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 released from the unit, which can put the system in violation of permits or inhibit disinfection systems.

To maintain, a healthy secondary clarifier, follow these tips.

1. Monitor the upstream biological system very closely
Clarifier performance is married to the upstream biological system, whether the process is a trickling filter, rotating biological contactor, pure oxygen aeration or extended aeration. Because poor settling solids can occur with biological process, be sure to check these process controls:

• Maintain an optimal dissolved oxygen level for your treatment train. If possible, automate the blower control to appropriately react to changing conditions of the hydraulic or organic loadings.
• Check the plant’s operations and maintenance (O&M) manual to compare design criteria with current operations. The O&M provides guidance on the engineering capabilities of a biological system, therefore deviations can be detrimental.
• At extended aeration plants, monitor key parameters such as alkalinity, temperature and nitrification. As the plant nitrifies, the potential for rising sludge increases because of denitrification in the clarifier. Alkalinity and nitrifying bacteria must be present — in the right ratio — to achieve complete nitrification.
• Perform daily microscopic examinations of the aeration basin to monitor filamentous growth, and take prompt corrective action if abnormal conditions are observed.

2. Use a uniform sampling location
Choosing the right location for sludge sampling and clarifier core testing — commonly called sludge judge testing — is important for assessing a clarifier’s health. If operators do not use a uniform location and speed for the sludge judge test, it becomes difficult to trust data collected from shift to shift. Review sampling procedures, location labeling and operator techniques semi-annually. Also, create a written sampling standard operating procedure to create consistency.

3. Use trending data to predict conditions

Trending data on SCADA terminals is a wonderful way to help identify positive and negative plant trends. Maximize a plant’s SCADA data by following these tips:

• Check available flow data (raw flow, recycled flows, return activated sludge flow, etc.) in conjunction with dissolved oxygen concentration, which will give you a correlation to the impact of hydraulic loading to dissolved oxygen use.
• Monitor the mixed liquor suspended solids concentration in the aeration basin and plant effluent total suspended solids to the temperature in the aeration basin. This will show how temperatures affect settling. Note: Settling can also be affected by concentration alone. Lighter flock settles slower.
• Trend pounds of solids in the aeration basin, waste activated sludge, and effluent TSS to determine plant sludge retention time (SRT). Targeting flow might give you an incomplete picture of the process characteristics.

4. Act quickly to correct poor trends or conditions

Gathering data is only a small portion of what is required for top performance. Once you analyze data, 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 aeration basin) should be given one full SRT cycle. Then, verify the change has met the expectations.

5. Prepare 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 decide if a preemptive strike — such as raising the plant solids loading — is 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 clarifier performance can be optimized by following these simple tips. You should also consider having a quality preventative maintenance program that monitors the mechanical integrity of the system. Through a concerted effort of plant maintenance and operations staff, the secondary clarifier will reach peak performance.