m e following are good references to obtain infomtion on sludge treatment technologies and applications:
1. Brunner, Calvin. Design of Sewage Sludge Incineration Systems. Noyes Data Corporation: 1980.
TREATING THE SLUDGE 593
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
Michael Ray Overcash and Dhiraj Ray. Design of Land Treatment Systems For Industrial Wastes. Michigan : Ann Arborscience , 1979.
P. Aarne Vesilind. Treatment and Disposal of Wastewater Sludges.
Michigan : Ann Arbor Science , 1979.
Stanley E. Manahan. Enviromenfal Chemistry. Florida : CRC Press, 1994.
JFWEF and ASCE. Design of Municipal Wastewater Treatment Plants, Volume II. Book Press, Inc.: Brattleboro, Vermont, 1991.
Davis, M. L. and Cornwell, D. A. Introduction to Environmental Engineering. McGraw-Hill, Inc.: New York, 1991.
Craig Cogger . Recycling Municipal Wastewater Sludge in Washington. Washington State University, November 1991.
DEC Devision of Solid Waste.Municiple Sewage Sludge Management Practices in New York State, April 1989.
Chaney and J.A. Ryan.The Future of Residuals Management Afer 1991. AWWA/WPCF Joint Residuals Management Conference, Water Pollution Control Federation, Arlington, Va. ,August 1991.
U.S. Environmental Protection Agency, 1993.40 CFR Parts 257,403 and 503. Standards for use or disposal of sewage sludge. page 3, Federal Registry 58.9248-941 5.
Cheremisinoff, N. P. and P. N. Cheremisinoff, Wafer Treament and Waste Recovery: Advanced Technologies and Application, Prentice hall Publishers, New Jersey, 1993.
m e following are good references to obtain pollution prevention information on, many of which cover water m g e m e n t and treatmentpractices. With the exception of reference 12, they can all be obtained through the US. EPA at minimal to no cost:
12. Cheremisinoff, Nicholas P. and Avrom Bendavid-Val, Green Proflts: R e Manager's Handbook for I S 0 14001 and Pollution Prevention, Butterworth- Heinemann and Pollution Engineering Magazine, MA, 2001,
13. ERIC: DB54 Cleaning Up Polluted Runoff with the Clean Water State Revolving Fund, March 1998 832/F-98-001 NSCEP:
14. Enforcement Requirements: Case Studies [Fact Sheet] 832/F-93-007 NSCEP:
15. Environmental Pollution Control Alternatives: Centralized Waste Treatment Alternatives for the Electroplating Industry, June 1981 62515-81-017.
16. Environmental Pollution Control Alternatives: Municipal Wastewater, 1976 17. Environmental Pollution Control Alternatives: Municipal Wastewater,
November 1979 625/5-79-012 ERIC: W438; NTIS: PB95-156691.
18. Environmental Pollution Control Alternatives: Sludge Handling, Dewatering, and Disposal Alternatives for the Metal Finishing Industry, October 1982 832lF-93-OO7.
6235-76-012 ERIC: W437; NTIS: PB95-156709.
594
19.
20.
21.
22.
23.
24.
25.
26.
27.
WATER AND WASTEWATER TREATMENT TECHNOLOGIES
625/5-82-018 NSCEP: 832/F-93-007; ERIC: W439; NTIS: PB95-157004.
Facility Pollution Prevention Guide, May 1992 600/R-92-088 NSCEP:
Guides to Pollution Prevention: Non-Agricultural Pesticide, July 1993 625/R- Guides to Pollution Prevention: The Commercial Printing Industry, August 110023.
Guides to Pollution Prevention: The Fabricated Metal Products Industry, July 110015.
Guides to Pollution Prevention: Wood Preserving Industry, November 1993 Pollution Prevention Information Exchange System (PIES): User Guide Version 2.1, November 1992 600/R-92-213 NSCEP: 600/R-92-213; ERIC:
W390.
Pollution Prevention Opportunity Checklists: Case Studies, September 1993 Waste Minimization Opportunity Assessment Manual, July 1988 625/7-88- Water-Related GISs (Geographic Information Systems) Along the United States-Mexico Border, July 1993 832/B-93-004 NSCEP: 832/B-93-004;
600/R-92-088; ERIC: W600; NTIS: PB92-213206.
93-009 NSCEP: 625111-93-009; ERIC: W316; NTIS: PB94-114634.
1980 625/7-90-008 NSCEP: 625/7-90-008; ERIC: WA06; NTIS: PB91-
1990 625/7-90-006 NSCEP: 625/7-90-006; ERIC: WA07; NTIS: PB91-
625/R-94-014 ERIC: WA08; NTIS: ~~94-436298.
832/F-93-006 NSCEP: 832/F-93-006; ERIC: W543.
003 ERIC: W423; NTIS: PB92-216985.
ERIC: W358; NTIS: PB94-114857.
QUESTIONS FOR THINKING AND DISCUSSING
1. We have an emulsion of oil in water that we need to separate. The oil droplets have a mean diameter of m, and the specific gravity Of the oil is 0.91.
Applying a sedimentation centrifuge to effect the separation at a spedd of 5,000 rpm, and assuming that the distance of a droplet to the axis of rotation is 0.1 m, determine the droplet's radial settling velocity.
2. Determine the settling velocity of a particle (d = 4 X lo4 m and pp = 900 kg/m3) through water in a sedimentation centrifuge operating at 4,000 rpm. The particle velocity is a function of distance from the axis of rotation, as shown by the following data:
Distance, m Settling Velocitv, Remolds Number
m/sec
0.01 0.0155 0.62
0.03 0.0465 1.90
TREATING THE SLUDGE 595
Distance, m Settling Velocitv, Remolds Number
-
m/sec
0.05 0.070 2.8
0.10 0.1125 4.5
0.20 0.185 7.4
0.30 0.245 9.8
1 .oo 0.437 17.5
3. A solid-bowl centrifuge has the following dimensions: R, = 0.30 m, R3 = 0.32 m, l! = 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, D,, 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 pm. (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 assumptionsfor head); (c) If the process requirements demand that the return water only contain 1 weight % solids, will additional units
@.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 pm 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.
596 WATER AND WASTEWATER TREATMENT TECHNOLOGIES
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 Litedhr at a temperature of 95" F. The average particle size is 65 pm. (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 landfi 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 45O 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 meters.
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
TREATING THE SLUDGE 597
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 .O 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 ft*.
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 fiiter 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 mglLiter 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: http://www.w-ww.com., 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
598 WATER AND WASTEWATER TREATMENT TECHNOLOGIES
BOD and SS, PPD Average Annual, 424 Maximum Month, 694 Maximum Day, 868 Headworks Bar Screen
Type, MANUALLY CLEANED Number, 1
Size, inches, 16 Bar Spacing, inches, 1 Comminutor
Type, IN-LINE Number, 1 Size, inches, 12 Motor, hp, 6 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/l, 3,000
Design Sludge Mean Cell Residence Time, Days, 40
Design Temperature, "C Low Month, 10
Average Month, 15 Aerators
Type, HIGH SPEED MECHANICAL FLOATING SURFACE
Number, 2 Size, hp (ea), 25
Design Maximum Oxygen Transfer, ppd, 1,500
Secondary Sedimentation Number of tanks, 1 Diameter, ft, 35 Overflow rate, gpd/sf Average Annual, 3 12 Peak Hour, 946
Solids Loading Rate, ppdhf Average Annual (50% return), 12 Peak Hour (100% return), 47 Return Sludge Pumps Number, 2
Design Maximum Capacity, gpm, 630 Intiitration Basins
Number, 8 Surface Area, 0.44
Net Hydraulic Loading rate, Wwk Average Annual, 2 1 .9
Maximum Month, 36.6 Sludge Disposal System
Type, LIQUID SLUDGE LAND APPLICATION
Land Area, acres, 13
Sludge Loading, Tons per acre per year, 5.3
The wastewater enters the plant through the headworks where it passes through a bar screen, comminutor and Parshal flume. Following the headworks, the wastewater enters the aeration basin where floating surface aerator aerate and mix
TREATING THE SLUDGE 599
the sewage. Biological growth in an aeration basin is carried with the effluent to secondary sedimentation. Here the growth settles to the bottom of the tank. It is raked to the center of the tank by rotating arms and flows to the return activated sludge pump station. The clarifier effluent flows by gravity to the infiltration basins where it seeps into the ground. The sludge from the return activated sludge pump station is pumped back into the aeration basin on the anoxic zone side. The return sludge seeds the incoming waste water and increases the BOD removal capacity.
Excess sludge is removed from the clarifier of the aeration basin by the waste activated sludge pump. The excess sludge is disposed by spraying on an adjacent forest land with a permanent spray irrigation system. Based on the above information, do the following exercises:
(a) Develop a list of any terms that you are not familiar with and not covered in detail in this volume. Obtain the definitions and an understanding of those terms as they apply to this design case.
(b) Develop a detailed process flowsheet for the plant. Show flow rates and mass flows for major process streams on your system diagram.
(c) Develop an inventory list of the chemicals needed for water conditioning in this
(a) Develop an estimate for the horsepower requirements needed for the return sludge pumps.
(e) Develop a plot plan layout for the plant based on the information given above.
Roughly determine the amount of plot area needed for this plant.
(f) Work with your design team to develop a estimate for the Cost of installing such a plant. Include in your estimates engineering, site preparation, start-up, and training costs.
(g) Based on the cost estimate you develop, discuss with your team options for financing such an investment.
plant.
15. A large settling lagoon (approximately 0.5 ac in area and 75 ft deep) is used to separate a solid waste product whose particle density is roughly 1,700 kg/m3. The density of the dilute slurry is roughly 1,300 kg/m3 and its viscosity is 3.2 cp. The particles are spherical in nature with a 50 wt% size of 210 pm. If the lagoon is filled to 90% capacity with a solids concentration of 40%, how long will it take to achieve an 85% separation of sludge from the slurry? First analyze this problem by ignoring any evaporation losses. Next, analyze the problem considering evaporation losses. Assume that pan evaporation data from a local weather station show a yearly average of 53 in./yr. (Note - a standard evaporation pan is about 2
fr in diameter and 36 in. deep).
16. The lagoon described in the above question operates in the summer months at a mean temperature of 65" F. The mean ambient air temperature between the
600 WATER AND WASTEWATER TREATMFNT TECHNOLOGIES
months of June and September is about 75" F. Assuming an average wind velocity of 5 m / s , determine the following: (a) estimated losses due to evaporation; and (b) the concentration of the dilute slurry at then end of four months.