Drainage beds

The sohd can be contacted with the solvent in a number of different ways but traditionally that part of the solvent retained by the sohd is referred to as the underflow or holdup, whereas the sohd-free solute-laden solvent separated from the sohd after extraction is called the overflow. The holdup of bound hquor plays a vital role in the estimation of separation performance. In practice both static and dynamic holdup are measured in a process study, other parameters of importance being the relationship of holdup to drainage time and percolation rate. The results of such studies permit conclusions to be drawn about the feasibihty of extraction by percolation, the holdup of different bed heights of material prepared for extraction, and the relationship between solute content of the hquor and holdup. If the percolation rate is very low (in the case of oilseeds a minimum percolation rate of 3 x 10 m/s is normally required), extraction by immersion may be more effective. Percolation rate measurements and the methods of utilizing the data have been reported (8,9) these indicate that the effect of solute concentration on holdup plays an important part in determining the solute concentration in the hquor leaving the extractor. 

Batch Extractors. Coarse soHds are leached by percolation in fixed or moving-bed equipment. Both open and closed tanks (qv) having false bottoms are used, into which the soHds are dumped to a uniform depth and then treated with the solvent by percolation, immersion, or intermittent drainage methods. 

Static holdup depends upon the balance between surface-tension forces tending to hold hquiciin the bed and gravity or other forces that tend to displace the liquid out of the bed. Estimates of static holdup (for gravity drainage) may be made from the following relationship of Shulman et al. [Am. Jn.st. Chem. Eng. J., 1, 259 (1955)] ... 

Calvert (R-12) has tested the correlation in cross-flow packed beds, which tend to give better drainage than countercurrent beds, and has found the effect of gas-flow orientation insignificant. However, the onset of reentrainment was somewhat lower in a bed of 2.5-cm... 

A means of drainage, collection, and transportation of excess water within the filter bed... 

Good bed drainage characteristics are necessary to ensure that reaction products are easily transported out of the filter media. The leachate is generally recycled through the humidification process to reduce the wastewater stream. 

Soils sampled at two sites in creek beds and drainage ditches in an agricultural area in the Point Mugu watershed near Oxnard, California, contained endosulfan at concentrations between 20 and 30 ppm. The majority of the other sites had much lower concentrations (Leung et al. 1998). 

The values quoted above are for FYM from overwintered cattle, but the method of housing influences nutrient loss. With cattle in covered yards sufficient bedding must be used to absorb the liquid manure and there is little or no drainage the dung is kept well compressed by the animals and fermentation is reduced to a minimum. Under such conditions very little loss takes place. 

The veins of skin are organized along the same lines as the arteries in that there are both subpapillary and subdermal plexuses [11]. The main arteriole communication to these is the capillary bed. Copious blood is passed through capillaries when the core body is either feverish or overheated, far more than needed to sustain the life force of the epidermis, and this rich perfusion lends a red coloration to skin. When there is opposite physiological need, the capillary bed is short-circuited as blood is passed directly into the venous drainage by way of the arteriovenous anastomoses. Fair skin noticeably blanches when this occurs. These mechanisms act in part to regulate body temperature and blood pressure. 

It is probably beneficial to a passive system to lay a network of perforated drainage pipes in the gravel bed beneath the slab prior to the pour. The use of horizontal pipe runs and elbows in a passive system may greatly lessen the effectiveness the system and should be a voided. Some builders use a 6-in. PVC pipe in a passive system to help lessen the pressure drop. 

The capillary pressure PC(S) exhibits a marked hysteresis phenomenon when the liquid is alternately withdrawn (drainage) and introduced (imbibition) into the particulate bed. Consequently, capillary pressure changes as a result of variations in saturation do not follow a unique functional relationship. In fact, the suction is always higher on the drainage side of the imbibition-drainage cycle (M8). In Fig. 7 the suction curve starts at zero when S = 1. 

These acidic waters are toxic to plant and animal life, including fish and aquatic insects. Streams affected by acid drainage may be rendered nearly lifeless, their stream beds coated with unsightly yellow and red precipitates of oxy-hydroxide minerals. In some cases, the heavy metals in acid drainage threaten water supplies and irrigation projects. 

To see how this process works, we construct a model in which reaction of a hypothetical drainage water with calcite leads to the precipitation of ferric hydroxide [Fe(OH)3, which we use to represent HFO] and the sorption of dissolved species onto this phase. We assume that the precipitate remains suspended in solution with its surface in equilibrium with the changing fluid chemistry, using the surface com-plexation model described in Chapter 10. In our model, we envisage the precipitate eventually settling to the stream bed and hence removing the sorbed metals from the drainage. 

We do not concern ourselves with the precipitate that lines sediments in the stream bed, since it formed earlier while in contact with the drainage, and hence would not be expected to continue to sorb from solution. Smith et al. (1992), for example, found that in an acid drainage from Colorado (USA), sorption on the suspended solids, rather than the sediments along the stream bed, controls the dissolved metal concentrations. 

Stream beds, drainage ditches, Nova Scotia ... 

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