Tratamiento Anaerobio de Aguas Salinas Altas en Sulfato y Amoniaco

ELSEVIER 0 9 6 0 - 8 5 2 4 ( 9 5 ) 0 0 1 4 3 - 3

Bioresource Technology 54 (1995) 269-278 © 1996 Elsevier Science Limited

Printed in Great Britain. All fights reserved 0960-8524/95 $9.50 +0.00

ANAEROBIC TREATMENT OF SALINE WASTEWATERS U N D E R HIGH SULPHIDE AND AMMONIA CONTENT

Francisco Omil, Ram6n M6ndez* & Juan M. Lema

Department of Chemical Engineering, University of Santiago de Compostela, E-15706 Galicia, Spain

(Received 15 August 1995; accepted 19 September 1995)

Abstract The anaerobic treatment of wastewaters from the sea- food-processing industry was studied in a 15 m 3

industrial pilot-plant. These effluents have a high organic content (10-60 g COD/litre), with protein percentages between 25 and 70%, and a salinity similar to sea water: sodium (5-12 g/l), chloride (8-19 g/l) and sulphate (0"6-2"7 g/l). This high concentration of salts, together with the production of sulphide and ammonia due to sulphate reduction and protein breakdown, respectively, produces important inhibitory~toxic effects on non-adapted biomass. A mixture of anaerobic sludges from the treatment of non-saline wastewaters was used as the inoculum. After an initial start-up procedure, where the acclimation of the biomass was the objective, 70-90% organic matter removal was achieved, operating at dissolved sulphide (DS), total ammonia (TA) and sodium concentrations in the ranges of 0"25-0"5 g S-DS/I, 1.0-3"0 g N-TA/I and 6-10 g/l, respectively. The adaptation of the biomass to the salinity and the antagonistic effects on sodium toxicity caused by the presence of other ions made it possible to operate at these high sodium concentrations. Due to the strong buffering capacity of the process, pH was maintained above 7.25, resulting in levels of free hydrogen sulphide (FS) that were lower than 100 mg S-FS/I, which were not high enough to produce inhibition effects on adapted sludges. However, the control of the influent protein content is necessary, since values higher than 200 mg N-FA/I of free ammonia (FA) were shown to be inhibitory for this process.

Key words: Anaerobic treatment, sea-food-processing wastewaters, sulphate reduction, sulphide, ammonia, high salinity, inhibition.

INTRODUCTION

Sea-food-processing industries generate large amounts of highly polluted wastewaters (Prasertsan et al., 1994; Veiga et al., 1994; Aguiar & Sant'Anna, 1988) and, until recently, little attention has been

* Author to whom correspondence should be addressed. 269

paid to the treatment of these effluents and only a rough removal of a fraction of solids and fats has been carried out. Although the characteristics of these effluents largely depend on the raw material and the type of the process employed, usually all of them have high contents of chemical oxygen demand (COD) and protein and a very high salinity (sodium, chloride, sulphate).

The presence of high sodium and/or chloride concentrations was considered as inhibitory for the anaerobic wastewater treatment (Rinzema et al., 1988), but the possibility to adapt the microorgan- isms and the antagonistic effects caused by the presence of other ions can make this process fea- sible (Lema et al., 1991).

Sulphate reduction The presence of sulphate during the anaerobic degradation of these wastewaters enables sulphate- reducing bacteria (SRB) to use part of the carbon substrate, resulting in hydrogen-sulphide production, this process being competitive with methanogenesis. Sulphide, mainly in the undissociated form, H2S, can cause inhibition of methanogenic and also sulphate- reducing bacteria (Rinzema & Lettinga, 1988). Free hydrogen-sulphide concentration depends strongly on the pH of the medium, being around 50% of total dissolved sulphide at neutral values. Literature about hydrogen-sulphide inhibition of methanogenesis is quite confusing, Table 1 summarises some previously reported data, from which it can be con- cluded that ranges of 150-1100 mg S-DS/1 of total dissolved sulphide and 50-250 mgS-FS/1 of free hydrogen sulphide can produce inhibitory effects. It can be observed that, in general, granular biomass or biofilms present a higher threshold toxicity, which agrees with Parkin and Speece (1983).

Besides, Parkin et al. (1991) and Maillacheruvu et al. (1993) have also found that sulphide toxicity is produced at lower concentrations in suspended- growth systems than in anaerobic filters, confirming that biofilm or granular/flocculent sludges present a much more complex system than do completely mixed reactors in the context of sulphide toxicity.

270

Table 1. Sulphide

E Omi~ R. M~ndez, J. M. L em a

toxicity in methanogenic processes (T in *C, dissolved sulphide (DS) and in mg S/l)

free hydrogen sulphide (FS)

Biomass Substrate DS FS Inhibition T pH Ref.

Suspended Acetate 160 50 50% 30 7.0-7.4 1 Suspended DW 390 130 50% 37 7.0-7.2 2 Suspended Acetate 295 125 50% 35 6.5-7.4 3 Suspended Acetate 1060 100 50% 35 7.7-7.9 3 Suspended Lactate 250 100 50% 35 7.0 4 Suspended Lactate 1630 100 50% 35 8.0 4 Suspended C2, C3 145-195 60-65 * 35 7-0-7.2 5 Suspended C2, C3 150-200 60-75 ** 35 7.0 6 Biofilm Propionate 1000 200 ** 35 7.4 6 Biofilm Acetate 400 125 NR 35 7.2 6 Granular Acetate 676 250 50% 30 6-4-7-2 7 Granular Acetate 1045 90 50% 30 7.8-8.0 7

References: 1. Kroiss & Plahl-Wabnegg (1983); 2. Karhadkar et al. (1987); 3. Oleskiewicz et al. (1989); 4. McCartney & Oleskiewicz (1993); 5. Parkin et al. (1990); 6. Maillacheruvu et al. (1993); 7. Koster et al. (1986). (*) Process failure. (**) Inhibition threshold for adapted sludges. NR, Not reported; DW, distillery wastewater; C2, acetate; C3, propionate.

The ratio COD:sulphate appears to be a key factor in the regulation of the competition between methanogenic and sulphate-reducing bacteria (Col- leran et al., 1995). Until recent years, a common recommendation for a successful anaerobic treatment was to operate at COD:sulphate ratios higher than 10, and lower ratios were thought to be inhibitory for methanogenesis because of the high sulphide concentrations expected in the system. However, Hilton and Archer (1988) mentioned 8 as the inhibitory level, whereas in other studies methanogenesis was successful at a COD:sulphate ratio of 5 (M6ndez et al., 1989) and Derycke and Pipyn (1990) report operation at a ratio around 3, after a 2 month start-up period. At lower values the percentage of organic matter used by SRB is more important and for values lower than 1 complete sul- phidogenic systems can be developed with COD removal carried out only by SRB (Alphenaar et al., 1993; Visser et al., 1993a).

Ammonia Protein degradation results in high ammonia concentrations during the anaerobic treatment (Guerrero et al., 1993). According to Anderson et al. (1982), ammonia inhibition is mainly related to the presence of the undissociated form, becoming, therefore, more important as the pH increases. Table 2 shows a summary of data previously reported corresponding with different reactors and substrates. Free ammonia (FA) inhibitory concentrations for mesophilic treatment ranged from 25 to 140 mg N-FA/I, whereas during the thermophilic digestion of cattle manure very high values were achieved after a long acclimation period (390-700 mg N-FA/1)

Soto et al. (1991) reported that sludges adapted to the mesophilic anaerobic treatment of fish-canning wastewaters had 50% of toxicity at 2.8 g N-TA/I of total ammonia, and did not observe any methanogenic activity at 6 g N-TA/I. Other works carried out

Table 2. Ammonia toxicity in methanogenic processes (T in °C, total ammonia (TA) in g N/! and free ammonia (FA) in mg N/I)

Biomass Substrate N-TA N-FA Inhibition T pH Ref.

Suspended C2, C3, C4 3.0 24 NR 30 7.0 1 Suspended Piggery manure NR 125 * 37 NR 2 Suspended Cattle manure 6.0 700 ** 55 7.5 3 Suspended Cattle manure 4.0 390 ** 55 7.4 4 Suspended Acetate 5-0 55 *** NR 7.0 5 Suspended SFPWW 2-8 85 50% 37 7.4 6 Suspended C2 4.0 30 50% 30 7.0 7 Biofilm Acetate 12.0 85 50% 35 6.8 8 Granular Potato juice 4.1 145 50% 30 7.5-7-8 9

References: 1. Van Velsen (1979); 2. Braun et al. (1981); 3. Angelidaki et al. (1993); 4. Hashimoto (1986); 5. Bhattacharya & Parkin (1989); 6. Soto et al. (1991); 7. Soubes et a t (1994); 8. De Baere et al. (1984); 9. Koster & Lettinga (1988). (*) Inhibition threshold for unadapted sludges. (**) Inhibition threshold for adapted sludges. (***) Maximum value achieved without process failure. NR, Not reported; SFPWW, sea-food-processing wastewater; C2, acetate; C3, propionate, C4 butyrate.

Anaerobic treatment of saline wastewaters 271

in non-saline environments also report the importance of the acclimatisation period (Koster & Lettinga, 1988; Angelidaki et al., 1993).

In this paper, the effects of sulphide and ammonia concentrations on the operation of a pilot-plant treating effluents from a fish-canning factory (Omil et al., 1994) is described and discussed. The study of the dependence of removal efficiencies on sodium, free hydrogen sulphide and free ammonia concentrations, the sulphate removal capacity at different COD:sulphate ratios and the selection of a proper pH to minimise sulphide and ammonia inhibitory effects were the objectives of this work.

M E T H O D S

Wastewater characteristics Different mixtures of the most polluted streams generated by a fish-canning factory were treated in the pilot-plant (Omil et al., 1995a). Several dilution waters were used to adjust the characteristics of the final mixture according to a predefined strategy of evolution of the applied organic loading rate and the salinity in the digester to achieve the biomass acclimation. Table 3 shows the main characteristics of the wastewaters generated and the dilution waters used.

Industrial # l o t - p l a n t A industrial wastewater-treatment pilot-plant with a 15 m 3 central activity digester (CAD) was used. This technology is conceptually close to the anaerobic contact system and it has been previously described (Omil et al., 1995a). Temperature was maintained in the mesophilic range (37°C). All pertinent heating, pumping and gas management systems were avail- able.

Analytical methods Determinations of pH, total suspended solids (TSS), volatile suspended solids (VSS), chloride and sulphate were carried out as proposed by Standard Methods ( A P H A , AWA, WPCF, 1985). A semimicro method was used to accurately determine the chemical oxygen demand (COD) in samples with high chloride concentrations (Soto et al., 1989). Total ammonia (TA) was determined by anion-

selective electrode, while volatile fatty acids (VFA) and gas (N2, CH4, CO2) were analysed by gas chromatography (Fernfindez et al., 1995). Sodium was measured by an atomic absorption/flame spec- trophotometer (Perkin-Elmer 5000).

Balance calculations From gas composition, temperature and pH and considering equilibrium conditions, free hydrogen sulphide (FS) and total dissolved sulphide (DS) in the digester were obtained according to the procedure described below. In a similar way, from pH values and total ammonia (TA) content in the digester, free ammonia (FA) concentrations were calculated.

Determination o f total dissolved and free hydrogen sulphide The final products of the sulphate-reducing process are hydrogen sulphide and carbon dioxide. The sulphide is distributed between HES in the gas phase and HES, HS- and S 2- in solution, and insoluble metallic sulphides. The equilibrium of H2S between the gas and the liquid is regulated by Henry's law. The value for Henry's constant at 370C is 0"375 atm HES/(g H2S/1).

Hydrogen sulphide in solution is a weak acid and its equilibrium reactions are:

H2S~HS- +H +

kl = (0"382 x T(°C)+1.892) x 10 -8 mold

H S - ~ S 2 + H +

k2 = 1 x 10-19 mol/1 (at 25°C)

where kl and k: are the equilibrium constants. At pH of 7-8 only the first equilibrium is signi-

ficant. The concentration of total dissolved sulphide (H2S + HS- + S : - ) can be calculated from the concentration of free H2S (obtained by Henry's law) and the fraction of FS, fs (obtained from kl and pH) (Isa et al., 1986):

FS 1

DS-

Table 3. Characteristics of the different waters used in the pilot-plant (flow in ma/day, concentrations in g/l, subscripts t and s refer to total and soluble contents, respectively)

Flow CODt CODs Protein Na + SO 2-

Effluents treated Tuna cooking 51 12.36 9.71 4.55 10.88 2.37 Mussel cooking 96 16.90 15-76 3.34 8.85 2-15 Fish meal 25 55-40 16.50 19-20 5.05 0.59

Dilution waters General effluent - - < 4-3 -- - - 12.61 2.27 Sea water . . . . 12.63 2.70 Fresh water . . . . . .

272 E Omil, R. M~ndez, J. M. Lema

Determination of free ammonia Ammonia generated during anaerobic breakdown of proteins will be present in the liquid in its ionic (NH4 +) and undissociated form (FA). FA has been suggested to be the active component causing ammonia inhibition and its concentration can be calculated from the total ammonia concentration in the liquid and the fraction of FA, fN, using the following equation:

_ m

FA 1 m (

where kb and kw are the dissociation constants for ammonia and water, respectively (1.855 × 10 -5 and 2.355 x 10-14 mol/l at 37°C).

Percentage of COD used by sulphate-reducing bacteria The percentage of COD used by SRB can be obtained from the amount of sulphate reduced and the total amount of COD removed. COD for hydrogen sulphide is 64 g O2/mol, therefore, 64 g of COD are required per mol of sulphate reduced. The amount of sulphate reduced was calculated from a balance taking into account the sulphate concentrations in the influent, the flow and composition of biogas produced (considering an average temperature of 15°C) and the content of dissolved sulphide in the effluent.

RESULTS

Industrial pilot-plant operation Different experiments were carried out during 2 years to study the anaerobic treatment of the most polluted effluents produced in a fish-canning factory in the same conditions as they were generated. The different operational periods carried out, as well as the preliminary considerations on effluent management, are explained in detail elsewhere (Mefidez et al., 1992; Omil et al., 1995a).

As a summary of the performance, Fig. 1 shows the organic loading rate (OLR) progress referred to the influent, biogas and effluent; the overall efficiency (in terms of COD removal) and the alkalinity in the digester. The contribution of ammonia to the total alkalinity is important, oscillating between 1 and 3 g" CaCO3/1. However, remaining alkalinity present in the digester (without ammonia) was very constant, around 2 g/l.

Figure 2 shows that the evolution of sulphide (FS and DS), ammonia (FA and TA) and sodium. FS and FA varied depending on the characteristics of the influent, and being in most cases between 25-100 mg S-FS/1 and 25-150 mg N-FA/1, respectively, corresponding to 0"15-0.5 g S-DS/1 and 1"0-3.0 mg N-TA/I, respectively. During the first 75

days of operation, influent salinity was gradually modified by dilution with low salinity waters (fresh- water and other effluents from the factory) in order to favour biomass acclimation.

Sulphate reduction

Sulphate removal capacity Table 4 shows the sulphur balance calculated for the different periods of operation. Most of the sulphate contained in the influent was reduced to sulphide and distributed between the liquid and gas phases. The percentage of COD used by SRB was obtained by referring the COD used by SRB to the total COD removed.

It can be seen that during the periods of treatment of mussel-cooking wastewaters (P5, P6, P l l and P12) reduction percentages were higher, in spite of the sulphate concentrations in the influent also being higher. In P7, a negative value was obtained, due to the higher content of sulphur in the gas and liquid effluents than in the influent. This was explained by the small volume of wastewaters treated in this period and the low sulphate content of tuna-cooking and fish-meal effluents; this period being affected by the remaining sulphate in the reactor after P6. Remaining sulphate in the effluent was lower than 10% of the influent content on most days, with an overall averaged value of 4.0%.

The overall and specific sulphate-removal capacity of the digester is shown in Fig. 3. Maximum values of 0.35 g 5042--/1 reactor, day and 0"035 g 5042-/g VSS.day were obtained during periods 5 and 6. For the entire operation, the sulphate-removal capacity ranged from 0.05 to 0"35 g SO42-/1 reactor, day and between 0"005 and 0"035 g SO42-/g VSS.day with regard to the biomass.

Sulphide content in the reactor Figure 4 shows the COD:sulphate ratio for the influents treated in the pilot-plant. Minimum values of 7-8 were obtained in period 6 because of the high sulphate content of mussel-cooking effluent, which produced a biogas with a maximum H2S content of 4.25% at pH values around 7.25. The dispersion observed within some periods was due to the differ- ing sulphate contents in the influent because of the alternation of dilutions with sea water and fresh water (period 1), and the different mixtures of effluents employed (period 12). In periods 7 and 8 a wide dispersion could also be seen, but it was due to the different organic contents of the mixtures of tuna-cooking and fish-meal effluents.

Table 5 shows the average values of pH, HES in biogas, FS and DS in the reactor for each period of operation and overall. FS always kept a low value, below 100 mg S-FS/I, as a consequence of the stability of the pH around values higher than 7-25. These FS concentrations did not apparently affect the per-


formance of the system and only a slightly lower COD removal was observed during the first part of period 6.

Evolution of ammonia During the first periods of operation, TA remained between 1.0 and 2.5 g N-TA/I, with some oscillations which caused lower COD removals (around day 100, TA and FA were higher than 2.5 g N-TA/I and 200 mg N-FA/1, respectively). In period 8, when the behaviour of the process after a large increase in the protein content of the influent was studied, TA exceeded 3.5 g N-TA/1 (FA ranged from 200 to 300 mg N-FA/1), causing a strong inhibition in the process that was reflected clearly in the steep decrease of COD removal [Fig. l(b)]. This situation was over- come in a short period of time after decreasing the influent protein concentration (Omil et al., 1995a).

Table 5 shows the average ammonia concentrations in the digester for each period of operation. TA normally ranged between 1.2 and 2.3 g N-TA/I,

whereas FA was below 100 mg N-FA/I, except for periods 1 and 8, where it exceeded 150 mg N-FA/1.

DISCUSSION

It has been reported that the inhibitory effect caused by a high concentration of salts on methanogenic bacteria is mainly related to cations, sodium being among the most common ones. Sodium content in these wastewaters exceeded 10 g/l [Fig. 3(c)], a value reported to be inhibitory for methanogenesis (Rin- zema et al., 1988).

Figures 1 and 2 show that there was no clear toxic effect caused by the high salinity during these experiments. Only during P6 (mussel-processing wastewaters) was there a slight drop in the efficiency at high sodium contents, but the concentration of free hydrogen sulphide appears to be the reason, because similar values of sodium had been reached before without a noticeable decrease in efficiency. A similar situation was achieved at the end of period 9.

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( - - • - - ) and without the contribution of ammonia ( - - o - - ); P, period; see Table 4.


Methanogenic activity tests carried out with samples of biomass f rom this reactor showed the development of an active biomass adapted to these conditions. The inoculum used was taken from a low-saline digester and had a very low methanogenic

activity (0.045 g COD/g VSS-day). After periods P5 and P12 this activity was 0"75 and 0.53 g COD/g VSS.day, respectively, when the reactor opera ted at the highest levels of sulphide and sodium, the COD- removal efficiencies being around 8 1 -8 8 % (Omil et

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Table 4. Sulphur balance in the digester and percentage of COD removed used by SRB (GS refers to the biogas, whereas DS and the remaining sulphate refer to the digester)

Period Influent Sulphur balance COD removal Q (m 3) SO 2- (g/l) S-GS (%) S-DS (%) S-SO 2- (%) Total (kg) By SRB (%)

1, 2 79 1-35 26.0 61"3 12"7 984 6"3 3, 4 44 1.29 32.5 63.2 4.3 675 5.4

5 107 1.82 40"2 58"3 1"6 1449 8-8 6 126 2-24 49.7 47-7 2.6 1586 11.6 7 20 1.11 67-3 58.5 - 25 "7 689 2-7 8 47 1.14 39.4 51.2 9.4 578 5.6

9, 10 141 1.77 33.1 58-1 8"8 1541 9"8 11 133 1.78 43-9 54.0 2.2 1637 9.4 12 89 1.67 47-6 51.6 0-8 1179 8.4

Overall 786 1-72 41.3 54-7 4.0 10 316 8.4


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al., 1995b). These results show the importance of an adequate acclimation period, since sludges adapted to high sodium-contents are less sensitive than non- adapted ones. Similar results were obta ined at

lab-scale with similar wastewaters (M6ndez et al., 1995).

The antagonistic effect of other cations present in these wastewaters on the toxicity exerted by sodium

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Table 5. Sulphide and ammonia equilibria in the digester (GS is the percentage of hydrogen sulphide in the biogas, DS and FS are in mg S/l, TA and FA are in mg N/I)

Period pH GS DS FS TA FA

1 7'85 0.84 276 22 1940 162 2 7.62 1.43 292 38 2088 97 3 7"55 1.64 293 44 1887 82 4 7"53 1.00 171 27 1969 82 5 7.54 2-02 353 54 2145 91 6 7-25 3-47 356 92 1245 27 7 7-60 1"10 216 29 2356 114 8 7-57 1"05 195 28 3446 155 9 7-43 2-45 347 65 1841 61 10 7.45 1.99 292 53 1771 61 11 7"34 2"66 320 71 2051 55 12 7.32 2"48 288 66 1621 42

OveraU 7-45 2.19 322 58 1919 66

276 E Omi~ R. M~ndez, J. M. Lema

was clearly observed, the Na ÷ concentration that exerted a 50% inhibition being changed from 9 to 17 g/l (Omil et al., 1995b), in agreement with previous work (Feijoo et al., 1995; Soto et al., 1993).

The sulphate contained in the influent was almost completely removed in the digester, even at COD: sulphate ratios of 7, which caused high contents of sulphide (up to 4.25% in the gas phase and 500 mg S-DS/I of dissolved sulphide in the liquid phase). Previous lab studies on anaerobic filters have reported that the presence of 5% of H2S in the gas phase at pH 7 or lower in the liquid phase caused instability of the process, but at pH 7.25 or higher these systems could operate satisfactorily, due to the lower content of free H2S (Soto et al., 1991). How- ever, this strategy of increasing the pH to minimise the free hydrogen sulphide concentration in digest- ers operating with high levels of sulphide seems to be effective only with suspended sludge, as happened with the biomass developed in this digester, whereas for other systems the situation can be more complex, according to other results report- ing that the inhibition with granular sludge cannot be described only in terms of FS (Visser et al., 1993b). As reported by Vavilin et al. (1995), the toxic effect of hydrogen sulphide on methanogenic bacteria causes a VFA accumulation that decreases the pH, increasing the amount of free H2S in the medium. They showed that influents with COD:sulphate ratios of 10 and 5 make the process unstable. However, in the present work the system was able to operate at these ratios and the presence of up to 100 mg S-FS/1.

Ammonia inhibition appears in the present case to be the dominant factor for the stability of this

process. According to the model proposed by Angel- idaki et al. (1993), the toxic effect caused by the presence of high concentrations of ammonia on methanogenesis produces an accumulation of VFA which decreases pH, therefore reducing the content of free ammonia. Depending on the amplitude of these oscillations, the system will or will not be able to stabilise the process. Period 8 shows an example of instability which occurred as a result of the increase of the protein content in the influent, and the reversible nature of this inhibitory effect.

The selection of operating pH is a key factor for the successful treatment of these wastewaters and for this purpose the buffering capacity of the carbon dioxide/bicarbonate system is essential. Low pH maximises FS and high pH maximises FA. Taking into account both equilibria, pH values around 7"65 would minimise both concentrations. Figure 5 shows the variation of the COD removal, FS and FA in the digester with respect to the pH. In spite of the different situations during the entire period (influent characteristics, period of operation, etc.), it can be seen that pH values around 7.6 caused the minimisation of both FS and FA concentrations. COD-removal efficiencies were lower at higher values of pH, indicating a much more inhibiting effect of the highest concentrations of FA than of the concentrations of FS.

CONCLUSIONS

Performance and toxicity The anaerobic treatment of fish-canning wastewaters has been carried out successfully, achieving COD

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removals ranging from 70 to 90%, in spite of the high concentrations of salinity, sulphate and protein.

Wastewaters with COD:sulphate ratios from 7 to 50 were treated. As expected, the operation with ratios higher than 10 produced very low concentrations of sulphide that did not cause any significant effect. The lower values of 7-8 were proved to be compatible with a stable operation and with removal efficiencies higher than 80% (period 6). In addition, the inhibitory effect caused by hydrogen sulphide was not significant, because of the biomass acclimation and the maintenance of free H2S below 100 mg S-FS/I.

The percentage of sulphate reduced to sulphide in the digester was higher than 90, with an average value of 96%. The sulphate-removal capacity obtained ranged from 0.05 to 0.35 g SO42-/1 reactor. day. The percentage of the COD removed used by SRB was between 3 and 12%, with an average value of 8%.

Normal values of total- and free-ammonia during the operation were between 1.0 and 3.0 g N-TA/I and 25-150 mg N-FA/I, respectively. Higher contents of free ammonia were reached when high protein content wastewaters were treated, as can be observed during period 8. In this case, when values up to 300 mg N-FA/I were achieved, this caused a severe inhibition of the process. Normal efficiencies were recovered after 10 days, when the protein content of the influent was decreased.

Strategies to accomplish the treatment of these wastewaters Two strategies of operation are of importance to achieve a successful anaerobic treatment of these wastewaters: an adequate acclimation period and the minimisation of free hydrogen sulphide and ammonia in the reactor.

The strategy to be used in the start-up period depends upon the history of the inoculum. If it comes from a non-saline environment (as in this work) an adequate strategy of adaptation to salinity, sulphide and ammonia has to be designed, in order to achieve the optimum performance. A stepwise procedure for increasing these concentrations based on the efficiency of the process has to be followed.

Maximum free hydrogen sulphide in the liquid phase is limited by the sulphate concentration of these wastewaters (maximum 2-7 g/l) and by the stripping effect (which depends on pH). Based on these factors, if pH is maintained above 7.25, maximum free hydrogen-sulphide concentrations of 125 mg S-FS/1, and normally under 100 mg S-FS/1, are expected. These concentrations are not high enough to cause severe problems of inhibition, especially for previously adapted sludges.

The generation of ammonia becomes the main problem because the protein concentration in these wastewaters is quite high. The high buffering capacity of the system (3-4 g CaCOdl) ensured the

stability of the pH in the range 7.25-7"6, which are optimum values for the most common situations described in this work. However, when the influent protein concentration is very high, problems of inhibition will appear. In this case, it is not advisable to modify the pH by adding concentrated acid solu- tions, because of the risk of VFA accumulation; the best solution would be to dilute the influent (as shown in period 8) to decrease the organic protein content.

ACKNOWLEDGEMENTS

This work was supported by the Spanish Commision of Science and Technology (CICYT), Project BIO92-0598.

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