According to the American Petroleum Institute (API) Environmental Guidance Document (API 1989), tank bottoms are generally defined as the liquids and residue that collect at the bottom of the treating vessel or that remain at the bottom of storage tanks after a period of service. These residues include heavy hydrocarbons, solids, sands, emulsions, etcetera. Tank bottoms include combinations of hydrocarbon and non-hydrocarbon organic compounds as well as trace metals.
The API associated waste survey (API 1988) offers a comprehensive examination of the means by which tank bottoms and oil debris are managed in the United States. The survey data indicate that the volume sent to off-site commercial facilities accounted for approximately 55 percent of the total. The rest of the bottoms and debris were use in the applications listed in the sidebar below.
Many of the remaining tank bottoms were road spread (21 percent), reclaimed (14 percent) or land spread (7 percent). Here, road spreading refers to the application of road mixes or paving materials formulated with asphaltic tank bottoms and oily debris, and to the application of certain oilfield liquid wastes as in the case of road oiling.
In such instances, and when conducted in accordance with permitted regulations, road spreading can be considered a beneficial use of a material that would otherwise require disposal. Various oil field wastes may be applied to roads—if permitted by regulations—as dust suppressants, as surface deicers, to provide a better surface, or simply for disposal.
Petroleum Development Oman (PDO) generates approximately 18,000 tons of sludge annually. In addition, there are more than 20,000 tons of sludge accumulated at Mina Al-Fahl Terminal alone. Disposal of tank sludge is a significant cost item of tank maintenance for producers, refiners and transporters of petroleum materials (See Figure 1).
Land farming use by PDO to treat the petroleum sludge has not proved very successful due to the harsh climate in Oman. Experimental land strips in Marmul and Fahud have failed to consistently reduce the oil content below 2 to 3 percent. Furthermore, the process is very slow, thus does not represent a feasible method for treating the current stored inventory of sludge or for treating new sludge, which continues to be produced annually. Alternative available proven techniques of sludge remediation need to be investigated.
A research project was initiated to investigate the use of tank bottoms as a binder to construct and upgrade unpaved roads. To achieve this, various sludge samples were initially characterized for chemical composition, heavy metals, flash and fire points, and natural occurring radioactive minerals (NORM).
Then, three mixes were prepared using blends of aggregates and tank bottoms. No bitumen was used in the mixes. The mixes include a hot mix where aggregates and sludge were both heated; a heated sludge and cold aggregate blend; and a cold mix where no heating was applied.
The Marshall mix design (ASTM D1559) was followed in the preparation ad testing of the specimens.
After establishing an appropriate blend of aggregates, three mixes were prepared. The first mix was prepared by heating both the aggregate and the sludge for a period of two hours at 300oF (150oC). As readers know, with a normal asphalt cement (AC) mix, aggregate and asphalt are heated for no more than one hour in the lab setting. The time was extended for the sludge mix to evaporate moisture before starting any mixing.
The second mix was prepared by heating only the sludge and using cold aggregate. The third mix was prepared by mixing both the aggregate and sludge at room temperature.
Three aggregate sizes in addition to mineral filler were used in the mix design. The sizes were
- 10 mm and
- 0 to ½-inch.
A blend of these aggregates was performed to meet the gradation requirements of Oman’s specs for roads for a Class B wearing course (DGR 1994).
The blend composition was
- 14 percent ¾-inch,
- 32 percent 10 mm,
- 50 percent 0 to ½-inch and
- 4 percent mineral filler.
The Marshall mix design method (ASTM D1559) was followed in the preparation and testing of the specimens. The sludge content was varied from 2.0 to 7.5 percent by total weight of the mix.
The specimens containing 2 and 2.5 percent sludge partially crumbled before the stability and flow testing. The data show that patterns of variation for the Marshall properties are very similar to a normal HMA cement. The stability results indicate a significant increase in strength with the addition of sludge up to 5 percent—a stability of 15.9 kN was obtained—followed by a strength decrease with any further increase in sludge content. No sludge content was found to satisfy the Oman’s specs for a Class B wearing course; however, a sludge content of 6.5 percent can meet the Asphalt Institute criteria (1995) for a light trafficked surface or base material.
For the hot sludge and cold aggregate mix the specimens were prepared using the Marshall mix design procedure as mentioned above, with the exception of using the aggregate in a dry and cold condition. The sludge content was again varied from 2.0 to 7.5 percent by total weight of the mix. Once more, the specimens containing 2 to 3 percent sludge partially crumbled before the stability and flow testing.
The results indicated the same pattern of variation for the Marshall properties. Similar to the hot mix, no sludge content was found to satisfy the Oman’s specs for a Class B wearing course; however, a sludge content of 6.5 percent can meet the Asphalt Institute criteria (1995) for a light trafficked surface or base material.
The cold mix was prepared by mixing the aggregate in a dry and cold condition with the “as received” sludge without heating. The aggregate and sludge were blended and compacted into Marshall specimens. Marshall properties were determined. The same patterns of variation for the Marshall mix design data were observed. Moreover, samples containing 2 and 2.5 percent partially crumbled before the stability and flow testing. Similar to the previous two mixes, no sludge content was found to satisfy the Oman’s specs for a Class B wearing course; however, a sludge content of 6.5 percent can meet the Asphalt Institute criteria (1995) for a light trafficked surface or base material.
The sludge acts as a binder to the aggregate and provides a significant strength. The effect of heating both sludge and aggregate resulted in a significantly higher strength. Heating of either the aggregate or the sludge generally produces a higher strength mix compared to the cold one; however, the difference in strength was not significant at the optimum asphalt content between the second and third mixes.
It is recommended to use the hot mix. In case of economic constraints, the cold mix can be used. An optimum sludge content of 6.5 percent, by total weight of the mix, satisfies the requirements for low trafficked surfaces or base layers according to the Asphalt Institute specs. The results of the lab experiments indicate a potential use for the sludge in such applications.
No environmental harm should be anticipated from the use of the sludge in the construction of unpaved roads. However, advanced lab performance-based tests and the construction of experimental field sections are needed to better assess the engineering and environmental acceptability for the use of tank bottoms in road construction.