The secondary clarifier is a vital part of every wastewater treatment plant. Its health is determined by upstream variables and the mechanical integrity of the clarifier components. Some wastewater facilities do not have a filtration system to capture total suspended solids that are discharged from the unit. This can cause the treatment system to be in a violation state or inhibit the operations of a disinfection system such as Ultraviolet treatment units. This article will focus on the top 5 performance tips to achieve or maintain a healthy secondary clarifier.
Monitor the Upstream Biological System Very Closely
No matter if the process is a trickling filter, rotating biological contactor (RBC), pure oxygen aeration, or extended aeration the clarifier performance is married to the upstream biological system. Poor settling solids is a condition of the biological process. Here are some process control check for the operator:
- Maintain an optimal dissolved oxygen level for you treatment train. If possible automate the blower control to appropriately react to the changing conditions of the hydraulic or organic loadings.
- Check plant operations and maintenance (O&M) manual to compare design criteria with current operations. The O & M provides guidance as to the engineering capabilities of the biological system, therefore deviations can be detrimental to the system.
- Extended Aeration Plants: Monitor key parameters such as alkalinity, temperature, and nitrification. As the plant nitrifies the potential for rising sludge will increase, because of the denitrification that happens in the clarifier. Alkalinity and nitrifying bacteria must be present, in the right ratio, to achieve complete nitrification.
- Do microscopic examinations of the aeration basin daily to monitor filamentous growth and take prompt corrective action if any abnormal condition is observed.
Photo Credit: Loxahatchee River District taken by Sheldon Primus
Uniform the Sampling Locations
Choosing the right sampling location for sludge sampling and clarifier core testing (commonly called sludge judge testing) is important for assessing the health of the clarifier. If operators do not have a uniformed location and speed in which to conduct the “sludge judge” test, then it becomes difficult to trust the data that is collected from shift to shift. A review of sampling procedures, location labeling, and operator techniques should be made semi-annually. Also a written sampling standard operating procedure (SOP) will erase arbitrary actions by individual operators.
Use Trending Data to Predict Conditions
Trending data on Supervisory Control and Data Acquisition (SCADA) terminals is a wonderful way to help identify positive and negative plant trends on a given intervals. Here are some tips to maximize the use of the plant’s SCADA data (every plant does not monitor the same parameters, but one listed below are common):
- Check the available flow data (Raw flow, recycled flows, return activated sludge (RAS) flow, etc.) in conjunction with the D.O. concentration. This will give you a correlation to the impact of hydraulic loading to the D.O. usage
- Monitor the mixed liquor suspended solids (MLSS) concentration in the aeration basin and plant effluent total suspended solids (TSS) to the temperature in the aeration basin. This will give a connection to see how higher or lower temperatures affect settling. (Note: Settling can also be effected by concentration alone. Lighter flock settles slower)
- Trend the plant pounds of solids in the aerbay, waste activated sludge (WAS), and effluent TSS to give you the necessary information to determine the plant Sludge Retention Time (SRT). Targeting flow may give you an incomplete picture of the process characteristics.
Take Quick Action to Correct Poor Trends or Conditions
Gathering data is only a small portion of what is required for top performance. Once you analyze the data, then you must make prompt corrective action to avoid catastrophes. Corrective actions should be made strategically and incrementally to the process. Each major change (i.e. lower concentration of MLSS through wasting more, the addition or subtraction of a process train, or switching the aeration pattern of the aerbay) should be given 1 full SRT cycle. Then verify the expected changes has meet the expectations of the actual plant performance.
Be Prepared for Seasonal Weather Conditions or Process Conditions
Seasonal weather or process changes can throw the proverbial monkey wrench in any well working treatment system. Being aware of the latest weather trends will help the plant superintendent to decide if a preemptive strike such as raising the plant solids loading will be necessary. Remember, the secondary clarifier will react to concentration changes, filamentous bacteria, and organic or hydraulic loadings. The more predictable the seasonal condition the better prepared the operator should be to protect the clarifier and aeration operations.
The secondary clarifier is dependent on the upstream process, but the clarifier performance can be optimized through following these simple tips. An important tip that didn’t make the top five is to have a quality preventative maintenance program that monitors the mechanical integrity of the system. Through a concerted effort of the plant maintenance and operations staff the secondary clarifier can reach peak performance.