anaerobic digestion of wool scouring wastewater in a digester operated semi-continuously for biomass...

Agricultural Wastes 18 (1986) 27-38

Anaerobic Digestion of Wool Scouring Wastewater in a Digester Operated Semi-Continuously for Biomass

Retention

R. G. Cail, J. P. Bar ford & R. Lichacz

Department of Chemical Engineering, The University of Sydney, NSW 2006, Australia

A B S T R A C T

An anaerobic digester, operated semi-continuously in order to retain high concentrations of biomass in the digester, was used to treat wool scouring wastewater. At a space load of 9.gkg CODm -3 day -1 (hydraulic retention time, 2.8 days) > 56% of the COD and > 47% of the grease were removed. At these efficiencies, this rate was estimated to be at least 2.5-3.0 times greater than that which would be achieved in a continuously stirred digester. Preliminary studies of enzymatic pretreatment of the scouring effluent showed that significantly improved treatment rates and/or efficiencies could be achieved--i.e. > 70% removal of both the COD and grease at a space load of 12 kg COD m - 3 day- x. It is unlikely that any substantial levels of flocculation would develop in this system and it is expected that the moderate use of polyelectrolytes would be required to help maintain the VSS concentration in the reactor.

I N T R O D U C T I O N

The disposal of the high strength effluents produced by the wool scouring and carbonising industry represents a problem of considerable magnitude worldwide. Gibson et al (1981) reviewed the design, operation and economics of a wide variety of existing and prototype waste treatment processes for scouring effluents. These varied from traditional acid- cracking and lagoon systems to more recent processes such as hot-acid flocculation (an improved acid-cracking process), filtration, solvent extraction, chemical flocculation, evaporation/incineration and high rate

27 Agricultural Wastes 0141-4607/86/$03.50 © Elsevier Applied Science Publishers Ltd, England, 1986. Printed in Great Britain

28 R. G. Cail, J. P. Barford, R. Lichacz

activated-sludge reactors. While it is generally accepted that aerobic or anaerobic lagoons, followed by irrigation, offer the most cost effective solution, not all industries are fortunate enough to have adequate areas of land, or are in a favourable location, to install such systems.

The low energy requirement of anaerobic digesters, coupled with the fact that they produce methane which could be used in the mill for heating water, etc., gives them considerable advantages over other waste treatment systems. However, to date, this potential, with the exception of work on the development of stirred-tank digesters by Rodmell & Wilkie (1983) at a mill in New Zealand, does not appear to have been exploited by the wool industry worldwide. One of the/major problems with conventional stirred-tank reactors is that the treatment rates which can be achieved in such systems are comparatively low. Typically, hydraulic residence times of 10-20 days are required, depending on the type of waste and its strength. Thus, large digesters are needed and the high capital costs of such systems make them economically unattractive.

Recent developments in the design and operation of anaerobic digesters have resulted in significant improvements in the rate, process control and cost effectiveness of anaerobic treatment of effluents from agricultural and food-processing industries. All such designs rely for their effectiveness on retaining large amounts of active biomass in the reactor. These systems have been reviewed by Callander & Barford (1983) and Speece (1983). One process which has been successfully employed in the Department of Chemical Engineering, University of Sydney, is the semi-continuous digester system (Barford et al., 1986; Cail & Barford, 1985a,b). This is a simple, easily applied technology which, by virtue of the mode of operation, simulates a sludge-blanket reactor, enabling high cell densities to be maintained in the reactor and thus to achieve much higher treatment rates than are possible with conventional, continuously-stirred digesters.

The objectives of this programme were to investigate the feasibility of such a process for the treatment of wool-scouring effluent.

METHODS

Digester construction and operation

The construction of the 2-1itre semi-continuous digester used in this investigation is shown in Fig. 1.

Digestion of wool-scouring wastewater 29

MF18 Socket Cone Adapter With "T~ . Connect ion

Feed

ST20/2 St irr ing 'Gland Sealed With Water

~les

Surface Rake

Paddle B l a d e - Stirrer

2L Round Bo t tom Flask

Fig. 1. Semi-continuous digester used in this investigation.

The digester temperature was maintained at 35°C by partial immer- sion in a water bath. The digester pH varied between 7.1 and 7.4. No adjustment of the feed pH was required due to its high buffering capacity.

All feeding and effluent removal operations were controlled by a cam timer (Fig. 2). At the end of a 3 h settling period, fresh feed was added to the base of the digester during the latter part of the feed mixing cycle and the clarified effluent was simultaneously withdrawn from near the surface by means of peristaltic pumps. Following this operation, the reactor contents were briefly mixed at low speed, using the stirrer to distribute the feed, and then allowed to settle for the remainder of the 3 h period. In this way, relatively high levels of solids could be accumulated and retained in the reactor, thereby simulating the operation of a sludge-blanket process.


Feed Mixer Feed/Effluent Punm Digester M~er

locculanr l,'ump

III On Off

Fig. 2.

Analytical

180rain

I " i,.. 2o~. ~ --t I-,o., . "H ,~----?O.,n

Ffl----1~wn

I I

1 I

I

Digester operational sequence controlled by a cam-timer.

Volatile Fatty Acid (VFA) concentrations were measured daily by gas chromatography (Holdeman & Moore, 1975). Gas production was monitored by wet gas meter and the gas composition was determined using gas chromatography on a Poropak N column. The chemical oxygen demand (COD), biological oxygen demand (BOD), total Kjel- dahl nitrogen and Volatile Suspended Solids (VSS) were estimated by Standard Methods (APHA, 1975). The effluent was digested in aqua regia, and its elemental composition analysed by ICP emission spectro- photometry. Soluble COD removal efficiencies were determined on the supernatant of samples centrifuged at 10 000 rpm for 10 min in a Sorval RC2-B. Grease was estimated by extracting 50 ml of sample with 100 ml of a 50:50 mixture of carbon tetrachloride and absolute alcohol. This solution was mixed vigorously and then centrifuged; 30 ml of the carbon tetrachloride (bottom) layer was removed, evaporated to dryness in a tared crucible, and the residue weighed.

Flocculant

Zetag 88N (Allied Colloids, Sydney, Australia) was dosed at regular intervals to the digester. The quantity used was based on bench-scale settling tests as well as the amount of grease in the digester, since it was not desirable to accumulate too much wool grease in the reactor.

Enzyme

Actizyme, the enzyme used in the pretreatment studies, was obtained from Southern Cross Laboratories P/L, Dural, New South Wales, Australia.

Digestion of wool-scouring wastewater

TABLE 1 Typical Composition of Wool Scouring Effluent Used

in the Investigation

Component mg litre- x Component mg litre- l

COD 25 000 Na 252 BOD 8 000 K 1 470 SS 9 000 Mg 23 VSS 7 500 Ni 0.4 Grease 7 000 Mn 3 N 390 Mo < 1 P 13 Co <0.1 Fe 120 Cu 0.8 S 84 Zn 9.8 Ca 440

31

Effluent

Wool scouring effluent was obtained from a mill in New South Wales and stored frozen. The typical composit ion of the effluent is shown in Table 1. The nitrogen and phosphorous levels were supplemented to excess, with d iammonium hydrogen or thophosphate at the rate of 5 g litre-1. The digester was seeded with sludge from an anaerobic lagoon treating scouring effluent at the same mill where the effluent used in this study was obtained.

R E S U L T S A N D D I S C U S S I O N

In this investigation a semi-continuous digester, was used to assess its effectiveness in anaerobically treating wool-scouring wastewater both before and after pretreatment of the waste by an industrial enzyme product (Actizyme).

The organic space loading of the semi-continuous digester with and without actizyme pretreatment is shown in Fig. 3. Start up from the seed sludge was rapid, with a loading of 16-2 kg C O D per cubic metre per day (hydraulic retention time ( H R T ) = 1"4 days) being reached by day 57 (the Christmas break) despite pump failures on days 20, 40 and 44. This rapid start up is largely attributable to the fact that the hydrolysis of the wool grease components to VFA proceeds at a similar


2L. I 23i 22 21! 2O

17 16

E

Q

o, 12

9

6 S z, 3

2 1

F i g . 3 .

I 0 60 80

I I 20 z.O

Percent Achzyme added to Feed 2 2

0 LL 0 M, L | L • ~ .,.,, ~ ~ ° I ,

100 120 1£,0 160 180 200 220 2 0 60 280 l'=me (Days)

Space Ioadings applied to a semi-continuous digester treating wool scouring wastewater.

rate to the uptake and utilization of the VFA by the methanogens. This is in marked contrast to the situation which exists with easily hydrolysed compounds, such as sugars, which acidify rapidly and cause operational problems, with pH change and inhibition of the methanogens by high concentrations of VFA. Thus, the balanced nature of the digestion process, coupled with the high buffering capacity of the effluent itself, creates an inherently stable system, capable of rapid loading increases, without the risk of process failure, albeit at the expense of treatment efficiency.

Chemical flocculant, Zetag 88N, was added to the digester to improve the retention of solids, as shown in Fig. 4. It is clear from Fig. 4 that some polyelectrolyte addition is necessary in order to retain as much 'biomass' as possible (estimated as Volatile Suspended Solids (VSS)


5(>0

400

30-O

o 0 U

g20.o

O g o

F i g . 4.

Zetog 88N Addition (oJm 3 cligester doy)

I I 1 I I I l l I V l - - I I I 20 40 60 80 100 120 140 160 180 200 220 240 260 ~0

Tim<= (doys)

Effluent characteristics and solids concentrations in a semi-continuous digester treating wool scouring wastcwatcr.

minus grease) in the digester. An addition rate between 3 and 10 g per cubic metre of digester per day would seem to be the most practical. At a space load of 12 kg COD per cubic metre per day an addition rate of 3 g per cubic metre per day is equivalent to 9 to 10 g of Zetag per cubic metre of effluent treated, quite low by many water treatment standards. High rates of addition, such as those used initially, are not only expensive, but also cause problems of sludge flotation due to an excessive agglomeration of the solids present, which entrap the evolving biogas causing the particles to float. Following a shutdown of 11 days (days 57-68), the space load was rapidly increased to a loading of approximately 9 kg COD per cubic metre per day and allowed to stabilize at this level for some 8 to 9 weeks, (Fig. 3). A mass balance of the digester was carried out between days 130 and 134, inclusive. The results of this study are shown in 'Fable 2. At a space load of 9.9 kg COD per cubic metre per day (HRT, 2.8 days), the total COD removal, based on the production of methane, was estimated to be between 40% and 45%. On an effluent basis this figure was somewhat higher, at


TABLE 2 Performance Data for the Treatment of Wool Scouring Effluent with and without

Pretreatment, in a Semi-continuous Digester

Parameter No pretreatment

Actizyme pretreatment

Days 207-211 Days 238-242

Space load (kg COD m-3 day-1) 9.9 HRT (days) 2-8 Gas production (m 3 m- 3 day- ~)a 1.9 Methane (%) 76.8 Total COD removal, effluent (%) 58.6 Soluble COD removal (%) 65.5 Total BOD removal (%) 62.4 VSS digester (kg m- 3) 45-3 Grease in digester (kg m-3) 32-0 Sludge reduction (%) 34.0 Total grease removal (%) 47 Centrifuge grease removal (%) 66 Sludge loading (kg COD per kg

(VSS minus grease)) 0.75 Polyelectrolyte addition (grams per

cubic metre of digester per day) 3

11'9 12.2 2.1 2.1 2.9 3.3 68 67 78 > 66 b

38 20-25 29

> 70 71 75-80 est.

0.99

3 0

"Variable due to problems with feed and gas line bloCkages. b Includes loss of digester solids.

58.6% removal of the total COD (62.4% of the BOD). Soluble COD and BOD removals were 65.6% and 73.3%, respectively. Grease removal was 47% of the total going in and the reduction in VSS was 34%. The quantity of VSS discharged per kilogram of COD applied was 0.18 kg. Methane biogas production was 1.9m 3 CH 4 per cubic metre per day with methane averaging 76-8% of the biogas produced.

It is apparent that the major rate-limiting step in the process is the hydrolysis of the wool grease, due to its very complex chemical composition. As such, further increases in the space load would only result in a lowered COD removal efficiency. One way to overcome this situation is to pretreat the grease, so that it is either broken down to smaller molecules or becomes more amenable to breakdown. Actizyme was selected on the basis of the claims made for it commercially, its ready availability and ease of handling. It is an enzyme preparation sold in pellet form and contains proteases, amylases, cellulases and lipases


in a bran base. The product is described as being suitable for use in sewage treatment plants, septic tanks, abattoirs, piggeries, greasetraps, drains, etc., where it functions by increasing the rate of degradation of proteinaceous, starchy and greasy solids. The Actizyme pellets contained 15.5% moisture and consisted of 90% Volatile Solids with a COD of 1.3 g per gram of Volatile Solids. Thus, although Actizyme itself would add substantial quantities of COD to the feed, it was expected that its use would assist the degradation of some, or all, of the wool grease components. No attempt was made to optimise enzyme quantities and/or pretreatment times. Following some initial experimentation, a 1% w/v level of Actizyme was used to pretreat the scour liquor. The Actizyme was added to fresh feed and stirred continuously for 24 h prior to feeding it to the digester. Only a slight decrease in the feed COD (approximately 5%) was observed over this period. The use of the Actizyme increased the amount of biogas evolved, through increased COD conversion and through a slight lowering of digester pH, which liberated more of the dissolved CO 2. This had the undesirable effect of causing sludge flotation which resulted in frequent blockages of the gas lines and increased losses of digester solids in the effluent. A second problem with the use of Actizyme on a small scale was the fact that the extra solids present in the Actizyme pellets caused many feed line blockages, as shown in Fig. 2 (days 195, 214, 215, 221,223, 229, 235 and 251). This made it difficult to achieve steady-state operation. Two periods which had relatively trouble-free operation, between days 207 and 211 and 238 and 242, were analysed in detail; the results are shown in Table 2.

At an organic space loading rate of 11.9 to 12.2 kg COD per cubic metre per day (HRT, 2.1 days) the total COD removal in the effluent was a maximum during the period days 207 to 211 at 78%, compared with 59% achieved before pretreatment. Grease removal was high at >70%, compared with 47% on untreated waste. The measurement of biogas production was hampered by frequent line blockages and feed interruptions; however, by day 238 it had increased to at least 3.3 cubic metres per day with methane constituting about 67% of the total. When corrected for variations in loading and methane concentration, this rate is at least 30% higher than that achieved before enzyme pretreatment. This increase in gas production was not due to the cessation of polyelectrolyte dosing during the latter stages, as previous research (Callander, 1982; Callander & Barford, 1984) had


shown that, unlike a number of other polyelectrolytes tested, Zetag 88N did not appear to be inhibitory to the microorganisms or subject to biodeterioration.

As a consequence of the higher grease removal figures, the reduction of solids in the effluent was also higher, with > 70% of the Suspended Solids being removed.

From day 245, an attempt was made to increase the space loading rate to 18-19kg COD per cubic metre per day (HRT, 1.9-1.7 days). However, this was not particularly successful and the COD removal rate decreased to 50%-54%. The main reason for this was the relatively low level of VSS present in the digester at this time (about 20 to 25 g per litre). This situation arose due to problems with scum formation, feed line blockages and settling difficulties. An attempt to minimize scum formation by stopping polyelectrolyte addition was not particularly successful. By improving the operation of the scum breaker and making adjustments to the settling and mixing times, further loss of solids was minimized and the situation appeared to stabilize from day 260 on. Ideally, further adjustment of the polyelectrolyte addition rate would have been desirable to enable improved sludge retention but time did not permit us to achieve this.

While it is realized that the addition of 1% Actizyme to the effluent would be both expensive and impractical, a relatively high concentration was chosen in order to clearly establish the effect of such a pretreatment process. However, work in the literature (Bell & Oxham, 1971) and discussions with the manufacturers have indicated that concentrations of Actizyme as low as 0.01-0.001% w/v may also be effective. Addition- ally, the use of specific lipases (a number of which are becoming increasingly available commercially) may well prove even more effective than Actizyme, with the advantage that no additional COD would be added to the digester, as is the case with Actizyme which contains bran as an ingredient.

It is interesting to compare the results achieved in the semi-continuous digester with those achieved by Rodmell & Wilkie (1983) in conventional stirred anaerobic reactors. In a laboratory reactor they achieved 55% removal of the COD at a space load of 3.3 kg COD per cubic metre per day (0.9 kg BOD per cubic metre per day), while, at full scale and similar space loads, about 60% of the COD was removed. These loadings are significantly lower than those reached in the semi-continuous digester


where, at space loads of 9 and 12 kg COD per cubic metre per day, 59% and > 67% of the COD was removed before and after Actizyme pretreatment, respectively. Grease removal was also slightly higher in the semi-continuous digester.

One interesting finding by Rodmell and Wilkie was that, in general, the amount of BOD and grease removed did not vary greatly between 8, 10 and 20 days HRT, but that COD removals continued to increase with increasing HRT. Thus it would appear that there is a readily degradable fraction in wool scouring effluent which was removed in 8-10 days in their system. Increasing the retention time beyond this does not significantly improve BOD or grease removals. Furthermore, they also found that, as the waste strength increased, treatment efficiencies tended to improve. For example, with an influent COD of 66 g per litre, the COD removal was 55% at 20 days HRT, but with an influent COD of 100 g per litre (still HRT, 20 days) COD removal increased to 58%. At 10 days HRT the figures were 33% and 54%, respectively. At this stage, there would appear to be no ready explanation of this effect. However, the reason may be due to reaction kinetics and the K s for the breakdown of grease and other particulates. If the K s is relatively high, as is known to be the case for other difficult to degrade substances such as cellulose, then increasing the grease concentration (within limits), would, as a consequence, allow an increase in reaction rates (Verstraete, 1983).

In conclusion, it is clear from the results presented in this paper that the use of a sludge blanket process such as the semi-continuous digester, in which relatively high concentrations of biomass are retained with the assistance of moderate doses of polyelectrolyte, enabled significantly higher treatment rates to be obtained than with stirred tank reactors treating wool scour effluent. In addition, enzymic pretreatment of the raw effluent enabled further improvements in the treatment rates and/or efficiencies (and, as a consequence, increased gas production) to be achieved. At this stage, further studies are required at large laboratory and/or pilot scale in order to assess the design and operation of the process in more detail, on a wider range of scouring effluents and conditions likely to be encountered in industry. More research on the biochemistry of the hydrolysis of wool grease by anaerobic bacteria and/or enzyme would also be desirable, as an increased understanding of the mechanisms involved may lead to improved treatment processes.


A C K N O W L E D G E M E N T S

This work was supported by a grant from the Wool Research Trust Fund on the recommendation of the Australian Wool Corporation.

R E F E R E N C E S

APHA (1975). Standard methods for the examination of water and wastewater. (14th edn), American Public Health Association.

Barford, J. P., Cail, R. G., Callander, I. J. & Floyd, E. J. (1986). Anaerobic digestion of high strength cheese whey utilising semi-continuous digesters and chemical flocculant addition. Biotech. Bioeng. (In press).

Bell, K. W. & Oxham, J. S. (1971). Trial with Actizyme at country meatworks. Aust. Chem. Processing and Engineering, May, 18-19.

Cail, R. G. & Barford, J. P. (1985a). Mesophilic semi-continuous anaerobic digestion of palm oil mill effluent. Biomass, 7, (4), 287-96.

Cail, R. G. & Barford, J. P. (1985b). Thermophilic semi-continuous anaerobic digestion of palm oil mill effluent. Ag. Wastes, 13, (4), 195-304.

Callander, I. J. (1982). The development of the tower fermenter for anaerobic digestion. PhD Thesis, The University of Sydney.

Callander, I. J. & Barford, J. P. (1983). Recent advances in anaerobic digestion technology. Process Biochem., August 24-30.

Callander, I. J. & Barford, J. P. (1984). Improved anaerobic digestion of pig manure using a tower fermenter, Ag. Wastes, 11, 1-24.

Gibson, J. D. M., Morgan, V. V. & Robinson, B. (1981). Wool scouring and effluent treatments, Wool Sci. Rev., 57, 1-32.

Holdeman, L. V. & Moore, W. E. C. (1975). Anaerobic Laboratory.Manual, (3rd edn), Virginia Polytechnic Institute and State University, Blacksburg, Virginia, USA, 113.

Rodmell, P. A. & Wilkie, D. R. (1983). The biological treatment of wool scouring liquor. Transactions of the Inst. of Professional Engineers, NZ, 10, 2/CE, 33-45.

Speece, R. E. (1983) Anaerobic biotechnology for industrial wastewater treatment. Environ. Sci. Technol., 17, 416A-427A.

Verstraete, W. (1983). Biomethanation of Wastes: Perspectives and potentials. Biotechnology 83, Int. Conf., London, 4-6 May 1984, 725-42.

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