By Dr. Sara Heger, University of Minnesota
High-strength wastewater has been identified as a cause of early failure of on-site sewage treatment systems. With a large variation in failure types, it’s important for precast concrete manufacturers to know what causes these issues and what options are available to solve them.
What is high-strength wastewater? High-strength wastewater (HSW) can be defined in different ways based on the jurisdiction. The accepted national definition, according to the Consortium of Institutes for Decentralized Wastewater Treatment Decentralized Wastewater Glossary, is a biochemical oxygen demand (BOD5 ) less than or equal to 170 mg/L, total suspended solids (TSS) less than or equal to 60 mg/L, and oil and greases (O&G) less than or equal to 25 mg/L.
That being said, there is a large variation in types of HSW. It is helpful to consider this range when evaluating treatment options:
- Light HSW – Facilities with mostly toilet flushing tend to have elevated BOD5 and TSS (~2-3 times residential)
- Medium HSW – Facilities that have commercial kitchens have elevated BOD5, TSS and O&G (~4-10 times residential)
- High HSW – Industrial facilities with very high loads of BOD5, TSS and possibly O&G (10+ times residential)
The additional challenge with many commercial facilities can be their higher-than-normal use of cleaners and chemicals, which can negatively impact the needed microorganism in the treatment system.
When it comes to designing septic systems, often the first item brought up is how many gallons per day (GPD) the system is going to handle. However, for commercial systems, the organic loading rate is often more critical than the GPD. A one-size-fits-all approach does not work as the treatment starts in the facility, such as a restaurant, supermarket or slaughterhouse.
Wastewater from commercial facilities tends to contain more contaminants that need treatment than wastewater from residential sources. If excessive organic material or solids reach the soil treatment system, the biological clogging mat can become restrictive to the point of hydraulic failure. In addition, excessive O&G can clog pipes and the soil treatment system.
In septic systems, which are biological treatment systems, the bacteria uses available oxygen in the influent to consume the organic material. A lab test can be done to determine the BOD5. Whenever possible, the system should be sampled for the BOD5 along with the TSS and O&G if the HSW source includes food preparation or animal processing. If sampling is not an option, then data from similar facilities should be obtained. This information can be used along with the GPD to determine the overall organic load of the system.
Treatment options for HSW
Grease traps/interceptors are useful for HSW with high levels of O&G, including those that are food- or animal-processing related. Kitchen facilities should have and maintain a grease interceptor. Secondly, whenever possible, the external grease traps should collect only the kitchen wastewater and discharge to the downstream septic tanks, whereas the toilet and other wastewater should discharge directly into the septic tank. A cleanout outside the structure should be designed on the building sewer for line cleaning.
It is recommended to keep the building sewer short to limit the wastewater cooling and grease solidifying in the pipe. If fat and grease amounts are excessive, more and smaller tanks are better for cooling as there is more surface area contact with the cool soil surrounding many smaller tanks than one large one. A minimum of 24 hours of retention should be provided with 2-4 days recommended, particularly with high-temperature dishwashers, to assist with cooling and fat solidification (MPCA, 2013).i
The presence of a grease trap does not guarantee success if deep fryers are emptied into the drains, or if excessive disinfectants are used in the facility. The grease from fryers should be handled through a grease recycling program. Degreasers also should not be disposed of in the grease trap or septic tank as they can cause emulsification of the tanks and allow grease to move downstream into the treatment system. Undersized grease traps with inadequate detention capacity are of limited value in removing O&G as well.
Recent developments in micro-processor-based ultrasonic technology can assist in continuous monitoring of grease, sludge, liquid level and temperature in tanks and alert the owner when it is time to pump the tank. Even with automatic monitoring systems in place, owner awareness, managed maintenance and regulatory oversight are important to ensure the future of improved wastewater treatment and the environment.
Septic tanks for commercial applications should have increased hydraulic retention times. For example, Minnesota recommends at least four days of design hydraulic retention time for commercial applications. (MPCA, 2016).ii The septic tank provides anaerobic treatment (no free oxygen) of the influent. More retention time is likely needed for systems with grinder pumps or with large amounts of fats, oils and grease, dairy, alcohol or blood. Tank sizing also needs to consider the frequency of solids removal and the size of available pump trucks.
Septic tanks set in series are preferable to double-compartment tanks if temperature regulation is an issue. Effluent filters should be used on the final tank in series unless the system is going into a secondary treatment unit. The effluent screen should be rated for the flow and organic loading and have an automated alarm to indicate if the filter needs cleaning. Secured risers or hatches to the surface are required to service the tanks and effluent filter.
Flow equalization/pump tanks use a larger dosing tank and include an adjustable timer that controls pump rest intervals and run time for specific dosing regimens. In commercial systems, they should be considered when effluent is pumped from one system component to another. Flow equalization is a management concept that can help reduce stress on system performance due to high peak flows. In flow equalization, the peak flows are stored so they can be delivered uniformly over an extended period of time. Usually the flow for one day is equalized over a 24-hour period, but it can be done for longer, especially if peak flows last for longer than one full day. For this to be accomplished, the tank must be large enough to handle these flows, and the pump operation must be controlled by a timer as opposed to a float.
Secondary treatment units are often needed to deal with HSW. The secondary treatment is designed to provide an aerobic environment (with free oxygen). It is wise to use technologies with documented success with similar wastewater streams. Following the grease trap and/or septic tanks, the secondary treatment is used to reduce the wastewater’s oxygen demand at the soil treatment area to levels at or below residential strength.
A recirculating media filter (RMF) is a secondary treatment device designed to passively supply oxygen in a watertight structure containing a media. For HSW, an RMF typically uses coarse sand or gravel, foam or textile media. They are generally used on light HSW facilities because if the organic loading gets too high it may plug the media. Recirculation mixes the effluent that has passed through the filter with septic tank effluent to dilute the wastewater strength. The RMF must still treat the overall loading from the system. They can be constructed in tanks or in lined excavations or be manufactured in watertight containers. They are required to have an additional recirculation pump tank with a timer that contains the mixture of effluent dosed to the filter.
Aerobic treatment units (ATU) pretreat effluent by actively adding air to break down organic matter, reduce pathogens, and transform nutrients in what is known as the activated sludge treatment process. Naturally occurring microorganisms consume the organic material in sewage. Effluent treatment in an ATU is different from that in standard septic tanks, both in the speed and quality of treatment. Bacteria in an ATU use oxygen to break down organic matter efficiently, achieving relatively quick decomposition of organic solids and reducing the concentration of pathogens in the effluent. They are typically installed in a tank and have a blower, compressor or aspirator providing air to the system. There are varying sizes and configurations. Aerobic treatment units can be designed to handle light-to-high HSW and can successfully reduce BOD5, TSS and O&G to concentrations similar to residential waste strength or even cleaner.
Choosing the right system
Dealing with HSW requires careful analysis of the facility and, more importantly, the waste characteristics such as the organic loading rate. Designing an HSW system based only on flow rates can lead to issues downstream. When a precast tank supplier is approached to design tanks for an HSW system, they should:
- Request design flow
- Request the waste source
- Inquire if samples for BOD5, TSS and FOG can be obtained from an existing facility or if data is available from a similar facility
- Assist designer/engineering in determining the grease interceptor and septic tank sizing to reduce the loading to downstream components
- Suggest that with some waste streams, tanks alone will not be sufficient pretreatment prior to sending it to the soil distribution field and additional treatment steps may be needed.
Additional resources are available that offer some guidance from the Consortium of Institutes for Decentralized Wastewater Treatment1,2 and the Minnesota Pollution Control Agency.3
i Subsurface Sewage Treatment System Standards: Chapter 7080 – 7083 MN Rules. Minnesota Pollution Control Agency, Water Quality Div., St. Paul, MN.
ii Subsurface Sewage Treatment System Standards: Chapter 7080 – 7083 MN Rules. Minnesota Pollution Control Agency, Water Quality Div., St. Paul, MN.
- Subsurface Sewage Treatment System Standards: Chapter 7080 – 7083 MN Rules. Minnesota Pollution Control Agency, Water Quality Div., St. Paul, MN.