For commercial systems handling high-strength waste, it’s important to make sure enough oxygen is present for all necessary processes.
For commercial system design, the organic loading rate is often more critical than the hydraulic.
In a biological treatment system, the bacteria in the system with available oxygen first consume the biochemical oxygen demand (BOD5). BOD5 is a measure of the oxygen required by bacteria, chemicals and other organisms to break down organic matter over a five-day period. It is an indicator of the overall strength of the wastewater. If there is additional oxygen available, then nitrification will occur, which is the conversion of ammonium to nitrate. If an appropriate anoxic environment exists post-nitrification, then denitrification will occur. When designing commercial systems, enough oxygen must be available to both remove BOD and to nitrify. For existing systems, this process should be added by sampling the system:
- When: The system should be sampled within 18 hours of known peak usage. The optimum conditions are if the tank is not in need of pumping (worst case) and not pumped recently (best case) to get a representative sample.
- Where: It is best to sample from the outlet of the last septic tank or pump tank.
- How: Samples can be either pumped from the wastewater surface inside the baffle of the tank, or a bottle can be lowered into the gap, taking the sample as near to the wastewater surface as possible. The sides of the baffle should be avoided so that the FOG buildup on the baffle wall is not added into the sample. If a sample is taken from a pump tank be sure to move aside the scum layer if it exists.
If sampling data is not available, data from similar facilities should be obtained. This data can then be used to estimate the oxygen needed in the downstream components of the system using the equation:
Contaminant in mg/L x Flow in gpd x 8.34 ÷ 1,000,000 = pounds of contaminant
Following primary treatment in a grease trap and/or septic tanks, secondary treatment is often needed to keep the oxygen demand at the soil treatment area at normal amounts. The loading rates used on most state codes assume domestic strength waste is being applied to the infiltrative surface of the soil. If the levels exceed normal amounts, the biomat can become too thick due to solids and more organic material than oxygen available in the soil.
When choosing a secondary treatment unit, the designer is wise to utilize technologies with documented success in similar wastewater streams. Commonly, aerobic treatment units, sequencing batch reactors and other commercially available treatment systems can reduce BOD5, total suspended solids, and oil and grease to concentrations similar to residential waste strength or even cleaner. If nitrification is critical, additional oxygen may be needed, along with longer detention times in cold climates. Nitrification inhibitors (zinc, lead, ethanol, etc.) must also be avoided. The degree of reduction is individually dependent on the strength of the wastewater, the aeration process, soil properties, system design, use and management of the system, among other factors. When employed successfully, the resulting soil treatment systems can be sized much smaller than if designed to utilize organic loading, and the system should be a long-term solution for the property owner if properly managed.
About the author: Sara Heger, Ph.D., is an engineer, researcher and instructor in the Onsite Sewage Treatment Program in the Water Resources Center at the University of Minnesota. She presents at many local and national training events regarding the design, installation and management of septic systems and related research. Heger is education chair of the Minnesota Onsite Wastewater Association (MOWA) and the National Onsite Wastewater Recycling Association (NOWRA), and serves on the NSF International Committee on Wastewater Treatment Systems. Send her questions about septic system maintenance and operation by email to email@example.com.
This article is part of a series on the design and maintenance of commercial septic systems.