Bubble Size, Retention Time, Oxygen Transfer Efficiency are All Part of the Equation
Produced water management has undergone many changes over the years. Water recycling is more prevalent today and that means storage of more and more produced water.
The key to a successful produced water recycling program is the aggregation of enough produced water in the right area, in sufficient quantity and quality to allow for a high utilization rates. The process of aggregating produced water leads to larger pits and longer retention times. Even with a successful bacteria control program, stored water will exhibit bacteria regrowth. Historically, this has given rise to pit treatments but unfortunately, pit treatments are highly inefficient and very expensive.
Pit treatments can be plagued with poor mixing and poor oxidizer or biocide transfer throughout the water body and ultimately, higher costs. Even if mixing and transfer issues can be normalized, the process requires multiple repetitions since individual pit treatments are short lived. The resulting higher treatment costs led the industry to a search for a better way. This gave rise to aeration.
Aeration is the introduction of air into the water body to allow oxygen to be absorbed into the water. Oxygen is a mild oxidizer and can provide control of bacteria, iron and sulfides. Although it sounds simple, effective aeration is complicated and there are many variables to be considered.
There are two distinct types of aerators: submersible and surface aerators. As expected, submersibles sit as close to the bottom as possible and surface aerators are positioned on or near the surface.
Surface aerators serve a specific purpose, to provide an odor cap. Surface aerators do an excellent job aerating the top few feet of water, providing a few feet of sufficiently aerated water to absorb odors from unaerated water below. This is a useful feature for certain types of lagoons and ponds. However, in a produced water pit where water is pumped from below, there is no practical benefit from this type of aeration. Water will be of poor quality until pumping reaches the top few feet of the water body.
A second reason surface aerators are used in ponds and lagoons is that some natural lagoons and ponds can contain a significant amount of sediment which makes submersible aerators difficult to operate and impractical to install. Surface aerators also are useful in mitigating lighter organics that tend to float on top of the water. Trihalomethane (THM) is a byproduct common to chlorine-based disinfection. Surface aerators are effective in mitigating THM. Surface aerators are also used to increase evaporation.
Surface aeration such as shown here provides surface agitation to enhance evaporation. In produced water pits, surface aeration is considered easy to deploy since there is no need for the pit to be emptied. Despite that, surface aerators are a poor choice for aerating produced water pits of 10-foot depth or more due the generally poor oxygen transfer efficiency at lower depths.
The other type of aerating devices are submersibles. Since the primary objective of aeration is to transfer oxygen into water for maintenance or improved water quality, submersibles are a better choice. Allowing air to enter at the bottom of the water column allows more efficient oxygen transfer.
Most of the systems used in aeration basins across the world are submersible. By allowing the air to enter at the bottom and travel upwards, retention time of air in the water is increased. Increasing retention time provides more time for oxygen to be absorbed.
The rate of oxygen transfer is affected by many factors including water quality, temperature and pressure. With so many variables to consider, what is the best way to evaluate a submersible system?
Among equipment choices, for example, there are two types of diffusers, fine and coarse. To manage air flow, there are venturi, turbine systems, propeller type and nozzle systems.
Submersibles increase retention time, providing more time for oxygen to be absorbed.
The following discussion considers the primary factors in determining the right aeration system for the job.
One of the most common ways to evaluate these systems is to use Standard Oxygen Transfer Efficiency (SOTE). SOTE is measured at Standard Temperature and Pressure (STP) with clean water. Another key parameter is Standard Oxygen Transfer Rate (SOTR). SOTE and SOTR are related and represented by the following equation:
SOTE = (SOTR/oxygen mass flow) x 100
SOTE is represented as a percent. Another key term is the Standard Aeration Efficiency (SAE); it is calculated using the following equation:
SAE = SOTR/Power Input (watts)
SOTR is a bit more complicated but it is represented by the following equation:
SOTR = kL a (DOsat - DO) x V
where kL a = liquid-side mass transfer coefficient (h-1) DO = dissolved oxygen in water (kgO2 m-3) DOsat = dissolved oxygen in water at saturation (kgO2 m-3) V = water volume (m3 ).
This group of equations allows a comparison of aeration systems to not only evaluate the SOTE and SOTR of each system but also SAE, which tells you how much power it takes to deliver the needed oxygen. These equations help develop a baseline for performance at STP in clean water. When applied to produced water, adjustments must be made. Calculations are the same but for process conditions, simply adjust the oxygen saturation calculation and the mass transfer coefficient.
These calculations have turned the wastewater industry towards greater use of fine bubble diffusers. The primary difference between fine and coarse bubble diffusers is fine diffusers have a higher pressure drop. This might lead the operator to conclude that more energy is needed. In fact, smaller bubbles significantly increase oxygen transfer efficiency, so the extra energy is compensated by needing a lot less air.
When evaluating an aeration system, the proper amount of air in relation to the produced water content must be considered. Everything in produced water that will consume oxygen should be evaluated and there are different ways to do this. Chemical oxygen demand (COD) can be used as a surrogate but other elements such as ammonia, which is not typically tested for in produced water, should be included when evaluating oxygen uptake.
An evaluation of oxygen consumers normally tested for in produced water such as bacteria, iron and sulfides, can leave out other contaminants that consume oxygen. As a result, water quality and retention time in a pit or tank will likely be affected. These factors should be included within design parameters so additional air can be added when needed. An increase of 20 percent should do the trick. Once the oxygen needed is established, the oxygen transfer efficiency determines total air required.
AVOIDING DEAD ZONES
The next step is distribution of that oxygen throughout a pit or tank. Understanding Computational Fluid Dynamics (CFD) modeling can help with decision making. Use CFD to model the transfer of oxygen to ensure sufficient oxygen distribution is taking place throughout the pit or tank.
Evaluation of proper flow and in what direction in order to get the proper amount of mixing is not easy and becomes more difficult in larger pits. This is partly the reason for use of fine diffusers across the bottom of aeration basins. Fine diffusers are preferred in order to get oxygen distribution throughout the basin and avoid voids or dead zones with insufficient oxygen. Without uniform distribution, the aeration process does not add enough air where it is needed and too much air is lost to the surface. This is where bubble size plays a vital role.
The proper bubble size can make or break an aeration system. In simple terms, big bubbles rise to the surface faster than small bubbles. Small bubbles greatly increase the surface area needed for oxygen transfer as opposed to big bubbles. The combined slower rising and greater surface area results in improved oxygen transfer efficiency. As a result, less air is needed to operate the system. In general, larger bubbles tend to rise quickly while small bubbles bounce around in a more random fashion. As they move both horizontally and vertically, smaller bubbles collide with each other and exhibit more horizontal and less vertical movement. This random movement is called Brownian Motion. Smaller bubbles improve aeration efficiency needed for iron and sulfide control. At every point air is injected into water, a radius of influence is created. This means there is an area around an injection point where dissolved oxygen influences the water.
Smaller bubbles provide greater mixing, increasing the area of influence, resulting in fewer injection points needed. With fewer injection points, air volumes can be reduced and operating costs are lowered.
Fine diffusers improve oxygen distribution throughout the basin and help avoid dead zones with insufficient oxygen.
As mentioned earlier, the wastewater industry has advanced the concepts of SOTE, SOTR and SAE to help evaluate the many options available for efficient aeration. Many companies report that fine diffusers meet aeration objectives while helping lower overall cost to operate.
So, what about venturi systems? There are applications best suited for venturi. Pump-fed tanks already have much of what is needed for retrofitting a venturi aerator. A venturi system is essentially a pump and venturi eductor, so adding a venturi to an existing pump system is an option. Confirm that the planned flowrate will draw enough air to supply the needed oxygen. This may not quite match but there is no reason not to take advantage of the free energy you are getting from the pump, since the power to operate the pump is already supplied.
In tank systems it is much easier to get the mixing you need from the pump system, but you should verify this with some computational fluid dynamics (CFD) modeling.
Potential pitfalls to be considered include having a specific flowrate in mind but the pressure drop of the venturi eductor will restrict flow which can affect overall water management plans. Another concern is the venturi may not draw enough air into the water to meet oxygen demand. This can be solved by adding diffusers.
When planning an aeration process that includes the venturi approach, consider a pump size that provides adequate flow and draws enough air. This may increase your cost, but your SAE will be lower than other approaches.
Application of a venturi eductor does not translate into pits very easily. Pit water flow can be gravity fed which reduces the efficacy of a venturi system. When considering venturi systems, higher flowrates allow more intake air resulting in smaller bubbles. Conversely, lower flowrates reduce air volumes resulting in larger bubbles. System performance can be greatly reduced if flowrates change.
An important factor to consider when designing a pit system is pit size. In a large pit, distributing oxygen throughout the pit usually means adding several injection points. Deeper and larger pits require longer air intake lines, reducing total flow. Also, the addition of filters reduces flow even more. All these factors must be considered, or water could become starved of oxygen resulting in an ineffective treatment program.
New aeration systems, including hybrid designs that are on the drawing board, take advantage of free pump energy or water flow to inject air. Adding fine diffusers helps prevent dead zones and enhance bacteria, iron and sulfide control. Stay tuned for new developments.
Authored by Mark Patton