Sediment Oxygen Demand of a Leachate Pond at an Offshore Municipal Solid Waste Disposal Site 2 Years after the Site Was Closed

Various water quality parameters of a leachate pond at an offshore municipal solid waste disposal site were monitored. The pH, dissolved oxygen (DO), and water temperature at the bottom of the leachate pond were measured during Sep. (the 1st period) and Nov.-Dec. (the 2nd period) of 2011. The results suggested that the stratification of water temperature in the pond had gradually broken down due to convection occurring between the end of the 1st period and the 2nd period. The pH was almost constant at 10 11 during the 1st period and was approximately 11.5 during the 2nd period. The DO was almost zero during both periods. An anaerobic batch experiment with sampled sediment was undertaken to elucidate the mechanism of material leaching from the sediment. DO decreased under all experimental conditions. With respect to oxidation reduction potential (ORP) and total sulfide in addition to DO, the condition most closely mimicking that of the site became the most anaerobic. The average sediment oxygen demand, SODave, was calculated using a brief numerical model based on batch experiment data. The SODave was 1114.7 mg/m/d, indicating that at least 434 g/d of oxygen must be supplied to the leachate pond to maintain the DO.


Introduction
In Japan, approximately 80% of municipal solid waste (MSW) is incinerated, both to reduce its volume and for sanitary treatment [1].Particularly in urban areas, finding new sites for final disposal of wastes has become increasingly dif-ficult.It is thus necessary to develop strategies to manage MSW disposal sites in a more environmentally safe manner and to promote the reduction, reuse, and recycling of wastes at both the technological and legal levels.
Leachate discharged from a MSW landfill must be treated appropriately in accordance with its quality and volume [2].Urban residents living near disposal sites may experience some unpleasantness due to odors associated with the landfill as well as potential health hazards due to gases emitted from the landfill site [3].In addition, the color of the leachate can be worrying to residents [2].Complaints regarding leachate color and/or hydrogen sulfide odors can be lessened by careful management of leachate pond water quality.It is therefore necessary to consider measures that will prevent deterioration of pond water quality.
For the purpose of generating data that would aid in development of appropriate methods for controlling the water quality in leachate ponds, the authors investigated a leachate pond at an offshore disposal site 2 years after the site was closed.Pond water and sediment were sampled and analyzed in 2011 (9 Aug.), and the water mass balance in the leachate pond was also examined [4].At offshore disposal sites, most of the waste is submerged under seawater, and the velocity of water percolating through the waste layer is very slow [5].Thus, water quality in the leachate pond may deteriorate significantly due to high nutrient and heavy metal levels, especially after disposal is completed, necessitating long-term management of sea area disposal sites [6] [7] [8].
In order to obtain detailed water quality data for the offshore MSW site leachate pond, the authors measured the pH, DO, and water temperature at the bottom of the leachate pond over an approximately 1-month period in September and November-December of 2011.Anaerobic batch experiments using sediment sampled on 9 Aug. were then conducted in order to characterize the elution of various materials from the leachate pond sediment.Based on DO data, sediment oxygen demand was calculated for use in developing a strategy to improve the water quality of the leachate pond.

Correlation between Various Parameters in the
Water-Sediment of the Leachate Pond [4] An overview of the targeted offshore MSW disposal site in Hiroshima Prefecture, Japan, is shown in Table 1.This disposal site is a semi-aerobic type with a final cover soil after closing and has a structure to prevent the percolation of precipitation water.Schematic of the pond structure, and photo taken in 2011 of the leachate pond is shown in Figure 1.
The leachate pond was anaerobic type without stirring.Leachate from the disposal site is collected in a leachate pond and sent to a wastewater treatment facility for treatment by coagulation-sedimentation, neutralization, and sand filtration.Particularly in summer, there is an offensive odor associated with the site, mainly due to hydrogen sulfide gas, and there are many chironomids in    Water and sediment in the leachate pond were sampled on 9 Aug., 2011.
Samples were taken to the laboratory and kept frozen until analysis.They were thawed at every analysis and kept with the refrigerator until analysis.In this study, we analyzed pH (HM-50G, TOA DKK Corp.), EC (CM-30G, TOA DKK Corp.), SS, COD Mn (KMnO 4 titration un der acidic condition),  SO − (Hach 8051), Cl − (AgNO 3 titration), Ca, total Cr (Total Cr), Mn, Fe, Cu, Zn, Cd, and Pb using an ICP quantometer (SPS7800, Seiko Instruments, Inc.) after passing through a 1 µm membrane filter in the pond water.Sediment samples were dried at 100˚C, and ignition loss (IL) was measured by heating at 800˚C for 2 h in an electric furnace (FUL230FA, Advantec Inc.).Element contents of Pb, Cd, Zn, Cu, Fe, total Cr, Mn and Ca were analyzed by ICP after digestion in nitric and perchloric acid.Total sulfide in each sediment sample was analyzed by the gas detector tube (Hedoro-tec S, Gastec Corp.).These results have been reported [4].It was thought that various chemical and microbial reactions would occur under conditions of high alkalinity and high salt concentrations in the water and sediment of the leachate pond [9] [10].
Thus, in this study, correlation coefficients for various parameters in both samples were calculated to examine this possibility qualitatively.

Continuous Measurement of pH, DO, and Water Temperature in the Leachate Pond
To examine the water quality of the leachate pond, continuous in situ measurements were made at the offshore landfill between 8-29 Sept., 2011 (1st period) and 20 Nov.-21 Dec., 2011 (2nd period).Data regarding pH, DO, and water temperature were collected using a pH and DO meter equipped with a data logger (DM-32P, TOA DKK Corp.).The meter was placed above the pond sediment.Measurements were taken every hour, and the data were collected after each measurement period.

Batch Experiments with Sediment to Estimate Pond Water Characteristics
For the purpose of estimating the pollution mechanism particularly in summer, sediment oxygen demand was determined in batch experiments with sediment sampled from the leachate pond.Sediment was sampled on 9 Aug, 2011 and then air dried at 50˚C.Leachate pond water sampled at the same time or pure water was used as a solvent.The properties of the sediment and leachate water are shown in Table 2.The Fe and Ca content in the sediment were high.The water was highly alkaline (pH 9.5) and contained high concentrations of Cl − and SO − .The particle size distribution of the sediment by sieving and a dynamic light scattering particle size analyzer (LB-550, Horiba Ltd.) is shown in Figure 2.
The 50% cumulative passing diameter (d 50 ) was 0.024 mm, and visual inspection revealed a considerable amount of silt and slag.The number of sulfate-reducing bacteria in the sediment (7.1 × 10 5 cells/g-dry sediment) was determined using the MPN protocol with modified ISA medium [11].
For batch experiments, sediment and solvent were placed in a polyethylene bottle (50 mm diameter, approximately 65 mm height) with an inner cap, and the bottle was then sealed and thoroughly mixed.The experimental conditions are shown in Table 3.For each experimental run, some preliminary trials were first performed to decide experimental conditions.A total of 11 undisturbed sediment cores were sampled on 9 Aug., 2011 using an acrylic pipe (44 mm inside diameter, approximately 1,000 mm height).From the relationship between the water depth in the leachate pond and the dry weight of sampled sediment [4], the liquid/solid ratio [L/S] ranged from 5 to 10.The bottles were placed in an incubator maintained at either 20˚C or 30˚C.The conditions for run 4, which most closely matched the conditions of the site, were leachate water used as the solvent, L/S 5, and incubation at 30˚C.
At the time the polyethylene bottle was removed from the incubator, the pH, EC, ORP (D-72, Horiba, Ltd.), and DO were measured immediately.Water was filtered using a glass-fiber filter with a pore size of 0.45 µm and then analyzed for   SO − , Ca, T-Cr, Mn, Fe, Cu, Zn, Cd, Pb, and Cl − .Sediment was collected from the bottle and kept frozen until analysis.After thawing, the water content of the sediment was measured, and T-S was analyzed.The experiment was conducted in the shade.Results are reported as the mean of 3 samples collected at each sampling time.NH + , Ca, and Fe in pond water parameters.In the pond water, COD, Ca, and pH were correlated more with sediment parameters.

Results and Discussion
The pH of the pond water was not correlated with any other water parameter, but it was correlated with IL, Ca, Fe, and Zn in the sediment.This indicates that the pH in the pond water is influenced by elution of Ca from the sediment and by the inflow of fresh leachate from the landfilled layer.
With respect to organic substances, the sediment IL, which was 7.2% -12.1% in each sample, was correlated only with pond water pH.Pond water COD was correlated with Ca, T-Cr, Mn, Cu, and Zn but was not correlated with any other water parameter.As chlorophyll-a was not detected in any of the samples, the pond water COD was thought to be derived from landfilled waste that was not incinerated.Especially in summer, it was anticipated that the DO would be almost completely consumed and that the sulfate-reducing activity would be high, SO − were negatively correlated (r = −0.87).Of the heavy metals identified in the pond water (Cr(VI), Mn, Fe, Cu, Zn, Cd, and Pb), none of them exceeded the effluent standard in Japan.

Continuous Measurement of pH, DO, and Water Temperature in the Leachate Pond
Figure 4 shows the results of the continuous measurement of pH, DO, and water temperature above the sediment of the leachate pond.Lost data between 23 Nov and 13 Dec in the 2nd period are indicated by dashed lines.The daily average temperature in Hiroshima City is shown for the same periods [12].Although the patterns of change in the pond water temperature and the temperature in Hiroshima City were similar during the 1st period, the water temperature was about 2˚C -10˚C higher than the temperature during the 2nd period.These data suggest that the temperature stratification in the water in the leachate pond had gradually broken down as a result of convection beginning in November.The pH during the 1st period was approximately 10 -11, and during the 2nd period, the pH was nearly constant at about 11.5.The pH was high because about 70% of the land filled waste was incineration residues.
The DO was almost zero during both the 1st and 2nd periods.During the 1st period, the pond water temperature was about 20-27˚C, and there was an odor, due primarily to hydrogen sulfide.In addition, the Fe content of the sediment was high (2.55%) and the color of the pond water seemed to be reddish-brown due to the presence of Fe(II).The absence of DO was attributed primarily to consumption by microorganisms.During the 2nd period, the water temperature was about 13˚C -21˚C, and there was no smell around the pond.The color of the pond water was blackish-brown.It was suggested that DO supplied to the bottom of the pond by convection was consumed by chemical oxidation of Fe(II) and sulfide generated in the summer rather than by microorganisms.COD Mn rose gradually and then became constant.As for the amount of increase in COD Mn over the experimental period, it was largest in run 3, at 131.1 mg/L, followed by run 4 (in which the condition most closely mimicked those of the during the experimental period, it was greatest in run 3 (pure water, L/S 5, 30˚C), at 6.1 mg/L, followed by run 4 (which most closely mimicked the site conditions), at 3.9 mg/L, a trend similar to COD Mn .

Batch Experiment with Sediment
The amount of increase in T-S was the greatest in run 4 (5.23 mg/L).This was thought to be due to the activity of sulfate-reducing bacteria following the decreases in DO and ORP.Run 3, which used pure water as the solvent, exhibited a 2.23 mg/L increase in T-S, which was attributed to the reduction of residual SO − in the sediment.In run 2, which used leachate water as the solvent and incubation at 20˚C, the T-S concentration rose 2.05 mg/L.
The optimum temperature for many sulfate-reducing bacteria is thought to be 25˚C -30˚C [13].Run 2 appeared to provide a micro-environment suitable for the growth of sulfate-reducing bacteria, even though some nitrate acid remained and the surrounding temperature was 20˚C.Other studies have reported that sulfate-reducing activity occurs at a 2 4
From the results of the batch experiment, sediment oxygen demand was calculated using a brief numerical model [16].When the oxygen consumption rate is in proportion to the DO concentration, the following equation holds: Solving Equation (1) under the initial condition (C = C 0 at t = 0), the following equation is obtained: From curve fitting of the batch experiment DO concentration data using Equation (2) (see Figure 5), k values of 2.70, 1.01, 4.32, and 1.23 L/kg/d were obtained for runs 1, 2, 3, and 4, respectively.
The average oxygen demand rate per unit area (designated average sediment oxygen demand [SOD ave ] in this study) [17] was calculated using Equation (3): where SOD ave is expressed as mg/m 2 /d, T represents the period during which the DO was almost zero (days), and S represents the sectional area of the polyethylene bottle (m 2 ).
When calculating the SOD ave using the results of run 4 (the conditions of which most closely matched the conditions of the site) and T = 3 days, a value of 1114.7 mg/m 2 /d was obtained.The area of the leachate pond was 389.5 m 2 (see Table 1), and the mean water volume of the pond was 1185 m 3 , with a range of 800 to 2000 m 3 after the disposal site was closed [4].It was concluded that an oxygen demand of approximately 434 g/d would be required to maintain DO in the pond in the summer.
Factors influencing leachate pond water quality include the composition of the landfilled waste, the time elapsed after landfilling, the landfilling method, chemical and microbial reactions occurring in the pond, and weather conditions, such as temperature and precipitation [18].To prevent deterioration of the pond water quality, it is necessary to consider measures such as dredging [19] in addition to supplying oxygen by aeration in relation with the total cost [20] [21] [22] [23].These measures must lead to a decrease in the load on leachate treatment equipment.

Conclusions
In this study, the pH, DO, and water temperature at the bottom of a leachate pond at an offshore MSW disposal site were measured in order to better understand factors affecting pond water quality following site closure.An anaerobic batch experiment using sampled sediment was also conducted to characterize the elution of materials from the sediment.The sediment oxygen demand was calculated from the resulting data for use in developing methods to improve the quality of leachate pond water.The results of this study are as follows:

Figure 1 .
Figure 1.Condition of the leachate accumulation pond: (a) schematic of the pond structure, and (b) photo [4].
mm) Journal of Geoscience and Environment Protection

3. 1 .Figure 3
Figure 3 shows the correlation between various parameters examined in the water and sediment samples collected on 9 Aug, and only correlations satisfying are shown in the figure.Positive correlations are shown by solid lines, and negative correlations are indicated by dashed lines.In the sediment, Fe was strongly correlated with pond water parameters (pH, EC,

Figure 3 .IL
Figure 3. Correlation between various parameters in the water-sediment of the leachate pond.Only correlation coefficients between each parameter (r) satisfying 0.8 r ≥ are shown.Positive and negative correlations are shown by solid lines and dashed lines, respectively.

Figure 4 .
Figure 4. Results of continuous measurements in the leachate pond.(a) Water temperature in the leachate pond.The daily average of the temperature of Hiroshima City is shown for the same time period [12]; (b) pH and DO.Note that the broken line between 23 Nov. and 13 Dec. in the 2nd period indicates data loss.

Figure 5 .
Figure 5. DO, ORP, pH, and EC results for the batch experiments.Lines in the DO graph show calculated data from curve fitting of the batch experiment DO concentration data using equation (2).The rate constant of oxygen consumption (L/kg/d), k, were 2.70, 1.01, 4.32, and 1.23 L/kg/d for runs 1, 2, 3, and 4, respectively.

Figure 6
Figure6shows the results of COD Mn ,

Run 1 (
pure water, 20℃) Run 2 (leachate, 20℃) Run 3 (pure water, 30℃) Run 4 (leachate, 30℃) the DO concentration (mg/L), t represents time from the start of the experiment (days), V represents solvent volume (0.1 L), m represents the weight of sediment used in the experiment (kg-dry), and k represents the rate constant of oxygen consumption (L/kg/d).