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Activated sludge is a common method used to biologically treat municipal wastewater. The method was first developed in England in the early 1900’s. In 1912, H. W. Clarke studied the use of microorganisms in the purification of sewage through aeration. Edward Arden and William Lockett followed by carrying out similar experiments. They determined that high levels of purification could be reached through this biological process. They also integrated the use of recycle to the aeration stage. Arden and Lockett reported their findings to the Society of Chemical Industry in 19142. Since then, the theory of activated sludge as a wastewater treatment process has continued to develop in small steps to the present day. The activated sludge method typically consists of a well-agitated and aerated reactor and a settling tank. Depending on the design, the reactor can either model a plug-flow reactor, or a CFSTR. If the reactor is designed to be long and narrow, it will model a PFR, while circular tanks approximate a CFSTR. It was discovered that wastewater could not be purified solely by aeration, but that it needed to be activated by the living organisms that are contained in it. Thus, the sludge consists of undecomposed materials as well as living organisms. The sludge is recycled to the reactor, but overgrown sludge is known as excess sludge, which needs to be disposed of. A schematic of a typical activated sludge process is shown in Figure 1. Figure 1-A schematic representation of an activated sludge process http://future.pis.hitachi.co.jp/eng/a_tech/a-10.html The activated sludge process has been extremely successful in treating wastewater, in spite of all the variable process conditions that can make purifying water complex. The process has the potential to metabolize a wide variety of organic compounds and can also remove compounds containing nitrogen, sulfur, phosphorus, etc. There are many variations of the process that have been developed to increase purification, such as membrane-coupled systems, and high purity oxygen systems. Some argue that activated sludge is the best method compared to all other biological processes, from both a technical and economical standpoint. Overview As mentioned before, wastewater treatment is a very complex process. Wastewater is made up of a vast amount of different compounds and solids. The purification process often includes a primary treatment before the wastewater even enters the sludge tank. The primary treatment may involve some type of screening or filtration to remove large solids. It also may involve conditioning treatments to adjust the pH, or add specific nutrients, which will be vital to the growth of cells. This treatment can remove 45-60 percent of the suspended solids, and even higher removals can be reached with chemical additions. After the primary treatment, the wastewater is then fed to the sludge tank. The activated sludge process generally occurs in two steps. The first step is the transformation of the contents of the wastewater into the activated sludge, and the second step involves the separation of the sludge from the cleaned water, typically through sedimentation. The contents of the wastewater can be transformed into activated sludge by hydrolysis, adsorption, enmeshment, accumulation or storage. Larger solids that are found in wastewater generally are transformed through adsorption or enmeshment. Colloids, which have a diameter of approximately 10-2 microns, mostly adsorb on sludge flocs, while larger particles tend to become enmeshed in the void spaces between the sludge flocs. Accumulation, like enmeshment and adsorption, is a fairly fast process. It involves the uptake of small organic molecules. Experiments have shown that mono- and disaccharides, lower fatty acids, alcohols, amino acids, and other similar substances can be accumulated under rich nutritional conditions1. Hydrolysis and storage, on the other hand, are rather slow processes. Hydrolysis is a process in which surface bound substrate is utilized by the cells. This process transforms some particles into a solved matter, typically a degradable organic matter. Solved matters can then be further transformed either into new cell substances by the bacteria or can be oxidized. Once the wastewater components have been transformed into the sludge, it then moves on to the final clarifier. The sedimentation process heavily relies on the formation of flake-shaped sludge flocs in order to be efficient. The flocs then settle to the bottom of the clarifier where they thicken and are scraped into the withdrawal cone.
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