Quick Fixes For Organic Overload Of A Refinery Wastewater

Abstract
Operators of a petroleum refinery wastewater treatment plant treating 5 MGD (800 m3/hr) of process wastewater were having throughput restrictions due to poor settling of solids in the secondary clarifiers. To avoid having to reduce crude oil charge by 10%, a quick debottleneck solution was desired.
The Problem
The refinery had three complete mix activated sludge (CMAS) and clarifier trains that were organically overloaded, and two all-in-one activated sludge units that were hydraulically fully loaded. Efforts were focused on the CMAS/clarifier trains. Organic overload led to excessive exocellular polysaccharide (EPS) production, resulting in viscous bulking and poor settling in the three secondary clarifiers. A 15% increase in organic loading each year for the previous two years caused the inevitable overload situation shown in Figure 1. Although plans to add capacity were progressing, the new unit would not be operational for another 18 months; therefore, a quick fix was desired. Figure 2 shows India ink-stained biota, indicating the presence of troublesome high levels of EPS material caused by organic overload.
Investigating Options
Through collaborative discussions, a list of quick mitigation ideas was generated. The ideas were ranked for relative ease of implementation, degree of impact on solving the immediate problem, probability of success, and risk. These items were compiled and summarized in a priority matrix chart, shown in Figure 3. Those items that could have the most impact in the shortest time, without undue risk, were pursued. The chart and its accompanying table (table not shown in this abstract) were updated throughout the process to reflect progress and better informed assessments.
Actions, Results
Five of the items listed in Figure 3 were pursued. Number 9, cascading of waste activated sludge (WAS) from units that were not exhibiting high levels of EPS-the other units were hydraulically fully loaded but had higher resident solids and lower food-to-mass (F:M) ratios-to the CMAS/clarifier trains, allowed for marginal improvement in settleability. Number 8, use of ballasted flocculation, produced modest gains. Number 3, micronutrient addition, and number 12, bioaugmentation, did not produce gains. Number 2, use of granular activated carbon (GAC) beds to trim chemical oxygen demand (COD) loading, most effectively relieved organic loading on the CMAS/clarifier trains and allowed for full throughput rates. The paper, if accepted, will provide details on actions and results and will include a section on lessons learned.