Agro-industrial
wastewater
297
Environmental Management and Health, Vol. 11 No. 4, 2000, pp. 297-306.#MCB University Press, 0956-6163
Cost-effective pre-treatment of
agro-industrial wastewater
Fatma A. El-Gohary, Fayza A. Nasr and Rifaat A.Wahaab
Water Pollution Control Dept, National Research Centre, Cairo, Egyptand
Hamdy I. Aly
Faculty of Engineering, Ain Shams University, Cairo, Egypt
Keywords Water treatment, Water policies
Abstract Pre-treatment of wastewater discharged from a potato-chips factory was the subject of this study. Wastewater discharged from this factory is characterized by high values of BOD, TSS and oil and grease (3,685, 4,115 and 169mg/l, respectively). Treatability studies via continuous aerobic and anaerobic methods treatment have been investigated. The results obtained showed marked improvement in the quality of the treated effluent using packing material versus the upflow anaerobic sludge bed reactor (UASB) without packing. An extra removal in COD (53 per cent), BOD (61 per cent), TSS (52 per cent) and oil and grease (46 per cent) was obtained. Corresponding residual values were 398mgO2/l, 235mgO2/l, 108mg/l and 38mg/l, respectively. Based on the laboratory results, engineering designs and economic evaluation of the different treatment systems were developed.
Introduction
Agro-industries are major contributors to the worldwide industrial pollution problem, Egypt being no exception. With the tremendous pace of technology development, substantial research is devoted to cope with wastes of ever increasing complexity generated by agro-industries. Therefore, agro-industries more than any other industrial sectors in this field require a dynamic and comprehensive approach for appropriate waste management.
Biological processes have long been used successfully to treat food industrial effluents (Bustenet al., 1990). The only difficulty occurs in the separation of the sludge from the treated effluent in the settling tank due to sludge bulking (e.g. Rensink and Donker, 1990). It has been proved that the feed pattern of the plant plays a predominant role in the occurrence or the absence of bulking sludge (Rensink, 1974). The traditional activated sludge process, however, is energy consuming and requires special skills for its operation and maintenance. A recent survey showed that the anaerobic technology has successfully been applied for the treatment of a number of organic wastes (Ni and Nyns, 1993). Among several anaerobic processes, the upflow anaerobic sludge bed reactor (UASB) is by far the most widely applied for wastewater treatment. It can be used both for very small scale and for large scale applications (Lettingaet al., 1991). It is an attractive alternative for the treatment of industrial effluents discharged from alcoholic and soft drink bottling industries, paper recycle and paper making mills, fruit and vegetable canneries, dairy industry and malting and brewing process (Lettinga and Hulshoff Pol, 1986 ).
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Extensive research has been focused on the methods of retaining a sufficient quantity of active biomass in the biological system and optimizing the microbial activity. Many packing materials were investigated. The support material that studies point out as useful design parameters are: pore size and shape of support (Dahab and Young, 1982) and distribute the flow (Song and Young, 1986). A porous ceramic carrier was used by Kawase et al. (1989) for immobilization of microorganisms inside the reactor. Polyvinyl chloride and backed clay carriers develop excellent methanogenic fixed films using acetic acid as substrate (Kawaseet al., 1989). The use of polyurethane foam sponges can be successfully made as a support material in anaerobic reactors fed on olive mill effluent (Rozzi, 1989). Chin (1989) also used charcoal chip and sand with a good performance of aerobic reactor treating high strength edible oil refinery wastewater.
To optimize microbial activity, the use of packing material has been tested by several investigators. A good biofilm medium must offer a high specific surface area, a good surface on which the bacteria can grow and can be held (Schulz, 1993) and it must avoid the clogging by a surplus of biomass. The permeation of nutrients and oxygen into all parts of the biomass layers must be assured (Dodwell Company, 1987). The main objective of the present study is to propose an appropriate low cost treatment technology for wastewater discharged from a potato chips factory.
Materials and methods
Biological treatment of settled end of pipe wastewater from a potato chips factory was carried out using continuous flow aerobic and anaerobic systems. Dimensions and operating conditions of the treatment schemes are presented in Table I.
Aerobic treatment
To develop the design parameters for the continuous system, batch laboratory experiments were carried out in two-litre plexiglass columns. MLSS volume was regulated to cover a range from 2.0 to 3.5g/l. Air supply was adjusted to maintain a minimum dissolved oxygen concentration of 2.0mg/l. Detention periods ranging from one hour to 24 hours were examined. The characteristics of the biologically treated effluent as indicated by COD values were determined
Table I.
Acting volume Litre 3 5 3.5
Hydraulic retention time Hours 6 18 13.5 Organic load Kg BOD/m3/day 0.9 2.9 3.9
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after 60 minutes' settlement. Sludge characteristics were also determined. Based on the results obtained, a completely mixed activated sludge unit was designed and manufactured (see Figure 1).
Anaerobic treatment
Two UASB-Reactors (packed and unpacked) were operated in parallel. Dimensions and operating conditions are shown in Table I. The two systems were fed continuously with wastewater. A schematic diagram of the UASB-reactor is shown in Figure 2. A bionet-structured tubular plastic medium is used as a packing material.
The performance of the treatment systems was evaluated by monitoring the quality of the feed and the effluent of each treatment unit. Physico-chemical analysis was carried out according to APHA (1997).
Results and discussion
Primary sedimentation
The wastewater contains considerable amounts of total suspended solids (4,115 mg/l) that may adversely affect the microbial activity. Therefore, sedimentation was necessary prior to the biological treatment step. To determine the optimum detention time, a settlability test, covering a range from 30 minutes to three
Figure 1.
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hours was carried out. The results indicated that a detention time of one hour is the optimum selection. COD, BOD and TSS values were reduced by 48 per cent, 40 per cent and 55 per cent respectively (Table II and Figures 3 and 4).
Aerobic treatment
The results of batch-experiments indicated that the highest BOD removal was achieved at a retention time ranging from five to six hours using a MLSS of 3g/l. Based on these results continuous treatment using completely mixed activated sludge system was carried out. The hydraulic detention time was kept constant at six hours.The average organic load was around 8.6 kg BOD/m3/day. The results obtained indicated significant reduction of the organic load. Average residual values of COD, BOD,TSS and Oil and Grease were 639, 316, 169 and 62mg/l, respectively (Table II and Figures 3 and 4). These values are in agreement with the standards set by Egyptian law for discharge into the sewerage system.
Anaerobic treatment
In an attempt to reduce energy cost, the use of packed and unpacked UASB reactors was investigated.
Unpacked UASB reactor
The reactor was operated at a detention time of 18 hours and average organic load of 2.9kg BOD/m3/day. Analysis of the UASB effluent showed reductions of 86 per cent and 82 per cent in COD and BOD. The corresponding residual values were 650 and 342mgO2/l, respectively. TSS ranged between 116 and
410mg/l with an average value of 203mg/l (Table III and Figures 3 and 4).
Figure 2.
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Table
II.
Performance
data
of
the
treatment
system
Raw wastewater Treated wastewater
Before sedimentation After sedimentaion
Percentage removal
Activated sludge effluent
Percentage removal
Parameter Unit Min Max Aver Min Max Aver Aver Min Max Aver Aver
pH 4.6 6.5 ± 5.6 7 ± ± 7.4 7.9 ±
Chemical oxygen demand mgO2/l 5,206 13,860 8,646 2,892 6,560 4,932 48 534 690 639 86
Biochemical oxygen demand mgO2/l 2,040 6,000 3,685 1,287 2,820 2,196 40 250 400 316 84
Total kjeldahl nitrogen mg N/l 132 399 227 84 227 164 40 22 100 63 62
Total phosphates mg/l 28 90 78 18 45 33 42 3.5 4.8 4.4 85
Total solids at 105ëC mg/l 5,654 12,830 8,213 2,307 8,584 5,133 37 1,001 2,002 1,343 69
Volatile solids at 550ëC mg/l 4,038 10,560 5,834 1,225 5,848 3,816 48 170 680 378 85
Total dissolved solids at 105ëC mg/l 1,802 6,577 3,848 1,207 5,954 3,087 20 885 1,770 1,140 56
Volatile dissolved solids at 550ëC mg/l 1,496 4,660 3,087 589 2,562 2,410 22 126 490 266 79
Total suspended solids at 105ëC mg/l 791 7,100 4,115 268 4,472 1,848 55 92 368 169 89
Volatile suspended solids at 550ëC mg/l 321 6,800 2,047 169 4,362 1,406 42 42 190 101 89
Settlable solids 10 ml/l 102 200 114 3 100 20 82 ± ± ± 100
Settlable solids 30 ml/l 40 240 131 6 90 23 82 ± ± ± 100
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The removal value reached 91 per cent. Average residual concentration of oil and grease was 63mg/l. The biogas production rate was 0.37m3/kg COD removed (Table III and Figures 3 and 4).
Packed UASB-reactor
The results from the operation of the packed reactor indicated that an improvement in COD, BOD, TSS and oil and grease of 53 per cent, 61 per cent,
Figure 3.
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52 per cent and 46 per cent, compared to the unpacked reactor was achieved (Table III and Figures 3 and 4). These results were obtained at a lower retention time (13.5h).
Design and economic study of the treatment systems
Based on the laboratory results a final process design was developed (Figures 5 and 6). Economic evaluation of the two treatment systems was carried out. The cost of construction of the treatment plant and supply of mechanical and
Figure 4.
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Table
III.
Performance
data
of
the
treatment
system
Settled wastewater
Unpacked anae reactor-effl.
Percentage removal
Packed anae
reactor-effl. Percentage
Parameter Unit Min Max Aver Min Max Aver Aver Min Max Aver removal
pH ± 5.6 7 ± 7.1 7.6 ± ± ± ± ± ±
Chemical oxygen demand mgO2/l 2,892 6,560 4,932 547 696 650 86 215 410 303 94
Biochemical oxygen demand mgO2/l 1,287 2,820 2,196 250 400 342 82 107 150 135 94
Total kjeldahl nitrogen mg N/l 84 227 164 64 128 95 37 54 78 67 77
Total phosphates mg/l 18 45 33 4.4 5 4.9 83 3.0 4.8 4.5 86
Total solids at 105ëC mg/l 2,307 8,084 5,133 1,062 2,450 1,414 68 905 1,481 1,281 75
Volatile solid at 550ëC mg/l 1,225 5,848 3,016 297 1,020 430 82 227 730 429 85
Total dissolved solids at 105ëC mg/l 1,207 5,954 3,087 890 2,040 1,212 53 820 1,380 1,009 67
Volatile dissolved solids at 550ëC mg/l 589 2,562 1,451 172 634 305 73 193 451 369 75
Total suspended solids at 105ëC mg/l 268 4,472 1,848 116 410 203 85 85 153 97 94
Volatile suspended solids at 550ëC mg/l 169 4,362 1,406 28 386 126 86 34 100 60 96
Settlable solids 10 ml/l 3 100 20 0.1 0.5 0.3 98.5 Nil Nil Nil 100
Settlable solids 30 ml/l 6 90 23 0.2 1.0 0.7 97 Nil Nil Nil 100
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electrical equipment is estimated as $1.6 million and $0.735 million for activated sludge and UASB systems, respectively. Therefore, it is clear that the cost of the anaerobic treatment using UASB reactor is relatively low an compared to the activated sludge system. Also, the operation and maintenance cost are lower due to the savings in energy consumption.
References
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Dahab, M.F. and Young, C.I. (1982), ``Retention and distribution of solids in fixed bed anaerobic filter'',Proc. 1st Int. Conf. Fixed-film Biological Process, Vol. 3, pp. 1337-51.
Figure 5.
Schematic diagram of the anaerobic treatment process
Figure 6.
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Dodwell Company (1987), ``Ring-lace as fixed biomass reactor'',Report, Tokyo (obtainable at the company Grunbeck, D-8884 Hochstadt).
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