Bioremediation of oil sludge contaminated soil by co-composting with sewage sludge

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1 Journal of Scientific & Industrial Research Vol. 65, April 2006, pp Bioremediation of oil sludge contaminated soil by co-composting with sewage sludge Chua Choon Ling and Mohamed Hasnain Isa* School of Civil Engineering, Engineering Campus, Universiti Sains Malaysia, Nibong Tebal, Penang, Malaysia Received 24 June 2005; revised 12 December 2005; accepted 06 February 2006 This study determined the optimum ratio of contaminated soil to sewage sludge (organic amendments) in enhancing the biodegradation of refinery oil sludge. It also evaluated the treatment efficiency of refinery oil sludge by composting and the effect of sewage sludge addition on the biodegradation of refinery oil sludge. Experiments were conducted in six laboratory scale composting bins, each with 5 kg of soil. The composters were kept at C in a laboratory. The soil was spiked with petroleum refinery oil sludge (10%, dry weight basis). All composters were supplemented with grass (bulking agent) at a soil-to-grass ratio of 1:0.5 (v/v). The ratios of contaminated soil to sewage sludge were 1:0 (control), 1:0.1, 1:0.3, 1:0.5, 1:0.7 and 1:1 (w/w). The contents of each composter were tilled and well mixed weekly after which samples were drawn for analysis. The experiment showed enhanced biodegradation of oil and grease due to sewage sludge addition. The maximum oil and grease removal achieved was 65.6% over the 9 week study period under low temperature conditions, for the contaminated soil to sewage sludge ratio of 1:0.5. The remaining (34.4 %) was considered recalcitrant; stubbornly adsorbed onto soil particles and/or trapped in soil macro and micro pores and thus non-available to microorganisms. Keywords: Bioremediation, Co-composting, Refinery oil sludge, Sewage sludge IPC Code: C05F7/00; E02D31/00 Introduction Oil refineries are experiencing problems with environmental contamination because of difficulty in the safe disposal of oil sludge (OS) generated during the refining process. A typical Malaysian refinery (manufacturing capacity, 105,000 barrels/day) produces about 50 tons/y of sludge 1. This is a tremendous amount of hazardous waste; primarily consisting of oil, water and inorganic sediments. It is critical that OS be carefully handled because many of its constituents are carcinogenic and potent immunotoxicants. Methods such as incineration, pyrolysis and burial of the sludge in secured landfills are expensive. OS composting is a cost-effective and environmentally friendly technique for the biological treatment and stabilization of organic matter. It results in the partial or complete biotransformation of organic matter to microbial biomass and stable and innocuous end products such as CO 2 and H 2 O. A number of studies 2-5 has been conducted utilizing composting techniques in treating petroleum-contaminated soil and petroleum based oil wastes. *Author for correspondence Tel: ; Fax: mhisa@eng.usm.my; hasnain_isa@yahoo.co.uk Another problem of concern is the disposal of sewage sludge (SS) from wastewater treatment plants. Not only is the sludge difficult to dispose but also the cost of disposal is exorbitant. It is expected that the capacity of existing sludge processing and disposal facilities will be insufficient to meet the increased future requirements. Safety and environmental concerns limit the discharge of sludge into oceans or landfills and related legislations are anticipated to become more stringent over time. Composting can be effectively used as a safe and environment friendly method to stabilize SS. Hence co-composting of OS contaminated soil and SS could provide a common solution to both problems. SS, which is rich in organic matter, can be added to contaminated soil as a source of co-substrates, nutrients and microorganisms and to ameliorate the structure and water-retaining capacity of the soil 2. In addition, SS (organic amendments) also provides the carbon source, which is crucial in facilitating the degradation of the organic contaminants 3. SS is also generally rich in nitrogen, which is essential for the growth of microorganisms. Furthermore, it has a high microbial diversity, with the total microbial population being higher than that of soil. Increasing

2 LING & ISA: BIOREMEDIATION OF OIL SLUDGE CONTAMINATED SOIL 365 Table 1 Characteristics of the soil, sewage sludge and grass Parameter Soil Sewage sludge Grass (Bulking agent) ph COD, mg/g Total Kjeldahl Nitrogen, mg/g Phosphate, mg/g Phosphorus, mg/g C: N ratio 110:1 105:1 149:1 C: P ratio : : :1 Potassium, mg/g Nitrate, mg/g Sulfate, mg/g Total iron, mg/g Nickel, mg/g 0.02 N. d 1.25 Zinc, mg/g Manganese N. d N. d N. d N.d, not detected the microbial density can accelerate contaminant removal, as microbes are the primary agents for biodegradation. Contaminated soil often needs supplements because it experiences deprivation in organic matter and generally has a low microbial activity 4. The introduction of organic matter (SS) into the soil will help speed up the microbial activity and also the activity of specific degraders, which may be found in the contaminated soil or introduced along with the organic material 4. It is imperative that the ratio of contaminated soil to organic amendments be determined because an inappropriate amount may retard or inhibit microbial activity 6. Namkoong et al 3 showed that degradation rates of total petroleum hydrocarbons (TPH) and n-alkanes were highest at the ratio of 1:0.5 of contaminated soil to organic amendments on a wet weight basis. However, Dibble & Bartha 7 noted that the addition of micronutrients and organic supplements were not beneficial; SS interfered with hydrocarbon biodegradation. This study evaluates the biodegradability of OS by composting, ascertains the effect of SS addition on the biodegradation of OS and determines the optimum amount of SS needed for the bioremediation of OS contaminated soil. Materials and Methods Materials The principal materials (Table 1) are soil, OS, SS and grass (bulking agent). The soil was taken from a location about 3 m from the riverbank of Sungai Kerian, Nibong Tebal, Penang, Malaysia. It was a subsurface soil, sandy in nature and leftover from a previous boring (excavation) job. The soil was finegrained as only a small percentage (< 10 %) of it was retained on the 2 mm sieve. To prepare the soil for usage, large stones and pebbles (> 10 mm) were removed. Then, using a rubber pestle, parts of the soil that were in clumps were broken down. The OS was obtained from the Shell Refinery Plant in Negeri Sembilan, while SS was obtained from the sludge drying bed of one of Indah Water Konsortium s wastewater treatment plants for a housing estate in Sungai Bakap, Nibong Tebal, Penang. The grass was cut and collected from a park in Seberang Jaya, Penang. It was air dried for preservation and subsequently cut (size, < 5 cm) manually. Experimental Set-up The experimental set-up comprised laboratory scale composting bins. The composters were inverted food covers (66 cm 24 cm) that were placed on top of salad/fruit bowls (40 cm 10 cm) and had holes all over to facilitate aeration. Their circular shape allowed easy mixing of the contents. The salad/fruit bowls enabled composters to stand stably and also acted as leachate collection receptacles. Experimental Conditions Six composters, each with 5 kg of sandy soil spiked with refinery OS (10%, dry wt basis), were set-up and kept at room temperature (23-25 C). Ratio 8 of contaminated soil to bulking agent was 1:0.5 (v/v). Experiments were conducted with different contaminated soil to SS ratios [1:0 (control), 1:0.1, 1:0.3, 1:0.5, 1:0.7 and 1:1 (w/w)] for 9 weeks. Physical and Chemical Analyses The composting mixtures were manually tilled and well mixed once per week prior to drawing samples for analysis. The compost matrices were analyzed for oil and grease using Soxhlet Extraction Method (Standard Methods 503D) 9. Soil moisture was measured by drying at 105 C for 24 h. ph was measured in distilled water using a soil:water ratio of 1:5 (w/v). Carbon was determined using Chemical Oxygen Demand (COD) Open Reflux Method (Standard Methods 508A) 9. Total Kjeldahl Nitrogen (TKN) test was done in accordance with Standard Methods 420A 9. Phosphate, nitrate, sulphate, potassium and heavy metals (Fe, Zn, Ni, Mn) were analyzed using a direct reading spectrophotometer DR 2000 (Hach) after digesting the samples with concentrated hydrochloric acid.

3 366 J SCI IND RES VOL 65 APRIL 2006 Initial Amendments Initial tests conducted in week 0 suggested that the composting environment was not suitable. The composting mixtures were acidic (ph, ). The nitrogen and phosphorus contents were also very low. Therefore, amendments were made in week 1 to achieve the optimum conditions. ph ( ) was adjusted using sodium hydroxide (NaOH). Nitrogen was supplemented using urea (H 2 NCONH 2 ) while potassium di-hydrogen phosphate (KH 2 PO 4 ) was added as supplement for phosphorus. The C:N and C:P ratios (based on total mixture) were adjusted to 20:1 and 800:1 respectively. The level of moisture (20%) was maintained using ordinary tap water. This may seem to be less then the generally reported values (50-60%) of moisture for composting. However, during the experiment, specific nature of the soil and oil mixture would not allow higher values. Vasudevan & Rajaram 10 have also used the same level of moisture in their study on bioremediation of oil sludge contaminated soil. Results and Discussion ph At the start of the experiment, the ph of all composters was acidic (Fig. 1) due to acidic nature of the composting materials as follows: soil, 2.0; SS, 4.2; and grass, 5.9 (Table 1). The ph was then amended to the optimum value ( ) using NaOH 7 in week 1. The amended ph, however, shot up to in week 2 as NaOH does not produce a buffering effect. The Fig. 1 ph in composters at different weeks of incubation addition of urea to supplement nitrogen in the composting piles also caused the increase in ph 11. The ph then gradually declined beginning from week 3. This indicated that there were microbial activities that subsequently resulted in the generation of large intermediary pools of organic compounds that were acidic in nature 12. At the end of week 9, the ph of all composters was close to neutral, except for composter 1, which was somewhat more acidic. This trend shows that the biodegradation of oil and grease in the composters was approaching an end. A near neutral ph was anticipated because as microorganisms finish biodegrading the available hydrocarbons, they do not produce any more organic acids. Oil Degradation The effect of organic amendments on the biodegradation rate of oil and grease (Fig. 2) shows fluctuating pattern mainly due to the heterogeneity of the composting piles. Efforts were taken to obtain a homogenized sample by collecting a sample that was composed of several sub-samples after uniformly mixing the whole pile. Nevertheless, there still lies a possibility that the collected sample may contain oil and grease significantly more or less than the pile average. The maximum (65.6 %) oil and grease degradation in the 9-week experimental period (Fig. 2) was in Composter 4 (contaminated soil: sewage sludge ratio of 1:0.5). Biodegradation of oil and grease for week 1 was considerable even though

4 LING & ISA: BIOREMEDIATION OF OIL SLUDGE CONTAMINATED SOIL 367 nutrients (N and P) were not supplemented until the end of the week. This could probably be explained by the nitrate concentration variation during the composting period (Fig. 3). Nitrate removal was high during the first week. It may have been used as a nutrient source or another possibility is the occurrence of anaerobic respiration (denitrification), wherein nitrate was used as the terminal electron acceptor. This may have happened due to the presence of anaerobic microorganisms (either indigenous or inoculated through the addition of SS) in the composting mass. Denitrifiers could have been present initially in the sludge if the material was anaerobic in nature when it was collected from the drying bed. Anaerobic micro sites may also have existed in the aerobic composting matrix. It is suspected that the microorganisms in the soil used were anaerobic in nature because subsurface microorganisms can only survive in that condition if they are capable of using alternative electron acceptors such as nitrate. In a complex mixture such as the composting masses under discussion, there would be many other terminal electron acceptors (e.g. CO 2, SO 4 2-, Fe 3+, Mn 4+, etc.) that would all contribute towards the reduction in organic content. In the second week, the percentage of oil and grease removal deteriorated drastically. This was likely due to ph adjustment and amendments addition Fig. 2 Oil and grease removal in composters at different weeks of incubation Fig. 3 Nitrate concentration in composters at different weeks of incubation

5 368 J SCI IND RES VOL 65 APRIL 2006 at the end of week 1 which resulted in an acclimatization period in week 2. The aerobic microorganisms in their lag phase would be synthesizing the molecules necessary for growth and replication, inducing necessary metabolic enzymes as well as adjusting to new growth conditions 13. After the second week, the results show that all the composters generally showed a rising trend in the removal of oil and grease. Aerobic respiration has also started as the concentration of nitrate seemed to have stabilized. The negative oil and grease removals observed may be due to the effect of ph change on the detection of oil and grease or due to heterogeneity of the samples. With an adequate amount of SS, the rate of oil and grease biodegradation was enhanced. Nonetheless, a higher mix ratio of organic amendments does not always result in a higher rate of biodegradation. Instead, it may create an adverse effect whereby the degradation rate is inhibited by the excessive amount of organic amendments 3. This probably explains why the percentage of oil and grease removal was lower in Composter 6, which was amended with the highest ratio of SS (1:1), as compared to Composter 1, which was the control (not amended with SS). However, insufficient amount of SS may also retard the rate of biodegradation. Less SS would mean that the inoculated amount of microorganisms was also less. The smaller population of microorganisms initially may have lead to the slow rate of biodegradation as they need more time to produce an appropriate amount of microorganisms to speed up the biodegradation process. This is seen in Composter 2 and Composter 3 where the overall removal was 43.1 percent and 33.4 percent respectively. Overall, the oil and grease removal seemed to have stabilized after the 6 th week of composting for all the composters. The remaining hydrocarbon in the contaminated soil is considered as non-removable biologically even for long duration of treatment 11. From the highest removal of oil and grease (65.6 % for Composter 4), it may be concluded that the remaining (34.4 %) are hydrocarbons that are recalcitrant as they stubbornly adsorb onto inorganic soil constituents and onto soil organic matter. The hydrocarbons may be trapped in remote areas like soil macro and micro pores rendering them difficult for microbes to degrade. The heavier the distillates are, the higher is the amount of residual 4. The temperature of the composting mixtures (23-25 C) was lower than the normal temperature of composting piles (55-65 C), which is due to heat produced from biodegradation of organic matter. In this case, however, the temperature of the composting mixtures remained low as the temperature maintained in the laboratory they were kept in affected it. At a higher temperature, thermal desorption is expected. This would help to remove organic matter from the soil and make it available to the microorganisms by transferring it to the aqueous phase. It is easier for microorganisms to degrade organic compounds in the aqueous phase. Thus, it is expected that under normal full-scale operations, where the temperature would be higher, the increased bioavailability of organic matter would increase the amount and rate of oil and grease removal. For the present study, the volatilization effect of the hydrocarbons in the oil sludge is presumed to be minimal and therefore negligible. It is presumed to be minimal because oil sludge was collected from the bottom most rung of the distillation tower. Most of the volatile component of the hydrocarbon would have been released earlier. Namkoong et al 3 reported that when the mix proportion of SS is increased, the volatilization of individual n-alkanes is decreased because of the relatively high adsorption capacity of SS compared to soil. Nitrate The initial drop in the concentration of nitrate (Fig. 3) could be attributed to the microbial use of nitrate as a nutrient or as an electron acceptor (in denitrification). As the experiment progressed, it can be seen that the amount of nitrate remained fairly constant indicating that aerobic conditions were achieved or that the use of nitrate as a nutrient kept the concentrations low. Nevertheless, it can be observed that from week 3 onwards, the concentration of nitrate started to rise. The increase became more obvious in week 7. The rising concentration of nitrate could be explained in the light of nitrification of ammonia. From this, it can be deduced that the environment in the composting piles was suitable for microbial metabolism and growth. Conclusions The 9-week study has shown that there is much potential in using composting as a method for treating OS contaminated soil. SS supplementation did indeed enhance the rate of biodegradation. The optimum

6 LING & ISA: BIOREMEDIATION OF OIL SLUDGE CONTAMINATED SOIL 369 contaminated soil to SS ratio was 1:0.5 at which 65.6 percent biodegradation of oil and grease was achieved. The experiment showed that the soil used had a high amount of viable microorganisms (probably because of the site, from which soil was collected). In other situations where soil may not be as rich in microbes, effect of sewage sludge addition would be even more pronounced. The oil and grease reduction was very low from week 6 onwards, indicating that rest of the oil and grease remaining in the composting piles was recalcitrant. This treatment was unable to treat 34.4 percent of the oil and grease within 9-week period of experimentation. However, under the normally higher temperature condition of full scale composting, the present results are expected to improve due to the greater bioavailability of organic matter consequent of enhanced thermal desorption. Acknowledgements Authors thank Managements of Shell Refinery in Negeri Sembilan and that of Indah Water Consortium in Seberang Jaya for providing oil sludge and sewage sludge respectively. References 1 Agamuthu P, Solid Waste: Principles and Management: with Malaysian Case Studies (Institute of Biological Science, Universiti Malaya Press, Malaysia) 2001, Van Gestel K, Mergaert J, Swings J, Coosemans J & Ryckeboer J, Bioremediation of diesel oil-contaminated soil by composting with biowaste, Environ Pollut, 125 (2003) Namkoong W, Hwang E Y, Park J S & Choi J Y, Bioremediation of diesel contaminated soil with composting, Environ Pollut, 119 (2002) Jorgensen K S, Puustinen J & Suortti A M, Bioremediation of petroleum hydrocarbon-contaminated soil by composting in biopiles, Environ Pollut, 107 (2000) Beaudin N, Caron R F, Legros R & Ramsay B, Identification of the Key Factors Affecting Composting of a Weathered Hydrocarbon-Contaminated Soil, Biodegradation, 10 (1999) Thomas J M, Ward C H, Raymond R L, Wilson J T & Loehr R C, Bioremediation. Encyclopedia of Microbiology (Academic Press, San Diego, California) Dibble J T & Bartha R, Effect of environmental parameters on the biodegradation of oil sludge, Appl Environ Microbiol, 37 (1979) Rhykerd R L, Crews B, McInnes K J & Weaver R W, Impact of bulking agents, forced aeration and tillage on remediation of oil-contaminated soil, Bioresource Technol, 67 (1999) APHA, Standard Methods for the Examination of Water and Wastewater, 16th edn, Vasudevan N & Rajaram P, Bioremediation of oil sludgecontaminated soil, Environ Int, 26 (2001) Margesin R, Zimmerbauer A & Schinner F, Monitoring of bioremediation by soil biological activities, Chemosphere, 40 (2000) Yateem A, Balba M T, Al-Awadhi N & El-Nawawy A S, White rot fungi and their role in remediating oilcontaminated soil, Environ Int, 24 (1998) Baker K H & Herson D S, Bioremediation (McGraw-Hill, Inc.) 1994, 9-17, ,