| dc.description.abstract | 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been withdrawn from environmental effects of suspended oil, and a further reduction of the 30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
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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
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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. | en_US |
