The activated sludge process is the most widely used biological water and wastewater treatment. As is well known, by its means, suspended-growth microorganisms are applied to breakdown wastes. Aeration, the mixing of air and a liquid, is the means used to speed the reactions involved. Aeration methods include spraying the liquid into the air; diffusing air into the liquid or agitating the liquid to promote surface adsorption of air.
Aeration can be “conventional rate” based on mechanical aeration, or “extended rate” based on diffused aeration.
An extended-aeration system includes capabilities for aeration & mixing, settling, return of activated sludge and solids removal, this last in the form of a biomass known as waste-activated sludge. An activated sludge process re-circulates part of the biomass as an integral part of the process.
Awareness of the basic parameters measured in controlling an extended-aeration system can ensure proper treatment is carried out, such that legal and regulatory requirements for effluent discharge quality are met.
As mentioned, there are two commonly used process-loading ranges for operating activated-sludge systems. A “conventional-rate” process involves 5 to 15 days residence time, while an “extended aeration-rate” process has a slower, or lower, residence time of between 20 to 40 days residence time.
Plants operating within an extended-aeration range are usually small and without 24-hour-per-day operator attendance. They ordinarily operate within a mean cell residence time (MCRT) in a range of 20-40 days and a food/microorganism (F/M) ratio between 0.05-0.15 lbs.
MCRT is the average time a microorganism will spend in the activated sludge process under aeration. F/M ratio is a measure of food provided to bacteria under aeration in a tank. “Food provided” refers to the biochemical oxygen demand (BOD) entering the aeration tank per day, measured in pounds.
BOD must be at a rate sufficient for the organisms to use the oxygen in water or wastewater while stabilizing decomposable organic matter under aerobic conditions. In decomposition, organic matter serves as food for the bacteria and energy results from its oxidation. BOD measurements are used to characterize the organic strength of wastes in water. The phrase “bacteria under aeration” refers to the pounds of BOD in the aeration tank.
It is important to measure mixed liquor volatile suspended solids (MLVSS) and not just MLSS because MLVSS measures the live bacteria portion of MLSS. A healthy percentage of MLVSS to MLSS is =>75%.
An extended aeration process is sometimes referred to as a “total oxidation process,” which means all the primary treated or influent BOD is converted to CO2.
This is why some say removal of waste-activated sludge, commonly referred to as “sludge wasting,” isn’t necessary for extended-aeration systems. In actuality, it is not practical to achieve a total oxidation process, and even after quite long periods of aeration, suspended matter remains in the effluent. Although sludge wasting may not be required on a daily basis in plants operating with extended aeration, occasional wasting is surely preferred.
Often, extended-aeration effluent contains small pin-size suspended solids known as “floc,” which may be seen as passing over the weirs of the secondary clarifier. When the loading in an extended-aeration plant is in the higher portion of the loading range, operating issues may occur. Because the entire extended aeration range is in the nitrification zone, de-nitrification and rising sludge issues may exist. Moreover, greasy foam, filaments, and poor settle-ability at low loading may occur under these conditions. If possible, these dilemmas can be addressed using additional aeration capacity or decreasing MLSS and MLVSS under aeration.
While some sludge wasting is preferred, many operators believe wasting is not necessary, and indeed, many small extended-aeration plants have no facilities for wasting. Under these circumstances, it may be common for solids to carry over the clarifier weirs whenever fluctuations in flow occur. Unfortunately, this can cause a meaningful reduction in removal efficiency percentages.
If the operator of an extended aeration plant periodically experiences the loss of solids over the effluent weirs, two countermeasures can be used: regular sludge wasting and flow equalization. Of the two, sludge wasting is certainly the most important. A meticulous operator should keep track of the solids intentionally wasted, and the solids that unintentionally go over the effluent weirs. Based on the results, the plant can be operated to achieve a specific value of MCRT between 20-40 days.
Even if a plant isn’t equipped for sludge wasting, the operator should attempt to improvise a temporary or permanent scheme. Depending on plant design, geography and environmental conditions, the operator may be able to arrange for wasting. The sludge from plants of this sort may already be 'aerobically' digested. Therefore, it may not have an offensive odor.
Even when regular wasting is carried out, a high degree of flow deviation in extended aeration plants will cause loss of solids. This is probably due to the particular characteristics of the floc produced in the low loading range, as well as changes in flow. In certain cases, if the aeration tank is large enough, the operator may be able to arrange a substitute that will allow the use of the aeration tank as a flow equalizer. Strategic modifications of this sort may dramatically improve suspended solids removal in plants where losses are primarily due to flow-rate inconsistency.