Mesophilic semi-continuous anaerobic digestion of palm oil mill effluent

Mesophilic semi-continuous anaerobic digestion of palm oil mill effluent

Biomass 7 (1985) 287-295 Mesophilic Semi-continuous Anaerobic Digestion of Palm Oil Mill Effluent R.G. Cail and J.P. Barford Department of Chemical E...

398KB Sizes 9 Downloads 114 Views

Biomass 7 (1985) 287-295

Mesophilic Semi-continuous Anaerobic Digestion of Palm Oil Mill Effluent R.G. Cail and J.P. Barford Department of Chemical Engineering,The University of Sydney, NSW 2006, Australia (Received: 3 September. 1984)

ABSTRACT A semi-continuous digester was used to degrade palm oil mill effluent (POME) under mesophilic conditions. A space loading off2. 6 kg COD/m 3 day (hydraulic residence time o f 5. 6 days) was maintained for a month, with soluble COD removals being greater than 97%. This rate is significantly faster than the rates achieved in previously reported mesophilic studies. Key words: anaerobic digestion, semi-continuous, sludge blanket, palm

oil effluent, waste treatment.

INTRODUCTION Tile production of palm oil from the fruit ofElaeis guineensis is a major industry in S.E. Asia, notably in Malaysia. Some of the methods used to process the fruit have been described by Stanton.1 The principal sources of effluent are the steam sterilizer and the sludge separator (used in tile clarification stage). The effluent resulting from all processing operations contains high concentrations of free and dissolved oil and fatty acids, sodium soaps, glycerine, crude oil solids, starches, proteins and plant tissue. Such effluents typically have a biological oxygen demand (BOD) of 2 0 0 0 0 - 4 0 0 0 0 mg/litre; chemical oxygen demand (COD) 45 000287 Biomass 0144-4565/85/$03.30-© Elsevier Applied Science Publishers Ltd, England, 1985. Printed in Great Britain

288

R. G. Call, J. P. Barford

75 000 mg/litre; total solids, 4 0 0 0 0 - 6 0 0 0 0 mg/litre; suspended solids 2 0 0 0 0 - 3 0 0 0 0 mg/litre; oil and grease 4 8 0 0 - 1 2 0 0 0 mg/litre and pH 4.5-5.0. ~-5 The increasingly stringent water quality regulations being enforced in many S.E. Asihn countries have forced factories to investigate a wide range of approaches to the treatment of palm oil mill effluent (POME).1,6-8 Some of the treatment options which have been tried ranged from simple skimming devices, land disposal, use as animal fodder, chemical coagulation and flotation through to various aerobic and anaerobic microbiological processes. However, all systems have their disadvantages, due to incomplete treatment, large land requirements or high capital and/or running costs. The use of conventional anaerobic lagoons or digesters to treat POME. 2'4' s is characterized by long residence times, often in excess of 20 days, and thus large areas of land or large digesters are needed. Little research has been reported regarding the application of modem high rate mlaerobic digester technologies 8 such as upflow or downflow filters, fluidized beds, upflow anaerobic sludge blanket (UASB) systems or Upflow Floc digesters for the disposal of POME. Peyton et al. 4 investigated a contact stabilization process at laboratory scale using 10 days retention time, but ran into operational problems of poor sludge settleability. Based on this work, they subsequently built large-scale conventional mesophilic digesters with 20 days residence times. Chin a described a series of bench-scale experiments comparing fully mixed CSTR digesters with similar digesters operating with cell recycle. At hydraulic residence times of 10 days, a cell recycle system achieved 80% reduction of the COD on a settled fraction, but for greater than 90% COD removal hydraulic residence times of 20 days were required. The Department of Chemical Engineering has had considerable experience in the application of semi-continuous digesters for the anaerobic digestion of a number of wastes. In many cases the performances achieved have been considerably better than other reported investigations. Such digesters are simple to operate and by operating in a mix/settle mode it is possible to accumulate and relain high biomass concentrations in the digester, thereby simulating the sludge blanket principle of the UASB system. This investigation reports on the mesophilic semi-continuous digestion of POME.

Semi-continuous anaerobic digestion of palm oil mill effluent

289

MATERIALS AND METHODS Equipment A semi-continuous digester was constructed as shown in Fig. 1 using a 2 litre Quickfit vessel equipped with an independent stirrer and scum rake, through a rotating seal. The operational cycle (Fig. 2) was based on a fill and draw system, controlled by a cam timer. Feed was added to the base of the digester every 3 h with sinmltaneous effluent withdrawal, by peristaltic pump, from the clear settled zone at the top of the reactor. The feed was mixed briefly for 10-20 min at 40-50 rpm by the paddle blade stirrer and the contents were then allowed to settle for the remainder of the 3 11 period. By this method, biomass accumula-

MF18 SocKet Cone Adapter With " T ~ Connection \\ Feed ~' '\

I

ST 20/2 Stirring With Water /~Gas I1~ E!fflaent '

J ~-Somples

Surface RGKe

Paddle

B4ade

QuicKfit 2L Round 6ottom Flask

Stirrer

Fig. 1.

Two litre semi-continuous digester.

R. G. Call, Jr. P. Barford

290

L_

180 rnin

-~ ~ - lOmin

/'fl

I'-

11-"t

h'- I rain

2omJn

Feed Mixer

F(red/Effluerft Pum"D Digester

Floccukant

Mixer Pump

off

Fig. 2.

Semi-continuous operation - timing sequence.

tion and flocculation were encouraged. The digester was immersed in a water bath controlled at 35°C. Digester pH was maintained at 7.0-7-5 by manual adjustment of the feed pH with sodium hydroxide. Analytical methods Volatile fatty acid (VFA) concentrations were measured daily by gas chromatographyfl 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), total Kjeldahl nitrogen and volatile suspended solids (VSS) were estimated using standard methods. 1° The palm oil mill effluent (POME) was digested in aqua regia, and its elemental composition analysed by ICP emission spectrophotometry. Soluble COD removal efficiencies were determined on the supernatant of samples centrifuged at 10 000 rpm for 10 min in a Sorval RC2-B. Palm oil mill effluent (POME) The effluent was obtained from a commercial mill. It was thoroughly mixed and dispensed into 20 litre drums and frozen to prevent deterioration. Subsamples were thawed and stored at 4°C prior to use each day. The composition of the effluent per litre was: 70 g COD, 32.8 g SS, 28.8 g VSS, 920 mg Kjeldahl N, 171 mg P, 482 mg Fe, 296 mg S, 506 mg Ca, 17 mg Na, 1260 mg K, 446 mg Mg, 8 m g C u , 18 mg

Semi-continuous anaerobic digestion of palm oil mill effluent

291

Zn, 0.8 nag Mo, 0.05 mg Co, 3 mg Mn, 3 mg Ni, 229 mg A1, 5 mg B, 1 mg Ba, 131 mg Si. As such, the strength of the effluent was near the top of the range of literature values. Seed sludge The sludge was obtained from a mesophilic anaerobic lagoon treating POME.

RESULTS AND DISCUSSION

Space loading The digester was initially ted once a day to allow the seed sludge to acclimatize. As shown in Fig. 3, tile space load averaged about 3 kg COD/m 3 day during the first 2 weeks, with the load being occasionally reduced to keep the VFA concentrations below 100 mg/litre. The loading rate was then increased in steps of 0.5-1 kg COD/m 3 day consistent with the maintenance of low VFA levels (generally below 50 rag/ litre). On day 20, the digester was converted to automatic semicontinuous operation (Fig. 2). By day 34 the space loading reached

2O >,, cI

~-E

15

o y-

lO

"0 ID 0

,,,,,I

ID [0.. .,..)

0

50

100

150

Days

Fig. 3.

Mesophilicpalm oil effluent digestion

performance data.

292

R. G. Call, J. P. Barford

a maximum of 13.5 kg COD/m 3 day. The digestion process was hampered by numerous feed and effluent line blockages caused by grease and oils separating out of the POME at room temperature (20°C) and depositing in the lines. However, between days 40-67 the operation of the digesters was reasonably stable and an average space load of 12-6 kg COD/m 3 day was maintained, at a hydraulic residence time of 5-6 days. Further increases in loading rates were not possible due to continued line blockages and sludge flotation caused by the combination of high suspended solids concentrations in the digester and the rapid gas production. Many of these problems are the direct resL~lt of the small scale of the operation, so that further loading increases may be possible in a well-designed large-scale digester in which particular attention was given to preventing sludge flotation. COD conversion efficiencies Soluble COD conversion efficiencies were never less than 97% and were generally in the region of 99% even at space loads in excess of 12 kg COD/m 3 day. It is difficult to compare these treatment efficiencies directly with other investigations where the COD on total or settled fractions of the effluent were reported. Neither of these parameters was measured in this study, mainly due to the very variable discharges of sludge from the digester which caused large variations in the measured COD. However, the total COD removed can be estimated on the basis of sludge production figures using the value of 1 g VSS - 1.42 g COD. 11 Such a ratio would give a value of approximately 75% removal of total COD. By way of comparison, Peyton et al. 4 achieved total COD removals of approximately 70% after hydraulic residence times of 20 days, 3.6 times longer than the present study. Gas production Tt~e biogas methane concentration averaged 63.7% at space loads of approximately 12 kg COD/m a day. The conversion efficiency at these loadings was 234 ml CH4 per g of COD applied. The relatively low conversion of COD to methane indicated that a considerable portion of the influent COD was not being degraded in the digester despite the relatively high soluble COD conversion efficiencies. This was verified

Semi-co ntinuous anaerobic digestion of palm oil mill effluent

293

qualitatively by microscopic examination of the digester effluent which showed that considerable numbers of plant cells were only partially broken down. This is not unexpected due to the biochemicaUy complex nature of plant cell walls which are difficult to microbiologically hydrolyse. Thus, if the residence times are too short, cell breakdown will be incomplete. However, these residues do not appear to present a problem in the effluent as they settle rapidly and can be readily removed before final effluent discharge. Sludge characteristics The VSS concentration in the digester was relatively constant, at 21-122.4 g/litre, despite the operational problems described earlier. A mass balance on the production of sludge at the steady state space toad o1 12.6 kg COD/m 3 day showed that 0.25 kg VSS per kg COD applied was produced. This value is considerably higher than the usual value of 0.10.15 kg VSS per kg COD applied, due to the low conversion of plant cell material to methane and its subsequent presence in the digester effluent. Tile sludge loading (kg COD applied/kg VSS (digester) day) was found to be 0.57 at the steady state of 12.6 kg COD/m 3 day. While this figure is consistent with many values found in the literature it is somewhat lower than that achieved in continuous Upflow Floc digester operating on other types of wastes, where values in excess of 1-0 with sucrose-based stillage wastes have been attained. However, in view of the high proportion of plant cell material present in the digester, these lower values are not unexpected. While good flocculation was apparent in the digester, no pelletization of the sludge occurred during this investigation. Although Pol et al. 12 have reported that a highly active pelletized sludge can be developed in as little as 1-2 months from a poorly adapted sewage digester seed sludge fed a mixture of acetate, propionate and other nutrients, previous studies in this department using unadapted or poorly adapted seed cultures, on industrial wastes containing considerable quantities of suspended matter, have indicated that upwards of 12 months may be required before significant pelletization becomes evident. This is in general agreement with tile comments of Lettinga et al. 13 that pelletization can be hindered or prevented by the presence of finely dispersed poorly flocculating material. Thus it would appear that the duration of

294

R. G. Cail, J. P, Barford

the experiment was too short to draw firm conclusions about the possible development of pelletization.

CONCLUSION This investigation involved the application of a high rate semicontinuous anaerobic digester for the mesophilic treatment of palm oil mill effluent (POME). By keeping the mixing cycle short (in order to keep floc disruption to a minimum) the comparatively long settling times resulted in the accumulation of a high biomass concentration in the digester and the achievement of rapid and efficient degradation of the POME. As such, this process resembles a sludge blanket digester (e.g. UASB) in which an internal gravity separator assists biomass retention. A space load averaging 12.6 kg COD/m 3 day (HRT, 5.6 days) was maintained for about a month with total COD removal efficiencies in the region of 75% and soluble COD removals greater than 97%. These rates are 3 - 4 times faster than lagoon or conventional digester systems and are approximately twice as fast as other reported cell recycle digesters.Z, 4 Although some plant material passed through the digester without undergoing significant degradation, resulting in a higher discharge of sludge compared to a completely soluble waste, the sludge settles reasonably well and could be easily removed in an external clarifier. The high VSS concentrations present in the digester and the slightly viscous nature of the sludge, coupled to the high gas production rates, resulted in problems with buoying of the solids and subsequent losses of VSS. While this problem was partially overcome by the use of an independently rotatable top rake which served to break up the scum and release the entrapped gas, further experimentation as to the optimum stirring pattern is required. The relatively high quality effluent (in terms of soluble COD removal) discharged from the digester, while not meeting the stringent discharge to water course standards such as applying in Malaysia, would require only minimal further treatment to do so. Thus the rapid treatment rates reported here would lead to significant reductions in capital costs for a full-scale semi-continuous or UASB system.

Semi-contin uous anaerobic digestion of palm oil mill effluent

295

ACKNOWLEDGEMENTS The authors would like to express their appreciation to Miss J. Tanner and Mr E. F l o y d for their assistance. The financial assistance of Bizorba - Sanamatic is a c k n o w l e d g e d .

REFERENCES 1. Stanton, W. R. (1983). Agric. Wastes, 6, 3t-63. 2. Southworth, A. (1979). Proc. 34th indust, waste conf., Purdue University, Lafayette. 3. Chin, K. K. (198t). Water Research, 15, 19%202. 4. Peyton, T. O., Cooper, I. W. & Quah, S. K. (1979). Proc. 34th indust, waste conf., Purdue University, Lafayette. 5. Sinnappa, S. (1978). Int. conf. on water poll. control in developing countn'es, Bangkok, pp. 525-37. 6. Davis, J. B. (1978). Trop. Sci., 20(4), 232-62. 7. Davis, J. B. & Reilly, P. J. A. (1980). Oleagineaux, 35(6), 323-9. 8. PORIM (1982). PORIM regional workshop on palm oil mill technology and effluent, August, The Palm Oil Research Institute of Malaysia, Kuala Lumpur. 9. Holdeman, L. V. & Moore, W. E. C. (1975).Anaerobic laboratory manual, 3rd edn, Virginia Polytechnic Inst. Anaerobic Lab., Blacksburg. 10. APHA (1975). Standard methods for the examination o f water and wastewater. 141h edn, American Public Health Association, New York. 11. McCarty, P. L. (1975).Prog. Water Technol., 7(1), 157-72. 12. Pol, L. H., Dolfing, J., de Zeeuw. W. & Lettinga, G. (1982). Bioteeh. Letts., 4(5), 329-32. 13. Lettinga, G., van Velsen, A. F. M., Hobma, S. W., de Zeeuw, W. & Klapwijk, A. (1980). Biotech. Bioeng., 12,699-734.