Environ. Sci. Technol. 2004, 38, 2457-2469
Landfarm Performance under Arid Conditions. 2. Evaluation of Parameters R A M Z I F . H E J A Z I † A N D T A H I R H U S A I N * ,‡ Environmental Protection Department, Saudi Aramco, Box 5487, Dhahran 31311, Saudi Arabia, and Faculty of Engineering and Applied Science, Memorial University of Newfoundland, St. John’s, Newfoundland, Canada A1B 3X5
Various parameters that influence the degradation processes in landfarming operations include moisture content, microbial density and composition, nutrients, and tilling. In this paper, a detailed evaluation of each parameter is presented based on field experiments. This paper also addresses kinetic of weathering, volatilization, and biodegradation mechanisms under natural attenuation and enhanced conditions including aeration in the soil pores by tilling; combined effects of tilling and nutrients in the soil; tilling and addition of water to maintain a minimum moisture level; and combination of tilling, water, and nutrients. The effect of double loading under enhanced condition was also studied in this paper. The study shows that the tilling is very effective to to enhance the loss of oil and grease (O&G) fractions under arid conditions. However hydrocarbon loss in the absence of fertilizer and water was mainly due to weathering (volatilization). In the absence water and nutrients, the microbial counts were found to be low. The addition of water and fertilizer combined with the tilling helped in the significant reduction of O&G through both volatilization and biodegradation mechanisms with early reduction of n-alkanes through a biodegradation process followed by weathering. However, in order to determine the contribution of each of these two processes to the whole degradation, further work is required. The high loading rate resulted in retaining moisture content in the soil, and it delayed weathering and biodegradation. The high loading rate caused bacterial counts to increase, as it provided them with a plentiful source of food and water; however, it did not stimulate the biodegradation process for almost 6 months after the highest rate of sludge application to the soil.
Introduction The effectiveness of landfarming processes depends on several important parameters that can be grouped into three categories; sludge composition, soil characteristics, and temperature (1, 2). The information available in the literature specifies the optimal parameters for each category. However, after evaluating the existing literature, two important issues were noted. The first is that most of the literature provided a wide range of values for each parameter. For example, in * Corresponding author telephone: (709)737-8781; fax: (709)7374042; e-mail:
[email protected]. † Saudi Aramco. ‡ Memorial University of Newfoundland. 10.1021/es026045c CCC: $27.50 Published on Web 03/16/2004
2004 American Chemical Society
the case of temperature, the optimal temperature range was reported between 20 and 35 °C (3) while Dotson et al. (4) conclude that the optimal range was between 30 and 40 °C. Sandvik et al. (5) and Brown et al. (6) reported the optimal temperature at 18 and 30 °C, respectively. The wide range of temperatures that have been reported as optimal for bacteria likely are a reflection of the wide range of environmental conditions under which oil sludge is biodegraded. The composition of sludge and the loading rate are also important factors that affect the degradation process. The rate at which the compound is biologically broken down may increase or decrease depending on the presence of functional groups in the hydrocarbon chain or the aromatic ring (7). Bindra and Zestra (8) added that straight chain paraffin is the easier group of hydrocarbons to degrade and that the rate of decomposition decreased drastically by branching of the paraffin chain. Polycyclic aromatic hydrocarbons and asphalt fractions are also resistant to degradation. Dibble and Bartha (9) conducted a laboratory study in order to optimize the environmental parameters of oil sludge biodegradation. These parameters included moisture content, pH, nutrients, and temperature. Their results showed that the optimal degradation rate was achieved at a soil water holding capacity of 30-90%, a pH of 7.5-7.8, and a carbon to nitrogen to phosphorus (C:N:P) ratio of 100:10:1 at a temperature at or above 20 °C. The authors concluded by stating that the laboratory results can be helpful but will not represent actual field conditions due to limitations in parameters that could not be tested in the lab such as the fluctuating temperature in the field, aeration, and to some extent soil texture. Considering the entirely different climatic conditions prevailing in Saudi Arabia, it was decided to conduct an in-depth investigation under field conditions to determine the mechanism of kinetic conditions for degradation of oily sludge. Field experimental setup, sampling protocols, parameters analyzed, and analytical methods used have been discussed in detail in a companion paper (10). In this paper, a detailed investigation on the kinetic rate of oil and grease (O&G) with whole and selected n-alkanes is carried out to determine the extent of degradation processes under natural and enhanced conditions. A detailed investigation is presented to assess weathering, volatilizations, and biodegradation mechanisms on a cell by cell basis. Analytical methods, equipment, and the step-by-step approach to collect samples and analyze them in the laboratory are discussed in detail by Hejazi and Husain (10, 11).
Background Analysis of Soil and Sludge Baseline data on soil and sludge were collected to determine background levels for the parameters of interest. To assess the degradation process in the soil zones, samples were collected to a depth of 6 in. on a monthly basis for a period of 12 months and composited to analyze for physical, chemical, and microbiological parameters as listed Table 2 of the companion paper (10). To ensure that the collected samples were representative, composite samples were prepared by mixing samples taken from nine different locations by dividing each cell into nine square subgrids and taking three samples randomly from each grid. Baseline data showed no sign of O&G, with low soil moisture content (0.6%) and high pH (9.6). The low TKN (
