A slurry phase bioreactor was used to degrade the hydrocarbon component of oil-based drilling mud residues to an acceptable level for land disposal. The initial oil and grease concentration in the residue was approximately 26% by weight. Laboratory treatability studies had indicated a potential for biological degradation of the material. Initial target levels of nutrients were set at 200 ppm P as phosphoric acid, and 500 ppm N as ammonium nitrate. Daily samples were taken from the bioreactor to monitor pH and microbiological activity (as oxygen uptake rate, OUR). Samples were analyzed weekly for oil and grease content, cations and anions, and phosphate, ammonia, and nitrate levels.
Despite adequate nutrient additions, sufficient aeration, and a pH within physiological range, the OUR rarely indicated bacterial activity sufficient to degrade the oily waste. The full-scale bioreactor behavior was contrary to that of bench-scale reactors used in the treatability studies. Several parameters which were investigated as potential causes of the variations in activity included aeration (as dissolved oxygen, DO), mixing, pH, and nitrogen-phosphorous ratio (N:P). The calcareous calcium chloride-based drilling waste was found to reduce the bioavailable phosphate, and resulted in low levels of soluble phosphate and a subsequent high N:P.
The pH of the system was lowered to 6.0-6.3 by phosphoric acid addition to increase phosphate availability. This resulted in a dramatic increase in the OUR (from ≤ 0.1 mg O2 L−1min−1 to 0.5 mg O2 L−1 min−1) and a subsequent downward trend of the total oil and grease in the slurry prior to the dismantling of the bioreactor and land disposal of the waste.