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0.15 F:M 0.05 F:M <br /> High F:M Ratio Ideal F:M Ratio Low F:M Ratio <br /> I <br /> Food(BODICOD/TOC) ( I <br /> I Bacterial Population I <br /> o Dispersed Floc Region <br /> 1n <br /> IFloc Formation Region • <br /> IDispersed Growth Region � � • <br /> � ® I <br /> o ® I <br /> oFree-Swimming Ciliates <br /> • ® -- _ I <br /> • • ° <br /> Stalked Ciliates I <br /> o <br /> I� Rotifers <br /> Time <br /> Courtesy of wastewaterblog.com <br /> Food to Microorganism Ratio <br /> Cell Residence Time <br /> Cell Residence Time (CRT), also known as Sludge Age (SA), or Solids Retention Time (SRT), is the <br /> average length of time in days that an organism remains in secondary treatment. Sludge should <br /> be between 15 and 25 days old. There are several ways the operator may determine the relative <br /> age of the sludge. <br /> Microscope <br /> One way is through the use of a microscope. When the CRT is low (< about 4 days) the simpler <br /> life forms are present. This includes amoebas and flagellates. As sludge age increases, more <br /> complex organisms such as free-swimming ciliates and stalked ciliates appear. At high CRT levels <br /> multi-celled animals such as rotifiers and nematodes may be found. As seen, as CRT increases, <br /> complexity and size of organisms increase, allowing the operator to use a microscope to <br /> determine the quality of the biomass in the plant. <br /> Bacteria make up 95% of microorganisms in activated sludge and are primarily responsible for <br /> removing organic material. About 4 % of the microorganisms are protozoa which remove free <br /> swimming bacteria and other suspended particles. <br />