Tank Types Demystified

Different names used to describe sewage tanks in onsite systems
Tank Types Demystified

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I receive a lot of questions about the different names used to describe sewage tanks in onsite systems. The Consortium of Institutes for Decentralized Wastewater Treatment wrote a glossary of terms for this reason. Depending on the region where a contractor works, components of onsite systems have different names, but in discussions it was often found that the functions were the same. In this article I will take a look at the types of sewage tanks we commonly use in onsite systems and briefly describe their functions.

Onsite sewage tanks can be loosely grouped into two categories: treatment and dosing tanks. Treatment tanks include septic tanks, trash tanks, grease traps and processing tanks. The main purpose of treatment tanks is to separate, decompose and store solids. They separate and hold solids by slowing down the flow of wastewater in the tank. The time the effluent stays in the tank is called detention time and is critical to the separation of solids.

Detention time is determined by dividing tank volume by estimated daily flow. Inlet baffles and other devices encourage separation by assuring effluent takes a longer path through the tank. This allows better separation by slowing down the flow.

 

TREATMENT TANKS 

We’re most familiar with the septic tank. It receives raw sewage from a residence or other commercial building, separates the settleable and floating solids from the liquid, breaks down the organic material by anaerobic (which means without oxygen) digestion, stores the solids and allows a clarified effluent to move out of the tank to the soil dispersal area for final treatment.

The partially digested solids settle to the bottom of the tank and the floating solids – including fats, oils, and grease – are held in the tank by the outlet baffle. Periodic removal of these solids is a key to the proper operation of a septic tank. Septic tanks are usually sized to provide two days of detention time.

Grease traps are another type of tank pumping contractors deal with on a daily basis. They accept the waste stream from a kitchen. It has been well documented that the kitchen waste stream is high in organic material, including fats, oils and grease (FOG) and if this “high strength” waste is discharged directly to a septic tank and the soil treatment area, excessive biomat often develops leading to premature system failure.

Grease traps are designed to retain these solids until they are removed. This means these tanks need to be cleaned on a regular basis, set by amount of wastewater being generated in the facility.

I often get the question: “What is a trash tank? Isn’t that just a septic tank?” Trash tanks are tanks or compartments of tanks that are located just ahead of an aerobic treatment unit (ATU). Their purpose is to protect the ATU from large non-degradable solids including plastic products, feminine hygiene products, cleaning products, etc. The size and, to a certain extent, function is dictated by the manufacturer of the ATU. They are usually sized with a one-day detention time based on estimated flow. In some states, a septic tank is required in front of the trash tank.

The final type of treatment tank we often see is a processing tank. This is a septic tank that receives a combination of raw sewage and recirculated effluent to enhance nitrogen removal. Processing tanks are often used with media filters, where effluent is run through the aerated media (to convert nitrogen to the nitrate form NO3- and then back into the processing tank to facilitate de-nitrification (conversion of nitrate nitrogen to nitrogen gas).

Processing tanks can actually be a combination of septic tank, surge storage tank, pump tank and recirculating tank all in one. Many times, though, a processing tank follows a septic tank.

 

DOSING TANKS

Dosing tanks are the other major group of tanks we deal with, and they include pump tanks, siphon tanks, flow equalization tanks and recirculation tanks. A dosing tank is a tank or a compartment in a tank that provides storage of effluent and contains a pump or siphon to send the effluent on to another part of the treatment system. Dosing tanks are sized according to the required dosing regimen.

At a minimum, for a demand dosed system this means the capacity of the tank must allow for enough volume to keep the pump submerged, the dose volume, an alarm activation volume and any required reserve volume above the alarm. For a timed dose system, the tank capacity must be enough to keep the pump submerged, the average daily flow, the surge volume, and the reserve volume above the alarm activation level.

The volume must be adequate to store the average daily flow for the system while it is delivered in timed doses to the final soil dispersal area. A timer controls when the pump is on and when it is off.

A flow equalization tank is a specialized type of timed dose tank. It stores effluent for a period of time and delivers the effluent uniformly to the next treatment component over a period of several days. This provides more uniform effluent quality and increases treatment efficiency of that
next component.

These tanks are typically designed to hold at least twice the normal daily flow and dose it over the course of more than a single day. Where two or more pumps are used, a reserve volume might not be included. The flow in a flow equalization tank is controlled by a timer that controls pump operation according to fixed on (dose) and off (rest) cycles. These types of configurations are helpful where flows can vary highly from day to day. They allow flows to be spread out more evenly and help prevent any part or component of the treatment system from being overloaded.

The final type of dosing tank is the recirculation tank discussed earlier as part of the processing tank. It is a tank or compartment where effluent from two or more components in the treatment system pass, such as a septic tank and media filter, and ensures that partially treated effluent will move through one of the components more than once.

 

 



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