Water Management
In the introductory column, we mentioned the growing number of produced water and brackish water pits. More and more, and larger and larger, allowing for more produced water recycling. As fracs increase in size and more water is needed, the ability to store and aggregate water is significant. As water is stored and aggregated, quality worsens over time. Bacteria blooms, hydrogen sulfide will be generated, and now you have lost control of your pit.
The out-of-control pit gave rise to the pit treatment, which involves adding a chemical biocide to the pit to prevent bacterial growth. Pit treatments don’t last, thus pits may require multiple treatments over time for bacterial control. You must ensure that the chemical biocide is sufficiently mixed throughout the pit, which is difficult. The most common method is a pump, some hose and a chemical injection system.
Most experienced water professionals know that a static system with one entrance (influent) and one exit (effluent) will cause a short circuit. This is where the water entering the pit connects to the water exiting the pit, and you end up recirculating the same water over and over without distributing the chemical throughout the pit. It is not unusual to pull an effluent water sample with great bacteria kill, but the pit is still out of control; the short circuit strikes again. As a result, pit treatments can be very expensive or just ineffective. As pits grow larger, distributing your chemical throughout them grows more difficult. There must be a better way, and there is—aeration.
Aeration is simply defined as the introduction and mixing of air with another medium. In our case, we are introducing air into water. The oxygen in the air acts as an oxidant and kills the bacteria, which can be either aerobic, requiring oxygen, or anaerobic, not requiring oxygen. Oxygen creates an environment where anaerobic bacteria dies, but
aerobic bacteria will survive. Most produced water is oxygen-poor, so the bacteria living in it are mostly anaerobic bacteria, which makes aeration a great control measure.
The oxygen from aeration also will oxidize sulfides and iron. If you ever had a fish aquarium, you know what happens when you stop diffusing air into the tank—the fish die, the water gets cloudy, algae forms and, in time, the smell becomes horrific. Now imagine all of that on a large scale, and you have an out-of-control pit; but with some air, water quality is maintained and there is no smell.
So, how do I introduce air? There are many methods. The first method we discuss is pumps and venturi eductors, where the water flowing through a venturi creates suction when air is introduced. This aerated water is then pumped into the pit. You need multiple injection points throughout the bottom of the pit to distribute the oxygen, and allow the air to rise and oxygen to get absorbed. This oxygen being absorbed is referred to as dissolved oxygen.
Now, let’s talk bubble size, the smaller the bubble, the more surface area there is, allowing more oxygen to be absorbed. Then there is bubble rise—the smaller the bubble, the slower it rises. The longer the bubble stays in the water, the more oxygen is absorbed. To make small bubbles with a pump system, we need a high flowrate and a small gas inlet, which restricts the amount of air that is introduced.
Most pump systems are designed for municipal applications where much less air is required—when you apply these systems to produced water they don’t supply enough air. If you enlarge your gas inlet to allow more air, you only create large bubbles that rise quickly and don’t allow oxygen absorption. Pumps also require significant energy. In many pump applications, the aerated water is introduced too close to the surface, which doesn’t allow the oxygen absorption, making the system ineffective. The next option to consider is floating or submersible systems.
Floating systems are easy to deploy, but they only treat the top few feet. Submersible systems are deployed at the bottom, allowing air to rise, so more oxygen is absorbed. Floating systems require running electrical power to them in a pit, potentially creating a shock hazard. Floating systems are good for tanks with shallow depths, but not large pits. That brings us to diffuser systems.
Diffuser systems use low-energy blowers to flow air to a diffusion system. This could be individual diffusers or diffusion tubing. Because they sit at the bottom, you improve retention time and they can be evenly distributed throughout the pit. The problem is sediment/solids will build up in pits and bury these diffusers, effectively ruining their efficiency. So what is needed is a portable/relocatable diffuser system. The mixing prevents the solids from settling over them and increases the transfer of oxygen. Making it relocatable allows you to inspect them without draining or taking a pit out of service. There are only a few of these types of systems on the market.
So you found your portable mixing diffuser system—what now? Regularly inspect it for leaks. Significant bubbles coming to the surface are a sign of a leak. Pulling regular samples at different depths and locations, and testing for dissolved oxygen (DO), will ensure you have an efficient system. Doing this once a week is usually sufficient, and will ensure you are maintainig your water quality.
Please feel free to e-mail me questions or input on future topics that you would like to see covered at info@hydrozonix.com. 
