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WASTEWATER TREATMENT PLANT PROCESS DESIGN
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BIOLOGICAL PROCESSES The material offered provides links to tools that you
may find useful in Concept Designs for Wastewater Treatment Plants.
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When the project involves an existing treatment plant that is to be expanded, thoughts about interpretations or existing data is found in the above document “SELECTING A DESIGN BASIS FOR WWTP EXPANSION" |
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An Excell spreadsheet, “Activated
Sludge Performance Estimate", is offered that may be
useful in interpretation of existing process performance. The spreadsheet attempts to estimate
Influent soluble biodegradable organic material by subtracting a BOD5
for VSS (say 0.8 mg/mg) from the BOD5 values. Assuming that
soluble COD values are not available, an attempt is made to convert soluble
BOD5 to soluble biodegradable COD using a ratio for biodegradable
COD to BOD5 (usually assumed, say between 1.8 and 2.0). The same procedure is repeated for effluent data,
but the assumed BOD5 of VSS is reduced to 0.3 to 0.5 mg/mg. If the project involves industrial waste or a
municipal system treating a large fraction of industrial wastes, an idea of
kinetics (how fast the microorganisms work) is needed. The spreadsheet offers a way to estimate
kinetics from available performance data. Don’t be surprised if the attempt to estimate
soluble BOD5 results in negative numbers. I normally throw out negative numbers or
adjust the data if there is an apparent reason, such as an obviously
out-of-line VSS number. Is the
effort worthwhile? Possibly. It can be
a guide, but in the final analysis the entire effort is supported by
assumptions. The entire effort is an attempt to develop the oxygen equivalent of biodegradable organic material, i.e. soluble biodegradable COD. This number can be measured using respirometry and a procedure is identified. This is the kind of information that we should be developing; not “screwing around” with BOD5 |
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The Activated Sludge Model is more an engineer’s model as opposed to a scientist’s model. Physically, the reactor is composed of one to 10 completely mixed stages. A separating wall may create a stage or staging may reflect a degree of hydraulic staging due to configuration (for example, long and narrow reactions basins). Any stage may:
Return of sludge from a clarifier is mathematically represented by an initial settling velocity (ISV) relationship. Within each stage there are estimates of:
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A discussion of theory for the
Activated Sludge Model is available
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Sludge production is highly important to a practical estimate of activated sludge performance, particularly nitrification. The model provides an estimate of sludge production for any selected set of wastewater inputs and operating conditions. The Excel spreadsheet above will also provide an estimate. Its estimate is somewhat lower that that obtained from the model because the model takes into account solids from biological stabilization of organic material from VSS destruction. A pitfall in Vendor’s process design is a failure to reasonably estimate sludge production. |
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The Activated Sludge Mathematical Model, a zipped file, can be down loaded. A Model Operation Manual can be found at: |
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The activated sludge model provides an estimate of oxygen requirements. It is the engineer's task to select and specify an oxygen transfer system. Information about converting actual oxygen demand to a demand at standard conditions is discussed in "Supplying Oxygen Transfer Requirements" . Thoughts about dealing with vendors transfer data are also provide.
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