ELSEVIER

International Biodeteriorution & Biodegradution, Vol. 39. No. 4 (1997) 287-293

C 1997 Published by Elsevier Science Limited

All rights reserved. Printed in Great Britain

PII: SO964-8305(97)00022-X 09&I-8305/97 $17.00 + 0.00

Anaerobic Degradation of Paper Mill Sludge

E. Ratnieks” & C. C. Gaylardeb

“Riocell S.A., Guaiba, R.S., Brazil ‘Depto. de Solos, UFRGS, Porto Alegre. R.S., Brazil

(Received 6 August 1996; revised version received 19 September 1996; accepted 7 January 1997)

The effect of pH adjustment and surfactant addition on the anaerobic degradation of sludge from a pulp and paper mill waste treatment plant was monitored by COD (Chemical Oxygen Demand) and EOX (Extractable Organic Halogen) analysis. COD decreased by 29-64% and EOX by 22250%, in all treatments (including control, non-adjusted, sludges). Adjusting the initial pH to 7.0 with sodium bicarbonate, but not with potassium hydroxide, accelerated degradation. Maximum decreases in dry weight, COD and EOX, compared to the control, were seen in sludges receiving bicarbonate; after 5 months the reductions were 60% (COD) and 50% (EOX). The results indicate that the use of bicarbonate to adjust pH could be advantageous in anaerobic treatment of organochloride-containing residues. (6 1997 Published by Elsevier Science Limited

INTRODUCTION

The presence of halogenated xenobiotic substances in the environment is a subject of growing concern. The requirement for studies on the environmental impact and the treatment of industrial effluents containing these compounds has intensified recently. Almenmark et al. (1991) noted that Swedish environmental legislation for the pulp and paper industry related to the disposal of organochlorides from bleaching units is becoming restrictive and hence more effective treatment systems are necessary.

The types of chlorinated compounds found in the effluents from the production of paper from eucalypts include polyphenols and condensed and hydrolysable tannins (Smith et al., 1994). The greater part of the organochlorinated material is associated with high molecular weight breakdown products of lignin (M > lOOO), and degrades slowly. Some, for example lipophilic chlorinated phenols, are adsorbed on to microbial biomass. Secondary waste treatment in the paper factory eliminates only a portion of the chlorinated compounds. Between 22 and 40% of organic chlorides (measured as adsorbable organic halides -AOX) has been shown to be degraded by activated sludge treatment (Gergov et al., 1988; Nevalainen et al., 1991). A relatively small

proportion (4-l 1%) of the total AOX entering the system is incorporated into the sludge. This is about lo-25g kg-’ dry solids. The subsequent removal of such compounds from the anoxic sludge is attributed to volatilisation, sedimentation, or enzymic dehalogenation (Quensen III et al., 1988; Suflita, 1982). Boman and Bergstrom (cited by Almenmark et al., 1991) showed that a variety of chlorinated compounds are degraded in sludge inoculated with sediment exposed to industrial wastes. They identified a microbial consortium capable of effective anaerobic degradation. The organisms retained their degradative ability for more than two years, thus showing that anaerobic degradation is economically viable. A number of other studies on anaerobic degradation of chlorinated organics have been published (e.g. Mikesell & Boyd. 1986; Stevens et al., 1988; Tiedje et al., 1986). Methanogenic (Suflita, 1986) and sulphidogenic bacteria (Stevens et al., 1988; Dolling, 1990) have been shown to be important in this process.

The present study investigates anaerobic degradation in an industrial sludge treatment plant. The rate of degradation of general organic material and of lipophilic chlorinated organics was monitored and the effects of pH adjustment and of the addition of various types of surfactants were determined.

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288 E. Ratnieks, C. C. Gaylarde

MATERIALS AND METHODS

Solid waste

The solid industrial waste was a mixture of residues from the primary, secondary and tertiary treatment stages, collected from the Riocell S.A. pulp and paper factory. The final waste mixture contained approximately 20% solids and had a pH of 6.

Inoculum

The waste was mixed with 5% (w/w) of the same material which had previously been allowed to stabilise in anoxic cells for 6 months.

Modifying treatments

Samples were modified by the addition of surfactants or by pH adjustment. Various types of surfactant were utilised. The cationic compound was cetyl trimethyl ammonium chloride (Hoechst, Slo Paulo). Non-ionic surfactants with hydrophobic or hydrophilic characteristics were prepared from mixtures of polyethoxyethers linked to a nonylphenyl group (Arkopal, Hoechst, Sao Paulo) with varying degrees of ethoxylation. The hydrophilic mixture was 60:40 (w/w) Arkopal N090:N060 and the hydrophobic, 60:40 (w/w) Arkopal N090:N040. All surfactants were added at 500mg kg-’ (w/w) of sludge. In some treatments, pH was adjusted to a value of 7 by the addition of potassium hydroxide or sodium bicarbonate. The effect of hydrophobic surfactant together with pH adjustment with bicarbonate was also studied. The sludge was finally adjusted to 10% solids content (w/w). Digestion was carried out in sealed polythene vessels of 201 for a period of 5 months, with three replicates per treatment.

Sampling

Monthly samples were taken using a specially developed sampling device which collected a vertical sample from the containers without overdue disturbance of the material (in order to maintain anoxic conditions). Each sample was analysed by chemical oxygen demand (COD, Greenberg et al., 1992) and organohalide determination in acetone-hexane extracts from acidified sludge (EOX). All analyses were done in triplicate. EOX was determined as described by the Euroglass Analytical Instruments Users’

Manual, ECS 2000, Netherlands (1992). pH and oxygen were measured with standard electrodes. At the end of the total period, each treatment was homogenised and analysed for residual dry solids.

Statistical analysis

COD and EOX concentrations as a function of time were compared using the Spearman signed rank test. Final values for total COD, EOX, wet wt and dry wt at the end of the 5 month period, were analysed by the student t-test modified for non- equal variances and compared using the Kruskal- Wallis (non-parametric analysis of variance) test. The expected values (original values corrected for all samples withdrawn and assuming no degradation) were compared with the observed values. A 1% level of probability was accepted as significant for multiple tests and 5% for single 2-tailed t-test values.

RESULTS

O2 and pH

No oxygen could be detected in the vessels, indicating that conditions were anoxic. The original pH of the sludge was 6.3. In all treatments, including those which were initially adjusted to pH 7, pH fell to around 5.0-5.5 after the first 2 weeks. During this period there was intense gas production. Only in the bicarbonate treatments did the pH increase to more neutral values at the end of the first month and this tended to increase further over the entire 5 months (Table l), rising to above 7.5. Gas production remained high only in these bicarbonate-treated samples.

COD and EOX

There was a decrease of COD and EOX kg-’ dry wt with time in all treatments. This decrease was statistically significant in all treatments with the exception of COD in the KOH treated samples (Spearman rank order correlation; Table 2). Figure la shows the concentration curves for COD for control (non-adjusted), bicarbonate, and bicarbonate + hydrophobic surfactant treatments. The results of treatments with KOH and surfactants are compared with the control in Fig. lb. COD increased or remained constant during the. first month except in the case of the

Anaerobic degradation of paper mill sludge 289

Table 1. pH Values of the Various Treatments over 5 months (Means of 3 Replicates)

Treatment Month

0 1 2 3 4 5

Control 6.3 5.1 5.3 5.3 5.2 5.2 Cationic surfactant 6.3 5.2 5.3 5.2 5.1 5.1 Hydrophilic surfactant 6.3 5.2 5.4 5.5 5.2 5.3 Hydrophobic surfactant 6.3 5.1 5.4 5.4 5.3 5.2 KOH 7.0 5.3 5.6 5.4 5.4 5.5 HCOs 7.0 6.33 7.0 7.1 7.4 7.5 HC03 + hydrophobic 7.0 6.3 7.2 7.1 7.8 7.7 surfactant

(89% of the values differ from the mean by less than 0.1 pH unit).

Table 2. Spearman Correlation Coefficients for Time vs. COD and EOX Concentrations

Treatment

Control Cationic surfactant Hydrophilic surfactant Hydrophobic surfactant KOH HC03 HC03 + hydrophobic surfactant

Spearman correlation coefftcient

COD EOX

-0.7242” -0.8308” -0.7430” -0.7308a

-0.6301” -0.8684”

-0.5771” -0.7179”

-0.3513 ns -0.8469” -0.8532” -0.5058” -0.8500” -0.6928”

’ significant at the 5% level (n= 18) ns not significant. COD Chemical Oxygen Demand. EOX Extractable Organic Halogen.

bicarbonate and cationic surfactant treatments. After the first month, COD decreased continuously with time in most cases. The additional presence of hydrophobic surfactant did not change the degradation rate. Treatments in which pH levels remained around pH 5 appeared, after the first month, as static, low density, sludges, accumulating gases within them. Treatments which recovered their initial pH values of around 7 (bicarbonate treatments) showed continued decrease of COD, and appeared as flocculent, dense, sludges which continued to actively liberate gases throughout the incubation period.

Absolute COD and EOX values

Figure 2 and Table 3 show the values at the end of the 5 month digestion period, after correction for weight reductions due to sampling. The absolute

values of COD and EOX on a dry weight basis, total dry weight and total wet weight all show a significant fall in the bicarbonate treatments, compared to the control. In all treatments where pH remained between 5 and 5.5 after the first month, there were no significant differences from the control values; COD fell by 28.5-30.6% and EOX was more variable, values falling by 22.3- 33.6% in the same period in these treatments. The addition of bicarbonate caused reductions of 58.9- 63.6% in COD and 43.549.7% in EOX. These figures were significantly different from the control values at the 5% level.

DISCUSSION

Effects of pH adjustment

The addition of sodium bicarbonate resulted in a fall in COD in the first month, whereas there was an increase, or no change, in the other treatments apart from the addition of cationic surfactant (Fig. 1). In all treatments, there was a very active evolution of gas at this time. The rise in COD in non-bicarbonate treatments indicates that organic compounds unreactive with dichromate were converted to substances reactive with this reagent. During this early period, pH values fell to around 5 after 2 weeks, indicating the production of organic acids. In the bicarbonate treatments, the pH increased to 6.3 after 1 month and rose steadily thereafter, whereas in the other treatments pH remained low (Table 1). This suggests that the presence of bicarbonate stimulates metabolic pathways leading to subsequent decarboxylation of organic acids.

Chantry & Boyle (1986) found that the removal of chlorinated organics (polychlorinated biphenyls) was higher (by about 10%) at neutral, as opposed to acidic, pH. Speece & Parkin (1983) state that various factors can cause a fall in pH in the system and that the effect of the accumulation of acids can persist for a prolonged period. The fall in pH to a value of 5 for 15 days is only recuperated with new alkalisation, and even then only after 3 weeks. The authors also note that methanogens present in biofilms are not subjected to the same pH conditions as the suspended organisms. These facts may help to explain the partial effectiveness of treatments in which initial pH was not recovered.

Adjustment of the initial pH values to 7 using potassium hydroxide did not have the same effect

290

(4

E. Ratnieks, C. C. Gaylarde

1000 r

900 -

3 . 800 - N zil

700 -

600 -

500 -

0 2 4

(b)

1000 -

950 -

900 -

G A . 850 - N

El - - 800

750 -

700 -

650 -

600 -

0 Cationic

* Control

0 Hydrophobic

A Hydrophilic

V KOH

500 ' ' I I

0 2 4

Time (months)

Fig. 1. Change in Chemical Oxygen Demand (COD; median values) with time. For statistical analysis, see Table 2(a) Control and treatments adjusted to pH 7 with HC03; (b) Control, KOH and surfactant treatments.

as treatment with bicarbonate. This might have been because of the better buffering capacity of the latter, but is more likely to be due to stimulation of methanogenesis by bicarbonate ions (Stanier et al., 1987), leading to increased biodegradation. Additional evidence for this was seen in the continued high levels of gases liberated by the bicarbonate-treated sludges. McCarty (1964) considered that pH should be corrected when it tends to fall below 6.5 and that sodium bicarbonate is suitable for pH adjustment without inhibitory effects up to 5-6 gl-‘. Although the bicarbonate treatments showed increased pH values after 5 months, they did not approach the inhibitory levels quoted by Speece and Parkin (1983) of 9-9.5. The increasing pH values indicate that free hydrogen chloride, liberated by reductive dehalogenation, does not accumulate in the sludge

of the bicarbonate treated samples in significant amounts. It must therefore be postulated that some of the chlorine may be removed as volatile organochlorides, such as methyl chloride. This compound is known to be produced by some terrestrial fungi (Neilson, 1996). Methane from anaerobic digestors is a valuable product, but if methyl chloride is present in the gas in more than trace amounts, severe corrosion problems will occur on combustion and a pretreatment step may be needed. If present in sufficient amounts, methyl chloride would be a valuable by-product for use in the chemical industry.

Effects of surfactants

Organic compounds may adsorb to solids in heterogeneous systems and this factor has been

Anaerobic degradation of paper mill sludge 291

8 Reduction in COD & EOX after 5 months

(Means of 3 replicates) 70

60

50

.? 40 z B

H 30 8

20

10

0 -

0 COD

n EOX

i 1 I Con Hf Hb

I Cat OH HCO, HCO,+Hb

Fig. 2. Reduction in Chemical Oxygen Demand (COD) and Extractable Organic Halogen (EOX) levels in sludges after 5 months incubation. Values corrected by mass balance. Mean of three replicates. Data from which this figure is derived, and statistical

analysis, is to be found in Table 3.

Table 3. Final Values for Dry and Wet wt and Absolute COD and EOX after 5 Months

Observation Con Hf Hb Cat KOH HC03 HC03 + Hb

Expected wet wt (kg) 18.12 18.44 18.21 17.75 17.95 15.73 16.07 Observed wet wt (kg) 17.60 17.70 17.60 16.23 17.33 14.67 15.20 Change (%) 2.81 4.00 3.33 8.54 3.44 6.76 5.43 SD 0.56 1.18 0.64 3.10 0.53 0.55 1.38 Expected dry wt (kg) 1.82 1.84 1.84 1.76 1.80 1.51 1.63 Observed dry wt (kg) 1.66 1.68 1.62 1.47 1.59 1.01 1.10 Change 8.52 9.01 11.80 16.44 11.59 33.28 32.67 SD 1.80 1.28 1.49 4.25 2.00 3.33 2.16 Expected COD (g kg-‘) 1464.33 1485.67 1480.33 1421.33 1455.00 1216.00 1316.67 Observed COD (g kg-‘) 1045.00 1048.33 1026.67 996.33 1040.33 443.00 541.00 Change (%) 28.64 29.44 30.65 29.90 28.50 63.57 58.91 SD 7.79 9.24 5.20 9.26 4.13 6.46 6.47 Expected EOX (mg kg-‘) 1861.33 1888.33 1883.00 1805.33 1848.67 1546.67 1672.33 Observed EOX (mg kg-‘) 1446.67 1414.33 1349.00 1198.00 1334.33 874.00 841.33 Change (%) 22.28 25.10 28.36 33.64 27.82 43.49 49.69 SD 4.17 6.01 4.42 5.10 4.93 3.57 5.00

Con = Control; Hf = hydrophilic surfactant; Hb = hydrophobic surfactant; Cat = cationic surfactant; HC03 = bicarbonate; COD = Chemical Oxygen Demand; EOX = Extractable Organic Halogen. Statistically significant changes (p < 1%) are underlined.

postulated to explain occasional failures in the biodegradation of organic compounds (Block et al., 1993). Ageing of sludges may result either in the liberation of the organics, or in bond strengthening to surfaces. In the latter case, the organics become inaccessible to biological degradation. The results of ageing cannot be predicted and have to be determined empirically (Neilson, 1996).

Surfactive agents can free organic compounds bound by van der Waal’s forces from a solid matrix (Tiehm, 1994; Pendergrass, 199 1; Rosiers, 1983), and may make them more available to

microorganisms, and increased organochloride removal from sludge by such treatment has been shown (Chantry & Boyle, 1986). Ethoxylated nonylphenol detergents (Arkopal) with a grade of ethoxylation of 6-13 moles are stated to be tolerated by various groups of bacteria. In the present study, no significant effect of surfactants was observed. This failure might be explained by the fact that up to 75% of ethoxylated nonylphenols with an ethoxylation level of less than 10 moles are degraded within 30 days in rivers (Schick, 1966) and, presumably, in sludge; their effects, if any, would be short-lived.

292 E. Ratnieks, C. C. Gaylarde

Cationic surfactants have a solubilising effect on organochlorine compounds (Botre et al., 1978) but are generally toxic to microorganisms. Once again, this surfactant may well be broken down relatively rapidly under the conditions of these experiments, thus allowing early solubilisation of the adsorbed organics, but restricting eventual degradation.

The general lack of effectiveness of the surfactant treatments (Fig. lb) may also indicate that lipophilic substances in this system, as determined by the EOX analysis, are already adequately available. There are opposing views in the literature with respect to the bioavailability or otherwise of chlorinated organics (Parker et al.,

1993; Gergov et al., 1988).

Reduction of COD and AOX

The reductions in COD (Fig. 1) and EOX (not shown) with time indicate that the composition of the dried sludge changes with time, The results suggest that more than 50% of the initial dry weight of the sludge consists of recalcitrant material, probably inorganic in character. The presence of this non-reactive material explains why the COD and EOX values per unit dry weight fall as their contribution to the total weight of the sludge falls (Table 3).

The maximum degradation levels found in this study (5060%, Fig. 2) are well beyond those which have been previously quoted. Puhakka & Rintala (1987) showed a reduction in COD of 32-37% in a 10 day anaerobic digestor with controlled (optimum) conditions of temperature and pH, using a mixture of chemical sludge and activated (biological) sludge. Modesto Filho (1994) reported 45% dechlorination of a concentrated liquid industrial waste incubated for approximately 38 months in a fixed-film stationary-bed reactor inoculated with a methanogenic consortium from a pilot-scale anaerobic reactor and supplemented with citric acid fermentation waste plus 47mM sodium bicarbonate, but no previous reports of EOX degradation in solid wastes from the pulp and paper industry have been found. In the present study, EOX is reduced concomitantly with COD, even though the concentration of the former is ten times smaller. This was unexpected in view of the generally held belief that chlorinated organics are slowly degraded. However, Neilson (1996) mentions that chlorinated material present in sludge may, be only poorly recalcitrant, especially in anoxic conditions. There is evidence that chlorinated

phenols, catechols and anilines are better degraded under anoxic conditions, even when there is a high level of chlorine substitution in the molecule. This might be explained by the fact that reductive dechlorination generates ATP, which is scarce in anoxic environments (Dolling, 1990).

The initial results of this study are considered promising and further work is in progress. Amongst other things, it will be important to discover the fate of the degraded organochlorides, employing analytical methods to measure volatile and dissolved chloride levels.

CONCLUSIONS

General and chlorinated organic material was degraded in all treatments, including the control (unadjusted) sludge. Maximum reductions, found in treatments using bicarbonate to adjust pH to 7, were 60% for COD and 50% for EOX. The surfactants caused no statistically significant increase in organics removal, possibly because they were degraded together with the general organic matter.

The results indicate that the use of bicarbonate to adjust pH to 7 could be advantageous in anaerobic treatment of organochoride-containing residues.

ACKNOWLEDGEMENTS

We wish to thank Riocell S.A.. Guaiba, Brazil, for support, Peter Gaylarde for helpful discussions and assistance with statistical analysis and the following for further assistance: Manlio Gallotti, Steven Lammiman, Celso Mazzoli, Nina Mariaana Gullsten, Ervin Mora and Patricia de Oliveira.

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