Evaluating diffuser vs. venturi-type aeration systems calls for knowing your bubbles.
Simply defined, aeration is the introduction of air into a media. In wastewater, aeration is used primarily to introduce oxygen into the water. Oxygen is a mild oxidant and will work well to oxidize iron and sulfides, while inhibiting bacterial growth in produced water. Aeration has become popular in the oilfield as a low-cost alternative to pit treatments.
Pit treatments are the introduction of an oxidant into a pit to hinder runaway bacterial growth, or to control iron and sulfides, or a combination of the three. Anybody who has stored produced water over time is familiar with the increased odor and sulfides that can occur when produced water is left unmanaged.
In smaller tanks, recirculating the water with an oxidant is not a challenging task. But as tanks get larger, or if using a pit, the complexity increases, and problems can develop.
Pit oxidation treatments can be expensive and are typically difficult to properly execute. Transferring oxidant throughout a pit, especially a large pit can be very difficult. Depending on how the pit treatment is set up, short circuits can develop.
Short circuits are where an influent line begins to connect to an effluent line causing a retreatment of the same water while most of the pit is left uncirculated. When short circuits leave untreated water in a pit, the result is an unsuccessful treatment. Challenges encountered with pit treatments give rise to aeration as a simpler solution.
Although not complete bacteria control program, aeration can reduce overall cost of recycling.
Aeration allows even distribution of oxygen and helps avoid short circuit problems. However, a proper aeration system requires planning and design.
Oxygen is a slower acting oxidant and needs time to work. Salinity slows down the dissolution of oxygen in water; the higher the salinity, the slower the oxygen dissolves into water. It is the dissolved oxygen that reacts with iron, sulfides and bacteria.
Although there is some interaction between oxygen bubbles and iron and sulfides, the overall solubility of oxygen in water is reduced by increased salinity, and it is understood that salinity is very high in most produced waters.
To manage high salinity water, aeration systems must be designed to allow enough retention time for the process to work. This is why aeration is preferred for pits and large tanks. The longer retention times allow oxygen to dissolve into water and react with iron, sulfides and bacteria. Unfortunately, as effective as aeration can be for water treatment, it is not a cure-all for bacteria. Aeration works best as an anaerobic bacteria control method.
BACTERIA AND OXYGEN
Aerobic bacteria need oxygen, while anaerobic bacteria can survive without oxygen. The bacteria of most concern in the oilfield is bacteria which causes microbial induced corrosion and that tends to be anaerobic.
Oxygen introduced by aeration will kill anaerobic bacteria but conversely, it can allow aerobic strains to thrive. In a typical produced water recycle scenario, oxidation can be included before aeration as a pre-treatment step or after as an on-the-fly option to manage aerobic and anaerobic bacteria. When designing aeration systems, it is important to understand that aeration is not a complete bacteria control program, but since oxygen is a cheap oxidant, it can reduce the overall cost of recycling.
Aeration does a great job of preserving the disinfection of produced water after it has been treated by a stronger oxidizer, which provides a solution for treating aerobic bacteria. Aerobic bacteria can cause biofilms on lines and equipment that restrict flow and cause plugging. Because of these impacts, an aeration program should be combined with a stronger oxidation process to ensure aerobic bacteria is controlled as well.
In general, aeration is simply a way to deliver oxygen into water. There are many options for introducing oxygen into water, but the most important consideration is that aeration must contact as much water as possible for the best oxygen transfer.
To reach that goal, submersible aeration is preferred, which may eliminate the need for surface aerators. Surface aerators can provide an odor cap, but because they aerate only a few feet of water near the surface, only that zone absorbs odors from below. This does very little to improve overall water quality.
The two most common options for submersible aerators are the diffuser type and the venturi or eductor type.
DIFFUSER VS VENTURI
Diffuser aerators allow air to pass thru, usually from a blower or compressor. The amount of air can be easily controlled and increased and therefore increased oxygen is delivered.
Venturi-type aerators require flowing water to create a vacuum to suck in the air. Air volume is limited by water flow and pump pressure. Increase the water flow and the pressure increases air flow. The primary advantage comes when there is an existing pump system present in the pit or tank.
A venturi-type aerator on an existing pump helps reduce overall cost.
Adding a venturi-type submersible aerator to an existing pump helps reduce overall cost even with the added electrical demand. A concern with such a system is that the pressure drop at the venturi may limit the pump to where it does not provide enough pressure to allow the venturi to work properly. Using an existing pump can provide advantages so long as the pump is properly sized and takes into consideration performance decline over time.
In general, when comparing diffuser-type systems to venturi-type systems, capital expenditure on the diffuser system can be 50 to 70 percent higher than a venturi system. On the other hand, operating expense on the venturi system can be more than 300 percent higher than a diffuser. Careful evaluation of the aeration design is advised.
AERATION COST COMPARISON, 400,000 BBL PIT
In the case shown above, the venturi system carries a lower initial cost. However, the additional utility cost means that by year two, the venturi system is more expensive. In larger pits, pump performance will also affect long-term effectiveness of the venturi system.
Because both types of aerators have applications in the oilfield, Hydrozonix offers both venturi and diffuser-type aeration systems. Unfortunately, there have been instances of venturi systems being misapplied.
It is incorrect to compare a blower and diffuser system to a venturi without a pump. Without a pump, a venturi system does not operate. In cases where an existing pump may not be included in an analysis of aerator efficiency, the increase in power demand must to be factored into the operating cost estimate.
More importantly, can the flow rate of the existing pump provide enough vacuum to supply all air required considering that the pressure drop across a venturi will change the overall flow rate?
To properly compare systems, some basic information is required:
1. Calculate oxygen demand of the water. This done by combining the oxygen consuming compounds such as iron, sulfides and bacteria that are present and adding a little extra oxygen to maintain positive oxidation reduction potential;
2. Determine how much air is needed to achieve the desired amount of oxygen to be introduced. The oxygen transfer efficiency should be calculated for each option considered. This determines pump size for a venturi system or blower size for a diffuser system;
3. With the dimensions of a given pit or tank, will there be enough flow to distribute oxygen throughout?
BACTERIA AND OXYGEN
At this point of the discussion, the size of the bubbles doing the aerating comes into play.
Bubbles rise and larger bubbles rise faster. The faster a bubble rises, the less time for oxygen to be absorbed into the water. So small bubbles are better.
Smaller bubbles also have more surface area which helps increase oxygen transfer. Taken together, smaller bubbles provide a combination of slower rise and more surface area for additional oxygen transfer.
Most venturi systems can provide micro-bubbles or larger. Diffusers, on the other hand, can provide different bubble sizes with costs increasing from coarse to finebubble diffusers.
However, a cost comparison is not as simple as that. The increasing cost of smaller bubbles can be greater than the efficiency gains in oxygen transfer. As a result, it may be more economical to go with larger bubbles.
Additionally, the cost of moving the bubbles to where they are needed should be evaluated. In a larger, wider pit, the bubbles may rise before reaching the middle resulting in uneven oxygen distribution. One solution is to place submersible venturis in different locations to allow for better distribution.
Experience with venturis in treating produced water shows they will develop scale and require periodic cleaning. Emptying a large pit regularly to remove venturis for cleaning can be a difficult, expensive proposition. Similarly, pump performance degrades with time, especially if not serviced properly. Accordingly, a venturi system will encounter slowing flow rates resulting in reduced distribution of oxygen, reduced delivered air volumes and larger bubble size. Ultimately, oxygen transfer will be reduced. Problems with existing venturi aeration systems are already being observed. For our customers, we recommend diffuser-type aeration for larger pits to avoid the pitfalls noted above. Venturi systems are only recommended for above ground tank systems that can be easily cleaned and maintained. Also, in smaller tanks it is easier to match flow rates of existing pump systems, which makes it easier to obtain the designed oxygen diffusion rate regardless of pump performance.
Next, consider how an aeration system can be modeled to ensure oxygen is distributed where it is needed.
Computational fluid dynamics modeling uses numerical analysis and algorithms to simulate fluid flow, distribution and the interactions of liquids and gases. CFD modeling allows virtual testing of oxygen distribution to determine the right size blower, placement of diffusers, pump size, and distribution of nozzles. Without CFD modeling, an aeration system will not operate at maximum efficiency.
Recently, Hydrozonix has developed a new aeration product that fits simply into an existing aboveground storage tank. The candy cane aeration system was designed for quick installation and can easily be relocated under a rental plan. The design allows the venturi to sit out of the water for easy cleaning and replacement.
One of the newest trends in aeration is the use of nanobubble systems, which can be either diffuser or venturi type. Why are nanobubbles important? Unlike small bubbles and even micro bubbles, nanobubbles don’t rise. They rely on random Brownian motion and stay suspended in the water.
Use of nanobubbles in aeration is a promising new area, but initial evaluation shows the gain in oxygen transfer efficiency does not justify the additional cost. This is expected to change as manufacturers reevaluate their price points.
Other tests indicate extremely high dissolved oxygen levels with nanobubble systems, some even exceeding the theoretical saturation point. Dissolved oxygen is a primary goal for aeration systems so super-high DO results tend to cause a lowering of overall air demand.
Recent field tests show high DO levels even in the presence of ferrous iron. Ferrous iron is easily oxidized by oxygen, so very small bubbles may appear to be dissolved when tested but actually are not.
Oxygen nanobubbles are a benefit because they create a reservoir of oxygen to draw from. However, the oxygen saturation rate of produced water will determine how much dissolved oxygen is available. To truly take advantage of nanobubbles, more retention time may be needed. Testing of nanobubble systems is continuing and additional results will be reported in Part 2 of this series.
FOR BEST RESULTS
Aeration is a powerful tool in reducing the overall cost of recycling produced water. The best results will come from proper applications of the right system. Other variables include system performance over time and for venturi systems, pump size must be considered.
Evaluating diffuser systems is easier, but there are other considerations. Oxygen transfer efficiency is directly related to depth of water, so pits and larger tanks can be good applications for diffusers. Shallow, aboveground tanks are not as good.
There is no perfect answer and every application is different. We have seen applications where both venturi and diffusers are used in combination. Look for further insights in Part 2 of this report in the next issue of SPWM.
Authored by Mark Patton, President of Hyrdozonix