Aeration in Lagoons
For years, wastewater lagoon systems have provided secondary treatment performance to many small to medium sized communities. The attributes of these processes have been attractive as cost effective options for the treatment of municipal wastewater. The aeration segment in these systems is the most critical component and is the core of their biological treatment process. A lagoon systems ability to aerate the incoming sewage has a direct impact on the level of wastewater treatment it achieves. This page will focus on the heart and soul of these systems - aeration.
An ample oxygen supply in a wastewater pond system is the key to rapid and effective wastewater treatment. Oxygen is needed by the bacteria to allow their respiration reactions to proceed rapidly. The oxygen is combined by the bacteria with carbon to form carbon dioxide. Without sufficient oxygen being present, bacteria are not able to quickly biodegrade the incoming organic matter. In the absence of dissolved oxygen, degradation must occur under septic conditions which are slow, odorous and yield incomplete conversions of pollutants. Under septic conditions, some of the carbon will react with hydrogen and sulfur to form sulfuric acid and methane. Other carbon will be converted to organic acids that create low pH conditions in the ponds and make the water more difficult to treat. For example, treated ponds designed to biodegrade wastewater pollutants without oxygen often must hold the incoming sewage for six months or longer to achieve acceptable levels of pollution removal. This is because the bio-degradation of organic matter in the absence of oxygen is a very slow kinetic process.
The designers of wastewater lagoon systems must take into account the fate of the biological cells, called sludges, that would eventually settle to the pond's bottom. One advantage to using an aeration system to treat wastewater is the lack of sludge volumes that require treatment. Unlike conventional wastewater treatment plants that must remove excess sludges daily, a pond type system can go from ten to twenty years without ever needing cleaning. This is because, in the presence of sufficient oxygen, bacterial cells that settle to the ponds bottom are eventually biodegraded into carbon dioxide and inert materials. Since a large portion of municipal wastewater consists of biodegradable organic carbon matter, much of the settled sludge in the lagoon can be quickly decomposed by the remaining active bacteria. If sufficient oxygen is not present in the ponds, the sludge layer will accumulate faster than it can be biodegraded. When this occurs, the sludges may build up to a point that it must be removed at a faster rate than would be expected for a pond. This effectively eliminates the main advantage of a lagoon system which is supposed to be an infrequent sludge removal need.
Adequate aeration is also an important element in keeping the lagoons content mixed and in suspension. Even in a partially-mixed hydraulic regime, mixing is very important to the overall treatment process. With adequate mixing, incoming pollutants and wastewater are better distributed throughout the entire lagoon volume. This results in more uniform and efficient treatment. In addition, solids that settle can be re-suspended by the aerator's mixing action and brought back into contact with the microbial population floating throughout the pond. Poor mixing has the effect of creating thick solids deposits that fall to the lagoon floor before proper treatment has occurred. This causes improperly treated solids to fall away from the active, overhead treatment process. It also creates septic conditions on the lagoon bottom which, in themselves, pull available oxygen out of the upper layers of the pond and reduce the effectiveness of treatment in the upper zone.
Clearly the proper aeration and mixing of a lagoon is critical if the system is to properly treat the influent wastewater pollutants. A plant that has never had proper aeration and mixing will often result in a lot of money spent on electrical power for aeration purposes with little lagoon oxygen to show for this expenditure as well as deep accumulations of partially treated sludge on the lagoons floors which requires frequent removal.
The proper oxygenation of a treatment system also has important implications for an emerging wastewater treatment plant toxicity issue. Raw sewage contains large amounts of ammonia (NH3). The typical ammonia concentration of raw sewage is 30 ppm. This ammonia is present as a natural consequence of the degradation of protein-based compounds in wastewater. Nitrogen is a major building block of protein which is present in wastewater due to the discharge of protein based wastewater materials. As the organic compounds that contain protein are degraded, ammonia is released into the raw sewage stream. This is problematic because ammonia has been found to be toxic to aquatic organisms in the water body. In the presence of adequate oxygen, the nitrogen in ammonia can combine with the oxygen to form non-toxic nitrate compounds (NO3-). In order for this reaction to occur, the plant must have sufficient dissolved oxygen available.
Both coarse and fine bubble diffused aeration systems transfer oxygen into the water by creating small bubbles. As the bubbles travel through the water, oxygen is transferred across the bubble's surface and into the water. Mechanical aerators work in the opposite way by creating small droplets of water using a mixer. These droplets are propelled through the atmosphere above the ponds surface. Oxygen in the air is transferred into the small water droplets which then fall back into the water.
Factors to Consider
There are many factors that will act to hinder the transfer of the oxygen load in a wastewater lagoon system. All of these factors must be considered to ensure that sufficient air is added to allow the necessary pounds of oxygen per day to be transferred. Some of these factors include:
- The alpha factor in oxygen transfer relates how well oxygen will diffuse into wastewater as compared to clean tap water. This is an important consideration because most aeration equipment is tested and rated in clean water laboratory testing and these results must be correlated to actual wastewater applications. An alpha factor of 0.80 is common for fine bubble aeration in wastewater lagoons.
- The beta factor in oxygen transfer considers how dissolved solids in the wastewater will hinder the diffusion of oxygen as compared to being in clean water with few dissolved solids. This is an important consideration because aeration equipment is usually tested and rated in clean water. A typical beta factor of 0.95 is common for most wastewater applications.
- The theta factor in oxygen transfer relates how temperature changes will affect the rate at which oxygen can be transferred into the water. This is important because most aeration equipment is sized and rated at standard temperature conditions of 20 degrees Celsius. It must then be adjusted using the theta factor for the expected worst case field temperature conditions. Typically, a theta factor of 1.024 is used and the hottest temperatures of the year are considered since it is more difficult to transfer oxygen into hot water than cold water
- Biological activity in the ponds is optimized when a minimum dissolved oxygen saturation concentration of 2.0 ppm is maintained at all times. The aeration equipment should be sized for this basis.
- The atmospheric pressure at the treatment plant site is an important factor in determining how much oxygen can be transferred. It is more difficult to transfer oxygen at higher elevations than at sea level because of changes in the local air pressure. The maximum allowable oxygen saturation concentration that can occur at the field temperature and lagoon depth conditions must be considered.
- The maximum amount of oxygen that water can hold at 20 degrees Celsius is 9.09 ppm. As the water temperature increases in the summer months, lesser concentrations of oxygen can be held by the warmer water.
All of the above factors must be considered collectively to properly size aeration equipment for any treatment system. These factors can be mathematically related to match the actual oxygen requirement (AOR) needed to meet field conditions, with the standard oxygen requirements (SOR), at which aeration equipment is rated in the laboratory. The relationship between AOR and SOR is very important in properly designing a pond's aeration system. AOR represents the actual amount of oxygen that needs to be added to the ponds under full design loading, full ammonia conversion, and worst case temperature conditions. SOR represents the excess oxygen that must be added in order to make sure that the AOR will be met. Blowers and diffusers are sized on the basis of SOR which represents clean water conditions at 20 degrees Celsius and sea level air pressures. They are purchased on the basis of SOR after first relating the required AOR conditions to SOR conditions.
Having determined the standard oxygen requirement of a pond system, it is next important to consider how much air volume in SCFM (standard cubic feet per minute) will be needed to deliver that mass of oxygen. Each cubic foot of air added to the lagoon will contain about 0.0173 pounds of oxygen. The oxygen transfer efficiency (OTE) of a diffuser system is a function of its depth in the ponds. Typically, an OTE of about 1.6% per foot of depth is found for fine bubble diffusers in a pond setting. For a lagoon with ten feet of depth, a transfer efficiency of about 16% could be expected. This means that 16% of the air added at a depth of ten feet will actively be transferred into the water while eighty-four percent will be excess and will bubble to the surface. This seems like an excessive air loss rate, but it is the best now available with current technology.
Would your wastewater treatment plant benefit by replacing an older aeration system such as coarse bubble diffusion with fine bubble technology? It's well worth investigating. It is important to understand some critical terms, such as oxygen transfer efficiency and alpha, before beginning your evaluation. The oxygen transfer efficiency of an aeration system is the ratio of the amount of oxygen that actually dissolves into the water to the total amount of oxygen pumped into the water. Only the dissolved oxygen is available for treatment, and any portion of oxygen that does not dissolve is a waste of energy and, therefore, money.
Finally, aeration basin and aerator layout geometry can dramatically alter oxygen transfer efficiency. Standard clean water transfer efficiency tests are usually based on full floor aerator coverage. This creates a nearly ideal situation for oxygen transfer. A turbulent counter-current flow regime is established with a volume of water being dragged upward with the rising bubbles, being opposed by an equal flow of water traveling downward. In large basins, such as lagoons, full floor coverage is not normally needed to meet the biological aeration demands. As a result of incomplete coverage, large scale currents can form a spiral roll in the basins. The air bubbles flow with the water, which significantly reduces the transfer efficiency. Compared with the full floor configuration, many more air bubbles short-circuit their way to the top of the aeration basin.
Although it may be true that fine bubble systems can provide an appreciable savings in many situations, those who purchase and operate these systems without verification do so at the peril of their own bottom line cost. How can you avoid this pitfall? First, test the system proposed with your wastewater. Do not trust a vendor who does not recommend oxygen transfer testing. Second, make sure that oxygen transfer testing mimics the actual conditions, including the configuration of your basin. The cost of aeration system testing can be high and may not be affordable for many plants. Before testing, the potential cost savings of switching to fine bubble aeration should be evaluated. Literature, vendor, and EPA values for oxygen transfer efficiently and alpha can be used. Find plants with similar wastewater to refine from their experiences the values and contact plants to learn from their experience. If the investment in fine bubble diffusion appears favorable, consider testing before replacing the system.
Whether it be a plant upgrade or a new facility, a lagoon system will not work well without proper aeration. As sludge disposal regulations become stricter and disposal sites become scarce, proper aeration will reduce the amount of solids that may have to be removed from the lagoons since more of the incoming pollutants will be oxidized.