an evaluation of single- and separated-phase anaerobic industrial wastewater treatment in fluidized...

An Evaluation of Single- and Separated- Phase Anaerobic lndust rial Wastewater Treatment in Fluidized Bed Reactors

Michael A. Bull,* Robert M. Sterritt, and John N. Lestert Public Health Engineering Laboratory, Imperial College, London S W7 2BU, England

Accepted for Publication December 30, 1983

Four fluidized bed reactors were used to evaluate single- and separated-phase anaerobic treatments of a high strength wastewater. Two reactors were fed with a synthetic wastewater, containing glucose as the primary carbon source, with a COD of 1.2 x lo4 mglL while the remaining pair were fed with a wastewater with a COD of 6000 mglL. At each influent strength, one fluidized bed reactor was operated as a single-phase system while the other was operated as a methanogenic reactor which was preceded by an acidification reactor in a separated- phase system. The reactors were operated under steady- state and variable process conditions. The separated- phase system consistently gave a better quality effluent with lower effluent suspended solids and total COD, and the methane yield was also improved. Under variable process conditions, the separated-phase system was in- herently more stable and recovered more rapidly following a shock loading. Propionate and acetate degradation studies indicated that the biomass in the methanogenic fluidized beds of the two-phase systems was more adapted to volatile acid degradation than the biomass in the single-phase fluidized beds.

INTRODUCTION

Anaerobic degradation can be considered a two-stage process. In the first stage, complex organic material is first solubilized by extracellular enzymes and subsequently hy- drolyzed and fermented to organic acids by the acid- forming bacteria. The organic acids are then degraded in the second phase by methanogenic bacteria to produce methane and carbon dioxide. Methanogenic bacteria can only utilize the lower-molecular-weight acids, acetic and formic, as substrates.* The longer chain acids are degraded to acetic acid by a separate group of bacteria, the obligatory hydrogen producing acetogenic (OHPA) bacteria.] The OHPA bacteria must grow in close association with the methanogenic bacteria since the latter remove the hydrogen produced to maintain favorable thermodynamic conditions. The two phases of anaerobic digestion, the nonmethanogenic (solubilization and acidification) phase

*Present address: British Gas Corporation, London Research Sta-

tTo whom all correspondence should be addressed. tion, Michael Road, London SW6 2AD, England.

Biotechnology and Bioengineering, Vol. XXVI, Pp. 1054-1065 (1984) 0 1984 John Wiley&Sons, Inc.

and the methanogenic (acid degradation and gas formation) phase, differ widely in their physiological and nutri- tional requirements; thus it has been proposed that if the two phases are physically separated by using two reactors in series, they may both operate under more optimal con- d i t i o n ~ ~ leading to improved efficiency of anaerobic waste treatment processes. Phase separation has been achieved by kinetic control^,^ chemical inhibition of the methanogenic b a ~ t e r i a , ~ or by dialysis.6

Two-phase anaerobic degradation has been extensively studied in stirred tank and upflow reactors and has been shown to improve the treatment efficiency, increase the maximum possible organic loading, and improve reactor stability.’g8 There are problems, however, with the settle- ability of the biomass, particularly that from the methanogenic reactor’; for this reason, certain reactor types would not be suitable for treatment of wastewaters where high organic loadings and, hence, high volumetric loadings are required. Such problems may be overcome by using a reactor design such as the fluidized bed that is able to retain high concentrations of biomass immobilized on a solid support.

Previous experiments conducted in our laboratory with anaerobic fluidized beds having designs similar to those described in this article have been concerned with estab- lishing their performance under conditions of constant and transient shock loading and with ‘the development of techniques to accelerate the initial development of an active biomass film.9-16 In reviewing the literature on anaerobic fluidized processes, l 7 and comparing the findings with our own data, it became apparent that fluidized beds may have further potential as methanogenic reactors in a two-phase system. Hence, the present article describes a comparison of fluidized bed performance in single- and two-phase systems.

EXPERIMENTAL

Apparatus

Four fluidized bed reactors were used during this study, each constructed from extruded acrylic tubing of 2 m

CCC 0006-3592/84/091054-12$04.00

height, 0.05 m i.d., and 4 L volume. The influent synthetic wastewater was pumped to a recycle chamber and mixed with recycled effluent. The flow to the reactors was taken from the recycle chambers and distributed at the base of the reactors using a conical shaped distributor. The support material used was silica sand with a mean diameter of 0.22 mm. Each reactor contained ca. 2.5 L sand and the flow rate up through the reactor was adjusted to achieve a bed expansion of 20%. This expansion was chosen in order to maintain a true fluidized condition, as opposed to an expanded bed, while maximizing the quantity of support material and, hence, biomass contained within the bed, and minimizing shear stresses which may lead to detach- ment of the biomass. The upflow velocity required to maintain the bed expansion was between 4.6 and 7.6 m/h, which was equivalent to a flow rate of 9-15 L/h. Liquid recycle ratios of between 1 : 12 and 1 : 40 were used. The reactor temperature was maintained at 35°C by passing the feed to the column through a small heat-exchanger. A detailed description of the fluidized bed reactor construc- tion has been given previou~ly.’~

The acidification reactors were constructed from Quick- fit borosilicate glass components (Corning Ltd.). Each consisted of a 500-mL flat bottom fermentation vessel fitted with a flange lid fitted with connections for the supply of sterile synthetic wastewater and a gas outlet. The effluent left via an overflow positioned to maintain a constant volume of 300 mL and was transferred to the methanogenic fluidized bed via a U-tube which acted as a gas seal. The contents of each reactor were mixed using a magnetic stirrer and the temperature was maintained at 35 2°C by the use of an electric heating belt.

Wastewater Characteristics

The influent was based on a glucose wastewater previously used in an anaerobic contact reactor study. l 8 The influent contained (g/L distilled water) glucose, 8.0; pep- tone (Oxoid, L34), 2.4; meat extract (Oxoid “Lab Lemco” L29), 0.8; KH2P04, 0.24; NaHC03, 0.32; CaCI2 . 6H20, 0.22; and MgS04 - 7H20, 0.048. All components were “Analar” grade except where specified. The synthetic wastewater was prepared in 20-L quantities and sterilized at 121°C for 1 h to maintain a constant effluent quality. Two reactors were fed with wastewater with an influent chemical oxygen demand (COD) of 1.2 X lo4 mg/L and the remaining two with an influent COD of 0.6 X lo4 mg/L obtained by diluting the influent solution with distilled water. At each influent concentration, one fluidized bed reactor was operated as a single-phase anaerobic system and the other as a separate methanogenic reactor receiving the effluent from an acidification reactor. The operating pH of each system was maintained ai 7.0 and the synthetic wastewater contained no suspended solids.

Sampling and Analyses

The performance of the acidification reactors was rou- tinely monitored by measuring effluent COD, suspended

solids (SS), total volatile acids, and pH by standard meth- ods. l9 Individual volatile acids were measured by gas chromatography using direct injection onto a column of Chromosorb 101. Formic acid (10%) was injected between samples to avoid hysterisis effects, as recommended by Banfield and Lowden.20 Gas composition was measured using a modified gas chromatograph as described previously.21 Hexose sugars were determined by the method of Dubois et a1.22

The performance of the fluidized bed reactors was measured using the same parameters. In addition, alkalinity was measured by acid titration to pH 4.5.19 Gas production was determined by acidified water displacement.

Acetate and propionate kinetic parameters were measured using the method reported by Kaspar and Wuhr-

the sodium salts of the acids were used to raise the acid concentration in the reactor and the decline in acid concentration determined hourly. Under conditions of constant loading, the acetate degradation rate (Vo,,) was assumed to be 0.7 times the methane production rate. After the acetate concentration was raised, degradation occurred at conditions of saturation and its rate could be determined from

When the concentration dropped to a value where degradation became concentration dependent, Lineweaver- Burk plots could be constructed to determine the maximum degradation rate VmaxAC and the half-rate coefficient K, from the equation:

1 + - (2)

Propionate degradation parameters were determined similarly except that the propionate degradation rate (Vo,,,) was assumed to be 0.08 times the methane production rate under conditions of constant loading.23 Al- lowance was made in all calculations for dilution caused by the influent wastewater.

1 Ks ____ - 1 v s Vmax V,ll,X _ -

Experimental Procedure

The reactors were tested under both steady and un- steady state conditions. During the constant organic loading experiments each reactor was operated at six fluidized bed COD loadings, in the range 3- 18 kg/m3/day at an influent COD of 1.2 X lo4 mg/L and 1.5-9 kg/m3/day at an influent COD of 0.6 X lo4 mg/L. To improve reactor stability, pH was controlled by the addition of sodium hydrogen carbonate (Analar). At the higher organic loadings, its concentration in the influent was increased proportionally in order to maintain the same effective buffering capacity. The influent flow rate was adjusted for the required loading and the reactors were allowed to stabilize for three hydraulic residence times or 14 days before sampling. The operational parameters of the reactor were measured daily over three days to ensure stabilization had taken place; the means of the three measurements are reported in all

BULL, STERRITT, AND LESTER: ANAEROBIC WASTEWATER TREATMENT 1055

cases. Previous studies on the same reactors indicated that steady-state conditions could be maintained for periods up to 80 days."

During and for 10 h following a shock load or a transient temperature reduction, fluidized bed effluent samples were taken for analysis. All analyses apart from total volatile acids were performed immediately while samples for volatile acids determination were filtered, stored at - 10°C, and analyzed within 48 h.

Fluidized Bed Reactor Startup

The reactors used already had a fully developed biofilm following a reactor startup experiment.I6 Two months were allowed for the reactor biomass to adapt to the glucose substrate and to the separated-phase system. Initially the reactors were fed with wastewater at the required strength at 0.25 mL/min and the flow rate increased step- wise to achieve a fluidized bed COD loading of 12 or 6 kg/m3/day depending on the influent COD. No operational difficulties were encountered during this period.

Acidification Reactor Startup

Prior to initial startup, the reactors were filled with waste solution and the contents were allowed to stabilize at the required temperature. The reactors were both inoculated with 10 mL of freshly collected activated sludge and allowed to operate in a batch mode for 48 h before wastewater was allowed to enter at a flow rate of 0.25 mL/min and the loading gradually increased. An active bacterial population was rapidly established as demonstrated by a rapid increase in effluent suspended solids, and a reduction in

0 E

l o o t

pH. Since the reactors were connected in series with the fluidized bed reactors, they were subjected to the same range of influent flow rates although the COD loadings on the acidification reactor were proportionally higher due to their lower volumes.

RESULTS AND DISCUSSION

Performance of the Acidification Reactors

Suspended solids in the reactors were affected by both influent concentration and flow rate. The suspended solids were approximately twice as high in the reactor fed with the higher strength solution than in the reactor fed with the lower strength substrate. Suspended solids declined linearly with hydraulic residence time in both the reactors except at the two highest loadings in the high strength influent reactor. After 30 days operation, both reactors developed a yeast-type biomass (identified by direct visual microscopy) which at the higher loadings attached itself to the reactor walls. The reactors were emptied, cleaned, and reinoculated. However, the new population of microor- ganisms still contained this type of biomass to a limited extent, although increasing the stirrer speed reduced wall growth. Reactor suspended solids over the range tested are given in Figure 1.

The biomass in the acidification reactors was fairly typical of such systems.'q8 It was white, occasionally formed flocs of between 1 and 2 mm diameter, and appeared to have good settling properties. The biomass concentration in the reactor was also similar to that reported previously. Z4,25

1 30 6 0 90 1 2 0 150 180

0

COD LOADING LrgCODm' 6'

Figure 1. 0.6 X 10'' nig/L.

Effluent suspended \olids of acidification reactors: ( ) influent COD 1.2 X lo4 mg/L ( 0 ) inllucnt COD

1056 BIOTECHNOLOGY AND BIOENGINEERING, VOL. 26, SEPTEMBER 1984

The reactor operating pH generally varied between 3 and 5. At the lower loadings with the reduced buffering capacity, the pH was relatively stable over the three COD loadings, with a typical value of 3.5. The operating pH increased to over 4.5 when the influent sodium hydrogen carbonate concentration was increased but declined with increasing loading. These phenomena are shown in Fig- ure 2. Emphasis has been placed on simplicity of reactor operation, with little attempt at controlling such parameters as pH since the economic advantages of these processes would be lost to an extent if additional monitoring and control systems were required. This has meant reactor operation has been somewhat less than optimum. The operating pH of these reactors, is considerably less than the optima of ca. pH 6 reported by Zoetemeyer and co- w o r k e r ~ ~ ~ and Ishida et Operation at a lower pH not only affects the reaction rates but also affects the product distribution from the reactor.

The total volatile acid concentrations generally decreased with increased loading. At the higher pH values, however, acid concentrations were higher indicating more favorable conditions for acid production. Acetic acid was the main acid product, generally accounting for over 90% of the total acids. Propionic and butyric acids were also detected in measurable concentrations. Propionic acid concentrations in the high strength reactor generally increased with increasing loading, while in the lower strength reactor the concentration peaked at the intermediate load. Butyric acid concentrations were low (less than 30 mg/L) and were relatively unaffected by the organic load. Higher-molecular-weight organic acids were also detect- able, but not in measurable concentrations. Individual volatile acid concentrations are given in Table I . Ethanol was also a major product from the acidification reactor. Although its accurate determination was not possible, concentrations of over 500 mg/L were noted.

Zoetemeyer and c o - ~ o r k e r s ~ ~ studied the effect of pH on the acid product distribution at various dilution rates. The major carbon source was glucose and the experiments took place at 3OoC in a stirred tank reactor. The major acid product was reported to be butyrate when the reactor was operated at pH 5.69 (the lowest pH reported). The

I 1 I 1 I 1 30 60 90 120 150 180

COD LOADING kgni3d-'

Figure 2. 1.2 X lo4 mg/L and ( 0 ) influent COD i \ 0.6 X 10' nig/L.

Operating pH of acidification reactors: ( ) influcnt COD is

Table I. reactors.

Individual volatile acids produced from the acidification

~~

Influent 0.6 x lo4 1.2 x 104

th) (mg/L) (mg/L)

COD HRT Acetic Propionic Butyric Acetic Propionic Butyric

10 833 23 11 1191 47 15 5 584 72 22 1102 39 21 3.33 415 99 10 1126 56 25 2.5 944 43 7 1419 47 15

1.66 761 41 27 1258 130 22 2 786 56 15 1296 106 16

high acetate concentrations found here were probably due to the low relative sludge loadings, since it has been reported previously that acetate production is highest at a low sludge loading.25

The acidification reactors operated at very high dilution rates compared to those reported by previous a ~ t h o r s , ~ ~ . ~ ~ and this, together with the persistence of a yeast-type biomass, could account for the high ethanol concentrations found.24 Ethanol is often reported to be a product of acid fermentati~n,~' particularly from glucose,28 and although high concentrations are detected in failed digesters, it has been shown to degrade rapidly under anaerobic conditions. l7

Carbon dioxide, hydrogen, and nitrogen were found in the gas phase of the reactor and ca. 80% of the gas was carbon dioxide, which is typical of a system operating at low pH. Gas production could not be determined accu- rately due to level fluctuations in the reactor vessel and leakage from the effluent overflow.

Effluent hexose sugar concentrations increased with increasing organic loadings except in the reactor supplied with wastewater with a COD of 0.6 X lo4 mg/L where the effluent concentration dropped at a COD loading of 60 kg/m3/day. This was probably due to an increased con- version rate because of the more favorable pH. In both cases, the concentration tended to reach a plateau at the higher loadings, as shown in Figure 3 . Although the effluent hexose concentration cannot be directly correlated to effluent glucose since intra- and extracellular polysac- charides will give a positive reaction to the test,29 the hexose concentration can give an indication of the extent of glucose degradation. This was relatively low in comparison to other reported systems and this is probably an indication of the unfavorable pH and lower reaction rates of this substrate compared to a totally glucose based substrate.

Little COD reduction was achieved in the acidification reactor, with a maximum of 8% found in the higher strength feed reactor. Since the primary aim of the acidification reactor is fermentation, the expected COD removal would be low and this was found to be the case. Most of the COD removal is through hydrogen evolution and the extent to which this occurred is consistent with previously reported values.


30 60 90 120 150 180 COD LOADING kgm’d-’

Figure 3. lo4 mg/L and ( 0 ) influent COD is0.6 X lo4 nig/L.

Effluent hexose concentration of acidification reactors: ( ) ) influent COD is 1.2 X

Performance of Fluidized Bed Reactors reactor was being operated at COD loadings at and above 9 kg/m3/day.

Overall COD removal in all reactors (both soluble and insoluble) is shown in Figures 4 and 5 over a range of COD loadings. Filtered COD removal was over 75% at COD loadings up to and including 15 kg/m3/day, but above this loading COD removal declined to less than 50%. In the reactors treating the higher strength influent, there

All reactors operated satisfactorily over the range of organic loadings tested except the separated-phase reactor treating the lower strength wastewater. Poor distribution of liquid at the base was found to be the cause of large quantities of biomass and sand being carried over in the effluent. The problem was traced and rectified when the

5 0

I I I I 2 4 6 8

C O D LOADING kgm-’d-’

0 I D

Figure 4. single phase, ( A ) filtered separated phase, and ( ) unfiltered separated phase.

Effluent COD from fluidized bed reactor. where influent COD is 6000 nig/L: ( A ) filtercd 5ingle phare. ( 0 ) unfiltcrcd


1 I I 5 10 15 1

- 3 -I COD L O A D I N G k g m d

Figure 5. phase, ( 0 ) unfiltered single phase, ( A ) filtered separated phase, and ( ) unfiltered separated phase.

Effluent COD from fluidized bed reactor, where influent COD is 1.2 X lo4 nig/L: (A) filtcrcd Gnglc

was a significant difference in performance between the single- and separated-phase reactors in both soluble and insoluble COD removals, the separated-phase reactor (when operating satisfactorily) consistently giving improved performance. In reactors operating with the lower strength feed, the separated-phase reactor achieved superior insoluble COD removals but soluble COD removal was generally similar over most of the range tested. Treat- ment efficiency was also superior in the reactors treating the higher strength wastewater at the same organic ioad- ing. Loadings higher than 15 kg/m3/day have been achieved previously, but pH control was necessary.30

Effluent suspended solids were very high even at the lowest loadings; values below 150 mg/L were never re- corded. This precludes the use of the reactor for complete treatment. In the reactors treating the lower strength wastewater, the suspended solids concentration increased with increasing organic loading except at a COD loading of 6 kg/m3/day where the concentration decreased. This coincided with an increased degradation of glucose in the acidification reactor and the higher sodium hydrogen carbonate concentration in the influent, factors that could improve reactor stability and operation. The reactors treating the influent with a COD of 1.2 X lo4 mg/L ex- hibited a sharp increase in effluent suspended solids at a COD loading of 6 kg/m3/day. However, above this loading there were only relatively small changes. Effluents from the separated-phase reactors had considerably lower suspended solids concentrations, generally having a value about 60% of those from the single-phase fluidized bed reactors. Figure 6 shows the effluent suspended solids over the range of COD loadings examined. The high effluent suspended solids appear to be due to the fact that the reactors are self-regulating with respect to their biomass concentrations, it being rarely necessary to waste any bio-

mass. The fluidized bed biomass concentration was lower (5-20 g/L) in comparison to the expanded bed reactor.3' This is probably due to the higher bed expansions used in the fluidized bed reactor. Anaerobic films on small parti- cles are very thin (up to 15 ~ m ) ~ ' and, hence, biomass concentration per unit volume will decrease with increasing bed expansion. However, running the reactor at higher expansions will avoid any blockage of the bed with inert solids, a problem with reactors that operate with low upflow velocities.32

The pH of the effluents from all reactors varied between 6.25 and 7, with the exception of the reactors receiving the high strength waste at a COD loading of 18 kg/m3/day where the pH dropped to less than 5 in both the single- and separated-phase systems. This was indicative of failure due to acid accumulation since an increase in total volatile acids from concentrations of 300-580 mg/L at COD loadings up to 15 kg/m3/day to 2000 mg/L at a loading of 18 kg/m3/day was observed.

Influence of Varying Process Conditions on the Performance of Single- and Separated-Phase Systems

The transient changes in process conditions studied were reductions of 10°C in operating temperature, increases in influent flow rate of 100% and increases in influent COD of 10070, each lasting for 4 h . Generally, the pattern of response of the reactors was similar in each case, although it varied in magnitude. Hence, the response to a single 100% increase in flow rate is shown in Figure 7 as an example of this pattern, and the results of the other experiments are summarized in Table I1 in terms of maximum change in each parameter during the test period.

In the periods during and immediately after the 4-h


1501

0-

a E

v) u)

1001

5 0 (

0 5 10 15 18

COD LOADING k g 6 ' d - l

Figure 6. Effluent suspended solidsfrom fluidized bed reactors: ( 0 ) influent COD is 1.2 X lo4 mg/L single phase; ( ) influent COD is 1.2 X i04 mg/L separated phase; (A) influent COD is 0.6 X lo4 mg/Lsingle phase; (A) influent COD is 0.6 X lo4 mg/L single phase; and ( A ) influent COD is 0.6 X lo4 mg/L separated phase.

shock periods, the response of the reactors was character- ized by increased effluent COD, volatile acids, and suspended solids and decreases in alkalinity. Although these effects were seen in both the single- and separated-phase systems, the latter appeared to have a greater stability. In particular, the effluent COD from the separated-phase system took longer to stabilize after the cessation of the shock than did the effluent COD from the single-phase reactor. The effluent suspended solids remained stable or decreased with the separated-phase system, in contrast to the single-phase reactor where the solids tended to increase and were much more unstable. Under these conditions, both reactors performed similarly to those in experiments reported p rev iou~ ly . '~~ '~ The reactors were only subjected to mild overload conditions since the principal aim of the experiment was to determine only differences in operating behavior between the single- and separated-phase reactors.

While evaluating the stability of single- and separated- phase stirred tank reactors under conditions of shock loading, Cohen et found that while both reactors ac- cumulated volatile acids the separated-phase reactors re- turned to typical operating conditions in 20% of the time taken by the single-phase reactors. Improvements in stability in these experiments concerned with fluidized beds are small. However, the inherent stability of a single-

phase fluidized bed reactor has been shown to be very g o ~ d . ' ~ . ' ~ One parameter usually taken to be an indicator of the stability of an anaerobic system is the alkalinity: volatile acids ratio. In the separated-phase system, this ratio may be up to 60% higher than in the single-phase system, which is indicative of greater stability. The organic acids consisted almost entirely of acetic acid over most of the loadings tested. Propionic acid concentrations were generally below 20 mg/L, except at COD loadings above 15 kg/m3/day. At a COD loading of 18 kg/m3/day, propionic, butyric, isobutyric, and valeric acid were all found in concentrations above 200 mg/L.

Gas production and methane composition are given in Table 111. Both methane composition and methane production decreased with increasing COD loading. Gas production from the separated-phase systems was slightly greater and had a marginally higher methane composition than that from the single-phase reactor.

Use of separated-phase digestion appears to principally affect the insoluble phase and improves the gas yield. The separated-phase reactor also produced superior results for soluble COD removal at the higher influent COD. The improvement in effluent suspended solids from the separated-phase reactors is probably due the greater sludge buildup in the separated-phase fluidized bed recycle


, O 0 I so0 1 4

350

mg i' 300

250-

1100-

1000-

900

mgi '

8 0 0

7 0 0

300 mgi'

-

-

- -

-

. I 800 L - -

I l l l l l l l l l l l l l l

1 m g i ' I I COD

2oOt 1 YL t t t ' t ' A-

V O L A T I L E

1

Figure 7. tors: ( c ,)separated phase and ( 0 ) single phase.

Effect of 4-h 100% influent flow rate increase on the perforrnancc of fiuidizcd bcd reac-

chambers, indicating that more biomass was settling within the reactor system than in a single-phase reactor. Effluent suspended solids from all the fluidized bed reactors were extremely fine and had very poor settling properties, thus it is important to reduce the solids concentration as much as possible to improve the final effluent quality. The in- complete acidification in the first reactors means that complete phase separation had not occurred and thus final effluent quality could also be improved by optimiz- ing the performance of the acidification reactors.

A second factor that improves effluent quality from the separated-phase reactors is the substrate composition. Pipyn and V e r ~ t r a e t e ~ ~ showed that formation of lactate and ethanol in the acid stage was preferable to volatile acid production, since the former route distributed a greater proportion of the free energy change of anaerobic degradation to the methanogenic bacteria (see Table IV).

In an experimental two-phase process, biomass production was found to be 40% less if the fermentation in the first (acidification) stage was directed to ethanol and lactate rather than volatile fatty acids, and this was attributed to the difference in the free energy change between the two possible metabolic pathways. Since ethanol fermentation was occurring in the system reported here, and although lactic acid was not measured it is generally found in equimolar quantities to ethanol in acid f e r m e n t a t i ~ n , ~ ~ this effect was also occurring in these experiments.

Kinetic Parameters of Propionate Oxidation and Acetate Degradation in Single- and Separated-Phase Fluidized Bed Reactors

In order to compare the performance of the single- and separated-phase fluidized reactors and to compare the


Table II. mance of fluidized beds.

Influence of temperature reductions and increases in influent COD and flow rate on the perfor-

Single phase Separated phase

Initial Maximum Initial Maximum Shock load Parameter value change value change

~

PH Temperature SS (mg/L)

COD (mg/L) VA (mg/L) Alkalinity (mg/L)

PH

reduction (10°C. 4 h)

Flow rate SS (mg/L) COD (mg/L) VA (mg/L) Alkalinity (mg/L)

PH

increase (10070, 4 h)

COD increase SS (mg/L)

VA (mg/L) Alkalinity (mg/L)

(loo%, 4 h) COD (mg/L)

6.85 540

1960 300

1040

6.90 530

1880 310 780

6.60 500

1860 310 750

-0.10 + 40 + 200 + 75 - 140

-0.30 + 105 + 360 + 40 - 75

-0.10 + 150 + 600 + 60 - 45

7.05 +0.15 200 - 70

1350 + 250 300 + 120

1700 - 300

7.25 -0.50 1 80 + 20

1300 + 330 290 + 75

1080 - 140

6.70 fO.15 125 +90

1300 + 500 290 + 125 900 - 85

Table III. reactors.

Gas yield and methane composition from fluidized bed

Single phase Separate phase

Methane Methane Methane Methane COD loading yield composition yield composition (kg/m3/day) (m3/kg COD) (70) (m3/kg COD) (70)

Low strength feed (0.6 X lo4 mg/L) 3 0.257 73 0.343 76 6 0.268 73 0.356 77 9 0.263 73 0.347 74

High strength feed (1.2 X lo4 mg/L) 6 0.271 72 0.349 77

12 0.213 68 0.322 71

Table N. Distribution of the total free energy change of the two-phase anaerobic fermentation of glucose to methane over the different microbial groups (after ref. 33).

Free energy change Free energy available for the

change available synthetic bacteria as for the

End acid-phase fermentatives Hz gas" Otherwise product (70) (700) (%)

Acetic acid 51.1 33.6 15.4 Propionic acid 88.7 0.0 11.3

Ethanol 55.9 0.0 44.1 Lactic acid 49.0 0.0 51.1

Butyric acid 63.0 16.8 20.2

'This is potentially subject to loss if headspace gases are not reintroduced into the methane reactor.

performance of conventional anaerobic systems with the fluidized bed reactor, the kinetic parameters of propionate oxidation and acetate degradation were determined.

The kinetic parameters of propionate oxidation were determined by raising the propionate concentration using a sodium propionate solution (50 g/L) injected slowly into the fluidized bed reactor through the lowest sample point. To determine degradation under conditions of saturation the concentration was raised to 200 mg/L, while to collect data for Lineweaver-Burk plots a lOO-mg/L spike was used. Effluent samples were collected hourly and the individual volatile acids concentration determined immediately. The results from a typical lOO-mg/L propionate increase are shown in Figures 8 and 9. As the propionate concentration decreased, the acetate concentration increased for a period of up to two hours following the initial addition of propionate. Lineweaver-Burk plots were drawn and values of Vmaxpn,p and K, are shown in Table V. The highest maximum degraxiiion rates were found in

Table V. Kinetic parameters of propionate oxidation.

Experiment V",,, K , No. V, (mmol/L/h) V, (rng/L) r 2

Single-phase reactor 1 0.154 1.908 0.877 166 0.993 2 0.161 1.872 0.856 143 0.875 3 0.157 1.890 0.866 150 0.890

Separated-phase reactor 1 0.215 1.589 1.115 125 0.947 2 0.210 1.670 1.078 130 0.870

0.217 1.620 1.092 130 0.832 3


I I I I I 2 0 0 . I _

B I- z f 180-

P i

y1 " 180-

2 0

I- 7.

I I I I I I I I 0' : 2 3 4 3 s I I) 0

1 I M E houc.

Figure 8. Effect of 100 mg/L propionate increase on volatile acid concentrations in a single-phase fluidized bed reactor: ( A ) acetate and ( ,) propionate.

, I I I 1 I 1 I

f \-q /

the single-phase system, while the lowest KMProP values were found in the separated-phase system.

Kinetic parameters of acetate degradation were determined in a similar manner. However, satisfactory Line- weaver-Burk plots could not be drawn since degradation was so rapid during the concentration-dependent phase that insufficient data was available. Steady-state and experimentally measured maximum degradation rates are given in Table VI. Maximum experimental removal rates occurred in the single-phase system.

The anaerobic degradation rates of volatile acids allow a comparison to be made between the performances of different reactors. The maximum degradation rates in fluidized bed reactors were extremely high in comparison to a conventional system; previously reported data for conventional sludge digestion is given in Table VII. The K, values are very high in comparison to some reported values but this could be a reflection of the high maximum turnover

\ \ .

I I I I 1 I 2 3 5 6 7 8

T I M E , h o u r i

Figure 9. Effect of 100 mg/L propionate increase on volatile acid concentrations in a separated-phase fluidized bed reactor: ( A ) acetate and (c,) propionate.

Table W. acetate degradation in a conventional sludge digester (after ref. 16).

Average kinetic parameters of propionate oxidation and

VO V E vmax K , (mmol/L/h) (mmol/L/h) (mmol/L/h) (mmol/L)

Table VI. Kinetic parameters of acetate degradation.

Acetate 0.27 0.41 0.63 0.32 Propionate 0.03 0. I9 0.233 0.094

rates. The lower K, values in the separated-phase system indicate the superior ease of substrate assimilation in these reactors. la Experimentally measured V,,, values were higher in the separated-phase system indicating that they were operating closer to their maximum theoretical values. The data for proprionate degradation is presented as maximum specific turnover rates in Table VIII in order to compare the results with those of Cohen et Values of V,,, in separated-phase reactors are very similar, although the single-phase fluidized bed reactor V,,,, value is three

Single-phase reactor Separated-phasc reactor

Experiment VO VE i-2 vo VE 2 No. (mmol/L/h) (mmol/L/h) (mmol/L/h) (mmol/L/h) (mniol/L/h) (mmol/L/h)

1 1.34 1.76 0.977 1.88 2.33 0.967 2 1.41 1.57 0.960 1.83 2.31 0.98 3 1.37 1.89 0.948 1.89 2.37 0.947


Table VIII. bed reactor and conventional sludge digesters (latter data from ref. 8).

Specific propionate and acetate degradation rates in single- and separated-phase fluidized

Fluidized bed Sludge digester

"€A,. VER,,,, VLAC Experiment (mg/L biomass/h) (rng/L biomasslh) (rng/L biomasdh) (mg/L biomasslh)

Single phase 1 8.2 13.25 1.84 2.89 2 8.0 11.82 I .54 1.74 2 8.1 14.15 2. I8 3.h2

Separated phase 1 16.15 27.3 17.42 20.94 2 15.62 27.06 14.03 13.02 3 15.82 27.77 12.34 20.31

times the conventional value. These values cannot be directly compared since the substrates are different, but the higher V,,,,, values in the separated-phase reactors indicate that the biomass is more adapted to degradation of volatile acids. The accumulation of acetate during the oxidation of propionate reveals that in both reactors aceto- clastic reactions are rate limiting. This is in agreement with the findings of Kaspar and W ~ h r m a n n , * ~ however Cohen et aL8 found that carbon dioxide reduction was rate limiting.

The rate-limiting step has important consequences during organic overload. In this system, with a substrate based on glucose, a shock load will lead to rapid generation of NADH by glycolysis. Reducing equivalents may be dis- posed of either by hydrogen transfer leading to reduction of carbon dioxide to methane or by electron disposing fermentation leading to the formation of propionate and b ~ t y r a t e . ~ ~ , ~ ~ Should carbon dioxide reduction be rate limiting, acid accumulation will occur and with significant quantities of hydrogen in the digester gas, inhibition of propionate and butyrate degradation will lead to a failed digester.

Cohen et aL8 found that V,,, values increased after suc- cessive shock loads of a mixture of volatile acids. They assumed that the system adapted itself by an increase in the population of acid degrading (OHPA) bacteria to aid process stabilization during organic overload. This effect was not found here, but it is possible that a highly adaptable microbial population had developed during the preceding experiments when the reactors were subjected to wide ranges of operating conditions.

CONCLUSIONS

Anaerobic fluidized bed reactors treating a glucose based substrate achieved good COD removal up to a COD loading of 15 kg/m3/day. Above this loading, volatile acid accumulation indicated failure of the reactors. The addition of a stirred tank reactor to achieve a separated-phase system with an acidification reactor followed by a methanogenic fluidized bed reactor improved effluent quality

1064 BIOTECHNOLOGY AND BIOENGINEERING, VOL.

compared to a single-phase fluidized bed reactor. The separated-phase system produced an effluent with a considerably lower suspended solids concentration, and this was attributed to the superior settling properties of the acidification reactor sludge and the formation of ethanol and lactate in the acidification reactor which has been shown to reduce acid sludge production. Under shock load conditions, the separated-phase system was inher- ently more stable. In calculation of the kinetic parameters of acetate and propionate degradation, it was found that acetate degradation was 4.3 times greater and propionate degradation 9.9 times greater than values reported for a conventional anaerobic sludge digester. Specific degradation rates for acetate and propionate were 2.08 and 1.96 times higher, respectively, in a separated-phase system compared to the single-phase system.

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