High-solids aerobic decomposition: pilot-scale reactor development and experimentation

E~.SEVIER P I I :

Process Biochemistry, Vol. 32, No. 5, pp. 361-375, 1997 Copyright © 1997 Elsevier Science Ltd

Printed in Great Britain. All rights reserved 0032 -9592/97 $17.01) + 0.00

S 0 0 3 2 - 9 5 9 2 ( 9 6 ) 0 0 0 7 3 - 8

High-solids aerobic decomposition: pilot-scale reactor development and experimentation

J. S. V a n d e r G h e y n s t , a'c J. M. G o s s e t t b and L. P. Walker a*

~Department of Agricultural and Biological Engineering, Riley Robb Hall, Cornell University, Ithaca, NY 14853-5701, USA bSchoo l of Civil and Environmental Engineering, Hollister Hall, Cornell University, Ithaca, NY 14853-3501, USA

CDepartment of Biological and Agricultural Engineering, Bainer Hall, University of California, Davis, CA 95616-5295, USA

(Received 25 June 1996; accepted 27 July 1996)

Abstract

Pilot-scale reactors have been constructed to mimic the central core of an aerated static bed or in-vessel composting process. The 770 litre reactors were instrumented to measure temporal and spatial variations in temperature, oxygen and moisture content. Experiments were performed with a synthetic food waste (SFW) and digested biosolids using four different aeration rates and two initial moisture contents.

An analysis of the temporal and spatial temperature and oxygen profiles has shown the systems replicate well and represent a process with one-dimensional spatial variation. An analysis of oxygen gradients has shown that cumulative oxygen depletion and oxygen depletion rates within the bed increased with increasing aeration rate in the SFW experiments, however, they decreased with increasing aeration rate in the biosolids experiments. The SFW studies showed that a 10% variation in initial moisture content had little influence on cumulative 02 consumed, but had a significant influence on the location of maximum biological activity within the bed.

Maximum temperatures varied from 58 to 74°C in the SFW experiments and from 43 to 60°(; in the biosolids experiments. In all experiments the maximum temperatures and the positions where they occurred varied with initial moisture content and aeration rate. In the SFW experiments maximum axial temperature differences coincided with significant axial differences in moisture content, while in the biosolids experiments maximum axial temperature differences coincided with minimal axial differences in moisture content. © 1997 Elsevier Science Ltd

Keywords: food waste, biosolids, composting, oxygen uptake, temperature gradients, moisture gradients.

I n t roduc t i on

There has been an increased interest in using biological t-.rocesses to address environmental concerns such as solid waste disposal, hazardous waste remediation and I reatment of contaminated water. One biological pro- tess which has received considerable attention for the lemediation of organic solid waste is controlled aerobic t~ecomposition, more commonly called composting. The commercial success of composting is dependent on successful odour control and stability and quality of the lna l product. These goals are met by managing oxygen, 1 loisture and energy transport within the process. Poor q,xygen transfer inhibits the growth of aerobic micro- q,rganisms and enhances the growth of anaerobes which t~ave been reported to increase odour product ion/ '2 l,ow moisture content and high temperature inhibit vlicrobial activity, which may result in an unstable

To whom correspondence should be addressed.

361

product and reduce product quali ty) Addressing these issues involves developing a better understanding of the interactions between the process degradation kinetics and the mechanisms of heat and mass transport.

A common method used to gain a better understanding of these interactions is to perform in-vessel degradation studies on bench- and pilot-scale levels. Studies performed at the bench-scale level are the most prevalent in the literature, however, both types of systems have been used to evaluate process parameters such as airflow rate, 4- ~ 1 temperature 4.s, 12-15 and moisture. 4'~'13'16 Also, the effects of material properties such as bulking agent, 6" 17, J8 recycle ratio, 19"z° carbon to nitrogen ratio 2~'22 and particle size z3 have been evalu- ated using in-vessel systems.

The most common reactor configuration used in bench-scale systems consists of a vertical packed bed with forced aera t ion . 4-6"8'9'14"2L24-29 These types of

systems mimic an aerated static pile or in-vessel static

362 J.S. VanderGheynst et al.

bed, where mass and energy exchanges between the organic matrix and the gas stream often are revealed through gradients in temperature, oxygen and water content. Therefore, to understand the coupling between degradation kinetics and heat and mass transport within an aerated static bed process, reactors must be designed to allow analysis of gradients across the reactor bed. Several bench-scale studies have moni- tored temperature gradients, 6"s'9'14"26 but there has been limited study where gas or material sampling has been performed at more than one location in the reactor. s

The in-vessel, pilot-scale biodegradation studies 5'7'j1"15'~8"3° have also been limited in the measurement and analysis of temporal and spatial variation in temperature, gas and substrate composition. The level of temperature monitoring varied from measuring temperature at one location 15 to measuring temperature at 16 locations. 5 In these studies gas analysis typically was performed on the reactor exhaustS.7.11,18 and moisture analysis was performed on a composite sample. 5'7' ~1,15, ~8 Thus, little information is available on gradients in gas composition or water content from previous pilot-scale or bench-scale vertical packed bed studies.

Although the logistics of materials handling and reactor operations are generally more favourable for bench-scale systems, there are some limitations to using bench-scale data to analyse the heat and mass transport processes within full-scale systems. One limit- ation is the larger surface area to volume ratio (A/V) associated with bench-scale reactors. As A/V increases, the potential for heat loss from the reactor wall increases. Some of the bench-scale studies took advan- tage of the larger A/V by submersing the vessels in a temperature regulated environment and controlling process temperature and heat loss from the vessel walls by controlling ambient temperature, 4'25-29 while other studies used insulation to minimise heat loss. 6'9"~4

It is apparent from the research that has been con- ducted in high-solids degradation studies that temporal and spatial variations in state variables, such as moisture content, 02 or CO2 concentration and temperature, are driven by basic mass and energy transport mechanisms and changes in chemical composition and microbial ecology of the organic matrix. Thus, pilot-scale reactors must be developed that will allow researchers to 'capture' these temporal and

spatial variations in order to gain insight into the basic physical, biochemical and microbial mechanisms that are at the core of the solid-state degradation process. This report presents the effort of the authors to develop pilot-scale static bed reactors that mimic the central core of an in-vessel or aerated static pile composting system. It is assumed that the central core has minimal radial spatial variation in the state variables, therefore, all spatial variation in the state variables is one-dimensional along the axis of the reactor bed. In addition, general process performance results, such as cumulative oxygen consumption and maximum reactor temperatures, are reported for two different substrates, four aeration rates and two moisture contents. An assessment of the temporal and spatial variations in temperature, moisture content and Oz concentration is also presented.

Materials and Methods

Substrate preparation

Two substrates were used in the pilot-scale studies. One was synthetic food waste (SFW) which consisted of dry dog food (Big Red Puppy Food, Pro-Pet Inc., Syracuse, NY) and the other was anaerobically digested, polymer-dewatered biosolids obtained from the Ithaca Wastewater Treatment Plant (Ithaca, NY). Dog food was chosen because it has a fibre, fat and protein content similar to food waste or garbage and because it's physical composition is fairly uniform from batch to batch. Hard maple-wood chips (Coastal Lum- ber, Cayuta, NY) were added to both substrates as a bulking agent and as a carbon source. To allow for long-term storage, the chips were dried below a moisture content of 10% wet basis (g water x 100/g total weight) immediately after they were received. Prior to experimentation, the chips were size-reduced through a 12.7 mm screen using a hammer mill (Buffalo Ham- mermill Co., Buffalo, NY, model WA-8). Physical properties for the dog food, biosolids and maple-wood chips are given in Table 1.

The carbon to nitrogen dry mass ratios (C:N) and moisture contents of the experimental SFW and biosolids mixtures are given in Table 2. The C:N of the SFW mix was selected from previous unpublished bench-scale studies using this substrate, while the C:N of the biosolids mix was constrained by the desired

Table 1. Physical properties of raw materials

Substrate Carbon Nitrogen Moisture Volatile solids (% of dry solids) (% of dry solids) (% wet basis) (% of dry solids)

SFW 44.6 5.3 7.0 89.5 Biosolids 29.7 4.1 81.0-85.6 55.8 Maple chips 45.6 0.2 5.0-9.0 99.5

High-solids aerobic decomposition 363

h,wer end moisture content of the mix (50%). The raw materials were weighed and then mixed in a 1 m 3 Davis mixer (H.C. Davis Sons, Bonner Springs, KA) for 45-70 min. The mixtures were loaded into the pilot- s~:ale vessels using a conveyor belt to a height a!~proximately 2-5 cm from the top of the vessel. The s,tbstrate was unloaded manually and mixed to obtain a c )mposite sample from the experiment.

1, eactor design and construction

1 our 770 litre pilot-scale reactors were constructed to cbserve the behaviour of an aerated static bed bio- ciegradation process. The reactor vessels were constructed using 265 cm sections of smooth-lined, cor- lagated, high-density polyethylene (HDPE) pipe with ~n inner diameter of 60 cm as shown in Fig. 1. H D P E i', non-toxic, non-corrosive and can withstand tempera-

r! able 2. Carbon to nitrogen ratio and initial moisture of ~perimental mixtures

I~dixture C:N Initial (kg:kg) moisture

(% wet basis)

,~,FW and maple-wood chips tfiosolids and maple-wood chips

18:1 45, 55 43:1 50, 60

tures up to 120°C. Two 20 cm diameter ports were located 55 and 30 cm from the bot tom of the vessel to permit material unloading and vessel cleaning as illustrated in Fig. 1. The ports were constructed from 20 cm SCH 40 PVC pipe, and a flange with an outer diameter of 33 cm was permanently mounted to the outside of each 20 cm port. During experiments, the port open- ings were sealed with a 20 cm T-cone pressure fitting. The material was supported by a perforated plate, 60 cm in diameter and 12"7 mm thick, which was fabricated from 12.7 mm PVC. The plate was secured to a PVC ring which was mounted 50 cm from the bottom of each reactor. Air was introduced to each vessel below the perforated plate through a 32 mm SCH 80 PVC pipe mounted 20cm from the bottom of the vessel (see Fig. 1). Air exited each reactor through a 32 mm SCH 80 PVC pipe mounted 5 cm from the top of the vessel. A 120 x 120 × 275 cm wood frame consist- ing of 2 × 4 foot planks and retaining panels was built to support each vessel as illustrated in Fig. 1.

To minimize radial temperature gradients, the reactor walls were insulated with 12.7 mm thick sheets of foam insulation with an R value of 10"5 K/W (Armaflex AP, Armstrong, Lancaster, PA). The bot tom of each vessel was insulated using 1 m 2 pieces of 5 cm poly- isocyanurate foam board with reflective aluminum foil facers. The top of each vessel was insulated using 61 cm diameter pieces of 12.7 mm Armaflex AP insulation.

Stainless-steel tubes with an outer diameter of 12.7 mm and 78 cm in length were located at 20, 40, 60,

2

_j

-~(--

4 7._

12

13

(:ig. 1. Schematic diagram of pilot-scale system where 1 =pressure regulator; 2=flow meter; 3=humidifier; 4=insulated inlet air line; 5=HDPE pilot-scale reactor; 6=unloading and cleaning port; 7=perforated plate; 8=stainless steel tubes containing lhermocouples and gas sampling lines; 9=oxygen sensors; 10=sampling port; 1 l=angle iron flange; 12=exhaust line; 13=con- lenser; 14=refrigeration unit; 15=condensed vapour collection.


90, 120, 135, 150 and 180 cm above the perforated plate. The tubes were used to secure and protect ther- mocouples and 3.2 mm OD Teflon gas sampling lines. The tubes were fabricated with two holes which were positioned to measure temperature and permit gas sampling at 30 and 2 cm from the wall of the vessel. Each reactor was constructed with 38 x 32 mm NPT PVC bushings to permit oxygen sensing at 30, 75 and 180 cm above the perforated plate and material sampling at 10, 30, 50, 75, 105 and 135cm above the perforated plate (see Fig. 1). Oxygen probes were fabricated with a 32 mm PVC threaded housing which fitted directly in to the bushings. 3~

Airflow regulation and treatment

Compressed air was used to aerate the reactors. The initial line pressure of the compressed air, 400-500 kPa, was reduced using a regulator (7Z553, Grainger, Syracuse, NY). The flow rate of the pressure-reduced air was controlled using precision valve flow meters (Gilmont Instruments, Barrington, IL). The metered air was humidified by bubbling it through a water column with a height of 35-80 cm and volume of 20-115 litres. Each humidifier was constructed with a submersible aquarium heater to maintain the water temperature at approximately 25°C. To produce bub- bles, the air was forced through a manifold of 8 to 16 3.1 mm PVC tees. The humidified air was supplied to the pilot-scale reactors via insulated Nalgene tubing.

Effluent gas treatment

The effluent vapour from the pilot-scale reactors was condensed to obtain a mass balance on the water removed from the process. The effluent from each reactor was fed into a Plexiglas condensing unit containing approximately 10-15 litres of water (Fig. 1) and was bubbled through approximately 15-19 cm of liquid. To keep the liquid cool, each condenser contained 6.3 mm OD copper cooling coils through which ethyl- ene glycol was pumped and continuously cycled through a refrigeration unit. The exhaust line from each condenser contained a thermocouple to monitor the temperature leaving the condenser. Condensed water vapor was collected in 201itre bottles and weighed daily.

The refrigeration unit did not have the cooling capa- city to support all four pilot-scale systems, therefore the cooling unit was only used to support the three lower-aeration systems, and the effluent vapour leaving the system with the highest aeration rate was not com- pletely condensed. However, a mass balance on the water removed from the high-aeration system was obtained by monitoring the relative humidity and temperature of the reactor's exhaust.

Oxygen and humidity measurement

Oxygen concentration and humidity were measured using gas sensing probes developed by NeuwGhent Technology (LaGrangeville, NY). The probe electron- ics, calibration and documentation of successful in-situ 02 monitoring are discussed in detail elsewhere? ~ In some experiments 1 ml gas samples were obtained manually from gas sampling lines and analysed for 02 using gas chromatography.

Temperature measurement

Temperature was measured using copper, constantan thermocouple wire (PP-T-24, Omega Engineering, Stamford, CT). The tip of each thermocouple was spot- welded and coated with epoxy resin to prevent corrosion during the degradation process, and subse- quently threaded into the stainless steel tube to the 30 or 2 cm sampling hole.

Data acquisition system

Temperature, oxygen and humidity measurements were recorded with a computer data acquisition system. The system consisted of analogue to digital (A/D) conver- ters discussed elsewhere 32 and an IBM XT computer. It was designed to allow temperature measurement at 16 locations and oxygen and humidity measurement at four locations in each reactor. The data acquisition program was written to store hourly averages of temperature, oxygen and humidity data from all four reactors to one storage file.

Pilot-scale experiments

Pilot-scale experiments were performed using the SFW and biosolids mixtures at four different aeration rates and two initial moisture contents. Table 3 provides an overview of the various operating conditions imple- mented in the decomposition experiments. The range of moisture contents, aeration rates and the two substrates were chosen to mimic process conditions typically used in industry and to capture the effect of each of these process variables on process performance.

Samples ranging from 5 to 30g wet weight were removed from the reactor one or two times a week and analysed for total and volatile solids. A core sampler 2.5 cm in diameter and 76 cm in length was used to remove samples from the reactor. Once samples were removed, a narrow bar was used to push material into the void created by the core sampler. Total solids were measured by oven drying at 103°C for 24 h. Volatile solids were measured by placing dried samples in a muffle furnace at 560°C for at least 8 h. Prior to weigh- ing dried and ashed samples, the samples were cooled in a desiccator to prevent water sorption.


Each of the experiments using the SFW and wood- chip mixture were performed in duplicate to assess our ability to replicate process behaviour for a given initial moisture content and aeration rate. Duplicates were ro t performed for the biosolids mix because of the cifficulty in obtaining material with a uniform composi- l,on. All experiments were run for 20 days or until 02 ~ epletion was undetectable.

• 'able 3. Pilot-scale experimental process variables

Mixture Initial Aeration moisture rate (% wet (litre/min/kg basis) initial

dry solids)

~;FW and 45 0.06 wood chips 45 0.15

45 0.31 45 0-63

~;FW and 55 0.06 wood chips 55 0-15

55 0-31 55 0.60

3iosolids and 50 0"22 wood chips 50 0.47

50 0.94

~3iosolids and 60 0.08 wood chips 60 0"21

60 0-43 60 0.83

R e s u l t s a n d D i s c u s s i o n

Reproducibility of reactor performance

An assessment of the reproducibility of the pilot-scale system can be attained from an analysis of temperature and oxygen profiles from two experiments with identical initial conditions and inlet air conditions. Figures 2 and 3 show profiles of temperature and oxygen vs time for two synthetic MSW experiments. Both experiments were similar, having an initial moisture content of 45% and an aeration rate of 0-06 litres/min/kg (litres per rain per kg of initial dry weight). Trial 2 was performed 1 month after trial 1. These particular trials were selected because they represented the largest gradients observed in both temperature and oxygen among all the experiments. The temperature profiles shown in Fig. 2 correspond to those along the centre axis of the vessel at 20 and 150 cm above the perforated plate. At 150 cm both trials showed an initial 54-56°C temperature peak between 35 and 40h and a stabilising temperature profile after 140 h. Between 140 and 400 h the average temperature difference between the trials' 150 cm temperature profiles was less than 2°C, while between 0 and 400 h the difference was less than 2-5°C. These differences were not much greater than the 2 ° C accuracy of the data acquisition system. At 20 cm both trials showed an initial 55-56°C temperature peak at 35 h. Although the profiles at 20 cm never reached a stable temperature because of drying, the average difference between replicate profiles for the duration of the experiment was still less than 3°C.

7O

50 h.~. i

.o 40 -

8. ~: 30 - - -

~ ° Trial 1" 150 cm, centre

20 - ----~--- Trial 2 :150 cm, centre

- - ~- - Trial 1: 20 cm, centre 1 0 -

- - [ ] - - Trial 2 :20 cm, centre

I I I [ I I I

0 50 100 150 200 250 300 350 400

Time (h)

Fig. 2. Temperature profiles obtained 150 and 20 cm above the pilot-scale vessel perforated plate. Profiles are from two SFW experiments with initial moisture content of 45% and aeration rate of 0-06 litre/min/kg.


25

2 0 -

e -

e . .

1 5 - e -

1 0 - ©

5

0

" s " ~ "-~" ~-- :~:"~-- ~ ' - ~ : - - ~ ~

9 "* "~j " ~ ~ . . . . ¢ , . . . J r

. . . . A---- Trial 2:75 cm

1 I I I

Tna 30cm II - - [] - - Trial 2 :30 em

' ' ' . . . . f . . . . I . . . . 1

50 100 150 200 250 300 350 400

Time (h) Fig. 3. 02 concentration profiles obtained 30 and 75 cm above the pilot-scale vessel perforated plate. Profiles are from two SFW experiments with initial moisture content of 45% and aeration rate of 0'06 litre/min/kg.

Further replication was observed in the oxygen profiles shown in Fig. 3. These profiles correspond to oxygen concentrations measured at 30 and 75 cm above the perforated plate. Both trials showed a sharp drop in oxygen concentration between 20 and 25 h at 30 and 75 cm. At 75 cm oxygen concentration initially fell below 10% and then increased to 15-15.5% between 30 and 40 h. Both 75 cm profiles showed an upward trend in O2 concentration after 125 h. Between 125 and 400 h, the average difference between the 75 cm profiles was 0"7% 02. At 30 cm both trial profiles showed a general trend of increasing oxygen concentration after 60 h and then both levelled off to constant 02 between 170 and 190h. Between 60 and 190h the average difference between the 30 cm profiles was 1.5% 02. An analysis of the difference between trial profiles shows the pilot-scale system was able to duplicate behaviour well.

Verification of minimal radial gradients

Another requirement of the pilot-scale systems was minimal heat loss from the reactor walls. One method to determine if the vessels meet this requirement is an analysis of radial temperature gradients within the bed. Figure 4 shows radial temperature profiles and an ambient temperature profile for an SFW high-aeration experiment (0.6 litres/min/kg) with an initial moisture content of 55%. The magnitudes of the radial gradients observed in Fig. 4 are typical of the gradients observed in most of the pilot-scale experiments. At 20 cm above the perforated plate the difference in temperature between the wall and the centre of the vessel varied

between 0 and 5°C, with an average difference less than 2°C. Maximum temperature differences (gradients >4.5°C) were observed between 40 and 115 h, which also corresponded to the onset of drying in the lower portion of the bed as shown in Fig. 5. At 90 cm above the perforated plate the difference in temperature between the wall and the centre of the vessel varied between 0 and 8°C with an average difference less than 3.5°C. At 90cm maximum temperature differences occurred between 160 and 230 h, which corresponded to the onset of drying in the upper portion of the bed (Fig. 5). If heat loss from the reactor walls were the cause of radial temperature differences, we would have expected to see the maximum heat loss and the maximum radial temperature difference when the difference between the wall temperature and ambient temperature was maximal. For this experiment the maximum difference in wall temperature and ambient temperature was approximately 33°C at 90 cm and the resulting radial gradient was less than 3°C.

For both of the positions the magnitude of the radial temperature differences did not appear to be related to the ambient temperature, but did appear to be linked to drying. A further case for the link to drying can be obtained from an analysis of the radial gradients observed in the low-aeration experiment which had very little drying (Fig. 5). For the low aeration experiment the average radial temperature differences observed at 20 and 90 cm were less than 1-5 and 2.6°C, respectively.

Radial gradients in oxygen were also measured manually for an SFW experiment. Figure 6 shows profiles of oxygen vs time 90 cm above the perforated plate at


6 0 -

50

4 0

2

'~ 30

20

10

0

i

0 50 100 150

Time (h)

: 90 cm, centre - - ~ - - 90 cm, wall - - - -~ - - - 20 cm, centre - - [ ] - - 20 cm, wall

× ambient

200 250

iqg. 4. Ambient and reactor temperature profiles obtained 20 and 90 cm above the perforated plate and 2 and 30 cm from the eactor wall. Profiles are from an SFW experiment with initial moisture content of 55% and aeration rate of 0.6 litre/min/kg.

and 30 cm from the wall. The max imum dif ference !~etween radia l m e a s u r e m e n t s was 1.6 pe rcen tage i~oints and the average d i f ference was 0-64% O2.

Femporal and axial variation in the state variables

In add i t ion to thei r abil i ty to r ep roduce t e m p e r a t u r e and oxygen profi les, ano the r r equ i r emen t of the pi lot-

scale vessels was to capture temporal and axial gradients in temperature, moisture and oxygen concentration. Temporal and axial gradients in temperature are shown in Figs 7 and 8. The rapid transition from mesophilic to thermophilic temperatures observed in these figures is characteristic of all the degradation studies. Generally, this transition occurred within the first 10-30 h of the process. For the low aeration rate

-q.

e..

o

L.

O

60

50

4 0 -

3 0 -

20

10

_i-.- _ - . .

• \

- \

• \

[ ] , •

• 1 3 5 c m - low aerat ion

- - Jr- - 135 c m - high aerat ion - - - -~- - - 30 c m - low aerat ion

- - [ ] - - 30 cm - high aerat ion

0 . . . . I . . . . I ' ' ' t . . . . I . . . .

0 50 100 150 200 250

T ime (h)

Fig. 5. Moisture content profiles obtained 30 and 135 cm above the perforated plate. Profiles are from an SFW experiment with initial moisture content of 55% and aeration rate of 0"06 litre/min/kg and 0"6 litre/min/kg.


22

2 0 -

= 1 8 - o

'~ 1 6 - e - .

= 1 4 - o

~0 12

© 1 0 -

8

0

t .... • .... 90 cm wall

' " t " " l ' " t . . . . ,' " , . . . . ,' . . . . ,' '

50 100 150 200 250 300 350

Time (h)

• 90 cm centre

1

400

Fig. 6. Oxygen profiles obtained 90 cm above the perforated plate and 2 and 30 cm from the reactor wall. Profiles are from an SFW experiment with an initial moisture content of 55% and aeration rate of 0.06 litre/min/kg.

(Fig. 7) temperature increased and peaked first at 20cm followed by 60cm and then 150cm, which peaked 10 h later, while for the high aeration rate (Fig. 8) temperature increased and reached an initial peak first at 60cm followed closely by 150cm and then 20 cm which peaked 15 h later. For the high-aeration experiment, spatial variation in temperature between 20 and 150cm ranged from 0°C at 35 h to 25°C at 100 h. In contrast, for the low-aeration experiment the spatial variation in temperature ranged from 0°C at 35 h to only 4°C at 100 h.

Although the 25°C vertical temperature difference observed in Fig. 8 was significant, it was not the largest observed in the pilot-scale studies. Table 4 lists the maximum temperature differences observed between

20 and 150 cm, as well as the moisture differences measured at approximately the same time between 30 and 135 cm. For the SFW experiments the maximum differences were not significantly influenced by aeration rate, however, they were less, on average, for the experiments with a lower initial moisture content. The time when the maximum occurred increased with decreasing aeration rate and increasing initial moisture content. For all of the SFW experiments the occurrence of the maximum temperature difference coincided with a significant moisture difference.

The temperature differences observed for the biosolids mix are given in Table 5. On average the gradients in the biosolids experiments were smaller than those observed in the SFW experiments and typi-

60 ".g.L.~ "'-~:J

5 0 -

,il 30-

_ . _ . ~ . . ~ . ~ [ . . . . A---- 60 cm, centre I - - o-- - 20 cm, centre I

] 0 ' ' ' l ' ' ' i ' ' ' l ' ' ' i ' ' ' 0 20 40 60 80 100

Time (h) Fig. 7. Temperature profiles obtained 20, 60 and 150 cm above the perforated plate. Profiles are from an SFW experiment with initial moisture content of 55% and aeration rate of 0"06 litre/min/kg.


60

50- / %'% C %

40-

30- ~

20 - ----A---- 60 cm, centre - - o - - - 2 0 c m , c e n t r e

0 20 40 60 80 100 Time (h)

Fig. 8. Temperature profiles obtained 20, 60 and 150 cm above the perforated plate. Profiles are from an SFW experiment with initial moisture content of 55% and aeration rate of 0"6 litre/min/kg.

Fable 4. Maximum temperature gradient values and time of occurrence between 20 and 150 cm above the perforated plate and corresponding moisture gradients between 30 and 135 cm in the SFW experiments

Initial moisture Aeration ATm,x Time A Moisture (% wet basis) rate (150-20 cm) (h) (135-30 cm)

(litre/min/kg) (°C) (% wet basis)

45 0.06 32 463 30 45 0-15 30 344 26 45 0-31 26 143 33 45 0.63 29 70 21 55 0.06 37 476 38 55 0.15 36 341 35 55 0.30 38 242 24 55 0.60 29 219 26

Table 5. Maximum temperature gradient values and time of occurrence between 20 and 150 cm above the perforated plate and corresponding moisture gradients between 30 and 135 cm in the biosolids experiments

Initial moisture Aeration ATm~x Time A Moisture (% wet basis) rate (150-20 cm) (h) (135-30 cm)

(litre/min/kg) (°C) (% wet basis)

50 0'22 31 48 - 1 50 0.47 31 90 - 3 50 0.94 23 10 0 60 0-08 23 369 - 6 60 0"21 23 107 - 1 60 0"43 19 170 - 3 60 0-83 18 15 0

370 J.S. VanderGheynst et ai.

cally occurred earlier in the process. The one exception was the 0.08 litres/min/kg dry weight experiment with 60% initial moisture. Unlike the SFW results, the maximum temperature differences did not coincide with significant moisture differences and tended to decrease with increasing aeration rate.

Temporal and axial variations in molar 02 concentration are illustrated in Figs 9 and 10. For the low aeration rate (Fig. 9) oxygen concentration at 75 cm and at the exhaust dropped below 10 and 5% 02

between 30 and 35 h, while for the high aeration rate (Fig. 10), 02 dropped to only 17.5%. Other packed-bed studies have shown that aerated systems with residual 02 concentrations of 10 and 2% have as much as 25 and 65% less biological activity than those with residual O2 concentrations of 18%. 4 Therefore, the low 02 concentration observed in the low aeration experiment could have resulted in reduced microbial activity above 75 cm, and could be an explanation for the temperature lag observed at 150cm (Fig. 7). The higher 02

25

:

= - .: .,,.."A.. \ "

I~ i I# e l )

O 5

. . . . . A---- 75 cm exhaust

0 ' t ' ' I ' I ' ' 0 20 40 60 80 100

Time (h) Fig. 9. Oz concentration profiles obtained 30 and 75 cm above the perforated plate and at the exhaust. Profiles are from an SFW experiment with initial moisture content of 55% and aeration rate of 0"06 litre/min/kg.

2 5 -

O ° ~

¢.9

o ¢9

~9 0 / ) ;:m

©

2 0 -

1 5 -

1 0 -

5

0

- - o-- - 3 0 c m . . . . A---- 75 cm

-- exhaust

0 20 40 60 80 100 Time (h)

Fig. 10. 02 concentration profiles obtained 30 and 75 cm above the perforated plate and at the exhaust. Profiles are from an SFW experiment with initial moisture content of 55% and aeration rate of 0-6 litre/min/kg.


~oncentration observed in the high-aeration experiment ~_:ould explain the minimal difference between the rate ,ff temperature increase at 60 and 150 cm (Fig. 8). !Turther discussion of the observed effect of aeration m 02 depletion will be presented later.

Moisture content profiles for the two extreme aera- ion experiments are presented in Fig. 5. At 50 h the )rofiles representing both experiments showed minimal ~patial variation in moisture, while for the high-aera- ion experiment at 140h the variation in moisture ~etween 30 and 135cm was 24 percentage points. ¢,tudies have observed maximum oxygen uptake at moisture contents ranging between 50 and 6 0 % 4. lo and !aave shown uptake to be reduced by 50% below a moisture content of 40%. j6 Thus, the profiles in Fig. 5 suggest that moisture content was not limiting to microbial activity in the low aeration experiment, but could have been limiting to microbial activity in the high aeration experiment. The drop in the temperature profile at 20 cm after 50 h (Fig. 8) could be explained by the rapid drying observed in the lower section of the bed.

Influence of aeration rate and initial mo&ture content on ~tate variables and rates and extent of degradation

Table 6 lists the maximum temperatures (Tm~,x) and minimum molar 02 concentrations (O2,m~.) observed in the SFW experiments. For both initial moisture contents Tma~ was found to decrease and O2.m~, increase with increasing aeration rate. The position at which T,,~× occurred also increased with increasing aeration rate. The low O2 concentrations in the effluent of the low-aeration experiments indicated that 02 could have been limiting to microbial activity and heat production in the upper section of the bed, which would have resulted in the middle and lower sections of the bed being hotter than the upper section. For the 45% moisture experiments, the time for Tmax to occur decreased more rapidly with increasing aeration rate than for the 55% experiments. O2.m,, and the time to reach this level were not influenced by moisture and in all cases

02.mi n occurred within the first 24-32 h of the process. The occurrence of Oz.min also coincided with the occurrence of the maximum rate of O~ depletion for the entire system, but in no case did it coincide with Tmax.

The temperature and oxygen results for the biosolids experiments are listed in Table 7. Overall, the observed T, .... values were lower and the 02,min values were higher for the biosolids than for the SFW experiments, and O2~,,,~,, occurred 10-15 h earlier in the process. This is not surprising given that this is a pre-digested sludge which is expected to have a lower content of readily available substrate. Similar to the SFW, in the biosolids experiments Tin.× decreased and 02,mi n

increased with increasing aeration rate and the position of Tmax increased with increasing aeration.

Tables 8 and 9 list cumulative 02 consumed in the SFW and biosolids experiments. In the SFW experiments cumulative consumption increased with increasing aeration rate and moisture content did not have a significant influence on cumulative 02 consumed. However, the fraction of consumption between the lower and upper sections of the bed was influenced by the initial moisture content and in some cases aeration rate. For a moisture content of 45%, a larger fraction of the 02 was consumed in the lower section of the bed at 50 h, while at 100 h in the high-aeration experiments the larger fraction of consumption shifted to the upper section of the bed. This shift in activity could be explained by a reduction in the moisture content which led to a reduction in the rate of substrate decomposition,

The cumulative 02 consumed in the biosolids experiments (Table 9) was less than that consumed in the SFW experiments. Unlike the SFW experiments, the cumulative consumption decreased with increasing aeration rate and the fraction of 02 consumed in the lower section of the vessel increased with increasing aeration rate. This information, combined with the lower T,,,x and higher 02 ,mi n values observed in Table 7, suggests that there was less readily available substrate for microbial activity in the biosolids than in the SFW.

Table 6. Maximum temperature and minimum oxygen concentration observed in the SFW experiments

Initial Aeration T,,.x Position of moisture rate (°C) T, .....

(% wet basis) (litre/min/kg) (cm)

Time of 02,mi n in effluent Time of T . . . . (%) 02,mi n (h) (h)

45 0.06 64 150 397 3 24 45 0-15 63 180 208 7 27 45 0.31 61 180 87 16 27 45 0"63 58 180 70 18 26 55 0-06 74 90 288 2 32 55 0.15 69 120 290 5 31 55 0-30 65 180 224 14 29 55 0'60 59 180 208 18 28


Table 7. Maximum temperature and minimum oxygen concentration observed in the biosolids experiments

Initial Aeration rate Tm~x Position of Time of O2,mi n in Time of moisture (litre/min/kg) (°C) Tma x T.,~x effluent 0 2 , m i n

(% wet (cm) (h) (%) (h) basis)

50 0.22 58 180 38 16 8 50 0.47 54 180 96 19 8 50 0.94 45 180 11 20 7 60 0.08 60 120 139 11 13 60 0.21 56 150 132 17 14 60 0.43 50 180 172 19 9 60 0.83 43 180 16 ND ND

ND, not determined.

Table 8. Cumulative 02 consumed in the SFW experiments at 50 and 100 h and corresponding fraction of 02 consumed between the inlet and 75 cm and between 75 and 180 cm

Initial Aeration moisture (litre/min/kg)

(%)

50 h 100 h

C u m u l a t i v e f,-75 cm f75-18{I cm C u m u l a t i v e

02 consumed 02 consumed (g/kg VS) (g/kg VS)

f 0 -75 cm f75-- 180 ¢m

45 0.06 20 0-59 0.42 48 0"56 0.44 45 0.15 26 0-48 0-52 54 0-53 0.47 45 0-31 21 0.52 0.48 58 0.41 0"59 45 0-63 27 0-66 0"34 77 0.35 0.65 55 0.06 17 0.70 0"30 33 0.64 0.36 55 0.15 25 0.65 0"34 56 0.72 0.28 55 0-30 25 0-63 0'37 60 0.53 0.47 55 0.60 32 0-76 0.24 73 0.66 0.34

Table 9. Cumulative 02 consumed in the biosolids experiments at 50 and 100 h and corresponding fraction of 02 consumed between the inlet and 75 cm and between 75 and 180 cm

Initial Aeration moisture (litre/min/kg)

(%)

50 h 100 h

Cumulative fo-75 cm f75-180 cm Cumulative 02 consumed 02 consumed

(g/kg VS) (g/kg VS)

f 0 -75 cm f75 - 181} cm

50 0"22 23 ND ND 49 0"26 0"74 50 0"47 ND ND ND 30 0.41 0"59 50 0"94 11 0'87 0"14 21 0.81 0-19 60 0"08 21 0-43 0"57 46 0.29 0"71 60 0"21 22 0-49 0"51 40 0.49 0"51 60 0-43 12 ND ND 26 ND ND 60 0'83 14 0-60 0"40 32 0.56 0"44

ND, not determined.


Maximum rates of oxygen depletion are given in 'Fables 10 and 11. Oxygen depletion rate was computed using the following equation:

m * ( fo . , i - , - fo , , i ) Ro2,i- (1)

pv,(Av)

where Ro2.i=rate of oxygen consumption between position i - 1 and position i, g OJh/kg VS; m = m a s s flow ~ate of dry air, g dry air/h; fo_..~- 1 =mass fraction of 02 ~n air at position i - l , g Oz/g dry air; fo..i=mass fraction of O2 in air at position i, g O2/g dry air; pvs=bulk density of volatile solids, kg VS/m3; Av=volume between position i and i - I, m 3.

For the SFW experiments (Table 10), the maximum ~ate of oxygen depletion, RO2 . . . . . generally increased with increasing aeration rate and the time at which the naximum occurred fell between 25 and 40 h, with the exception of the experiments with an initial moisture of 55% and the highest aeration. In all cases the greatest ~O2 . . . . values were observed in the section of the bed

between the inlet and 30 cm and values tended to decrease along the axis. For all sections of the bed, RO2 . . . . increased with increasing moisture, however, this increase was not reflected in an increase in the cumulative 02 consumed as shown in Table 8.

For the biosolids experiments RO2 . . . . between the inlet and 30 cm occurred before 20 h (Table 11). The other sections of the bed showed RO2.max values occur- ring at various times between 8 and 125 h. In most cases RO2 . . . . decreased with increasing aeration rate with the exception of the 30-75 cm section of the 60% initial moisture, high-aeration experiment. Similar to the SFW experiments, the greatest RO2 . . . . values were observed in the section of the bed between the inlet and 30 cm.

Summary and Conclusions

Pilot-scale systems have been presented which mimic the central core of an aerated static pile or in-vessel

Fable 10. Maximum rate of oxygen depletion between the inlet and 30 cm, 30 and 75 cm, and 75 and 180 cm for the SFW ~xperiments

Initial Aeration rate moisture (litre/min/kg)

(%)

0-30 cm 30-75 cm 75-180 cm

RO2 . . . . Time of RO2 . . . . Time of RO2 ..... Time of (g/h-kg VS) R02 . . . . (h) (g/h-kg VS) no2 . . . . (h) (g/h-kg VS) RO 2 . . . . (h)

45 0.06 1.8 26 1.2 27 0.7 24 45 0.15 1.9 33 1.7 27 1.7 27 45 0.31 2-0 34 1.4 27 1-3 26 45 0.63 3.3 36 1.8 29 2.2 106 55 0.06 1.6 32 1-5 33 1.1 41 55 0.15 2.2 36 2-0 36 1.7 36 55 0.30 2.7 39 1.7 30 1.7 29 55 0.60 3.6 55 3-1 37 1.8 176

Table 11. Maximum rate of oxygen depletion between the inlet and 30 cm, 30 and 75 cm, and 75 and 180 cm for the biosolids experiments

Initial Aeration rate moisture (litre/min/kg)

(%)

0-30 cm 30-75 cm 75-180 cm

RO2 . . . . Time of RO2 . . . . Time of RO2 . . . . Time of (g/h- kg VS) RO2 . . . . (h) (g/h-kg VS) RO2 . . . . (h) (g/h-kg VS) R02 . . . . (h)

50 0"22 1'2 7 50 0.47 1 "2 6 50 0-94 ND ND 60 0"08 1"3 16 60 0.21 1"2 13 60 0-43 1'4 9 60 0'83 0"6 8

0"6 8 1 "0 96 0-5 111 0-9 8 ND ND 0-9 117 0"7 27 0.9 124 1'0 119 0'6 22 ND ND ND ND 1-2 9 ND ND

ND, not determined.


high-solids degradation process. An assessment of the ability of the system to duplicate reactor pe=formance was done by analysis of temperature and oxygen profiles from two experiments with identical initial and inlet air conditions. The average difference observed between temperature profiles was less than 3°C and the average difference between oxygen profiles was less than 1.5% 02. Radial temperature gradients were measured to assess whether heat loss from the reactor walls was significant. On average, radial temperature differences were less than 3.5°C and their occurrence has been linked to drying rather than ambient temperature and heat loss from the walls. Also, horizontal oxygen measurements showed that radial differences in 02 concentration were less than 2 percentage points. Based on these analyses it can be concluded that the experimental system replicates well and can be assumed to represent a process with one-dimensional spatial variation in the state variables.

Measurements of spatial and temporal gradients in temperature, 02 concentration and moisture content illustrate the coupled interaction between process variables. In the SFW experiments the trends of reduced rate of temperature increase at low oxygen concentrations has shown the influence of microbial activity on temperature and the coupling of degradation rates to oxygen transport and energy transport. The coinci- dence of axial temperature gradients with axial gradients in moisture has displayed the coupling of mass transport to energy transport, and the decrease in fraction of oxygen depletion in the lower section of the bed as moisture decreased has shown the coupling of moisture to degradation rates. The observation of maximum temperatures at higher axial positions and of decreased maximum temperatures as aeration rate increased has shown the influence of aeration rate on 02 supply for microbial activity as well as its influence on cooling. However, the observation of decreasing RO2 ..... with increasing aeration rate in the biosolids experiments has illustrated the influence of excessive cooling on microbial activity and its result of lower maximum temperatures observed in these experiments.

A presentation of the SFW and biosolids values for Tmax, O2.min, cumulative 02 consumption and RO2 . . . . has shown the difference in behaviour between the two substrates. For the SFW experiments RO2 ..... and cumulative 02 consumption increased with increasing aeration rate, while for the biosolids experiments, R O 2 . . . . and cumulative 02 consumption decreased with increasing aeration rate. These differences illustrate the strong influence of substrate in process performance.

A presentation of Tmax, RO2 . . . . . and cumulative 02 consumption values for the SFW experiments has shown the influence of initial moisture content on process behaviour. Although no significant difference between cumulative 02 consumption values for initial moisture contents of 45 and 55% was observed, Tmax

and RO2 ..... values were found to increase .with increasing moisture, and the fraction of oxygen consumed in the lower section of the bed was found to decrease with decreasing moisture. The fact that initial moisture content was found to have minimal effect on the cumulative oxygen consumed, yet have noticeable effect on RO2 . . . . and the distribution of 02 consumption shows the importance of measuring gradients in the state variables for gaining insight into process behaviour.

It is apparent from the results presented that within a static bed degradation process there exists considerable temporal and spatial variation in state variables such as 02 concentration, temperature and moisture content. Now that the pilot-scale reactors have been charactcrised and their limitations defined, the temporal and spatial gradients in process parameters can be used to gain insight into transport phenomena and biokinetic mechanisms within aerobic high-solids fermentation systems.

Acknowledgements

This research was supported by Agreement Number 3027-ERER-MW-94 from the New York State Energy Research and Development Authority, by a fellowship from the US Environmental Protection Agency, and support from the New York State Center for Advanced Technology in Biotechnology at Cornell University. The authors are grateful to Timothy Nock, Carolyn Baker and Josh Watson for their help with pilot-scale experimentation.

References

1. Miller, F. C., Minimizing odor generation. In Science and Engineering of Compost: Design, Environmental, Microbiological and Utilization Aspects, ed. H. A. J. Hoitink and H. M. Keener. Renaissance Publications, Worthington, 1993, pp. 219-241.

2. Murray, C. M., Controlling odor. In Solid Waste Composting Council Second Annual Conference. SWCC, Falls Church, VA, 1991.

3. Strom, P. F., Effect of temperature on bacterial species diversity in thermophilic solid-waste corn- posting. Applied and Environmental Microbiology, 1985, 50(4), 899-905.

4. Suler, D. J. and Finstein, M. S., Effect of temperature, aeration and moisture on CO2 formation in bench-scale, continuously thermophilic composting of solid waste. Applied and Environmental Micro- biology, 1977, 33, 345-350.

5. Bach, P. D., Nakasaki, K., Shoda, M. and Kubota, H., Thermal balance in composting operations. Journal of Fermentation Technology, 1987, 65, 199-209.

6. Bach, P. D., Shoda, M. and Kubota, H., Compost- ing reaction rate of sewage sludge in an


autothermal packed bed reactor. Journal of Fer- mentation Technology, 1985, 63, 271-278.

7. Nakasaki, K., Kato, J., Akiyama, T. and Kubota, H., A new composting model and assessment of optimum operation for effective drying of corn- posting material. Journal of Fermentation Technology, 1987, 65, 441-447.

8. Campbell, C. D., Darbyshire, J. F. and Anderson, J. G., The composting of tree bark in small scale reactors - - adiabatic and fixed temperature experiments. Biological Wastes, 1990, 31, 175-185.

9. Lau, A. K., Lo, K. V. and Liao, P. H., Yu, J. C., Aeration experiments for swine waste composting. Bioresource Technology, 1992, 41, 145-152.

~0. de Bertoidi, M., Rutili, A., Citterio, B. and Civilini, M., Composting management: a new process control through 02 feedback. Waste Management Research, 1988, 6, 239-259.

1. Elwell, D. L., Keener, H. M. and Hansen, R. C., Controlled, high rate composting of mixtures of food wastes, yard waste and chicken manure. In Seventh International Symposium on Agricultural and Food Processing Wastes. American Society of Agricultural Engineers, Chicago, IL, 1995.

2. Jeris, J. S. and Regan, R. W., Controlling environmental parameters for optimum composting - - I. Compost Science, 1973, 14, 10-15.

13. Bach, P. D., Shoda, M. and Kubota, H., Rate of composting of dewatered sewage sludge in continuously mixed isothermal reactor. Journal of Fermentation Technology, 1984, 62, 285-292.

14. Nakasaki, K., Sasaki, Y., Shoda, M. and Kubota, H., Effect of temperature on composting of sewage sludge. Applied and Environmental Microbiology, 1985, 50, 1526-1530.

15. Campbell, A., Engebretson, R. and Tripepi, R., Pulp and paper sludge composting. Tappi Journal, 1991, 74, |83-191.

16. Schulze, K. L., Relationship between moisture content and activity of finished compost. Compost Science, 1961, 2, 32-34.

17. Nakasaki, K., Sasaki, Y., Shoda, M. and Kubota, H., Effects of bulking agent on the reaction rate of thermophilic sewage sludge composting. Journal of Fermentation Technology, 1986, 64, 539-544.

18. Hansen, R. C., Keener, H. M., Marugg, C., Dick, W. A. and Hoitink, H. A. J., Composting of poultry manure. In Science and Engineering of Composting: Design, Environmental, Microbial and Utilization Aspects. Renaissance, Worthington, 1993, pp. 131-153.

19. Jeris, J. S. and Regan, R. W., Controlling environmental parameters for optimum composting - - II. Compost Science, 1973, 14, 8-15.

20. Nakasaki, K., Sasaki, Y., Shoda, M. and Kubota, H., Change in microbial numbers during thermophilic composting of sewage sludge with reference to CO2 evolution rate. Applied and Environmental Microbiology, 1985, 49, 37-41.

21. Kubota, H. and Nakasaki, K., Accelerated thermophilic composting of garbage. Biocycle, 1991, 32, 66-68.

22. Nakasaki, N., Yaguchi, H., Sasaki, Y. and Kubota, H., Effects of C/N ratio on the thermophilic composting of garbage. Journal of Fermentation and Bioengineering, 1992, 73, 43-45.

23. Nakasaki, K., Nakano, Y., Akiyama, T., Shoda, M. and Kubota, H., Oxygen diffusion and microbial activity in the composting of dehydrated sewage sludge cakes. Journal of Fermentation Technology, 1987, 65, 43-48.

24. Walker, I. K. and Harrison, W. J., The self-heating of wet wool. New Zealand Journal of Agricultural Research, 1960, 3, 861-895.

25. Ashbolt, N. J. and Line, M. A., A bench-scale system to study the composting of organic wastes. Journal of Environmental Quality, 1982, 11, 405 -408.

26. Hogan, J. A., Miller, F. C. and Finstein, M. S., Physical modeling of the composting ecosystem. Applied and Environmental Microbiology, 1989, 55, 1082-1092.

27. Campbell, C. D., Darbyshire, J. F. and Anderson, J. G., The composting of tree bark in small scale reactors - - self heating experiments. Biological Wastes, 1990, 31, 145-161.

28. Bono, J. J., Chalaux, N. and Chabbert, B., Bench- scale composting of two agricultural wastes. Bioresource Technology, 1990, 40, 119-124.

29. Hamelers, H. V. M., A theoretical model of corn- posting kinetics. In Science and Engineering of Composting: Design, Environmental, Microbial and Utilization Aspects, ed. H. A. J. Hoitink and H. M. Keener. Renaissance Publications, Worthington, 1993, pp. 36-58.

30. Whang, D. S. and Meenaghan, G. F., Kinetic model of the composting process. Compost Science/ Land Utilization, 1980, 21, 44-46.

31. VanderGheynst, J. S., VanderGheynst, G. B. and Walker, L. P., Development and analysis of oxygen sensing probes for in-situ monitoring of solid-state biodegradation processes. Journal of Air and Waste Management, in press.

32. Hall, S. G., Aneshanseley, D. and Walker, L. P., Instrumentation in experimental compost modeling. Paper presented at the ASAE 1995 Annual International Summer Meeting, Chicago, Illinois, July, 1995.

Documents

High-solids aerobic decomposition: pilot-scale reactor development and experimentation