Pond Depth

A summary of the BURRO test series parameters is given in Table VII. The colunm entitled spill-plate location needs some comment. In BURRO-2 through -8, the location of the spill plate relative to the water level changed due to normal pond evaporation. That is, the pond depth... 

Keywords Costs Environmental concerns Evaporation pond design Evaporation rate Liners Pond area Pond banks Pond depth Social impacts... 

The minimum depth of a pond is directly proportionate to the rate of evaporation. This depth needs to allow for increases in volume, the precipitation of salts, as well as for rainfall and waves. It is estimated that the best evaporation rate can be achieved with pond depths of 0.03-0.45 m, however ponds with depths of up to 1.02 m have been shown to have reasonable evaporation rates (Mickley 2006). Similarly to pond area, a safety factor can be applied to the calculated minimum pond depth to increase the capacity and prevent the pond from overflowing. This extra depth will depend upon the expected additional discharge volume at the beginning of plant operation (Mickley 2006), and the ambient conditions during winter, at which time the pond may store water rather than reduce its volume (Ahmed et al. 2000). 

For a stand-alone photobiological (sulfur-deprived, algal) H2-production facility producing 300 kg/day of H2, the total capital investment was estimated to be 5 million with a H2 selling price of approximately 14/kg of hydrogen and a 15% return on investment. This system assumed moderate improvements in the H2-production rate and included PSA purification with high-pressure compressed H2 storage. The total photobioreactor area was 110,000 m with a 10-cm pond depth, 0.2 g/1 cell concentration, and 10/m reactor cost. ... 

Temperature Highly variable, some control possible by pond depth Cooling often required (by spraying water on PBR or immersing tubes in cooling baths)... 

Prooess oontrol and reproducibility Limited (flow speed, mixing, temperature only by pond depth) Possible within certain tolerances... 

Applying all possible combinations of incident photon flux density, pond depth, and hydraulic retention time, Kroon et al. built a model to predict the algal biomass... 

An advance on the plain design (see Section 2.2.4.2) is to recess the weir plates into holes machined into the hub. A thicker cover plate would secure each weir plate. The outer edge of the weir and cover plate would be circular to locate in the recess. Different pond depths are then obtained with different weir plates. 

An alternative to the centripetal pump is the skimmer pipe, by means of which the centrate is skimmed or scooped from the pond surface in the bowl. The position of the skimmer in the bowl can be altered from the outside while the bowl is rotating, and so afford a means of pond depth adjustment during operation. This is done using a lever on the outside of the casing attached to a pipe positioned over and through an annular plate mounted on the front face of the front hub of the bowl. The skimmer pipe thus dips into the bowl pond... 

The gaps under cones are usually around half the pond depth. However, much smaller gaps have been used in vegetable oil refining (in a three-phase decanter). Uniquely in vegetable oil refining a double concentric cone has been used where the caustic soda wash is fed between the cones, rhe wash exits at the periphery of the cones under the surface of the oil. 

Compare E of equation (4.39) with E of equation (4.32). Note the extra numeral 2 in equation (4.38) compared with equation (4.33). This is to be expected if all the particles that have to be separated have the advantage of starting at half pond depth ... 

Figure 4.7. Graph comparing the various formulae for Sigma at various pond depths.

All the formulae indicate that a better clarification capacity is achieved at the shallowest pond depth, whereas in practice it is generally the opposite. Therefore, the simple formula is generally considered sufficient for practical purposes. However, when scaling from one machine to another, it is imperative that the same formula is used for both machines. It is also recommended that one should not normally scale between machines of dissimilar geometry. 

Classification, the fractionation or separation of particles by size, could be considered as merely inefficient clarification. The cut, or desired classification, is adjusted by altering the centrifuge s efficiency. This is most easily done by altering the feed rate or bowl speed. However, adjustment of pond depth or differential may, in certain circumstances, be used. 

As the factor I is increased, cake dryness decreases and solids recoveries increase, and vice versa. Very good correlations can be found between cake dryness and and with solids recovery and for fixed pond depths and fixed polymer dosages. 

In t he dewatering of compressible cakes, as much pressure as possible is put on the cake, before adversely affecting capacity or centrate clarity, in dry solids operation, which will he discussed in more detail in other sections, it has been reported [1] that increasing cake height, and thus pressure in the centrifuge bowl, improves dryness capability. Thus, for such applications, it would be advantageous to maximise pond depth. To estimate the pressure within the pond, refer to equation (4,61),... 

For a cake drainage dewatering, scale up would be by one of the cake conveyance formulae, equations (4.65) or (4.68), and pond depth and conveyor differential would he judiciously adjusted to ensure the residence time in the critic.nl areas was kept the same. 

It has just been shown that the perft)rmance of a dry solids decanter is related to conveyor torque achievable, bowl speed, pond depth, and Uoccuiant usage. Once the cake dryness has been fixed, it is useful to be able to assess the maximum capacity possible on a given decanter. 

The precise design of the test decanter needs to be adapted to the process to be tested. Oil/water separation requires a three-phase design, solvents require flameproof electrics, effluents need abrasion protection. While manufacturers will have a pool of test decanters of basic design, it is essential that they are able to adapt them for special application testing. For instance, they will have special decanter conveyors that can be introduced, when necessary, and will be able to make small changes to them where necessary, such as the addition of a floater disc for floating solids. With a test decanter it is essential to be able to adjust bowl speed, differential and pond depth to suit particular applications or difficulties as they arise. 

The laboratory decanter was limited by its small gearbox torque, and by its relatively small pond depth. However, its performance was sufficiently encouraging to warrant the larger scale tests and was confirmed in practice. The last laboratory test recorded stretched the limits of the decanter to demonstrate the feasibility of extra dryness. Because of the lower recoveries of the last three test runs, these are not included in the graphs. For the first pilot plant series, no centrate samples were analysed, but adjudged good and so a nominal figure is used for the sake of the calculations. 

Table A. 5 in the Appendix contains a sample of data from a test series on thickening municipal WAS in a 737 mm diameter decanter bowl. Salient data from this table are plotted on the graphs in Figures 6.18 and 6.19. From these graphs it will be seen that conveyor differential controls both dryness and solids recovery. Feed rate also has a marked effect on both cake dryness and recovery. Increasing bowl speed with a slight decrease in pond depth makes the cake dryness a little more sensitive to conveyor differential change. Recovery is unaltered. Extra pond depth may have helped recovery, but this may have impaired dryness control.

It will be seen that just as good a dryness was achieved at the low g level, with the benefit of reduced polymer usage and cleaner centrate. It is apparent that with this relatively heavy process material, the high g produced high torques and low scrolling efficiencies, even with extra pond depth. Lower differentials were possible at the lower g level, to compensate for the smaller g in obtaining the required cake dryness. 

The data series in the graph in Figure 6.35 are separated, for easier definition, in Figures 6.36-6.39. It can be seen from these that all but the smaller machine lines are coincident. The slight difference of the smaller one is probably due to its slightly reduced pond depth. 

A centrate clarity limiting application is characterised by a fall off in centrate quality, when feed rate is increased, independent of conveyor differential, once pond depth has been optimised. Spent wash dewatering, discussed in the previous chapter, is one such example. 

The pond depth relative to the bowl diameter is larger for no. 1 decanter and smaller for no. 2. This would suggest there is some performance in hand with the estimate for no. 1 and suggest some caution with the estimate for no. 2. This would bring the performance levels for both closer, and thus the choice would probably be biased towards the smaller of the two. 

These are mainly the feed and polymer pumps, and the decanter brake torque or speed. However, in special cases, the actual bowl speed could be a part of a control strategy. The pond depth itself, using the inflatable dam, could be used in a thickening control strategy. 

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