By Sara Heger, Ph.D.
A trash tank is generally installed prior to aerobic treatment units. Some propriety ATUs require one while others do not. The key benefit of these tanks is they will separate, partially decompose and store solids that are part of the wastewater stream. If one is not required, you should consider adding one, as it will assist with system performance.
Some key characteristics and benefit of trash traps are covered below.
Treatment
Trash tanks can serve as an anaerobic/facultative treatment device but are mainly intended to remove non-degradable items such as plastic from the wastewater stream. Trash traps are typically used to remove larger items or inorganic material in the wastewater stream (e.g., grease, garbage grindings and trash). This serves to protect the aeration system from damage that can occur from such items. Trash traps can provide a certain level of anaerobic treatment. Oxygen enters the trash tank as dissolved oxygen in the wastewater and is used by the facultative microbes living in the tank. This tank typically vents through the house plumbing to the house roof.
These tanks separate solids by slowing down the flow of wastewater in the tank. The time the effluent stays in a treatment tank is called detention time and is a critical parameter in effective physical separation of solids. Detention time is calculated by dividing the tank volume by daily flow. Inlet devices and the configuration of compartments affect solids separation as well by creating a less direct flow path for effluent to travel. This allows for better separation by slowing down the flow.
Sizing
Trash traps have limited detention time, typically as short as one-half of the daily design flow volume for residential applications. The capacity is smaller than the septic tank code requirements to assure there is sufficient food for the aerobic bacteria and to limit the production of hydrogen sulfide. The size is generally specified by the manufacturer of the unit but may be code driven. With commercial applications sizing may be larger particularly if fats, oils and grease are a concern. Some proprietary ATUs may have a trash tank as an integral part of the unit. In other cases, there is a combination septic/trash tank installed in the treatment train before the ATU.
Benefits
- Removes items from waste stream that are not biodegradable
- Removes items that can clog pipes, damage aeration units and plug pumps
- Reduces floating debris in clarifiers
- Reduces loading to the ATU which is critical for homes with high occupancy and/or that use a garbage disposal and high use commercial properties
- Provides some flow attenuation/reduction in velocity
- Allows for troubleshooting of the waste stream from the home/structure as the trash trap can be evaluated and laboratory tested if needed. Trash traps can be upset by chemicals or any other product that affects the microbes in the treatment unit. Temperature and pH affect the treatment process as well. Mixing due to the loss of baffles or high velocity flow can affect the settling.
- Can be used for denitrification but upsizing of capacity is likely and a return line from the clarifier needs to be installed
Maintenance
During service of an ATU, the trash trap should be evaluated with a sludge-measuring device. It should be looked at like a septic tank to assure the odor is appropriate, there are no non-biodegradable items, and that it is watertight. The service provider should confirm that there are three distinct layers in the tank. This tank should be cleaned based on the manufacturer’s requirements or when more than 25% of the liquid depth is storing sludge and scum.