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SEDIMENTATION Horizontal Flow
A sedimentation tank of the rectangular horizontal flow type has been installed. Flow to this unit can be varied by means of a metered bleed on the inlet pipe. The tank is designed to operate at flows between 1 O and 57 gpm corresponding to detention times of 200 to 50 minutes and surface loadings of 260 to 1490 gpd per sq. ft. Tests have been aimed at finding the maximum overflow rates which will give a settled water turbidity of about 5 Jackson turbidity units. Because of viscosity changes in water with variation of temperature, sedimentation efficiency is very temperature sensitive as shown below: Overflow Rates (gpd/sq. ft.) Temperature 0 c 1400 25 1170 20 960 13 ¡ Sludge concentration varies from solids content of 1.4 per cent after one day retention to 2A per cent after about seven days retention in the sedimentation tanks. Distribution characteristics of the inlet conditions
have proved very critical if optimum performance is to be obtained from the settling basin. Uneven distribution will result in short circuiting along the basin with resultant high levels of turbidity in the settled water and consequential additional solids load to the filters. Design practice has been to adopt horizontal openings in the baffle walls between the flocculators and the settling basin. Approximately 30 per cent openings in these walls has proved effective in maintaining floe
formation. Solids Contact Clarifier A small conical bottom solids contact clarifier 6 ft. in diameter has been installed. Water from the mixer or flocculator can be fed to the tank at rates up to 41 gpm corresponding to a surface load of 2,100 gpd per sq. ft. On one side a slurry weir between the hopper is provided, the height of the slurry weir between the hopper and the upflow section being <1diustable. Coagulated water enters at the bottom via a central vertical pipe and turbulent mixing occurs. It then flows upwards through the suspended slurry blanket and over the circumferential weir. The clarified water may be passed to the rapid sand filters or directed to waste.
This unit has proved to be very sensitive to changes in. operation and varied in its efficiency. Flocculation was found necessary after rapid mixing with the best surface loading rates being found by establishing a rate which gave a stable floe blanket in the clarifier. Hydraulic
and
temperature
variations
can
cause
'boiling¡ of the sludge blanket and carry over of floe becomes difficult to control with the result that satisfactory sedimentation may result with overflow rates varying over a range of 1 500 - 21 00 gpd/ sq. ft. Tube Settler Clarifier A small tube settler unit has also been installed. The actual clarifier compartment is 2 ft. 6 in. square and
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contains plastic modules forming 2 in. square tubes at 60 degree to the horizontal. The depth of the modules . is 21 in. Chemically dosed and flocculated water enters a chamber on one side, flows down, then up through the module tubes and is then decanted over the offtake weirs. Floe settling in the tubes rolls down and falls into the sludge hopper at the base of the unit. Sludge is withdrawn at intervals from the bottom of the hopper. The clarified water may be passed to the filters or directed to waste. Results indicate that an increase of 3.5 times the loading in a horizontal flow basin can be achieved, i.e.
5,500 gallons per day per sq. ft. The evaluation of many factors, will be needed before a final decision is made on the use of these units. ,, Adequate sludge removal facilities must be provided for this type of unit. FILTERS Four filters have been provided designed to filter up to 8 gpm per sq. ft. Each consists of a perspex tube between 7 and 8 ft. high, filters A, B and C being 8-in. square and filter D being circular with an area of 1 sq. ft. Each has a metered inlet at the top and an outlet fitted with a rate of flow controller at the bottom. Backwash and air scour facilities for rates of up to 20.5 gpm per sq. ft. and 4.5 cubic feet per minute per sq. ft., respectively, have been provided. Head loss gauges indicate when backwashing should be in'itiated. Specification for Filter Media
SAND Depth (in.) Effective Size (mm)
12 0.45 Uniformity Coefficient 1.50
ANTHRACITE Depth (in.I
18 1.0 1.40
Effective Size {mm) Uniformity Coefficient
All filters have 9 in. of graded gravel under their media beds, filter A was originally set up as an upward flow filter with 4 ft. 6 in. of sand (effective size 0.45 mm). After sufficient tests of the upward flow principle had been completed the unit was converted to the conventional downflow type. Filter D was installed later in the testing programme and was designed to allow testing of the various types of commercially available underdrain nozzles. In addition piezometers were installed at various depths in the filter to enable head loss studies to be made. Currently tests are proceeding with the packed as under: B C Mixed Media 18 in. Anthracite 18 in. 12 in. Sand E = 0.45 Anthracite (coarse) 9ln. Sand E = 0.55 U = 1.50 U = 1.50 9 in. Graded 3 in. IIlmenite Sand Grave! 3 in. Garnet 9 in. Graded Gravel Gravel 9in. Graded Gravel
A 54 in. Sand
filters
D 18 in. Anthracite 12in. Sand
E =0.45 U = 1.50 9 in. Graded Grave\
At this stage no comparative figures are available on respective filter performances.