Wastewater Septicity - Causes and what is going on with the biology in septic wastewaters

Step-Feed aeration basin to reduce impact of high soluble BOD (septic) wastewater.

A sudden color change to gray or black accompanied by odors is often called septicity or septic wastewater. Septicity can be caused by several factors and environmental conditions, so let's review what makes wastewater septic and how to correct septic conditions.

How to detect septic wastewater

• Dark gray or black color
• Negative ORP (Redox) & D.O. absent
• Odors from sulfides and/or volatile organic acids

Color is usually the first thing noticed by operators as it is a very distinct from normal wastewater. As D.O. meters do not do well below 0.5 mg/L O2, test the water with an ORP/Redox meter. Negative ORP (usually < -125 to -150 mV) confirms the reducing environment where sulfides and organic acids are the end products of microbial activity.

Biochemistry of Septicity
When oxygen is depleted, bacteria do not stop growing. Without oxygen, many microbes begin to use nitrate (NO3) as an alternative electron acceptor (oxygen source). (This is the process seen in denitrification systems). Once nitrate is depleted - ORP drops below -100 mV - you begin to see bacteria capable of reducing sulfate (SRB) which generates S= and H2S. You also have growth by fermentative organisms that produce organic acids. Organic acids are highly soluble and often a source of odors in wastewater collection systems and EQ basins.

Why septic water entering aeration basin can be a problem?
High soluble BOD5 (organic acids) and sulfides result in very low D.O. near the inlet. This environment favors the growth of many problematic filamentous bacteria. In biological phosphorus removal systems, you need the "septic" waster in the anaerobic zone to provide the soluble organic acids used by PAO.

What to do if you have excessive septic influent flows

• Add aeration to EQ basin or tank - monitor via ORP/Redox
• Nitrate additions are used to prevent septic conditions. Especially in collection systems. 
• At WWTP, you can change influent to step feed. This will help with low D.O. filaments by reducing the low D.O. zone severity.
• Adjust aeration to put more D.O. near the influent. If you run biological nutrient removal zones, you may also need to make adjustments to operations.