"ALTERNATIVE COMPOSTING SYSTEMS" Donald T. Garbrick, P.E. Vice President. Presented at. Sponsored By

Transcription

1 "ALTERNATIVE COMPOSTING SYSTEMS" BY Donald T. Garbrick, P.E. Vice President J. N. Pease Associates Presented at A Conference on Sludge Disposal in North Carolina and An Update on Regulations and Trends Sponsored By American Water Works Association (AWA) Water Pollution Control Federation (WPCF) Professional Engineers of North Carolina (PENC) February 15, 1989 Jane S. McKinnon Center Raleigh, North Carolina

2 TABLE OF CONTENTS Title Page Introduction... 1 Existing Wastewater Treatment Plant Discussion of Alternatives Cost-Effective Analysis The Selected Alternative... 33

3 INTRODUCTION On December 28, 1987, J. N. Pease Associates prepared an Amendment to the Davidson County 201 Facility Plan which addressed alternatives for sludge disposal at Lexington's Regional Wastewater Treatment Plant. The authorization to prepare the Amendment resulted in the County's refusal to accept the disposal of dewatered sludge in the new County landfill. Initially, the scope of services required evaluation not only of compost facilities, but on-site landfilling of dewatered sludges and land application. Because the Amendment was evaluating a facility constructed under a Federal Grant, EPA and State Agencies required that existing facilities be utilized under the selected alternative or the funding previously received would be deducted from the Grant. EXISTING WASTEWATER TREATMENT PLANT The Lexington Regional Wastewater Treatment Plant was constructed in This facility has been well managed, operated, and maintained by the City of Lexington. The present influent wastewater flow to the facility averages approximately 2.9 million gallons per day with a design capacity of 5.5 million gallons per day. When operating at design capacity, the plant should produce 2,000 gallons of sludge for every 1 million gallons treated. This translates into 535,000 cubic feet of sludge per year. The following is a'description of the existing facility: I. PRELIMINARY TREATMENT 1. Metering a. Parshall Flume 2. Screening a. Mechanical Bar Screen b. Bypass Bar Screen (hand operation). 3. Grit Removal a. Collector: Square tank with circular scraper mechanism, washer, elevator and discharges into portable storage boxes Feb-89 ALTERNATIVE COMPOSTING SYSTEMS GARBRICK-RPT - 1

4 11. PRIMARY CLARIFIERS 1. Size - Two Units a. Side Water Depth Feet. b. Diameter - 75 feet ROUGHING FILTERS 1. Two Units a. Depth = 5 ft. b. Diameter = 100 ft. IV. AERATION TAkS 1. Six Tanks a. 28' wide x 216' long x 15' deep. b. Detention Time = 8.3 hours. v. VI. VII. SECONDARY CLARIFIERS 1. Two Units a. Diameter = 110 ft. b. Side water depth = 16'-3". POST AERATION LAGOON 1. One Lagoon 2. Detention Time = 24 hours on average flow. 3. Use four aerators. CHLORINE CONTACT BASIN 1. Tank Volume = 15,278 CF. 2. Detention = 30 minutes. VIII. SLUDGE DIGESTION FACILITIES 1. Four Units (One Uncovered For Storage) a. 60' diameter. b. Side depth 30' Feb-89 ALTERNATIVE COMPOSTING SYSTEMS GARBRICKeRPT - 2

5 c. Cone depth 12. d. Gas recirculation - perth type in 3 units. e. Heat exchanger. IX. SLUDGE DRYING BEDS 1. Area = 178,444 sf drying cycles per year. DISCUSSION OF ALTERNATIVES The following alternatives for disposing of sludge are discussed in this Section; they are: Alternative Alternative I - Composting of Dewatered Sludge - Sludge Dewatering I1 - On-Site Landfilling of Dewatered Sludge Alternative I11 - Land Application Each alternative was evaluated as to its suitability to dispose of the sludge in an environmentally safe and economical manner. ALTERNATIVE I - COMPOSTING Composting of sewage sludge is the aerobic thermophilic decomposition of its organic constituents. The sludge is dewatered to 20% solids and mixed with a bulking agent, usually wood chips or shredded bark. The mixture is then aerated for two or three weeks, maintaining a temperature within the mixture of at least 131 degrees F. This is the minimum temperature required to destroy harmful bacteria and other pathogenic organisms. The result of the decomposition is a relatively stable, humus-like material that can be used as a soil conditioner and as a low grade fertilizer. Composted sludge is a safe material, but should not be used on areas where root or leaf crops for direct human consumption are to be grown within one year of the last application of compost to the site. The two types of composting that will be discussed in this 201 Amendment are the aerated static pile method and the enclosed composting method. In the static pile method, sludge is conditioned with the bulking agent and placed in piles over a network of perforated pipes that are connected to blowers. The blowers draw air down through the piles, increasing the aerobic reaction and, thus, increasing the temperature. The blowers are used to regulate the temperature and keep it above 131 degrees F. After 21 days, the piles are removed from the pipe network and placed in curing piles for at least 30 days, before being screened for reusable wood chips. The finished compost can then be stockpiled for future use Feb-89 ALTERNATIVE COMPOSTING SYSTEMS GARBRICK-RPT - 3

6 Enclosed composting is a mechanical process that is accomplished within an enclosed vessel. Because the system is enclosed, there is more control over the composting process. Variables such as oxygen concentration, temperature, and airflow can be carefully regulated, minimizing process time and odor problems. The sludge and bulking agent mixture is composted for 14 days and then cured for at least 21 days. The compost is then screened and stockpiled. Composting sewage sludge, by either means, can be used advantageously in potting mixes, on lawns, as mulch, and for nursery production of trees and ornamental plants. Its main value is as a soil conditioner. Compost improves the soil in the following ways: Enhanced Aggregation - Compost increases the tendency of soil particles to stick together. Increased Water Retention - Allows sandy soils to hold water better. Improved permeability - Helps make clay and loose, reducing compaction. Improved Aeration - Looser soil encourages root growth. or other heavy soils more friable Buffering - Increases capacity of soil to absorb sudden changes in acidity or alkalinity. Composted sewage sludge may be assumed to be free of dangerous pathogens and toxic substances but contact with children and pets should be minimized and direct ingestion of composted sludge should be avoided. SLUDGE DEWATERING (COMPOSTING) The first step in composting sewage sludge is to mix the sludge with a bulking agent. The most favorable solids content for the sludge is between 20 and 22 percent. Sludge is being produced from the digesters with a solids content of 4.75 percent and, therefore, must be dewatered. The present method of dewatering is with a system of drying beds, making it difficult to monitor and control the solids content. Of the other methods of sludge dewatering, belt presses are rated highest in overall comparison of environmental factors, capital cost, 0 & M cost, and dewatering effectiveness. A belt press has been designed to meet the needs of the Lexington Wastewater Treatment Plant. The design was based on a conservative 3 percent sludge solids content, and assumed a 40-hour workweek yielding a flow rate of 57 gpm and a dry solids rate of 770 lbs. per hour. The budget price for the belt press is $135,000. This includes the belt press, hydraulic power pack, controls and master control panel. The ancillary equipment consisting of a sludge grinder, two sludge feed pumps, polymer feed system, two wash water booster pumps and conveyor is $130,000. The total cost, including installation, piping, steel building, grading, and Feb-89 ALTERNATIVE COMPOSTING SYSTEMS GARBRICKaRPT - 4

7 electrical work, is estimated at $384,000. This price has been included in all composting system cost evaluations. ALTERNATIVE I1 - ON-SITE LANDFILLING OF DEWATERED SLUDGE The dewatered sewage sludge may be disposed of at an on-site landfill, to be operated and maintained by the plant. An excavated trench must be formed and lined with a plastic liner to prevent leachate from entering the ground water. The sludge would be dewatered on the drying beds and then transferred to the landfill area. Once the sludge is placed in the trench, cover soil is usually placed on top that same day in order to maximize odor control. The trenches are formed with scrapers, dozers, and backhoes. The sludge is transported to the trench by dump truck and dumped directly into the trench. Excavated soil is used to cover the trench and should be at least 4' deep.. Since there is excavated material available for covering the trenches, no imported soil is required. The sludge removed from the drying beds will have a solids content of 50%. At this concentration, the sludge will not spread out evenly when dumped from the trench sidewall. The trucks shall enter the trench from one end and dump the sludge directly on the trench floor. Sewage sludge contains many harmful bacteria and pathogenic organisms. These bacteria and organisms can be a serious health risk if introduced into the water table. A plastic liner must be placed in each trench to control the harmful leachate. One potential problem with the liners is that they might break or tear, contaminating the surrounding area. Another disadvantage of on-si te landf illing is the limited availability of usable land at the existing site. Eventually, more land will be required to meet future sludge disposal needs. ALTERNATIVE I11 - LAND APPLICATION OF DEWATERED SLUDGE Sewage sludge may be applied directly to the soil as a fertilizer, because sludge contains both crop nutrients and organic matter. The sludge can either be injected with a low solids content, or spread with a high solids content. For this 201 Amendment, only land spreading will be considered. The majority of Davidson County is farmland and woodland. There will be available farmland in the southeastern section of the county to be used for land application. Due to the harmful contents in sewage sludge, such as pathogens and heavy metals, land application must be carefully regulated. Each site has a limit to how much sludge can be applied. Sludge will be dewatered on the existing drying beds. From there, the sludge will be hauled to the site and stockpiled. A large commercial spreader and front en3 loader are needed to apply the dewatered sludge. These pieces of equipment will have to be transported from site to site Feb-89 ALTERNATIVE COMPOSTING SYSTEMS GARBRICK-RPT - 5

8 COST-EFFECTIVE ANALYSIS This section compared basis. Several methods each method accompanies TOTALLY ENCLOSED COMPOSTING SYSTEM the cost of each alternative on a net present worth of composting were considered. A brief description of each estimate. The totally enclosed composting system consists of a cylindrical bioreactor, 43 high with a 23 diameter, and an optional cure reactor of identical size. The bioreactor is equipped with a continuous gas analyzer to monitor the oxygen and carbon dioxide levels. This allows the bioreactor to maintain optimum composting conditions by controlling the airflow through the reactor. The composting process requires approximately thirteen days in the bioreactor plus additional time to cure. Curing the compost can either be accomplished in a cure reactor or in piles on the ground. The advantage of the cure reactor is a reduction in overall cure time. A cost analysis has been performed for both a bioreactor with a cure reactor and a bioreactor without a cure reactor. TOTALLY ENCLOSED COMPOSTING SYSTEM (BIOREACTOR AND CURE REACTOR) TOTAL COST Enclosed System Contingency (10%) SUBTOTAL Interest During Construction (6-118%) Legal, Administrative, and Technical (15%) TOTAL CAPITAL Present Worth of 0 & M $86,010 (8.51) Present Worth of Salvage $2,284,000 (0.1486) Present Worth of Compost Credit $95,300 (8.51) NET PRESENT WORTH S 2, ; 600 $ 2,668,906 $ 163, ,300 $ 3,232, , , ,000 $ 2,814, Feb-89 ALTERNATIVE COMPOSTING SYSTEMS GARBRICK-RPT - 6

9 TOTALLY ENCLOSED COMPOSTING SYSTEM (BIOREACTOR AND CURE REACTOR) COST ESTIMATE 1. Enclosed System $ 1,900, Grading 1, Concrete 20, Miscellaneous - Painting, Lighting, HVAC, etc. 20, Stockpile Area - A/C Pavement 100, Belt Press 384, 000 TOTAL 2,426,300 0 C M COSTS 1. Electrical $ 17, Labor 21, Maintenance 9, Bulking Agent - 10, Belt Press 26, Fuel 1,950 TOTAL 0 & M $ 86,090 * Feb-89 ALTERNATIVE COMPOSTING SYSTEMS GARBRICKeRPT - 7

10 DESIGN CALCULATIONS FOR THE TOTALLY ENCLOSED COMPOSTINC; SYSTEM Sludge Produced: 2,000 gal/mg 3.0% dry solids. 2,000 gal/mg x 8.34 lbs/gal x 0.03 = 500 lbslmg 5.5 mgd, 500 lbdmg x 5.5 mgd = 2,750 lbs/day. Estimated Output: Input solids 8,970 lbs/day Total Finished Compost 10,280 45% solids. Recyclable Compost 2,311 lbs/day. Compost for Disposal 7,469 lbs/day or 10.3 cy/day Feb-89 ALTERNATIVE COMPOSTING SYSTEMS GARBRICKaRPT - 8

11 PARTIALLY ENCLOSED COMPOSTING SYSTEM (BIOREACTOR ONLY) Partially Enclosed System Contingency (10%) SUBTOTAL Interest During Construction (6-1/8%), Legal, Administrative, and Technical (15%) TOTAL CAPITAL Present Worth of 0 & M $64,550 (8.51) Present Worth of Salvage $2,084,000 (0.1486) Present Worth of Compost Credit $95,300 (8.51) NET PRESENT WORTH TOTAL COST $ 2,217, ,700 $ 2,439,000 $ 149, T" 549, , ,000 S ' Feb-89 ALTERNATIVE COMPOSTING SYSTEMS GARBRICK-RPT - 9

12 PARTIALLY ENCLOSED COMPOSTING SYSTEM (BIOREACTOR ONLY) 1. Partially Enclosed System 2. Grading 3. Concrete 4. Stockpile Area - A/C Pavement 5. Miscellaneous - Painting, Lighting, HVAC, etc. 6. Belt Press TOTAL 0 & M COSTS 1. Labor 2. Electrical 3. Equipment Maintenance 4. Bulking Agent 5. Belt Press 6. Fuel TOTAL 0 & M COST ESTIMATE $ 1,700,000 1,000 12, ,000 20, ,000 $ 2,217,300 $ 10,500 9,219 6,600 10,297 26,000 1,950 $ 64, Feb-89 ALTERNATIVE COMPOSTING SYSTEMS GARBRICKeRPT - 10

13 DESIGN CALCULATIONS FOR THE PARTIALLY ENCLOSED COMPOSTING SYSTEM Sludge Produced: 2,000 gal/mg 3.0% dry solids. 2,000 gallmg x 8.34 lbs/gal x lbs/mg 5.5 mgd, 500 lbs/mg x 5.5 mgd = 2,750 lbs/day. - Estimated Output: Input solids 8,970 lbs/day To tal Finished Compost 10,280 45% solids. Recyclable Compost 2,311 lbdday. Compost for Disposal 7,469 lbs/day or 10.3 cy/day Feb-89 ALTERNATIVE COMPOSTING SYSTEMS GARBRICK-RPT - 11

14 AUGER AERATOR ENCLOSED COMPOSTING SYSTEM The auger aerator digester is a circular vessel with aerator augers suspended from a bridge that travels around the top of the digester wall. These aerator augers constantly aerate and move the material towards the center of the vessel for discharge. Air is drawn through the mixture by a network of pipes attached to a blower. The auger speed, rate of rotation of the carriage assembly, and the amount of air introduced into the digester are carefully monitored to produce the optimum temperature and correct retention time in the digester. A cost analysis has been done on the auger aerator enclosed composting system and the results are on the following two pages Feb-89 ALTERNATIVE COMPOSTING SYSTEMS GARBRICKaRPT - 12

15 AUGER AERATOR ENCLOSED COMPOSTING SYSTEM Auger Aerator System Contingency (10%) SUBTOTAL Interest During Construction (6-1/8%) I Legal, Administrative, and Technical (15%) TOTAL Present Worth of 0 6 M $90,800 (8.51) Present Worth of Salvage $1,234,000 (0.1486) Present Worth of Compost Credit $95,300 (8.51) NET PRESENT VORTH TOTAL COST " $ 1,699, , 900 $ 114, I Feb-89 ALTERNATIVE COMPOSTING SYSTEMS GARBRICK-RPT - 13

16 AUGER AERATOR COMPOSTING SYSTEM 1. Equipment 2. Paved and Covered Storage 3. Building (Concrete Floor, Electric, Plumbing, HVAC) 4. Installation of Reactor Walls and Foundation 5. Installation of Equipment 6. Stockpile Area (A/C Pavement) 7. Grading and Site Work 8. Belt Press TOTAL 0 S M COSTS 1. Labor 2. Bulking Agent 3.' Electrical 4. Fuel 5. Maintenance 6. Belt Press TOTAL COST ESTIMATE $ 850,000 73,400 27,000 32, , ,000 20,000 $ 20,800 4,446 27,558 3,510 8,500 26,000 $ 90, Feb-89 ALTERNATIVE COMPOSTING SYSTEMS GARBRICK-RPT - 14

17 DESIGN CALCULATIONS FOR THE AUGER AERATOR COMPOSTING SYSTEM Sludge Produced: Dry tonslday Wet tonslday Estimated Output: Input solids Water Total Total Finish Compost 47,230 Recyclable Compost 37,396 lbslday tonslday tonslday 28,747 lbslday 43,120 lbslday 31,867 lbs/day 40% solids. Compost for Disposal 9,734 lbslday or 10.8 cy/day Feb-89 ALTERNATIVE COMPOSTING SYSTEMS GARBRICK-RPT - 15

18 ENCLOSED DYNAMIC COMPOSTING SYSTEM The enclosed dynamic composting is done in parallel eight-foot wide troughs. Sludge and bulking agent are placed at the loading end of the troughs. A mechanical mixer moves along the top of each trough, throughly mixing the sludge and bulking agent. Air is drawn through the mixture by a network of blowers and pipes. The mixture is turned and aerated by the mixer as it moves from one end of the trough to the other over 2 to 3 weeks. The finished compost is removed from the end of the trough by a front end loader and stockpiled. Some finished compost can be mixed in with the sludge and bulking agent to reduce material handling and bulking agent requirements. A cost analysis was done on the enclosed dynamic system and the results are on the following pages Feb-89 ALTERNATIVE COMPOSTING SYSTEMS GARBRICK-RPT - 16

19 ENCLOSED DYNAMIC COMPOSTING SYSTEM Enclosed Dynamic System Contingency (10%) SUBTOTAL Interest During Construction (6-1/8%) ' Legal, Administratlve, and Technical (15%) TOTAL Present Worth of 0 & M $76,800 (8.51) Present Worth of Salvage $1,234,000 (0.1486) Present Worth of Compost Credit $95,300 (8.51) NET PRESENT WORTH TOTAL COST $ 1,453, ,300 s 1,598,306 $ 97, , , ,000 $1,587,ooo Feb-89 ALTERNATIVE COMPOSTING SYSTEMS GARBRICKaRPT - 17

20 ENCLOSED DYNAMIC COMPOSTING SYSTEM 1. Equipment 2. Building 3. Ins tall-equipmen t 4. Stockpile Area 5. Grading and Site Work 6. Belt Press TOTAL 0 & M COSTS 1. Labor 1 2. Bulking Agent) 3. Electrical ) 4. Fuel ) 10% Equipment Cost 5. Maintenance ) 6. Belt Press 0 & M TOTAL COST ESTIMATE $ 500, , , ,000 50, ,000 $ 1,453,000 $ 50,000 26, 000 $ 76, Feb-89 ALTERNATIVE COMPOSTING SYSTEMS GARBRICK.RPT - 18

21 DESIGN CALCULATIONS FOR THE ENCLOSED DYNAMIC COMPOSTING SYSTEM Sludge Produced: 2,000 gal/mg treated at 4.75% dry solids. 2,000 gal/mg x 8.34 lbs/gal x I mgd, 800 lbs/mg x 5.5 mgd = 4.4 tons/day. Estimated Output: Input solids Water Total 59,200 lbslday 27,345 lbs/day 86,545 lbs/day Total Finished Compost 78,129 lbs/day Q 65% solids. Recyclable Compost 40,000 lbslday. Compost for Disposal 38,129 lbs/day or 32 cy/day Feb-89 ALTERNATIVE COMPOSTING SYSTEMS GARBRICKaRPT - 19

22 AERATED STATIC PILE COMPOSTING SYSTEM The aerated static pile system is a more simplistic method of composting sewage sludge than is the enclosed composting system. Dewatered sewage sludge is mixed with bulking agent and placed in piles over a network of perforated pipes connected to blowers. The blowers draw air through the piles, increasing the temperature of the mixture to ensure the destruction of pathogenic organisms and produce sufficient evaporation. The composting process requires at least 21 days, and then the piles are broken down and screened for reusable bulking agent. The remaining compost is then stored in curing piles to further stabilize the material. All composting will take place inside a pre-engineered steel building. There will be twelve piles of 110' each, with a blower at the end of each. compost in order to control odor problems. There are two options for removing the material from the compost piles. One option is to provide a conveyor belt for every two static piles. Composted material would be loaded onto the conveyor belt and transported out the back of the building, and deposited in curing piles. The other option is to remove the material by front end loader and transport it to the drying beds to cure. The drying beds can be used for curing piles since the sludge will be dewatered by a belt press. After sufficient curing time, the compost is moved to a stockpile area. A cost analysis was done with bo.th options the following pages. considered. The results are on Feb-89 ALTERNATIVE COMPOSTING SYSTEMS GARBRICK.RPT - 20

23 AERATED STATIC PILE COMPOSTING SYSTEM, USING DRYING BEDS FOR CURING PILES TOTAL COST Compost System $ 1,569,400 Contingency (10%) 156,900 SUBTOTAL 1,726,300 Interest During Construction (6-118%) Legal, Administrative, and Technical (15%) TOTAL $ 105, ; 900 $ 2,090,906 Present Worth of 0 & H (10% over 20 years) $154,500 (8.51) 1,314,800 Present Worth of Salvage Value $572,500 (0.1486) a5, 100 Present Worth of Compost Credit $91,250 (8.51) 776,500 NET PRESENT WORTH S 2,544, Feb-89 ALTERNATIVE COMPOSTING SYSTEMS GARBRICK-RPT - 21

24 AERATED STATIC PILE COMPOSTING SYSTEM, USING DRYING BEDS FOR CURING PILES COST ESTIMATE 1. Building - Pre-Engineered Steel Building with Concrete Floor $. 724, Equipment - Mixer, Screen, Loader, Hopper System 3. Earthwork 4. Concrete - For Drying Beds 5. A/C Pavement for Stockpile Area 6. Belt Pr-ess 7. Miscellaneous - Blowers, Pipes TOTAL 0 & M COSTS 1. Labor 2. Equipment Maintenance 3. Bulking Agent 4. Electrical TOTAL 215, ,000 71, , ?000 28,500 $ 1,569,400 $ 44,400 30? , Feb-89 ALTERNATIVE COMPOSTING SYSTEMS GARBRICK-RPT - 22

25 DESIGN CALCULATIONS FOR THE AERATED STATIC PILE COMPOSTING SYSTEM Wet Sludge Production: 2,000 g/mg x 5.5 mgd x 365 days x U7.5 = 535,333 cf/yr Compost Pile Size: Triangular Shape: Width = 12 ft. Height = 6 ft. Area = 1/2 x 6 ft x 12 ft = 36 sf Linear Footage of Compost: (535,333 cf/yr sludge + 267,667 cf/yr bulking agent) + 36 sf = 22,305 lf/yr Compost Detention Time: Compost Cycles Per Year: cycles/year. 21 days or 3 weeks. 3 weeks/52 weeks = 17.3 cycles/year, say 17 Linear Feet Per Cycle: 22,305 lf/year c 17 cycles = 1,312 lf/year. Number of Rows: Length = 110' 1,312 If 1200~ = rows Feb-89 ALTERNATIVE COMPOSTING SYSTEMS GARBRICK-RPT - 23

26 AERATED STATIC PILE COMPOSTING SYSTEM WITH CONVEYORS Compost System Contingency (10%) SUBTOTAL Interest During Construction (6-1/8%) Legal, Administrative, and Technical (15%) TOTAL Present Worth of 0 & M (10% over 20 years) $160,440 (8.51) Present Worth of Salvage Value $869,500 (0.1486) Present Worth of Compost Credit $91,250 (8.51) NET PRESENT WORTH TOTAL COST $ 1,907, ; 800 $ 2,098,400 $ 128,500 1,365, , ,500 3,001, Feb-89 ALTERNATIVE COMPOSTING SYSTEMS GARBRICK-RPT - 24

27 AERATED STATIC PILE COMPOSTING SYSTEM WITH CONVEYOR SYSTEM COST ESTIMATE 1. Building - Pre-Engineered Steel Building with Concrete Floor Equipment - Conveyors, Hopper, Mixer, Screen, F.E.L. Earthwork - Clearing and Grading 4. A/C Pavement for Stockpile Area 5. Belt Press 6. Miscellaneous - Blowers, Pipes: TOTAL 0 & M COSTS 1. Labor 2. Equipment Maintenance 3. Bulking Agent 4. Electrical TOTAL $ 805, , , , ,000 28,500 $ 1,907,600 $ 44,400 36,240 47,800 32,000 $ 160, Feb-89 ALTERNATIVE COMPOSTING SYSTEMS GARBRICK.RPT - 25

28 DESIGN CALCULATIONS FOR THE AERATED STATIC PILE COMPOSTING SYSTEM Wet Sludge Production: 2,000 g/mg x 5.5 mgd x 365 days x U7.5 = 535,333 cf/yr Compost Pile Size: Triangular Shape: Width = 12 ft. Height = 6 ft. Area = 1/2 x 6 ft x 12 ft = 36 sf Linear Footage of Compost: (535,333 cf/yr sludge + 267,667 cf/yr bulking agent) + 36 sf = 22,305 lf/yr Compost Detention Time: 21 days or 3 weeks. Compost Cycles Per Year: 3 weeks/52 weeks = 17.3 cycles/year, say 17 cycles/year. Linear Feet Per Cycle: 22,305 lf/year + 17 cycles = 1,312 lf/year. Number of Rows: Length = 110 1,312 LF 110 ft/row = 3 rows Feb-89 ALTERNATIVE COMPOSTING SYSTEMS GARBRICK-RPT - 26

29 ON-SITE LANDFILLING OF DEWATERED SEWAGE SLUDGE Sewage sludge can be disposed of at an on-site landfill. This requires excavating a trench, lining it with a plastic liner, filling the trench with dewatered sludge, and covering the trench with the excavated material. In order to operate and maintain a landfill, the City of Lexington will need a scraper, dozer, loader and backhoe, along with two laborers to run the equipment. The site must be relatively flat and the trenches must be at least ten feet wide. After the scraper forms the trench, a liner must be put in to contain any leachate that may occur. Sludge will be dewatered on the drying beds to at least 50% solids. A front end loader will remove the sludge and a dump truck will haul it to the trench site. Because the sludge is so dry, the truck must enter the trench and dump directly on the floor of the trench. Dumping from the side walls of the tench will not work because sludge is too dry to spread out evenly. The trench should be approximately five feet deep. Once full, the trench should be covered with a minimum of four feet of excavated material. No imported cover material will be required. There are some potential disadvantages to landfilling sewage sludge. Sludge contains harmful bacteria, pathogenic organisms, and heavy metals that can be dangerous to animals and humans. Liners are intended to prevent these materials from entering the ground water, but they may leak. Another disadvantage to landfilling is that there is a limited amount of land available at the site. In the future, sludge would have to be hauled off site for disposal Feb-89 ALTERNATIVE COMPOSTING SYSTEMS GARBRICK-RPT - 27

30 ON-SITE LANDFILL Capital Contingency (10%) SUBTOTAL Interest During Construction (6-1/8%) Legal, Administrative, and Technical (15%) TOTAL Present Worth of 0 & M $130,500 (8.51) Present Worth of Salvage Value $441,800 (0.1486) NET PRESENT WORTH TOTAL COST $ 552,300 45,700 $ 598,000. $ 30,800 75,400 $ 704,200 1,110, ,700 $ 1,749, Feb-89 ALTERNATIVE COMPOSTING SYSTEMS GARBRICK-RPT - 28

31 ON-SITE LANDFILL 1. Liner Cost 2. Equipment - Scraper, Loader, Dozer, Backhoe, 6 Monitoring Wells TOTAL CAPITAL 0 & H COSTS 1. Labor 2. Equipment Maintenance TOTAL COST ESTIMATE $ 103, ,300 $=r" $ 55,500 75,000 $ 130, Feb-89 ALTERNATIVE COMPOSTING SYSTEMS GARBRICK-RPT - 29

32 LAND APPLICATION OF DEWATERED SEWAGE SLUDGE Sludge may be applied directly to the soil as a source of fertilizer nutrients and as a soil amendment. The amount of sludge applied must be carefully regulated, due to the harmful bacteria and pathogenic organisms found in sewage sludge. The drying beds will be utilized to dewater the sludge to approximately 50% solids. Once dewatered, the sludge is removed from the beds with a front end loader and hauled by dump truck to the application site. A front end loader and a spreader will be brought to the site to apply the sludge. This equipment must be transported to each application site. Land application of sewage sludge has potential disadvantages. Sludge contains constituents which can be harmful to crops, or the animals and humans that consume them, if not carefully regulated. Another potential problem is finding enough farmland to utilize sludge application over the next 20 years. Public approval of sludge application is also difficult Feb-89 ALTERNATIVE COMPOSTING SYSTEMS GARBRICK-RPT - 30

33 LAND APPLICATION Capital Contingency (10%) SUBTOTAL Interest During Construction (6-1/8%) ' Legal, Administrative, and Technical (15%) TOTAL Present Worth of 0 & M $131,100 (8.51) Present Worth of Salvage Value $300,000 (0.1486) NET PRESENT WORTH TOTAL COST $ 489,400 48,940 $ 538 f 346 $ 33, , ;600 s 1,723, Feb-89 ALTERNATIVE COMPOSTING SYSTEMS GARBRICK.RPT - 31

34 LAND APPLICATION COST ESTIMATE 1. Hauling Cost - Labor, Fuel, Loading, Unloading $ 161, Equipment Cost - Spreader, Tractor, Front-End Loader 300, Stockpile Area $ 27,700 TOTAL CAPITAL 489,400 0 & M COSTS 1. Labor (3 Men) 2. Equipment Maintenance TOTAL $ 83, Feb-89 ALTERNATIVE COMPOSTING SYSTEMS GARBRICK.RPT - 32

35 THE SELECTED ALTERNATIVE Based on the cost-effective analysis and environmental impact, the enclosed dynamic composting system was selected as the most cost-effective with the least environmental impact Feb-89 ALTERNATIVE COMPOSTING SYSTEMS GARBRICK.RPT - 33