Produced Water Treatment Objectives
Produced water usually represents a waste product in the petroleum industry; it is more often than not only a cost that must be controlled to enhance project economics.
Treatment of produced water may be required in order to meet pre-disposal regulatory limits or to meet beneficial use specifications.
The parameters might be specific constituents of concern such as ammonia or barium that can be toxic to sensitive animal and plant-life. Or the parameters may be more broadly-based such as Total Dissolved Solids (TDS) or Sodium Adsorption Ratio (SAR) that can affect several aspects of the environment.
Produced Water Characteristics
Parameter | Typical Range – Offshore | Typical Range – Onshore |
pH | 6.0 – 9.0 | 5.0 – 9.0 |
Oil & Grease, mg/l | 100 – >10,000 | 10 – >10,000 |
TSS, mg/l | 20 – 500 | 20 – 1000 |
TDS, mg/l | 10,000 – 50,000 | 2000 – 250,000 |
Total Hardness, mg/l CaCO3 | 2000 – 8000 | 200 – 50,000 |
Chloride, mg/l | 5000 – 30,000 | 1000 – 125,000 |
Sulfide, mg/l | 0 – 500 | 0 – 500 |
Silica, mg/l | 0 – 10 | 0 – 250 |
COD, mg/l | – | 500 – 2500 |
Boron, Barium, Strontium, Iron, mg/l | 0 – 100 | 0 – 10,000 |
Treated Produced Water Quality
Parameter | Offshore Disposal | Onshore (Deep Well Disposal) | Onshore (Irrigation/ Surface Water) | Onshore (Recycle as OTSG Feed Water) | Onshore (Recycle as Drum Boiler Feed Water)1 |
pH | 6.0 – 9.0 | 6.0 – 9.0 | 6.0 – 9.0 | 6.0 – 9.0 | 8.8 – 9.6 |
O&G, mg/l | 15 – 40 | 1 – 10 | <10 | 1 – 10 | 0.2 |
TSS, mg/l (<5 µm) | – | < 1 | < 30 | < 1 | Nil |
TH, mg/l CaCO3 | – | – | SAR < 10 | < 1.0 | < 0.3 |
BOD5, mg/l | – | – | < 30 | – | <0.2 |
DO, mg/l | – | < 0.01 | 2 | < 0.007 | <0.007 |
TDS, mg/l | – | – | 500 – 2100 | < 8,000 | 5 – 10 |
Silica, mg/l | – | – | – | < 50 | 0.2 – 0.4 |
The General Objective of the produced water treatment technologies are
- De-oiling – Removal of free and dispersed oil and grease present in produced water. (API ,CPI , Nano-DAF™)
- Walnut Shell Filter , Walnut shell media filters designed for oily water filtration.
- Soluble organics removal – Removal of dissolved organics.
- Disinfection – Removal of bacteria, microorganisms, algae, etc. (RiteDOS™ , BioCell-C™ ,BioCell-B™ , Bio-Zone™ , UVDOS™ )
- Suspended solids removal – Removal of suspended particles, sand, turbidity, etc.
- Dissolved gas removal – Removal of light hydrocarbon gases, carbon dioxide, hydrogen sulfide, etc.
- Desalination or demineralization – Removal of dissolved salts, sulfates, nitrates, contaminants, scaling agents, etc.
- Softening – Removal of excess water hardness.
- Sodium Adsorption Ratio (SAR) adjustment – Addition of calcium or magnesium ions into the produced water to adjust sodicity levels prior to irrigation.
- Miscellaneous – Naturally occurring radioactive materials (NORM) removal.
Selection of produced water treatment structure is often a challenging problem that is steered by the overall treatment objective. The general plan is to select the cheapest method – preferably mobile treatment units which assure the achievement of targeted output criteria. In this way technology can be positioned in the field for optimum convenience and the technology can be fine-tuned to meet specific end-uses for the water.
Produced Water Treatment Technologies
Treatment objectives can be addressed by treatment technology options. The options can differ in their inherent facility requirements, capital costs, operating expense, and waste streams; all three factors can be important to the oil and gas operator. Some technologies may have large space requirements that may not be possible in some oil and gas installations.
Walnut Shell Filter
Deep Bed Nutshell Filters are downflow, deep bed, nutshell media filters designed for oily water filtration. As the liquid passes through the walnut shell media filter’s media bed, oil and solids are efficiently retained in the bed and can easily be backwashed out of the media bed with the patented rotating media scrubbing assembly. Nut Shell filtration is a water polishing technology used to remove hydrocarbon liquids from water down to very low concentrations (in some cases <3 ppm).
WTPL Nut Shell Filters are designed to remove fine solids and hydrocarbon liquids from water as a secondary or tertiary stage of water treatment. Oily water is introduced into the vessel and distributed to ensure even flow throughout the vessel. As the water flows through the vessel the Nut Shell media adsorbs the oil and fine solids allowing clean water to exit the vessel. Nut Shell Filters are typically used to achieve outlet oil concentrations of <5 ppm, although concentrations down to <1 ppm can be achieved.
Nut Shell Filters are also capable of removing particulate solids down to 3 microns in size. During operation, oily water enters the vessel and flows through the Nut Shell media. The media physically adsorbs the oil and fine solids and prevents these from passing through the vessel. Over time the oil and fine solids accumulates on the media. To regenerate the filter media, vessel flow is reversed thereby washing the adsorbed oil and fine solids off the media.
There are several methods of regenerating Nut Shell Filters. These methods all require the back flow of the produced water and a physical disturbance of the Nut Shell media bed, whether with an agitator or physical motion of the Nut Shell media. This movement displaces the media bed and releases the filtered oil and fine solids from the media
Ion Exchange
Heavy oil recovery processes have traditionally used “once through” steam generators (OTSG) to
produce high pressure steam for injection into geological formations containing heavy oil. The heat
given up by the condensing steam fluidizes the heavy oil and allows the oil/water mixture to be
brought to the surface. The oil is recovered as product and the water, referred to as produced water, is de-oiled and treated for feedwater to the OTSG.
The typical treatment method for produced water is warm or hot lime softening (WLS or HLS), filtration,and weak acid cation exchange (WAC).The ion exchange process effectively removes arsenic, heavy metals, nitrates, radium, salts, uranium, and other elements from the produced water. Ion exchange is a reversible chemical reaction wherein positively or negatively charged ions present in the water are replaced by similarly charged ions present within the resin. The resins immersed in the water are either naturally occurring inorganic zeolites or synthetically produced organic resins. When the replacement ions on the resin are exhausted, the resin is recharged with more replacement ions.
Ion Exchange Resins
Ion exchange resins are classified as cation exchangers, which exchange positively charged ions and anion exchangers, which exchange negatively charged ions. The resins are further classified as:
- Strong Acid Cation (SAC) Resins
- Weak Acid Cation (WAC) Resins
- Strong Base Anion (SBA )Resin
- Weak Base Anion (WBA ) Resins
Hot Lime and Cold Lime Softening Process
The objective of hot or cold lime softening is to reduce the levels of impurities, specifically the level of hardness species such as calcium and magnesium, dissolved in the raw water supply. The treated water must of sufficient purity to provide boiler make-up water of acceptable quality for steam generation, and to accomplish this at reasonable cost. The basic process is the reduction of hardness and metal ions by precipitation as low solubility solids.
Cold Lime Softening Process CLSP:
- It is carried out at room temperature (25 – 30 degree C)
- Reactions take place at slow rate.
- The precipitates formed are very fine so the use of coagulants is essential.
- Filtration of the precipitates formed is difficult.
- Dissolved gases are not removed.
Hot Lime Softening Process HLSP:
- It is carried out at elevated temperature around (95 -100 Degree C ).
- Reactions take place at fast rate.
- Coagulants are not required.
- Filtration is easy and faster at higher temperature.
- Dissolved gases such as CO2 are removed.
NanoWater is one of the world’s leading companies in the Water and Wastewater treatment industry, NanoWater Design , Engineer , Procure ,installation , commissioning with Operation and Maintenance .Please feel free to contact to discuss your project at sales@nanowaterusa.com