Hydraulic underflow

The term passive interception is used to describe recovery systems that rely upon natural groundwater flow to deliver free-phase NAPLs to the collection facility without the addition of external energy (such as pumping). These systems often include linear interception-type systems such as trenches (or French drains), subsurface dams ( funnel-and-gate structures), combined hydraulic underflow with skimming, and density skimming units. 

Construction of a subsurface structure that penetrates the water table only a short distance can be an effective LNAPL retention technique. Most underflow structures function in the same way as a surface baffle in an oil water separator or a septic tank. The structure must be carefully installed perpendicular to groundwater flow, and have some arrangement to collect the free-phase LNAPL. Often, a simple French drain (constructed parallel to the retaining wall) leading to a recovery well (with skimmer) is effective. A schematic diagram of a hydraulic underflow structure with a skimming unit is presented in Figure 7.3. 

FIGURE 7.3 Schematic of hydraulic underflow with a skimming unit. 

Gives very clean sands and has relatively low hydraulic water requirements (0.5 t/t underflow). One of the most efficient single-stage classifiers available for closed circuit grinding and washing. Relatively expensive. 

Hydraulic cylindrical tank classifier Om tp (M-F) Hydraulic form of overloaded thickener. Siphon-Sizer (N-F) uses siphon to discharge underflow instead of rotating rake. 1.0 to 40 1.4 mm to 45 im (25 mm) 1 to 150 Not critical 0.4 to 15 20 to 35 0.75 to 11 Two-product device giving very clean underflow. Requires relatively little hydraulic water (2 t/t solids feed). Used for washing, desliming, and closed circuit grinding. 

Hydraulic cone classifier V°W (M-F) Open cylindrical upper section with conical lower section containing slowly rotating mechanism. 0.6 to 1.6 400 im to 100 im (6 mm) 10 to 120 Not critical 2 to 15. 30 to 50 3 to 7.5 Used primarily in closed circuit grinding to reclassify hydrocycloue underflow... 

Basically a tube with hydraulic water fed near bottom to produce hindered settling. Underflow withdrawn through valve at base. Column maybe filled with network to even out flow. 

Efficient separations, but requires 3 t hydraulic water/t solids feed. Used to produce exceptionally clean underflow fractioned into... 

Principal Option for Containment/ Recovery Excavation Vacuum extraction Temporary cap/cover Hydraulic modification No action Groundwater pumping Subsurface drains Hydraulic barriers Low permeability barriers No action Overflow/underflow containment (i.e. oil booms) Run off/run on control Diversion/collection No action Capping/ nsulation Operations modifications Gas collection/removal No action... 

The second-stage (ammonia removal) effluent contained unacceptable levels of F and P and had to be subjected to third-stage lime treatment. This raises the pH from 8.5 to 11.4 and produces an effluent with concentrations of F and P equal to 25 and 2 mg/L, respectively. The hydraulic design parameters were a 15 min reaction time and a 265 gpd/ft (10.8 m /m /day) clarifier overflow rate. The resulting precipitated solids underflow concentration was 0.6% by wt. In all three stages, an anionic polymer was used to aid coagulation, solids settling, and effluent clarification. 

As long as the hydraulic conductivity of the aquifer is less than that of the PRB, underflow of contaminated groundwater should not occur. 

An essential factor for good table operation is that the rate of feed must be uniform, both as to tonnage and as to physical properties. No one factor will cause more trouble to the table operator than to have a surging feed. The feed to tables may be unsized, or it may be either screened or hydraulically classified. For treating fine coals a common procedure is to use hydrocyclones both to deslime the material and to give a cyclone underflow of about 40 percent solids, which constitutes the table feed. 

Hydraulic flow (underflow) of raw sludge from primary clarifier (1 lbH2O/1.051b raw sludge 6.51b dry solids/100 lb raw sludge). 

Hydrocyclone Hydraulic device for separating suspended solid particles from liquids by centrifugal action. Cyclone action splits the inlet flow, a small part of which exits via the lower cone, the remainder overflowing the top of the cylindrical section. Particles are separated according to their densities so that the denser particles exit via the cone underflow and less dense particles exit with the overflow. 

Sulphur Pit Calibration of each section of the pit position of each overflow and underflow covers and grids on the pit hydraulic test for each steam coil ... 

The simplest way to operate a hydrocyclone is to open both the underflow and overflow to the atmosphere because this ensures correct hydraulic balance between the two outlets, independent of what happens downstream of either of the outlets. VirmaUy aU test data available on different cyclone designs, as well as theoretical or empirical formulae for performance characteristics, are for such conditions of equal back pressure on both discharge streams. 

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