Experimental Studies on the Co-Treatment of Landfill Leachlate and Sewage in Fredericton, N.B., Canada R. C. Berry* & K.C.Li

Experimental studies on the co-treatment of

landfill leachlate and sewage in Fredericton, N.B., Canada

R. C. Berry* & K.C.Lin**

* ADI Systems Inc., 1133 Regent St., Fredericton N.B., Canada E-mail: ronberry@brunswickmicro. nb. ca

*^Department of Civil Engineering, University of New Brunswick, Fredencfofz, A^.A, EJB j^3 C^Wa E-mail: [email protected]

Abstract

Problems in co-treating landfill leachate and sewage in the wastewater treatment plant in Fredericton. N.B.. Canada, led to the study offiel d samples and laboratory experiments. Batch reactors fed with settled sewage were operated under different leachate contents to evaluate some kinetic parameters The waste characteristics were analyzed. Leachate toxicity was sufficiently reduced after on-site partial treatment and by sewage dilution. However, an increase in sludge production overloaded the sludge processing unit. Too much leachate addition to the sewage would inhibit biological oxidation.

1 Introduction

Landfill leachate is mixed with sewage for co-treatment in the city of Fredericton,

N B, Canada. The leachate is generated from the Fredericton Region Solid Waste Commission (FRSWC) landfill site. It is partially treated by surface aeration in one pond and settling in another pond before being discharged into the municipal sewer lines en route to the Fredericton Area Pollution Control Commission

(FAPCC) sewage treatment plant (STP) approximately 7 km away The periodic discharge of the landfill leachate into the FAPCC STP has caused several problems and difficulties in plant operation In the past, shock loadings from the high strength leachate wastewater have caused upsets to system performance. The STP operator found it difficult to adjust the food-to- microorganism (F/M) ratio to a suitable value to maintain plant treatment efficiency. More power was required to force air into the aeration tank to maintain a dissolved oxygen (DO) level in the aeration tank high enough for biological oxidation. Finally, more sludge was also produced, which needed processing

before final disposal. All these problems added to the increased cost of operation. The objectives of this study were (1) to analyze field data to detect any toxicity in the waste mixture, and (2) to evaluate some kinetic parameters from laboratory experiments. The purpose was to gain further insight for better operation and control of the wastewater treatment plant.

2 Literature review

2.1 Leachate characteristics

The quantity and characteristics of leachate generated at a given landfill location are site-specific. Leachate is an extremely high strength wastewater when compared to a typical municipal wastewater. Zolten' compiled a list of historical testing results of leachate at many municipal solid waste landfills and contrasted them with municipal wastewater characteristics. Indicator (lumped) parameters used to indicate the overall leachate strength include dissolved and suspended solids, organic material (measured in terms of oxygen demand and total organic carbon), pH, alkalinity and hardness/ According to McGinley and Kmet/ some major contaminants contributing to dissolved solids include NH/, Organic-N, NO/, NO/, PO/, Organic-P, SO/, Organic-S, S*~,

Cl, Ca^, Mg*, Na% K , Fe*\ Fe* and Mn*. Uloth and Mavinic* observed that high concentrations of heavy metals inhibited biological growth Some trace organic compounds in leachate can be health hazards. In contrast to natural organic compounds, anthropogenic compounds are relatively resistant to biodegradation. These man made organic compounds include aliphatics, aromatics, polycyclic aromatics (all the above can be halogenated or nonhalogenated), pesticides, nitrosamines and phthalate esters/ Leachate characteristics change with time as a landfill ages. Many processes contribute to the overall leachate quality and quantity as described by Ehrig/ Freshly landfilled waste normally undergoes aerobic degradation. It takes several years to reach a steady state between acid and methane production.

2.2 Leachate and sewage co-treatment at STPs

Leachate and sewage co-treatment in an STP is an attractive approach. Leachate is generally amenable to biological treatment after dilution by sewage. No additional nutrients are required because excess nitrogen is normally available from leachate and excess phosphorus from sewage/ Also, the cost of discharging leachate into a municipal STP is relatively low compared to on-site leachate treatment/ However, some researchers foresee that this method may not be possible in the future because of laws preventing such a practice, economics unfavourable for leachate treatment, and/or increasing fees for co-treatment/ Potential problems associated with leachate and sewage co-treatment are high organic strength of leachate, variability of organic loads, toxicity to treatment

biomass, increased sludge production, and metal accumulation in the sludge.* The effects of leachate addition to various types of activated sludge systems have been studied by several researchers, e.g., Boyle & Ham/ Chian & DeWalleJ and Cummins '^ Leachate addition to an STP increases its organic load and influent strength.

The acceptable amount of leachate addition depends on the excess capacity of the plant." Co-treatment of leachate and sewage is possible even at low temperatures close to 5°C."'" Mavinic" showed that the minimum sludge age increased with increasing leachate strength and decreasing temperature Palit & Qasim'* found that the minimum mean cell residence time, 6%, should be greater than 4.7 days for activated sludge treatment of sewage-diluted leachate. Sludge production in co- treatment systems varies with organic loading/ '* Land disposal of sludges with high metal content can be a concern. In general, the metal concentrations in sludges produced from leachate treatment depend on the metal species and the leachate strength; they may or may not be higher than those from domestic sewage treatment/

3 Field studies: leachate processing and characterization

3.1 Leachate management

The leachate generated from the FRSWC landfill site is normally recycled back to the landfill for biodegradation. Periodically, the leachate flow is diverted from recirculation to two holding ponds for serial aeration and sedimentation As the settling pond is filled, a certain amount of settled leachate is discharged to a municipal sewerline en route to the FAPCC STP for treatment At the time of this study, tanker trucks were used for transportation of the settled leachate to the sewerline about 2.5 km away from the ponds. On a regular dumping day, five or six truck loads were used. It took 1.5 to 2.0 hours to empty each truck load of 15 m* (3300 Impgal) into the sewer. The leachate-sewage mixture is treated by a step-aeration activated sludge process at the FAPCC STP

3.2 Leachate characteristics

Leachate samples had been collected by the FRSWC personnel periodically between 1991 and 1994 for content analysis. The samples were taken from the leachate collection main before recirculation or partial treatment on the site. Table

1 summarizes the test results. Included on the list of measured parameters are some organic and inorganic substances, anthropogenic compounds and coliforms. The number of samples collected and the average, minimum, maximum and standard deviation of each parameter value are shown together with the corresponding range of each parameter inhibitory to the activated sludge system

Table 1: FRSWC landfill leachate characteristics and concentrations inhibitory to the activated sludge process.

Parameter Unit # Avg. Max. Min. Std. Inhibi- Obs Dcv. tory Range'*

BOD, mg/L 10 19750 26 100 16 100 2992 >4:1 COD mg/L 9 35289 40500 26900 3936 increase DOC mg/L 10 11 300 18800 8210 2771 in 4hrs SS mg/L 2 139 206 72 67 0 pH <5,>8 TKN mg/L 8 549 673 166 160 NOvNO, mg/L 11 15 160 <0.1 46 480; (asN) 1600 Sulfate mg/L 11 394 1260 5 454 >100 Sulfide mg/L 1 0.10 0.3 <0.l 0.1

Phenol mg/L 1 0.90 2.5 0.1 0.9 0.05-0.2 Cyanide mg/L I 0.15 0.5 <0.1 0.17 0.1-5.0 Cd mg/L 1 0.01 0.02 <0.01 0.01 10 Cr mg/L 1 0.05 0.12 <0.02 0.05 15-50 Ni mg/L 1 0.15 0.42 0.03 0.12 1.0-2.5 Fe mg/L 18 560 1840 3 645 1000 Mn mg/L 18 65 137 0.52 51.5 10 Pb mg/L 11 0.09 0.24 <0.02 0.10 0.1 Total P mg/L 8 3.6 11.2 <0.1 3.2 Hg ppb 11 <1 <1 <1 <1 0.1-5.0 Zn mg/L 11 2.28 10.7 0.02 391 0.08-10 Benzene Ppb 7 3.9 24.0 <0.4 8.2 100-500 Toluene PPb 7 218 1100 <0.4 3.62 200 Ethyl ppb 5 233 110 <0.4 43.4 Xylenes PPb 7 79.4 490 <0.4 168 Higher PPb 7 38.5 160 <0.4 59.9 1120 Alkyl Benzenes Naphtha- PPb 7 6.6 39 <0.4 13.3 500 lenes Fuel Oil 7 22000 PPb 3143 <0.4 7698 Total mg/L 9 3.4 9.4 ND 3.8 Petroleum Hydro- carbon Total # 11 5792 36000 0 11562 Col i form /100 Fecal mL 11 3072 17000 0 5783

Col i form E.Coli 11 2728 17000 0 5848

Water Pollution 467

It can be seen from Table 1 that the organic strength of the FRSWC landfill leachate was high in terms of five-day biochemical oxygen demand (BOD,), chemical oxygen demand (COD) or dissolved organic carbon (DOC.) For example, the maximum BOD, concentration in the leachate was 26 100 mg/L compared to the maximum value of 30 000 mg/L reported by McGinley & Kmef and Davis & Cornwell." On the other hand, the suspended solids (SS) concentration in the leachate was low. The maximum SS concentration measured at the site was 206 mg/L compared to a maximum of 1000 mg/L in Davis & Cornwell." The low SS level was likely due to the recirculation of leachate and the filtering effect of the solid waste piles above the collection system at the site. The organic strength of the leachate remained fairly constant from 1991 to

1994. The coefficients of variation for BOD,, COD and DOC were 0.15,0.11 and 0.25, respectively, much less than one standard deviation relative to the mean. The leachate had a BOD/COD ratio of 0.56 based on the average values. Using the classification system proposed by Henry el a/.,™ the leachate is beyond the

"young" age and approaching a "mature" age. Table 1 also shows that some parameters, such as BOD,, COD, DOC, SO/ , CN , Mn, Hg, Zn, phenol and toluene, had exceeded the inhibitory limits for the activated sludge process. However, it is expected that these would be diluted by sewage to a noninhibitory level by the time it entered the FAPCC STP. Most of the heavy metal and aromatic compound concentrations in the leachate were below the toxic levels inhibitory to the biological treatment process. The coliform concentrations in the leachate were much lower than those measured in sewage For some unknown reasons, the pH of the leachate was not recorded Presumably, it was slightly acidic because of the carbon dioxide and organic acids produced during anaerobic decomposition of the wastes.

The leachate was generally low in nutrients. Total Kjeldahl nitrogen (TKN) and total-P averaged 549 and 3.6 mg/L compared to reported values of 41.6-1409 mg/L^ and 0-130 mg/L,* respectively. NO?" plus NO/ nitrogen concentrations were high at times (reaching 160 mg/L in contrast to the 0.2-10.29 mg/L levels reported by Zolten) * A commonly accepted BOD:N P ratio for biodegradation is 100:5:1. Thus, the leachate was nutrient limiting for biodegradation. However, once mixed and diluted by sewage, nutrient deficiency should not be a problem as sewage contains a lot of N and P relative to BOD

3.3 Leachate partial treatment and dilution en route to the FAPCC STP

The strength of the leachate was greatly reduced after partial treatment at the site through aeration in one pond and sedimentation in the other. The leachate COD concentration was decreased to l/14th to 1/15th of its original level When truck loads of the settled leachate were transported to the sewerline en route to the FAPCC STP, the leachate was further diluted by sewage. Wastewater samples were collected at several locations along the sewerline in five different days between February and March, 1993 for COD analysis Figure 1 shows its COD attenuation along some 7 km of sewerline from the dumping station to the STP.

1000 2000 3000 4000 5000 6000 7000 Distance from Dumping Point (m)

Figure 1: Leachate dilution along the sewerline en route to the FAPCC STP

It can be seen from Figure 1 that the leachate COD concentration was 2000-

2700 mg/L at the dumping station on the sewerline; the COD strength of the leachate-sewage mixture decreased to approximately 500 mg/L near the FAPCC STP. This represents 75% to 81% reduction in COD simply by sewage dilution along the approximately 7 km long sewerline. The average sewage flow to the

STP for 1993 was 15 600 nf/d The leachate flow to the STP was about 1.2%- 1.5% of this average flow rate during the short dumping periods. Although the volume of leachate added to the STP influent was small on a daily basis, the BOD; mass flow rate during the dumping periods could be significant. Figure 2 shows a typical BOD; mass flow rate over one day. Leachate mass flow is superimposed onto the sewage mass flow It can be seen that the leachate BOD, mass flow amounted to 9%-14% of the sewage BOD, mass flow to the plant.

4500

04:00 08:00 12:00 16:00 20:00 00:00 02:00 06:00 10:00 14:00 18:00 22:00 Time (hr:min)

Figure 2: Typical BOD, mass flow to the FAPCC STP

4 Laboratory studies: batch reactor experiments

4.1 Method

Two aerobic biological batch reactors were used to study the response of the FAPCC STP biomass to various leachate loadings. Both units were 2-L Plexiglas cylinders with a porous air diffuser in the conical bottom of each The experimental methods that were used to conduct the bio-kinetic tests follow those in Reynolds'* and Cook & Forced

Acclimated culture was taken from the aeration tank of the STP. The feed into one reactor (Rl) was primary effluent (PE) collected from the plant every day; the feed into the other reactor (R2) was a mixture of PE and settled leachate (SL) obtained from the FRSWC site. Both reactors were fed regularly at 24-h intervals.

Aeration was set at 550-650 mL air/min At the end of each day, some mixed liquor was wasted from each reactor as required to maintain an F/M ratio of 0.08- 0.20 g DOC applied/g MLVSS-d (MLVSS = mixed liquor volatile suspended solids). Then, aeration was stopped, settled liquor was withdrawn from each reactor and replaced by new feed. Finally, aeration was resumed to start another cycle. MLVSS and specific oxygen uptake rate (SOUR) were measured at the start and end of each cycle for each reactor. DOC and SS were tested on the feed and the decanted "effluent" of each reactor every day. Occasionally, SOURs were monitored over a 24-h period for both reactors for comparison. Reactor 2 was initially fed with a composite wastewater consisting of 1% SL and 99% PE by volume. Later, leachate was increased to 5% by volume. The experimental period lasted for three months. Daily results were used to estimate the cell yield (Y^ in g VSS produced/g DOC removed) and the decay rate (b, in d *) from the slope and intercept, respectively, of a linear plot of the net specific growth rate (u, in g VSS/g MLVSS-d) versus the specific substrate utilization rate

(q, in g DOC/g MLVSS-d).

4.2 Results

Figure 3 shows typical DOC, MLSS and MLVSS results for Rl over 24 h DOC decreased exponentially from 46 mg/L to 9.4mg/L in 1380 min, whereas MLSS and MLVSS increased exponentially from 918 and 671 mg/L to 1055 and 778 mg/L, respectively. The Y, and b values obtained from the n-versus-q plots for the two reactors together with the typical and range values obtained from Metcalf & Eddy^ are summarized in Table 2 for comparison. The Y* values for the two reactors have been adjusted by a BOD/DOC ratio of 2.5 observed for the PE wastewater in this study. The results appear to be reasonable. The decay rates were a little bit high, reflecting the high endogenous respiration rate of the cells in the experiments. An important point observed is that adding 1% leachate by volume to the PE wastewater increased the cell yield, Y,, and the decay rate, b The average net growth rate, u, of the cells in Reactor 2 at 1% leachate was 0.022

g VSS/g MLVSS-d. This is higher than that of 0.013 g VSS/g MLVSS-d for

Reactor 1 without leachate addition. This increase in net cell growth supports the findings in the plant data analysis,^ i.e., a less than 2% leachate addition to the FAPCC STP inflow would enhance cell yield and sludge production. This also explains why the sludge processing unit at the STP was overloaded.

1100 1050 1000 _

950 g -MLSS - MLVSS - -DOC 900 | 850 2

800 $ •750 700

650 200 400 600 800 1000 1200 1400 Time (minutes)

Figure 3: Typical DOC, MLSS and MLVSS results for Reactor 1.

Table 2: Cell yield and decay rates.

Parameter (unit) Reactor 1 Reactor 2 Range^ Typical^

Y; (g VSS/g BOD,) 0.68 0.84 0.4-0.8 0.6

b (d ') 0.10 0.14 0.025 - 0.075 006

Figure 4 shows typical SOUR data for each reactor over a 24-h period It can be seen that SOUR in R2 (with 5% leachate) was higher than that in Rl (with no

200 400 600 800 1000 1200 1400 1600 Time (min) Figure 4: Specific oxygen uptake rates for reactor 1 and reactor 2.

leachate). The areas under these curves yield oxygen uptake rates of 1086 and 1166 mg 0%/g MLVSS-d for Rl and R2, respectively. This indicates that the presence of 5% leachate in the wastewater feed may increase the microbial metabolic rate. Further spot checks on SOUR showed that leachate additions of 20% and 40% of the wastewater feed reduced SOURs by about 20% and 62%, respectively.^ This demonstrates the inhibitory effect of large amounts of leachate in sewage on microbial activities, especially in the form of shock loads.

Conclusion

The FRSWC landfill leachate is approaching a mature age (BOD/COD = 0.56). Concentrations of BOD,, COD, DOC, SO/, CN , Mn, Hg, Zn, phenol and toluene in the leachate exceeded the inhibitory limits for activated sludge treatment.

However, toxicity became negligible after partial treatment on site and dilution by sewage before co-treatment at the FAPCC STP. From batch reactor studies, the microbial yield, decay rate and net growth rate were all higher for a 1% leachate-primary effluent (PE) feed than for a PE feed (Y,

= 2.1 and 1.7 g VSS/g DOC, b - 0.14 and 0.10 d\ and n = 0.022 and 0.013 g VSS/g MLVSS-d, respectively). A 5% leachate-PE feed yielded a higher SOUR than a PE feed (1166 and 1086 mg O/g MLVSS-d, respectively). With 20% and 40% leachate additions to the PE feeds, SOUR decreased by 20% and 62%, respectively. The increased sludge production at the FAPCC STP was due to the acceptably small amount (<2%) of leachate addition to the sewage flow for co-treatment A larger sludge processing unit was required. High shock loads of leachate should be avoided for better operation and control of the treatment plant.

Acknowledgment

This study was supported financially by a research grant sponsored by the Fredericton Region Solid Waste Commission, Fredericton, N.B., Canada.

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