The water we have on earth is all we have. New water is not created as years pass. The hydrologic cycle explains the circuitous route that water takes. Water that is here now is consumed, transpired or evaporated and then journeys away, only to come back again as precipitation or groundwater to start the cycle again. In municipal water treatment, water is simply a transport mechanism. It was clean to start with prior to washing clothes or flushing the toilet. Reclamation facilities exist to return the water back to its natural state and discharge it to a local water body. And the water cycle goes on.

DO

By merely looking at water one cannot tell if it has oxygen in it other than the hydrogen and oxygen molecules. But the molecule is not the oxygen that improves water quality and allows aquatic biota to survive. Dissolved oxygen (DO) must be present in ample concentration to support robust water health and is widely understood as a powerful oxidant. Even a stagnant pond can be brought back to life in short order by effectively infusing DO into the water. In nature, the cascading of a mountain stream over rocks enriches the DO level, ensuring trophy species of fish thrive. Healthy DO levels promote beneficial biologic activity from naturally occurring microbes. Increasing DO in flowback impoundments initiates a plethora of water quality improvements.

Evaporation

Many centralized treatment ponds also are known as evaporative ponds or pits. Two key components to evaporation in a water impoundment are heat (sunshine) and surface air movement (wind). Other factors include altitude, vapor pressure at the water surface and vapor pressure of the surrounding air (Potts, 1988). One can discover myriad evaporation formulas based on geography, latitude, climate and the like, but the aforementioned factors are in play regardless of the play. During interviews with three different evaporation pit operators, it was discovered that none deployed any scientific methodology to determine evaporation rate but all were convinced that their aeration methods helped.

Advanced aeration improves evaporation

There are multiple phenomena occurring when fine bubbles and coarse bubbles are introduced in a subsurface fashion in a water impoundment. Firstly, the rising bubbles cause surface disruption and may add momentum to the air, increasing the rate at which the humid air is removed from the surface, a critical aid to evaporation (Brutsaert, 1982). Secondly, when air is introduced into water, bubbles are formed, and diffusing vapor in the water migrates to the interior of the bubble (Burkard and Van Liew, 1994). The resultant migrant vapor in the bubble reaches 100% relative humidity and is released during the breakup process at the surface (Helfer, Lemckert & Zhang, 2012). The combination of coarse and fine bubbles delivered from the water pit floor, effectively oxygenating and destratifying the entire water column, is considered advanced aeration.

As mentioned earlier, a casual observation of existing fractured water impoundments notes some sort of surface aeration, weeping systems, periodic air jamming or other air sparging. By field practice, the operators realize the advantages of using air in water management.

Scorecard for aeration efficiency

In 1984 the American Society of Civil Engineers adopted aeration efficiency standards. The Standard Oxygen Transfer Efficiency (SOTE) test was developed to put all vendors/methods on an even playing field. The key to SOTE is evaluating the efficiency of introducing oxygen into a body of water. The trial is recorded at sea level in clean water at 20 C (68 F). Bubble size is the single biggest factor to determine SOTE. Fine bubbles (less than 1/8 in.) tend to rise more slowly, providing better oxygen transfer efficiency. Coarse bubbles (greater than 1/4 in. and often 1 in. to 2 in.) are more effective at mixing. Other factors in analyzing an aeration system are flow rate, depth, temperature, elevation, layout, alpha and beta factors, and maintenance. Chart 1 shows Standard Aeration Efficiency (SAE) for various aeration types at different water depths. The higher SAE rating is the most desired as it takes into account implied utility costs. The measurement is pounds of oxygen per hp-hr expressed as lb-O2/hp-hr.

When advanced aeration is introduced to an impoundment, many water quality parameters begin improving in correlation to residence time or exposure. Fine bubbles can effectively introduce DO into the target water body. Assuming anaerobic conditions, a transformation from anaerobic (DO < 1.0 mg/l) to aerobic (DO > 1.0 mg/l) water occurs. Sulfate-reducing bacteria (SRBs) are susceptible to the oxidation potential of DO (Characklis, L., Lee, W. 1994). Initiating the oxidation of SRBs inhibits the formation of hydrogen sulfide (H2S), a colorless gas that is corrosive, poisonous, flammable and explosive. Effective infusion of DO also begins the oxidation of H2S, which helps mitigate multiple issues, including odor (Sharma, K., Yuan, K., 2010). With increased DO, the pH level in the water body begins to move toward stabilization. This is valuable when it is understood that a target pH value between 7.0 and 8.0 is optimum for maximum H2S removal (Chen, K., Morris, J., 1972). The water body also begins stripping CO2, light hydrocarbon gases and volatile organic compounds due to redox, which is a reflection of its oxidative and reductive capacity.

DO transforms metals such as iron and manganese to their oxidized state. This allows them to be filtered out or settled to the bottom of the pit. Guar residues and other organics also begin oxidizing, which fosters increased light transmissivity or clarity to the water. The surface disruption caused by advanced aeration also reduces surface matting and encrustable formations. This benefit is twofold as it allows deeper access to the water body by UV rays and provides a naturally corralled surface area for potential skim harvesting of migrant hydrocarbons. These documented water quality improvements offer contaminant load reduction for downhole reuse or follow-on treatment trains such as electro-coagulation, reverse osmosis and the like.

Anaerobic water pits cause certain treatment chemicals to be oxidized or partially consumed. The “serendipitous uptake” by nontarget contaminants wastes chemicals and money. This can cause the chemicals to be less effective than expected, leading to costly overdosing. Increasing DO with advanced aeration will optimize subsequent chemical use and increase operator control and understanding of water pit behavior.

Taking control of fractured water impoundments is a critical first step in subsequent treatment, recycling, evaporation or other intended reclamation of the water. Cost control and process improvement are to be gained by addressing the water chemistry early on in the process, promoting improvement each step of the way. A cursory observation of the literature coupled with the data presented in the chart suggest that using subsurface aeration deploying coarse and fine bubbles would offer the water quality benefits outlined herein. Notwithstanding nuances from site to site, advanced aeration merits serious consideration in the continuum of water management.

At press time Arcadian Technologies just completed installing a full-scale advanced aeration system for a client in the Uintah Basin. The company also is partnering with an instrumentation company to provide online real-time water quality parameters from the installation site. These data will afford regional drillers valuable information to consider reclaimed water for downhole reuse.

References available.