and information
- VDI-Association German Engineers
- ATV/DWA- German Waste water association
First of all, it must be determined for what purpose cleaned water should be used for:
- Recycling and then reuse as potable water, flow tests, fire protection pumps
- Direct discharge into the sea with/without dilution (authorities requirements)
- Discharge in a river or a lake
- Discharge to disposal or injection back into the underground
- Human use
Then the effluent standards are defined. Depending on the effluent requirements, different technologies are used in combination, or the water is divided into separate waste streams. The produced water, generated by the onshore production process of oil or gas depends on the rock, the drilling depth and the extraction chemicals.
Depending on the age of the drilling field, this production water (mostly hypersaline salty solution) can be produced to the same volume as the oil actually extracted. The specifications for a project for the purification of production wastewater are very extensive, essentially these typical aspects have to be considered:
Scales in pipes and technical devices
Precipitates from brine build a layer on all surfaces. This creates incrustations and deposits in pipes, as well as in technical devices. By adding inhibitors, i.e. surface-active substances, incrustation should be reduced. A scale cannot be completely avoided, and as a result the inhibitors further pollute the water. If the oil wells dry up, more water and brine must be injected, which can cause a scale thicknesses of up to 10 cm. The deposits or possibly entire pipelines must be mechanically removed regularly. This results in high costs and pollution.
The Clearfox modules minimize these costs. The modules are installed in parallel. This enables bypasses to be switched in a simple manner, backflushing to be carried out and complete pipe sections to be replaced even during operation. The focus is here a fast and simple scale management, as well as a high grade of redundancy.
NORM and TENORMS
Naturally occuring radioactive materials (NORM) occur predominantly as solids directly at the borehole and the further pipelines, but are also subsequently concentrated wherever there is a separation of oil and water.
These technically modified nuclides (TENORM= technical enhanced and concentrated of naturally occuring radioactive materials) then behave similarly as their radiating heavy metal relatives and they are even contained in the water phase and as gaseous secondary products through complex formations. The exposure of radioactive substances can lead to health damage. Furthermore, all technical facilities that come into contact with it are radioactively contaminated.
Clearfox modules prevent dangers to the health of operating personnel. Almost all modules are constructed as closed containers in the format of an ISO sea container. There is no direct connection to gases or liquids. All parts in contact with the product are easy to exchange and dispose of. Aeration and ventilation lines can be forwarded separately to another location.
Heavy metal pollution
Depending on the drilling field, the wastewater produced can contain a high concentration of heavy metals, with only individual heavy metal groups usually being present in high concentrations.
Below heavymetal concentrations in a sample of produced water from the Southern Mediterranean [mg/l]:
These have to be removed from the produced water. This can be done by oxidation or chemical precipitation, but in some cases complex compounds must first be formed, which in turn are then removed from the wastewater by means of sedimentation and filtration. This is usually in competition with ion exchangers, when there are just slightly present above permitted limits.
In many cases the operational philosophy is based only on one method during operation.
The Clearfox module system allows various combinations of heavy metal removal technologies., always considering the long time operating costs.
Sludge and sediments
In the case of a phase change due to temperature variation, pressure change or pH change, sludge is produced during crude oil production. This sludge must first be separated from the oil and water phase. Further sludge is created by wastewater treatment of the produced water. In particular through the breakdown of carbon compounds and the addition of precipitants, flocculants. To reduce the disposal volume, the sludge must be dewatered. This is usually done by centrifuges or presses. The radioactive substances (norm, TENORM) concentrate more in the sludge than in the water phase. For this reason the sludge must be considered as special radioactive waste.
The sludge amount can be estimated as follows.
Clearfox modules for sludge dewatering are preferably housed in sea containers. During normal operation, the operator only comes into contact with the discharged / dewatered sludge The sludge presses work with a system that has been tested for oily-water sludge. In these specially modified dewatering machines, discs are rotating and they separate the sludge without clogging by using centrifugal force and pressure force. The operating effort is minimized. The risk for the operational personnel is extremely reduced.
- Group 1 with an elimination level below 20% radionuclides of the elements sodium, potassium, rubidium, cesium, strontium, arsenic, antimony and iodine.
- Group 2 with a degree of elimination between 20% and 70% of the radionuclides elements tellurium, uranium, niobium, manganese, technetium and cobalt.
- Group 3 with a degree of elimination of over 70% radionuclides of the elements beryllium, tin, silver, gold, zinc, cadmium, mercury, scandium, yttrium, cerium, gandolinium, chromium and iron
Biocides and inhibitors
By adding emulsifiers and bioinhibitors, the produced water can reach a high degree of toxicity. If biological processes are used, this can mean (in addition to high salt contents) a reduction in the biological activity up to total breakdown. On-site tests can be used to estimate the ability of the water regarding biodegrability. Generally, biological processes should be used which are protected against toxicity by building EPS matrix.
Clearfox modules work with biofilm growth carriers after the fully immersed ventilated fixed bed. Through previous laboratory analysis or better degradation tests with the wastewater on site, it can be estimated relatively accurately how the systems should be dimensioned for each project. Since this is also subject to uncertainties, it is advantageous that the bioreactor modules can simply be expanded with the addition of further modules. Basically, the biofilm typical of process technology (i.e. non-floating organisms) is considered the best available technology.
This is based solely on the fact that the biofilm protects itself with extracellular polymers. This is far superior than conventional activated sludge processes. In order to be able to make an accurate treatment efficiency determination, a pilot test is required. Clearfox systems, which are properly designed are a robust and stable process technology.
Salinity (high TDS)
Biological treatment in order to reduce COD and BOD loads is the most cost effective and most environmentally friendly process in the long run.
However, the high salt content makes biological cleaning of produced water in oil and gas fields very difficult. In order to colonize halophilic (salt-loving bacteria that reduce carbon and nitrogen in this environment), sessile methods with a sludge age independent of flow must be used. Simple activated sludge or membrane biological treatment systems (MBR=Membrane Bio Reactor) cannot be operated effectively because the bacterial concentration cannot be achieved, or dead end filtrations can clog & contaminate. Furthermore, when using dilution effects the hydraulics are limited very quickly.
The Clearfox FBR module works with a constantly submerged aerated fixed bed (FBR=Fixed Bed Reactor), a sludge return or a filtration membrane is not necessary. By cascading with different biocenoses,relatively high salinity levels can be tolerated. The system is open and operated by gravityflow. The degradation process is freight related, not quantity related. Therefore, dilution effects can be used in extreme salt contents. The secondary sludge that is formed does not have to be recycled, but is simply removed by gravity. The Clearfox FBR module is regarding Capex and Opex unbeatable, when it is used for carbon and nitrogen degradation.
Corrosion, material resistances
All parts of the WWTP in touch with the wastewater must be resistant to hypersalinity, chlorides, extreme pH values, various hydrocarbons and sulphur compounds.
As far as available, Clearfox always uses the best available materials for the pipwork, the technical equipment and the conveyor equipment with moving parts. For example, the proven principle of airlift pumps is used for the feed pumps. They do not contain any moving or blockable parts, and the part in contact with the product is made of either silicone/resistant thermoplasts or super duplex steel of category 4.
The austenitic-ferritic steels are welded together according to the guidelines DVS 0946 of the German Welding Association (DVS).
Another example are the plastic components used within the reactors, these are seamlessly made from highly cross-linked plastic in one piece (rotation process) and are resistant to aggressive attack at a high temperature range. As soon as welded connections are necessary, they are carried out by qualified personnel in accordance with EN 13067
Author’s note: Diffusion effect of hydrocarbons is a separate topic and will be discussed elsewhere
Required space and Mobility
Onshore drilling companies usually have the right for a limited time to use a limited area for their business. The cleared Well Pad area must be efficiently used for active wells, additional drillings and geosonic measurements. It is adviced to keep this area clean and keep it free from obstacles.
A waste water treatment system realizes not a direct profit out of the drilling, it covers environmental and health tasks. It should take up less space and must be able to be rebuilt elsewhere if necessary. (mobile sewage treatment), possibly it must be stackable in order to safe space. In our view, however, a compromise must be found between space-saving solutions, which usually have high operating costs, and large-scale wastewater treatment plants that are simple and cheap to operate.
With the Clearfox modular system (in ISO seacontainer) we meet every demand. The containerized system is robuste and well proven in thousands of applications.
Generally the housing has an ISO conformity as following:
|668||–||Series 1 freight containers-Classification, external dimensions and ratings|
|6346||–||Coding, identification and marking for freight containers|
|1161||–||Specification of corner fittings for series 1 freight containers|
|1496/1||–||Specification and testing of series 1 freight containers.|
Part 1 : General cargo containers for general purposes
On the one hand, the Clearfox container system is mobile, on the other hand we have different cleaning principles to be installed- we can choose from, all of them are designed, produced and assembled and finally tested in our factory in Germany.
Fire protection, Electrical safety
In the past, oil drilling fields have always been the focus of attention due to explosion and fire risk. The conveyed or separated liquids logically create a high hazard potential due to gases, mists or dusts. Risk analysis must first determine exactly where and how high the risk potential is. The system technology must be selected accordingly. In the simplest case, the motors and drives are explosion-proof, in extreme cases the entire area must be sealed off and separately ventilated. Furthermore, it must be considered in advance how best to react in the event of a disaster. The action plan can range from simple fire extinguishers to evacuation measures for limited areas.
This includes every technical part of the plant and thus also the wastewater treatment plant (as well as sludge treatment) of the water produced. With the Clearfox modules, the individual requirements are taken into account beforehand, in accordance with the client`s specification. Generally according American Standard fire protection ANSI/NFPA 70
This can mean to partially equip the electrical equipment redundantly, or equipping appropriately encapsulated drives. However, it is also relatively easy to organize that, if certain areas of the ISO container are ventilated separately or given decentralized shutdown options. The Clearfox module system suits perfectly here, because the areas are clearly defined.
Usually in this branch the clients specification follows guidelines as in example below:
- American Electro association
- EIA RS232/485
- International Elektro commission
- IEC 60529/60079-7 and various from IEC 61xxx-part
European Elektro Standards
- CENELEC EN50020
- Normengemeinschaft Mess und Regeltechnik
- NAMUR Standards, as NE 43
display und Visualizing for processes
- ISA-S 18.1/51.1/S5.5/S71.04
Basically we meet with our process technology all guidelines, which are binding for this branch oil and gas, such as:
- Electronial manufactorer association/ american engineers
- ANSI/Nema ICS 2-6
- ANSI / IEEE 488, 472/802
Our inhouse engineering team provides all knowhow and services due to our international experience in order to meet all international guidelines
Characteristics of produced water
The usual sewage parameters for assessing the water quality or the efficiency of the treatment technology are as follows: (different analyses from Sibiria, USA, South America and Mediterranean Sea)
- pH 4-7
- Oil content can be up to 1 g / l but is mostly removed from the produced water before (since it is the actual valuable substance)
- Salt concentration (salinity) The salinity is an typical characteristic of the water produced. It can contain over 180.00 mg / l. This can usually be seen from the TDS, which essentially consists of sodium chloride. The chloride content is also an important indicator. Many produce water are considered hypersaline, i.e. oversaturated with salt.
- Totally dissolved solids (TDS) up to 300.000 mg/l, mainly caused from NaCl
- Totally suspended solids (TSS) between 50 and 1000 mg/l
- Temperature 15 bis 35 Grad Celsius regional depending
- Aromatic- (BTEX) as Benzene, Toluol, Etylenbenzol, Xylol, Polyaromatic (PAH), Alkyl phenols
(PAH and alkyl phenols are not so good solable in the waste water, so often they are in the dispersed oil, which are suspended smaller drops oil. Amount depends on pretreatments and the kind of well.)
- Organic acids such as Benzoic acids
- Dispersed hydrocarbons, naphta residues
- Sum parameter for oxygen demand
- Chemical (COD)/ Biological (BOD)
These 2 typical sum parameters and the relationship to each other provide information on how well the water is degradable by bacteria.
BOD values differ from 500 to 3000 mg / l
COD values differ from 2000 to 20000 mg/l
- Nitrogen, Phosphorus (only seen as traces in relation to carbon, but may contain an excess of nitrogen, which must be removed for some direct discharges)
- Sulphides, depending on injection water
- Heavy metals (Boron, Cadmium, Copper,Mercury, Iron and much more)
- Radioactive materials (NORM, technically enhanced), as already discussed before
- Uranium, thorium, radium with his decay products as well as radon Lead 210, potassium 40, polonium (Partially gaseous, especially concentrated in the sludge and deposits, loads of up to 15000 Bequerel / gram, average waste load 100 Bq / g)
What Process Technology for Treating Produced Water from Oil and Gas?
Process technology is a challenge. There is no universal process technology for the purification of produced water from oil or gas fieldsr. Of course Manufacturers always present their own product as if it were a panacea. If one evaluates according to criteria such as cleaning performance, space requirements, operating costs, investment costs, as well as sustainability (further pollution), one comes to the conclusion that different process technologies have to be combined.
Is necessary to remove easily screenable materials from the system and to protect the downstream units. Mostly already included in the pretreatment for the demulsifiers. Sedimentation tanks are simple, but only suspended solids are removed and the space required is high. Screening systems mechanically remove all particles, they are more effective, save space, but require more maintenance.
The goal of oxidation is to bring pure carbon compounds from a dissolved to the suspended condition, in order to make compounds biodegradable, to oxidize heavy metals , to remove organic and inorganic components from the wastewater. Is more or less universally applicable and always suitable. In the typical process, strong oxidizing agents such as ozone, hydrogen peroxide with and without UV light expansion (e.g. Fenton process) are necessary.
In the catalytic wet oxidation, compressed air for the oxidation at high pressure is supersaturated in the water and the temperature is increased. Requires reaction volumes and expensive vessel construction. In both cases, the wastewater has to be post- treated for the subsequent processes, ozone requires high safety regulations, and the electricity requirement is high.
In the electrochemical advanced oxidation / reduction (AEO), a potential is applied between two surface-coated electrodes, no chemicals are added, the process is non-pressure, only harmless hydrogen is produced, the oxidation takes place on the surface or indirectly through radical formation. The doping of the electrode material must be adapted to the challenge (heavy metals, NORM, COD, AOX, etc.).
The initial investment (e.g. diamond –type doping) can be large with high-quality doping. Electricity costs are moderate, if the wastewater is highly conductive, the electrical consumption drops. Can be used for almost all contaminants in the waste water produced, up effluent required for direct discharge.
Dissolved Air Flotation (DAF) / with precipitation (DAP)
Flotation (in connection with precipitation and flocculation) has a very high physical separation effect, especially for oils or emulsions that have been splitted before. This is necessary as standard technology, particularly in the case of wastewater produced, to relieve the loads to subsequent process steps. Residues are the removed / precipitated un-dissolved substances that are contained in the wastewater. The investment costs are low, the type of sludge removal determines the operating costs.
Chemical treatment (precipitation, hydroxide formation)
Adding precipitants to specifically bind substances in-soluble in water (heavy metals, COD, phosphorus) which then have to be removed (flocculation, flotation, filtration). The investment costs are negligible, the technology is simple. With the addition of chemical substances, more and more sludge accumulates, due to the resulting compounds. The cost of the chemicals can increase operating costs extremely. The resulting sludge may contain compounds that are toxic. At high NORM concentrations, the system must be designed without any deposits or chemical sinks inside the system components.
Microfiltration (sand anthracite, filter drums)
Depending on grade of filtration, as filter drums or multilayer sandfilters (anthracite) to remove suspended solids (e.g. from the precipitation). Low investment costs, relatively simple, with high NORM – concentrations sinks must be avoided.
Biological treatment (aerobic)
Suitable microorganisms oxidize and reduce carbon and nitrogen. These must be in touch with the wastewater long enough under suitable milieu conditions (oxygen, no inhibitors, oxygen present, nutrients). A very high sludge age (high bacterial concentration ) is required for the wastewater produced. This can only be achieved with sessile processes (FBR) or increased sludge concentration, by passing the water through membranes (MBR).
Depending on the salinity, the biological activity drops, the wastewater has to either be diluted or designed larger. Halophilic bacteria reduce BOD and thus the COD load in produced wastewater. Very high space requirement for reaction volumes (only possible in rare cases for offshore). Little residues, very low operating costs, investment costs depending on the container requirements, mostly moderate, can only be used if biodegradable.
Basically, the wastewater produced can be inoculated with specially bred organisms or concentrated bacteria, or the naturally occurring ones in the system can be increased. This takes a long start-up phase, but it is safer and permanent at Zero cost. In any case, however, the BOD must prove before that, the wastewater is degradable, the availability with the TOC and the ratio of the chemically oxidizable to the biologically oxidizable are sufficient for an economic process. Submerged fixed bed reactors (FBR) are very complex to build, but cascaded ones are ideal for treating produced wastewater.
Sequencing Batch (SBR) reactors and MBR reactors are not ideal because the free-floating organisms only build up insufficient EPS and only monocultures. Membrane bioreactors (MBR) are like a combination of activated sludge biology and filtration through membranes. Investment costs are low, but can only be used to a limited extent with wastewater produced and can be used due to the high degree of pre-cleaning. Membrane resistance and good floc formation in biology are the prerequisites for stable operation. Trickling filters are very well suited to the wastewater produced, but only in small quantities, since the space requirement exceeds the costs.
Basically, biological processes for waste water from the oil and gas industry can be favoured, if a high effect is to be achieved with little effort and the biodegradabilty has been checked before. This is usually justified even if the wastewater has to be diluted to lower salt concentrations (up to 40-60,000 mg / l NaCl) in order to provide the right environment for halophilic bacteria
Ultrafiltration, Nanofiltration, reverse Osmosis
All are used to remove fine suspended particles that have arisen from previous steps or that were already in the wastewater. Ultrafiltration (depending on the cut off of the membrane) competes with simple microfiltration. For each produced water, attention must be paid to the achievable effect and the effort. This must be in proportion. The nanofiltration has high investment costs, the operating costs are increased.
Depending on the process requirements, however, this can be a good option in connection with previous technologies to bring the wastewater to a discharge quality via the direct filtration path. At first glance, reverse osmosis is suitable as a universal separation option for almost all ingredients in the produced water. However, reverse osmosis is usually only worthwhile here if a partial flow is to be prepared for human use.
High salt levels generally reduce the amount. Reverse osmosis requires good pre-treatment (mostly biology + nanofiltration) and is very expensive to invest and operate (chemicals, membranes). In order to keep the flux rates high, chemicals must be added to prevent membrane fouling.
Adsorption and ion exchange
For small quantities and when choosing the right materials, a selective removal of many ingredients is possible. Requires good pre-treatment, moderate investment cost, operating costs can be extremely high. Usually only suitable as a partial flow for polishing.
Comparison of process technologies for produced water
|process||Capex||Opex||stability||secondary pollution||efficiency||Space required|
|Mikro/Sand Antracit Filters||+++||+++||+++||+++||+||+|