Understanding Septicity in Wastewater: Risks, Impact, and Solutions
What is Septicity?
Septicity occurs when bacteria in wastewater consume oxygen and, in its absence, switch to alternative sources such as nitrate. When nitrate levels are insufficient, bacteria turn to sulphate, converting it into sulphide. This process produces hydrogen sulphide (H₂S), a corrosive compound with significant consequences for wastewater systems.
The Risks of Septicity
The presence of H₂S in wastewater introduces a range of challenges, from infrastructure damage to operational disruptions:
Structural Damage
Concrete Erosion: Bacteria in septic wastewater can break down concrete by feeding on sulphate, which results in the degradation of structures like discharge chambers. This damage can lead to loose concrete fragments entering pumps, causing further damage.
Corrosion of Metal Components: Hydrogen sulphide also corrodes manhole covers and pump components, weakening their integrity and functionality.
Electronic and Fire Hazards
Silver Whisker Formation: In pump station control panels, H₂S can create "silver whiskers," which bridge gaps between electronic connections. This phenomenon not only damages electronics but can also pose a fire hazard.
Rusting of Control Kiosks: Even in non-coastal areas, rusted kiosks may indicate H₂S infiltration through cable ducts. This can be confirmed using gas detectors and loggers in both wet wells and control panel areas.
Health and Safety Concerns
Hydrogen sulphide gas in the air poses a lethal risk to engineers working in these environments. Exposure to high concentrations can lead to serious injury or even death.
Operational Challenges
Septic wastewater is harder to treat than fresh wastewater. Its lack of oxygen disrupts primary settlement and aeration processes in treatment plants, potentially leading to operational shutdowns.
Addressing Septicity in Wastewater Systems
Managing septicity requires a combination of preventive measures and targeted treatments:
Nitrate Dosing
Adding nitrates to the wastewater prevents bacteria from turning to sulphate as an oxygen source, mitigating H₂S production.
The required nitrate dosage depends on the length and diameter of the rising main and the flow rate of the wastewater.
Nitrate Types: Sodium nitrate (e.g., Animex 64) and calcium nitrate (e.g., Animex 76) are commonly used. Calcium nitrate is stronger and more cost-effective for high-demand systems, while sodium nitrate is preferred for standard applications.
Regular Monitoring
Use gas detectors and loggers to measure H₂S levels in wet wells and control panel areas. This data helps identify problem areas before they lead to significant damage.
Proactive Maintenance
Inspect and reinforce vulnerable components like discharge chambers, manhole covers, and pump station control panels.
Address rust and corrosion promptly to maintain system integrity.
Conclusion
Septicity in wastewater systems is a complex issue that demands a proactive and informed approach. By understanding its causes, impacts, and solutions, utilities and engineers can safeguard infrastructure, protect workers, and ensure the uninterrupted operation of treatment plants. Effective nitrate dosing and regular monitoring are key strategies to combat this pervasive problem, reducing risks and maintaining system efficiency.
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