Gravity sedimentation is one of the most widely used processes for separating solids from liquids whether it be for ore and mineral processing or for treating municipal sewage or industrial wastes. Investment in sedimentation equipment can represent a significant portion of the total capital costs associated with a processing or treatment facility. In spite of the importance of sedimentation, little attention has been devoted in recent years to improving this equipment by applying the basic principles involved in sedimentation. In the water and waste treatment field, design criteria established one half-century ago is still the accepted standard and in reflection of this lack of progress, sedimentation and thickening equipment has remained virtually unchanged.
It is apparent from the theoretical aspects discussed above that shallow tubular passageways provide one shape which would satisfy the basic requirements for ideal settling conditions in that they could provide shallow depth, a large wetted perimeter, laminar flow conditions, and reasonable overflow rates. For example, a tube diameter of 4 inches at a flow rate of 10 gpm per square foot of tube end area would provide a Reynolds number of less than 100. A one-inch diameter tube, 4 feet long, at a flow rate of 10 gpm per square foot would provide a Reynolds number of 24 and an equivalent surface overflow rate of 235 gpd per square, foot while having a detention time of only three minutes. The very short detention times make the space- saving potential of such configurations readily apparent.
The authors have described two basic tube settling systems steeply inclined. In water and tertiary treatment applications the operation of the low angle tube settlers is coordinated with that of the filter following the tube settler. Each time the filter backwashes, the tube settler is completely drained. The tubes are inclined only slightly in the direction of flow (7°) to promote the drainage of sludge during the backwash cycle. The falling water surface scours the sludge deposits from the tubes and carries them to waste. The water drained from the tubes is replaced with the last portion of the filter backwash water, thus no additional water is lost due to this draining procedure.
Some practical means had to be developed to incorporate angle tube settlers into a modular form which would be economical to build and could be easily supported and installed in a sedimentation basin. Following preliminary evaluation of a great many potential designs, a tube module design (patent pending) in which the material of construction is normally PVC and ABS plastic, was developed. Extruded ABS channels are installed at a 60° inclination between thin sheets of PVC. By alternating the direction of inclination of each row of channels forming the tube passageways, the module becomes a self-supporting beam which needs support only at its ends.
Water Treatment Applications
Tube clarifiers have had fairly extensive application in the clarification of water and wastewater since introduction some two years ago primarily since we are most actively engaged in the Sanitary Engineering field. In the area of minerals classification or ore thickening we have had only limited exposure confined to laboratory bench scale studies. In the ensuing sections several water treatment and waste treatment applications will be discussed which will serve to illustrate how the tube clarifiers are applied in this area.
In water treatment practice, for both municipal and industrial plants, tube modules can be applied in existing clarification facilities to double unit capacity without construction of new structures. Through pilot plant and full scale plant experiences we have found the tube clarifier to be effective solids separation devices at overflow rates of from 3 to 4 gpm per square foot in comparison to overflow rates of 0.5 to 1 gpm per square foot required to provide equivalent performance in conventional clarifiers. In converting the existing clarifier.
One particular municipal water treatment plant in which tube modules were recently installed. The original plant is conventional rapid sand filter plant and utilizes horizontal flow sedimentation basins. The plant capacity has been tripled by the installation of tube modules and mixed media filtration materials within existing structures. Approximately 3,000 square feet of modules were installed in the three sedimentation basins to increase the capacity from 8 to 22 MGD. Note that the filters were also, converted to high-rate filtration by installing mixed media filter materials and enlarging the filter piping to operate at 8 gpm/ft².
Waste Treatment Applications
An important prerequisite for successful clarification of any liquid waste or process stream is that the material being separated have a reasonably fast settling rate. If rates are low the clarification equipment becomes exceedingly large and costly. Very fine clays and minerals with average particle sizes ranging downward from approximately 0.002 mm settle very slowly and hence steps must be taken to improve settleability. Particles in this size range may have free settling velocities ranging from 1 x 10-² to 1 x 10 -4 cm per second (2-200 gpd/ft²) and will definitely require coagulation to increase settling rates if they are to be removed successfully and economically in a gravity clarifier. Coagulation with aluminum sulfate, ferric chloride, ferrous sulfate, etc., or with organic polymers can destroy the stability of this finely divided particle, allowing them to coagulate and to settle at much greater rates. Well flocculated alum floc will settle at velocities ranging from 0.1 to 0.3 ft/min. (1000 gpd/ft² – 3250 gpd/ft²) depending upon the amount of associated turbidity, water temperature, etc while calcium carbonate particles produced in water softening applications will settle at rates as high as 0.6 ft/min.