Behnaz has a PhD degree in Environmental Engineering from Texas Tech university. She has a combined eight years of research and work experience in the field of water and wastewater treatment processes. Her interest in ears is designing water and wastewater treatment processes. She worked in NASA research lab at Texas Tech University as a researcher for five years. Her research was focused on using combinations of wastewater treatment processes to treat space-based waste streams with the aim of water reuse. She joined Ardurra in August 2022 as a project engineer. Some of her roles are process design, biological process modeling, and treatment data analysis.

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Annie graduated from Colorado School of Mines with Bachelor’s degree and Master’s degree in Environmental Engineering in 2020. She joined Ardurra Austin as an Engineer-In-Training in July 2021. Annie’s area of interest is treatment process design, with an emphasis on wastewater treatment. Some of her roles include but are not limited to assisting with process design, biological process modeling, and treatment data analysis. She has been involved with multiple treatment facility evaluations and improvement projects such as Canyon Region Water Authority Hays Caldwell Water Treatment Plant, Oklahoma City Deer Creek WWTP, and City of Laredo Zacate Creek WWTP.

A properly operating secondary clarifier in a wastewater treatment plant is essential. Secondary clarifiers have a high impact on effluent characteristics and are a critical unit process to achieving permit compliance. Undesirable conditions such as high solids blanket depth, pin floc, and dispersed growth problems can cascade into settling failure. Secondary clarifiers are also inevitably linked to aeration basin treatment efficiency, and their performance suffers when aeration basin nutrient imbalances, bulking, or foaming issues arise. Even sudden changes in weather conditions (e.g., cold weather, flood events) or inadequate return activated sludge (RAS) flow may upset solids consolidation.

Troubleshooting is essential in transforming a failing or inefficient system into a reliable treatment process. In a recent project at the City of Laredo, the project team assessed the aeration basin and clarifier performance data from the wastewater treatment plants over the last six years. The assessment concluded an absolute correlation between decreased TSS and BOD removal efficiencies and solids blanket depth in four secondary clarifiers. With increased solids blanket depth, TSS and BOD removal efficiency decreases and vice versa. The project team used a commonly available commercial wastewater process modeling software to closely examine solids settling and plot state point graphs and solids compaction profiles. Our presenters will discuss solids-settling patterns and expected functions of the secondary clarifier while demonstrating when and why failures occur and how to prevent them.

The analysis also included stress testing of the clarifiers. The project team developed diurnal flow and loading curves and implemented them into dynamic wastewater simulation. The team used data from 2017 to 2022 to calibrate and validate the model. We tested five scenarios which include:

1. One-day flood unit hydrograph
2. One-week cold weather snap
3. Increasing aerating efficiency
4. Increased wasting (i.e., lower SRTs), and
5. Incorporation of an anoxic selector

Each of the five scenarios was modeled at the current AADF of 10 MGD and permitted flow of 14 MGD. The state point graphs demonstrate that the secondary clarifiers were under stable operation during current-day average daily flow model simulations. However, the secondary clarifiers lose treatment efficiency as flow increases to 14 MGD. We will describe our state point graphs, explain how we can identify and prevent settling failures and inefficiencies, and provide potential corrective measures. We will outline these corrective measures from the standard operating procedure we developed for the City of Laredo.

We will also show how solids compaction profiles show a higher solids blanket depth in high flow events and provide better strategies to control blanket depths. The results of the clarifier performance analysis helped the WWTP operators to develop a better understanding of the operations and control of the clarifiers. The primary purpose of this talk is for the audience to understand the standard practices and operational challenges of secondary clarifiers. Our presentation will utilize our case study to demonstrate operating principles and provide the experienced operator with strategies for enhancing sludge settleability.