Biological Wastes 34 (1990) 215-226

Anaerobic Digestion of Pesticide-Plant Wastewater

C h i u - Y u e Lin

Department of Hydraulic Engineering, Feng Chia University, Taichung, Taiwan 40724

(Received 8 November 1989; revised version received 6 June 1990; accepted 19 June 1990)

A B S T R A C T

Experiments on anaerobic digestion using an organic phosphorus pesticide- plant wastewater containing a high percentage ( 6 2 % ) o f acetate were conducted with chemostat-type digesters maintained at 35°C. The influent COD concentration was 9650mg/liter. The digesters operated stably at the retention times of 15, 20 and 30 days, with COD removal efficiencies o f 87.0, 89.4, and 91.5%, respectively, The biokinetic constants were Vma x = 3.37/day, K S = 4077 mg COD~liter, Yg = 0.148 mg VSS/mg COD, and K a = O.050/day. Bacilli and sarcinae were the predominant morphological species and the dominance was independent of retention time. The optimal retention time was 15 days.

I N T R O D U C T I O N

Pesticide manufactur ing industries are essential to the development of agriculture and economics in Taiwan. However, the need for better environmental quality requires industry to pay more attention to pollution problems, particularly toxic wastewater. There are many processes for treating high-strength toxic wastewater, but few have been satisfactorily used in pesticide-plant wastewater treatment.

Some pesticide-plant wastewater contains a high percentage of acetate, which is known to be a major intermediate product of anaerobic digestion and also a major methane precursor. Most of the pesticide-plant wastewater

215 Biological Wastes 0269-7483/90/$03"50© 1990 Elsevier Science Publishers Ltd, England. Printed in Great Britain

216 Chiu- Yue Lin

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PESTICIDE WASTEWATER CONCENTRATION (rag/I)

Former study results indicating that the pesticide-plant wastewater could be treated with glucose addition (Lin el al., 1987b).

in Taiwan is not adequately treated and there is little reported on anaerobic treatment of the wastewater.

Some pesticides have been reported to be anaerobically degradable (Guthrie et al., 1984; Kiene & Capone, 1986). Figure 1 summarizes the results, which indicate that pesticide-plant wastewater can be treated with glucose addition (Lin et al., 1987b). In the present study, the wastewater from an organic phosphorus pesticide plant located in Southern Taiwan was used as the substrate in anaerobic digestion. This wastewater contained a high percentage (62%) of acetate. The objective of this research was to investigate the digestibility and operation parameters of the process.

METHODS

Digesters

The experimental system consisted of four cylindrical digesters (A, B, C and D), each with a 2.5 liter working volume. The contents of the chemostat- type anaerobic digesters were completely mixed with magnetic stirrers. Each digester was connected to a gas collection cylinder which was placed in an acidified-saturated salt solution• These digesters, with fill-and-draw feeding, were placed in a thermostat with temperature controlled at 35 ___ I°C.

Anaerobic digestion of pesticide-plant wastewater 217

TABLE I Wastewater Characteristics

Parameter Magnitude"

pH 6-2-6"8 COD 223 300-296 500 BOD 5 155000-180 100 TKN 6 620-7 300 Org.-N 5 260-6 070 TP 20 480-24 960 Org.-P 14 880-20 160

" Expressed in mg/liter except pH.

Seed sludge and substrate

The seed sludge for the experiments was obtained from a mixture of a mesophilically digested sewage sludge and the wastewater settling tank sludge of a starch-producing plant.

An organic phosphorus pesticide-plant wastewater was used and its characteristics are listed in Table 1. The raw wastewater was stored at 4°C after collection. First, glucose was added to the digester as a supplementary carbon source. Sufficient inorganic nutrients were added to the substrate. These nutrients have been used by Speece and McCarty (1964), as summarized in a former paper (Lin et al., 1987a).

Experimental methods

The sludge in Digester A was first acclimated with glucose-mixed pesticide wastewater (GPW) with a ratio of glucose:pesticide wastewater =4:1 (12000:3000mg COD/liter). After a period of operation the 10000mg COD/liter pesticide wastewater (see Table 2) was used and the retention time was controlled at 15 days to obtain steady-state data. Digester B was operated as Digester A, but the retention time was shortened to 10 days to investigate the digestibility. In Digesters C and D, the diluted raw pesticide- plant wastewater (RPW) was used to obtain steady-state data. Table 2 summarizes the experimental conditions.

The digesters were monitored for pH values, alkalinity, gas production and composition, volatile fatty acid (VFA) distribution, and solids concentration. The gas volumes were corrected for water vapor content, assuming vapor-saturated gas, and to standard temperature and pressure (STP). The observation of types of bacteria in the digesters was carried out by phase-contrast microscopy.

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Anaerobic digestion of pesticide-plant wastewater 219

The mixed liquor volatile suspended solids (MLVSS) used to express biomass concentrations were measured according to Standard Methods (APHA, 1980). Gas analyses were carried out with a gas chromatograph equipped with a thermal conductivity detector. VFA were analyzed with a gas chromatograph having a flame ionization detector.

Steady-state conditions are defined as those during which product concentration variations were small (approx. 10%). Each value for steady- state data is an average of five to six repeated analyses undertaken for two weeks during steady-state conditions.

RESULTS AND DISCUSSION

Denaturation of the substrates

The quality of the concentrated RPW did not change during storage. However, the GPW and diluted RPW (the substrates prepared for digester operations) changed in quality.

In the prepared substrates, turbidity and scum appeared after 3-5 days and black deposits appeared after 7-10 days, the solution turning entirely black after 10 days. Microscopic examinations of the bacteria in each denatured substrate revealed that there were many kinds of microorgan- isms, including bacilli and spirilla. This suggested that the substrate in the GPW and RPW could be utilized by microorganisms, i.e., these wastewaters could be treated biologically. Observations on the denaturation of the GPW are summarized in Table 3.

Performance of anaerobic digesters

Successful operation of the anaerobic digestion process depends upon the balance of various factors (e.g. pH level, alkalinity, and organic acids) which affect the microorganisms responsible for acidogenesis and methanogenesis. Figures 2 to 4 show the experimental data for the effluent pH, COD and daily gas production of the digesters. Negative values of gas production resulted from power failure. The methane contents of the gas were stable at about 69-71% throughout the experiments.

As an example, Fig. 2 shows the performance of Digester A in which the pesticide wastewater portion was increased from 3000 to 9000mg COD/liter. It was then increased to 10 000 mg COD/liter after 35 days with no glucose; the retention time was shortened from 20 to 15 days (see Table 2). The VFA concentration was shown to increase instantaneously and linearly as the retention time was shortened. But once the concentration reached 1300 mg COD/liter, it remained constant.

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Fig. 2. The performance of Digester A. The pesticide wastewater portion was increased from 3000 to l0 000 mg COD/liter and the retention time was shortened from 20 to 15 days.

Digester C had a longer retention time (30 days), but it took more time to reach another operation state. The author also found the same phenomenon in conducting an experiment on temperature characteristics of the methanogenesis process in anaerobic digestion (Lin et al., 1987a).

Digester B was operated similarly to Digester A, except for retention time. However, the digester failed completely when the retention time was further shortened to 10 days, due to washout of microorganisms, with high residual COD and no gas production. These phenomena show that the critical detention time was longer than 10 days.

Data under steady-state conditions

Figure 3 shows the acetate, COD and MLVSS concentrations in the effluents, and gas production, under steady-state conditions. Data described here are presented as mean values with standard deviations of _ 3.8% to +6.7%. The figure indicates that the effluent COD, effluent acetate concentration, and gas production increased, but MLVSS concentration decreased, with reduced retention time. The effluent COD were 1257, 1022 and 818 mg/liter for the digesters with retention times of 15, 20 and 30 days, respectively, while the influent concentration was 9650mg COD/liter. Therefore, the COD removal efficiencies were 87"0, 89.4 and 91-5%, respectively.

When the influent acetate concentration was 5980 mg COD/liter (62% of the influent COD), the effluent acetate concentrations were 135, 81 and 87 mg COD/liter for Digesters A, C and D, respectively; the acetate-removal efficiencies were 97.7, 98.6 and 98.5%, respectively. A comparison of the

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COD removal with acetate removal indicates that non-acetate organics (as COD) were degraded readily at longer retention times.

Hourly fluctuations of digester contents

Since the digesters were fed once a day on a fill-and-draw basis, the hourly fluctuations of the substrate in the digesters were investigated. Figure 4 shows the hourly changes of COD in the digester liquids under steady-state condition. The COD removal rate for each digester was rapid for the first three hours and then slowed down. This shows that the substrate removal rate decreased as the substrate concentration reduced. The COD removal rates for the first three hours decreased as the retention time increased and their values were 109, 83-3 and 79.3 mg COD/liter/h for the 15, 20 and 30-day retention-time digesters, respectively. Observations on the failure of the 10- day digester and the COD removal rate indicate that the optimal detention time was 15 days.

Fig. 4.

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Anaerobic digestion of pesticide-plant wastewater 223

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The variations for the COD and HAc concentrations in the digester showed similar trends.

The acetate concentrations in the digesters under steady-state conditions were quite low and acetate was the only VFA which could be detected. As an example, Fig. 5 shows the variation of the acetate concentration in the 20- day retention-time digester. The trend of variation for the COD and acetate concentrations almost coincided with each other after three hours, but the COD removal rate for the first two hours was higher than that of the acetate. The reason is that the COD-bearing organics were degraded into acetate. This is a general pathway of organic degradation in anaerobic digestion (Bryant, 1979; Zeikus, 1982). No VFA other than acetate was observed.

Sludge yield and gas conversion

Figure 6 shows the relationships between sludge yield, gas conversion, and retention time for the digesters under steady-state conditions. One of the

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224 Chiu- Yue Lin

characteristics of anaerobic treatment is the smaller sludge production than that of the aerobic treatment. The sludge yields for the digesters at the retention times of 15, 20 and 30 days were 0.084, 0.073, and 0.060 kg VSS/kg COD, respectively. The digester with shorter retention time had higher sludge yield and gas conversion. This indicates that the microorganisms in this digester were more active in substrate utilization. This coincided with the finding that shorter retention time gave higher COD removal rate in the observation on hourly variations in digester contents.

Kinetic constants

One of the objectives of this study was the evaluation of the kinetic constants of biological growth and substrate utilization. The method of least squares was used to determine the line of best-fit for the experimental results and these were arranged in the forms usually used in kinetic analysis. Equations 1 and 2 have been used successfully in analyzing anaerobic digestion kinetics (Chang e t al., 1982; L i n e t al., 1987a). These equations were used to determine the kinetic constants (Fig. 7), and their values are Vma x = 3"37/day, K~ = 4077 mg COD/liter, Y, = 0.148 mg VSS/mg COD, and K d = 0"050/day. These values are similar to those reported for anaerobic digestion (Sundstrom & Klei, 1979; Chang et al., 1982).

VmaxS v - - - (1)

K s + S

1 K. - - - v = D (2)

where v = specific substrate utilization rate, mg COD/mg/day S = effluent substrate concentration, mg COD/liter

K s = substrate saturation concentration, mg COD/liter Vm.~ x = maximum specific substrate utilization rate, mg COD/mg/day

Yg = growth yield of microorganism, mg/mg COD K a = endogenous decay coefficient of microorganism, 1/day D = dilution rate, the reverse of retention time, 1/day

Morphological type of organisms

Hydrolytic, acetogenic, and methanogenic bacteria are generally found in anaerobic digesters fed with complex organics (Zeikus, 1982). Microscopic examination of the digester liquors in this study showed that the microorganisms could be morphologically classified as bacilli (or filamen- tous rods), sarcinae, vibrio, spirilla, and other kinds of bacteria which were

Anaerobic digestion of pesticide-plant wastewater 225

small in size but large,in number. There was no apparent difference in morphological types of microorganisms in the digesters at different retention times. However, bacilli and sarcinae were dominant in all digesters and this dominance was found to be retention-time independent.

In appearance the predominant bacilli and sarcinae resembled those dominant in methanogenic digesters (Zehnder et al., 1980; Chang et al., 1982) because the major organic component was acetate in the RPW. The predominant species appeared to be Methanobacterium (or Methanothrix) and Methanosarcina.

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Some species of the microorganisms in the digesters also seemed like those that appeared in the deteriorated glucose-mixed pesticide wastewater. They included bacilli, spirilla and other tiny bacteria. Their common character- istic was high motility, and some of the bacteria were difficult to separate by centrifugation at 3500 rpm for 20 min.

CONCLUSIONS

The following conclusions can be drawn from the results of the anaerobic digestion at 35°C using the wastewater (of 9650mg COD/liter) from an organic phosphorus pesticide plant. Anaerobic digestion of the wastewater is feasible; the COD removal efficiencies reached 87-0, 89.4 and 91.5% for retention times of 15, 20 and 30 days, respectively. The kinetic constants were Vma x = 3"37/day, K s = 4077 mg COD/liter, Y~ = 0-148 mg VSS/mg COD, and K s = 0.050/day. There was no volatile fatty acid other than

226 Chiu- ]rue Lin

acetate in the effluent; the acetate in the wastewater was almost completely degraded. The optimal retention time is 15 days. Bacilli and sarcinae are the predominant microbial species; the predominance is independent of retention time.

A C K N O W L E D G E M E N T S

The author expresses his gratitude to Mr J. H. Chang for his technical assistance. This work was supported by a grant from the Taiwan National Science Council (NSC77-0410-E305-03Z). A version of this paper was presented at the IAWPRC Asian Workshop on Anaerobic Treatment, 7-8 November 1988, in Bangkok.

R E F E R E N C E S

APHA (1980). Standard Methods for the Examination of Water and Wastewater, 15th ed. American Public Health Association, Washington, DC.

Bryant, M. P. (1979). Microbial methane production--theoretical aspects. J. Anita. Sci., 48, 193-201.

Chang, J. E., Noike, T. & Matsumoto, J. (1982). Effect of retention time and feed substrate concentration on methanogenesis in anaerobic digestion. Proc. Japan Soc. Civil Engrs, No. 320, 67-76 (in Japanese).

Guthrie, M. A., Kirsch, E. J., Wukasch, R. F. & Grady, Jr, C. P. L. (1984). Pentachlorophenol biodegradation, II. Anaerobic. Water Res., 18, 451-61.

Kiene, R. P. & Capone, D. G. (1986). Stimulation ofmethanogenesis by aidicarb and several other n-methyl carbamate pesticides. AppL Environ. Microbiol., 5, 1247-51.

Lin, C. Y., Noike, T., Sato, K. & Matsumoto, J. (1987a). Temperature characteristics of the methanogenesis process in anaerobic digestion. Wat. Sei. TechnoL, Vol. 19, pp. 299-310.

Lin, C. Y., Chang, J. H., Hung, Z. M. & Lin, T. P. (1987b). The feasibility of treating pesticide wastewater by anaerobic digestion. Ind. Pollut. Control, 24(10), 85-92 (in Chinese).

Speece, R. E. & McCarty, P. L. (1964). Nutrient requirements and biological solids accumulation in anaerobic digestion. Adv. Wat. Pollut. Res., 2, 305-22.

Sundstrom, D. W. & Kiei, H. E. (1979). Wastewater Treatment. Prentice-Hall, pp. t 68-72.

Zehnder, A. J. B., Huser, B. A., Brock, T. D. & Wuhrmann, K. (1980). Characterization of an acetate-decarboxylating, non-hydrogen-oxidizing methane bacterium. Arch. MicrobioL, 124, 1-11.

Zeikus, J. G. (1982). Microbial intermediary metabolism in anaerobic digestion. In Anaerobic Digestion 1981, ed. D. E. Hughes et al., Elsevier Biomedical Press BV, Amsterdam, pp. 23-36.