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Utilization of dehydrated sewage sludge as an alternative nutrient to stimulate lipid waste degradation by the thermophilic oxic process
JOURNALOFBIOSCIENCE AND BIOENGINEERING Vol. 94, No. 2, 113-I 18. 2002
Utilization of Dehydrated Sewage Sludge as an Alternative Nutrient to Stimulate Lipid Waste Degradation by the Thermophilic Oxic Process KAZUNORl Institute ofApplied
NAKANO’
Biochemistry,
University
AND
MASkTOSHI
ofTsukuba,I-I-1
MATSUMURA’*
Tennodai, Tsukuba, Ibaraki 3058572,
Japan’
Received 14 February 2002/Accepted 10 May 2002
Dehydrated and dried powdered sewage sludge (SW) was examined for use as an alternative to yeast extract (YE) to promote the degradation of lipid materials by a thermophilic oxic process (TOP). Its stimulatory effect on lipid degradation was found to be superior to that of YE. When 1.5 g of SW was added in combination with urea and 60 ml of a trace-element solution, the degradation effkiency was 82.9% for a 120-h treatment of 15 g of salad oil while that attained with YE was 68.3%. Although the degradation efficiency attained for animal fat, lard, was 77.8% which was lower than for vegetable oil, salad oil, it was still comparable to that obtained with YE, 76.9%. The applicability of SW to lipid degradation was confirmed in tests on three kinds of highly concentrated lipid wastes. With a nutrient supplement consisting of SW, a constant degradation effkiency of around 75% in 120-h treatment was attained for all lipid wastes despite their different features. The results of an elemental analysis suggested that the effectiveness of SW as a nutrient to stimulate thermophilic microbial activity in TOP was attributable to both a suffkient quantity and variety of amino acids and mineral components. [Key words: thermophilic oxic process, lipid waste, nutrient supplementation, sewage sludge]
Lipid materials in wastewater discharged from kitchens, restaurants and the food industry are usually separated by a trap system such as air flotation to avoid contamination of the main treatment system and problems in the activated sludge process (1, 2). The treatment of trapped and highly concentrated lipid materials, however, remains a problem. Large quantities of highly concentrated lipid wastes (HCLW) are currently treated by incineration or landfill dumping despite drawbacks such as secondary air pollution (3) and a lack of space. Hence, an alternative treatment process for HCLW is urgently needed. Although biological wastewater treatment is relatively safe and attractive, its application to HCLW treatment has not been realized. Difficulty in the biodegradation of HCLW, due to physical properties such as a low solubility in water, existence in a solid form at general treatment temperatures and a high lipid concentration, has been considered the major drawback (2). The thermophilic oxic process (TOP), however, offers several advantages to overcome problems of biodegradability. TOP is a biological process involving a thermophilic solid state culture, i.e., above 6O”C, conducted under aerobic conditions (4). Under such conditions, favorable changes in the physical properties of HCLW, especially an increase in diffusion coefficients and the improved solubility of lipid materials in aqueous media enabling better mass transfer, can be expected (5). Furthermore, a thermo-
biostimulation,
biodegradation,
philic solid state culture system could also avoid the problems associated with a liquid system. The solid medium allows better contact between the low water soluble lipid materials and the microorganisms. From such points of view, TOP is advantageous and can be expected as a novel biological treatment strategy for HCLW. In our previous study, a nutrient supplement which contained the components necessary to promote the biodegradation of lipid materials in TOP was developed, and the importance of a balanced supplement consisting of organic and inorganic components was demonstrated (6). Regardless of the source of either vegetable oil or animal fat, the combination of 1.5 g of yeast extract (YE), 1 g of urea (U) and 60 ml of trace-element (TE) solution was effective in stimulating the degradation of 15 g of lipids. A significant effect of YE on the stimulation of TOP in spite of the presence of inorganic components, U and TE, was also elucidated. However, the difficulty in developing a defined medium to realize the effect of YE was confirmed. In actual waste treatment, expensive nutrients such as YE cannot be used. Consequently, a disposable biomass such as spent yeast from a brewery or excess sludge generated by wastewater treatment plants was proposed as a cheaper and more abundant alternative to YE in HCLW treatment. According to OECD reports, annual production of sewage sludge by public sewage plants in Japan was 1.7 million tons dry-weight and 86% was disposed by landfill and incineration (7). The use of sewage sludge as nutrients for HCLW treatment by TOP will not only help compensate for
* Corresponding author. e-mail: [email protected] phone/fax: +81-(0)298-53-6624 113
114
NAKANO AND MATSUMURA
the cost of nutrients but also aid in recycling through the reuse of abundant disposable sludge. In the present study, therefore, excess sewage sludge (SW) was examined for use as an alternative to YE. Test treatments by TOP stimulated by nutrient supplementation using SW were conducted both for fresh lipid materials and actual HCLW. The applicability of SW as a nutrient to stimulate TOP in treating HCLW was evaluated by comparing the degradation efficiency with that of YE. Based on a knowledge of the stimulatory function of the supplement developed previously (6) and an elemental analysis of SW, the stimulative mechanism of SW was discussed. MATERIALS AND METHODS Seed thermophilic microorganisms The mixed thermophilic microorganisms used in this study came from a thermophilic compost material. The compost was used to inoculate a pre-culture flask. A solid state pre-culture was operated as described previously (6). When the CO, concentration in the exhaust gas of the pre-culture flask reached around 5%, the wood chip carriers containing thermophilic microorganisms were taken from the pre-culture flask and used as a seed inocuhnn. The solid media used in this study were Reaction system commercial wood chips (mean size 1.5 mm; SNM-HKl; Sanyo Denki, Tokyo) for use in a household-garbage composter. The thermophilic oxic process reactors consisted of 1-Z Erlemneyer flasks half filled with wood chips (140 g, 52% moisture content), and set in a 60°C water bath as described previously (6). Aeration at 0.1 wm was done through a sparger placed at the bottom of the flask. The amount of commercial food oil or lipid waste added was 15 g. Supplemental compounds such as SW, YE, U and TE for testing the stimulation of biodegradation of lipid materials were suspended in 60 ml of distilled water and added to the reactor after adjusting the pH to 8. The amount of water supplied with the 60 ml of supplement solution was sufficient to adjust the moisture content of the system to about 60%. Treatment by TOP was initiated with an inoculation of about 7 g of seeded wood chips. In order to distribute any components added to the flasks homogeneously, mixing with a spatula was carried out manually every 24 h. The CO, concentration in the exhaust gas was monitored as an indicator of degradation activity (8). The stimulative effect of the nutrient supplement was evaluated baaed on degradation efficiency by measuring the amount of residual lipid materials in the system al&r treatment. Sewage sludge material In this study, SW was examined for use as an alternative to YE. The sludge material was obtained from a sewage plant for domestic waste water in Mito-city. A dehydrated excess sludge cake (Mean moisture content: 85.7%) was collected from the plant. For convenience in the laboratory, the sludge cake was dehydrated and powdered to use for SW by drying at 110°C for 48 h followed by grinding using a coffee mill (KMM30; Braun Japan, Yokohama). Total nitrogen and phosphorus contained in SW were 80.5 mg-N/g and 24.7 mg-P/g, respectively. The loss on ignition of SW was around 74%. Two kinds of commercial cooking ‘Qpes of lipid materials oils and three kinds of HCLW were used in this study. The fresh cooking oils were a commercial salad oil (Nisshin Seiyu, Tokyo) and lard (Yokozeki Yushi Kogyo, Ibaraki). Their average chemical oxygen demand (COD& was 2840 and 3 134 mg/g-oil, respectively. The HCLW were used cooking oil from a restaurant (HCLW 1), and trapped wastes from a food oil company (HCLW2) and from a meat factory (HCLW3). The average fat content of these wastes was 98.3, 92.4 and 55.3 wt%, respectively. Their
J. BIOSCI.BIOENG..
average COD,, was 2847,3273 and 2084 mg/g-fat, respectively. Analytical methods The amount of residual lipid material in the reaction system was measured from the weight of residue after the soxhlet extraction of wood chips with an appropriate solvent. For determination of extracellular residual lipids (ERL), n-hexane was used: for total residual lipids (TRL), which included intracellular lipids, a chloroform/methanol (2/l) mixture was used (9, 10). The COD,, was determined by calorimetric analysis using test kits @R/4000; Hach, Colorado, USA) as described (6). The CO, concentration in the exhaust gas was measured using a carbon dioxide detector tube (No.2L; Gastec, Kanagawa) after collecting the gas in a plastic bag. The pH of the reaction system was determined from a mixture of 1 g of sample and 10 ml of distilled water. The moisture content of samples was determined after drying at 105°C for 24 h. The T-N and T-P content of the SW was analyzed calorimetrically with phenate and ascorbic acid (1 1), respectively, after pre-treatment by Kjeldahl digestion (12). The loss on ignition of SW was obtained by ashing SW at 750°C for half hour using an electric hearth (KM-280; Advantec, Tokyo). Trace metals of SW were determined with an Inductively Coupled Argon Plasma Atomic Emmision Spectrophotometer (ICAP-757V; Nippon Jarrell-Ash, Tokyo), following pre-treatment by digestion using nitric acid and perchloric acid (9). Samples were also subjected to amino acids analysis (Amino Acid Analyzer: D-502; Dionex, Osaka) after deproteinization with sulfosalicylic acid.
RESULTS AND DISCUSSION Stimulative effect of SW as an alternative nutrient for lipid degradation by TOP In our previous study, a disposable biomass containing similar elements to YE was proposed as a substitute for use in HCLW treatment by TOP (6). For such a material, spent yeast from a brewery or excess sludge generated in a wastewater treatment plant may be appropriate. In this study, therefore, commercial dried baker’s yeast (DBY) and SW were examined as substitutes. To evaluate the stimulative effect of the substitutes on TOP, the degradation efficiency based on TRL obtained after a 120-h treatment was compared. Table 1 summarizes the degradation effkiency obtained with various supplements when 15 g of fresh salad oil and lard was used as the test material. As shown in the table, minimal degradation of lipids was observed when no supplements (only 60 ml of water) were added with more than 90% of lipids remaining even after 120-h treatment. A significant improvement in TABLE 1. Degradation efficiency of salad oil and lard in TOP stimulated by nutrient supplements of various organic materials Nutrient supplement No supplements
Degradation efficiency (%) Lard Salad oil
U 1.0 g+TE
8.6 42.5
3.9 22.2
YE 1.5g+U 1.0 g+TE YE 3.og+u l.Og+TE DBY 1.5 g+U l.Og+TE
68.3 68.2 67.6
76.9 70.7 -
SW 1.5 g+U l.Og+TE SW 2.0 g+U 1.0 g+TE SW 3.0 IZ+U 1.0 e+TE
82.9 78.2
63.9 75.8
77.8 79.6 Degradation efficiency (%) is defined as the percentage of material that disappeared after 120-h treatment of 15 g of fresh lipid. Abbreviations: YE, yeast extract; U, urea; TE, trace-element solution; DBY, dry baker’s yeast; SW, dry sewage sludge.
UTILIZATION OF SEWAGE SLUDGE AS AN ALTERNATIVE NUTRIENT
VOL.94,2002
the degradation effkiency was, however, possible by supplying nutrients that promote thermopilic microbial activity. Even when an inorganic nitrogen supplement, urea (U), was added in combination with the trace element solution (TE), the improvement in degradation was obvious. Further improvement in lipid degradation was realized by combining organic and inorganic suppiements (YE+U+TE). With the nutrient supplement developed in the previous study (6), YE 1.5 g+U 1.0 g+TE, a degradation efficiency of 68.3% and 76.9% was attained for salad oil and lard, respectively. On the other hand, with 1.5 g of commercial DBY, a similar degradation efficiency for salad oil (67.6%) was obtained when added in combination with 1 g of urea and TE. Thus, dried yeast was confirmed to have a similar stimulative effect as YE, revealing the applicability of spent yeast as an alternative material to YE. A further improvement in the degradation efficiency was obtained by using another substitute, SW. With a combination of 1.5 g of SW, a degradation efficiency of 82.9% after 120-h trea~ent was attained for salad oil. The degradation efficiency obtained for lard with the same combination was, however, not as high. It was also lower than that obtained with 1.5 g of YE. To ensure a sufficient amount of SW, further additions in combination with 1 g of U and TE were examined. An improved de~adation efficiency in lard with addition of SW was observed, but the improvement was not observed in the case of salad oil. The best degradation efficiency for lard (77.8%) was attained with a combination of 3.0 g of SW. From these results, it was confirmed that the degradation efficiency for fresh lipid materials obtained with a readily free and abundant material like SW was high compared to that obtained with the expensive nutrient material, YE. Stimulative function of components of SW in lipid degradation by TOP The nutrient supplement devel-
oped in our previous study consisted of a biomass component, a nitrogen source and some trace mineral components, which were supplied with YE, U and TE, respectively (6). TABLE 2. Comparison of degradation efftciency of salad oil in TOP obtained with various combinations of nutrient supplements using YE and SW
Since SW consists of not only bacterial cell-mass, but also inorganic nitrogen and mineral components derived from waste water, the nutrient supplement may not require such inorganic components to have a stimulative effect on lipid degradation by TOP. Thus, a test batch treatment to investigate the possibility of sole addition of SW as nu~ient to promote degradation of salad oil was carried out (Table 2). Although the addition of YE resulted in a low degradation efficiency of 28.3% after 120-h treatment, that of SW resulted in a very high degradation efficiency of 79.0%. This value was much higher than that attained by the model supplement with YE and even comparable to that attained with a combination of SW, U and TE. Further addition of TE did not change the degradation efficiency and addition of U resulted in an even lower efficiency than addition of SW. These results suggested that SW could satisfy both organic and inorganic components which were essential to promote lipid degradation by TOP. Previously, the signi~c~ce of the complements effect of amino acids and inorganic components was demonstrated when the stimulative effect of the nutrient supplement was investigated (6). Therefore, the mineral components and amino acids included in SW could be considered to function as nutrients for the degradation of lipid by TOP. Thus, an analysis of SW for amino acids and mineral components was conducted and their amounts were compared with those supplied by the model nutrient supplement with YE. Table 3 shows major amino acids detected in the solution of 1.5 g of YE and SW. Although YE dissolved in acidic water completely, SW dissolved only partially. However, the amount of amino acid detected in the SW solution was much larger than that in the YE solution. As shown by the relative value (SW/YE) for each kind of amino acid, amounts in SW solution were rich compared to YE solution. Thus, as an amino acid source, SW was confirmed to be superior to YE. Table 4 shows a comparison of representative mineral components detected in 1.5 g of SW and TE used for the nutri~t supTABLE 3. Comparison of major amino acids detected in the solution of YE and SW Amino acids
Nutrient supplement No suppleme& u l.Og U l.Og+TE YE YE YE YE
1.5g 1.5 g+U l.Og 1.5 g+TE lSg+U l.Og+TE
Degradation efficiency (%I 8.6 -3.0 42.5 28.3 36.1 52.4 68.3
SW 1.5g SW 1.5g+u l.Og SW 1.5 g+TE
79.0 59.6 79.6
SW 1.5 g+lJ 1.0 g+TE
82.9
YE-ash+U-l.Og+TE SW-ash+U l.Og+TE
54.4 67.2
Degradation efficiency (%) isdefined as the percentage of material that disappeared after 120-h treatment of 15 g of fresh salad oil. Abbreviations: YE-ash, ash obtained from 1.5 g of YE; SW-ash, ash obtained from 1.5 g of SW. Other abbreviations are the same as in Table 1.
115
Phosphoserine Phosph~th~ol~ine Aspartic acid Threonine Serine Glutamic acid Glycine Aianine Amino-~-bu~ric acid Valine Isoleucine Leucine ‘Tyrosine Phenylalanine @-Alanine ~-Aminoisobu~ri~ acid Ethanolamine Lysine Arginine
YE (nmol/mi)
SW (nmollml)
Relative value (SW/YE)
18 16 123 95 85 40 210 235 6 80 40 56 36 25 4 0 30 64 38
94 113 234 161 137 199 120 655 127 355 296 459 I44 245 126 366 53 223 197
5.22 7.06 1.90 1.69 1.61 4.98 0.57 2.79 21.17 4.44 7.40 8.20 4.00 9.80 31.50 1.77 3.48 5.18
1.5 g of YE and SW was used for preparing the solution.
116
NAKANO AND MATSUMURA
J. BIOSCL BIOENG..
TABLE 4. Comparison of amount of inorganic components contained in SW and TE used as nutrient supplements Elements Ca
Mg K Al Fe Mn cu Na Zn P
11.3 3.4 5.4 42.8 11.2 0.0 0.2 1.1 6.8 31.1
14.0 10.0 40.0 0.0 0.1 0.7 0.0 190.0 0.1 20.0
Amounts of inorganic components contained in 1.5 g of SW and 60 ml of TE are shown.
plement. It was found that SW had a variety of mineral components and a phosphorus content higher than that of TE. The significance of phosphorus in inducing a supplemental effect on other trace mineral components was demonstrated previously (6). A large amount of phosphorus may be one of the reasons for the good stimulatory effect of SW. To confirm the stimulative function of the mineral components of SW, degradation efficiency was examined with SW-ash. As shown in Table 2, the degradation efficiency obtained with SW-ash was higher than that with YE-ash when U and TE were added. Thus, also in terms of effective mineral components, SW was superior to YE. These results demonstrated the effectiveness of SW as a nutrient to satisfy the essential components necessary to promote the degradation of lipid materials. However, the total amount of nitrogen supplied by SW seemed critical to the degradation efficiency. Baaed on the total amount per unit of SW described previously, 1.5 g of SW can supply 121 mg-N, which corresponds to the amount of nitrogen supplied with 0.26 g of U. In the previous study, a sufficient amount of U per 15 g of lipid materials was around 0.5 g (6). Moreover, input inorganic nitrogen was converted into ammonia during the lipid degradation process and was evaporated under alkali conditions described below. Considering both the critical amount of nitrogen in SW and the loss of nitrogen as ammonia gas during treatment, the addition of a nitrogen source to SW may be needed as a nutrient supplement for TOP especially in long-term treatments. Degradation of salad oil by TOP stimulated by nutrient supplement using SW To compare the stimulative
effect of various kinds of nutrient supplements, degradation efficiency based on TRL obtained after 120-h treatment was adopted as described previously. The best degradation efficiency, X2.9%, was obtained with SW 1.5 g+U 1.0 g+TE. Even under such conditions, however, around 20% of the lipid material remained in the system after 120-h treatment. Whether the residual material after 120-h treatment will be degraded further or accumulated in the system at a later phase is important for establishing a continuous treatment process. To elucidate this, longer term test batch treatments were conducted. A representative time course of TOP obtained when the batch treatment was continued until 2 10 h is shown in Fig. 1. In this treatment, 15.4 g of fresh salad oil was used as the test material and a nutrient supplement, SW
4 -
2 0
50
100
150
200
Time (h)
FIG. 1. Time course of batch treatment of salad oil by TOP stimulated by nutrient supplement using SW. A combination of 1.5 g of SW, and 1 g of U and TE was used. (a) Changes in residual lipid materials. (b) CO, concentration in exhaust gas. (c) Degradation and removal efficiency of lipid materials. (d) Moisture content and pH of the system.
1.5 g+U 1.0 g+TE, was added to stimulate the process. To assess the lipid degradation process, TRL and ERL were compared as shown in Fig. la. The amount of ERL was half that of the salad oil even at 24 h, revealing a rapid absorption of lipid materials into cells after hydrolysis. Subsequently, a decrease in TRL was observed. The degradation efficiency calculated based on the amount of TRL reached 65.8% at 45 h (Fig. lc). This rapid degradation was followed by a phase of gradual degradation in which a decrease in intracellular lipid (difference between TRL and ERL) was observed. The lipid degradation by TOP was found to be divided into such two steps. The final amount of TRL after 210-h treatment was 1.Og, corresponding to a degradation efficiency of 93.5%. Consequently, further degradation of residual lipid materials after 120-h treatment was confirmed although the rate became slower at the later phase. The hydrolyzed lipid materials will be converted to cellmass or removed from the system as CO, gas. During the treatment, the exit gas was collected in a plastic bag to monitor the biodegradation activity in TOP and also to calculate the output of lipid degradation. The average CO, concentration in the exhaust gas is shown in Fig. lb. A peak in the concentration appeared two days after addition of the oil. The removal efficiency shown in Fig. lc was calculated based on the carbon balance considering total output carbon as CO, in the exhaust gas and total input carbon derived from salad oil, SW and U. The removal efficiency obtained at 45 h was 19.0%, and was much lower than the corresponding degradation efficiency obtained at the same treatment time point. It has been reported that oxidation limited
VOL.94,2002
UTILIZATION OF SEWAGE SLUDGE AS AN ALTE~ATIVE
the overall lipid degradation of activated sludge when the lipid concentration exceeded 0.8 g/l (1). Since the amount of lipid used in this study was much higher, the reduced removal efficiency compared with degradation efftciency could be considered due to rate limitation by the oxidation step. Although the inhibition of microorganisms by longchain fatty acids formed after lipid hydrolysis has been a problem for anaerobic microbial processing (13, 14), no such problem was observed for TOP. The thermophilic microbial activity was maintained and the difference in degradation and removal efficiency decreased at the later phase. The removal efficiency eventually reached 88.4% at 210 h. The relative value {removal efflciency/de~a~tion efliciency) at 210 h was 0.94, revealing a high rate of conversion of degraded lipid materials into CO, gas. It suggested that the smooth conversion of lipid material into CO, was eventually possible by TOP stimulated by nutrient supplements using SW. Moisture content was important to keep the microbial activity in the solid state culture system (15). Although water was not added after starting the treatment, the amount of moisture in the reactor was stable at around 65% as shown in Fig. Id. The amount of water formed by lipid degradation might be equivalent to that lost due to evaporation under the~ophilic aerated conditions. The pH of the system was also monitored (Fig. Id). Since no control system for pH was used in this study, the pH of the system would be expected to be lowered by fatty acids generated after lipid hydrolysis as shown in other studies (5). However, the pH was stable at around 8.5. This was found to be due to the generation of ammonia derived from U during treatment, resulting in the detection of ammonia in the exhaust gas (maximum concentration; 0.2%). Since such a function of U to maintain the pH of the system against acidifiers is useful to the feasibility of the process, the addition of enough U would be impo~~t for actual HCLW treatment by TOP. Degradation performance of HCLW with TOP stimulated by nutrient supplements using SW In the previous study, the achievable degradation efficiency for HCLW with TOP stimulated by nutrient supplements consisting of a semi-defined medium based on the model supplement was investigated (6). The result that the nutrient supplements giving the best degradation efficiency for different kinds of HCLW differed for each type of waste demonstrated a difference in requirements to stimulate the process to treat HCLW. Thus, to examine the applicability of SW as a nutrient to stimulate TOP for treating HCLW, tests were carried out under the same the~ophilic oxic conditions using 15 g of three kinds of actual HCLW, namely, HCLW 1, HCLW2 and HCLW3. The degradation efficiency after 120-h treatment was compared and results are shown in Table 5. For all of HCLW, the degradation efficiencies attained with 1.5 g of SW in combination with 1 g of U and TE were much higher than those obtained with the model nutrient supplement using I .5 g of YE. Moreover, an improvement in the degradation efficiency was possible by further addition of SW. With 3.Og of SW, the highest degradation efficiency could be attained for HCLWl and 2. Although the degradation efficiency for HCLW3 was lower than that attained with a semi-defined medium using YE, the supplement formulated
NUT~ENT
li?
TABLE 5. Comparison of degradation efficiency of HCLW in TOP stimulated by nutrient supplements formulated with semi-defined medium and SW.
Nutrient supplement
Degradation efficiency (%) HCLWI
HCLW2
HCLW3
No supplements U 1.0 g+TE
13.0 42.5
0.5 22.2
1.3 35.3
YE lSg+U YE3.0g+U YE 1.5g+U YE 1.5g+U
59.9 70.1 72.9 68.1
57.5 57.7 36.8 32.2
50.6 39.9 60.0 81.9
71.6 73.9
74.9 79.0
62.0 77.9
I.Og+TE I.Og+TE l.Og+TE+PO,O.l5g l.Og+TE+PO, I.OOg
SW 1.5g+U I.Og+TE SW 3.0 g+U 1.0 g+TE
Degradation efficiency (%) is defined as the percentage of material that disappeared after 120-h treatment of 15 g of actual HCLW. Abbreviations: HCLWl, used cooking oil from a restaurant; HCLW2, trapped waste from a food oil company; HCLW3, waste from a meat factory; PO,, N$HPO,. 2H,O. Other abbreviations are the same as in Table 1.
with SW worked well for all HCLW tested despite the different features of HCLW+ A degradation efficiency of around 75% was attained for all kinds of HCLW with a supplement consisting of 3.0 g of SW. No difference in the requirements to stimulate TOP for treating HCLW using a semi-defined medium was observed for TOP stimulated using SW. The sufftcient quantity and variety of amino acids and mineral components contained in SW may allow such flexibility for satisfying the nutrient requirements needed for HCLW treatment. Therefore, a readily available and possibly cost-free SW will be useful not only as an alternative to YE, but also as an appropriate nutrient that is effective for HCLW having different nutrient requirements. For convenience in the laboratory, dehydrated and powdered sewage sludge dried at 110°C for 48 h was used in the present study. However, a similar degradation efficiency could be obtained using a dehydrated non-drying sewage sludge cake (mois~re content; 85%) for salad oil degradation (data not shown). Therefore, a dehy~ated sewage sludge cake without drying will be applicable for HCLW treatment although the moisture adjustment of the system considering the amount already present in the sludge cake must be taken into account, In the present study, it took 2 10 h to attain a removal efficiency of 88.4% for salad oil in batch treatment. In repeated batch treatment, however, the acclimation of microorganisms can be expected and a shorter treatment time to attain a similar removal efficiency may be possible. Although no obvious inhibition of microorganisms by long-chain fatty acids formed after lipid hydrolysis was observed in batch treatment, whether contamination by new lipids added to residual lipid materials which often consist of long-chain fatty acid would influence the treatment efficiency or not would be important for developing a continuous treatment process. Although the applicability and conditions to use SW as a nutrient to promote lipid degradation by TOP were elucidated by batch treatment, further investigation concerning optimization of the input lipid loading or interval is needed to apply the proposed process to actual HCLW treatment.
118
NAKANOAND MATSUMURA ACKNOWLEDGMENTS
This study was partially supported by Grants-in-Aid for the Encouragement of Young Scientists from The Ministry of Education, Culture, Sports, Science and Technology, Japan. We wish to thank Dr. Catalino Alfafara for many useful suggestions in the manuscript preparation. REFERENCES 1. Hsu, T., Hanakl, K., and Matsumoto, J.: Kinetics of hydrolysis, oxidation, and adsorption during olive oil degradation by activated sludge. Biotechnol. Bioeng., 25, 1829-1839 (1983). 2. Lefebvre, X., Paul, E., Mauret, M., Baptiste, P., and Capdeville, B.: Kinetic characterization of saponified domestic lipid residues aerobic biodegradation. Wat. Res., 32, 3031-3038 (1998). 3. Chang, M. B. and I-hang, T. F.: The effects of temperature and oxygen content on the PCDD/PCDFs formation in MSW fly ash. Chemosphere, 40, 159164 (2000). 4. Liu, B. G., Noda, S., and Mori, T.: Complete decomposition of organic matter in high BOD wastewater by tbermophilic oxic process. Proc. Environ. Eng. Res., 29,77-84 (1992). 5. Becker, P., Koester, D., Popov, N., Markossian, S., Antranikian, G., and Mae&l, H.: The biodegradation of olive oil and the treatment of lipid-rich wool scouring wastewater under aerobic thermophilic conditions. Wat. Res., 33, 653-660 (1999). 6. Nakano, K. and Matsumura, M.: Improvement in the treatment efficiency of thermophilic oxic process for highly concentrated lipid wastes by nutrient supplementation. J. Biosci.
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Bioeng., 92, 532-538 (2001). 7. Organization for economic co-operation and development: OECD environmental data compendium 1999, p. 8283. OECD, Paris (1999). 8. Nakasaki, K., Sasaki, M., Shoda, M., and Kubota, II.: Change in microbial numbers during thermophilic composting of sewage sludge with reference to CO, evolution rate. Appl. Environ. Microbial., 49,37-41 (1985). 9. Japanese standards association: JIS handbook environmental measurements 10, p. 909-1015. Japanese standards association, Tokyo (1995). (in Japanese) 10. Dudd, S. N., Regert, M., and Evershed, R P.: Assessing microbial lipid contributions during laboratory degradations of fats and oils and pure triacylglycerols absorbed in ceramic potsherds. Org. Geochem., 29,1345-1354 (1998). 11. Petts, K W. and Belcber, M.: Au automated method for the simultaneous determination of nitrogen and phosphorus in plant material, sludge and soil. Wat. Pollut. Control, 79, 399404 (1980). 12. Greenberg, A. E., Clesceri, L. S., and Eaton, A. D.: Standard methods for examination of water and wastewater, p. 80-l 15. American Public Health Association, Washington, DC. (1992). 13. Hwu, C. S., Donlon, B., and Lettiuga, G.: Comparative toxicity of long-chain fatty acid to anaerobic sludges from various origins. Wat. Sci. Tech., 34,351-358 (1996). 14. Angelidaki, I. and Abring, B. K.: Effects of free long-chain fatty acids on thermophilic anaerobic digestion. Appl. Microbiol. Biotechnol., 37, 808-812 (1992). 15. Nakasaki, K., Aoki, N., and Kubota, H.: Accelerated composting of grass clippings by controlling moisture level. Waste Manage. Res., 12, 13-20 (1994).
Utilization of Dehydrated Sewage Sludge as an Alternative Nutrient to Stimulate Lipid Waste Degradation by the Thermophilic Oxic Process KAZUNORl Institute ofApplied
NAKANO’
Biochemistry,
University
AND
MASkTOSHI
ofTsukuba,I-I-1
MATSUMURA’*
Tennodai, Tsukuba, Ibaraki 3058572,
Japan’
Received 14 February 2002/Accepted 10 May 2002
Dehydrated and dried powdered sewage sludge (SW) was examined for use as an alternative to yeast extract (YE) to promote the degradation of lipid materials by a thermophilic oxic process (TOP). Its stimulatory effect on lipid degradation was found to be superior to that of YE. When 1.5 g of SW was added in combination with urea and 60 ml of a trace-element solution, the degradation effkiency was 82.9% for a 120-h treatment of 15 g of salad oil while that attained with YE was 68.3%. Although the degradation efficiency attained for animal fat, lard, was 77.8% which was lower than for vegetable oil, salad oil, it was still comparable to that obtained with YE, 76.9%. The applicability of SW to lipid degradation was confirmed in tests on three kinds of highly concentrated lipid wastes. With a nutrient supplement consisting of SW, a constant degradation effkiency of around 75% in 120-h treatment was attained for all lipid wastes despite their different features. The results of an elemental analysis suggested that the effectiveness of SW as a nutrient to stimulate thermophilic microbial activity in TOP was attributable to both a suffkient quantity and variety of amino acids and mineral components. [Key words: thermophilic oxic process, lipid waste, nutrient supplementation, sewage sludge]
Lipid materials in wastewater discharged from kitchens, restaurants and the food industry are usually separated by a trap system such as air flotation to avoid contamination of the main treatment system and problems in the activated sludge process (1, 2). The treatment of trapped and highly concentrated lipid materials, however, remains a problem. Large quantities of highly concentrated lipid wastes (HCLW) are currently treated by incineration or landfill dumping despite drawbacks such as secondary air pollution (3) and a lack of space. Hence, an alternative treatment process for HCLW is urgently needed. Although biological wastewater treatment is relatively safe and attractive, its application to HCLW treatment has not been realized. Difficulty in the biodegradation of HCLW, due to physical properties such as a low solubility in water, existence in a solid form at general treatment temperatures and a high lipid concentration, has been considered the major drawback (2). The thermophilic oxic process (TOP), however, offers several advantages to overcome problems of biodegradability. TOP is a biological process involving a thermophilic solid state culture, i.e., above 6O”C, conducted under aerobic conditions (4). Under such conditions, favorable changes in the physical properties of HCLW, especially an increase in diffusion coefficients and the improved solubility of lipid materials in aqueous media enabling better mass transfer, can be expected (5). Furthermore, a thermo-
biostimulation,
biodegradation,
philic solid state culture system could also avoid the problems associated with a liquid system. The solid medium allows better contact between the low water soluble lipid materials and the microorganisms. From such points of view, TOP is advantageous and can be expected as a novel biological treatment strategy for HCLW. In our previous study, a nutrient supplement which contained the components necessary to promote the biodegradation of lipid materials in TOP was developed, and the importance of a balanced supplement consisting of organic and inorganic components was demonstrated (6). Regardless of the source of either vegetable oil or animal fat, the combination of 1.5 g of yeast extract (YE), 1 g of urea (U) and 60 ml of trace-element (TE) solution was effective in stimulating the degradation of 15 g of lipids. A significant effect of YE on the stimulation of TOP in spite of the presence of inorganic components, U and TE, was also elucidated. However, the difficulty in developing a defined medium to realize the effect of YE was confirmed. In actual waste treatment, expensive nutrients such as YE cannot be used. Consequently, a disposable biomass such as spent yeast from a brewery or excess sludge generated by wastewater treatment plants was proposed as a cheaper and more abundant alternative to YE in HCLW treatment. According to OECD reports, annual production of sewage sludge by public sewage plants in Japan was 1.7 million tons dry-weight and 86% was disposed by landfill and incineration (7). The use of sewage sludge as nutrients for HCLW treatment by TOP will not only help compensate for
* Corresponding author. e-mail: [email protected] phone/fax: +81-(0)298-53-6624 113
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the cost of nutrients but also aid in recycling through the reuse of abundant disposable sludge. In the present study, therefore, excess sewage sludge (SW) was examined for use as an alternative to YE. Test treatments by TOP stimulated by nutrient supplementation using SW were conducted both for fresh lipid materials and actual HCLW. The applicability of SW as a nutrient to stimulate TOP in treating HCLW was evaluated by comparing the degradation efficiency with that of YE. Based on a knowledge of the stimulatory function of the supplement developed previously (6) and an elemental analysis of SW, the stimulative mechanism of SW was discussed. MATERIALS AND METHODS Seed thermophilic microorganisms The mixed thermophilic microorganisms used in this study came from a thermophilic compost material. The compost was used to inoculate a pre-culture flask. A solid state pre-culture was operated as described previously (6). When the CO, concentration in the exhaust gas of the pre-culture flask reached around 5%, the wood chip carriers containing thermophilic microorganisms were taken from the pre-culture flask and used as a seed inocuhnn. The solid media used in this study were Reaction system commercial wood chips (mean size 1.5 mm; SNM-HKl; Sanyo Denki, Tokyo) for use in a household-garbage composter. The thermophilic oxic process reactors consisted of 1-Z Erlemneyer flasks half filled with wood chips (140 g, 52% moisture content), and set in a 60°C water bath as described previously (6). Aeration at 0.1 wm was done through a sparger placed at the bottom of the flask. The amount of commercial food oil or lipid waste added was 15 g. Supplemental compounds such as SW, YE, U and TE for testing the stimulation of biodegradation of lipid materials were suspended in 60 ml of distilled water and added to the reactor after adjusting the pH to 8. The amount of water supplied with the 60 ml of supplement solution was sufficient to adjust the moisture content of the system to about 60%. Treatment by TOP was initiated with an inoculation of about 7 g of seeded wood chips. In order to distribute any components added to the flasks homogeneously, mixing with a spatula was carried out manually every 24 h. The CO, concentration in the exhaust gas was monitored as an indicator of degradation activity (8). The stimulative effect of the nutrient supplement was evaluated baaed on degradation efficiency by measuring the amount of residual lipid materials in the system al&r treatment. Sewage sludge material In this study, SW was examined for use as an alternative to YE. The sludge material was obtained from a sewage plant for domestic waste water in Mito-city. A dehydrated excess sludge cake (Mean moisture content: 85.7%) was collected from the plant. For convenience in the laboratory, the sludge cake was dehydrated and powdered to use for SW by drying at 110°C for 48 h followed by grinding using a coffee mill (KMM30; Braun Japan, Yokohama). Total nitrogen and phosphorus contained in SW were 80.5 mg-N/g and 24.7 mg-P/g, respectively. The loss on ignition of SW was around 74%. Two kinds of commercial cooking ‘Qpes of lipid materials oils and three kinds of HCLW were used in this study. The fresh cooking oils were a commercial salad oil (Nisshin Seiyu, Tokyo) and lard (Yokozeki Yushi Kogyo, Ibaraki). Their average chemical oxygen demand (COD& was 2840 and 3 134 mg/g-oil, respectively. The HCLW were used cooking oil from a restaurant (HCLW 1), and trapped wastes from a food oil company (HCLW2) and from a meat factory (HCLW3). The average fat content of these wastes was 98.3, 92.4 and 55.3 wt%, respectively. Their
J. BIOSCI.BIOENG..
average COD,, was 2847,3273 and 2084 mg/g-fat, respectively. Analytical methods The amount of residual lipid material in the reaction system was measured from the weight of residue after the soxhlet extraction of wood chips with an appropriate solvent. For determination of extracellular residual lipids (ERL), n-hexane was used: for total residual lipids (TRL), which included intracellular lipids, a chloroform/methanol (2/l) mixture was used (9, 10). The COD,, was determined by calorimetric analysis using test kits @R/4000; Hach, Colorado, USA) as described (6). The CO, concentration in the exhaust gas was measured using a carbon dioxide detector tube (No.2L; Gastec, Kanagawa) after collecting the gas in a plastic bag. The pH of the reaction system was determined from a mixture of 1 g of sample and 10 ml of distilled water. The moisture content of samples was determined after drying at 105°C for 24 h. The T-N and T-P content of the SW was analyzed calorimetrically with phenate and ascorbic acid (1 1), respectively, after pre-treatment by Kjeldahl digestion (12). The loss on ignition of SW was obtained by ashing SW at 750°C for half hour using an electric hearth (KM-280; Advantec, Tokyo). Trace metals of SW were determined with an Inductively Coupled Argon Plasma Atomic Emmision Spectrophotometer (ICAP-757V; Nippon Jarrell-Ash, Tokyo), following pre-treatment by digestion using nitric acid and perchloric acid (9). Samples were also subjected to amino acids analysis (Amino Acid Analyzer: D-502; Dionex, Osaka) after deproteinization with sulfosalicylic acid.
RESULTS AND DISCUSSION Stimulative effect of SW as an alternative nutrient for lipid degradation by TOP In our previous study, a disposable biomass containing similar elements to YE was proposed as a substitute for use in HCLW treatment by TOP (6). For such a material, spent yeast from a brewery or excess sludge generated in a wastewater treatment plant may be appropriate. In this study, therefore, commercial dried baker’s yeast (DBY) and SW were examined as substitutes. To evaluate the stimulative effect of the substitutes on TOP, the degradation efficiency based on TRL obtained after a 120-h treatment was compared. Table 1 summarizes the degradation effkiency obtained with various supplements when 15 g of fresh salad oil and lard was used as the test material. As shown in the table, minimal degradation of lipids was observed when no supplements (only 60 ml of water) were added with more than 90% of lipids remaining even after 120-h treatment. A significant improvement in TABLE 1. Degradation efficiency of salad oil and lard in TOP stimulated by nutrient supplements of various organic materials Nutrient supplement No supplements
Degradation efficiency (%) Lard Salad oil
U 1.0 g+TE
8.6 42.5
3.9 22.2
YE 1.5g+U 1.0 g+TE YE 3.og+u l.Og+TE DBY 1.5 g+U l.Og+TE
68.3 68.2 67.6
76.9 70.7 -
SW 1.5 g+U l.Og+TE SW 2.0 g+U 1.0 g+TE SW 3.0 IZ+U 1.0 e+TE
82.9 78.2
63.9 75.8
77.8 79.6 Degradation efficiency (%) is defined as the percentage of material that disappeared after 120-h treatment of 15 g of fresh lipid. Abbreviations: YE, yeast extract; U, urea; TE, trace-element solution; DBY, dry baker’s yeast; SW, dry sewage sludge.
UTILIZATION OF SEWAGE SLUDGE AS AN ALTERNATIVE NUTRIENT
VOL.94,2002
the degradation effkiency was, however, possible by supplying nutrients that promote thermopilic microbial activity. Even when an inorganic nitrogen supplement, urea (U), was added in combination with the trace element solution (TE), the improvement in degradation was obvious. Further improvement in lipid degradation was realized by combining organic and inorganic suppiements (YE+U+TE). With the nutrient supplement developed in the previous study (6), YE 1.5 g+U 1.0 g+TE, a degradation efficiency of 68.3% and 76.9% was attained for salad oil and lard, respectively. On the other hand, with 1.5 g of commercial DBY, a similar degradation efficiency for salad oil (67.6%) was obtained when added in combination with 1 g of urea and TE. Thus, dried yeast was confirmed to have a similar stimulative effect as YE, revealing the applicability of spent yeast as an alternative material to YE. A further improvement in the degradation efficiency was obtained by using another substitute, SW. With a combination of 1.5 g of SW, a degradation efficiency of 82.9% after 120-h trea~ent was attained for salad oil. The degradation efficiency obtained for lard with the same combination was, however, not as high. It was also lower than that obtained with 1.5 g of YE. To ensure a sufficient amount of SW, further additions in combination with 1 g of U and TE were examined. An improved de~adation efficiency in lard with addition of SW was observed, but the improvement was not observed in the case of salad oil. The best degradation efficiency for lard (77.8%) was attained with a combination of 3.0 g of SW. From these results, it was confirmed that the degradation efficiency for fresh lipid materials obtained with a readily free and abundant material like SW was high compared to that obtained with the expensive nutrient material, YE. Stimulative function of components of SW in lipid degradation by TOP The nutrient supplement devel-
oped in our previous study consisted of a biomass component, a nitrogen source and some trace mineral components, which were supplied with YE, U and TE, respectively (6). TABLE 2. Comparison of degradation efftciency of salad oil in TOP obtained with various combinations of nutrient supplements using YE and SW
Since SW consists of not only bacterial cell-mass, but also inorganic nitrogen and mineral components derived from waste water, the nutrient supplement may not require such inorganic components to have a stimulative effect on lipid degradation by TOP. Thus, a test batch treatment to investigate the possibility of sole addition of SW as nu~ient to promote degradation of salad oil was carried out (Table 2). Although the addition of YE resulted in a low degradation efficiency of 28.3% after 120-h treatment, that of SW resulted in a very high degradation efficiency of 79.0%. This value was much higher than that attained by the model supplement with YE and even comparable to that attained with a combination of SW, U and TE. Further addition of TE did not change the degradation efficiency and addition of U resulted in an even lower efficiency than addition of SW. These results suggested that SW could satisfy both organic and inorganic components which were essential to promote lipid degradation by TOP. Previously, the signi~c~ce of the complements effect of amino acids and inorganic components was demonstrated when the stimulative effect of the nutrient supplement was investigated (6). Therefore, the mineral components and amino acids included in SW could be considered to function as nutrients for the degradation of lipid by TOP. Thus, an analysis of SW for amino acids and mineral components was conducted and their amounts were compared with those supplied by the model nutrient supplement with YE. Table 3 shows major amino acids detected in the solution of 1.5 g of YE and SW. Although YE dissolved in acidic water completely, SW dissolved only partially. However, the amount of amino acid detected in the SW solution was much larger than that in the YE solution. As shown by the relative value (SW/YE) for each kind of amino acid, amounts in SW solution were rich compared to YE solution. Thus, as an amino acid source, SW was confirmed to be superior to YE. Table 4 shows a comparison of representative mineral components detected in 1.5 g of SW and TE used for the nutri~t supTABLE 3. Comparison of major amino acids detected in the solution of YE and SW Amino acids
Nutrient supplement No suppleme& u l.Og U l.Og+TE YE YE YE YE
1.5g 1.5 g+U l.Og 1.5 g+TE lSg+U l.Og+TE
Degradation efficiency (%I 8.6 -3.0 42.5 28.3 36.1 52.4 68.3
SW 1.5g SW 1.5g+u l.Og SW 1.5 g+TE
79.0 59.6 79.6
SW 1.5 g+lJ 1.0 g+TE
82.9
YE-ash+U-l.Og+TE SW-ash+U l.Og+TE
54.4 67.2
Degradation efficiency (%) isdefined as the percentage of material that disappeared after 120-h treatment of 15 g of fresh salad oil. Abbreviations: YE-ash, ash obtained from 1.5 g of YE; SW-ash, ash obtained from 1.5 g of SW. Other abbreviations are the same as in Table 1.
115
Phosphoserine Phosph~th~ol~ine Aspartic acid Threonine Serine Glutamic acid Glycine Aianine Amino-~-bu~ric acid Valine Isoleucine Leucine ‘Tyrosine Phenylalanine @-Alanine ~-Aminoisobu~ri~ acid Ethanolamine Lysine Arginine
YE (nmol/mi)
SW (nmollml)
Relative value (SW/YE)
18 16 123 95 85 40 210 235 6 80 40 56 36 25 4 0 30 64 38
94 113 234 161 137 199 120 655 127 355 296 459 I44 245 126 366 53 223 197
5.22 7.06 1.90 1.69 1.61 4.98 0.57 2.79 21.17 4.44 7.40 8.20 4.00 9.80 31.50 1.77 3.48 5.18
1.5 g of YE and SW was used for preparing the solution.
116
NAKANO AND MATSUMURA
J. BIOSCL BIOENG..
TABLE 4. Comparison of amount of inorganic components contained in SW and TE used as nutrient supplements Elements Ca
Mg K Al Fe Mn cu Na Zn P
11.3 3.4 5.4 42.8 11.2 0.0 0.2 1.1 6.8 31.1
14.0 10.0 40.0 0.0 0.1 0.7 0.0 190.0 0.1 20.0
Amounts of inorganic components contained in 1.5 g of SW and 60 ml of TE are shown.
plement. It was found that SW had a variety of mineral components and a phosphorus content higher than that of TE. The significance of phosphorus in inducing a supplemental effect on other trace mineral components was demonstrated previously (6). A large amount of phosphorus may be one of the reasons for the good stimulatory effect of SW. To confirm the stimulative function of the mineral components of SW, degradation efficiency was examined with SW-ash. As shown in Table 2, the degradation efficiency obtained with SW-ash was higher than that with YE-ash when U and TE were added. Thus, also in terms of effective mineral components, SW was superior to YE. These results demonstrated the effectiveness of SW as a nutrient to satisfy the essential components necessary to promote the degradation of lipid materials. However, the total amount of nitrogen supplied by SW seemed critical to the degradation efficiency. Baaed on the total amount per unit of SW described previously, 1.5 g of SW can supply 121 mg-N, which corresponds to the amount of nitrogen supplied with 0.26 g of U. In the previous study, a sufficient amount of U per 15 g of lipid materials was around 0.5 g (6). Moreover, input inorganic nitrogen was converted into ammonia during the lipid degradation process and was evaporated under alkali conditions described below. Considering both the critical amount of nitrogen in SW and the loss of nitrogen as ammonia gas during treatment, the addition of a nitrogen source to SW may be needed as a nutrient supplement for TOP especially in long-term treatments. Degradation of salad oil by TOP stimulated by nutrient supplement using SW To compare the stimulative
effect of various kinds of nutrient supplements, degradation efficiency based on TRL obtained after 120-h treatment was adopted as described previously. The best degradation efficiency, X2.9%, was obtained with SW 1.5 g+U 1.0 g+TE. Even under such conditions, however, around 20% of the lipid material remained in the system after 120-h treatment. Whether the residual material after 120-h treatment will be degraded further or accumulated in the system at a later phase is important for establishing a continuous treatment process. To elucidate this, longer term test batch treatments were conducted. A representative time course of TOP obtained when the batch treatment was continued until 2 10 h is shown in Fig. 1. In this treatment, 15.4 g of fresh salad oil was used as the test material and a nutrient supplement, SW
4 -
2 0
50
100
150
200
Time (h)
FIG. 1. Time course of batch treatment of salad oil by TOP stimulated by nutrient supplement using SW. A combination of 1.5 g of SW, and 1 g of U and TE was used. (a) Changes in residual lipid materials. (b) CO, concentration in exhaust gas. (c) Degradation and removal efficiency of lipid materials. (d) Moisture content and pH of the system.
1.5 g+U 1.0 g+TE, was added to stimulate the process. To assess the lipid degradation process, TRL and ERL were compared as shown in Fig. la. The amount of ERL was half that of the salad oil even at 24 h, revealing a rapid absorption of lipid materials into cells after hydrolysis. Subsequently, a decrease in TRL was observed. The degradation efficiency calculated based on the amount of TRL reached 65.8% at 45 h (Fig. lc). This rapid degradation was followed by a phase of gradual degradation in which a decrease in intracellular lipid (difference between TRL and ERL) was observed. The lipid degradation by TOP was found to be divided into such two steps. The final amount of TRL after 210-h treatment was 1.Og, corresponding to a degradation efficiency of 93.5%. Consequently, further degradation of residual lipid materials after 120-h treatment was confirmed although the rate became slower at the later phase. The hydrolyzed lipid materials will be converted to cellmass or removed from the system as CO, gas. During the treatment, the exit gas was collected in a plastic bag to monitor the biodegradation activity in TOP and also to calculate the output of lipid degradation. The average CO, concentration in the exhaust gas is shown in Fig. lb. A peak in the concentration appeared two days after addition of the oil. The removal efficiency shown in Fig. lc was calculated based on the carbon balance considering total output carbon as CO, in the exhaust gas and total input carbon derived from salad oil, SW and U. The removal efficiency obtained at 45 h was 19.0%, and was much lower than the corresponding degradation efficiency obtained at the same treatment time point. It has been reported that oxidation limited
VOL.94,2002
UTILIZATION OF SEWAGE SLUDGE AS AN ALTE~ATIVE
the overall lipid degradation of activated sludge when the lipid concentration exceeded 0.8 g/l (1). Since the amount of lipid used in this study was much higher, the reduced removal efficiency compared with degradation efftciency could be considered due to rate limitation by the oxidation step. Although the inhibition of microorganisms by longchain fatty acids formed after lipid hydrolysis has been a problem for anaerobic microbial processing (13, 14), no such problem was observed for TOP. The thermophilic microbial activity was maintained and the difference in degradation and removal efficiency decreased at the later phase. The removal efficiency eventually reached 88.4% at 210 h. The relative value {removal efflciency/de~a~tion efliciency) at 210 h was 0.94, revealing a high rate of conversion of degraded lipid materials into CO, gas. It suggested that the smooth conversion of lipid material into CO, was eventually possible by TOP stimulated by nutrient supplements using SW. Moisture content was important to keep the microbial activity in the solid state culture system (15). Although water was not added after starting the treatment, the amount of moisture in the reactor was stable at around 65% as shown in Fig. Id. The amount of water formed by lipid degradation might be equivalent to that lost due to evaporation under the~ophilic aerated conditions. The pH of the system was also monitored (Fig. Id). Since no control system for pH was used in this study, the pH of the system would be expected to be lowered by fatty acids generated after lipid hydrolysis as shown in other studies (5). However, the pH was stable at around 8.5. This was found to be due to the generation of ammonia derived from U during treatment, resulting in the detection of ammonia in the exhaust gas (maximum concentration; 0.2%). Since such a function of U to maintain the pH of the system against acidifiers is useful to the feasibility of the process, the addition of enough U would be impo~~t for actual HCLW treatment by TOP. Degradation performance of HCLW with TOP stimulated by nutrient supplements using SW In the previous study, the achievable degradation efficiency for HCLW with TOP stimulated by nutrient supplements consisting of a semi-defined medium based on the model supplement was investigated (6). The result that the nutrient supplements giving the best degradation efficiency for different kinds of HCLW differed for each type of waste demonstrated a difference in requirements to stimulate the process to treat HCLW. Thus, to examine the applicability of SW as a nutrient to stimulate TOP for treating HCLW, tests were carried out under the same the~ophilic oxic conditions using 15 g of three kinds of actual HCLW, namely, HCLW 1, HCLW2 and HCLW3. The degradation efficiency after 120-h treatment was compared and results are shown in Table 5. For all of HCLW, the degradation efficiencies attained with 1.5 g of SW in combination with 1 g of U and TE were much higher than those obtained with the model nutrient supplement using I .5 g of YE. Moreover, an improvement in the degradation efficiency was possible by further addition of SW. With 3.Og of SW, the highest degradation efficiency could be attained for HCLWl and 2. Although the degradation efficiency for HCLW3 was lower than that attained with a semi-defined medium using YE, the supplement formulated
NUT~ENT
li?
TABLE 5. Comparison of degradation efficiency of HCLW in TOP stimulated by nutrient supplements formulated with semi-defined medium and SW.
Nutrient supplement
Degradation efficiency (%) HCLWI
HCLW2
HCLW3
No supplements U 1.0 g+TE
13.0 42.5
0.5 22.2
1.3 35.3
YE lSg+U YE3.0g+U YE 1.5g+U YE 1.5g+U
59.9 70.1 72.9 68.1
57.5 57.7 36.8 32.2
50.6 39.9 60.0 81.9
71.6 73.9
74.9 79.0
62.0 77.9
I.Og+TE I.Og+TE l.Og+TE+PO,O.l5g l.Og+TE+PO, I.OOg
SW 1.5g+U I.Og+TE SW 3.0 g+U 1.0 g+TE
Degradation efficiency (%) is defined as the percentage of material that disappeared after 120-h treatment of 15 g of actual HCLW. Abbreviations: HCLWl, used cooking oil from a restaurant; HCLW2, trapped waste from a food oil company; HCLW3, waste from a meat factory; PO,, N$HPO,. 2H,O. Other abbreviations are the same as in Table 1.
with SW worked well for all HCLW tested despite the different features of HCLW+ A degradation efficiency of around 75% was attained for all kinds of HCLW with a supplement consisting of 3.0 g of SW. No difference in the requirements to stimulate TOP for treating HCLW using a semi-defined medium was observed for TOP stimulated using SW. The sufftcient quantity and variety of amino acids and mineral components contained in SW may allow such flexibility for satisfying the nutrient requirements needed for HCLW treatment. Therefore, a readily available and possibly cost-free SW will be useful not only as an alternative to YE, but also as an appropriate nutrient that is effective for HCLW having different nutrient requirements. For convenience in the laboratory, dehydrated and powdered sewage sludge dried at 110°C for 48 h was used in the present study. However, a similar degradation efficiency could be obtained using a dehydrated non-drying sewage sludge cake (mois~re content; 85%) for salad oil degradation (data not shown). Therefore, a dehy~ated sewage sludge cake without drying will be applicable for HCLW treatment although the moisture adjustment of the system considering the amount already present in the sludge cake must be taken into account, In the present study, it took 2 10 h to attain a removal efficiency of 88.4% for salad oil in batch treatment. In repeated batch treatment, however, the acclimation of microorganisms can be expected and a shorter treatment time to attain a similar removal efficiency may be possible. Although no obvious inhibition of microorganisms by long-chain fatty acids formed after lipid hydrolysis was observed in batch treatment, whether contamination by new lipids added to residual lipid materials which often consist of long-chain fatty acid would influence the treatment efficiency or not would be important for developing a continuous treatment process. Although the applicability and conditions to use SW as a nutrient to promote lipid degradation by TOP were elucidated by batch treatment, further investigation concerning optimization of the input lipid loading or interval is needed to apply the proposed process to actual HCLW treatment.
118
NAKANOAND MATSUMURA ACKNOWLEDGMENTS
This study was partially supported by Grants-in-Aid for the Encouragement of Young Scientists from The Ministry of Education, Culture, Sports, Science and Technology, Japan. We wish to thank Dr. Catalino Alfafara for many useful suggestions in the manuscript preparation. REFERENCES 1. Hsu, T., Hanakl, K., and Matsumoto, J.: Kinetics of hydrolysis, oxidation, and adsorption during olive oil degradation by activated sludge. Biotechnol. Bioeng., 25, 1829-1839 (1983). 2. Lefebvre, X., Paul, E., Mauret, M., Baptiste, P., and Capdeville, B.: Kinetic characterization of saponified domestic lipid residues aerobic biodegradation. Wat. Res., 32, 3031-3038 (1998). 3. Chang, M. B. and I-hang, T. F.: The effects of temperature and oxygen content on the PCDD/PCDFs formation in MSW fly ash. Chemosphere, 40, 159164 (2000). 4. Liu, B. G., Noda, S., and Mori, T.: Complete decomposition of organic matter in high BOD wastewater by tbermophilic oxic process. Proc. Environ. Eng. Res., 29,77-84 (1992). 5. Becker, P., Koester, D., Popov, N., Markossian, S., Antranikian, G., and Mae&l, H.: The biodegradation of olive oil and the treatment of lipid-rich wool scouring wastewater under aerobic thermophilic conditions. Wat. Res., 33, 653-660 (1999). 6. Nakano, K. and Matsumura, M.: Improvement in the treatment efficiency of thermophilic oxic process for highly concentrated lipid wastes by nutrient supplementation. J. Biosci.
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