Overcoming wastewater challenges of opportunity crude processing
Refinery wastewater facilities need new practices and solutions for efficient and sustainable operation with heavy opportunity crudes.
Veolia Water Technologies & Solutions
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The challenges associated with processing opportunity crudes have been well-documented over recent years. Their high content of small, suspended solids, tramp amines, asphaltenes, naphthenic acids, and many other problematic substances combined with high variability can make these crudes difficult to treat from beginning to end in a refinery. Increased difficulty desalting these crudes can lead to brine effluent heavily contaminated with hydrocarbons sent to the wastewater treatment plant. Even when the desalting operation is working well, the increased loading can overwhelm some wastewater plants, forcing production to be curtailed.
When faced with this higher loading, many wastewater assets can become overburdened and more likely to push past the breaking point into upset conditions. Symptoms of these upset conditions can include foaming, poor sludge settling and compaction, internal or external toxicity, reduced nitrification rates, and reduced chemical oxygen demand (COD) removal.
Identification and treatment of the symptoms and root causes are important to maintain compliance with increasingly tight water discharge limits. This article discusses common wastewater management problems associated with processing opportunity crudes, holistic monitoring strategies to identify potential upsets early, and operation strategies and treatment techniques used to maintain effluent discharge compliance while processing opportunity crudes reliably.
Opportunity crude contaminants
The blending of tight oils and heavy ‘opportunity’ crude oils is now a common practice for many industry players. This practice certainly benefits operators by sourcing crudes based on availability and price. However, it does present a risk to the performance and reliability of the various process units within the refinery. The resulting blend sent to the crude unit can exhibit less-than-ideal properties that vary daily, even hourly, making it very hard to maintain the unit’s production quality.
Increased use of ‘opportunity’ crudes has been shown to create challenging conditions in wastewater treatment as the crude unit struggles to separate water and oil efficiently. Working upstream by optimising crude blending strategies and desalter operations should certainly be part of a holistic plan to manage heavy crudes. However, conditions should also be in place downstream to address potential desalter upsets. If a comprehensive plan to prevent and address wastewater systems’ contaminations and upsets is not put into place, there is a real risk that production may need to be curtailed as contaminants in the effluent discharge get too close to allowable limits.
Much has been written about the connection between opportunity crudes and wastewater difficulties, such as foaming, poor sludge settling and compaction, internal or external toxicity, reduced nitrification rates, and reduced COD removal. Problematic features associated with the opportunity crudes that have been blamed for the wastewater difficulties include increased quantity of solids that are smaller in size, increased amine loading, more naphthenic acids, and high variability in the crudes.
The use of opportunity crudes in a refinery’s crude diet is expected to lead to a higher potential for desalter upsets due to variability, emulsions, pH, viscosity, stability, and asphaltene precipitation. These contaminations and fluctuations would provoke more frequent primary wastewater treatment problems as the desalter effluent brine undercarries process compounds such as asphaltenes, small, suspended solids and oil, all in variable quantities. In the secondary wastewater system, we expect to see potential problems due to elevated biochemical oxygen demand (BOD) and COD, elevated levels of both long- and short-chain organic acids, more tramp amines, higher surfactant loads, and inhibitory substances.
While it is true that any one of these properties can detrimentally impact wastewater treatment, a single bad actor is not usually identified, as multiple parameters typically act together during an upset event. These combined properties push refinery wastewater treatment assets ever closer to the edge, and contingency plans cannot contain these conditions indefinitely.
Each refinery wastewater treatment plant has its own unique set of potential constraints based on its system’s design and level of contingencies. However, the lack of a plan to predict and address upsets takes the system ever closer to the breaking point. Once the breaking point is hit, several things can occur, including foaming of aeration basins and clarifiers, floating solids on clarifiers, poor sludge settling and compaction, internal and external toxicity, reduced nitrification rates, and reduced COD removal rates.
Predicting and addressing stressed conditions on a complex and dynamic system such as a refinery’s wastewater plant requires the implementation of a holistic monitoring strategy. The strategy should prioritise areas found to be most vulnerable for a given system, which should not be assumed to be the same as another wastewater system.
For example, a wastewater system with limited secondary system capacity may need to focus on ensuring fast COD oxidation and rapid settling sludge, which may need to be enhanced with bioaugmentation and chemical aids. Closely monitoring the bioaugmentation and chemical systems will be important in this case. A system with adequate secondary system capacity but limited primary system capacity may be better served by honing in on monitoring for source control, primary chemical treatment programmes, and optimisation of the primary system. Once vulnerabilities are understood, set up a programme to monitor each wastewater asset. Some typical monitored parameters are found in Table 1.
Monitoring of primary treatment operations should include, at minimum, oil and grease (O&G), total suspended solids (TSS), COD, and turbidity. In addition to the influent and effluent of the primary treatment assets, other streams that should be monitored are specific influent streams (desalter brine), equalisation systems, API systems, floatation, and clarifier systems.
Secondary biological treatment is the heart of most complex wastewater treatment plants, so a holistic monitoring programme becomes ever more important as to the sensitivity and criticality of the process. Standard chemical tests combined with dissolved oxygen uptake (DOUR), microscopy, and advanced tools such as Veolia’s BioHealth Adenosine Triphosphate (ATP) can provide unique insights to better understand the condition of the secondary system’s biology ahead of performance showing decline. BioHealth DNA genomics testing can also help detect shifts in the microbiome due to changes in food source or other stressors, enabling better long-term process decisions.
Because BioHealth ATP testing considers multiple intrant factors to simulate the impact on the secondary treatment biomass, it can provide earlier detection of potential upset conditions than other monitoring tools. Key output information includes Biological Stress Index (BSI), Active Biomass Ratio (ABR), Active Volatile Suspended Solids (AVSS), and True Food to Mass ratio (True F:M).
BSI provides the ratio of ATP released from deteriorating cell membranes compared to the total quantity of ATP in a sample, which is a good indicator that inhibitory conditions are present. This allows operators to make smart decisions when considering discretionary loads and can help direct efforts in searching upstream for problematic flows and detrimental environmental conditions such as elevated temperature.
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