Southern Methodist University School of Engineering and Applied Science SMU SSH 8321 SMU ME 5315

1. Southern Methodist University School of Engineering and Applied Science SMU SSH 8321 SMU ME 5315 NTU HW 741-N Treatment Technology I - Physical and Chemical Methods March 28, 2000 Dr. Roger Dickey

2. Conventional GMF’s operate in a semi-continuous, cyclical fashion as individual filter cells are removed from service for backwashing.

3. Continuous Flow GMF Equipment: • Recently, two types of continuous flow GMF’s have become common in wastewater treatment applications, • moving bed, continuous backwash filter • automatic backwashing filter

4. Moving Bed, Continuous Backwash Filter - The typical moving bed filter is a continuous backwash, upflow, deep-bed GMF. These filters are characterized by high hydraulic loading rates similar to multi-media filters although they contain only a single media, typically sand. The sand bed depth is often in the range of 3 ft to 4 ft.

5. Coarse sand with an effective size of 2 mm to 3 mm is often used and the sand remains unstratified due to the continuous flow nature of the filter. These filter units are typically constructed as steel or FRP cylindrical tanks. Backwash water requirements range from 2% to 7% of the influent feed rate.

6. The wastewater flow is introduced at the bottom of the filter and flows upward through the sand bed. The sand bed moves downward in a counter-current fashion. An airlift is used to pump the sand from the filter bottom through a central pipe to a washing chamber located at the top of the filter unit. The washing chamber is separated from, but is connected hydraulically to, the filter effluent.

7. In the sand washer, the accumulated wastewater solids are removed from the sand grains by abrasion and fluid shear forces. The cleaned sand grains are then redistributed on top of the sand bed, allowing for continuous uninterrupted flow of filtrate and reject backwash water.

8. A weir is used to maintain the water in the sand washing chamber at a slightly lower level than the filter effluent causing a positive upward flow of filtered water through the sand washer. The wastewater solids are flushed over the weir by the flowing reject backwash water for recycle, treatment, and disposal.

9. Automatic Backwashing Filter - The typical automatic backwashing filter is a downflow, shallow-bed GMF. These filters are characterized by high hydraulic loading rates similar to multi-media filters although they contain only a single media, typically sand. The shallow sand bed typically has a depth of 1 ft or less.

10. Relatively fine sand with an effective size of 0.35 mm to 0.6 mm is often used. The sand bed is divided into multiple rectangular compartments, each backwashed separately by a moving carriage. The carriage contains an isolation hood and other required equipment, instrumentation, and controls.

11. During the backwash cycle, the carriage and an attached hood move slowly over the filter bed, consecutively isolating and backwashing each cell. Wastewater is continuously filtered through the cells not being backwashed.

12. (2) Cartridge and Bag Filters Cartridge and bag filters are used for small to very small volumetric flow rates and for point-of-use systems. These filters are basically pressure vessels containing a cartridge with a wound-filament filter or woven bag.

13. Water passes through the wound-filament cartridge or bag, and particles large enough to be trapped in the pores of the filtering medium are removed. These filters should be used only for relatively low influent TSS concentrations because they can clog rapidly and allow solids to pass through into the effluent.

14. As the filter element gradually becomes clogged over the duration of the filter run, the pressure drop across the filter eventually increases to the point that the filter run must be terminated. Cartridge and bag filter elements are not backwashed, but rather the cartridge or bag is simply thrown away and replaced by a clean one.

15. (D) Flotation Flotation is a unit process by which solid particles or oil droplets in suspension become attached to microscopic air bubbles, giving the air-solids particles or air-oil droplets buoyancy. Given the right conditions, the agglomerate will rise to the liquid surface to form a “blanket” that can be removed by a mechanical skimmer.

16. Flotation is particularly applicable to wastes • having significant concentrations of: • fibrous material • finely divided solids • solids of low specific gravity • oil and grease

17. Flotation is also a common sludge thickening unit process (e.g., for waste activated sludge from a biological treatment system).

18. Flotation has the following advantages and disadvantages when compared to gravity sedimentation for solids-liquid separation: • Advantages - • Higher overflow rates and lower detention times can be used meaning smaller tank sizes and space requirements

19. Advantages (continued) - • Odor nuisance is minimized because of the short detention times and the presence of dissolved oxygen (for air flotation) • In some cases, thicker sludges are obtained

20. Disadvantages - • Additional mechanical equipment is required resulting in higher capital and O&M costs • Does not yield as high a treatment efficiency • Operation may be temperamental

21. Dissolved Air Flotation (DAF) is the most common method of flotation in wastewater treatment. Other gases are sometimes used for flotation and the process is then referred to as Dissolved Gas Flotation (DGF). For example, nitrogen gas is often used when an explosive gas mixture would result with the use of air.

22. DAF consists of pressurizing the wastewater with air at 15 to 50 psig and then suddenly releasing the pressure to atmospheric. When the pressure is released, the dissolved air in excess of saturation at atmospheric pressure forms extremely fine bubbles; 10 to 120 m with a mean of 40 m in diameter.

23. The air bubbles increase the buoyancy of suspended solids (or oil droplets) by entrapment of the bubbles in the particle structure and adhesion of the bubbles to the particle surface.

24. Principal components of a DAF unit include: • Pressurizing pump • Air dissolution tank • Pressure release valve • Air injection equipment • Flotation tank with skimmer

25. Two common configurations for DAF units: • Recycle pressurization - Figure 21.2, p. 633 Figure 23.2, p. 726 • Full flow pressurization - Figure 21.3, p. 634

26. DAF units are commercially available as prefabricated rectangular or cylindrical steel tanks and are often shipped complete with a skid mounted auxiliary equipment package ready for use. Tank depths vary from 3 ft to 10 ft. Chemical coagulants and flocculation aids are often used prior to DAF units. Increasing floc size facilitates the entrapment of rising air bubbles.

27. Lab bench-scale and/or pilot scale testing is almost always required to develop design data. Bench-scale (Figure 23.3, p. 727) and pilot DAF units are available for treatability studies from most manufacturers of DAF systems.

28. Design Parameters Include: (1) Air:Solids Weight Ratio, A:S where, = air mass flow rate (lb/min) = solids mass flow rate (lb/min) A:S is dimensionless [M/M] (e.g., lb air/lb solids)

29. (2) Overflow Rate, VO, where, Q = volumetric flow rate [L3/T] AP = flotation tank surface area [L2] VO is typically reported with units of gpm/ft2 or liter/min/m2

30. (3) Solids Loading Rate, RS where, = solids mass flow rate [M/T] AP = flotation tank surface area [L2] RS is typically reported with units of lb/hr/ft2 or kg/hr/m2

31. Type ranges for design parameters: (1) A:S = 0.005 to 0.20 lb air/lb solids (2) VO (including recycle) = 1.0 to 4.0 gpm/ft2 (3) RS = 0.5 to 5.0 lb solids/hr/ft2

32. (E) Sludge Concentration, Dewatering, and Disposal Concentration (Thickening) - The solids content of waste treatment sludges varies widely. Total solids concentration from various treatment operations ranges from 0.5% to 10% with 2% a fairly typical value.

33. Thickening is a procedure used to increase the solids concentration of sludges by removing a portion of the liquid fraction. Chemical conditioning is usually employed before thickening processes by the addition of organic polymers (flocculation aids). Dosages range from 1.0 to 100.0 lb polymer/ton dry solids depending upon the polymer selected, sludge characteristics, and thickener type.

34. Thickening is typically accomplished using physical unit processes including: • Gravity settling and compression - Figure 21.1, p. 631 • DAF - Figure 21.2, p. 633 Figure 21.3, p. 634

35. Centrifugation - Figure 21.16, p. 649 Figure 21.17, p. 650 • Gravity filter belts -

36. Dewatering - • Dewatering involves a mechanical unit operation to reduce the moisture content of sludge for one or more of the following reasons: • Reduce the volume to minimize transportation costs for disposal

37. Dewatered sludge is easier to handle using bulk material handling equipment, e.g. tractors with buckets or blades, belt conveyors, auger conveyors • Dewatering is required before incineration of organic sludges to increase the energy content by removal of excess moisture.

38. Dewatering before composting of orgainc sludges to minimize the need for supplemental bulking agents or amendments • Dewatered is required prior to landfilling to reduce leachate production at the landfill site

39. Mechanical dewatering devices use many physical means to remove excess moisture including filtration, squeezing, capillary action, vacuum withdrawal, centrifugal separation, and compaction.

40. Selection of a dewatering device depends on: • Characteristics of inlet sludge • Characteristics of dewatered sludge • Space available • Economics

41. Chemical conditioning with organic polymers (i.e., flocculation aids) is almost always used prior to mechanical dewatering devices. Dosages range from 1.0 to 100.0 lb polymer/ton dry solids depending upon the chemical characteristics of the polymer, physical characteristics of the sludge, and the type of dewatering device. Alum, ferric chloride, and lime are also sometimes used.

42. Common dewatering equipment include: • Vacuum filters - Figure 21.7, p. 639 Figure 21.8, p. 640 • Centrifuges - Figure 21.16, p. 649 Figure 21.17, p. 650

43. Common dewatering equipment include: • Belt filter presses - Figure 21.15, p. 649 • Pressure filter presses - Figure 21.12, p. 647 Figure 21.13, p. 647 Figure 21.14, p. 648

44. Disposal - • Common sludge conveyance methods to disposal sites: • Pipeline • Truck • Rail • Barge

45. Common methods of disposal: • Landfilling • Incineration • Land application • Ocean disposal