Sewage treatment plant manufacturer and their design of diffused aeration equipment.
Sewage treatment plant manufacturer who are involved in the design of diffused aeration equipment for wastewater treatment should understand the impact that process type, maintenance issues and economic considerations can have on the selection of equipment. Like many other engineering challenges, these factors are frequently interrelated and trade-offs of one aspect versus another are required for most application. This design guide presents information that has been obtained and developed from a variety of sources. Some of this information has been developed from actual test data, some is condensed from other published sources, some is based on good engineering judgement and practical field experience.
Oxygen must be provided in biological treatment systems to satisfy several types of demands. These are referred to as actual oxygen requirements or AOR. AOR is always expressed as “field conditions”. Each wastewater treatment plant has its own unique field conditions that include site elevation, temperature, working DO level, diffuser submergence and alpha and beta factors. The sewage treatment plant manufacturer must use these factors to convert AOR to standard oxygen requirements (SOR) to properly apply the aeration equipment and determine the amount of process air required to satisfy the biological treatment oxygen demands.
Common units of expression for AOR and SOR are pounds of oxygen per day per unit volume. SOR values will always be larger than AOR values. Confusion and misunderstanding can be minimized between designer and equipment sewage treatment plant manufacturer if the sewage treatment plant manufacturer expresses his desired oxygen demands in terms of SOR values. If this is not possible, then clearly identify the oxygen demand as an AOR value and provide as much information as possible for the sewage treatment plant manufacturer to assist you in making the appropriate AOR/SOR conversion
Equipment sewage treatment plant manufacturers can show engineers and sewage treatment plant manufacturers a rational method to convert AOR to SOR and can offer advice on the probable values used in the AOR to SOR conversion. However, it is the engineer’s responsibility to determine the AOR of a particular system or process and select the appropriate conversion factors to relate AOR to SOR. Specifying an SOR value is the best way to prevent confusion and problems in the specifications.
ACTIVATED SLUDGE AND BIOLOGICAL TREATMENT
Activated sludge aeration tanks are the largest applications for diffused aeration equipment. These tanks and the associated air diffusion equipment are the heart of the activated sludge process and typically are the single largest energy user associated with plant operations. Energy costs for aeration will typically be 50% to 90% of all energy consumed at a wastewater treatment plant.
Oxygen must be provided in biological wastewater treatment systems to satisfy several types of demands. One demand is that associated with the oxidation of organic or carbonaceous materials. Carbonaceous oxygen demand is associated with two cellular functions: cell synthesis and endogenous respiration. Cell synthesis carbonaceous oxygen demand occurs when organic matter is first metabolized by the microorganisms contained in the mixed liquor. It is related to the oxygen required to oxidize a portion of the organic matter to provide the energy necessary for cell synthesis. Endogenous respiration carbonaceous oxygen demand occurs as the synthesized organisms are retained in the treatment system and it represents the essential life processes. The net result is that increasing amounts of oxygen are required as lower process organic loadings are used. Lower process organic loadings are characterized by operation at a longer solids retention time (SRT) and a lower food-to-microorganism (F:M) loadings.
CALCULATING ACTUAL OXYGEN REQUIREMENTS –
AOR
A number of approaches have been used to estimate the oxygen requirements caused by the biochemical oxidation of organic matter. Many regulatory agencies specify oxygen design criteria for various unit processes and some requirements are probably based on various empirical or rule-of-thumb techniques. More sophisticated approaches to estimating the oxygen demand in aeration systems may be obtained from various computerized process models.
Unfortunately, most process models either require certain values that need to be experimentally determined for the particular waste or else must rely on past experience and judgement in selecting the model values. One rational approach to determine the oxygen requirements in the biological process is to total the oxygen demand due to the sources described below. The summation of all of these contributions must be considered in the sizing of the aeration system.
BOD LOADING
The relationship between SRT and pounds of oxygen required per pound of BOD removed at various temperatures for domestic wastewater in an activated sludge system. For typical SRTs of 5 to 10 days, the pounds of oxygen per pound of BOD removed varies, from 0.92 to 1.07. A value of 1.0 pound of oxygen per pound of BOD removed is commonly used. On occasion, some sewage treatment plant manufacturers use a more conservative value of 1.1 pounds of oxygen per pound of BOD removed. In processes with long detention times (more than 18 hours) and low organic loadings where excess sludge is also oxidized in the aeration tanks, a higher value is justified. Examples where higher values are justified are extended aeration and oxidation ditches. In these cases, supplying 1.25 to 1.80 pounds of oxygen per pound of BOD removed or higher is appropriate.
AMMONIA LOADING
The oxidation of one pound of ammonia requires 4.3 to 4.6 pounds of oxygen. Typical domestic wastewater contains 25-30 mg/l of ammonia. Do not underestimate the oxygen demand to oxidize the ammonia. Oxidizing 25 mg/l of ammonia is equivalent to an additional 115 mg/l of BOD loading. Be award that even if a plant is not specifically designed to nitrify, that under favorable loading, temperature and SRT conditions, nitrification can and will occur. This may exert a large unanticipated oxygen demand.
