Revenues Expenses Profits

The most important aspect of this is that profits can be increased by either an increase in revenues or a decrease in expenses. Water treatment operations are by and large end-of-pipe treatment technologies, and hence from the standpoint industry applications that must treat water, the investments required increase expenditures and decrease profit. Municipal facilities view their roles differently, because their end-product is clean water which is saleable, plus they may have addon revenues when biosolids are developed and sold into local markets. There are different categories of revenues and expenses, and it is important to distinguish between them.

Obviously, revenue is money coming into the company; from the sale of goods or services, from rental fees, from interest income, etc. The profit equation shows that an increase in revenue leads to a direct increase in profit, and vice versa if all other revenues and expenses are held constant. Note that we are going to assume that the condition of other expenses/revenues are held constant in the discussions below.

Revenue impacts must be closely examined. For example, companies often can cut wastewater treatment costs if water use (and, in turn, the resulting wastewater flow) is limited to nonpeak times at the wastewater treatment facility. However, this limitation on water use could hamper production. Consequently, even though the company's actions to regulate water use could reduce wastewater charges, revenue could also be decreased, unless alternative methods could be found to maintain total production. Conversely, a change in a production procedure as a result of a technology change could increase revenue. For example, moving from liquid to dry paint stripping can not only reduce water consumption, but also affect production output. Because clean-up time from dry paint-stripping operations (such as bead blasting) is generally much shorter than from using a hazardous, liquid based stripper, it could mean not only the elimination of the liquid waste stream (this is a pollution prevention example), but also less employee time spent in the cleanup operation. In this case, production is enhanced and revenues are increased by the practice. Another potential revenue effect is the generation of marketable byproducts such as biosolids. Such opportunities bring new, incremental revenues to the overall operation of the plant. The point to remember is that the project has the potential to either increase or decrease revenues and profits - and that's the reason for doing a financial analysis.

Expenses are monies that leave the company to cover the costs of operations, maintenance, insurance, etc. There are several major cost categories:

• Insurance expenses

• Depreciation expenses

• Interest expenses

• Labor expenses

• Training expenses

• Auditing and demo expenses

• Floor-space expenses

Each of these should be carefully considered in your analysis. Insurance Expenses. Depending upon the project, insurance expenses could either increase or decrease. Insurance premiums can be increased depending on the technology option chosen for a plant design.

Depreciation Expenses. By purchasing capital equipment with a limited life the entire cost is not charged against the current year. Instead, depreciation expense calculations spread the equipment's procurement costs (including delivery charges, installation, start-up expenses, etc.) over a period of time by taking a percentage of the cost each year over the life of the equipment. For example, if the expect life of a piece of equipment is 10 years, each year the enterprise would charge an accounting expense of 10 percent of the procurement cost of the equipment. This is known as the method of straight-line depreciation. Although there are other methods available, all investment projects under consideration at any given time should use a single depreciation method to accurately compare alternative projects' expense and revenue effects. Because straight-line depreciation is easy to compute, it is the preferred method. Note that even though a company must use a different depreciation system for tax purposes (e.g., the Accelerated Cost Recovery System, or ACRS), it is acceptable to use other methods for bookkeeping and analysis. In any event, any capital equipment must be expensed through depreciation. Interest Expense. Investment in equipment implies that one of two things must occur: Either a company must pay for the project out of its own cash, or it must finance the cost by borrowing money from a bank, by issuing bonds, or by some other means. When a firm pays for a project out of its own cash reserves, the action is sometimes called an opportunity cost. If you must borrow the cash, there is an interest charge associated with using someone else's money. It is important to recognize that interest is a true expense and must be treated, like insurance expense, as an offset to the project's benefits. The magnitude of the expense will vary with bank lending rates, the interest rate offered on the corporate notes issued, etc. In any case, there will be an expense. The reason companies account for equipment purchases as a cost is this: If cash is used for the purpose of pollution control, it is unavailable to use for other opportunities or investments. Revenues that could have been generated by the cash (for example, interest from a certificate of deposit at a bank) are treated as an expense and thus reduce the value of the project. But again - we may not have a choice if the project is driven by legal requirments such as the CWA.

Although the reasoning seems sound, opportunity costs are not really expenses. Though it is true that the cash will be unavailable for other investments, opportunity cost should be thought of as a comparison criteria and not an expense. The opportunity forgone by using the cash is considered when the project competes for funds and is expressed by one of the financial analysis factors discussed earlier (net value of present worth, pay back period, etc.). It is this competition for company funds that encompasses opportunity cost, so opportunity cost should not be accounted directly against the project's benefits.

Many companies apply a minimum rate of return, or hurdle rate, to express the opportunity-cost competition between investments. For example, if a firm can draw 10 percent interest on cash in the bank, then 10 percent would be a valid choice for the hurdle rate as it represents the company's cash opportunity cost. Then, in analyzing investment options under a return-on-investment criteria, not only would the highest returns be selected, but any project that pays the firm a return of less than the 10 percent hurdle rate would not be considered.

Labor Expenses. In the majority of situations, projects will cause a company's labor requirements to change. This change could be a positive effect that increases available productive time, or there could be a decrease in employees' production time depending upon the practice. When computing labor expenses, the Tier 1 costs could be significant. Labor expense calculations can be simplistic or comprehensive. The most direct and basic approach is to multiply the wage rate by the hours of labor. More comprehensive calculations include the associated costs of payroll taxes, administration, and benefits. Many companies routinely track these costs and establish an internal 'burdened' labor rate to use in financial analysis.

Training Expenses. Your project may also involve the purchase of equipment that requires additional operator training. In computing the total training costs, the enterprise must consider as an expense both the direct costs and the staff time spent in training. Remember that some of the technologies discussed require more extensive worker training than others. In addition, any other costs for refresher training, or for training for new employees, that is above the level currently needed must be included in the analysis. Computing direct costs is simply a matter of adding the costs of tuition, travel, per diem, etc., for the employees. Similarly, to compute the labor costs, simply multiply the employees' wage rates by the number of hours spent away from the job in training.

Auditing and Demo Expenses. Labor and other expenses associated with defining the engineering project are often overlooked. Although these tend to be small for low-investment projects, some contemplated operations may require pilot testing, or sending personnel off-site to work with vendors in their shops. This can happen when dealing with exotic sludge or unique waste waters that require treatment. Pilot or plant trials can incur significant up-front costs from production down times, personnel, monitoring equipment, and laboratory measurements, as well as engineering design time and consultant-time charges. Some enterprises may prefer to absorb these costs as part of their R&D budget - for organizations these expenses simply are a part of the baseline cost of operations. Floor Space Expenses. As with any costs, the floor-space costs must be based on the value of alternative uses. Unfortunately, computing floor-space opportunity cost is not always straightforward, as it is in the case of training costs. In instances where little square footage is required, there may be no other use for the floor space, which implies a zero cost. Alternatively, as the square footage required increases, calculating floor-space costs becomes more straightforward. For example, if a new building is needed to house the water treatment equipment, it's easy to compute a cost. The average-square-foot cost for a new or used warehouse (or administrative or production space) that would be charged to procure the space on the local market is the average market worth of a square foot of floor space. Unless there is a specific alternative proposal for the floor space, this market analysis should work as a proxy.

Though cash flow does not have a direct effect on a company's revenues or expenses, the concept must be considered. If the project involves procurement costs, they often must be paid upon delivery of the equipment - yet cash recovery could take many months or even years. Three things about any project can affect a firm's available cash. First, cash is used at the time of purchase. Second, it takes time to realize financial returns from the project, through either enhanced revenues or decreased expenses. Finally, depreciation expense is calculated at a much slower rate than the cash was spent. As a result of the investment, a company could find itself cash-poor. Even though cash flow does not directly affect revenues and expenses, it may be necessary to consider when analyzing your project.

Though most companies use only revenue and expense figures when comparing investment projects, income-tax effects can enter into each calculation if either revenues or expenses change from the baseline values. More expenses mean lower profits and less taxes, and vice versa. If an company needs to know the effect of income taxes on profit, the computations are simple and can be done during or after the analysis. As with expenses and revenues, you do not need to compute the total tax liability for each option. Instead, you only need to look at the options' effect on revenues and/or expenses, and the difference in tax liability resulting from deviations from the baseline. The profit equation reflects gross or pretax profits. Income tax is based on the gross profit figure from this equation and cannot be computed until you know what effect the options will have on revenues and/or

We have introduced some concepts above as they relate to total-cost accounting and the total-cost assessment. Total-cost accounting is applied in management accounting to represent the allocation of all direct and indirect costs to specific products, the lives of products, or to operations as we have considered in this volume. It should be thought of as a long-term, comprehensive analysis of the entire range of costs and savings associated with the investment. Life-cycle cost assessment represents a methodical process of evaluating the life-cycle costs of a product, product line, process, system, or facility - starting with raw-material acquisition such as the chemical additives used in water conditioning, and going all the way to disposal of the sludge - by identifying the environmental consequences and assigning monetary value. A more detailed discussion of this subject is beyond our scope, but you will find some good references to refer to below. The references provided below are organized by general subject category. I have looked at all of these and relied on quite a few in y own consulting practice. The last section provides you with some challenging exercises that you can work

RECOMMENDED RESOURCES FOR THE READER

The following are good references to obtain information on sludge treatment technologies and applications:

1. Brunner, Calvin. Design of Sewage Sludge Incineration Systems. Noyes Data Corporation: 1980.

2. Michael Ray Overcash and Dhiraj Ray. Design of Land Treatment Systems For Industrial Wastes. Michigan : Ann ArborScience , 1979.

3. P. Aarne Vesilind. Treatment and Disposal Of Wastewater Sludges.

4. Stanley E. Manahan. Enviromental Chemistry. Florida : CRC Press, 1994.

5. JFWEF and ASCE. Design of Municipal Wastewater Treatment Plants, Volume II. Book Press, Inc.: Brattleboro, Vermont, 1991.

6. Davis, M. L. and Cornwell, D. A. Introduction to Environmental

7. Craig Cogger. Recycling Municipal Wastewater Sludge in Washington. Washington State University, November 1991.

8. DEC Devision of Solid Waste.Municiple Sewage Sludge Management

9. Chaney and J.A. Ryan.77k? Future of Residuals Management After 1991.AWWA/WPCF Joint Residuals Management Conference, Water Pollution Control Federation, Arlington, Va..August 1991.

10. U.S. Environmental Protection Agency, 1993.40 CFR Parts 257,403 and 503. Standards for use or disposal of sewage sludge, page 3, Federal

11. Cheremisinoff, N. P. and P. N. Cheremisinoff, Water Treatment and Waste Recovery: Advanced Technologies and Application, Prentice hall Publishers,

The following are good references to obtain pollution prevention information on, many of which cover water management and treatment practices. With the exception of reference 12, they can all be obtained through the U.S. EPA at minimal to no

12. Cheremisinoff, Nicholas P. and Avrom Bendavid-Val, Green Profits: The Manager's Handbook for ISO 14001 and Pollution Prevention, ButterworthHeinemann 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 625/5-81-017.

16. Environmental Pollution Control Alternatives: Municipal Wastewater, 1976 625/5-76-012 ERIC: W437; NTIS: PB95-156709.

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

625/5-82-018 NSCEP: 832/F-93-007; ERIC: W439; NTIS: PB95-157004.

19. Facility Pollution Prevention Guide, May 1992 600/R-92-088 NSCEP: 600/R-92-088; ERIC: W600; NTIS: PB92-213206.

20. Guides to Pollution Prevention: Non-Agricultural Pesticide, July 1993 625/R-93-009 NSCEP: 625/R-93-009; ERIC: W316; NTIS: PB94-114634.

21. Guides to Pollution Prevention: The Commercial Printing Industry, August 1980 625/7-90-008 NSCEP: 625/7-90-008; ERIC: WA06; NTIS: PB91-110023.

22. Guides to Pollution Prevention: The Fabricated Metal Products Industry, July 1990 625/7-90-006 NSCEP: 625/7-90-006; ERIC: WA07; NTIS: PB91-110015.

23. Guides to Pollution Prevention: Wood Preserving Industry, November 1993 625/R-94-014 ERIC: WA08; mis. PB94_136298.

24. Pollution Prevention Information Exchange System (PIES): User Guide Version 2.1, November 1992 600/R-92-213 NSCEP: 600/R-92-213; ERIC: W390.

25. Pollution Prevention Opportunity Checklists: Case Studies, September 1993 832/F-93-006 NSCEP: 832/F-93-006; ERIC: W543.

26. Waste Minimization Opportunity Assessment Manual, July 1988 625/7-88003 ERIC: W423; NTIS: PB92-216985.

27. Water-Related GISs (Geographic Information Systems) Along the United States-Mexico Border, July 1993 832/B-93-004 NSCEP: 832/B-93-004; 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 10"4 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 10"* 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 Velocity, m/sec

Reynolds Number

0.0155 0.0465

Distance, m

Settling Velocity, m/sec

Reynolds Number

0.05

0.070

0.10

0.1125

0.20

0.185

0.30

0.245

1.00

0.437

17.5

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: 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

Design Temperature, °C

Low Month, 10 Average Month, 15

Aerators

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

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

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, 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

Type, LIQUID SLUDGE LAND

APPLICATION

Land Area, acres, 13

Sludge Loading, Tons per acre per year,

Solids Loading Rate, ppd/sf

Average Annual (50% return), 12 Peak Hour (100% return), 47

Infiltration Basins

Number, 8 Surface Area, 0.44

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 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 plant.

(d) 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.

(i) 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.

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 ^m. 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 ft 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 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.

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