Faron Anslow, Department of Chemistry, Oregon State University, Corvallis, OR 97331-4003.
In partial fulfillment for the special project component of Experimental Chemistry II, CH 461 Fall term 1998

Leachate is a major environmental pollution concern of sanitary landfills. An analysis was done on the effectiveness of a dual stage leachate treatment site using six pass forward osmosis in the first stage and reverse osmosis in the second stage. Samples of raw leachate, first pass leachate, sixth pass leachate, and treated permeate were taken. ICP Atomic Emission Spectrophotometry was used to measure the concentrations of As, Se, Zn, Pb, Fe, Cr, and Cu in the samples. The permeate had As, Pb, Cu, and Cr levels below the detection limit. The concentrations of the other metals were 0.009±0.01ppm for Se, 0.004±0.002ppm for Zn, 0.004±0.002ppm for Fe, and 0.003 for Cu. The raw leachate had the following metal concentrations: 0.059±0.008ppm for As, 0.02±0.01ppm for Se, 0.250±0.008ppm for Zn, 4.7±0.5ppm for Fe, 0.091±0.005ppm for Cr, and 0.056±0.003ppm for Cu. Lead was detected in the raw leachate at levels very close to the instrument detection limit. The leachate concentration ratio of between the first and sixth forward osmosis pass is 3.7± 1.3. These data show that the treatment facility is effective at removing the measured metals from the leachate.

All of the municipal solid waste generated by Corvallis, OR is sent about 5mi N. of town to the Coffin Butte Landfill. The Corvallis region experiences 42 inches of rain annually with the bulk of that precipitation falling between the months of November and May (Oregon Climate Service 1998). The rain that falls on the landfill waste percolates through the debris collecting metal, inorganic, and organic contaminants. This contaminated water is termed leachate and if left untreated would be a serious environmental hazard. Federal law requires that this leachate be prevented from entering subsurface water supplies without treatment (Quasim 1994). The leachate collected at Coffin Butte is purified by a dual stage treatment plant utilizing both forward and reverse osmosis.

The first step in the leachate treatment is acidification. This step brings metal precipitates and other solids into solution. The acidified leachate is then passed through a six step forward osmosis stage. In each stage the water in the leachate is osmotically drawn from the leachate into a 6% solution of NaCl. This process concentrates the leachate from the raw state to the final concentrated state. The high concentration leachate is then mixed into portland cement and returned to the land fill. The water that has been extracted from the leachate into the 6% NaCl solution is then sent to the reverse osmosis stage. The second, final stage of purification is accomplished by three passes through reverse osmosis purifiers to extract the purified water from the concentrated saline solution. Reverse osmosis is based on the same principles as forward osmosis except the water is forced, under pressure, to pass through a semipermeable membrane from the high concentration NaCl solution into the low concentration pure water. The concentrated saline solution is then recycled and used again for the forward osmosis step. The pure water, or permeate, is pumped to a holding pond where it is oxygenated and then used to irrigate a tree farm.

To determine the effectiveness of the leachate treatment system samples from four points along the treatment line were analyzed. Samples of raw leachate, leachate after it has passed through the first forward osmosis step, leachate after it has passed through all six steps of the forward osmosis stage, and permeate after full treatment were drawn. It is important that the discharged permeate have toxic metal concentrations below those set by the EPA and the Clean Water Act. The metal content of the samples were analyzed with an inductively coupled argon plasma atomic emission spectrophotometer (ICP-AE). As, Se, Pb, Zn, Fe, Cr, and Cu were measured in this experiment. As, Se, Pb, and Cr were analyzed due to their potential toxicity; while the metals Zn, Cu, and Fe were measured to further characterize the samples.

As, Se, Pb, and Cr are toxic metals, so their release into the environment is restricted by the federal government as outlined in table 6. Arsenic is most commonly used to manufacture agricultural products such as pesticides, herbicides, and preservatives. Arsenic has been shown to increase the rates of various cancers in humans and is hazardous in drinking water concentrations higher than 10ppb. Typical background drinking water levels are 2.4ppb. Arsenic is most toxic in its +5 oxidation state (Eisler 1988). Since only total arsenic was determined, no conclusions can be drawn its toxicity in the leachate. Selenium is interesting in that it is both an essential nutritional metal to humans and other mammals while being toxic at high doses. Humans require 22-220mg/day of selenium in their diet. If drinking water is the sole source of selenium then the concentration should be no more than 0.5ppm (Eisler 1985). Global average Se concentrations in drinking water are 0.12-0.44ppb. Chromium highly toxic in its +6 state. In this form it is carcinogenic to humans and other mammals. Chromium is present in drinking water with total Cr concentrations of less than 8ppb (Eisler 1986).

Sample Collection

Four 150ml wide mouth bottles were washed with 80ml of 8% HNO₃ and allowed to soak for 15 minutes. These bottles were then allowed to dry completely. Four samples were collected in these bottles at the Coffin Butte Leachate Treatment Plant. The following samples were taken

~140ml of acidified raw leachate

~110ml of post first forward osmosis step leachate

~130ml of post sixth forward osmosis step leachate (full conc. leachate)

~140ml of fully treated permeate

The samples were stored tightly capped at room temperature.

Acidification

The samples were acidified to dissolve possible metal containing solids and precipitates. The acidification was also necessary to match the pH of the standard solutions. All acidification steps were performed under a ventilation hood. The three leachate samples were split in half. One of each of the three leachate samples was filtered through Whatman #1 qualitative filter paper. While the remaining three were left unfiltered. Seven 25ml volumetric flasks were rinsed three times with 5ml aliquots of 2% HNO₃. The following procedure was repeated for each of the seven samples. The volumetric flask was rinsed twice with the leachate to be contained. 1ml of 70% HNO₃ was added to the volumetric flask. The solution was brought to volume with leachate giving a final nitric acid concentration of 2.8%. These acidified solutions were capped and stored at 25°C. The higher concentration leachate solutions formed a precipitate upon acidification. Immediately Prior to measurement all samples were filtered to remove precipitate formed during the acidification step. Eight small, ~20ml, test tubes were rinsed three times with 5ml aliquots of 2% HNO₃. Immediately prior to measurement the solutions were filtered through Whatman #1 qualitative filter paper. The test tubes were rinsed twice with the incoming filtrate before collection for measurement. After about 10ml of filtrate was required for the measurement.

Measurement

Measurement of the metal concentrations was done with the ICP-AE under the following instrument parameters.

ICP instrument parameters Table 1

The specific method used to measure the different metals is attached to this report. The wavelength at which the elements were measured was chosen to optimize the instrument response, detection limit, and the multi element interference characteristics of the metal being measured. As was measured on two different wavelengths to minimize potential interference with other metals that may be in the leachate. Multi element standards of 0.5ppm, 0.25ppm, 0.05ppm, and pure 2% HNO₃ were used to create the calibration curves for the individual metals. The multi element standards had equal concentrations of Ca, Cu, Fe, Mg, Zn, As, Be, Co, Cr, Li, Mn, Mo, Ni, Pb, Sb, Se, Sr, Ti, Tl, and V. After calibration the seven solutions were measured proceeding from low concentration too higher concentration. Each element was measured three times with a 3 second integration time. Finally, a blank of 2% HNO₃ was measure ten consecutive times for each element to determine the detection limit of the instrument for the measured metals.

The instrument calibration yielded the following calibration curve data in table 2.

After the ICP-AE was calibrated the leachate samples were measured. The results are given in table 3. Table 4 lists the metal concentrations of the permeate. Note: all reported uncertainties are 95% confidence intervals.

The detection limit of the instrument was determined yielding the results in table 5.

 Detection Limits Table 5

Of primary concern is the effectiveness of the treatment facility at removing the metals analyzed in this experiment. It is also interesting to draw conclusions about the leachate treatment process and the experimental procedure.

Based on the permeate data in table 4 compared to the raw leachate data in table 3 one can see that the treatment facility is effective in reducing the metal concentrations in the leachate to environmentally acceptable levels. The concentrations of all metals studied have been reduced significantly. Arsenic measured at 189.042nm has been reduced below the detection limit of the instrument; while at 197.262nm arsenic was detected, but the confidence interval does not allow conclusive determination of its presence. Both Pb and Cr concentrations have been reduced below the detection limit as well. The concentrations of Se, Zn, Fe, and Cu have been reduced below the acceptable federal water quality standards for the analyzed metals, which are outlined in table 6. The permeate discharged from the treatment facility could therefore be safely discharged assuming the concentration reduction of these metals is a good indicator of the concentration reduction of other metals.

At the cost of generating clean water from the raw leachate a more concentrated effluent is generated. The concentrated leachate metal concentrations are 3.7± 1.3 times greater on average than the concentrations in raw leachate as shown in table 7. The average concentration ratio was calculated using only the data from the unfiltered measurements. These data were chosen over the filtered data because of their greater apparent validity, which will be discussed later. This concentrated leachate is mixed into portland cement at the treatment site and then disposed of in the adjacent landfill.

 Concentration Ratios between Concentrated and Raw Leachate Table7

In addition to characterizing the effectiveness of the treatment plant, the results of this experiment lend insight to possible procedural improvements. A quite apparent discrepancy in the data lies in the difference between the data that were filtered before acidification and those that weren't. Those samples that were filtered prior to acidification showed a reduction in metal concentration compared to the unfiltered samples as shown in table 8.

b refers to unfiltered minus filtered

The difference between the two acidification procedures leads me to believe that a more thorough acidification or digestion would appropriate for samples of this type to achieve more accurate results.

Another point that is interesting to note is the difference found between the measurements of As at the two different wavelengths. The concentration of As at 197.262nm was 1.30± 0.04 times greater than that at 189.042nm. In trial runs of 0.05ppm standard this factor was not encountered leading to the belief of the author that the discrepancy is due to interfering species in the leachate samples. Due to this interference it can’t be said which of the two As wavelengths yielded the most accurate measurements.

During the course of this experiment several items were uncovered that would be interesting to investigate further. One would be to analyze the runoff from the tree farm on which the permeate is sprayed and compare its composition to that of the permeate. Another would be to analyze background levels of the analyzed metals in uncontaminated runoff from the area to compare with the composition of the leachate. In addition to these I think it would be valuable to analyze the content of all metals in both the leachate and permeate yielding a more complete characterization and to conclusively determine the treatment effectiveness.