effects of mixing on anaerobic treatment of potato-processing wastewater

Effects of mixing on anaerobic treatment of potato-processing wastewater

K . C. LIN A N D M. E. J. PEARCE Deparrtnenr of Civil Engineering, University of New Brunswick, P .O. Box 4400, Frederictotl. N.B., Catzada E3B 5A3

Received January 8, 1990

Revised manuscript accepted September 26, 1990

Four laboratory-scale reactors were used to study the effects of mixing intensity and mixing duration on the anaerobic treatment of potato-processing wastewater at 20°C. The mixing intensities were set at impeller speeds of 0. 20, 50, and 100 rpm. Two mixing durations were studied: 45 and 15 minih. It was found that both mixing intensities and mixing durations studied and their joint effect significantly affected the steady-state performance of the anaerobic reactors in treating the potato- processing wastewater with respect to organics and solids removals and methane production.

Key words: mixing effects, anaerobic treatment, potato-processing wastewater, organics and solids removal, methane production.

Quatre reacteurs B l'bchelle de laboratoire ont CtC utilisCs pour Ctudier les effets de I'intensitC et de la durCe de mtlange sur le traitement anakrobique des eaux rCsiduelles du traitement de la pomme de terre B 20°C. Les intensitis de melange ont CtC rCglCes B des vitesses de 0, 20, 50 et 100 tours B la minute. Deux durees de mClange ont CtC CtudiCes, soit 45 et 15 minlh. On a constat6 que les intensites et les durCes de melange CtudiCes ainsi que leur effet combine influaient sur la performance en regime permanent des rCacteurs anakrobiques en ce qui concerne I'Climination des solides et des matikres organiques des eaux rCsiduelles et la production de methane.

Mors elks : effets de melange, traitement anakrobique, eaux rCsiduelles du traitement de la pomme dc terre, Climination des solides et des matikres organiques, production de mithane.

[Traduit par la rCdaction]

Can. J . Civ. Eng. 18. 501-514 (1991)

Introduction

Anaerobic treatment of high-strength industrial wastewaters is a proven alternative to aerobic treatment. It offers substantial waste stabilization with an added benefit of utilizable biogas as a source of energy. A lot of research work has been directed at the development of variations in process and reactor design of anaerobic treatment facilities. Examples include anaerobic contact reactors, fluidized beds, upflow anaerobic sludge blanket (UASB) reactors, baffled tanks, and several other reactor configurations. However, the role mixing plays in each design variation has not been clearly established.

This study was planned to complement previous research at the University of New Brunswick (UNB) involving the treatment of potato-processing wastewater in physically unmixed anaerobic reactors. A summary of the research performed in this area at UNB can be found in Lakerides (1986). The pur- pose of this investigation is to examine the effects of mixing intensity and mixing duration on the performance of laboratory- scale anaerobic reactors in treating potato-processing wastewater

Literature review

The historical evolution of anaerobic technology is described in a number of papers, including Sheridan (1982) and McCarty (1982). McCarty (1964) described, in detail, anaerobic waste treatment fundamentals and its applicability in the treatment of a wide variety of industrial wastes.

The effects of mixing on the anaerobic process are not well understood (Verhoff et al. 1974; Stuckey 1983). Monteith and Stephenson (1981) listed the following benefits: (i) minimum solids deposition and dead space; (ii) uniform substrate distri-

NOTE: Written discussion of this paper is welcomed and will be received by the Editor until October 31, 1991 (address inside front cover).

bution, thus reducing short circuiting; (iii) uniformity of envi- ronmental factors; and (iv) no scum formation at the sludge surface. However, there were investigations that showed that the benefits of mixing were not realized (Tenney and Budzin 1972; Verhoff et al. 1974; Zoltek and Gram 1975; Smart 1978; Monteith and Stephenson 1981; Noone and Brade 1982).

Mixing can be provided continuously or intermittently. Inter- mittent mixing in the anaerobic digestion of livestock waste under mesophilic temperature conditions has been recommended by Mills (1979) and Smith et al. (1979). Hashimoto (1982) found higher gas production from beef cattle wastes under both continuous mixing and vacuum than under intermittent mixing and normal pressure conditions. Ho and Tan (1985) reported higher growth constants and greater gas production for a continuously mixed digester than for an unmixed digester. On the contrary, Ben-Hasson et al. (1985) observed 75% lower methane production rate from a continuously mixed reactor than from an unmixed reactor when treating dairy cattle manure anaerobically.

A lot of controversies and uncertainties have been shown in the literature on the effect of mixing in anaerobic wastewater treatment. Therefore, further studies on the subject are worth- while to provide more insight in better understanding of its role in anaerobiosis.

Experimental setup

A schematic diagram of the laboratory apparatus is shown in Fig. 1. Four identical reactors (designated R1, R2, R3, and R4) were constructed using plywood sideplates and a Plexiglas cover plate. All interior plywood surfaces were covered by four layers of marine quality enamel. Connecting joints were assembled with screws and sealed with silicon. The four corner edges were rounded to minimize dead spaces. The cover plate and removable cover lid were fabricated from

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FIG. 1. Schematic diagram of laboratory apparatus (only one reactor shown).

Plexiglas to allow visual inspection of the reactor contents. A gas-tight seal was accomplished by a neoprene O-ring positioned in a groove outside the circumference of the cover plate opening.

Each reactor was baffled and fitted with a 4-blade impeller except R1 (this unmixed reactor was used as a control unit). Two baffle blades were positioned on opposite sides of the mid length of the tank. The mixing shaft entered the reactor through the centre of the cover lid, passing through a lubri- cated, self-sealing tube guide. Impeller speeds were adjusted by the use of speed controllers attached to the mixing motors. All tubings entering the reactor through the cover plate and lid were submerged below the liquid level to eliminate biogas losses.

Feed from a stirred bucket was delivered to the reactors through peristaltic pumps controlled by a timer. The effluents flowed to separate effluent bottles from a small settling tube. The latter was provided to trap any washed-out sludge solids for return to the reactors and save chemicals for pH and alkalinity control through recirculation. Biogas was collected in 10-L gas collection bags. All the experimental units were housed in a walk-in incubator for temperature control. The dimensions of the reactor and the impeller are summarized in Table 1.

Operation and testing

The anaerobic reactors were seeded with anaerobic sludge (625 mL/unit) obtained from a UASB reactor previously run on potato-processing wastewater. The feed in this study con- sisted mainly of primary clarifier influent (sometimes mixed with blancher water) obtained from a potato-processing plant. The feed was prepared once every 4 days. Since the initial pH was low (typically 4 - 4 3 , 0.25-2.15 g of NaHC03 were added to each litre of the wastewater to adjust the pH and provide alkalinity in the feed. The average chemical oxygen demand (COD) loading rate was maintained at 0.5 kg/(m3 . d). At an average flow rate of 1 L/d to each reactor, the hydraulic retention time (HRT) was kept at 7 d. Effluent recirculation from the bottom of the settling tube to the reactor influent was provided at 100% of the inflow. This helped to reduce the

TABLE 1. Reactor and impeller dimensions

Liquid volume Reactor length Reactor width Liquid height Baffle width Impeller diameter Blade width Blade length Impeller elevation

amount of NaHC03 consumption and minimize solids loss in the system. Biogas was continuously collected from each reactor. Temperature was controlled at 20°C for the entire duration of the experiments.

Two main aspects of operation were changes in mixing duration and mixing intensity. Continuous operation was approximated by regular intermittent pumping. A cycle time of 1-h duration was selected. This was the time interval required to feed the reactors (with simultaneous withdrawal from overflows), mix the individual reactor contents, and provide quiescent settling of the suspended solids in the reactors in the above sequence. Feeding was turned on for 1 minute every hour. While R1 was not provided with any mixing, R2, R3, and R4 were subjected to alternate on-off mixing periods. Two mixing durations were studied: 45 and 15 min/h (designated as TM1 and TM2, respectively). In this way, the operation of the reactors with mixing simulated intermittent mixing in a normally unmixed fermenter.

Mixing intensities were indirectly measured in terms of impeller speeds (rpm). The primary objective in this case was biological solids suspension at low mixing intensities. Three speed levels were used: 20, 50, and 100 rpm (for R2, R3, and R4, respectively). The lowest mixing intensity at 20 rpm was much greater than that required for laminar flow (Reynolds number Nr I 400), but was just sufficient to achieve "off- bottom suspension" of the sludge bed. At 100 rpm, the mixing intensity approached the level of fully turbulent flow (Nr r 10 OOO), achieving complete uniformity of the reactor contents.

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0.60

0.54

0.48

- - a 0.42 m' e 0.36 a 3 0.30 0 2 0.24 9 0.18

0.12

0.06

- 5 0 0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320

TIME (d)

FIG. 2. Average loadings to the reactors.

To evaluate the effects of mixing on the anaerobic reactors under investigation, the following data were monitored and recorded:

1. Operational data. Preset rates of feed flow and recirculation as well as impeller speeds were checked regularly. Efflu- ent pH, alkalinity, and volatile acids (VA) concentrations were monitored to insure system balance.

2. Performance data. These include total COD (TCOD), soluble COD (SCOD), suspended solids (SS), and volatile suspended solids (VSS) concentrations in the feed and effluents, as well as biogas production. Grab samples were taken at the beginning and the end of each new feed interval for the feed and effluent, respectively. Biogas volume and composition (CH4, COz, and other gases) were analyzed at the end of the feed interval. Percent removals of TCOD (SCOD) and SS (VSS) as well as methane production rates and methane yields were determined based on the average volumetric feed and gas rates.

Data collection was terminated 320 days after starting. Results on reactor performance are presented and analyzed below.

Experimental results Feed characteristics

During the study period, five separate batches of potato- processing wastewater were shipped to the laboratory for use. Unfortunately, the raw waste characteristics were not consistent in all shipments. While the average TCOD loading rate was adjusted to 0.5 kg/(m3 . d), average SCOD, SS, and VSS loadings varied significantly. As seen in Fig. 2, large increases in SCOD loading from batch 1 to batch 2 and from batch 3 to batch 4 were noted; there were corresponding decreases in SS and VSS loadings during these periods. These were due to the large amount of blancher water in the raw wastewater received. The ranges of average TCOD, SCOD, SS, and VSS concentrations in the five batches were 3266-3595, 121 1-2806, 544- 1128, and 489- 1067 mg/L, respectively; the corresponding overall average concentrations

were 3458, 2218, 830, and 753 mg/L, respectively. It was observed that batch 4 had the highest TCOD and SCOD and lowest SS and VSS concentrations among the five batches.

Operational control paratneters Overall pH, alkalinity, and VA concentrations in the reactor

effluents were quite stable over the duration of the experiments. The values of pH ranged from 7 to 8; alkalinity ranged from 854 to 1634 mg/L (as CaC03). Reactor start-up appeared to be accelerated by increasing mixing intensities under the conditions studied. Volatile acids concentrations dropped to below 10 mg/L (as CaC03) in 69 d for R 1, 35 d for R2, and less than 10 d for R3 and R4. Variations in biogas production rates appeared quite normal. There were noticeable decrease and immediate recovery in gas production at each batch change.

Based on impeller speed, motor torque delivered to the shaft, and power loss due to tube guide friction, the impeller power delivered to each reactor provided with mixing can be estimated according to Holland and Chapman (1966). Thus, the power-to-volume ratios (P/V) corresponding to impeller speeds of 20, 50, and 100 rpm in this study were approximately 270, 700, and 1500 W/m3, respectively. These are much higher than the power level of 13 W/rn3 suggested by Owen (1982) to be adequate for effective mixing. However, it was not the intention of this work to study power economy. The minimum impeller speed of 20 rpm was necessary to achieve off-bottom suspension of sludge and, hence, adequate mixing for the given reactor configurations. The relative impeller speeds used here were appropriate to serve the pur- pose of demonstrating the effect of mixing intensity on anaerobic reactor performance. If desired, the effective power input (P/V) can be approximated in proportion to the impeller speed at constant torque, since the frictional loss was small (2 % - 11 %). Moreover, the P/V ratio can be reduced significantly for larger, geometrically similar reactors while maintain- ing constant N, using scale-up techniques (Oldshue 1983) - a subject beyond the scope of this study.

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0 1 I I I I I I 1 I I I I I I f I 1 0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320

TIME (d)

FIG. 3. TCOD and SCOD concentrations in the feed.

0 1 I I I I I I I I I I I I I I I 1 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320

TIME (d)

FIG. 4. SS and VSS concentrations in the feed.

Overall pe$ormance Reactor input and output in terms of organics and solids con-

centration units are graphically presented in Figs. 3 -6 for the entire study period. Figure 3 shows the variations in concentrations of TCOD and SCOD in the feed. Variations in feed TCOD were minimized in the experiments so as to maintain a constant TCOD loading to the reactors. However, such variations (2850-3998 mg/L) for the most part were inevitable, considering the nature of the waste and the preparation requirements. The uncontrolled feed SCOD concentrations increased significantly from batch 1 to batch 2 and from batch 3 to batch 4, explaining for the corresponding increases in SCOD loadings during these periods.

Figure 4 presents the variations in SS and VSS concentra-

tions in the feed. Significant variations in feed SS and VSS concentrations throughout the investigation are illustrated. The decreases in SS and VSS concentrations roughly between days 60 and 200 and between days 200 and 300 complement the corresponding increases in SCOD concentrations to give an approximately constant TCOD loading in these periods.

Figure 5 shows the effluent TCOD and SCOD concentrations of the four reactors. These concentrations appeared to gradually decrease with time in R1 (without mixing). The overall response of R2 (with mixing at 20 rpm) was very consistent, showing relatively little variations in effluent TCOD and SCOD concentrations with time. The higher degrees in mixing in R3 and R4 compared to R2 increased effluent TCOD variability. This was due to the higher concentrations

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MIXER SHUTOFF I

TIME (d)

FIG. 5. TCOD and SCOD concentrations in reactor effluents.

of solids escaping the reactors at higher mixing intensities, especially in the case of R4 (with mixing at 100 rpm). The high variability of effluent TCOD concentrations compared with SCOD concentrations was, of course, due to the incorpo- ration of solids in the TCOD tests. On the whole, the effluent SCOD concentrations in R2, R3, and R4 were seen to be lower than those in R1, apparently attributed to the effect of mixing.

Figure 6 shows the effluent SS and VSS concentrations of the four reactors. Similar to the variability in effluent TCOD concentrations, the variability in effluent SS and VSS concentrations was the lowest in R2 and the highest in R4. The effluent SS and VSS concentrations in R4 between days 206 and 239 were even higher than those in the feed. As a result, there was a net loss of biological solids from R4 during this period. Usually, this would lead to reduced biodegradation. However, effluent SCOD concentrations from R4 in the same period appeared to be unaffected (Fig. 5d); also, VA concentrations did not reveal any abnormality. The high degree of mixing in R4 that provided high uniformity in reactor content might have acted in a remedial manner.

Figures 5 and 6 reveal a tendency towards increased process instability in terms of TCOD and solids concentrations in the effluent as mixing intensity was increased from 20 to 100 rpm. As mixing duration was reduced (during TM2 after day 210), more time was available for solids settling. As a result, effluent TCOD and SS (VSS) concentrations started to stabi- lize. However, it took approximately 40 days before R3 and R4 reached steady state.

The biogas methane content from each reactor was very consistent, showing no trends or major deviations from the mean with changes in feed characteristics, reactor maturity, or mixing duration. Over the entire period of study, the mean CH4 contents in the biogas were 73.5 % , 76.5 %, 76.4 %, and 75.5 % for R l , R2, R3, and R4, respectively. The correspond-

ing C 0 2 contents were 22.5%, 20.7%, 21.0%, and 21.1%, respectively. Visually, there was not much difference in biogas composition among the four reactors. However, it can be shown statistically by means of t-tests (at a = 0.05 and n = 18) that there was a significant difference between methane contents in reactors with and without mixing (i.e., any one of R2, R3, and R4 versus R1) and between those in R2 and R4 (at 20 and 100 rpm, respectively). This suggests that the degree of mixing had an influence on the methane content of the biogas. The above also shows that R2 generated the highest methane content. In terms of methane yield during the entire period of study, R1, R2, R3, and R4 produced 0.315, 0.354, 0.352, and 0.370 m3 CH4/kg TCOD removed, respectively. This shows that methane yield tended to increase with mixing intensity within the range of impeller speeds used.

It may be noticed in Figs. 5(c-d) and 6(c-d) that disturb- ances in effluent qualities of R3 and R4 occurred in days 138- 162. These were due to the shutoff of the mixers in R3 and R4 during this period. The impact of mixer shutoff will not be discussed in this paper.

Steady-state results The performance of the reactors under steady-state condi-

tions was further examined for the two mixing durations (TM1 and TM2) studied. Here, the selection of the steady-state periods was based on the minimization of the standard devia- tion of the mean effluent TCOD concentrations from each reactor. The steady-state period during TM1 (mixing duration at 45 min/h) was from day 82 to day 130; the steady-state period during TM2 (mixing duration at 15 min/h) was from day 250 to day 298. These are designated as steady-state periods MD1 and MD2, respectively.

Table 2 presents the steady-state performance results of the reactors. The following ranges of steady-state percent

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I EFFL SS (b) R 2

I I I I I 1 I I I I I I I I I 1 0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320

TIME (d)

FIG. 6. SS and VSS concentrations in reactor effluents

removals of organics and solids were observed: TCOD from 85.0% to 96.9%; SCOD from 87.8% to 98.0%; SS from 52.7% to 92.5%; and VSS from 59.3% to 93.2%. In general, percent removals of SCOD and VSS were higher than those of TCOD and SS, respectively. Methane production rates ranged from 0.86 to 1.19 L/d during the same periods.

Among the effluent organics and solids parameters listed in Table 2, only effluent SCOD decreased in concentration at higher mixing intensities during MDI and MD2. The others (effluent TCOD, SS, and VSS) all showed higher values in R1 (without mixing) and R4 (with highest mixing) than in R2 and R3. For these latter group of parameters, there seemed to be a minimum value within the range of mixing intensities studied. On the other hand, methane production rate appeared to be the highest from R2 (at 20 rpm) during both steady-state periods.

Figure 7 shows the percent removals of TCOD, SCOD, SS, and VSS with respect to mixing intensities for both MDI and MD2. The general behaviours of these parameters during both

steady-state periods are similar. During MD 1, percent removals of TCOD, SS, and VSS revealed an optimum mixing intensity towards the lower side of the 20 -50 rpm range. Dur- ing MD2, the optimum mixing intensity appeared to have shifted to the higher side of this range. This indicates that when mixing duration is reduced, mixing intensity must be increased to achieve optimal removals of organics and solids in the systems studied.

In contrast, SCOD percent removals kept on increasing with mixing intensities. During MDI, the increase was greatest between 0 and 20 rpm (from 87.7% to 95%), then gradually reaching 97.6% SCOD removal at 100 rpm. As mentioned before, an impeller speed of 20 rpm for the configuration used was just sufficient to cause off-bottom suspension of the settled sludge. It seems that when the power level exceeded the off- bottom suspension point during MDI, the rate of SCOD removal was reduced (i.e., not much improvement was achieved). During MD2, SCOD removals also increased with mixing intensities (from 95.3 % at 0 rpm to 98 % at 100 rpm).

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TABLE 2. Steady-state performance results

Effluent

Parameter Period Feed R 1 R2 R3 R4

Organics and solids concentrations (removals in parentheses) TCOD (mg/L) MDl 3290 399 (87.9%) 196 (94.0%)

MD2 3654 298(91.8%) 209(94.3%) SCOD (mg/L) MD1 1970 240 (87.8%) 97 (95.1 %)

MD2 2865 133 (95.4%) 118 (95.9%) SS (mg/L) MDl 843 116 (86.2%) 63 (92.5%)

MD2 526 179 (66.0%) 110 (79.1 %) VSS (mg/L) MDl 725 105 (85.5%) 49 (93.2%)

MD2 483 125 (74.1 %) 92 (81.0%)

Methane production

CH, rate (L/d) MDl - 0.86 1.07 MD2 - 1.06 1.19

However, the greatest increase had shifted to a higher mixing intensity range of 20-50 rpm; further increase was small beyond this range. This also shows that when mixing duration is reduced, mixing intensity has to be increased to achieve optimal SCOD removal.

Table 3 summarizes the steady-state results on methane gas production from the reactors. It can be seen that all values are higher for R2, R3, and R4 than for R1. While the differences in methane production among R2, R3, and R4 are not large, those between the reactors with and without mixing are more substantial. Thus, mixing the reactor content at the intensities and durations studied enhanced methane gas production rate and methane yield.

Methane yields based on TCOD removal were not much different during MDl and MD2 for all reactors; the yields based on SCOD removal were all lower during MD2 than during MDl. However, methane production rates (L/(m3 . d)) were higher during MD2 than during MDl for reactors with or without mixing. The latter contradicts some results reported in the literature indicating that methane production decreases with a reduction in mixing duration (Mills 1979; Smith et al. 1979; Hashimoto 1982). The methane production rates and yields from R1 were also inconsistent in the two steady-state periods (supposedly, they should be the same during MD 1 and MD2, since there was no mixing in Rl).

The above controversies are due to one important uncon- trollable factor which has not been accounted for - the soluble fraction, SF (=SCOD/TCOD), of organics in the wastewater supply. Soluble fractions of the feed during MD1 and MD2 were 0.584 and 0.780, respectively. Thus, higher SF, rather than shorter mixing duration (MD2), actually resulted in a higher methane production rate (L/(m3 . d)). When the organics are more soluble, they are more readily and easily biodegradable; hence, more biogas is produced. As seen in Figs. 7a and 7b, higher SCOD removals occurred during MD2 than during MD1 (or more correctly, at higher SF than at lower SF of the organics). This means that lower methane yield (m3/(kg SCOD removed. d)) would be expected at higher SF, as is shown in the last two columns of Table 3. It can be shown that there is actually a significant negative correlation between methane yield (in terms of SCOD removal) and SF at the 5% level of significance ( r = -0.83).

The above analysis shows that a change in the SF of the feed, due to a change in SCOD concentration, significantly

TABLE 3. Methane production during steady-state periods

CH, yield (m3/kg COD removed)

CH, production rate (L/(m3 . d)) TCOD basis SCOD basis

Reactor MDl MD2 MDl MD2 MDl MD2

affected the methane production rate and methane yield. To remove the possible influence of different feed characteristics on reactor performance, the results of each reactor with mixing are expressed as a ratio to those of R1 without mixing in the following analysis.

Figure 8 shows the performance of the reactors normalized with respect to R1. In general, percent removals of TCOD and SCOD by reactors with mixing were better than those by R1 without mixing (Fig. 8a). Furthermore, the reduction in mixing duration from MDl to MD2 is seen to have a negative effect on both TCOD and SCOD percent removals, with the exception of TCOD percent removal by R4. As observed before, the normalized SCOD percent removal curves in Fig. 8a also show higher SCOD percent removals at higher mixing intensities, but the increase is negligible between 50 and 100 rpm for both MD 1 and MD2. The normalized TCOD percent removal curve for MD1 peaks at a mixing intensity of 30 rpm, whereas that for MD2 peaks at 50 rpm.

The normalized solids removal curves in Fig. 8b clearly show the adverse effect of high-level mixing. Suspended solids and volatile suspended solids percent removals by R4 were all lower than those by R1 during both MD1 and MD2. This was due to the substantial loss of solids in the effluent at the high mixing intensity used at 100 rpm. However, moderate degree of mixing is obviously advantageous. The normalized SS and VSS percent removal curves in Fig. 8b appear to peak at a mixing intensity of 30 rpm during MD1 and at 50 rpm during MD2. The reduction in mixing duration from MDl and MD2 resulted in a considerable improvement of SS and VSS percent removals for all reactors with mixing, especially for R4. This

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- 3 ss- - 2 0 gp. 2 LU a: 5 04-

TCOD

LU 0 a: 2 92-

nl (b) TCOD AND SCOD -- MD2

m I , I , , , , , , I I a ! , , , r r I , E , I o 1 o a m a a m 7 o m a 1 m o 1 o a m a a m 7 o m m 1 m

MIXING INTENSITY (rpm) MIXING INTENSITY (rpm)

- 2. a-

E 70- vss

0 a: W V55 a m-

(c) SS AND VSS -- MD1 (d) SS AND VSS -- MD2 SS 55

5 3 ~ l t l r l l l l l ~ 5 3 . I I I I I I I 8 i , o 1 o a x a ~ 1 m 7 o m a 1 m o 1 o a m a m m 7 o m a 1 m

MIXING INTENSITY (rpm) MIXING INTENSITY (rpm)

FIG. 7. Steady-state percent removals of organics and solids versus mixing intensities.

explains for the improvement in TCOD percent removal by R4 form MDl to MD2 (Fig. 8a).

Figure 8c shows methane production rates by all reactors compared to R1 during the steady-state periods. It is clearly seen that normalized methane production rates from reactors with mixing decreased from MDl to MD2, instead of the apparent increases shown in Table 3. Similar results are seen from the normalized methane yield curves illustrated in Fig. 8d. In particular, methane yield based on TCOD percent removal was considerably more sensitive to the reduction in mixing duration than that based on SCOD percent remoal. With the effect of changing feed characteristics removed, the advan- tages of longer mixing durations in terms of higher methane production rate and higher methane yield are clearly revealed.

Both Figs. 8c and 8d show higher methane production rates and higher methane yields with mixing than without mixing. They also show that the optimum mixing intensity for maximum methane production appears to be between 20 and 50 rpm for the conditions of this study.

The above analysis clearly demonstrates that mixing intensity and mixing duration and their joint effect significantly affect the steady-state performance ofthe anaerobic reactors in treating potato-processing wastewater. Indeed, statistical tests can be used for verification. A two-factor analysis of variance based on normalized data was actually performed. The results - . did show that reactor performance in terms of percent removals of TCOD, SCOD, SS, and VSS as well as methane production rate and yield were significantly affected by the main effects of the mixing intensity and mixing duration levels studied and by their interaction (a = 0.05).

Some additional information on mean cell retention time. 0,, and substrate utilization rate are presented and discussed

below. Mean cell retention time is expressed as the amount of mixed liquor volatile suspended solids (MLVSS) in the reactor divided by the amount of VSS lost in its effluent per day. Sub- strate utilization rate is expressed in kg SCOD removed/ (kg MLVSS . d). These values are given in Table 4.

It is seen that the 0, values range from 83 to 1128 d. The values are lower for MDl than for MD2 for all reactors. This indicates a higher loss of solids from the reactor for a longer mixing duration. The 0, values are highest for R2 during MDl and for R3 during MD2 (606 and 1128 d , respectively) Reactor R4 had the shortest 0, among all reactors. This shows the adverse effect of high mixing intensity on solids loss in the effluent. It may be noted that R1 did not have the longest 0, among all the reactors. While there was no mixing in R1 to cause solids loss in the effluent, periodic bursts of gas pockets in the sludge sediment and rising sludge brought about by gas bubbles might have contributed to solids loss from the reactor. The much higher values of 0, compared with HRT point out that loss of biological solids from the reactors was not a problem. The tube settlers were quite effective in trap- ping washed-out solids for return to the reactors. In fact, for the majority of operating time, the solids content of the settling tubes was minimal with occasional plugs of solids observed.

The average TCOD and SCOD percent removals for each reactor during MDl and MD2 are plotted against the corresponding 0, values in Fig. 9. On the whole, both TCOD and SCOD percent removals increased marginally with increasing 0, values. In the case of R4, SCOD percent removals (Fig. 9b) were slightly higher than those of the other reactors, which were operated at higher 0, values than that of itself. TCOD percent removal from R4 during MD2 (Fig. 9a) was also compatible with the other reactors at higher 0, values.

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MIXING INTENSITY (rprn)

R 2 R3 R4

0 1 0 2 0 3 0 4 0 5 3 6 0 7 0 8 0 W l ~ MIXING INTENSITY ( r p )

MIXING INTENSITY ( rpm)

FIG. 8. Normalized steady-state reactor performance versus mixing intensities.

I .'J

- n (dl E 1.20-

0 + a

SCOD-MD2 ,, -. n- - - - -

---_. - - _ _ - - _ _ \ --.?

R3 R4

o 1 0 m z o ~ x 1 m 7 o a o m 1 m MIXING INTENSITY (rpm)

FIG. 9. TCOD and SCOD percent removals versus O, during steady-state periods.

This implies that high organic removal efficiencies could still from a two-way analysis of variance that the four mixing be achieved at lower 0, values using higher mixing intensi- intensity levels had a significant effect on SCOD utilization ties, provided that the mixing duration was not very long. rate (a = 0.05). Also, the rates of SCOD utilization in R4

Substrate utilization rates were observed to range from were significantly higher than those in the other reactors in 0.041 to 0.110 kg SCOD removed/(kg MLVSS . d) for R2 and both steady-state periods (a = 0.05). Although the above R4, respectively, both during MD2 (Table 4). It was found ANOVA test did not reveal a significant effect of the two mix-

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LIN AND PEARCE

TABLE 4. Mean cell retention time and substrate utilization rate during steady-state periods

Mean cell retention time Substrate utilization rate (4 (kg SCOD removed/(kg MLVSS . d))

Reactor MDI MD2 MD I MD2

ing durations on SCOD utilization rate, individual t-tests did show significant changes in the rates from MD 1 to MD2 in the reactors with mixing. While the rates of SCOD utilization in R2 and R3 decreased from MDl to MD2, the rate in R4 increased.

Summary and conclusions Four laboratory-scale reactors were used to study the effects

of mixing intensity and mixing duration on the anaerobic treatment of potato-processing wastewater at 20°C. Mixing intensities for reactors R l , R2, R3, and R4 were set at impeller speeds of 0, 20, 50, and 100 rpm, respectively. Two mixing durations at 45 and 15 min/h were studied. The following conclusions can be made:

1. Reactor start-up appeared to be accelerated by increasing mixing intensities under the conditions studied.

2. The levels of mixing intensity and mixing duration used and their joint effect significantly affected reactor performance with respect to organics and solids removals.

3. process instability in terms of variations in effluent TCOD and SS (VSS) concentrations increased as mixing intensity was increased from 20 to 100 rpm. Effluent SCOD concentrations decreased at higher mixing intensities during both steady-state periods.

4. Steady-state percent removals of SCOD (87.8 % -98.0%) and VSS (59.3% -93.2%) were generally higher than those of TCOD (85.0% -96.9 %) and SS (52.7% -92.5 %), respectively, by all reactors under the conditions studied. ~.

5. The observed results suggest that when mixing duration is reduced for the same cycle time, mixing intensity must be increased to achieve optimal removals of organics and solids.

6. Mixing at the highest impeller speed studied (100 rpm) caused substantial solids loss in the effluent. Reduction in mixing duration from 45 to 15 minlh resulted in considerable improvement in SS (VSS) removal efficiencies for all reactors with mixing; however, this generally had a negative effect on TCOD and SCOD percent removals.

7. Methane production rate (L/(mL d)) and methane yield (m"kg COD removed) were enhanced by mixing; however, both decreased (relative to those from R l ) when mixing duration was reduced from 45 to 15 min/h. Over the entire study period, methane yield ranged from 0.315 to 0.370 m3/kg TCOD removed; methane content in the biogas was 2 % -3% higher from reactors with mixing than that without mixing. ~ i ~ h e r soluble fraction of organics in the feed was found to increase methane production rate but decrease methane yield.

8. Values of mean cell retention time, 0,, ranging from 83 to 1128 d, were lower at a mixing duration of 45 min/h than at 15 minlh during steady-state operations. Both TCOD and

SCOD percent removal efficiencies increased marginally with increasing 0, values. Nevertheless, high efficiencies could still be achieved at the lowest 0, value experienced (in associ- ation with the highest mixing intensity) when the shorter mixing duration was used.

9. The mixing intensities studied had a significant effect on SCOD utilization rate. The rates of SCOD utilization in R4 (where mixing intensity was the highest) were significantly higher than those in the other three reactors during both steady-state periods (a = 0.05).

Acknowledgements

The authors gratefully acknowledge the Natural Sciences and Engineering Research Council of Canada for support of this work under grant A93 14. Potato-processing wastewater samples were kindly provided by McCain Foods, Ltd., Florenceville, Fredericton, N.B., Canada. At the time of this study, M. E. J. Pearce was a graduate student under the super- vision of the first author at the University of New Brunswick.

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HASHIMOTO, A. G. 1982. Effects of mixing duration and vacuum on methane production rate from beef cattle waste. Biotechnology and Bioengineering, 24: 9-23.

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MCCARTY, P. I,. 1964. Anaerobic waste treatment fundamentals, Parts 1, 2, 3, and 4. Public Works, September, pp. 107-1 12; October, pp. 123- 126; November, pp. 91 -94; December, pp. 95-99.

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List of symbols

steady-state periods 1 and 2 at mixing durations of 45 and 15 minlh, respectively sample size Reynolds number simple correlation coefficient reactors 1-4 with impeller speeds of 0, 20,50, and 100 rprn, respectively experimental periods 1 and 2 at mixing durations of 45 and 15 minlh, respectively level of significance in statistical tests mean cell retention time

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effects of mixing on anaerobic treatment of potato-processing wastewater

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