- Frac flow-back water is water that is returned back to surface after the hydraulic fracturing process has occurred. Typically 25-40% of the volume of liquid injected returns to surface during the first 30 days. This wastewater can contain many of the following: hydrocarbons, proppant, guar gum, polyacrylamide, bacteria, chemical cross-linking agents, enzymes, reverse breakers and naturally occurring minerals and their salts.
- Treatment of this contaminated water is now possible using technologies such as the HydroPod™ to enable the treated water to be re-purposed for further use within oilfield processes or to be potentially cleaned further to a standard where discharge to the natural environment may be possible.
- In addition to frac flowback water, the oilfield produces large quantities of production water. This production water is water that is produced alongside the hydrocarbons from deep within the earth. This water is essentially water that has been locked up for millions of years alongside the hydrocarbons and returns to surface during the production process. This water is typically not as contaminated with chemicals as the frac flowback water but can contain higher concentrations of minerals and salts.
The HydroPod Module
- The HydroPod™ Module is self-contained within a 15ft container with all associated pumps, oxygen concentrators, ozone generators, measurement devices and electrocoagulation columns neatly plumbed into a single container.
- Water destined for treatment is typically stored in frac tanks, storage tanks or ponds. Baseline analysis of the water is undertaken to determine analyte levels prior to any treatment being undertaken.
- If oil levels are greater than 0.5% it is normally desirable to install an oil-water separator upstream of the HydroPod™ to ensure that the levels within the influent stream are below 0.5%.
- Ozone is used as a powerful and safe oxidizing agent that is easily generated in-situ using available oxygen in the air and an ozone generator. Ozone has a relatively short half-life (20 minutes) and has been used in a variety of different industries as an oxidizer and Ozone is used as a powerful and potent oxidizing agent. Ozone or O3 is a form of oxygen that has one more oxygen atom associated with the molecule than “normal” oxygen (O2). The addition of the third oxygen atom to form ozone from oxygen makes the molecule much more unstable, and within this instability lies the key to its oxidizing power. The ozone breaks down into oxygen radicles and oxygen molecules. The oxygen radicles are extremely powerful short lived oxidizing agents which will accelerate the natural breakdown of any molecule to is oxidized states. Hydrocarbons will break down to carbon dioxide and water, sulfur containing compounds, to sulfates, phosphorus containing compounds to phosphates, nitrogen containing compounds to nitrates, etc.
- Because ozone is unstable, it has a fairly short half-life under normal conditions and so it has to be produced and supplied continuously. The HydroPod™ uses an oxygen concentrator that is capable of concentrating oxygen present in the atmosphere (20.9%). This acts as the feedstock to the ozone generator which supplies ozone to the venturi valve into the process stream.
- The ozone is typically injected into the wastewater flow stream through a venturi to ensure the maximum amount of ozone is dissolved into solution upon entering the stream. In addition ozone pre-treatment may occur where ozone is sparged into a mixing tank prior to entering the HydroPod™ unit.
- Following ozonation, the wastewater flows through the electro-coagulation (EC system).
The Electro-Coagulation (EC) Process
- Electrocoagulation (“electro”, meaning to apply an electrical charge to water, and “coagulation”, meaning the process of changing the particle surface charge, allowing suspended matter to form an agglomeration) is an advanced and economical water treatment technology. It effectively removes suspended solids to sub-micrometre levels, breaks emulsions such as oil and grease or polysaccharides, and can oxidize and remove heavy metals from water without the use of filters.
- Coagulation is one of the most important physio-chemical reactions used in water treatment. Ions (salts and heavy metals) and colloids (organic and inorganic) are mostly held in solution by electrical charges. By the addition of ions with opposite charges, these colloids can be destabilized and coagulation can achieved through chemical or electrical methods or a combination of the two. The coagulant is added in the form of suitable chemical substances. The coagulant aid that is applied is a proprietary blend of aluminum salts and amines.
- Coagulation is brought about primarily by the reduction of the net surface charge to a point where the colloidal particles, previously stabilized by electrostatic repulsion, can approach closely enough for van der Waals forces to hold them together and allow aggregation. The reduction of the surface charge is a consequence of the decrease of the repulsive potential of the electrical double layer by the presence of an electrolyte having opposite charge. In the EC process, the coagulant is generated in situ by electrolytic oxidation of an appropriate anode material. In this process, charged ionic species—metals or otherwise—are removed from wastewater by allowing it to react with an ion having an opposite charge, or with floc of metallic hydroxides generated within the effluent.
- Electrocoagulation offers either an alternative or an adjunct to the use of metal salts or polymers and polyelectrolyte addition for breaking stable emulsions and suspensions. The technology removes metals, colloidal solids and particles, and soluble inorganic pollutants from aqueous media by introducing highly charged polymeric metal hydroxide species. These species neutralize the electrostatic charges on suspended solids and oil droplets to facilitate agglomeration or coagulation and resultant separation from the aqueous phase. The treatment prompts the precipitation of certain metals and salts.
- Chemical coagulation has been used for decades to destabilize suspensions and to effect precipitation of soluble metals species, as well as other inorganic species from aqueous streams, thereby permitting their removal through sedimentation or filtration. Alum, lime and/or polymers have typically been the chemical coagulants of choice. These processes, however, tend to generate large volumes of sludge with high bound water content that can be slow to filter and difficult to dewater. These treatment processes also tend to increase the total dissolved solids (TDS) content of the effluent, requiring the use of additional subsequent separation techniques to render the effluent suitable for re-use.
- Although the electrocoagulation mechanism resembles chemical coagulation in that the cationic species are responsible for the neutralization of surface charges, the characteristics of the electro-coagulated flock differ dramatically from those generated by chemical coagulation. An electro-coagulated flock tends to contain less bound water, is more shear resistant and is more readily filterable.
- In its simplest form, an electrocoagulation reactor is made up of an electrolytic cell with one anode and one cathode. When connected to an external power source, the anode material will electrochemically corrode due to oxidation, while the cathode will be subjected to passivation.
- An EC system essentially consists of pairs of conductive metal plates in parallel, which act as mono-polar electrodes. It furthermore requires a direct current power source, a resistance box to regulate the current density and a multimeter to read the current values. The conductive metal plates are commonly known as “sacrificial electrodes.” The sacrificial anode lowers the dissolution potential of the anode and minimizes the passivation of the cathode. The sacrificial anodes and cathodes can be of the same or of different materials.
- The arrangement of mono-polar electrodes with cells in series is electrically similar to a single cell with many electrodes and interconnections. In series cell arrangement, a higher potential difference is required for a given current to flow because the cells connected in series have higher resistance. The same current would, however, flow through all the electrodes. On the other hand, in parallel or bi-polar arrangement the electric current is divided between all the electrodes in relation to the resistance of the individual cells, and each face on the electrode has a different polarity.
- During electrolysis, the positive side undergoes anodic reactions, while on the negative side, cathodic reactions are encountered. Consumable metal plates, such as iron or aluminum, are usually used as sacrificial electrodes to continuously produce ions in the water. The released ions neutralize the charges of the particles and thereby initiate coagulation. The released ions remove undesirable contaminants either by chemical reaction and precipitation, or by causing the colloidal materials to coalesce, which can then be removed by flotation. In addition, as water containing colloidal particulates, oils, or other contaminants move through the applied electric field, there may be ionization, electrolysis, hydrolysis, and free-radical formation which can alter the physical and chemical properties of water and contaminants. As a result, the reactive and excited state causes contaminants to be released from the water and destroyed or made less soluble.
- It is important to note that electrocoagulation technology cannot remove infinitely soluble matter. Therefore ions with molecular weights smaller than Ca+2 or Mg+2 cannot be dissociated from the aqueous medium.