Petroleum refining is a complex operation and unit operations within
refineries vary depending on the type of crude and the type of end products.
Refining is a water intensive operation which generates large quantity of
waste water that needs to be treated so as to avoid environmental
degradation. A proper understanding of the unit operations as well as
pollutant loads is necessary in order to set up a suitable Effluent
Treatment Plant(ETP). This article discusses about various challenges that
we have faced in the designing and operating of several Refinery ETPs in
India.
Characterisation Of Waste Water
In order to design an ETP for an upcoming refinery, design parameters like
the flow rate, Biological Oxygen Demand (BOD), Chemical Oxygen Demand (COD),
Suspended Solids (SS), Oil & Grease (O & G), Sulfides, Phenols, Ammonia etc
., have to be assumed. The selection of the scheme and the estimation of
unit sizes depend on the inlet characteristics and hence it is imperative
that the assumed design parameters be as close to the actual parameters that
will be encountered once the plant is commissioned. It is quite normal to
assume higher values while designing the plants. However, one should keep in
mind that some processes do not perform efficiently when they are over
designed.
Typical Treatment Scheme
The treatment philosophy adopted to treat refinery waste waters consists of
a series of steps to sequentially remove the pollutants so that the treated
wastewater meets the required regulatory norms. The typical treatment steps
are:
- Removal of O & G: This is necessary as downstream
operations are sensitive to the presence of high levels of O & G. In a
refinery wastewater treatment system, two steps of oil removal are
typically required to achieve the necessary removal of free oil from the
collected wastewater prior to feeding it to a biological system. This
oil removal is achieved by using an American Petroleum Institute (API)
or Tilted Plate Interceptor (TPI) or equivalent oil water separator
followed by a Dissolved Air Flotation (DAF) or Induced Air Flotation
(IAF) unit.
- Flow Equalisation: Flow of wastewater generated
from refinery from various manufacturing process units is not uniform.
The flow variation is equalised by providing an equalisation tank of
sufficient capacity and constant flow is maintained in all downstream
treatment units. This makes the process control much easier.
- Removal of Suspended Solids: This is achieved using
a clarifier or it can be combined with O & G removal in a DAF unit
- Removal of Sulphides: Lower sulphide concentration
can be handled in biological treatment system. However, high
concentrations of sulphides need to be taken care before wastewater
enters into biological treatment system. Oxidation of sulphides using
Hydrogen Peroxide is most popular method of oxidising them.
- Biological Treatment: Various biological systems
have been adopted including Activated Sludge Process, Bio-tower,
Sequential Batch Reactor, Anaerobic reactors, Membrane Bioreactors etc.
Every process has its own inherent advantages and disadvantages and a
suitable system has to be selected by the designer based on factors such
as organic load, treatment efficiency required, land area available etc
.
- Polishing of Treated Waste Water
This step includes units like Pressure Sand Filters (PSF), Dual Media
Filters(DMF), Activated Carbon Filter (ACF) to remove the residual
suspended solids as well organic pollutants that have passed through the
earlier treatment steps.
- Reuse and Recycle : As the refinery consumes a
large quantities of water, it is imperative one must try and reuse the
treated waste waters back in the process. Additional treatment units
like Microfiltration, Ultrafiltration or Reverse Osmosis may be adopted
in order to meet the reuse water quality that is required
Challenges and Solutions in Biological Treatment of Refinery
Wastewater
The key treatment step in the refinery ETP is the biological treatment as
the bulk of the pollutants are removed in a cost effective manner in this
step. Hence it is important to design this system in a robust manner so that
it can operate efficiently. It is important to note that in the biological
treatment step, the pollutants are degraded by microorganisms and it is
imperative that a conducive environment is maintained such that the
microorganisms can operate up to their maximum potential. A list of various
factors that influence the design and operation of the biological system are
given below:
- Fluctuation of Wastewater Characteristics: One of
the main challenges in refinery wastewater treatment is its variation in
characteristics. As mentioned earlier, when assuming the pollutant load
for design of the ETP, it is normal to assume higher values so that the
designs are ‘safe’. This causes problems during the operation of the ETP
when the actual pollutant load is much lower. Also, based on variations
in quality of crude and refining process, wastewater quality also varies
. The biological system should be selected to take care of these
variations. The biological system should be able to vary process
parameters such as air supply, Mixed Liquor Suspended Solids (MLSS),
Food to Microorganism (F/M) ratio, etc., to suit variations in feed.
Figure1: Foam in refinery
Figure2: Typical Refinery ETP
Modular design of biological process also helps in handling variations
of wastewater quality. If provided with multiple process tanks, plant
can be operated based on the actual pollution load by using all process
tanks or part of them.
- Oil & Grease: The performance of the O & G removal
equipment is sometimes upset due to a surge in the inlet O & G quantity
due to dumping of oil within the refinery. This causes O & G to slip
into the biological system affecting the microorganisms. This also
causes foaming in the aeration tanks that affects the quality of the
treated water. A figure showing the foaming in a biological system of a
refinery is shown here (Figure 1).
- Sulphides: Refinery wastewaters contain high levels of
Sulphides that are toxic to microbes and hence have to be removed prior
to the biological treatment. In case sulphides slip into the biological
system, it exerts demand on the aeration system since it consumes the
oxygen that is meant for the respiration of the microorganisms. Hence it
is normal to consider a safety margin in the design of the air blowers
so that it can meet any contingency due to sudden air demand because of
sulphides. Care should be taken that sulphide concentration in
biological system are below toxic limits.
- Complex Organic Compounds and Heavy Metals: Complex organic
compounds such as phenols, benzene products, etc. are also part of
refinery wastewater. Their concentrations depend on the source and
quality of crude oil. These compounds are not easily biodegradable and
may be toxic to biological system at higher concentrations. Hence, the
ETP should be designed to handle these complex organics in case they
are present in wastewater. Heavy metals also pose problem to biological
treatment. Both complex organics and heavy metals should be taken care
in pretreatment if present in higher concentrations.
- Nutrients: The growth and performance of the
microorganisms depend on the availability of critical nutrients like
nitrogen and phosphorous in the waste water. Ideally, a BOD:N:P ratio of
100:5:1 is required for best performance. Normally abundant nitrogen is
present in the refinery wastewater. However, it is important to
constantly analyse the wastewater and add nitrogen (typically as Urea)
and phosphorous (as Phosphoric acid or Di Ammonium Phosphate) to make up
for any deficit.
Many a times, total nitrogen levels in the wastewater are much more than
required which necessitates treatment to reduce them. Hence, the
biological system selected should be able to remove excess nitrogen
along with organic pollutants.
- Energy efficiency: The oxygen required for the
microorganisms are delivered through fine bubble diffusers that release
small bubbles of air from the bottom of the aeration basins. Air blowers
are installed to supply air into biological process. Air blowers
constitute major portion of power consumption in the ETP. Any steps
taken to optimise energy consumption shall result in energy efficiency
of the plant.
Biological process should be provided with oxygen control system to
supply air as per the organic pollution load coming into it. Dissolved
Oxygen (DO) or Oxygen Uptake Rate (OUR) based blower control optimizes
both energy consumption and process performance. Energy efficient
blowers further improve the energy efficiency of the ETP.
- Process Automation: Due to variations in wastewater
characteristics, it is advisable to install process automation to
control vital process parameters such as DO, OUR, MLSS, Sludge
recirculation, Sludge wasting, etc. This helps in automatically keeping
the process parameters at their optimum levels and generates consistent
quality of treated wastewater.
Conclusion
Refinery wastewater characterisation is very complex exercise that requires
an understanding about the refining processes involved. The designing of the
ETP is a challenging job as treatment units have to be robust enough to
handle continuous variations in the concentration of pollutants. The
operation of the biological system governs the performance of the ETP, as it
removes the bulk of the pollutants. Ideal operating conditions have to be
provided for the microorganisms in order to ensure effective treatment. The
biological system should be selected to handle any variations in flow and
characteristics of refinery wastewater.
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