When assessing environmental risks from industrial discharges it is a prerequisite to properly estimate concentrations of many chemicals in the discharge effluent. For many chemicals (e.g. hydrocarbons, metals) analytical methods are available and effluent concentrations can directly be measured. However, for many man-added, process chemicals (e.g. biocides, corrosion inhibitors) no analytical methods are available and concentrations need to be estimated based on the amounts used and physical chemical behaviour of the components. For this purpose a designated model was developed to be applied to oil refineries and terminals. The Onshore Plant Effluent Model (OPEM) estimates the annual average effluent volume and concentrations of chemical components in the discharged water from upstream oil plants.

OPEM can be tailored to a specific facility by using a generic process diagram in which all relevant parts of the process can be included; creating a system that represents the facility that the model will be run for. To be able to run and provide accurate output, the model needs information including composition of each process chemical, physico-chemical properties, effect thresholds (i.e. PNECs) and basic toxicity data. This data is stored in databases that form an integrated part of OPEM. Another input for the model is the characterisation of the inflows. Here a selection can be made from flows of oil with traces of water and chemicals from a specific field, flows of chemicals that are applied directly to the system and additional water flows like rain and drainage water. Finally, mass balance data must be entered, which represents a quantification of the inflows or chemicals used.

OPEM uses dilution and mass balance equations to calculate the effluent concentration of each component in any part of the facility including the discharged effluent. For every chemical component OPEM produces a pie-chart presenting the origin of the component (where is it used and in which process chemicals).

Even though developed for upstream oil industry, OPEM is a generic model that can be used for complex effluent concentration calculations for any process facility. Estimated effluent concentrations of chemical components together with the information on the origin of the components are an important input for environmental risk management.

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