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Phenol removal by soil adsorption with activated sludge addition.

With the increasing public awareness of environmental pollution, the disposal of hazardous wastes is a major concern for the environmental professional. The most commonly used disposal techniques involve land-based options such as landfills, land treatment and surface impoundments. Since there are no completely environmentally safe ways of disposing of hazardous wastes, the removal of toxic organics by soils needs to be examined. With respect to operation and maintenance costs, the land treatment method is considered a low cost treatment for municipal and industrial wastewater. It is a suitable option for rural areas where land space is available.

There are several factors that affect the removal of toxic organics by soils or land treatment methods. The most important one that affected the disappearance rate of toxic organics was the chemical structure. Alexander and Lustigman (1) and Baker and Mayfield (2) reported that chemical structure had a notable effect on microbial decomposition of chlorophenols and other selected pesticides. Other factors include soil characteristics and environmental conditions: number of soil microorganisms, soil microbial populations, soil pH, soil texture, temperature and nutrient availability. The removal of phenolic compound in a well-characterized fine sandy loam has been examined by Namkoong, et al. (3).

For this paper, phenol was selected as a type of hazardous waste and was subjected to soil treatment such as the in-situ bio-remediation process. The objectives of this study were to examine the removal efficiency of phenol from phenol-containing wastewater using different types of soils, and to evaluate the effect of activated sludge addition into soil on the removal of phenol from the wastewater.

Methods and procedures

Four types of soils--loam, sandy loam, silty loam and silty-clay loam--were used in this investigation. They were collected within the Cleveland metropolitan area. The characteristics and distribution of these soils were reported by Musgrave and Holloran (4). The location of sample collection and the description of these soils are summarized in Table 1. All of these soils are of loam type with variation in sand, silt and clay contents. A loam by definition is "a soil material which is seven to 27 percent clay particles, 28 to 50 percent silt particles, and less than 52 percent sand particles" (4). A sand particle varies from 2 to 1/64 mm; a silt particle from 1/64 to 1/256 mm; and a clay particle is smaller than 1/256 mm. The sand and silt are composed mainly of quartz and the clays are composed of clay minerals such as kaolinite, montmorillonite, illite and vermiculite. The clays are generally formed from the chemical weathering of feldspars.

The soil samples were taken from a depth between 10 and 20 inches and were stored in Ziplock bags. They were air dried TABULAR DATA OMITTED for one to two days before being used for the experiment. The synthetic phenol wastewater was prepared from phenol and tap water. Tap water was dechlorinated by letting it sit in an open container for about two days. A predetermined concentration of total organic carbon (TOC) of phenol was used in this experiment.

The parameters used for the experiment were as follows:

* four different types of soils;

* volume ratios of soil/wastewater including 1:1, 1:2 and 1:5;

* treatment time intervals of one, two, three, five and seven days; and

* variable amounts of activated sludge added to the wastewater. A series of test tubes was assembled with the designed soil to wastewater ratios of 1:1, 1:2 and 1:5 for one, two, three, five and seven day treatment intervals. The test tubes were sealed with cork stoppers to allow passage of air into the test tubes in order to maintain an aerobic condition for effective biological treatment.

At the beginning of the experiment (during the first day), the samples were shaken for a few minutes and then were laid at a 15 degree angle until the next shaking. Starting the second day, twice a day the samples were agitated to renew the surface of contact between soil and wastewater. After each treatment time interval, the cork stoppers were removed and the samples were filtered and acidified to inhibit bio-oxidation. All samples were stored in a freezer prior to analysis. All filtered samples were diluted and tested for TOC concentration. For this paper, TOC was used as an indication of phenol concentration in the wastewater due to its rapid determination. Since the theoretical phenol of TOC ratio is 1 mg/l phenol:0.766 mg/l TOC, the phenol concentration can be calculated if TOC of phenol wastewater is known.

Results and discussion

Soil adsorption treatment -- Figure 1 shows TOC of wastewater as a function of treatment time for four types of soils. The soil to wastewater treatment ratio was kept at 1:1. Measurements of TOC indicate the relative level of the phenol in the wastewater. It was found that the TOC decreased sharply with increasing treatment time for loam, silty loam and silty-clay loam up to two days, then remained relatively constant at the longer treatment time intervals. The sandy loam did not decrease the TOC levels significantly. The removal efficiency of TOC ranged from 12 to 50 percent with the loam, silty loam and silty-clay loam. The silty-clay loam also appeared to be the best and the sandy loam the worst adsorbent. Similar variation trends were also noted in Figure 2, in which the soil to wastewater ratio was kept at 1:2. It was noted that the TOC showed an increase after one day of treatment for loam soil, which may be due to desorption of phenol. When the soil/wastewater ratio was 1:5 as shown in Figure 3, the silty-clay loam showed the greatest overall reduction of TOC levels. The TOC variation trends with treatment time appeared to be similar between silty loam and silty-clay loam, but the silty loam had a higher concentration of TOC.

Figures 4 and 5 depict concentration of TOC and percent of TOC removal versus types of soils. The soil/wastewater ratio was at 1:2 and the original concentration of TOC in the wastewater was controlled at 1160 mg/l, which was chosen because it more or less represents the upper limit for aerobic treatment. Five treatment time intervals--one, two, three, five and seven days--were shown in the figures for comparison. Concentration of TOC in the wastewater was generally reduced after the soil adsorption treatment. Loam, silty loam and silty-clay loam were quite successful in removing TOC or phenol from the wastewater; removal efficiency ranged from 30 to 55 percent. This is probably due to the smaller particle size of these types of soil with a larger surface area for adsorption of TABULAR DATA OMITTED phenol. The best removal efficiency seemed to occur at one day or two days of treatment time.

Sandy loam was relatively ineffective for phenol removal in this treatment experiment because of the larger particle size of this type of soil; it has a smaller surface area per unit volume available for adsorption for phenol and for bacteria biofilm attachment. These resulted in lower physico-chemical adsorption as well as lower biological oxidation performance. The percentage of sand contained in each type of soil has an important effect on the adsorption capacity of soil for phenol. Among the four types of soils tested, the percentage of sand in loam, silty loam and silty-clay loam was lower than that in sandy loam and thus had a better treatment efficiency.

Treatment by soil with activated sludge addition -- Figures 6, 7 and 8 show the TOC concentration in wastewater versus treatment time for the soil/wastewater ratios of 1:1, 1:2 and 1:5, respectively. Activated sludge was added to the wastewater at 10 percent of wastewater volume. The mixed liquor volatile suspended solids (MLVSS) was 3000 mg/l, and the TOC of filtered activated sludge sample was 15 mg/l. The TOC concentration reflected the phenol content in the wastewater. The original concentration of phenol and activated sludge was controlled to a TOC concentration of 3310 mg/l. All four types of soils were again used in this part of the experiment. It is noted that the TOC concentration decreased sharply after one day of treatment time for both silty loam and silty-clay loam, then remained relatively unchanged at longer treatment time intervals. The effluent TOC versus treatment time curves showed a similar trend for soil treatment systems using silty loam and silty-clay loam at (soil)/(wastewater + activated sludge) ratio of 1:1 and 1:2. The effect of (soil)/(wastewater + activated sludge) ratio on the TOC removal was observed. As the ratio increased, the TOC removal efficiency improved. This implies that at higher soil concentration in the mixture, the TOC removal by soil adsorption also increased.

The sandy loam did not reduce much of the phenol even with the addition of activated sludge. In general, at a higher soil concentration the larger surface area of soil particles will provide a larger surface area for bacteria biofilm attachment which will improve biological treatment efficiency of phenol removal. Silty loam and silty-clay loam were quite effective in phenol reduction with activated sludge addition. The addition of activated sludge enhanced the bio-oxidation and removal of phenol from wastewater. Their range of TOC removal efficiency was from 54 to 73 percent, compared to 30 to 55 percent for the soil treatment system without activated sludge addition. This additional 18 to 24 percent increase in TOC removal efficiency is attributed to bio-oxidation due to activated sludge addition. This implies that the bioaugmentation process using bacteria obtained from the activated sludge collected from the aeration tank of the municipal wastewater treatment plant is effective in improving treatment performance of the soil treatment system. The other advantage of using the activated sludge for bioaugmentation process for soil treatment system is the low cost of activated sludge as compared to commercially available bacterial culture products. Land treatment with bioaugmentation is an inexpensive way to removal phenol from phenol-containing wastewater. After the treatment, a complete phenol removal could be expected. The final products of bio-oxidation are carbon dioxide and water.

Figures 9 and 10 show TOC concentration and TOC removal efficiency versus types of soils. The (soil)/(wastewater + activated sludge) ratio was at 1:1. The initial concentration of TOC in the filtered sample of soil, wastewater and activated sludge mixture was 3310 mg/l. The TOC of filtered activated sludge was 15 mg/l, which consists of less than 0.5 percent of the 3310 mg/l. Both silty loam and silty-clay loam were highly effective in removal phenol from the wastewater with their removal efficiency ranging from 54 to 70 percent. The effect of treatment time seemed to be insignificant with the activated sludge addition treatment. Sandy loam had the worst performance in removing phenol from the wastewater.

The good performance of phenol removal by silty-clay loam is probably due to a higher proportion of clay particles and the larger surface area contained in this type of soil. Clays could be quite effective in removing TOC from the wastewater, as reported by Lo and Hung (5). In comparing Figure 1 with Figure 6, the effect of activated sludge addition to the soil and water mixture became apparent. The TOC removal efficiency for soil water mixture using silty loam was about 40 percent for the mixture without activated sludge addition, and was about 63 percent for mixture with activated sludge addition. The increase in 23 percent of TOC or phenol removal efficiency is due to the biological removal of phenol by microorganisms present in activated sludge. The addition of activated sludge to the soil is considered a type of bioaugmentation which can be used for in-situ bio-remediation. This treatment process can also be used for the treatment of other hazardous wastes.

Conclusions

* The soil adsorption treatment of wastewater in phenol removal appeared to be effective for loam, silty loam and silty-clay loam. Their phenol removal efficiency ranged from 30 to 55 percent.

* For the treatment using soil with addition of activated sludge, the silty loam and silty-clay loam were quite successful in decreasing phenol content of the wastewater. The phenol removal efficiency ranged from 54 to 73 percent, an increase of 18 to 24 percent over the soil alone.

* The soils with a better removal efficiency of phenol were the ones which consisted of a high proportion of fine particles such as silt and clay. This is due to a larger surface area for fine particles for physico-chemical adsorption and biofilm growth and attachment.

* The treatment with addition of activated sludge to soil had a better removal efficiency of phenol because the addition of activated sludge provides necessary biomass for biological oxidation of phenol in the soil treatment system. This treatment process can be used for the treatment of other hazardous wastes.

References

1. Alexander, M. and B.K. Lustigman (1966), Effect of chemical structure on microbial degradation of substituted benzenes, J. Agric. Food Chem. 14:410.

2. Baker, M.D. and C.I. Mayfield (1980), Microbial and non-microbial decomposition of chlorophenols and phenol in soil, Water, Air, Soil Pollut. 13:411.

3. Namkoong, W., R.C. Loehr and J.F. Malina, Jr. (1989), Effects of mixture and acclimation on removal of phenolic compounds in soil, J. Water Pollut. Control Fed. 61:242-250.

4. Musgrave, D.K. and D.M. Holloran (1980), Soil Survey of Cuyahoga County, Ohio, Ohio Dept. Natural Resouces and Ohio Agricultural Research and Development Center.

5. Lo, H.H. and Y.T. Hung (1991), Utilization of clays and zeolites for coagulation treatment of municipal wastewater, Intern. J. Env. Studies 37:203-212.
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Article Details
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Author:Yung-Tse Hung
Publication:Journal of Environmental Health
Date:Nov 1, 1993
Words:2270
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