anaerobic digestion of high-strength cheese whey utilizing semicontinuous digesters and chemical...

Anaerobic Digestion of High-Strength Cheese Whey Utilizing Semicontinuous Digesters and Chemical Flocculant Addition

J. P. Barford, R. G. Cail, 1. J. Callander, and E. J. Floyd The Department of Chemical Engineering, The University of Sydney, Australia

Accepted for publication March 2 1, 1985

Semicontinuous digesters were used to anaerobically treat high-strength whey (70 kg/m3 COD). A maximum loading of 16.1 kg COD/m3-day was obtained with soluble COD removal efficiencies greater than 99%. The use of a chemical flocculant resulted in an increased biomass concentration in the digester compared to a control, thus ena- bling correspondingly higher space loadings to be applied. With the onset of substantial levels of granulation of the biomass, flocculant dosage was able to be discontinued. This article discusses the performance of the digesters in detail and, briefly, the long-term operational difficulties experienced and the control strategies employed on such systems.

INTRODUCTION

Whey is a high-strength waste product of cheese manufacture. The introduction of stricter pollution control standards have provided an economic incentive for cheese manufacturers to reassess available methods of whey utilization and to investigate new ones. Whey contains a proportion of the milk proteins (lac- talbumin and lactoglobulin), most of water-soluble vi- tamins (riboflavin and thiamine), most of the lactose, and some mineral salts. Depending on the type of cheese being made, up to 9 L of whey are discharged for every kilogram of cheese produced, thus representing a problem of considerable magnitude in countries where dairy- ing and cheese manufacture is a major industry.

A number of solutions have been proposed to reduce the pollution level of whey, which rely on converting the whey or various components of it to marketable products. For example, whey can be used as a food additive, either in liquid form or as a dried product. However, because drying is very energy intensive, this solution is often uneconomic. Lactose may be re- covered by crystallization and used in infant foods or pharmaceuticals, but markets are limited and other car- bohydrates are generally cheaper. Reverse osmosis, which can be used to recover proteidlactose concen-

trates, results in a low-strength effluent virtually free of pollutants; however, the high capital costs and the contamination of the protein with large amounts of lactose reduces the economic potential of this procedure. Alternatively, ultrafiltration,’ a similar procedure to reverse osmosis, can be used to selectively recover various protein fractions with high market values, but the effluent discharged from such operations is still a very strong waste containing a11 the lactose and some protein. Another method which has been advocated is to use the whey as a substrate for the production of single-cell protein or for fermentation to Due to the relatively dilute nature of the feedstock (compared to alternative substrates such as molasses or sugar cane juice), ethanol fermentation systems are capital intensive and require costly processing steps to recover the product. Furthermore, for both single-cell protein production and fermentation to ethanol, the remaining waste still contains significant quantities of pollutants. Therefore, excepting reverse osmosis, all the above processes give rise to effluent streams of varying strengths and compositions that would require further treatment to meet river discharge standards if they were applicable.

Biological treatment systems, either aerobic, anaerobic, or combinations thereof, can be used to treat a wide variety of waste streams and are capable of re- ducing the levels of pollutants to meet even the most stringent requirements. However, one of the most commonly applied methods, the activated sludge pro- cess, is unsuited to the treatment of very high strength wastes such as whey, due to the large energy requirements for aeration and mixing, which lead to high running costs. In contrast, anaerobic systems have much lower running costs and produce methane, which can be used as an energy source. Despite this potential, anaerobic processes have not been highly regarded in the industry, in the past, largely due to the fact that

Biotechnology and Bioengineering, Vol. XXVIII, Pp. 1601-1607 (1986) 0 1986 John Wiley & Sons, Inc. CCC 0006-3592/86/111601-07$04.00

conventionally designed lagoons or digesters have relatively slow treatment rates and can suffer from poor stability unless the systems are overdesigned to com- pensate. These systems either require large areas of land or large digesters and, as such, have considerably higher capital costs compared to aerobic processes. Furthermore, lagoons also have problems of odor and offer little practical possibility of methane gas recovery.

Since the 1970s new digester technologies have been developed that have enabled much higher concentrations of biomass to be maintained in the digester, lead- ing to improved treatment rates, efficiencies, and pro- cess c o n t r ~ l . ~ , ~ These high-rate anaerobic digesters can be divided into three basic categories:

1. Upflow or downflow filter systems where the biomass is retained on packing material.

2. Fluidized bed reactors in which high concentrations of biomass are retained on particulate material (e.g., sand) that is fluidized by pumping liquid through the bed.

3. Sludge blanket digesters in which high biomass levels are accumulated and retained in the digester by means of an internal gas-liquid-solids separator that relies for its effectiveness on the development of natural or induced flocculation of the biomass which set- tles rapidly.

The anaerobic digestion of whey has been investigated by a number of researchers. Treatment efficiencies are affected by both reactor type and waste strength. In experiments with stirred tank reactors, Harish- chandra and Sovena6 achieved space loads of 2.92 kg BOD/m3 - day with 83% reduction of the total BOD, while Follmann and Mark17 reported a 98% reduction in the COD of centrifuged effluent at a space load of 5.6 kg COD/m3 - day. Holder and Sewards,' using a laboratory-scale contact digester, were able to achieve 90% reduction in the total COD at a space load of 4 COD/m3 . day, while Parker et al.9 achieved similar treatment efficiencies in a sludge blanket digester at a loading of 6 kg COD/m3 * day. Experiments with a fluidized bed system by Hickey and Owens'? gave 83.6% reduction of the total COD at a loading of 13.4 kg COD/m3 - day (HRT = 1.4 days). Much higher loadings were reported by the authors but at the expense of treatment efficiency. For example, at a loading of 37.6 kg COD/m3-day (HRT = 4.9 days), the total COD removal dropped to 72%. Sutton and Li," using the Dorr-Oliver, Anitron system removed 82% of the total COD at space loads of 5.3-7.4 kg COD/m3-day. Pre- liminary studies by Switzenbaum and Danskin,I2 using an expanded bed reactor, resulted in 92% removal of the COD at a space load of 8.2 kg COD/m3-day. The treatment of whey permeate (COD 48 kg/m3) in a fluidized bed digester was investigated by Boening and Lar- sen.13 They were able to remove 90% of the soluble COD at removal rates of 7.7 kg COD/m3.day. Van den Berg et al.I4 used a fixed film upflow reactor to treat

a simulated dairy waste (COD 4.4 g/L) in the laboratory. Only 45% of the COD was removed at a loading rate of 9.4 kg COD/m3-day.

The anaerobic research program at the University of Sydney has successfully developed the concept of a continuous Upflow Floc (Tower) digester (Callander and B a r f ~ r d ' ~ , ~ ' and Cail and B a r f ~ r d ' ~ , ' ~ that operates in a manner similar to the Upflow Anaerobic Sludge Blanket (UASB) described by Lettingal* but in which the retention of biomass is enhanced by the use of a chemical flocculant particularly during start up. In par- allel with this development has been the utilization of semicontinuous digesters to assess the amenability of a number of industrial wastes (starch effluent, palm oil mill effluent-both mes~philic '~ and thermophilic digestionZ0-and wool scouring wastewater) to anaerobic digestion. These digesters are operated on a mixkettle fill and draw basis. In this way, biomass accumulation in the digester is considerably enhanced over conventional stirred tank reactors. Biomass retention can be further improved by the addition of chemical polyelectrolytes to flocculate the particulate matter and promote better settling.2' This system, which can be considered similar to Upflow Floc or UASB reactors, is believed to give a conservative estimate of the treatment rates achievable in such reactors.

The maximum waste strength employed in previous studies of the anaerobic digestion of dairy wastes andor whey was less than 50 g COD/L. Furthermore, such studies have been of comparatively short duration. The aims of this investigation were to conduct a long-term study of the anaerobic digestion of very high strength dairy whey (approximately 70 kg COD/m3), using a 2-L semicontinuous digester, to assess the advantages of the addition of polyelectrolytes for flocculation and biomass accumulation compared to a control and to determine the maximum rates and efficiencies achievable in such systems. This article reports the complete set of data obtained over 478 days and describes the performance of the digesters, the effect of flocculants, the long-term operational difficulties experienced, the control strategies employed, and the likely effect of scale-up on such a system.

EXPERIMENTAL

Digester

The semicontinuous reactor was a 2-L Quickfit ves- sel (1.8-L working volume) equipped with a stirrer and scum breaker (Fig. 1). On day 334 the digester was modified as shown in Figure 2 in order to improve gas removal, foam breaking, and solids settling.

The digester was initially fed once daily manually, however, on day 27 this was converted to an automatic system operated by a cam timer. The operational cycle was based on a mixhettle, fill, and draw system. Feed

1602 BIOTECHNOLOGY AND BIOENGINEERING, VOL. 28, NOVEMBER 1986

Effluant L l Figure 1. Semicontinuous digester.

was added to the base of the digester every 3 h with simultaneous effluent withdrawal, by peristaltic pump, from the clear settled zone at the top of the digester. The feed was mixed for 20-155 min at 40-50 rpm by a paddle blade stirrer. The digester contents were then allowed to settle for the remainder of the 3-h period. By this method, biomass accumulation and flocculation is considerably enhanced compared to continuously stirred systems. At the start of the investigation, the feedinglmixing cycle was based on the sequence shown in Figure 3. However, this was later changed on day 413 to that shown in Figure 4, so that stirring was strictly limited, further minimizing floc disruption and improving solids settling. This policy proved to be very effective .

Analytical

Volatile fatty acid (VFA) concentrations were measured daily by gas chromatography using a simplified version of the method of Holdeman and Moore.22 Daily gas production was monitored by wet gas meter, and

Faad Muar

Digastor Mlxor

Figure 2. Cam cycle timer-sequence of operations (day 1412).

Gland Sealad With Water MF18 SOckQt Cone

Adaptor Wi th '' T I'

Connection

Food Flocc

- ulant

il Quickfit 2L Round

Paddle Blade Bottom Flask St i r re r

Figure 3. Modified semicontinuous digester.

the gas composition was periodically determined using gas chromatography on a Poropak N column. The chemical oxygen demand (COD), total kjeldahl nitrogen, and volatile suspended solids (VSS) were esti- mated in accordance with standard methods.23 The elemental composition of the whey was analyzed, after acid digestion, using atomic absorbtion spectropho- tometry. Soluble COD removal efficiencies were determined on the supernatant from samples centrifuged at 10,000 rpm for 10 min in a Sorval RCZB. Settled COD removal efficiencies were measured on the supernatant of the total effluent, which had been left standing in a 100-mL measuring cylinder for 1 h. This test simulated the effect of a clarifier on final effluent quality.

Flocculant Dosage

Callander and Barford** describe the selection and use of various flocculating agents for use in a Tower digester treating piggery wastes. On the basis of these

Faad Mlxor

Dlwstar Ukar Floccukmt Pump

Figure 4. Cam cycle timer-sequence of operations (day 412-478).

BARFORD ET AL.: ANAEROBIC DIGESTIONS OF CHEESE WHEY 1603

tests, Zetag 88N (Allied Colloids Sydney, Australia) was chosen. Dose rates were determined on the basis of bench settling tests. Initially the flocculant was added manually as a single dose. When the digester was modified, this addition was automated and the flocculant was added near the end of the mixing cycle (Fig. 4).

Whey Composition

The whey solution used was a nonhygroscopic cheese whey powder dissolved in a nutrient solution. Whey powder was obtained from Ibis Milk Products Ltd. (Shepparton, Victoria). The manufacturer supplied the following approximate analysis (all wt %): moisture 2.54.0; fat 0.8-1 S; protein 11 .O-14.0; nitrogen 1 .&2.2, NaCl 2.54.5; phosphorus 0.75-0.85; P205 1.6-2.0; calcium 0.65-0.75; sodium 0.65-0.75; potassium 2.1- 2.5; fiber 0; lactose 72. As whey is typically 6.6% solids, 73.01 g of this powder was added to 1 L of nutrient solution. The nutrient supplement was based on the work of Speece and M ~ C a r t y . ~ ~ The nutrient solution consisted of (all mg/L): (NH4)*HP04, 707;

FeCI,-6H20, 2048, giving the whey a final composition of: COD, 69,800; kjeldahl N, 1290; P, 570; K, 2000; Na, 870; Ca, 440; Mg, 214; Fe, 373; Co, 0.9; Ni, 1.1; Cu, 0.4; Zn, 0.8. The whey pH was adjusted to 6.5 with sodium hydroxide in order to control digester pH between 6.8 and 7.2. To minimize microbial breakdown of the feed, the feed containers were washed and replenished every day.

MgS04.7H20, 251; MgC12.6H20, 1,312; KCl, 491;

Seed Sludge

Each digester was seeded with 1.8 L of digested sludge taken from an anaerobic digester at Bondi Sew- age Works (Sydney). The sludge was first strained through cheesecloth to remove coarse solids, taking care to minimize contact with air. After transfer to the digesters, the head space was purged with 10-20 vol- umes of nitrogen.

RESULTS AND DISCUSSION

The objectives of this investigation were several- fold, namely, to determine maximum loading rates achievable in a semicontinuous digester treating high- strength whey; to assess the relative effectiveness of chemical flocculants for biomass accumulation over natural flocculation processes; to obtain long-term performance data and develop effective control strategies for stable operation.

The anaerobic digestion of whey was characterized by several distinct stages over a 478-day period. The discussion of the experimental results is most easily presented by considering each stage separately, viz;

Stage 1: Stage 2:

Acclimatization of seed sludge Comparison of the addition of flocculants in one digester (the flocculated digester) to a control (the unflocculated digester), culmi- nating in the failure of the control

Stage 3: Continued operation of the flocculated

la [ 16

a r - 1 0 a 0 c

' L a

0 " s

: : 4

a C XI .-

-I

a

n m

2

n

'I: - 0 50 100 150 200 250 300 350 400 450 500

TimQ (days)

Figure 5. formance, compared to a control.

Semicontinuous digestion of whey-effect of flocculant addition on per-


Stage 4: Stage 5 :

Startup after a downtime of 27 days Introduction of a modified digester to overcome the operational problems of poor solids settling and scum formation caused by gas flotation Further qualitative data and operational ex- perience to attempt to establish optimum solids (VSS) levels in digester

Stage 6:

Stage 1: Day 1-76

Acclimatization: Both digesters were seeded with a sample of domestic sewage digester sludge, and the space loading was gradually increased from 0.8-3.5 kg COD/m3-day over a 76-day period (Fig. 5). This procedure allowed sufficient time for the population of organisms to adapt to the whey. At the end of this time both digesters had nearly identical performance, that is, 98% of the soluble organic carbon was being removed. Biomass levels were 19.3 g VSS/L in the control and 17.4 g VSS/L in digester to be flocculated.

Stage 2: Day 77-147

The flocculated digester was dosed with polyelectrolyte flocculant from day 77, and the space loading was increased in small steps. It can be seen from Figure 5 that the performance of both digesters was similar up to a space load of 8.5-9.0 kg COD/m3-day (day 143). However, from this point, the control (unflocculated) began to fail, as characterized by rapidly rising VFA

* O I 70

0 5 0

/ --.

-l

concentrations, poor COD conversion, and a drop in pH. The reason for the failure of the control was organic overload, that is, there was insufficient biomass in the digester to assimilate the incoming organic load. Figure 6 clearly shows the marked advantage of polyelectrolyte addition on biomass accumulation. While the VSS in the control digester decreased slightly from day 69, the VSS in the flocculated system continued to rise quite rapidly. At the time of failure, the control (unflocculated) had 17.1 g VSS/L compared to 29.5 g VSS/L in the flocculated digester (i.e., 73% more biomass).

Stage 3: Day 148-204

The flocculated digester was operated for a further 60 days, by which time a maximum space loading of 12.4 kg COD/m3-day was reached and the VSS were 43.3 g/L. Treatment efficiency remained very high with conversion of more than 98% of the soluble COD. The VFA levels were low, (< 120 mg/L), indicating good digester stability. At this time, although the space loading was not maximal, the digester was shut down for the Christmas vacation. Column 1 of Table I summa- rizes the performance characteristics of the system at the time of shutdown. Total flocculant costs were small compared to the value of the methane generated (<4% based on fuel oil prices). The amount of flocculant added and the frequency of addition is shown in Figure 6.

---_-- Unt loccufatod - Flocculated

100 150 200 250 300 350 400 450 500

Time (days)

Figure 6. Semicontinuous digestion of whey-effect of flocculant addition on digester volatile suspended solids accumulation.

1605 BARFORD ET AL.: ANAEROBIC DIGESTIONS OF CHEESE WHEY


After a shutdown of 27 days (Christmas break) the operation of the whey digester was continued. Figure 5 clearly shows the robustness of a well-acclimqtized and flocculated system to prolonged shutdown. High space loadings were reestablished with the aid of flocculant in less than 2 weeks, and in 3 weeks the space loading had been increased to a maximum of 16.6 kg COD/m3-day (day 266). Treatment efficiencies continued to be greater than 98% removal of the soluble COD with low VFA (< 300 mg/L) concentrations, and the biomass levels in the digester ranging between 38 and 55.8 g VSS/L. The performance at this stage is sum- marized in Table I.

The variation in biomass levels in the flocculated digester during the period (Fig. 6) was primarily the result of two factors: (a) hindered settling at high biomass concentrations and (b) foam formation with sub- sequent biomass flotation as a result of the very rapid gas production at high space loads. The significant losses of biomass that occurred in the latter part of this stage resulted in a substantial decrease in the space load to half the previous maximum by day 333. Although, the stirring rate was increased to try and break up the surface scum, the static scum breaker device proved to be ineffective (Fig. 1). It is interesting to note that floc shear did not occur at the higher rpm, and micro- scopic examination showed considerable pelletization of the sludge.


From day 334 the culture was transferred, under a nitrogen blanket, to a modified semicontinuous digester, the main features of which were a rotating surface rake, an improved gas off-take port, and effluent withdrawal system to prevent line blockages. In view of the high degree of pelletization of the sludge, flocculant dosing was stopped on day 356. These changes were successful and virtually etiminated scum formation and improved sludge settling. As shown in Figure 5, a space load averaging 16.1 kg COD/m3-day was quickly reestablished and maintained for nearly 5 weeks. The average performance figures during this steady state (day 362-398) are shown in Table I.

The levels of both acetic and propionic tended to rise slowly but generally remained below 300 mg/L. Treatment efficiencies of the total effluent COD were usualiy above 90% and were better than 99% on the soluble COD. The supernatant from effluent samples left to settle for 1 h (to simulate a clarifier) showed COD conversion efficiencies greater than 98%. During this period, biomass levels rose from 24.0 g VSS/L to a maximum of 69.9 g VSSIL. Methane concentrations in the gas stream were in the range of 50-60%.

Unfortunately, the very high solids accumulated in

Table I. Summary of steady-state performance data for the semicontinuous anaerobic digestion of whey.

~ ~~~

Day Day Day Parameter 201-204 260-273 363-398

Space load 10.73

HRT (days) 6.4 (kg COD/m3.day)

Methane production 373 rate (l/kg COD in)

CH,% 57.2 Total COD 98

Settled COD (1 h) -

Soluble COD 99

VSS dig. (g/L) 43.3 VSS eff. (g/L) 1.1 VSS yield 9%

(removal %)

(removal %)

(removal %)

(kg VSS/kg COD in)

(kg COD/kg VSS)

propionic-mgll)

F/M 0.25

VFA (acetic/ I1 1/1

Flocculant addition Yes

16.1 16.1

4.3 4.3 29 1 325

52.8 53.0 93

98

99 99

51.3 62.4 5 5.8 - 8%

0.3 I 0.26

50/250 150/100

Yes No

-

-

the digester led to further operational problems. In particular, considerable foaming occurred that could not be easily controlled, and the rapidly settling sludge granules caused continual blockages of effluent withdrawal lines, further disrupting digester operation.


To overcome the problem of excess sludge and line blockages, the digester contents were strained through I-mm gauze in an attempt to remove the largest granules, and thus lower the digester solids. As Figure 6 shows, this procedure reduced the digester solids from

The space loading, which was initially lowered in accordance with the lower VSS present, was able to be increased as the biomass levels increased in the digester. However, continuing problems of effluent line blockages caused by the pelletized sludge and the re- sultant sludge buildup which caused excessive scum formation forced the eventual termination of the investigation. COD removal efficiencies were not re- corded during this period since the main aim was to establish a stable mode of operation. To a large extent, this aim was thwarted by the successful development of a highly granular sludge which settled very rapidly in lines. It is believed that these problems could be overcome in a large-scale digester through adequate design and a regular sludge wastage policy.

This investigation has highlighted a number of interesting areas for future research, particularly in re- lation to the effect of sludge age on biomass activity

64.3 g VSSIL to 43.7 g VSSIL.


and the consequent effect on sludge loading rates. There would appear to be an optimum between long sludge ages (20 days or more) in order to minimize sludge production and short sludge ages of 10 days or less. It would seem from our work that at long sludge ages biomass activity is poorer and only low sludge loadings <0.3 kg COD/kg VSS-day can be applied, thus re- quiring larger reactors. However, while biomass activity is often greater at shorter sludge ages, with sludge loadings of 0.9-1.5 being p o ~ s i b l e , ' ~ ~ ~ ~ sludge production is often higher due to less decomposition of microbial biomass and incomplete breakdown of effluent components. This situation is also reflected in lower conversion of the incoming COD to methane. Thus a balance exists between sludge age, sludge production, and conversion of COD to gas. Further research is required in this area in order to establish optimum operating policies.

CONCLUSIONS

This investigation was characterized by the following features:

1 . The use of a polyelectrolyte to flocculate the solids (including biomass) present in the digester resulted in a considerably higher biomass concentration compared to an unflocculated control. The consequence of this was that space loading rates were able to be increased much more rapidly without digester failure occurring.

2. Stirring the culture (even at low rpm) in the early stages of flocculation was detrimental to the formation of flocs and led to excessive flocculant consumption. The pattern of stirring and flocculant addition used initially was modified to minimize floc shear.

3. Start-up after a prolonged break (about 1 month) was rapid and trouble free.

4. Granulation of the sludge took about 10 months to develop to the stage where flocculant dosing could be stopped. These granules were highly resistant to shear.

5. A maximum loading of 16.1 kg COD/m3-day (HRT 4.3 days) was reached with over 99% removal of the soluble COD. This rate, which was obtained on a simulated whey effluent, was 2-3 times faster (for com- parable treatment efficiencies) than previously reported rates for the anaerobic digestion of whey and dairy wastes of considerably lower strengths.

6. A high degree of control was achievable using daily measurements of VFA concentrations. If the acid levels rose too high (above lo00 mg/L, total) or too rapidly, feed rates could be reduced to stabilize the system before it got out of control.

7. Further research is required in a number of areas, viz; design to minimize or eliminate problems caused by solids flotation at high gas production rates; the

effect of sludge age on biomass activity and the opti- mization of biomass activity and control of sludge production through specific sludge wastage policies.

Thus the use of a semicontinuous system to simulate a sludge blanket reactor for the anaerobic digestion of cheese whey enabled very high space loading rates and treatment efficiencies to be achieved. Furthermore, the addition of chemical flocculants during start-up gave significant advantages by assisting biomass accumulation, thereby allowing a shorter start-up time. Fi- nally, it could be reasonably expected that in a properly designed, continuous Upflow Floc digester or UASB significantly higher loadings might be possible.

The authors would like to thank M. Laginestra for his assis- tance in the initial stages of this investigation.

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BARFORD ET AL.: ANAEROBIC DIGESTIONS OF CHEESE WHEY 1607

anaerobic digestion of high-strength cheese whey utilizing semicontinuous digesters and chemical...

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