I ’

ity serving over 400,000 people in the City of Charlotte, North Car- olina and surrounding suburban areas. The CMUD operates five

wastewater treatment plants (WWTPs). The three largest WWTPs, located in the Sugar Creek basin south of the city center, have been integrated into a basin-wide fa- cilities plan (CH2M Hill, 1985) and current- ly generate biosolids totaling about 36 dry tons per day (dtpd).

Until 1986, CMUD disposed of its biosolids in municipal solid waste landfills. That year, Mecklenburg County ceased ac- cepting biosolids in its landfills. This action was followed in 1988 by a statewide ban on disposing of biosolids in MSW landfills. Biosolids were stockpiled on-site until a co- operative agricultural land application pro- gram was implemented. This program has been quite successful, and received first place honors in the U.S. Environmental Pro- tection Agency’s beneficial use of biosolids awards in 1992.

Mecklenburg County is becoming heavily urbanized, and CMUD is dependent largely on more rural neighboring counties for its land application program. Recognizing the potential risks to the program if public or po- litical opposition should develop, city offi- cials have carefully managed the program and cultivated relationships to promote its continued success. The lack of a diversified biosolids management program concerned CMUD, however, leading to the formation in 1992 of a Residuals Innovation Team to evaluate other pptions.

The team recommended addi t ional biosolids products that could have wider and more diversified markets for distribution throughout the year. The products that the team considered to be most cost-effective to produce and have a good demand potential are biosolids compost and alkaline stabi-

Dual approach for biosolids processing is carvied over to odor control, with both chemical scrubbers and biofiltration in the design.

Robert H. Forbes, Jr., Trille C. Mendenkall, Lynne 0. Young and Jeffrey D. Sckwisow

An aerial view of the McAlpine Creek Wastewater Manage- ment Facility, where the Residuals Management Facility is being constructed, depicts the development around the site. Within a mile radius are a shopping mall, two residential subdivisions and a freeway that is under construction.

lized biosolids. Both would have potential for use in landscaping, turf management, nurseries, agriculture, landfill cover, and by homeowners. The diversification of these markets could stabilize demand for biosolids use and make it less dependent on weather and crop growing seasons.

After considering several options, the de- cision was made to construct a facility that would produce compost and lime stabilized biosolids in roughly equivalent amounts, and locate it at the site of CMUD’s largest WWTP. The chosen processes were aerated static pile composting and dry lime stabi- lization. The two processes have similar re- quirements for materials conveyance and mixing, therefore both could use the same biosolids receiving, conveyance, and mixing equipment, and could be operated using the same programmable instrumentation and controls system (Forbes & Gullet, 1993).

Named the McAlpine Creek Residuals Management Facility (RMF), the biosolids processing facility would have firm capacity to produce 20 dtpd of compost and 20 dtpd of lime stabilized biosolids. The evaluation and conceptual design of the RMF were docu- mented in a regional facilities plan amend- ment (CH2M Hill, 1993). A federal waste- water grant was preserved for par t ia l funding of its construction, which is under- way and slated for completion by late 1995.

ODOR CONTROL STRATEGIES The McAlpine Creek site was chosen for

the RMF because sufficient city-owned was available, and the adjacent J?&F would contribute the majority of losolids, thus saving considerable transportation costs. The area surrounding the WWTP, however, has become suburban. Two resi- dential subdivisions and a large shopping mall are located less than a mile in the di-

BIOCYCLE AUGUST 1994 49

A target maximum odor level of three dilutions to threshold (D/T) was established at the property boundary and beyond.

rection of prevailing winds, and a major free- way will be located about half a mile away in the same direction.

These factors made it evident that all po- tential impacts of the RMF on the sur- rounding community, especially odor gener- ation, needed to be minimized. The WWTP had a history of odor complaints in the past that led to replacement of biosolids drying beds with enclosed dewatering centrifuges. Officials did not want the RMF to generate odors that might resurrect complaints from the surrounding area.

Because of the critical need for effective odor control, CMUD and CHBM Hill used a multifaceted approach to system design and public acceptance. Among the steps taken were an assessment of site conditions such as weather and topography to develop an on- site odor dispersion model and surveys of odor control systems installed a t biosolids composting and lime stabilization facilities around the U S . The findings were used to develop odor control strategies for the RMF, incorporating the most successful elements of odor control systems at facilities using composting and lime stabilization technolo- gies. The system also would need to satisfy all air quality permitting requirements.

Performance criteria were established for the odor control system that would reduce odors to levels not likely to cause complaints during worst case dispersion conditions. Flexibility and expandability were designed into the odor control systems to provide re- dundancy and allow for treatment of a vari- ety of odors. Finally, CMUD and its consul- tants are developing a public awareness and involvement program to explain the envi- ronmental benefits of the RMF and the steps

that are being taken to make the facility “good neighbor.” Included in this program, for instance, are plans to convert the former biosolids drying beds and stockpile areas into athletic fields for public use.

DISPERSION CONDITIONS AND MODELING RESULTS

Site dispersion conditions were analyzed initially by Odor Science & Engineering as part of plans to locate an in-vessel compost- ing facility on the site (Forbes, et al., 1992). A ‘‘pufr‘ model was used to predict short- term impacts approaching an instantaneous odor event. The RMF site has a relatively low wind speed averaging seven miles per hour, and its atmospheric conditions are neutral to stable 71 percent of the time. These conditions lead to relatively poor dis- persion of odors.

When the concept of the RMF changed to a combined aerated static pile composting and lime stabilization facility, the odor dis- persion model was revised to incorporate the emissions sources of the preliminary design (Odor Science and Engineering, 1993). A target maximum odor level of three dilu- tions-to-threshold (Dm) was established at the property boundary and beyond. This lev- el of odor is considered to be virtually unde- tectable over background levels by odor sen- sitive individuals. Several scenarios of emission rates, scrubber stack dilutions, and biofiltration rates were considered in the modeling runs. Modeling was conducted for the combinations of atmospheric stabili- ty conditions and varying wind speeds. Re- sults of the modeling runs (Table 1) were used to establish and confirm design criteria for the odor control systems.

Table 1. Maximum Allowable Odor level in Source Emissions, Based on Maintaining Maximum Odor level of 3 D / l at Property line and Beyond

Maximum Exhaust Stack Allowable Odor

Number of Volume Velocity Diameter Height Level in Stack Source Sources (GFM) (Wmin) fft) fft) ( D m

Lime stabilization scrubbers (no dilution)

Lime stabilization scrubbers (10:l dilution)

Lime stabilization scrubbers (5:l dilution)

Biofilter (compost odor control discharge)

Roof exhaust fansb Storage building Processing building Mixing/receiving area Screening area

2

2 2 2

2 2 2

1

8 6 1 1

32,000

320,000 320,000 320,000

150,000 150,000 250,000

30,000

70,000 89,000 45,000 21,300

3,326

3,080 3,080 3,080

2,984 2,984 2,984

3

2,110 2,015 2,292 2,214

3.5

11.5 11.5 11.5

8.0 8.0 8.0

113

6.5 7.5 5.0 3.5

40

40 1 00 150

40 100 150

0

30 30 30 30

42

10 17 33

18a 24 39

68a

6 6 6 6

D/l = Dilutions to threshold aRepresents selected odor control discharge Goof exhaust fans will likely need to be run continuously ( a t 12t air changes per hour) to maintain low odor levels

AUGUST 1994

ODOR CONTROL REQUIREMENTS

As shown by the dispersion modeling re- sults, good odor control and containment at the facility site will be required to alleviate the potential for odor migration beyond the property line. It was decided to enclose the complete RMF in a building and maintain negative air pressure to prevent fugitive emissions. In addition, odor sources would be enclosed as much as possible, with ex- haust air directed from the odor sources to odor control systems, which would be de- signed to treat specific characteristics of the odor sources as described below.

Biosolids Receiving and Compost Process Odors: The RMF will receive pri- marily anaerobically digested and dewa- tered biosolids from the three WWTPs in the basin. A variety of odors are associated with digested biosolids, and often change de- pending on the level of biosolids stabiliza- tion. Ammonia is the primary odorant asso- ciated with anaerobically digested biosolids, but various other nitrogen and sulfur based odorants also may be present. To control

these odors, all biosolids receiving bins and conveyors will be covered and maintained under negative pressure by blowers that will direct the air to the compost odor con- trol system.

Odors associated with the biosolids com- posting process are strong and contain both reduced sulfur and nitrogen related com- pounds. In addition, pervasive oxygenated compounds are usually produced during the aerobic activity of composting. One of the major problems with composting odors are partially oxidized compounds such as dimethyl disulfide (DMDS), produced by the partial oxidation of mercaptans. These com- pounds tend not to be very water soluble and are somewhat resistant to further oxidation, which make them difficult to remove by tra- ditional chemical scrubbing technology. They do tend to have strong sorption charac- teristics, however, making them amenable to removal by biofiltration (Card, 1993).

With aerated static piles operated in the negative pressure mode, process air can be directed to an odor control system and treat- ed separately from the building ventilation

- The lime stabilization process creates odors primarily associated with ammonia, amines and related compounds, especially when treating anaerobically digested biosolids.

I

Table 2. CMUD Residuals Management Facility Estimated Air Exhaust Rates, Odor Characteristics, and Control Techniques Recommended

Design Criteria for Estimated Selected Odorous Exhaust

Compounds Exhaust Rate Rate Air/Odor Source Odor Characteristics (PPN Criteria (cfm)

Composting process Strong, reduced sulfur 200 ppm NH3 1,200 cfm per static 25,200 ventilation and nitrogen related compounds, 100 ppm H2S pile, 21 active piles

pervasive oxygenated compounds 2 ppm DMDS % of blowers on

Truck reaiving bin, High strength, primarily ammonia 400 ppm NH3 500 cfm per bin and 2,300 feed bins, and compost with some reduced sulfur and other 100 ppm H2S 200 cfm per mixer mixer ventilation related compounds and conveyor

Total Air Exhaust to Compost Odor Control System (3-Stage Scrubbers and Biofilter) 27,500

Residuals/lime mixer Strong, mostly ammonia related 500 ppm NH3 200 cfm per mixer and 1,000 and conveyor ventilation compounds 250 ppm H,S 200 cfm per conveyor

Biosolids/lime reactor Medium strength, mostly 80 ppm NH3 12 air changes 62,600 area ventilation ammonia 10 ppm H2S per hour

Total Air Exhaust to Residuals/Lime Odor Control System (2-Stage Scrubbers and Dilution Fans) 63,600

Biosolids receiving and Light strength, primarily t5 ppm NH3 12 air changes 90,000 mixing area ventilation ammonia with some reduced sulfur <1 ppm H2S per hour

and related compounds

Ventilation of static pile Light strength because of dilution <1 ppm NH3 12 air changes 500,000 composting area as a result of high ventilation rate. per hour

Some faint compost odors

Total Exhaust to Building Ventilation System (High Velocity Upblast Fans) 590,000

Screening area dust Primarily dusty, particulate-laden 0.30-micron 150 fpm face velocity 21,300 control ventilation air; light strength with some mature size particles in a 22' x 8'

compost odors and longer exhaust hood over trommel screen

Total Exhaust to Dust Control (Industrial Type Cartridge Filter) 21,300

BIOCYCLE AUGUST 1994 51

- The building will be maintained under negative pressure by exhausting all ventilation air with roof mounted, high velocity upblast fans.

air. Compost aeration in the positive pres- sure mode does not allow process air to be collected and treated separately, and so will not be performed at this facility. The RMF design includes 28 static piles, each aerated by a dedicated blower that cycles on and off in response to pile temperatures. The total process air emission rate can vary consider- ably depending on how many blowers are operating at a given time. The maximum aeration rate is anticipated to be about 25,000 cubic feet per minute (cfrn) and the minimum rate about 7,000 cfm, therefore flexibility in treating a variable air stream will be required. A modulating damper will be provided for makeup air to create more

consistent air flow into the odor control SYF- tem. The temperature of compost process air will average about 40°C with a maximum of 70°C. Under most conditions, the process air stream will be saturated with water vapor.

Lime Stabilization Process Odors: Air flow and off-gases associated with the lime stabilization process have somewhat differ- ent odor characteristics than those arising from the composting process. Lime stabiliza- tion creates odors primarily associated with ammonia, amines, and related compounds, especially when treating the anaerobically digested biosolids that will be received at the RMF. Odors from the biosolids and lime mix- ing and reaction area will be highest, be- cause the elevation in pH and heat created by the reaction of quicklime with anaerobic biosolids will liberate ammonia and its re- lated alkaline gases. Measures have been taken in the design to enclose and negative- ly pressurize the mixers, conveyors, and re- action areas associated with this process. All exhaust air from these areas will be routed to the lime stabilization odor control system. As a result, the volume of air to be treated is considerably greater than the compost pro- cess air, averaging 63,600 cfm. Temperature and moisture content of the air stream will be somewhat less than compost process air because of dilution by ventilation air.

Product Screening, Storage, and Oth- er Building Odors: A basic philosophy of odor control at the RMF is to capture, en- close, and treat odorous air streams as close to the odor source as possible, minimizing odors within the building itself. The com- posting process will have a detention time of 28 days, resulting in a stable product with minimal odors. The finished compost will be screened to separate compost product from woodchips which will be recycled. Screening generates dust and particulates which need to be removed to maintain a good working environment and reduce particulate emis- sions. A cartridge filter of the type that has been proven in similar industrial environ- ments wil l be used for this purpose. Screened compost product will be taken to a separate curing and storage area within the building, where static piles using a low lev- el of aeration will be constructed.

The lime stabilization process reaction area has about a three day detention time, during which the product mix will be turned several times to exhaust off-gases. All ventilation air from this area will be directed to the lime stabilization process odor control system. The lime stabilized product leaving this area will have minimal odors, and will be stockpiled in the same storage area within the building as the finished compost product.

Maintenance of a healthy working envi- ronment is a primary goal, so ample ventila- tion rates capable of at least 12 air changes per hour have been provided throughout the building. Negative pressure will be main- tained in the building by exhausting all ven- tilation air with roof-mounted, high velocity upblast fans. Intake louvers on the sides of the building will provide a constant supply

52 BIOCYCLE AUGUST 1994

of fresh air. The high ventilation rate will prevent buildup of odors inside the building and the upblast fans will aid in dilution and dispersion of the low level odors.

SELECTED ODOR CONTROL SYSTEMS Two technologies for odor control - wet

chemical scrubbing and biofiltration - have been selected for use at the RMF among the options available (Forbes, et al., 1992). Ther- mal oxidation, activated carbon scrubbing, and activated sludge treatment were all ruled out as too costly for the large volume of air requiring treatment at this facility. Because of the unique odor characteristics of the different biosolids stabilization process- es, separate odor control systems have been designed for the compost process and lime stabilization process air. Table 2 summa- rizes the sources, anticipated odor charac- teristics, and volumes of odorous air to be treated by the different odor control systems described below.

Compost Odor Control: The history of wet chemical scrubbing systems at biosolids composting facilities has evolved whereby three stage systems are regarded to be most successful in terms of odor control. The use of biofilters for compost odor control also has in- creased in the past few years, with the tech- nology of biofilter design advancing as well. At the CMUD RMF, it was decided to use a hybrid of both chemical scrubbing and biofil- tration technologies to control compost odors.

There is an ongoing debate over the mer- its of packed tower versus mist tower chem- ical scrubbers in the field of compost odor control. Packed towers recirculate dilute chemical solution, which comes in contact with the air stream by cascading over packed media. The recirculating solution re- quires a large volume of water. Mist towers, on the other hand, use high pressure nozzles to create very small droplets of chemical so- lution which come in contact with the air stream. Less water is required because the fine droplets create a large amount of sur- face area for contact with the odorants in the air. Because of the “once through” nature of the chemicals in mist towers, overdosing or underdosing of scrubbant solution can easi- ly occur if odorous compound concentration or air flow rate change. Successful mist tow- er scrubbing systems depend on frequent monitoring of target odorants to optimize chemical feed rates, and often require some overdosing to account for variations in the air stream. Packed towers do not require such close attention because chemicals tend to be retained in the recirculation water un- til consumed. Packed towers were chosen for the RMF primarily because of their capital cost. Other reasons are the availability of a large amount of WWTP effluent to use as re- circulation water, and the packed towers tend to be more forgiving in their operation as mentioned above.

A first stage, vertical packed tower uses sulfuric acid and operates at a low pH (2-4) to remove ammonia, amines, and related ni- trogen alkaline compounds. The second

BIOCYCLE

stage also consists of a vertical packed tow- er and uses sodium hydroxide to operate at a high pH (8-9) for removal of hydrogen sul- fide and other reduced sulfur compounds. The second stage also will have the flexibil- ity to use sodium hypochlorite or hydrogen peroxide to oxidize odorous compounds. Both the first and second stages will operate in the counter-current mode with air flow upward against the falling scrubbant solu- tion. Provisions have been made to add par- allel mist scrubbing towers between the first and second stage packed towers, should it be considered necessary in the future. The sec- ond stage will be followed by a third stage, horizontal packed tower operating in the cross-current mode. The function of this stage is to remove any residual oxidant and neutralize the pH of the air stream prior to entering the final biofiltration stage.

A biofilter is used as the fourth and final stage because of its ability to remove insolu- ble odorous compounds such as DMDS. Biofilters depend on the surface area of the media to provide contact sites for adsorption of odorous compounds and a habitat for mi- croorganisms to biologically oxidize the odorants. Four cells of biofilters are provid- ed, so that any one cell can be taken out of service and still maintain a conservative loading rate with a detention time of at least 10 seconds. The primary biofilter medium is perlite, a silica-based granular material with a large amount of surface area. Perlite has been recommended as the pr imary biofilter medium instead of finished com- post, because continuing degradation of the compost media can itself create odors (Card, 1993). The perlite is underlain by a gravel bed to provide good air distribution and overlain by a layer of sod or leaf compost to protect the perlite, aid in maintaining mois- ture, and increase detention time through the biofilter.

Lime Stabilization Process Odor Control: The separate odor control system designed for the lime stabilization process odor stream utilizes vertical packed towers and dilution fans. Biofiltration is not con- sidered to be necessary because the odors created by lime stabilization tend to be less complex than odors created by the biochem- ical activity of composting. The configura- tion of this system is very similar to the first two stages of the compost odor control sys- tem. The first stage will remove ammonia and related compounds in a low pH envi- ronment, and the second stage will remove hydrogen sulfide and related compounds in a high pH environment. There will be flexi- bility to use an oxidant in the second stage if necessary. Two parallel trains of first and second stage scrubbers are required because the air flow to be treated is about twice as large as the compost process air stream.

Dilution fans will be employed at the dis- charge stack of each second stage scrubber to aid in dispersion and dilution of the scrub- ber exhaust. The fans will mix ambient air with the scrubber discharge at a 5:l volume ratio. The upward velocity created by the

- Four cells of biofilters are provided so that any one cell can be taken out of service and still maintain a conservative loading rate and a detention time of at least 10 seconds.

AUGUST 1994 53

Good facility management creates a more pleasing environment to customers and the public.

fans will increase the effective stack height safely above the roof of the RMF to prevent any downwash and provide good dispersion.

Capital costs for the total odor control sys- tem are $2.1 million. That includes $600,000 for seven packed tower scrubbers, $450,000 for odorous air duct work, $290,000 for ex- haust and dilution fans, $250,000 for pumps and piping and $70,000 for the biofilter. Scrubbing systems are being supplied by In- dusco Environmental of Atlanta, Georgia.

CONCLUSIONS The design of odor control systems for

CMUD's Residuals Management Facility has been based on making conservative pre- dictions of the types and strengths of odors that will be produced, coupled with conduct- ing extensive surveys and reviews of odor control technologies currently used in the U.S. for similar types of facilities. In addi- tion to the technologies finally selected, it must be recognized that good process opera- tions and housekeeping are important ele- ments of odor control at a biosolids manage- ment facility. Good process operations will create stable finished products with mini- mal odors. Housekeeping and site manage- ment will lessen the chance of fugitive odor emissions. Perhaps more important, good fa- cility management will create a more pleas- ing environment to the customers and the public, who determines the ultimate success of a biosolids management program.

Robert Forbes is Project Manager with CH2M HILL in Charlotte, North Carolina. Trille Mendenhall is the Residuals Program Manag- er with the Charlotte-Mecklenburg Utility De- partment (CMUD). Lynne Young and Jeffrey Schwisow are Process Design Engineers in CH2M HILL'S Gainesuille, Florida office. The authors would like to thank CMUD and the City of Charlotte, North Carolina, for their high standards for design of the Residuals Management Facility.

REFERENCES Card, T.A. (1993). Review Comments on Odor Con-

trol Design of CMUD RMF (unpublished). CH2M Hill (1985). 201 Facilities Plan Amendment

for McAlpine Creek WWTP Improvements. CH2M Hill (1993). 201 Facilities Plan Amend-

ment-Regional Facility for the Stabilization and Beneficial Use of Biosolids.

Forbes, R.H., and Gullet, B.M. (1993). Sludge Man- agement Facility Combines Two Processes. Pro- ceedings of the Joint CSCE-ASCE National Con- ference on Enuironrnental Engineering.

Forbes, R.H., Mendenhall, T.C., Ostojic, N., and Duffee, R.A. (1992). Incorporating Odor Control Into the Design of an In-Vessel Biosolids Com- posting Facility. Proceedings-The Future Direc- tion of Municipal Biosolids Management. Water Environmental Federation Specialty Conference Series.

Odor Science and Engineering (1993). Results of Re- vised Modeling Assessment for CMUD RMF (un- published).

WALINGA CHAMPION - " ~ " ~ - ~ -~ ~~~ - ~~~ - "

The Bigger The Hand The More You Can Carry!

54 BIOCYCLE AUGUST 1994