Part 1 of 8:
BOD Loading or Food
Aerated Stabilization Basins (ASBs) are the most prevalent type of wastewater treatment system found in the Pulp and Paper Industry in the Southeastern United States. An overview of this type of system can be found in the EBS article “What’s the Difference – Aerated Stabilization Basins and Activated Sludge Systems?” While ASBs come in a wide variety of configurations, virtually all ASBs exhibit at least some plug flow characteristics. Various parameters will change across the system. For example, soluble Biochemical Oxygen Demand (BOD) removal will drop throughout the system, as will Dissolved Oxygen Uptake Rate (DOUR). Total Suspended Solids (TSS) will likely increase across the aeration basin as more bacteria are produced and then will decrease across the settling ponds.
The critical driving force for the biological system is the organic loading (BOD). The more BOD entering the system the greater the pressure on the system to provide adequate amounts of the other key drivers and proper growth conditions. All BOD is not equal. Some compounds (methanol and sugars) are more easily biodegraded than others (turpentine and soaps). Consequently, the biological population will respond differently to different food sources. Also, bacterial populations become acclimated to particular food sources. An abrupt change in the quality or quantity of organic loading can force the biomass into an acclimation phase, resulting in a temporary decline in BOD removal efficiency.
When microorganisms find themselves in an abundance (excess) of food, beyond that which the cell can assimilate, reproduction occurs at a logarithmic rate. The log growth phase continues until the food is depleted. As food is used up, the reproduction rate slows down considerably. This period of decreasing rate of growth is called the declining growth phase.
When the concentration of available food has been exhausted, reproduction ceases. Cells are still alive, although they are starving. At this point, microorganisms use organic material within their own cells as a source of energy. This is the endogenous phase. During this phase, the total mass of microorganisms begins to slowly decrease as the cells use up their stored reserves and begin to die. BOD removal is usually very good when a biomass is in endogenous respiration, because the bugs have oxidized all of the available food. ASBs are oper ated to achieve endogenous respiration well before effluent is discharged from the pond. This ensures that the BOD has been assimilated and oxidized by the biomass.
Understanding the quantity, biodegradability, variability, and potential toxicity of the ASB influent is critical in optimizing performance and maintaining NPDES compliance. The wastewater experts at Environmental Business Specialists are among the most experienced in North America at evaluating the complexities of pulp and paper mill wastewater influent streams. Using respirometry, BOD surrogate development, and creative data analysis techniques, we can help troubleshoot performance problems and reduce operating costs.
This article is adapted from “Aerated Stabilization Basins in the Pulp and Paper Industry” by Paul Klopping and Mike Foster. © 2003. Callan & Brooks. For information on purchasing a copy of this manual, contact EBS.