Renewable Diesel

The justification for use of RD is compelling. RD is nearly chemically identical to petrodiesel and can therefore be used as a “drop-in” fuel in neat form in petrodiesel engines. It is also more valuable than other renewable fuels such as corn ethanol and biodiesel because it has a lower carbon intensity (CI).

Given the high cost to design and construct an RD refinery, major petroleum refiners are utilizing one or several of the following practices to optimize the capital investment:

The nature of the RD waste residuals presents a significant complicating factor to the use of existing refinery WRRF infrastructure. The RD refining process has two general components: pretreatment and hydrotreating. The unique challenges with RD come from the pretreatment step. (Wastewater from hydrotreating is similar to petroleum refining wastewater, which is well documented.)

Given these unique wastewater characteristics, existing refinery WRRF infrastructure is generally not suitable for treating the RD wastewater stream without significant modification. Facilities adding RD capacity or converting to RD must, therefore, do one or more of the following:

RD wastewater treatment is challenging and not currently well understood. However, the anticipated fourfold market growth over the next 5 years will require the development of treatment strategies for this complex wastewater. As potential RD refiners seek to build new or leverage existing wastewater treatment infrastructure, the unique nature of RD wastewater will have to be carefully considered...

In all cases, extensive treatment of the RD wastewater is required prior to final discharge. Figure 1 (right) presents a schematic of the most common current process flow for treatment of RD wastewater. Treatment alternatives include provisions for equalization, pH adjustment, cooling, primary treatment, biological treatment, and sludge processing. The biological component of the treatment train...

Given the low pH nature of some of the waste streams — consistently less than pH of 5 — and the significant coagulant dose required in primary separation, the first step in this treatment process is neutralization of select low pH process wastewater streams. These select process wastewater streams have temperatures as high as 80°C. At this facility, they are cooled by a spiral-type...

Historically, a series of operational challenges led to poor performance — only 50% total COD removal and intermittent poor performance of both the primary and secondary treatment processes. An investigation revealed operational practices as the cause. Table 2 (below) presents a summary of the challenges and solutions detailed below.

Typical approaches to breaking of the oily matrix and reducing organic loading by removing O&G and TSS were observed to be ineffective. In some cases, breaking this matrix required coagulant/flocculant chemistry in excess of 1,000 mg/L at the primary DAF. Primary coagulant/flocculant separation chemistry demands varied wildly from 400 to more than 1,400 mg/L and changes up to hundreds of...

Managing the pH considering the high doses of coagulant required caustic dosing. However, any caustic overdosing led to saponification and significant foaming in both equalization and aeration. The very stable foam reached heights in excess of 3 m (10 ft) and forced the facility to operate at reduced equalization and aeration basin levels and reduced aeration air flow to prevent overflow of foam.

Dewatering primary oily sludge required blending with waste biomass. Without proper blending, dewatering was unable to keep up with sludge production, and elevated TSS and O&G were frequently discharged downstream.

The wastewater was observed to be nitrogen deficient, hampering biological treatment. The recommendation was made to add urea into the wastewater prior to aeration. While not yet implemented, this action is anticipated to provide further optimization of COD removal.

High influent temperatures led to frequent aeration basin operating temperatures above 40°C with an average of 42°C and instances as high as 49°C. These conditions hampered mesophilic biological treatment to remove COD. The elevated temperatures were occurring even with the existing influent cooling, indicating that the influent cooling process was inadequately sized to manage aeration...

The net effect of these operational practices was improvement in activated sludge system total COD removal from 50% to between 80% and 90%. The addition of nitrogen and surface aeration is anticipated to provide additional and more consistent improvement of COD removal.

As part of the design of a new RD refinery, designers conducted a 12-week continuous flow treatability study. They sought to evaluate and compare activated sludge versus anaerobic contact treatment of the combined process wastewater and concentrated stream without primary separation of O&G and TSS. In this case, the RD refinery was seeking to treat the concentrated stream with the process...

Wastewater for testing was procured from an existing RD refinery owned by the entity designing the new RD refinery. In addition to serving as feed for the treatability units, the wastewater samples also provided characterization data for confirmation of the design basis loads, which were previously estimated. Mixed liquor seed was obtained from a food processing WRRF with an anaerobic contact...

Influent COD was measured for each new wastewater feed, and effluent COD was measured each weekday throughout the study for all reactors. The COD data are presented in Figure 7 (p. 27c).

O&G was analyzed for a subset of wastewater feed and reactor effluent samples. Table 4 (above) presents a summary of the O&G data. The average effluent O&G ranged from 5% to 10% of the influent O&G, demonstrating 90% to 95% removal in treatment. Additionally, many effluent values were reported as less than the detection limit. The detection limit varied from sample to sample but...

During testing, the designers evaluated the settling characteristics of the aerobic sludges for analysis of clarification performance. A total of six measurements were collected over the course of the study by placing a sample of aerobic biomass in a 2-L cylinder and measuring the stirred settling rate over 30 minutes. A summary of the results in terms of sludge volume index (SVI) is presented in...

The following conclusions regarding combined treatment of the RD process wastewater and concentrated streams were drawn from the treatability results.

The third case study presents the complexities observed during the design of a new RD refinery WRRF from concept to detailed design.

With the design basis established, the treatment technology had to be selected. Given the high COD, anaerobic treatment was identified as a potential cost savings option because of the significant aeration power demand and offsite residuals disposal costs associated with aerobic treatment. Notably, the decision to treat or haul the concentrated stream weighed heavily into the decision....