Bioresource Technology 47 ( 1994) 189 - 190

Short Communication

Effect of Pyrite Addition on Anaerobic Digestion

Abstract The effects of pyrite on anaerobic digestion of diluted distillery wastes were studied in batch digesters of 10 liters volume at doses of 10-40 mg per digester. During the experiments the methane production and the soluble COD removal increased with the pyrite doses in the range of 1-3 mg/liter. Process inhibition occurred at 4 mg/liter.

Key words: Pyrite, anaerobic digestion.

INTRODUCTION

The effects of iron, nickel and cobalt additions on anaerobic digestion have been widely studied. Eichen- berger (1984) found the role of iron in the bioconver- sion of acetic acid to methane. He showed that iron also acts as an inhibitor of sulfate toxicity. Hausinger (1987) demonstrated that nickel is essential for meth- anogenesis because it is present in the factor F420. Ahring and Westerman ( 1985) found that Methanobac- terium exhibited a peak in methane production at 5 mg/liter of nickel, but at higher concentrations inhibi- tion occurred. Takashima and Speece (1988) demon- strated stimulatory effects of nickel at 1 mg/liter and inhibition at 10 mg/liter. An increase of the acetate utilization rate was observed by Williams et al. (1986).

Oleszkiewicz and Sharma (1990) reviewed the effects of heavy metals on anaerobic digestion, report- ing both stimulatory and inhibitory effects, depending on the suspended solids concentration in the digestion and the doses of metal applied. Considering the infor- mation in the literature a study of the addition of pyrite (a low cost source of iron, nickel and cobalt) on the anaerobic digestion was decided on.

M E T H O D S

Pyrite powder of less than 0-5 mm particle size was used. The pyrite was analyzed by an atomic absorption technique and its composition was 49% iron, 0"63% nickel and 0"07% cobalt.

The substrate used in the digesters was distillery wastewater diluted 1 : 10 (v/v) with tap water. The sub-

Bioresource Technology 0960-8524/94/S07.00 © 1994 Elsevier Science Limited, England. Printed in Great Britain

strate characteristics were determined by the analysis of soluble COD, soluble BOD, total volatile solids, volatile acids, organic phosphorus and orthophos- phates, organic and ammonia nitrogen, Fe, Ni, Co and pH by Standard Methods (APHA, AWWA, WPCF, 1985).

The laboratory-scale digesters were two glass vessels of 10 liters volume. The vessels were closed by rubber taps provided with connections for sampling and a biogas outlet.

The digesters were inoculated with 2 liters of well- digested sewage sludge (obtained from a full-scale plant near the laboratory) having total and volatile solids concentrations of 5% and 64% respectively. The digester volume was completed with 8 liters of sub- strate.

Four batch runs with 36 days digestion time at a temperature of 30 + / - 2°C were done (at pyrite doses of 10 mg, 20 mg, 30 mg and 40 rag). In each run two digesters were employed: one as a control; the other received the pyrite. Each experiment was done twice.

In order to study the cumulative effect of pyrite on anaerobic digestion, the digesters were alternated as is presented in Table 1.

When run 1 finished, the digester content was set- tled. The supernatant was replaced by fresh diluted- waste while 2 liters of the sludge obtained remained as inoculum for run 2. The same procedure was done for runs 3 and 4.

During the digestion the pH was maintained at 7 by the addition of lime at a concentration of 40 g/liter in distilled water. The digester contents were mixed every 2 h by magnetic stirrers for 30 min at 500 rpm. Sam- ples (60 ml) of the supernatant after settling the solids for 16 h were taken twice a week for the determination of soluble COD and pH. The volume of methane pro- duced was determined by bubbling the biogas through a solution at 10% NaOH and measuring the gas volume by water displacement. The residual CO2 concentration was further checked by an Orsat apparatus.

Table 1. Arrangement of the experiments

Run no. Pyrite doses (mg)

Digester A Digester B

1 10 0 2 0 20 3 30 0 4 0 40

189

190 Effect o f pyrite addition on anaerobic digestion

Table 2. Characteristics of the diluted distillery waste

Parameter Range (mg/liter)"

SCOD 6 000-10 000 SBOD 3 000-4 500 TS 6 800-9 300 VS 4 000-6 200 TSS 160-580 VSS 140-500 Acetic acid 170-300 Propionic acid 5-62 Butyric acid 14-99 Orthophoshpate 2-8 Organic phosphorus 8-17 Ammonia nitrogen 2-45 Organic nitrogen 4-410 Fe 1-3 Ni 1-2 Co not detected pH 5-6

"Except pH.

Run 1 427 ~O

~ 8

.~ 6

' o

~ o m

a o o

Run 2

Run 3 S

0 8 t6 24 32 0 8 46 24 3 2

Run 4

Digestion time (days)

Fig. 1. Effect of the pyrite doses on the anaerobic digestion of diluted distillery waste: run 1 (10 mg), run 2 (20 mg), run 3 (30 mg), run 4 (40 mg). % SCOD variation in the control digester; +, SCOD variation in the digester with pyrite addi- tion; x, cummulative methane production in the control digester; v, cummulative methane production in the digester

with pyrite addition.

RESULTS AND DISCUSSION

In run 2, the addition of 20 mg gave a significant increase in methane production and soluble COD removal with respect to the control.

In run 3, when digester A received a dose of 30 mg the rate of soluble COD removal increased, with a sig- nificant increase of methane production, compared with the control. Besides this, the rate of the process in digesters A and B was higher than in run 2.

In run 4, the addition of 40 mg of pyrite to digester B gave higher cumulative methane production and soluble COD removal compared with the control for up to 24 days of digestion; after this time digester A was better than digester B. This behavior could be determined by process inhibition caused by the accu- mulation of the heavy metals reaching inhibitory con- centrations in the biomass.

The highest rates of soluble COD removal and methane production were observed in run 3, not only in the dosed digester (A) but also in digester B due to the effect of the pyrite added in run 2.

The activity of the methanogenic microorganisms appeared to be stimulated by the addition of pyrite which remained in the sludge. The microorganisms were capable of taking up the Fe, Ni and Co from the pyrite and storing metals.

R E F E R E N C E S

Ahring, B. K. & Westerman, E (1985). Sensitivity of thermo- philic methanogenic bacteria to heavy metals. Curr. Micro- bioL, 12,273-6.

APHA, AWWA, WPCF (1985). Standard Methods for the Examination of Waters and Wastewaters, 15th edn, McGraw-Hill, New York.

Eichenberger, E. (1984). Interrelation between essentially and toxicity of metals. In Metals Ions in Biological Systems, ed. H. Siegel. M. M. Decker, New York, pp. 67-96.

Hausinger, R. P. (1987). Nickel utilization by microorgan- isms. Microbial Rev., 51, 22-42.

Oleszkiewicz, J. A. & Sharma, B. K. (1990). Stimulation and inhibition of anaerobic process by heavy metals. A review. Biological Wastes, 31, 45-67.

Takashima, M. & Speece, R. E. (1988). Mineral nutrient requirements for high rate methane fermentation of ace- tate at low SRT. Proc. Ann. Water Poll. Control Fed., Dallas, Texas, 5 October, Wat. Poll. Control Fed., Wash- ington, DC.

Williams, C. M., Shih, J. C. H. & Spears, J. W. (1986). Effect of metals on biological methane generation from a labora- tory poultry waste digester. Biotech. and Bioeng., 28, 1608-10.

The characteristics of the substrate used in the experi- ments are given in the Table 2.

Figure 1 shows the variation of COD and the cumu- lative methane production with respect to the digestion time at different doses of pyrite.

At doses of 10 mg of pyrite the methane production and the soluble COD removal was higher than the control. The total amount of methane produced was around three times the value of the control.

E. P. Sfinchez Hermindez National Center for Scientific Research (CNIC), Environmental Pollution Department, PO Box 6990, La Habana, Cuba

(Received 15 April 1992; revised version received 3 July 1993; accepted 23 July 1993)