The lack of knowledge on the measurement of oil in water
Historically, the primary areas of application for oil in water (OiW) measurement have been municipal sewage treatment plants, shipping, and the oil and gas industry for monitoring and optimization of wastewater treatment processes, monitoring of regulatory compliance of discharged water and protection of the environment. .
The presence of oil can have a detrimental effect on the quality of drinking water, with even a tiny amount having a huge impact on taste. Similarly, for wastewater treatment plants, the presence of oil in commercial effluents can have an impact on the treatment process. Therefore a maximum amount of OiW in commercial effluents is usually specified, which in the UK and USA is usually around 100 mg/L.
For the shipping industry, the discharge of bilge water is strictly regulated by the International Maritime Organization (IMO). For treated bilge water, the discharge limit for oil content has been set at 15 ppm (parts per million).
In the upstream oil and gas industry, for every barrel of oil produced, approximately five barrels of water are co-produced. If produced water is to be discharged, whether for offshore or onshore production, the oil content of the discharged produced water must meet a discharge standard set by regulators around the world. For example, in the North Sea, a monthly average of 30 mg/L, as agreed by the OSPAR convention (Oslo-Paris), must be respected. However, for many newer installations, a reduced figure is often set and agreed between the operator and the relevant regulatory body/authority in the different countries.
The measurement of the OiW concentration therefore plays a very important role both for water treatment operations and for controlling the regulatory compliance of discharges. It can be measured in the laboratory using a benchtop method or in the field using an online analyzer.
Laboratory bench methods include reference methods – which are important for defining what OiW is and also for regulatory compliance monitoring – and non-reference methods, which are important for control, optimization and process trends. Reference methods include infrared absorption, gas chromatography and flame ionization detection (GC-FID) and gravimetric methods. Non-reference methods may include colorimetry, infrared absorption (based on transmission and attenuated total horizontal reflection), ultraviolet (UV) absorbance, and UV fluorescence.
Field measurement methods are often needed for a variety of reasons, including ease of use, rapid results, low cost, portability, and potentially no requirement for the use of solvent and other consumables. In the case of online analyzers, they can provide minute-by-minute (if not second-by-second) data, which is extremely useful for process control and optimization.
Filling knowledge gaps on uncertainty
All measurements have an associated uncertainty, and the measurement of OiW is no exception. The uncertainty of a measurement is defined as the size of the margin of doubt associated with the measurement.
For a typical result of oil in produced water of 15 mg/L obtained by manual sampling and analysis using the OSPAR GC-FID method, a previous estimate carried out at the national engineering laboratory TÜV SÜD showed that the measurement uncertainty could reach up to ±49% with a confidence of 95%. Such a high level of uncertainty could have serious implications for monitoring regulatory compliance, evaluating the performance of online OiW analyzers, and developing acceptance criteria for the use of non-referenced OiW measurement methods for release reports.
In the oil and gas industry, on-line and continuous OiW analyzers have been used for produced water management for many years. However, the guidelines for their use for reporting purposes had not been practiced, vetted or field-tested before being published.
Additionally, while there is growing interest in the development and deployment of normally inaccessible subsea separation platforms and systems, the release of significant amounts of produced water from such unmanned installations does not will not be possible without the availability and use of reliable online OiW analyzers. However, no guidelines currently exist to accept an OiW analyzer online to report the discharge of produced water from such facilities.
The National Engineering Laboratory TÜV SÜD therefore launched a Joint Industry Project (JIP) aimed at filling knowledge gaps, making recommendations to update existing guidelines and ultimately making the use of on-line analyzers for reporting produced water discharges a common practice. The JIP was conducted and completed in 2020 with the support of six international oil and gas operators and the UK Government’s Department for Business, Energy & Industrial Strategy (BEIS). A new JIP has now been launched, aiming to carry out field trials to ensure that the recommended changes to existing OSPAR guidelines will be practical and implementable.
The future of OiW analyzers for produced water discharge
OiW is a method defined parameter, therefore it is always important to mention the measurement method used when citing OiW concentration results, and to remember that all measurements have an associated uncertainty. However, due to the nature of the OiW measurement – primarily related to wastewater – few people have paid much attention to its uncertainty until recently. Yet uncertainty is vitally important.
The use of online OiW analyzers has increased in recent years, both for operations and for rejection reporting. As regulators demand ever-improving environmental performance and the oil and gas industry seeks to continually reduce operating costs, these advantages mean that on-line analyzers are likely to become increasingly popular. One of the key issues is the reliable operation of these analyzers – and keeping the analyzer’s optical window clear is vital. Ultrasound, water jet and mechanical wiping, as well as methods using fluid dynamics, have been developed and incorporated by vendors. In the future, we will see more and more maintenance based on condition monitoring, which will further improve the reliability of these OiW analyzers.
To make the use of on-line OiW analyzers common practice for reporting produced water discharges, knowledge gaps need to be addressed regarding analyzer acceptance, measurement uncertainty and guidelines for using these analyzers for an unmanned platform should be produced. Ongoing research programs will ensure that these gaps are properly addressed through the support and participation of all stakeholders, including regulators, operators and providers.
Ming Yang is a senior consultant at TÜV SÜD National Engineering Laboratory. TÜV SÜD National Engineering Laboratory is a world-class provider of technical consulting, research, testing and program management services. Part of the TÜV SÜD group, the organization is also a global center of excellence for flow and fluid flow measurement systems and is the UK’s designated institute for flow measurement.