3. A solid-bowl centrifuge has the following dimensions: R2 = 0.30 m, R3 = 0.32 m, i = 0.30 m. It is designd to operate at 5,000 rpm, separating particles from a suspension where the particle specific gravity is 7.8. Determine the required horsepower needed to set the centrifuge into operation.

4. A hydroclone will be used to separate out grit from cooling water that is recycled to plant process heat exchangers. The unit's diameter, Dc, is 32 inches. The waverage temperature of the suspension is 88° F and the specific gravity of the solids is 2.1. The volumetric flowrate of the susepnsion is 300 gpm and the solids concentration of the influent suspensnion is 7.8 % (weight basis). The average particle size is 300 ¿im. (a) Determine the overall separation efficiency of the hydroclone; (b) Determine the minium size horsepower requirements for the pump (you will need to make some assumptions for head)\ (c) If the process requirements demand that the return water only contain 1 weight % solids, will additional units (i.e., multiclones) be needed? If so, size these additional units.

5. For the above problem, develop a design basis for a settling chamber as an alternative.

6. Take the results for questions 4 and 5 and do a comparative cost analysis. First go the the Web and find suitable equipment suppliers that will provide the equipment in the size ranges you have calculated. Obtain some vendor quotes (rough ones will do). Then perform the fowllowing analysis: (a) What are the comparative costs between the two oprions for energy use?; (b) What are the comparative costs between the two options in terms of maintanance and labor costs?; (c) Can you combine both equipment options into a single process, and if so, can you justify this and how? Assume in the above that the reduction in solids concentration must meet the 1 % weirht criteria described in question 4.

7. A clarifying settler has the following characteristics: 750 mm bowl diameter; 600 mm bowl depth; 95 mm liquid layer thickness. The specific gravity of the susepnsion is 1.5, and that of the solids is 1.9. The particle cut size is 60 ¡j.m and the viscosity of the susepension is 15 cP. (a) Determine the capacity of the centrifuge in untis of gpm. (b) Determine the horesepoer requirments needed.

8. We wish to separate titanium dioxide particles from a water suspension. The method chosen is centrifugation. The unit is a continuous solid-bowl type with a bowl diamter of 400 mm, a length to width ratio of 3.0, and the unit operates at 2,000 rpm. The feed contains 18 % (weight basis) solids and is fed to the unit at 2,500 Liters/hr at a temperature of 95° F. The average particle size is 65 /¿m. (a) Determine the amount of solids recovered per hour; (b) Determine the solids concentration in the centrate; (c) Determine the horsepower requirments for the centrifuge; (d) Size a graviy settler to remove an additional 15 % of the solids.

9. The investment for a sludge dewatering and pasteurization process for a small municipal treatment facility is 4.5 million dollars. It is estimated that the operation can generate about 18 tons per year of a sludge suitable as a composting material that will support a local market. This offteake would represent about 10 % of the total market demand and resale values for the treated sludge range from $6.35 to $ 6.80 per ton. A market survey suggests that consumption will grow at a modest rate of 3.5 % per year over a five year projection. Labor and energy costs for the operation are estimated to be $ 165,000 per year. Determine whether this investment is practical and worthwhile.The current practice at the facility is to haul untreated wates off-site to a municipal landfill. Costs for transportation and disposal are typically 28 dollars per ton, and there is concern that these costs could escalate by 15 % over the next 5 years. In performing the analysis, consider several project investment paramters (e.g., payback period, ROI, B/C ratio, others).

10. For question 9, the municipal landfill has had public relations problems with the community. There has been concern over both odor issues and possible groundwater contamination. Taking these concerns into consideration, can you develop addional arguments that make the investment more finacially attractive?

11. We have an aeration basin that currently operates at 3.2 mg/Liter DO. Compare this operation where the DO concentration is 1.3 mg./Liter. The temperature of the basin is 18.0° C and 200 kW of aeration power is used. The average electricity cost is 8 cents per kWhr. Determine: (a) the current average electricity consumption for aeration; (b) the daily electricity costs for the operation; (c) what you could save on a daily basis and per year by lowering the DO concentration; (d) determine the yearly savings on a percentage basis.

12. When dealing with water treatment applications you cannot avoid pipe flow calculations. We have a pipeline in which the throughput capacity of 500 Liter/sec. The flow is split into two pipelines and the inside diamter of the pipe is 350 mm. The length of the pipeline is 55 m. The entry loss is 0.70 and the exit loss is 1.00. There are two 45° bends and two 90° bends in the lines, (a) Determine the flow per pipe; (b) determine the line velocity; (c) determine the resulting hydraulic loss in

13. Holly's (Holly, Michigan) original Wastewater Treatment Plant, WWTP, was built a trickling filter plant built in 1957 and had a design flow of 500,000 gallons per day. As the community grew it became necessary to construct a new plant. The majority of the plant was constructed in 1980 at a cost of approximately 6.3 million dollars. Seventy-five percent was funded by the Federal Government and five percent was funded by the State Government. The type of treatment used in the Holly Wastewater Treatment Plant is advanced treatment. Since the WWTP has a large impact on the receiving stream, the Shiawassee River, effluent and discharge limits are very stringent. For the past 10 years averages for BOD are 3.9 mg/Liter and suspended solids 4.2 mg/Liter. Plant was designed for an average flow of 1.5 million gallons per day (MGD). Presently average flow is 1.0 MGD. Sewage enters the plant via two thirty inch sanitary sewers, preliminary treatment consist of bar screen, aerated grit removal, and two 60,000 gallon primary clarifiers. The heart of the treatment system are rotating biological contactors. The RBC System consists of 3 rows of discs, with 4 discs per row with a total surface area of 1,500,000 ft2. Ferrous chloride is used at the head end of the treatment process (aerated grit tank) to aid in the removal of suspended solids and phosphorus. After the RBC's , wastewater enters into two final clarifiers. The sludge that is pumped out is much lighter in solids content (< 1 %). The sludge from the secondary clarifiers is then pumped back to the head end of the primary clarifiers. This helps to get the full use of the primary chemical added and thickens the sludge for better treatment and storage capacity in the digester. The sewage from the secondary clarifiers then flows into the filter feed wet well. Secondary effluent is pumped through four mixed media pressure sand filters. Filtration of secondary effluent is considered advanced or tertiary treatment and makes it possible to achieve excellent water quality. Effluent quality from the pressure filters averages below 2 mg/Liter for BOD and suspended solids during the summer months. Sludge stabilization process consists of an anaerobic digester. The digester provides anaerobic fermentation of the sludge in the enclosed tank. When operating, the destruction of the organisms produces methane gas. The gas is then used to heat the contents of the digester and other plant buildings. When the sludge is digested, it is transferred over the sludge storage tank. This simple tank holds 320,000 gallons of digested sludge and uses gravity to thicken the sludge. When the heavier sludge settles to the tank bottom, the remaining water or supernatant may be drawn off through a series of valves to the equalization basin. The treated biosolids is 8-9.5% solids is finally removed and injected 8 inches into farmland as a fertilizer supplement. Develop a detailed process flow sheet for the WWTP. Then develop a cost breakdown for each major component. Next, try to develop a qualitative energy audit, listing those operations in order of their highest energy consumption first. You can obtain more information on this plant's design by going to the following Web site:

14. The following information has been extracted from the design basis for an actual wastewater treatment plant:

Loadings Average Annual, 0.30

Population, 1,500 Maximum Day, 0.30

Flow, mgd Peak Hour, 0.91

Design Temperature, °C

Low Month, 10 Average Month, 15



Average Annual, 424 Maximum Month, 694 Maximum Day, 868 Headworks Bar Screen


Number, 1

Size, inches, 16

Bar Spacing, inches, 1



Number, 1

Size, inches, 12

Flow Measurement

Type, PARSHALL FLUME Number, 1

Throat Width, inches. 6 Aeration

Number of basins, 1 Volume, mgal, 0.67

Theoretical hydraulic residence in time hours:

Average Annual, 53 Maximum Month, 32

Design Waste Sludge Production, ppd, 420

Design Mixed Liquor Concentration, mg/1, 3,000

Design Sludge Mean Cell Residence Time, Days, 40


Design Maximum Oxygen Transfer, ppd, 1,500

Secondary Sedimentation

Number of tanks, 1 Diameter, ft, 35 Overflow rate, gpd/sf Average Annual, 312

Peak Hour, 946

Return Sludge Pumps

Number, 2

Design Maximum Capacity, gpm, 630

Net Hydraulic Loading rate, in/wk Average Annual, 21.9 Maximum Month, 36.6

Sludge Disposal System


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