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Equalization Capacity

Have you cut your plant some slack?

 

by Kristin Taormina

 

Unless a plant can perfectly match the flow of raw material (or wastewater) to the production process (or treatment plant), a need will almost certainly arise for upstream storage capacity to provide a continuous and constant flow to downstream processes to optimize their performance. In the wastewater treatment field, this is referred to as “equalization capacity.” Equalization capacity is the proverbial “wide spot in the line” that every industry needs, whether dealing with a production process or a wastewater treatment process.
 
In addition, many production plants generate wastewater with characteristics that vary significantly over a given time period. A common example of this is the difference between process wastewater and cleaning wastewater over a 24-hour period. Without equalization of wastewater quality (or loads), downstream treatment processes are generally oversized to handle the peaks that may occur. This results in excessive capital expenditures and exaggerated operating costs.
 
Okay, I think we all agree that we need equalization capacity, but how much? There are several established approaches to sizing an equalization system.
 
  1. The “best guess” approach. Based on the plant engineer’s comment that “I think 30,000 gallons oughta do her,” put in an old stainless steel tank from your plant’s boneyard. The problem with this approach is that it rarely works, since it does not take into account actual water usage and wastewater generation patterns in the plant. It only considers flow equalization and ignores load equalization.
  2. The second approach is a bit more scientific. Using hourly wastewater flow data (both from the production plant and to the downstream unit process or final discharge), a hydrograph can be constructed that allows a calculation of the maximum storage volume needed on any day to provide a near-constant discharge flow. However, like the “best guess” approach, this approach does not take into account variations in wastewater loads, so equalization of parameters like total suspended solids (TSS), chemical oxygen demand (COD), and pH may not be adequate for optimal performance of downstream processes.
  3. The third approach is more work but quite accurate. Hourly flow and wastewater characteristic data are collected and used to calculate the volume required to provide a combination of flow and load equalization.
  4. Assuming you arrive at the desired (and effective) equalization capacity, keep the following considerations in mind:
     
  • Prescreen the wastewater. If there are large solids that can settle in low flow areas, it is better to eliminate them before they settle and are stored in an equalization system.
     
  • The system should be mixed to minimize TSS settling and ensure that influent wastewater is blended with stored wastewater, maximizing the positive effects of equalization.
     
  • If wastewater temperature is a concern for downstream processes or for discharge, heating or cooling capacity can be incorporated into the equalization system.
     
  • If the downstream processes include biological treatment and pH adjustment or if nutrient addition is required, the equalization system is a good place to add the required chemicals.
     
Feel free to contact ATI with any questions regarding equalization applications.