Effective slop oil treatment is crucial for the oil and gas industry, not only to minimize environmental impact but also to recover valuable resources. The choice of treatment methods depends on the composition of the slop oil, regulatory requirements, and economic considerations.
Advances in treatment technologies continue to improve the efficiency and effectiveness of slop oil recovery, contributing to more sustainable practices in the industry.
In this article, we'll review:
- Why slop oil can be difficult to treat
- Common contaminants in slop oil
- KPIs to consider for slop oil treatment
What is Slop Oil?
Slop oil, also referred to as pit oil and waste oil, is like the kitchen junk drawer that we know we need to clean but we keep putting it off.
As the name implies, the oil isn't suitable for reuse or sale downstream, and it can represent environmental hazards if not managed properly.
The good news is that there are many possible solutions and it doesn't need to be a headache for you or your team.
Below, we'll delve further into how to treat slop oil and how to determine what products you may need to do so.
Why Slop Oil is Difficult to Treat
Slop oil is not simply a mixture of oil and water, as it is often a stabilized emulsion. Research shows that asphaltenes and resins can adsorb at the oil–water interface, creating a rigid film that prevents droplet coalescence. Surfactants, fine solids, salinity, pH, and temperature further influence stability.
Two slop oil streams with similar appearance may behave completely differently in treatment systems. Emulsion stability is strongly influenced by:
- Droplet size distribution
- Interfacial film strength
- Presence of solids or sludge
- Ionic strength and salinity
- Temperature and viscosity
Common Contaminants in Slop Oil Wastewater
Beyond recoverable hydrocarbons, slop oil-associated wastewater often contains a wide range of contaminants, including:
- Oil and grease (free, dispersed, emulsified)
- Suspended solids (TSS)
- Phenols and aromatic hydrocarbons
- Ammonia and sulfides
- Dissolved organics contributing to COD
These components are commonly reported in refinery wastewater characterization studies and significantly impact downstream treatment and discharge compliance.
A Practical Treatment Train for Slop Oil
Effective slop oil treatment typically requires multiple stages rather than a single chemical solution.
1. Free Oil Removal
Large oil droplets are removed via gravity separation (API separators, tanks, skimmers).
2. Demulsification and Coagulation
Chemical demulsifiers destabilize emulsions, while coagulants and flocculants promote droplet aggregation.
3. Physical Separation
- Dissolved air flotation (DAF)
- Induced gas flotation (IGF)
- Centrifugation
These technologies remove dispersed and emulsified oil after chemical conditioning.
4. Polishing/Advanced Treatment
When stricter discharge or reuse standards apply:
- Membrane filtration (UF, RO)
- Adsorption
- Advanced oxidation (in some cases)
What a Bottle Test Should Evaluate
Bottle or jar testing is critical, but it must replicate real-world conditions to be meaningful.
A robust evaluation should include:
- Oil separation rate
- Water clarity and turbidity
- Rag layer formation
- Chemical dose sensitivity
- Performance across varying pH, salinity, and temperature
Studies emphasize that demulsification efficiency depends heavily on operating conditions, meaning lab tests must simulate field variability.
Key Performance Indicators (KPIs)
To evaluate treatment success, operators should monitor both separation and water quality metrics. At Dober, we always talk about the importance of data to better inform your operations. In one way, that means better KPI tracking, which includes the following:
- Oil and grease
- Chemical oxygen demand (COD)
- Total suspended solids (TSS)
- Turbidity
- Phenols
- Ammonia and sulfides
- Recovered oil quality
These parameters are widely used in refinery wastewater management to assess both regulatory compliance and process optimization.
Best Practices for Slop Oil Treatment Optimization
So what should you do to optimize your slop oil treatment plan? Here are a few suggestions:
- Always characterize the stream before treatment
- Use bottle tests under realistic conditions
- Optimize both chemistry and process conditions
- Consider full treatment trains, not single solutions
- Track KPIs consistently to guide optimization
in Summary: what to do next for your Slop Oil treatment plan
We know slop oil can often be found in slop tanks, waste pits or be generated from tank cleanings that are left untreated and attended to as infrequently as possible. Without proper care, slop tanks can lead to onsite safety issues and waste pits can contaminate local groundwater. Our goal is to help prevent this by making the cleaning and management of slop oil a painless process.
Quite simply, slop is an emulsion of oil and water. An effective chemical treatment program consisting of emulsion breakers, reverse emulsion breakers and water clarifiers will help separate the oil from the water, so that your water achieves necessary discharge requirements (or reused), and your oil will be able to be used downstream. Since not all waste oils are created equal, there are two steps required to find an effective chemical treatment program.
- Request a sample kit from a chemical provider
- Perform bottle testing or jar testing to find the optimal sample for your waste oil
Now only if someone will help us with that messy kitchen drawer.
References
- Hongliang Guo, Qing Qin, Mingzhe Hu, Jo-Shu Chang, Duu-Jong Lee, Treatment of refinery wastewater: Current status and prospects, Journal of Environmental Chemical Engineering, Volume 12, Issue 2,
2024, 112508, ISSN 2213-3437, https://doi.org/10.1016/j.jece.2024.112508. (https://www.sciencedirect.com/science/article/pii/S2213343724006389) - Tian, Y.; Zhou, J.; He, C.; He, L.; Li, X.; Sui, H. The Formation, Stabilization and Separation of Oil–Water Emulsions: A Review. Processes 2022, 10, 738. https://doi.org/10.3390/pr10040738
- Medeiros, A.D.M.d.; Silva Junior, C.J.G.d.; Amorim, J.D.P.d.; Durval, I.J.B.; Costa, A.F.d.S.; Sarubbo, L.A. Oily Wastewater Treatment: Methods, Challenges, and Trends. Processes 2022, 10, 743. https://doi.org/10.3390/pr10040743
