Hydraulic residence time

In aerobic processes, the mean sludge residence time is typically 5 to 10 days. The hydraulic residence time is typically 0.2 to 0.3 days. Suspended growth aerobic processes are capable of removing up to 95 percent of BOD. 

The two micro bubble columns comprising the smaller micro channels reached nearly 100% conversion [5], The micro bubble column with the largest hydraulic diameter reached at best 75% conversion. The curve obtained displays the typical shape, passing through a maximum due to the antagonistic interplay between residence time and specific interfacial area. 

In situ oxygen supply requires aeration wells for the injection of oxygen. The criteria are that the aeration well zone must be wide enough to allow the total plume to pass through, and the flow of air must be sufficient to produce a substantial radius of aeration while small enough so as not to create an air barrier to groundwater flow. The required residence time tr for aeration can be calculated from Darcy s law as a function of the groundwater head and hydraulic conductivity ... 

It is important to know from Equation 27.7 that the performance of a complete mix with recycle system does not depend on hydraulic retention time. For a specific wastewater, a biological culture, and a particular set of environmental conditions, all coefficients Ks, b, Y, and km become constant. It is apparent from Equation 27.7 that the system performance is a function of mean cell residence time. 

The term (V/F) has units of time when V and F apply to an entire system, (V/F) is often called the "hydraulic residence time" or "detention time" of the water body.)... 

Hydraulic residence time (HRT = V/Q). Whatever be the ideal configurations of the continuous reactor, mass balances suggest that, for a given reactor and amount of biophase (V and X fixed), the conversion of the azo-dye should increase when Q decreases. In other words, DyeL at the reactor exit should decrease when the HRT increases. Along with this effect, the observation of Senan et al. [21], Ong et al. [33], Bras et al. [28], Georgiou and Aivasidis [37] and the relations reported by van der Zee and Villaverde [2] may be interpreted. 

Dividing throughout by F0, the recycle ratio, R = Fr/Fq and the hydraulic residence time, 6 = V/Fq, then ... 

More advanced water purification techniques, utilizing UV-irradiation, ozonization, or activated charcoal, may significantly improve the removal of these compounds, but these techniques are not broadly applied due to their high cost. Thus, current European activated sludge treatment plants, with a hydraulic residence time not greater than 14 h, can in most cases not completely eliminate all the estrogens and progestogens from the effluent [23]. 

Dump fill, instantaneous addition of waster water into the reactor is rarely used in the field but is implemented practically by including a static fill in the operating strategy. The mathematical representation for the SBR with dump fill is the same as that for the plug reactor at steady state, where the hydraulic residence time in the PFR compares to the time for react in the SBR. 

With hydraulic residence times ranging from months to years, lakes are efficient settling basins for particles. Lacustrine sediments are sinks for nutrients and for pollutants such as heavy metals and synthetic organic compounds that associate with settling particles. Natural aggregation (coagulation) increases particle sizes and thus particle settling velocities (Eq. 7.1) and accelerates particle removal to the bottom sediments and decreases particle concentrations in the water column. 

The experimental aspects of this study were focussed on two hard-water lakes in Switzerland, namely, the northern basin of Lake Zurich and Lake Sempach. The hydraulic residence time of Lake Zurich is 1.2 years. Most of the particles in the lake are produced directly or indirectly by biological processes within the lake itself (e.g., photosynthesis, CaC03 precipitation). Phosphorus removal has been implemented in recent years at all wastewater treatment plants discharging into the lake at present Lake Zurich can be described as between meso- and eutrophic. Lake Sempach has an average hydraulic residence time of 15.8 years as in Lake Zurich, particles in the lake waters are primarily autochthonous. Phosphorus concentrations have increased substantially and the lake is eutrophic. 

To this was added reactor hydraulics and microbial residence time (sludge age) and a given reactor configuration, resulting in the following Figure 2 ... 

The significance of this research showed that they could obtain excellent COD, BOD, TSS and NH3 removals in the effluent of their system with hydraulic residence times between 0.6 and 5 days and they obtained COD/N/P ratios of 167/3.3/1 and 127/2.8/1 which is substantially higher than the Metcalf and Eddy recommended 600 5 1 ratios for anaerobic reactors. Their work was performed at around 35oC + 2Co and their overall reactor treatment train looked like the following Figure 5. 

In a two-part series. Zeme discusses the importance of good separator hydraulics. A poor hydraulic design can make a good separation scheme ineffective. Zemel provides the methods and procedures to run a tracer test to identify short-circuiting, stagnant-flow regions, and shear forces. Analysis of the residence-time distribution curve that results is presented. Actual tests run on separators indicate that the most successful separator was the sequential dispersed-gas flotation cell, which closely followed the tanks-in-serie< model. This is contrasted with the poor performance of a conventional 2, 006-hbl [3 0-ms] wash tank The tracer responses of a pressurized flotation cell, a 15j000-bbl [2400 mJj wash tank, and a horizontal free-water knockout with and without baffles are also discussed. 

The problem in applying this residence time to field operations ties in that these are continuous operations. They will therefore have a distribution el residence times rather than a single time as in a batch calculation. The common assumption that the mean residence time in a vessel is equivalent to the treatment time of a batch test implies an ideal hydraulic behavior which doesn t occur in the field. 

The method of sizing a separator tank by multiplying the flow rate by the desired residence time must include a large factor of overdesign lo account for the real hydraulic characteristics of the system. The only other option is to define the hydraulic behavior as was done in the design of the API separator widely used in refineries. 

The application of the index-lake method presented here is only for lakes that are at hydraulic and isotopic steady states. A lake s steady-state isotopic composition is determined by the long-term averages of 8a, 8P, h, P, E, and water and air temperatures, which can vary greatly by the day and season and can vary to some degree annually. Therefore, it is only proper to apply the index-lake method to lakes of similar hydraulic residence time, during which time the averages of these controlling factors are determined. 

The isotopic compositions of four lakes (see Figure 3 for lake locations, in northern Wisconsin that are within 10 km of each other and that have hydraulic-residence times of about 10 years are plotted in Figure 4. The four lakes (Crystal, Pallette, Big Musky, and Sparkling) are groundwater flow-through lakes and have no surface inflows or outflows. Thus they have the same hydrological-budget components (they receive water from precip-... 

Application of Isotopic Methods to Non-Steady-State Groundwater-Lake Systems. Isotopic compositions of lakes whose hydraulic-residence times are relatively short (about 2 years or less) vary seasonally (32). Seasonal response occurs whenever a significant mass of water of a different isotopic composition is either added to or removed from the lake. Seasonal variations in P, E, G, 8P, 8a, and 8 are the principal driving forces behind observed variations in 8j for isotopically non-steady-state systems. In northern Wisconsin, many lakes are isotopically non-steady-state. 

In the simplest case, groundwater-flow rates for lakes at isotopic steady state (or those with relatively long hydraulic-residence times) can be estimated from data on average annual precipitation rates average annual evaporation rates the isotopic compositions of precipitation, lake water, and inflowing ground-water and relative humidity and lake temperature. 

---