Challenges in Setting Up ETP For Refineries
Satya Narayana Y V V
SFC Environmental
Technologies Pvt Ltd

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 thepollutant 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.

    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).

    Figure1: Foam in refinery

  • 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.