1. As more complete mixed reactors in series are provided, the system becomes mo
ID: 1822919 • Letter: 1
Question
1. As more complete mixed reactors in series are provided, the system becomes more plug flow. For a substance that decays according to first-order kinetics, what is the equivalent PF rate constant that would be used to describe treatment in a series of three CM reactors in terms of flow (Q), the total volume in the system (V), and the rate constant observed for the CM system?
2. An activated sludge plant is to be designed for a population of 4000 persons. The influent flow is 380 Lpcd and the influent BOD5 is 200 mg/L. Pertinent data are: Primary clarifier removes 33% of the influent BOD5, and the state regulatory agency allows a volumetric loading of 0.48 kg BOD5 /(m3.day). Determine the reactor basin volume.
3. Compare the total reactor volume required for 90% removal of a substance that decays according to first-order kinetics with a rate constant of 0.05 day
Explanation / Answer
1.. Mixing was studied in free water surface wetland receiving pumped river water, by measurement of the non-interacting tracer lithium. The flow pattern was found to be intermediate between plugflow and well-mixed. The nominal detention time, calculated from volume aand flow, was 50% larger than the mean tracer detention time. The peak time was found to be one-half the tracer detention time. Three models were constructed: plugflow with dispersion, tanks in series, and aseries-parallel network of tanks. All proved capable of fitting the exit tracer concentration curves but the network model provideda better fit to internal measurements. Pumping frequency was high enough to allow use of an average flowrate. The degree of mixing, as characterized by the variance of the exit tracer response curve, was comparable to that found by other researchers for wetlands, ponds and rivers. Keywords Detention time; Flow modeling; Hydrology; Mixing; Tracer testing; Wetlands 3. The anaerobic baffled reactor (ABR) contains a granulated, mixed anaerobic culture segregated into compartments. Operation of four reactors under a range of hydraulic retention times showed that this novel reactor design offers highly efficient performance in the conversion of carbon in the feed stream to methane and carbon dioxide. The design parameter varied was the number of compartments. COD removal at 20 h retention time was routinely over 95% in all reactors, with low washout of biomass. Very high specific reaction rates were achievable (although with a loss of efficiency) at low biomass concentrations and high loading rates. In order to optimize volumetric reaction rates, a tradeoff has to be made between high biomass concentration, granule size, and the resulting mass transfer limitations. Formate is shown to be an important intermediate in the process under conditions of high loading.
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