Energy yields from anaerobic digestion of palm oil mill effluent

Energy yields from anaerobic digestion of palm oil mill effluent

Biological Wastes 19 (1987)257-266 Energy Yields from Anaerobic Digestion of Palm Oil Mill Effluent W. J. Ng," K. K. Chin" & K. K. W o n g b Departme...

411KB Sizes 2 Downloads 102 Views

Biological Wastes 19 (1987)257-266

Energy Yields from Anaerobic Digestion of Palm Oil Mill Effluent W. J. Ng," K. K. Chin" & K. K. W o n g b Department of Civil Engineering, National Universit5 of Singapore. Singapore 0511 b Department of Environmental Science, Uni~ersiti Pertanian Malaysia, Serdang, Malaysia (Received 8 April 1986: accepted 6 June 1986)

ABSTRACT

Three different suq)ended-growth anaerobic digestion col!figurations--the mesophilic one-stage, the mesophilic two-phase amt the thermophilic onestage, were used to treat pahn oil mill effluent (PO.~IE) and their performances compared. The mesophilic two-phase process showed the highest energ.v yields which reached 2 0 5 4 2 J g -L COD utilized at a hydraulic retention time of 31 days. However, high energy yiehts did not coincide with high TCOD remot:als. The latter was a characteristic of the thermophilic process. The relatively poor TCOD remot'al in the two-phase system was due to its lower efficiency #z VSS removal. A t h v~b'aulic retention times of 25 days and more the mesophilic one-stage process had higher energy yields than the thermophilie process. Anaerobic digestion was fotmd to be an effectit:e means for P O M E treatment.

INTRODUCTION The cultivation of the palm oil tree, Elaeis guineemis, has expanded significantly over recent years. As demand for vegetable oils increases, the oil palm is likely to become an increasingly important crop. World production of palm oil currently stands at around 3-5 million tonnes per year of which ASEAN (Association of"South-East Asian Nations) countries account for no less than 80%. Production is expected to reach 4 million tonnes in 1986 and to further expand to 6.5 million tonnes by 1990 (Malaysian Business, 1985). 257 Biological Wastes 0269-7483/87/5;03-50 © Elsevier Applied Science Publishers Ltd, England, 1987. Printed in Great Britain

FRUIT BUNCHES~

~

STERILIZER . . . . . . .

BUNCH HOPPERS

PROCESS

SHELLS --,-

FI BERS~

KERNELS

DRIED

DRIER

I





°



HYDROCYCLONE WASTEWATER

CENTRIFUGE---'-CLARIFICATION SLUDGE

SCREEN

- " " HEAVY SLUDGE

CONDENSATE

",-.-BOILER BLOWDOWN

PROCESS WASTEWATERS

Palm oil milling process.

DRIER I PALM OIL STORAGE

I

VACUUM

l I II l

Fig. I.

CENTRIFUGE

I

HYROCYLONE - -l SEPARATOR I

CLARIFIER

CRAC KER

WAI'ER

HOT

SCREEN ~

O[L- I -

PRESS CIRUDE

SEPARATOR

I

PRESS CAKE

DIGESTER

EMPTY STALKS. . . . . . . . . . STRIPPER

SOLID WASTE

COOLING WATER

DISCHARGE

STEAM TRAP

SPILLAGE

CLEANUP WATER

OTHER WASTEWATER

"~ .~

c3

.~ .~

I'41 L/I O0

Energy from digestion of POME

259

Palm oil is extracted from the fresh fruit bunches by a five-stage process: steam sterilization, fruit bunch stripping, digestion, oil extraction from the mesocarp and clarification (Fig. 1). The kernel is also usually separated from the shell at the mill using a hydrocyclone separator. At each of these stages some wastewater is produced. Condensate from the sterilizer accounts for 17% of the wastewater volume while the clarification stage produces 65%, and the hydrocyclone, 8%, of wastewater volume. Current mill practice combines all process wastewater with spillage, washwaters and steam condensate to give a thick, brownish effluent (POME). Total wastewater amounts to 2-3 tonnes per tonne of finished oil (Tjeng & Olie, 1975). Although non-toxic, POME, which has a BOD 5 exceeding 20000mglitre -~, can be a cause of water pollution. In 1980, Malaysian mills discharged 6 million tonnes of effluent which, in terms of BOD load, was equivalent to the load generated by a population of 7.3 million. P O M E represents a considerable waste of harvested materials and attempts are being made to recover useful materials from it. P O M E is highly amenable to anaerobic digestion (Sinnappa, 1978; Chin, 1981; Chin & Wong, 1983; Ng et al. 1985). This means that energy in the form of biogas may be recovered from it. There are approximately 210 mills operating in Malaysia alone and, collectively, they generate about 130000 tonnes BOD 5 per annum. The energy that could be recovered is substantial. This paper reports on the energy yields that may be obtained from P O M E when it is digested anaerobically. The anaerobic digestion configurations investigated were the suspended growth mesophilic one-stage, mesophilic two-phase and thermophilic one-stage.

METHODS Anaerobic digester systems All the digesters used were fabricated from plexiglass. In the case of the mesophilic one-stage system, the digesters had effective volumes of 15 litres. Mixing was accomplished by recirculating the digester contents with a pump. The two-phase system consisted of two digesters connected in series. The first-phase digesters had effective volumes of 0-87 litres. The secondphase digesters had effective volumes of 12 and 15 litres to give the different hydraulic retention times desired. Stirrers were used to mix the contents of the two-phase digesters. The temperature of the mesophilic one-stage and two-phase systems was maintained at 32°C. The digesters used in the

260

W. J. Ng, K. K. Chin, K. K. Wong

thermophilic one-stage system had volumes of 21itres. Mixing was accomplished using a shaker water bath maintained at 55 _+ I°C.

Reactor operation The digesters of the mesophilic one-stage system were operated to give hydraulic retention times of 7, 14, 21, 25, 30, 35, 50, 75 and 100 days. Cell recycle was not practised. The first-phase digesters in the two-phase systems operated at a hydraulic retention time of 1 day while the second-phase digesters had hydraulic retention times of 10, 20 and 30 days. A portion of the whole effluents from the second-phase digesters was channeled back to the first tank. This served to reduce the strength of the feed P O M E by about a third. The thermophilic one-stage digesters had hydraulic retention times of 5, 15, 25 and 35 days. Again, there was no cell recycle. All the digesters were seeded with sludge drawn from an anaerobic digester of a domestic sewage treatment plant. No inorganic nutrient was supplemented: pH adjustment was not necessary during runs. At start-up, pH adjustment was occasionally required and sodium bicarbonate was used. Digester gas was collected by displacing an aqueous solution of 5% HzSO,~ and 20% NazSO a in a collection chamber. The volume of gas produced was measured daily by first equilibrating the collected gas to atmospheric pressure.

Feed material POME was collected from commercial mills regularly and stored at 4°C until required. The characteristics of the P O M E are detailed in Table 1. The feeds (Table 1) for the three anaerobic systems were made up from raw P O M E by blending different batches. Analyses Digester gas production rates were monitored daily and the methane content assessed by gas chromatography on a 2 m long Porapak Q 80/100 column. Other parameters measured at regular intervals were alkalinity, Volatile Acids, BOD 5, COD, Volatile Solids, pH and nitrogen. These tests were conducted according to the procedures in the Standard Methods (APHS, 1980). All the values reported represent the average of results from experimental runs in the steady-state condition. Steady-state was assumed after three hydraulic retention times.

Energy from digestion of POME

261

TABLE 1 Characteristics of the P O M E and Digester Feeds

Parameter

pH Alkalinity (mg litre- 1) Total Volatile Acids (mg litre-~) B O D 5 (mglitre -~) C O D (mglitre l) TS (rag litre -1 ) TSS (rag litre-1) VSS (mglitre-1) T K N (mglitre-~) TP (rag litre- ~)

POME

3"5-5'2 980-1 240 670-1 800 20000-35000 30000-73 100 30000-56000 26 150-36 500 14800-30900 500-1 100 68-315

Mesophilic One-stage

Mesophilic Thermophilic Two-phase One-stage

4

60 2 S4O

4-4

--25000 45000 35000 -17 300 800 140

1 300 14 100 37600 34600 18 600 15 100 ---

I 000 28200 67400 54000 31 800 26700 l 000 --

- - , Not determined. The two-phase feed analyses are of feed diluted by recycle.

RESULTS AND D I S C U S S I O N

Organic load conversion effieiencies Table 2 lists some of the digestion parameters monitored. Space loading rates used in this study ranged from 450 to 3210 mg C O D litre-day-~, 1210 to 3420mg CODlitre-day -~ and 1930 to 13500mg CODlitre-day -~ for the mesophilic one-stage, two-phase and the thermophilic one-stage, respectively. The thermophilic one-stage digesters were loaded more heavily on the expectation that the thermophilic process would show a higher rate of biological assimilation. This expectation was not misplaced as the thermophilic system did show the highest organic load conversion efficiencies. At hydraulic retention times of 15 to 35 days, C O D removals exceeded 90% while a hydraulic retention time of 5 days resulted in approximately 71% C O D removal. This was comparable with the performance of the semi-continuous anaerobic digester reported by Call & Barford (1985). However, it should be noted that the latter system endeavoured to retain the biological cells within the digester. In this study cell retention or recycle was not specifically practised in any of the three systems reported although, in the two-phase system effluent was recycled. Both the mesophilic one-stage and two-phase systems had significantly lower C O D conversion efficiencies than the thermophilic system. At a hydraulic retention time of about 30 days, there was little

Loading rates (rag C O D litre-~hty- i )

- - , Not determined.

Mesophilic One-stage 14 3 210 21 2 140 25 1 800 30 1 500 35 1 290 50 900 75 600 100 450 Mesophilic Two-stage phase 1 + I0 3 420 1 + 20 1 790 1 + 30 1 210 T h e r m o p h i l i c One-stage 5 13 500 15 4 490 '25 2 700 35 1 930

ll)'~b'aulic retention thne (thO's)

12 360 10 700 8 300 1 260 630 570 450 340 3 480 1 300 1 220 7 640 1 750 1 330 1 210

13 950 12 300 9740 19 800 4 720 3 240 2950

TBOD 5 (mg l i t r e - 1)

22 260 18 630 12 010 11 570 8 040 6 710 5 290 4 500

TCOD ( m g litre - i)

880 460 290 200

1 230 1 060 l 090

---

---

---

TVA ( m g litre - ~)

Effluent p a r a m e t e r s

9 960 6 620 4 680 3 540

8 700 8 280 7400

3 000 2 700 2 400 2 300 1 860 I 910 I 400 1 200

. . . VSS (rag litre - 1)

.

TABLE 2 I-tlluent Characteristics of the A n a e r o b i c Digestion Systems

7-3 7"4 7-4 7-5

7.2 7'2 7'2

6"8 7"4 7"5 7-5 7-5 7.6 7-8 7-8

. . pll

.

70"6 93"0 95-2 95-6

63-9 67-3 74.1

50"5 58"6 73-3 74"3 82"1 85. I 88-2 90"0

. . 7'COD

72.9 93.8 95"3 95"7

75"3 90-8 91.3

50"6 57"2 90-8 94-9 97'4 97-7 98"2 98.6

62-7 75"2 82"4 86"7

42.3 45"2 51"0

826 84"4 86" 1 86-7 89"2 88.9 91"9 93" 1

% R e d u c t i o n #1 . TBODs VSS

Energy from digestion of POME

263

difference between the COD conversion efficiencies of the mesophilic onestage and two-phase systems. However, the two-phase system was significantly more efficient at shorter hydraulic retention times. This was probably a result of the recycling of a portion of the treated effluent to the first-phase digesters. The COD and BOD values reported represent the total COD (TCOD) and BOD (TBOD) concentrations in the effluent. No attempt was made to separate the soluble and suspended fractions. As such, the reduction in COD, BOD and VSS reported should be due primarily to gasification and not conversion to biomass. This is significant because both the two-phase and thermophilic systems showed lower VSS removal efflciencies than the mesophilic one-stage process. If it is assumed that the VSS in the effluent was made up largely of biological cells, then the biological yields of the two systems must be higher than the mesophilic one-stage process. The Suspended Solids in the effluents of the two-phase and thermophilic systems were noted to settle readily. Consequently, solids removal from the effluents should not present great difficulty. Gas yields It was noted that gas yields per unit COD removed tended to increase as hydraulic retention times increased (Table 3). However, little appeared to be gained in terms of gas yields by increasing the hydraulic retention times of the mesophilic one-stage system to more than 35 days. Gas yields of the thermophilic system were the lowest. However, at a hydraulic retention time of 5 days the gas yield was still 0-21 litreg -1 COD utilized. It would therefore appear that, in terms of gas yields, the thermophilic system is only likely to show an advantage at relatively short hydraulic retention times. The mesophilic two-phase system had significantly higher gas yields in comparison with the other two systems. The higher gas yield of the twophase process was probably a result of more efficient gasification of the organic substrate in both the first and second reactors. It is easier to provide an optimal environmental condition for the acid and methane producers if the acid fermentation phase can be effectively separated from the methane fermentation phase. Data show that, in the two-tank system, the second reactor essentially served as a methanogenic reactor while the first reactor was primarily an acidogenic reactor; the system was thus a 'two-phase', rather than a 'twostage', digester. This was indicated by the large increase in Volatile Fatty Acids from 1300 mg litre-1 in the feed to 5458mg litre-1 in the first-phase effluent. This was significantly reduced in the second-stage and Volatile Fatty Acids content in the effluent ranged from 1090 to 1230 mg litre-1 for

W. J. Ng, K. K. Chin, K. K. Wong

264

TABLE 3

Gas Yields of the Anaerobic Digestion Systems Retention time (days)

Gas yield (litres per gram COD utilized)

Methane 1%)

Energy yieM (Joules per gram COD utilized)

0"19 0.25 0.67 0'50 0'63 0.58 0.57 0.56

53 56 58 58 59 62 60 64

3736 5 194 14417 10 759 13 790 13 341 12 688 13 297

0.44 0-75 0"98

60 61 57

9 794 16973 20 542

0"21 0.22 0'30 0'37

59 65 67 68

4 597 5 305 7 547 9334

Mesophilic One-stage 14 21 25 30 35 50 75 100

Mesophilic Two-phase 1 + 10 1 + 20 1 + 30

Thermophilic One-stage 5 15 25 35

hydraulic retention times of 30 to 10 days. The presence of an organic substrate composed largely of Volatile Fatty Acids could have promoted gasification in the second reactor. Energy yields The biogas methane concentration was above 50% for the three anaerobic systems investigated. Methane concentration generally showed an increasing trend as space loads decreased. This was particularly obvious in the thermophilic system. An exception to this was the two-phase system where the methane concentration showed no discernible trend with regard to decreasing space loads. Energy yields are linked both to gas yields and methane concentration. Although gas derived from the thermophilic system was slightly higher in methane concentration for any given hydraulic retention time, its gas yields in terms of COD utilized were the lowest. Consequently, energy yields from the thermophilic system were the lowest. The energy yield of the two-phase system was significantly higher than the remaining two systems at comparable hydraulic retention times. This came about primarily because of higher gas yields from the system.

Energy frorn digestion of PO.!4E

265

CONCLUSION This investigation involved the study of three anaerobic digestion configurations applied to the treatment of palm oil mill effluent. One of these, the suspended growth mesophilic one-stage, is commonly used in the industry. The remaining two can be readily applied. Bearing in mind that the temperature of P O M E at discharge is between 45:C and 60:C, current anaerobic treatment practice using the mesophilic systems requires a lagoon and cooling tower for cooling the wastes. The need to cool may be eliminated by opting for the thermophilic process. In addition to this, the relatively low heating requirement under tropical conditions makes thermophilic digestion an attractive alternative. Conversion to the twophase system can also be achieved with relative ease. In practice, there is no need for two gas-tight tanks since the first stage could be achieved in an existing anaerobic lagoon. 'Sour' lagoon effluent rich in Volatile Fatty Acids could then be pumped into a conventional methanogenic digester. When comparing the three digestion systems in terms of energy yields per gram C O D utilized and organic load removal efficiencies, it was found that high organic load removal efficiencies at a given hydraulic retention time were not matched by high energy yields. The mesophilic two-phase system produced the highest energy yields for any given retention time but its C O D removal was lower than the thermophilic system and only better than the mesophilic one-stage system at hydraulic retention times of 21 days and lower. Its VSS reduction was even more significantly lower and was the lowest among the three systems. Both the thermophilic and the two-phase systems offer the possibility of reduced tankage requirements. The more rapid treatment rates in terms of COD removal should lead to reduction in capital costs for full-scale systems. The thermophilic system is the more rapid of the two and maximum savings in tankage requirements would be realized in this system. However, higher energy yields were produced by the two-phase system. This system is slower than the thermophilic system, and, in addition, the system would also discharge more sludge on account of its lower VSS reduction efficiencies. REFERENCES American Public Health Association (APHA) (1980). Standard methods jbr the examination of waste and wastewater. (15th edn), APHA, AWWA, WPCF, Washington. DC. Call, R. G. & Barford, J. P. (1985). Mesophilic semi-continuous anaerobic digestion of palm oil mill effluent. Biomass, 7, 287-95.

266

W. J. Ng, K. K. Chin, K. K. Wong

Chin, K. K. (1981). Anaerobic treatment kinetics of palm oil sludge. Water Res., 15, 199. Chin, K. K. & Wong, K. K. (1983). Thermophilic anaerobic digestion of palm oil mill effluent. Water Res., 17, 993. Malaysian Business (1985). Feb. 16, 50-4. Ng, W. J., Wong, K. K. & Chin, K. K. (1985). Two-phase anaerobic treatment kinetics of palm oil wastes. Water Res., 19, 667-9. Sinnappa, S. (1978). Treatment studies of palm oil waste effluent. Paper presented at the International Conference on Water Pollution Control in Developing Countries, Asian Institute of Technology, Bangkok, Thailand. Tjeng, T. D. & Olie, J. J. (1975). Why solvent extraction of palm oil is not recommended. Oleagineux, 30, 523-8.