Replacement of a multiple hearth by a ¯uid bed incineratorthe Greensboro case history

Arthur White a, Dale Mayrose b, John F. Mullen c,*aT. Z. Osborne WWTP 2350 Hu�ne Mill Road, Mechanicsville, NC 27301, USA

bIn®lco Degremont, Inc., 7 Tudor Road, Freehold, NJ 07728, USAcIn®lco Degremont, Inc., 22 Little Fox Run, Shelton, CT 06484 USA

Accepted 6 December 1999

Abstract

The incinerator at the T.Z. Osborne Plant in Greensboro, North Carolina burns sludge from its own waste water treatment plantand sludge pumped from the nearby North Bu�alo plant. The two plants have a combined capacity of 36 million gallons per day ofwastewater. Because of the age of and increasing high maintenance on the existing multiple hearth incinerator, and the need toincrease treatment capacity, the Osborne plant concluded a study in 1992 evaluating its options for future municipal sewage sludge

disposal. Options which were evaluated during the study included; (i) rehabilitation of the existing eight-year old multiple hearth unit;(ii) addition of a new multiple hearth; (iii) addition of a new ¯uid bed system; (iv) drying, composting, or land application. The chosenoption, based on both economic and environmental considerations, was a new ¯uid bed system with a capacity of 2.55 tons per hour,

approximately double that of the existing multiple hearth. Design of the new ¯uid bed system began in December 1994 and equipmentdelivery for the incineration system was begun in April 1995. Initial operation occurred in August 1996. Primary and secondarysludge, dewatered to 28% dry solids by centrifuge, is delivered by piston pumps to the twenty-foot freeboard ID incinerator. A shell

and tube heat exchanger recuperates heat from the exhaust gas and preheats the combustion air to 1250�F, resulting in minimalauxiliary fuel use. The air pollution control device is a high-energy TandemNozzle1 scrubber. Greensboro was the initial installationof this scrubber design on a ¯uid bed incinerator and its characteristics and performance are discussed. Ash is dewatered in an ashthickener/belt press system prior to disposal to land®ll. The system includes a state of the art Programmable Logic Controller (PLC)

system for computer control of the operation. The unit was commissioned in August 1996 and has been in continuous operation sincethat time except for a one week inspection and maintenance shutdown in February 1999. The plant operates 24 h/day, 7 days perweek. The initial performance test showed the system to readily meet federal and state air emission standards. Particulate released was

0.002 grains per dry standard cubic foot, carbonmonoxide was 22.5 parts per million volumetric (ppmv) and opacity was 0.4%. Theseresults show a signi®cant emission reduction with the ¯uid bed when compared to the multiple hearth. Annual tests conducted sincethen and continuous emission monitoring have shown the unit to be in consistent compliance. Since the ¯uid bed system became

operational, the old multiple hearth system has been maintained on standby as a backup, but its use has not been required. Opera-tional experience is discussed, the most interesting of which is the relatively trouble-free operation. The minor problems whichoccurred and their solutions are detailed. Also included is a comparison of operation and maintenance experience of the ¯uid bed and

the multiple hearth. Current sludge disposal actual cost data are also provided including the average cost per ton of dry solids treated.The almost three years of operational experience to date has shown that the decision to install a new ¯uid bed system was the correctone on both an environmental and economic basis. It has provided bene®ts to all interested parties Ð the wastewater treatment plant,the regulators, the taxpayers, and the surrounding community. # 2000 Elsevier Science Ltd. All rights reserved.

Keywords: Fluid bed incinerator; Sludge; Multiple hearth system

1. Background

The City of Greensboro, North Carolina currentlyoperates two wastewater treatment plants. The North

Bu�alo Creek plant was built in 1938 and was expandedin capacity in 1959 from 8 to 16 million gallons per day(mgd). The Thomas Z. Osborne plant (formerly theMetro plant), which is the subject of this paper, wasauthorized in 1978. Construction began in 1981 anddedication was in September 1984. The installationincluded a multiple hearth incinerator with a capacity of1.24 dry tons per hour. The City also had a smaller

0956-053X/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.

PI I : S0956-053X(00 )00007-6

Waste Management 20 (2000) 703±709

www.elsevier.nl/locate/wasman

* Corresponding author. Tel.: +1-203-929-3577; fax: +1-203-929-

4098.

E-mail address: mullenj@idi-online.com (J.F. Mullen).

multiple hearth at the North Bu�alo Creek plant whichit operated from 1974 until 1984 when the Osborneplant was brought on line. The Osborne plant has thecapacity to treat an average ¯ow of 20 mgd and a peakrate of up to 50 mgd in wet weather. It also receives upto 6.5 mgd of wastewater pumped from the North Buf-falo Creek Plant some 5 miles away. In addition to itsdomestic users, it accepts wastewater from 23 signi®cantindustrial users, including organic chemical, electro-plating, metal ®nishing, and textile manufacturingplants among others. The plant is situated on a site of460 acres and designed for expansion to 80 mgd ormore, which would permit the phasing out of the NorthBu�alo Creek Plant and consolidation of Greensboro'swastewater treatment into a single facility. One suchexpansion to 30 mgd capacity is now ongoing withcompletion scheduled for 2001.

2. Review of sludge disposal options

In 1989, the City commissioned Hazen and Sawyer,their consulting engineer, to perform a study of theirfuture sludge disposal options because of a plannedexpansion of wastewater treatment capacity and a desireto provide a dependable backup for disposal to theexisting multiple hearth [1]. Other considerations forthis study were the then impending new emission reg-ulations (EPA 40 CFR 503) which the existing multiplehearth would have di�culty in meeting, the trend tobene®cial reuse and the age and increasing maintenanceof the multiple hearth. Among the options consideredwere rehabilitation of the existing multiple hearth, a newmultiple hearth furnace, a new ¯uid bed incineration sys-tem, drying, land application or composting. The Cityperformed the study with its own personnel using inputsfrom their consulting engineer and equipment and systemsuppliers.The original Osborne plant was constructed in a rural

community which opposed the siting. Members of thecommunity were very vocal during the permitting pro-cess and the construction phase and this opposition con-tinued during initial operation. The City was thereforevery sensitive to impact of future sludge disposal optionson the community, including emissions, odors and trucktra�c and these were considered in the evaluation.Both in-vessel composting and aerated static piles

were investigated. The City visited a number of di�erentoperating facilities across the United States. Because ofconcerns over odor and the ultimate disposal of thecompost, as well as an estimated capital cost between $8million and $10 million, this option was eliminated.The City pilot tested a sludge dryer to look at its

possible use in the disposal scheme as a preparatory stepto reduce the cost for either incineration or composting.This equipment would allow the City to produce a drier

cake for the incinerator but would require a workingheat exchanger in the exhaust ¯ue and a back up heatsource so that the system could be used when the incin-erator was down. The dryer alternative would not solvethe problem by itself, but could be incorporated withanother solution. The City received an installed cost esti-mate of $2.4 million just to get the cake drier and wouldstill have to use its existing belt press for dewatering.Because of both economics and concern for potential odoremission problems, this option was eliminated.The City visited a number of locations that were uti-

lizing lime stabilization and land application and notedthey demonstrated potential odor as well as o� sitetra�c problems. These alternatives also had potentialproblems due to the storage requirement for largequantities of sludge during wet weather and the need tofully contain the odor from such storage. The amount offarmland that could be permitted by the City was anotherconsideration. A number of smaller cities in the area hadland application programs in operation, had permittedmuch of the available farmland, and were looking for anoutlet for their sludge during wet periods.After review of the above disposal alternatives, the

issue of incineration was revisited. The Osborne plantwas originally designed for two multiple hearth incin-erators and two were permitted by the state at the timethe plant was bid. Although the building was sized fortwo units, only one was constructed due to fundinglimitations. However, because of plans for capacityexpansion, it was clear that a new unit would berequired. At the same time, the City looked at ways toimprove the operation of the multiple hearth furnaceand eliminate some of the operational problems inher-ent to its design. For instance, the City examined con-verting the existing furnace to a side outlet furnace witha recycle of the drying section air to improve air emis-sions and provide a uniform burn. The City visited theonly multiple hearth of this type unit that had beenconstructed (in San Mateo, California in 1974) andobtained an engineering proposal and cost estimates tomake the necessary changes. While it was consideredfeasible, the modi®cation of the multiple hearth to thiscon®guration would have required a down time ofapproximately one year, based on vendor estimates.This was unacceptable to the City at that time, since theexisting multiple hearth was their primary economicdisposal option. This option may be considered in thefuture if increasing sludge disposal requirements createa need for an additional operational incinerator.The choice then became between a new ¯uid bed and

a new multiple hearth incineration system. As a part oftheir evaluation, the City visited a number of ¯uid bedplants, including some which had experience with bothincinerator types. In contrast to its multiple hearthexperience, the City was impressed during these visitswith the low emissions achievable, the lack of odors, the

704 A. White et al. /Waste Management 20 (2000) 703±709

simplicity of operation and the ease with which the ¯uidbed system could be automated. While these factors arerelatively intangible, they were important in the decisionbecause they addressed previous objections in the com-munity to the multiple hearth installation. An additionaladvantage was the smaller footprint of the ¯uid bed,which would permit a unit of more than double the capa-city to be installed in the same space as that originallyallowed for the second multiple hearth.The ¯uid bed system also showed economic advan-

tages. Vendor quotations showed that the installed costof the complete multiple hearth system would be 5±10%higher than that for the ¯uid bed for systems of thesame capacity. Even more important were the potentialsavings in life cycle costs. Because of the ability of the¯uid bed system to preheat the combustion air using theexiting ¯ue gas, the ¯uid bed could operate without theneed for auxiliary fuel except during start-up and shut-down of the system. Further savings in fuel cost derivefrom the thermal ¯ywheel e�ect of the hot sand in thebed. Short maintenance shutdowns can be accomplishedwithout signi®cant heat losses from the unit. Return tooperating temperature would thus take less fuel and lesstime. The multiple hearth could be maintained at oper-ating temperature during such periods, but only with thecontinuous use of auxiliary fuel. The power costs for the¯uid bed would be higher because of the higher pressuredrop across the bed, but this would be mitigated in partby the lower air¯ow due to lower excess air requirements.It was also estimated that the ¯uid bed would provide a

reduction in operating and maintenance labor costs. Intotal, the projected total cost of all operation andmaintenance showed a substantial advantage for the¯uid bed.The study was completed in 1992 with a recommen-

dation to install a new ¯uid bed incineration systembased on economic, environmental and communityrelations considerations [2].In its performance test in 1984, the multiple hearth with

a venturi scrubber failed to meet the particulate controlrequirement of 1.3 pounds per dry ton of sludge. This wasapparently due to high concentrations of sub-micron silicainherent in the City's wastewater system. The venturi wasreplaced by a Dual TandemNozzle1 scrubber that solvedthe problem. It was therefore decided to include the DualTandem Nozzle1 scrubber as a part of the new system.As previously noted, a second multiple hearth incin-

erator had been permitted but not constructed at the plantlocation. A new permit was required by the regulatoryauthorities because of the technology substitution. Per-mitting e�ort was however signi®cantly eased by the sig-ni®cantly lower environmental impact of the ¯uid bed.

3. Fluid bed incinerator system description

A process ¯ow diagram for the Osborne incinerationsystem is shown in Fig. 1. The general physical character-istics and theoretical operating characteristics of a ¯uidbed incineration system have been detailed elsewhere

Fig. 1. Process ¯ow diagram.

A. White et al. /Waste Management 20 (2000) 703±709 705

[3,4]. At the Osborne plant, biosolids, dewatered to 28%dry solids utilizing high solids centrifuge decanters, arefed through a transfer box to two piston pumps whichfeed four nozzles in the ¯uid bed reactor. No. 2 oil isused as auxiliary fuel during start-up and operation asneeded, although normal reactor operation is auto-genous. The twenty-foot freeboard ID incineratoroperates at a design temperature of 1550�F. A smallamount of sand is lost from the ¯uid bed during operationdue to elutriation. This loss is usually replaced once aweek without an interruption of operation. A pneumaticsand system permits hands-free ®lling of the sand silo andautomatic sand addition to the ¯uid bed during operation.To minimize auxiliary fuel use, ¯uidizing air is preheatedto 1250�F in an external shell and tube heat exchangerutilizing the exiting ¯ue gas, which leaves the heatexchanger at 1000�F. Control of air pollution includesboth operational and design measures and the air pollu-tion control system. Control of excess combustion airand temperature are operational measures, while com-bustion residence time and optimum mixing conditionsare design measures. The air pollution control system isa Dual Tandem Nozzle1 scrubber. The resultant ashslurry is pumped to an ash thickener/belt press systemfor dewatering to approximately 50% dry solids. Thedewatered ash is hauled to a land®ll for disposal whereit is used as cover. No plume suppression is required.Since the Hydro-Sonic Dual Tandem Nozzle1 scrub-

ber at the Osborne plant is one of only three suchinstallations at municipal wastewater treatment plants,it merits further discussion. The system consists of aquench chamber, two subsonic nozzles in series, and acyclone separator. Water is atomized into the quenchchamber to cool the gas to its adiabatic saturation,condense most of the water vapor and remove most ofthe solids. The cooled gas enters two subsonic nozzles inseries, each followed by a turbulent mixing zone. Thefree jet action of the ®rst nozzle produces a ®ne waterspray which contacts the exhaust gases and condensesthe vapors of water and metals, removes the larger par-ticles and initiates the growth of the smaller particles onwater droplets so that they are more easily captured inthe second nozzle. Large droplets exiting the nozzles arethen removed by a cyclone separator. The total pressuredrop is about 50 inch wg. A more complete descriptionis provided in Ref. [5]. The use of this scrubber and the¯uid bed demonstrates the commitment at the Osborneplant to high performance and to the minimization ofenvironmental impact on the surrounding community.

4. Computer control

The new furnace is controlled utilizing a GE FanucPLC based control system that connects all of the com-ponents of the incinerator. The incinerator is monitored

from anoperator interface which is a PC based system thatcommunicates through the TCP/IP LAN system. Thecontract for the ¯uid bed also required the tie-in of theexisting centrifuge PLC panel with the new controls. Thissystem allows the operator to have a single access point tocontrol all components of the system, including the incin-erator, cake pumps, polymer feed system, centrifuge, con-tinuous emission monitoring system, ash press, etc. Theoperator interface may be used to download new controlset points or to start and stop the various equipment.The new furnace utilizes a start-up sequence that per-

forms all the steps that can be automated and alsoprompts the operator when he is required to carry outany actions that cannot be done by the control systemor for which operator action is desired. After the fur-nace start-up, the operator regulates the amount ofsludge supplied to the feed hopper of the cake pumps.The computer monitors the operations of the incin-erator and takes the necessary action to maintain propercombustion conditions.The PC based operator interface software provides

the operator with a full overview of the system opera-tion using custom graphics. The operator may choose tocontrol the equipment in manual, semiautomatic, or fullautomatic mode. The new control system also performsdata acquisition and storage for all signi®cant operatingparameters. This assists the operator and plant man-agement in providing solutions to any operating pro-blems and in ®ne-tuning the system for more e�cientoperation. It also can be used for generating requiredregulatory authority emission reports.

5. Commissioning and performance testing

In November 1996, testing was performed to demon-strate compliancewith environmental requirements for theemission of metals and particulates. These requirementsincluded:

. the US EPA 503 regulations for metal emissions(arsenic, beryllium, cadmium, chromium, lead,mercury and nickel) and for gaseous emissions ofCO and THC;

. the National Emission Standard for HazardousAir Pollutants (NESHAPS) requirements for ber-yllium and mercury,

. the New Source Performance Standards (NSPS)requirements for particulate, and

. the Speci®c Conditions and Limitations of theOsborne facility's Air Permit 4489R12 from theNorth Carolina Department of Environmental,Health and Natural Resources (DEHNR)

Testing for metals, particulate emissions and opacitywas performed in November 1996 [6]. Three runs were

706 A. White et al. /Waste Management 20 (2000) 703±709

conducted using EPA standard methods and correcting,as required, to standard conditions. Table 1 shows theaverage emissions and the compliance status for eachconstituent. Note that, with the exception of hexavalentchromium, all actual emissions are one to two orders ofmagnitude or more below the limit.Simultaneous with the metal emission tests, gaseous

emissions were tested [7] to show that the CO emissionswere within the regulatory requirements. Average emis-sions were 22.5 ppmv corrected to 7% O2, which is wellwithin the 100 ppmv regulatory requirement.

6. Operational history

Since the initial operation in August 1996, the planthas been operated 24 h/day, 7 days per week. With theexception of one unplanned outage of the ¯uidizingblower (discussed below), the plant has been operatedcontinuously with no cold starts required until the ®rstmaintenance shutdown in February 1999. The avail-ability of the unit, including the shutdown periods, hasbeen greater than 97%.Control of the ¯uid bed system is done by computer

and its operation is normally monitored by a singleoperator, who also is responsible for the centrifuge andash belt press operation. When the multiple hearth wasoperational, it alone required a dedicated operator with anadditional one required for the sludge dewatering opera-tion. There is an average of 0.5 mechanic per shift for thesolids handling building doing repairs and preventivemaintenance for the complete dewatering and incinerationoperation. There is also one equipment operator working4 h/day transporting ash to the land®ll.Many of the initial operational problems were due to

a lack of ¯uid bed system operating experience and hadrelatively simple solutions. Two exceptions to this were

problems with the ¯uidizing blower and the sludge feedpumps. A main bearing failure on the blower resulted indamage to the impeller and a requirement to rebuild theequipment. This resulted in a 12-day downtime, whichwas the longest shutdown to date. On the cake pumps,polymer and solids got into the hydraulic oil, causingpump shutdown when operating under high pressure.The source was apparently a ®lm of polymer on thepiston getting past the seals. The solution was to addredesigned and additional wiper rings. An additionaldi�culty on the sludge pumps was bridging in the hop-pers over the screw. Experience showed this could beavoided by maintaining the sludge levels relatively low(one foot maximum depth above the screw).Most of the other operational items were associated

with the ash system. The pneumatic diaphragm pumpsfrom the ash thickener would develop ash deposits if not¯ushed regularly. The ¯ushing was changed from amanual to an automatic operation. A similar depositproblem occurred on the belt press and was solved byautomatic, more frequent washing. Another problemwith the belt press would occur when a thick layer of ashwould jam the press. Automated control of ash thicknesson the belt was added. Initially belts on the pressrequired frequent replacement due to tears at the seams;however, going to a coarser, seamless belt signi®cantlyincreased belt life. Originally, ash from the belt presswent to a pre-existing bulk storage hopper. Due to anangle of repose for the wet ash that seemed to approach90�, it was virtually impossible to remove ash from thehopper. The dewatered ash is now dropped through avertical chute directly into the trucks which take it toland®ll.The results of the February 1999 inspection/main-

tenance shutdown, which included a complete internalinspection of all components, were very satisfactory. Thereactor itself, including feed guns, refractory, refractoryarch, and instrumentation, were in excellent shape. Ero-sion had occurred on the Hastelloy sheathing of the high-pressure water spray nozzles and it is planned to replacethem every 2 years. A small amount (approximately 15cubic feet) of sand had sifted through the refractory archor tuyeres into the windbox, but this does not impactoperation. The heat exchanger and ducting were free ofdeposits. Seventeen (of a total of 109) of the tubes in theair to gas heat exchanger had worked free of the lowertubesheet (in which they slide) and new sleeves wereincorporated to keep them in position. The entire shut-down including cooldown, inspection, minor repairsand startup to full operation took 7 days.The maintenance history of the ¯uid bed is in stark

contrast to past experience with that of the multiplehearth. A problem that constantly plagued the multiplehearth operation was slag build-up on the drop holesand the in-hearth ledge. The unit required frequentburner maintenance due to slag build-up on the burner

Table 1

1996 Emissions results and compliance status ¯uid bed incineration

system

Parameter Emissions

Actual Allowable

Particulate, pounds/ton dry solids 5.75E-02 1.30E+00

Opacity, percent 0.4 20.0

Metals, pounds per day

Arsenic 2.71E-03 NAa

Beryllium 8.40E-05 2.20E-02

Cadmium 7.44E-04 NA

Chromium < 5.83E-04 NA

Lead 3.24E-03 NA

Nickel 2.93E-03 NA

Mercury 2.62E-02 7.05E+00

Hexavalent chromium < 2.06E-06 2.54E-06

a NA Ð No individual permit limitation. Control is by limitation

on input of constituents in sludge

A. White et al. /Waste Management 20 (2000) 703±709 707

ports. This was apparently due to the inherent problemin the multiple hearth of controlling combustion zonetemperature. Temperature control was particularly dif-®cult when sludge input water content was varying andduring start-ups after maintenance shutdowns. Whenslag had begun to build up, it was necessary to stop thesludge feed, cool the furnace down to stop the build-up,and remove the auxiliary fuel burners. Slag was thenremoved by chipping o� the build-up with hooked toolsworking through access doors. Occurrence of the slagbuild up was such that shutdowns were required onaverage about once a month. About two or three timesa year, the incinerator would have to be cooled com-pletely so operating personnel could go inside the unitto do a more complete slag removal. Another problemwas build up of carbon deposits and subsequent ¯akingo� on the blades of ID fan, causing imbalance. The fanwould need to be taken out of service to chemicallyclean the impeller.In addition to improvements in operations and main-

tenance, the ¯uid bed system has also resulted inimprovements in emissions, as the comparison of Table2 against those of the multiple hearth system indicates.Note that the metal emissions rates are actual emissionswith no correction for the increased capacity of the ¯uidbed, which is over twice that of the multiple hearth. Fur-thermore, the multiple hearth is subject to wider excur-sions in peak emissions because of its basic design andoperation. The long residence time of solids (45 min ormore) in the hearth makes it di�cult to react quickly tochanging input feed or incinerator operating conditions.Beyond the quanti®ed emission numbers, the elimina-

tion of signi®cant visible emissions that would occur

during multiple hearth upset conditions and the reduc-tion in odors from the incinerator have been of majorbene®t in community relations. The plant had receivedfrequent complaints from local residents on smoke, stacknoise and incinerator odors during the multiple hearthoperation. These have been reduced to zero during ¯uidbed operation.

7. Cost data and cost comparisons

The total cost of the installation of the ¯uid bed sys-tem in 1994 was $6 million. This included the completeincineration system from the sludge pumps through thestack with all required ancillary equipment. Also inclu-ded in this cost were necessary modi®cations to theexisting building, including access doors for equipment,the ash dewatering equipment and integration of thesludge dewatering system control into the new PLC.Table 3 summarizes the 1998 operational cost experi-

ence for the system. Per ton of dry sludge, dewateringcost is about $45.50 while incineration cost is about $60.The current land®ll tipping fee (not including transpor-tation cost) for wet (after dewatering) sludge in the areais $34 per wet ton. This is equivalent to $115 per dryton Ð almost twice the cost of dewatered sludge dis-posal by incineration. Because the current computerizedcost accumulation program was not available in theperiod when the multiple hearth was operational, onlylimited de®nitive quantitative cost data is available. Forthe ®nal year of multiple hearth operation, fuel cost perton of dry sludge was $4.18. Since amortization (based on20 years and 5% interest) is calculated on a straight-linebasis (a constant value for 20 years) there would be anamortization cost for the original installation of the mul-tiple hearth as well as for the changes required to bring theunit into regulatory conformance. As previously noted,use of the ¯uid bed system also resulted in a reduction in

Table 2

Emission data comparison

Incinerator Multiple hearth Fluid bed

Date

Dry sludge feed rate, tons/h 1.24 2.55

Particulate emission rates, lb/dry

ton of sludge 0.77 0.058

Concentration, g /DSCF 0.02 0.0021

Volume of air ¯ow rate dry, SCFM 7150 8350

Opacity, % 0.40

Metal Emission Rates, lb/h

Arsenic 1.93E-04 1.13E-04

Beryllium 3.38E-06 3.50E-06

Cadmium 1.29E-02 3.10E-05

Chromium 5.14E-04 <43E-05

Lead 5.14E-04 1.35E-04

Mercury 4.44E-03 1.09E-03

Nickel 5.14E-04 1.221E-04

Hexavalent chromium < 1.80E-06

Gaseous emission rates, ppmv

CO Not measured 22.5

THC 80 9

Table 3

Operating cost per dry ton of sludge processed 1998 cost basis

Cost component Sludge

dewatering

Fluid bed incineration

including ash treatment

Polymer $31.00 NA

Power $3.58 $6.77

Fuel NA $1.85

Personnel $10.80 $19.36

Sand NA $0.48

Ash disposal NA $1.86

Subtotal $45.54 $30.34

Equipment amortization Note 1a $29.00

Total cost, $/ton dry sludge $59.36

a Because of the age of the solids handling building and the various

times at which installation of equipment has occurred, this number

cannot be easily expressed in 1998 dollars.

708 A. White et al. /Waste Management 20 (2000) 703±709

operating manpower, reduced maintenance andincreased system availability.

8. Conclusion

Operational experience has justi®ed the decision madein 1992 to use a ¯uid bed incinerator system as theGreensboro sludge disposal method. The existing mul-tiple hearth has been maintained on standby for usewhen the ¯uid bed was unavailable, but has not beenrequired for use since the ¯uid bed start-up in 1996. The¯uid bed system has proven to be reliable and easy tooperate. Fuel costs, operator man-hours, maintenancecosts and emissions have been substantially reducedwhen compared with the previous multiple hearthoperation. With improved air quality and lower sludgedisposal costs, the installation has brought bene®t to allinterested parties Ð the wastewater treatment plant, theregulators, the taxpayers, and the surrounding community.

References

[1] Osborne TZ. Wastewater treatment plant. In: Plant expansion

and upgrade study. Raleigh, NC: Hazen and Sawyer, 1991.

[2] Osborne TZ. Wastewater treatment plant. In: Sewage handling

facilities. Design report. Raleigh, NC: Hazen and Sawyer,

December 1992.

[3] Mullen JF. Consider ¯uid bed incineration for hazardous waste

destruction. Chemical Engineering Progress, June 1992.

[4] Mayrose DT, Cohen A. Fluid bed combustion of sewage

sludge, Sixteenth Annual Conference, Hawaii Water Pollution

Control Association, Honolulu, Hawaii, 2±4 February, 1994.

[5] Means JD, Holland OL. Utilization of hydro-sonic scrubbers

for abatement of emissions from hazardous/radioactive mixed

waste, Conference on Incineration of Low Level Radioactive

and Mixed Waste, Charlotte, NC, 22±25, April, 1986.

[6] Results of November 1996 Emissions Testing of the Fluidized

Bed Sewage Sludge Incinerator. General Engineering Labora-

tories Inc., Charleston, SC, December 16,1996.

[7] Fluidized Bed Incinerator Gaseous Emission Test Results, Gen-

eral Engineering Laboratories Inc., Charleston, SC, December

18,1996

A. White et al. /Waste Management 20 (2000) 703±709 709