HAZARDOUS WASTE & HAZARDOUS MATERIALSVolume 12, Number 2, 1995Mary Ann Liebert, Inc.

Submerged Filter Biotreatmentof Hazardous Leachate

in Aerobic, Anaerobic, andAnaerobic/Aerobic Systems

DANIEL P. SMITH

Department of Civil and Environmental EngineeringUtah State University

Logan, UT 84322-8200

ABSTRACT

Aerobic, anaerobic, and anaerobic/aerobic biotreatment of an industrial hazardous wastelandfill leachate was evaluated in bench scale biofilm reactor systems operated under steady- andnon-steady-state conditions. The leachate contained volatile and semi-volatile organics thatexceeded the best-demonstrated-available-technology (BDAT) standards established for multi-source leachate wastewater under the Resources Conservation and Recovery Act (RCRA). Theinfluent leachate stream was continuously applied to three parallel systems: 1) an upflowanaerobic filter followed by a submerged aerobic filter, both with plastic packing, 2) an anaerobicgranular activated carbon column, and 3) an upflow, plastic packed aerobic filter. All systemsachieved steady-state COD removals of 66-82 percent. The sequential anaerobic/aerobic filtersystem was most resistant to hydraulic and organic shock loading, whereas the aerobic filterperformance deteriorated significantly. Though transformations of specific chemical compoundswere achieved in both anaerobic and aerobic treatment, the sequential anaerobic/aerobic systemwas most effective for meeting BDAT standards for hazardous organics.

INTRODUCTION

Industrial hazardous waste landfill leachates frequently require treatment to meet bestdemonstrated available technology (BDAT) standards under the Resources Conservation andRecovery Act (RCRA). For a RCRA-hazardous multi-source leachate wastewater (MSLWW),BDAT standards consist of stringent numerical limits for numerous specific organic and inorganicchemicals [1]. Biological treatment in fixed-film reactors is one technology with the potential toachieve treatment goals. Limited information is currently available, however, on the design ofbiofilm systems that will reliably meet BDAT standards in the varying conditions to which theywill be exposed in the field. Leachate treatment systems must accommodate sudden and largevariations in hydraulic and organic loading and possible inhibition by toxic chemical compounds.These factors present formidable design challenges that must be addressed before the economicadvantages of fixed-film treatment are realized. The purpose of this paper is to compare theeffectiveness of anaerobic, aerobic, and sequential anaerobic/aerobic fixed-film bioreactor systemsin treating a RCRA-hazardous leachate under steady-state and non-steady-state conditions.

The leachate treated in this study was obtained from an industrial waste landfill with a 3hectare active disposal area. The dry weather leachate flow is 12 1/min. Leachate is delivered by

167

a collection and conveyance system to a 1500 m3 hydraulic surge tank which is currently batchdischarged. Leachate treated in this study was collected from the top meter of the water column inthe surge tank, below the tank surface layer where an oily layer sometimes forms. Chemicalanalyses of specific organic components in the leachate indicates the presence of a wide variety ofRCRA-listed volatile and semi-volatile components, many of which are above the MSLWW limits(Table 1). Typical leachate bulk chemical characteristics during dry weather conditions (Table 2)indicate a pH that is suitable for aerobic or anaerobic biological treatment and the presence of asubstantial volatile organic acids component that is typical of landfill leachates [2,3]. Asystematic decline in leachate chemical oxygen demand (COD) was observed through thebiological treatment study, and was attributed to dilution of leachate by seasonally varyingprecipitation (Figure 1).

TABLE 1Chemical Analyses of Landfill Leachate

MSLWW (FO-39) Number Average3 StandardChemical Analysis Regulatory Limit of Concentration Deviation

(mg/l)1 Samples (mg/i) (mg/1)

Total Organic Carbon No Limit 21 893 378pH No Limit 3 8.7 0.041Méthylène Chloride 0.089 2 0.144Acetone 0.28 23 6.13 3.36Chloroform 0.046 2 0.045

Methanol 0.332 14 1.17l-4Dioxane 0.12 0Methyl Isobutyl Ketone 0.14 0Toluene 0.08 2 0.062Ethyl Benzene 0.057 0Chlorobenzene 0.057 2 0.135Pyridine 0.014 4 0.0344 0.055Total Dichlorobenzenes 0.036 1 0.061Phenol 0.039 5 0.786 0.49

Chlorex 0.0242 1 0.786 0.49Napthalene 0.059 5 0.01

Phthalic Anhydride 0.0202 1 0Dibutyl Phthalate 0.057 5 0.301 0.29Bis(2-ethylhexyl) phthalate 0.28 5 1.07 0.749

1 Federal Register, Vol. 55, No. 106, June 1. 1990.2 Not listed in current FO-39 regulations.3 Summary of data over sixteen month period.

Biological Leachate Treatment

The application of biological processes for treatment of landfill leachates has beendescribed in numerous reports [3-7]. Successful leachate treatment has been accomplished usingpacked bed anaerobic reactors [3,8-10] and also with aerobic and anoxic suspended growthsystems [4,5,9]. Anaerobic treatment of hazardous leachates in fluidized bed bioreactors usingactivated carbon carrier media has been demonstrated to be highly effective for removal of a

variety of volatile and semi-volatile organics [8,10-12]. Sequential anaerobic/aerobic treatmentprocesses have been advantageously applied to complex leachate streams [13]. Completedegradation of several specific organic compounds which are often found in hazardous leachateshas been achieved in laboratory reactors using sequential anaerobic/aerobic laboratory reactors

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[13-15]. While laboratory studies indicate that fixed film reactors offer a viable option forhazardous leachate treatment, including the removal of specific regulated organic compounds,practical application of this technology under field conditions is limited.

TABLE 2Dry Weather Leachate Composition

Parameter Concentration, mg/L

OrganicsChemical Oxygen Demand (COD) 7,090Total Organic Carbon (TOC) 1,271Volatile Organic Acids (VOA) as Acetate 1,080

SolidsTotal Suspended Solids (TSS) 130Volatile Suspended Solids (VSS) 100

NutrientsAmmonia Nitrogen (NH3-N) 296Phosphorus (PO4-P) grjl

pH 7.!Alkalinity as CaC03 1,420

1 Below analytical detection limit

A fraction of the organic material from hazardous and non-hazardous landfill leachates hasbeen found to be non-biodegradable within the practical limits of biological treatment processes ,

and in some cases over 80% of the leachate COD can be refractory [2,4-12,16,17]. The degree ofCOD reduction achieved in biological treatment has been correlated with the age and stability ofthe landfilled material [8-11]. The fraction of non-degradable COD tends to be higher for older,more stable landfills, where bioactivity has removed the degradable components [2,17,18].

10,000

CDE

8,000

g 6,000O

$ 4,000COoCOa> 2,000

0

L • mi «".•.

7/8 9/4 11/1 12/29 2/25 4/24Date, 1993

FIGURE 1. Variation in untreated leachate unfiltered chemical oxygen demand.

169

Biotreatment can be quite efficient in reducing COD associated with relatively degradable organiccompounds, such as the volatile organic acids which frequently constitute a significant fraction ofleachate COD. It is important to realize that bioreactors will not accomplish removal ofnondegradable COD in landfill leachates. Evaluation of process performance under steady-stateand dynamic operation should consider the removal efficiency based on the degradable fraction ofthe organic material as well as removal efficiency based on the total COD.

While biofilm reactor models have been developed for simplified treatment cases usingsingle component biokinetic coefficients [19-21], these models cannot fully describe treatment ofcomplex leachates. Design of fixed film leachate treatment reactors is dependent on bench scaletreatability testing, pilot plant studies, and engineering judgment. This paper reports on a

laboratory treatability study that was conducted to develop performance data for alternativesevaluation and to support design of pilot and full scale reactors. The goal of this study was tocompare the ability of aerobic, anaerobic, and anaerobic/aerobic fixed film biological reactors toremove leachate bulk organic carbon and to meet BDAT standards for specific organiccompounds. Bioreactor performance was evaluated under steady-state conditions, and also intransient operation resulting from applied hydraulic and organic shock loadings.

MATERIALS AND METHODS

Experimental DesignParallel laboratory biofilm reactor systems were operated in the upflow mode without

recycle and received a continuous flow of leachate from a common reservoir (Figure 2). Threebiotreatment systems were examined: an upflow anaerobic filter (ANF) followed by an submergedaerobic filter (AF2), both with plastic packing, an anaerobic granular activated carbon column(AN-GAC), and an upflow, plastic packed aerobic filter (AF). In this study, the ANF and AF2reactors were also evaluated as single treatment systems. Thus, the performance of five biofilmtreatment systems was assessed: 1) anaerobic filter (ANF), 2) anaerobic activated carbon column(AN-GAC), 3) aerobic filter (AF), 4) aerobic filter that treated anaerobic filter effluent (AF2), and5) series anaerobic/aerobic filters (ANF/AF2). Details of the four systems are given in Table 3.

Aerobic Filter Anaerobic/Aerobic Filters Anaerobic GAC Column23 -C 36° C / 23° C 37°C

Air Off Gas Air Off Gas

Leachate Reservoir

FIGURE 2. Experimental Bioreactor Systems Showing Sample Points: A. Untreated Leachate,B. AF Effluent, C. ANF Effluent, D. AF2 Effluent, E. AN-GAC Effluent.

170

TABLE 3Bench Scale Bioreactor Design and Operating Parameters

Redox Environment

Anaerobic Aerobic

ANF AN-GAC AF AF2

Packed Column CharacteristicsInternal Diameter (ID), cm 6 3Packing Height, cm 53 46Cross Sectional Area, cm2 30.2 7.1Empty Bed Volume, 1 1.60 0.325Total Surface Area, m2 0.530 1.21Void Ratio 0.88 0.38

Media CharacteristicsTypeNominal SizeSpecific Surface Area, m2/m3

Reactor Operating CharacteristicsFlowrate2,1/d 1-23 0.246Empty Bed Contact Time2, days 1-30 1.32Temperature, °C 36 37Dissolved Oxygen, mg/1 0 0Steady-State Effluent pH 6.7-8.4 7.6-8.5Steady-State Operating Time, days 96 96

Nor-Pack1 GAC1.59 cm 20 x 40 mesh

331 3,720

6.253

30.21.60

0.5300.88

Nor-Pack11.59 cm

331

1.171.3723

5-97.4-8.3

96

922.8641.45

0.4800.88

Nor-Pack11.59 cm

331

1.231.1823

5-96.7-7.9

96

1 Nor-Pack 1.59 cm random polyethylene tower packing (NSW Corp.)2 Steady-state

Leachate samples were collected weekly and supplemented with nitrogen and phosphorusin excess of stoichiometric requirements for net biomass synthesis. Variable speed peristalticpumps (Watson-Marlow, 202U/AA) delivered leachate to the bioreactors continuously from a 40 1reservoir maintained at 4°C. Aeration to aerobic reactors was accomplished by introducinglaboratory air through a sparging stone located below a perforated liquid distribution plate whichalso served as a packing support. At the beginning of the study, the reactors were seeded fromthermophilic aerobic and anaerobic laboratory reactors treating waste solids from an industrial endof pipe biological wastewater treatment plant. The four reactors were operated at a 3 day EBCTduring an initial 130 day operating period, after which steady-state hydraulic loading was

commenced at the EBCT shown in Table 3. Reactor performance was quantified by routinemeasurements of pH, total organic carbon (TOC), COD, total suspended solids TSS), and volatilesuspended solids (VSS). Wet tip gas meters were used to continuously monitor gas productionfrom the anaerobic reactors. Volatile and semi-volatile organics, nitrogen, phosphorus, metals,and volatile organic acids in the anaerobic reactors were measured periodically.

The reactors were operated under constant hydraulic loading for 96 days to characterizesteady-state operation. Leachate quality fluctuated over the steady-state period due to varyingprecipitation levels and dilution effect, with a mean unfiltered COD of 3628 mg/1 and a standarddeviation of 1580 mg/1. At the end of the steady-state period, reactor responses to two hydraulicshock loads were evaluated. The steady-state systems were first subjected to a five-fold increasein flowrate over a period of seventy three hours (Transient I). Immediately following the initialsurge, the flowrate was further increased to ten or more times the steady-state flowrate, furtherdecreasing the EBCT to 0.12 to 0.14 days for the single reactors and 0.23 days for the sequentialanaerobic/ aerobic system (Transient II). The second hydraulic surge was applied for twenty fivehours, after which steady-state flowrates were resumed. Samples were collected hourly during theinitial transient periods and less frequently thereafter, and analyzed for TOC, VOA, and pH.

171

During shock loadings, reactor pHs did not fluctuate outside of the steady-state ranges (Table 3);the effluent COD was calculated by applying a conversion factor to TOC analyses of reactoreffluent samples.

Analytical Methods

Untreated leachate and reactor effluents were analyzed using standard methods [22]. AFisher Scientific Acumet 925 pH/Ion Meter, regularly calibrated with standard buffer solutionswas used to measure pH. Dissolved oxygen was monitored with a YSI Inc. Model 59 DissolvedOxygen Meter, regularly calibrated in air above deionized water. COD was analyzed by bothopen reflux/titration and digestion/colorimetric methods using a HACH DR/3000Spectrophotometer. The TOC was quantified using a Dohrman DC-190 High Temperature TOCAnalyzer. TOC was obtained by taking the difference between the measured total and inorganiccarbon.

Cumulative gas production rates from anaerobic reactors were measured using Wet TipGas Meters. Gas composition was analyzed using a HP 5890 Series II Gas Chromatograph withThermal Conductivity Detector, HP 3396 Series II Integrator, and an Alltech Concentric stainlesssteel column (CTR1). An area normalization procedure with a standard gas mixture (Scott Gases,Inc., Mix 237) was used to determine response factors for quantitative analyses of methanecontent. Volatile organic acids (VOA) were measured using a HP 5880A Series II GasChromatograph and Integrator, Flame Ionization Detector, and a Supelco glass column (1-1825)with 60/80 Carbopack C/0.3% Carbowax 20M/0.1% H,PO, packing for analysis of aqueous freeacids (C2-C5).

RESULTS AND DISCUSSION

The performance of the four biofilm reactor systems was compared under steady-state andnon-steady-state conditions. Results of steady-state and non-steady-state performance arediscussed below. The abilities of the test systems to meet BDAT standards are also compared.Steady-State Performance

Performance data for the anaerobic filter, anaerobic GAC column, aerobic filter, theaerobic filter treating ANF effluent,, and the sequential anaerobic/aerobic filters are summarizedin Tables 4 through 8. Landfill leachates contain organic materials, such as humic typesubstances, which are refractory under practical biological treatment conditions. Though thesehigh molecular mass materials exert a chemical oxygen demand which is included chemicalanalyses of untreated leachate and bioreactor effluents, they are inert in the biological reactors.The effectiveness and dynamics of bioprocess action can be compared on a more meaningful basisif the non-biodegradable fraction of COD is subtracted from overall measured COD. Based on

previous bench scale, dispersed growth aerobic treatment studies of this leachate and the overallresults of this study, the refractory fraction of untreated COD was estimated to be 20%. Studieson other landfill leachates have reported similar and greater non-degradable fractions (10,17).

ANF, AN-GAC, and AF systems achieved steady-state COD removals of 66 to 76% at 1.3day EBCT and 2.7 kg COD/m3/day organic loading rate. The estimated degradable andnondegradable fractions of influent COD are shown in Tables 4 through 8. The effluentdegradable COD from the bioreactor systems were estimated by subtracting influentnondegradable COD from the total effluent COD, as shown in Table 9. Removal efficienciesbased on estimated degradable COD in influent and effluent are shown in Table 10 for the fivesystems at steady and non-steady operation. The estimated biodegradable COD removals were

higher than total COD removals for all systems. At steady-state, ANF/AF2 achieved 81 % totalCOD removal and complete removal of degradable organics at 2.5 day EBCT and 1.46 kg/m3/dayorganic loading rate. The high performance efficiency of ANF/AF2 was achieved because AF2was effective at polishing ANF effluent; AF2 received a lower organic loading rate (1.05kg/m3/day) than the single stage reactors. While ANF/AF2 provided superior removal efficiency,it must be remembered that ANF/AF2 provided greater leachate retention time than did the othersystems. Anaerobic filter effluent COD (510 mg/1) was higher than aerobic filter (AF) effluent,though these were physically identical reactors with similar organic loading rates. The

172

TABLE 4Performance Summary of Anaerobic Filter (ANF)

Parameter Steady-State Transient I Transient II

Flowrate, 1/dayEBCT, daysOrganic loading rate, kg COD/m3dOrganic removal rate, kg COD/m3dCOD (Unfiltered)

Influent, mg/LDegradable Influent COD1Nondegradable Influent COD2

Effluent, mg/LPercent COD removal, %Gas production

Total, m3/m3 reactor-dPercent methane, %Percent of theoretical3- %

J avg4, g/m2/day

1.231.302.801.84

3,6282,902

7261,236

66

0.686864

5.56

6.080.2638.684.77

2,2841,827

4571,022

55

1.36755414.4

13.20.121

13.65.42

1,6431,314

32999040

1.50876116.4

1 Estimated as influent COD x 0.82 Estimated as influent COD x 0.23 CH4-COD Production Rate/Total COD Removal Rate x 1004 COD Removal Rate/Total Reactor Surface Area

TABLE 5Performance Summary of Anaerobic GAC Column (AN-GAC)

Parameter Steady-State Transient I Transient II

Flowrate, 1/dayEBCT, daysOrganic loading rate, kg COD/m3dOrganic removal rate, kg COD/m3dCOD (Unfiltered)

Influent, mg/LDegradable Influent COD1Nondegradable Influent COD2

Effluent, mg/LPercent COD removal, %Gas production

Total, m3/m3 reactor-dPercent methane, %Percent of theoretical3, %

J avg4, g/m2/day

0.2461.322.752.02

3,6282,902

726957

74

0.587051

0.54

1.100.2957.735.00

2,2841,827

457805

65

2.357893

1.34

2.280.143

11.55.36

1,6431,314

32987947

1328585

1.44

1 Estimated as influent COD x 0.82 Estimated as influent COD x 0.23 CH4-COD Production Rate/Total COD Removal Rate x 1004 COD Removal Rate/Total Reactor Surface Area

173

TABLE 6Performance Summary of Aerobic Filter (AF)

Parameter Steady-State Transient I Transient II

Flowrate, 1/day 1.17 5.69 13.9EBCT, days 1.37 0.28 0.115

2.65 8.12 14.32.01 3.41 2.55

Organic loading rate, kg COD/m3dOrganic removal rate, kg COD/m3dCOD (Unfiltered)

Influent, mg/L 3,628 2,284 1,643Degradable Influent COD' 2,902 1,827 1,314Nondegradable Influent COD2 726 457 329

Effluent, mg/L 872 1,323 1,349Percent COD removal, % 76 42 18J avg3, g/m2/day 6.08 10.3 7.70

1 Estimated as influent COD x 0.82 Estimated as influent COD x 0.23 COD Removal Rate/Total Reactor Surface Area

application of sequential anaerobic/aerobic filters at longer overall EBCT achieved the besteffluent quality.Non-Steady-State Performance

The increase in organic loading rate in Transients I and II was not proportional to theflowrate increase because the leachate applied during the transients was more dilute (Tables 4-8).For many of the reactors, the effluent COD was lower during shock loading than at steady-statebecause of the more dilute influent concentration. The organic removal rates generally increasedas the applied organic loading rate increased, with the exception of the aerobic filter (AF) which isdiscussed in more detail below. COD removal efficiencies declined in all biofilm reactors as

organic loading rates increased. However, the decline in COD removal efficiency among thebioreactor systems did not follow a pattern that was predictable from the steady-state results.

TABLE 7Performance Summary of Aerobic Filter (AF2)*

Parameter Steady-State Transient I Transient II

Flowrate, 1/day 1.23 6.08 13.2EBCT, days 1.18 0.238 0.11Organic loading rate, kg COD/m3d 1.05 4.29 9.0

Organic removal rate, kg COD/m3d °-47 2-36 3-8COD (Unfiltered)

Influent, mg/L 1,236 1,022 990Degradable Influent COD1 510 565 661Nondegradable Influent COD2 726 457 329

Effluent, mg/L 675 465 576Percent COD removal, % 45 55 42J avg3, g/m2/day 1-42 7.13 11.4

1 Estimated as influent COD x 0.82 Estimated as influent COD x 0.23 COD Removal Rate/Total Reactor Surface Area* Post anaerobic filter treatment

174

TABLE 8Performance Summary of Sequential Anaerobic/Aerobic Filters (ANF/AF2)

Parameter Steady-State Transient I Transient II

Flowrate, 1/dayEBCT, daysOrganic loading rate, kg COD/m3dOrganic removal rate, kg COD/m3dCOD (Unfiltered)

Influent, mg/LDegradable Influent COD1Nondegradable Influent COD2

Effluent, mg/LPercent COD removal, %J avg3, g/m2/day

1.232.481.461.19

3,6282,902

726675

813.60

6.080.504.553.63

2,2841,827

457465

8011.0

13.20.237.114.61

1,6431,314

329576

6513.9

1 Estimated as influent COD x 0.82 Estimated as influent COD x 0.23 COD Removal Rate/Total Reactor Surface Area

TABLE 9Estimated Effluent Degradable COD Concentration (mg/1)

Reactor Steady-State Transient I Transient II

AnaerobicPacked Filter 510GAC Column 231

AerobicPacked Filter 146Packed Filter (AF2) 0

Anaerobic/AerobicSeries Packed Filters 0

565348

8668

661550

1020247

247

TABLE 10Estimated Degradable COD Removal Efficiency (%)

Reactor Steady-State Transient I Transient II

AnaerobicPacked FilterGAC Column

AerobicPacked FilterPacked Filter (AF2)

Anaerobic/AerobicSeries Packed Filters

8292

95100

100

6980

5399

99

5058

2263

81

175

100

If) g ^~'5 o)< E<u Q)

(o 2o o>!<m tfío rer-

10 20 30 40 50Time (Hours)

60 70

FIGURE 3. Anaerobic filter effluent total volatile acids in Transient I.

The two anaerobic reactors responded with similar patterns of performance variationduring the transients. For both ANF and AN-GAC, organic removal rates in Transient I increased250% over steady-state, while estimated removal efficiencies for degradable COD decreased to50% in Transient II (Tables 4,5). Aerobic filter (AF) organic removal rates in Transients I and IIincreased only 70% and 30% respectively over those at steady-state (Table 6). The estimateddegradable COD removal in the aerobic filter (AF) fell off dramatically to only 52 and 22% inTransients I and II, respectively, leading to high effluent COD (Tables 6,9). Degradable organicmaterials that were not utilized during the applied shock loadings appear to have been the primarycontributors to the elevated effluent COD observed in the transients. The aerobic filter (AF)achieved the lowest steady-state effluent COD and the highest steady-state COD removalefficiency of the three 1.3 day EBCT systems (ANF, AN-GAC, and AF). However, AFperformance under shock loading showed a distinctly sharper deterioration than the anaerobicsystems.

Total volatile acids (TVA) are the major intermediates in the biotransformation of complexorganic substrates to methane in anaerobic systems, and TVA is a useful monitoring and

m w

100 110 120Time (Hours)

130 140 150

FIGURE 4. Anaerobic filter effluent total volatile acids in Transient II.

176

V)-o -**=**'o O)< E0)

« «

o>

s re

200

150

loo 4

50

10 20 30 40 50Time (Hours)

60 70

FIGURE 5. Anaerobic GAC column effluent total volatile acids in Transient I.

diagnostic tool for anaerobic bioreactor systems (23,24). TVA were monitored in ANF and AN-GAC effluents during the applied transients. In the anaerobic filter, TVA increased from 20 mg/1at steady-state to 60 mg/1 in Transient I (Figure 3). TVA further increased to 303 mg/1 inTransient II but declined to steady-state levels when the flowrate was returned to steady-statelevels (Figure 4). In the anaerobic GAC column, TVA reached 167 mg/1 in Transient 1, butreturned to steady-state levels before Transient I was completed (Figure 5). Following theinitiation if Transient II, TVA again increased and reached 314 mg/1 at 97 hr.; TVA then declinedto pre-perturbation levels after the termination of Transient II. Though TVA increased in bothanaerobic systems following both shock loadings, the magnitude of the increase in TVA was quitelimited, TVA rapid returned to steady-state levels during or after the shock loadings. These resultsindicate that the anaerobic reactors were quite resilient to upset due to organic overloading or tothe potential inhibitory effects of hazardous organics in the leachate.

The sequential anaerobic/aerobic filter was most resistant to performance deteriorationunder applied shock loading. In the five-fold hydraulic flowrate increase applied in Transient I,the COD loading rate tripled but the COD removal rates also tripled, resulting in minimal change

400

in!5 ^*'5 O)< E0)

'*z\reo>

re in S re

300

200

100

70 80 90 100 110 120 130 140 150Time (Hours)

FIGURE 6. Anaerobic GAC column effluent total volatile acids in Transient II.

177

in removal efficiency (Table 8). Only a moderate increase in effluent COD resulted when theflowrate was increase ten-fold in Transient II (Table 9). The superior performance of ANF/AF2 isdue to the longer overall EBCT, and to the lower organic loading applied to AF2 due to first stageanaerobic treatment. The ability of the anaerobic filter (ANF) to resist performance deteriorationunder shock loading was superior to the aerobic filter (AF). This suggests that an anaerobic firststage would give better response to shock loading than an aerobic first stage in a dual biofilterleachate treatment system.

Oxygen flux analysis of the aerobic filter

The dramatic deterioration in first stage aerobic filter removal efficiency during thetransients as compared to the anaerobic reactors suggests that oxygen flux limitation may haveinfluenced aerobic filter performance. A completely mixed biofilm reactor model was applied tothe aerobic biofilter to provide insight into this hypothesis. The completely mixed reactor modelassumes a uniform bulk substrate concentration in the reactor. The continuous mixing providedby aeration and long liquid residence times justified this assumption. A steady-state biofilmmodel, which includes substrate diffusion through a stagnant liquid layer, reaction and diffusion inthe biofilm, and biofilm thickness calculation, was used to predict the flux of substrate per crosssectional area of support surface at the given bulk substrate concentration [19,20]. The biofilmmodel was applied to steady-state AF operation using the parameters listed in Table 11 and themass balance equation:

a V (1)

where SeSoavQJ

effluent substrate cone, mg/cm3influent substrate cone, mg/cm3specific surface area, cm2/cm3volume, cm3flowrate, cm3/dayflux, mg/cm2/day

Solution of the CSTR biofilm model results in a bulk (effluent) degradable COD concentration of36.7 mg/1. (Table 12).

TABLE 11Aerobic Biofilm Model Parameter Values

Parameter, units Value Description

k, mg/mg/day 10.Ks, mg/cm3 0.20Y, mg VSS/g COD 0.45b, 1/day 0.15Xf, mg/cm3 40.0L, cm 0.0100Db, cm2/day 0.50Df, cm2/day 0.40

Maximum Substrate Utilization RateHalf Velocity ConstantYield CoefficientEndogenous Decay CoefficientBiofilm DensityStagnant Layer ThicknessDiffusion Coefficient In WaterDiffusion Coefficient In Biofilm

One assumption of the biofilm model is that substrate utilization throughout the biofilm islimited by the COD concentration only, and that oxygen diffusion into the biofilm is not limiting.A modified form of the Frank-Kamenetskii relationship [20] was used to predict if oxygendiffusion is flux limiting in the aerobic filter. To prevent flux limitation, the oxygen concentrationmust satisfy:

178

TABLE 12Anaerobic Filter Model Prediction

Parameter, units Model Prediction Description

Se, mg/1 36.7

J, mg/cm2/day 0.666Lf, cm 0.0499Total Reactor Biomass, mg 10,600

Effluent SubstrateConcentration (COD)Donor FluxBiofilm Thickness

Co2 > DçQDDo2 (1-f)Se (2)

where Co2 = dissolved oxygen cone, mg/cm3Se = effluent substrate cone, mg/cm3Dcod = diffusion coefficient for organics., mg/cm2/dayDo2 = diffusion coefficient for oxygen., mg/cm2/day"f = fraction of COD used for synthesis.

For Dcod = 1-6 cm2/day, Do2 = 0.5 cm2/day, f= 0.3, and Se = 36.7 mg/1, the bulk liquid dissolvedoxygen concentration must be at least 8.0 mg/1 for oxygen not to limit the flux into the biofilm.Since oxygen solubility in water at 23°C is only 8.6 mg/1, the model predicts that the steady-stateAF was close to oxygen flux limitation. Even if the bulk liquid was kept saturated with oxygen inequilibrium with the atmosphere throughout the transients, small increases in bulk substrateconcentration would lead to oxygen flux limitation into the biofilm. This analysis suggests thatthe aerobic filter may have suffered from oxygen flux limitations during the transients. Asubmerged aerobic biofilter in which oxygen is supplied by air sparging below the packing mediamay have limited ability to respond to the sudden increases in organic loading that are commonfor landfill leachates. Design of integrated equalization/ aerobic biofilter systems for landfillleachate treatment should account for the concentration of bulk organics in the leachate duringshock loadings as well as the magnitude of the flowrate increase.

Removal of Listed Organics in Steady-State Biofilm Reactors

Selected organic compounds specifically regulated for RCRA multi-source leachatewastewaters were monitored in untreated leachate and bioreactor effluents during the steady-stateperiod. Steady-state removals of RCRA-listed chemicals are shown in Tables 13 through 15 foranaerobic reactors, aerobic reactors, and the anaerobic/anaerobic system, respectively. In theuntreated leachate samples that were analyzed, not all of the indicator compounds were above theBDAT standards. In some cases the bioreactor effluent contained higher contaminantconcentrations than those reported for the untreated leachate. This apparent anomaly can beattributed to the wide variations in leachate organic composition due to stormwater infiltration andsample collection. It should be recognized that although stripping into the gas phase can beresponsible for organic compound removal in biotreatment systems [25], gas phase losses werenot evaluated in this study. Since air sparging results in a much higher gas flowrate thanmethanogenesis, emissions would be expected to be greater for aerobic systems than for anaerobictreatment.

Both anaerobic and aerobic reactors achieved high percentage removals for napthalene,phenol, and toluene. Partial to high percentage removals were achieved for acetone in anaerobicand aerobic systems and for acetonitrile in the aerobic systems. Results for other organics weremixed. Only the sequential anaerobic/aerobic treatment system removed all of the indicatorchemicals to below the BDAT standards. Though specific compounds were not analyzed inreactor effluent following shock loading, higher effluent levels of some indicator organiccompounds during the transients can be inferred from the decline in COD removal efficiency. The

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TABLE 13Steady-State Removal of Indicator Organic Compounds in Anaerobic Systems

Compound UntreatedLeachate

BDATstandard

Anaerobic Filter(AF)

Anaerobic GACColumn (AN-GAC)

EffluentCone.

%Removal

EffluentCone.

%Removal

Acetone, mg/LAcetonitrile,mg/LAniline, mg/LBis (2-ethyl-hexyl) phthalate,mg/LNaphthalene,mg/LPhenol, mg/LToluene, mg/L

2.210.430

0.0110.430

0.170

3.040.043

0.1620.097

0.810.278

0.059

0.0260.08

0.1200.90

0.1950.63

ND

NDND

95

100

100100

1.010.250

ND0.550

0.011ND

5442

100

100

99100

ND Below analytical detection limit.

TABLE 14Steady-State Removal of Indicator Organic Compounds in Aerobic Systems

Compound Untreated BDATLeachate Standard

Aerobic Filter(AF)

Aerobic Filter(AF2)

EffluentCone.

% Influent EffluentRemoval Cone* Cone.

Acetone, mg/L 2.21 0.162Acetonitrile, mg/L 0.430 0.097Aniline, mg/L 0.011 0.81Bis (2-ethyl-hexyl) 0.430 0.278

phthalate, mg/LNaphthalene, mg/L 0.170 0.059Phenol, mg/L 3.04 0.026Toluene, mg/L 0.043 0.08

0.5860.1040.0450.081

ND0.264ND

7476

10091100

0.1200.90

0.1950.63

NDNDND

0.018ND0.0030.031

NDNDND

%Removal

851009895

ND Below analytical detection limit.* ANF Effluent

TABLE 15Steady-State Removal of Indicator Organic Compounds

in Anaerobic/Aerobic System

Compound Untreated BDAT Anaerobic/Aerobic FiltersLeachate standard (AF)

Acetone, mg/L 2.21 0.162Acetonitrile, mg/L 0.430 0.097Aniline, mg/L 0.011 0.81Bis (2-ethyl-hexyl) 0.430 0.278

phthalate, mg/LNaphthalene, mg/L 0.170 0.059Phenol, mg/L 3.04 0.026Toluene, mg/L 0.043 0.08

EffluentCone

% Removal

0.018ND0.0030.031

NDNDND

991007393

100100100

ND Below analytical detection limit.* ANF Effluent

exact relationship between the removal efficiency of bulk organics (COD) and that of specificorganic compounds is not well established for either steady-state or transient operation.

CONCLUSIONS

The major conclusions from this study are listed below.• A bench scale anaerobic filter, anaerobic GAC column, and aerobic filter achieved 66 to

76% steady-state COD removal from a hazardous industrial landfill leachate at a CODloading of 2.7 kg/m3/day.

• A sequential anaerobic/aerobic filter system achieved 81% steady-state COD removal froma hazardous industrial landfill leachate at a COD loading of 1.46 kg/m3/day.

• Following applied hydraulic and organic shock loadings, both effluent COD and the CODmass removal efficiency decreased in all reactors The aerobic filter showed a dramaticdecline in removal efficiency, while the anaerobic reactors were more resistant to shockloading.

• The sequential anaerobic/aerobic reactors showed the best COD removal efficiency understeady-and non-steady-state operation, suggesting the advantages of a multiple stagesystem and longer overall retention times in the treatment system.

• About 20% on the COD in untreated leachate was non-biodegradable in either aerobic oranaerobic biofilm reactors. Alternative technologies would be required to treat therefractory COD in the leachate.

• All biofilm reactors were capable of treating some of the RCRA-listed organic chemicalsunder steady-state operation, but neither anaerobic or aerobic treatment met bestdemonstrated available technology standards for all compounds.

• The sequential anaerobic/aerobic filters achieved BDAT standards for indicator organiccompounds on the RCRA multi-source leachate wastewater list.

• Model and experimental results implicate oxygen diffusion as a key factor limiting aerobicactivity in the biofilter during applied transients.

• The design of integrated equalization/biotreatment systems for landfill leachates shouldprevent organic shock loadings to biofilm reactors as well as limit the hydraulic surges.

ACKNOWLEDGMENTS

A portion of this paper was originally prepared for the 26th Mid-Atlantic Industrial WasteConference, Newark, DE, August 1994. This investigation was conducted by Dr. Daniel Smith.At the time of the study, Dr. Smith was Senior Environmental Engineer with EnvironmentalProcess Design, Cranbury, New Jersey. The experimental work was performed by Mr. MicealCampbell.

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Reprint requests should be sent to:

Dr. Daniel P. SmithDepartment of Civiland Environmental EngineeringUtah State University

Logan, Utah 84322-8200.

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