Petroleum refineries have a range of systems and equipment for wastewater handling. However, these processes need to be reevaluated when considering renewable diesel refining because of the dramatic differences in wastewater streams that may result after the transition.

Renewable diesel refining includes two major process steps — the pretreatment unit (PTU) and hydrotreating in the renewable diesel unit (RDU). The PTU is used to reduce contaminants in raw materials that will be optimized in the RDU. The RDU converts the pretreated feedstock into renewable diesel and other products.

Both PTU technology selection and the variability of untreated renewable diesel feedstocks create unique wastewater streams that are difficult to effectively treat without improvements to existing refinery wastewater treatment systems. Refineries that convert to renewable diesel without adequate consideration of the unique characteristics of wastewater effluent can adversely affect project budgets, create processing inefficiencies and negatively impact the environment.

Determining a feedstock strategy is a critical part of project planning. Refineries need to decide if raw waste feedstock or pretreated feedstocks will be used for renewable diesel refining.

Feedstocks for renewable diesel production include animal fats (e.g., beef tallow, poultry fat and white grease), vegetable oils (e.g., soy, canola and corn) and used cooking oils. Due to the growth in renewable diesel production, demand for triglyceride feedstocks is increasing. As a result, refiners need to be flexible in their pretreatment requirements, depending on what is available in the marketplace.

Pretreatment of feedstock must meet stringent specifications to remove impurities, metals, chlorides, phosphorous and other contaminants. Removal of these contaminants is largely dictated by the need to prevent fouling or deactivation of the catalysts used in the downstream hydrotreating unit.

Pretreated feedstock is available to source but is priced at a premium. If pretreated feedstock is economically viable or preferred, refiners can use the raw material as soon as it arrives at the renewable fuel refinery. However, many refiners choose an on-site PTU to improve operational flexibility and overall processing economics. By understanding feedstock selection, refiners can identify the proper pretreatment steps and appropriate pretreatment technologies.

Due to pretreatment requirements and the resulting wastewater, refiners must be aware of and plan for high concentrations of wastewater contaminants and variability as part of the refinery energy transition. Accordingly, the cost of wastewater handling in any petroleum-to-renewable diesel refining conversion can be high, accounting for up to 15% of the total project cost.

Wastewater is characterized by identifying the constituents in the waste streams, including the quantity of organic compounds present. These organic loads largely dictate the wastewater treatment requirements, which are based on the oxygen demand needed for decomposition. Oxygen demand is detrimental to receiving bodies of water, as it will deplete levels and impact aquatic life. This pollutant load can be quantified by various parameters and must be reduced prior to discharge.

• Chemical oxygen demand (COD) quantifies the amount of oxygen required to chemically oxidize the organic pollutants in a waste stream.
• Biochemical oxygen demand (BOD) is used to approximate the amount of oxygen needed by microorganisms to biologically break down organic contaminants.

Renewable feedstock processing creates high-strength wastewater, with high COD and BOD content, due to the concentrated presence of organic compounds, phospholipids, metals and other impurities. Additionally, the changing quality of feedstocks (from vegetable oils to animal fats) can result in different organic loadings and other characteristics that need to be treated and accommodated on a continuous basis.

The impact of organic loadings, and the resulting strength of various renewable waste streams, can be up to 100 times greater than typical refinery wastewater. As a result, refiners must plan for the right wastewater handling strategies to avoid negative environmental and economic impacts. Unfortunately, because renewable diesel production is relatively new, treatment of this type of wastewater is a topic that is not well understood by many project planners and contractors.

Petroleum-to-renewable diesel refinery conversion can present a compelling business case for refiners. With awareness and understanding of the differences in wastewater streams from renewable diesel production, refiners can develop plans that address this critical area and reduce project and production execution risk. Upfront consideration of wastewater planning helps refiners leverage existing assets, determine the right treatment processes, identify the most effective technologies and maximize current and future opportunities.

Burns & McDonnell is supporting multiple refineries to map out and execute a transition to renewable fuels. Our approach is comprehensive, from feedstock selection to wastewater management, permitting and everything between.