Aerobic biological treatment of produced water from oil production
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Produced water is the largest waste stream generated from the oil and gas industry. Water of varying quantities is always produced along with oil and has to be separated from the oil. The amount of produced water generated generally increases as the oil field gets older, because more water has to be injected into the reservoir in order to force the oil out. The produced water can either be injected back into the reservoirs or be treated, typically by floatation units or hydrocyclones, and eventually be discharged to sea. The produced water still contains traces of oil, chemicals and a variety of dissolved compounds after this treatment. Experience has shown that the major contributors to environmental impact factor (EIF) are dispersed oil, volatile aromatics, heavy aromatics, alkylated phenols and different process chemicals. The requirements set by the authorities, regarding produced water treatment, does not involve removal of dissolved organic compounds from produced water. But, recently the focus has been withdrawn from environmental effects of suspended oil, and a further reduction of the 30 mg/l oil in water level is not considered. However, the focus is now on water soluble, heavy (non-volatile) aromatics and phenols since the long-term environmental effects of which is not fully understood. Research is ongoing in many oil and gas companies, in cooperation with Klif (klima og forurensingsdirektoratet). Recent research has detected negative effects on fish in open sea area caused by exposure to produced water. This thesis is a literature study on aerobic biological treatment technologies, for offshore use, for the removal of dissolved organic compounds and oil in water content from produced water. The aerobic treatment technologies assessed in this thesis was activated sludge (AS), biofilm (BF), membrane bioreactor (MBR) and aerated membrane biofilm reactor (MABR). The main focus, in the evaluation of the most beneficial biological treatment technology for produced water treatment, was put on required reactor volume due to the space limitations on offshore installations. A model for the produced water composition was defined for the calculations carried out in this thesis. The reactor volumes, sludge production and oxygen demand was calculated for the 3 different biological systems based on the assumptions made for the model produced water characteristics and values for the kinetic coefficients found from literature. The calculations clearly identified the relationship between the active biomass concentration and required reactor volume. A biological treatment system with a high active biomass concentration and high rate oxygen supply would be an advantage as it was found that the volume of the biological reactor decreased as the active biomass concentration of the system was increased. The formation of biofilm allow for a compact biomass formation compared with activated sludge systems. And therefore the required reactor volume for biofilm systems is typically smaller than for the activated sludge systems due to the high biomass concentration. The biomass concentration in biofilm systems largely depends on the specific surface area available for biomass growth, this was confirmed by the calculations carried out in this thesis. The calculations carried out also proved that the overall performance of the biological treatment systems largely depended on the temperature within the system. From the literature, a typical temperature for produced water was found to be 75 ºC, but for the calculations it was assumed that the temperature of the produced water was reduced to 30 ºC and 20 ºC during the pre-treatment. The results from the calculations in this thesis showed that the minimum sludge retention time (SRTmin) nearly doubled as the temperature was reduced from 30 to 20 ºC, from 0.33 days to 0.67 days. The SRT in turn, was found to largely affect the biological treatment processes in terms of required reactor volume. The effect of the SRT, at 20 times SRTmin, was seen as an increase in reactor volume of 73.5 % as the temperature was decreased from 20 to 30 ºC. For SRT of 8.1 times SRTmin the increase in reactor volume was calculated to be 83.6 % larger for systems operating at 20º compared with systems operating at 30 ºC. Last, at 2 times SRTmin the reactor volume was calculated to increase with 93.8 % as the temperature was decreased from 20 ºC to 30 ºC. The calculations in this thesis also showed that the volume of the biological reactor also depends on the active biomass concentration of the system, XA, which applies with literature. The relationship between biomass concentration and required reactor volume applies to all the biological treatment technologies, activated sludge as well as biofilms, therefore the relationship between active biomass concentration and reactor volume was calculated for XA concentrations up to 50,000 mg/l where the lower range represents the XA concentrations 4 found in AS systems and the higher range represents the possible active biomass concentrations of MABRs. For MBRs it was found that the active biomass concentration could get as high as 14400 mg/l. If the wastewater-loading rate is high, oxygen supply could limit the removal of organic substrate in biofilms. From literature it was found that MABRs outperformed both conventional biofilm reactors and activated sludge systems under conditions of high organic loading due to the fact that MABRs could contain an active biomass concentration higher than any other system because of the oxygen supply through the membrane. This technology would be able to provide the most compact biological reactor system of all the technologies assessed in this thesis. Further development of both MBRs and MABRs revolves around increasing the biomass concentration and, hence, reduce the reactor volume. But, the biomass concentration will eventually reach a limit due to physical constraints and/or substrate/oxygen transport limitations. The sludge production was found to depend on the MLSS concentration, reactor volume and SRT. The sludge production was lower for the system operating at 20ºC due to the increased SRT. The oxygen demand was found to be slightly lower at 30ºC due to the difference in reactor volume reaction rates for the two temperatures. It was calculated that the sludge production decreased with increased SRT and the oxygen demand was found to increase as the SRT was increased. It was concluded that that MABRs should be further investigated if biological treatment were to be used for produced water treatment on offshore installations. Because of uncertainties related to the produced water composition and other assumptions made in the calculations, it was recommended to carry out pilot testing of the actual water to be treated in order to provide the necessary design criteria.
Master's thesis in Environmental technology