The amount of water that a drainfield must be capable of absorbing is specifically defined by the Municipality of Anchorage (MOA) as 150 gallons per day, per bedroom, of the serviced residence. This corresponds to the following absorption requirements:
- 2 bedroom house: 300 gallons per day
- 3 bedroom house: 450 gallons per day
- 4 bedroom house: 600 gallons per day
- 5 bedroom house: 750 gallons per day
It is important to note that the septic system must be capable of absorbing the required volume and not just accepting it. In other words, the ability to introduce 450 gallons into a drainfield serving a 3 bedroom house does not mean that the system is adequate. The engineer must confirm that the water is being absorbed at an adequate rate.
In order to perform the SAT, a functional monitoring tube must be present in the drainfield. The monitoring tube allows the field engineer to measure the liquid level in the drainfield. If a monitoring tube is not present then arrangements must be made for a contractor to install one. A list of the typical costs associated with this can be found in our cost list.
Septic Tank and/or Drainfield Pumping
If the septic tank has not been pumped in the last 12 months it will be necessary to have it pumped in order to obtain a municipal Certificate of On-site Systems Approval. Even if the tank has been pumped in the last 12 months, it may be necessary to pump the tank in order to safely run the SAT and/or ensure the property owner the maximum chance of passing the SAT.
The advantages of pumping the septic tank immediately prior to performing the SAT are summarized as follows:
- SO THE BASEMENT WON'T GET FLOODED!: During a SAT, a considerable amount of water is added to the drainfield. If it is overfilled, it may back-up into the septic tank. If the septic tank is empty, the water will merely collect in the tank. However, if the septic tank is full (not pumped), the back-up from the drainfield may result in flooding the residence. When there is a significant drop in elevation from the house to the septic tank and/or drainfield, it may be physically impossible to cause a back-up. In such cases, it may be feasible to avoid pumping the septic tank. In most situations, there will be a potential for backing-up water into the house; therefore, the engineer will require that the septic tank be pumped. Under no circumstances should the drainfield be pumped, simply because it will reduce the chances of the drainfield passing an adequacy test (see next section).
- PROPERTY OWNER CAN RESUME NORMAL WATER USAGE: After the drainfield is filled with the required amount of water, the field engineer may need to monitor (for up to 24 hours) the recovery of the drainfield. If the septic tank is empty, the property owner will be able to use water in the residence normally since wastewater will merely flow into the empty septic tank and not reach the drainfield. However, if the tank is full (not pumped), any water used in the residence will flow into the drainfield and adversely affect the results of the adequacy test. In short, if the tank is not pumped, the resident may need to severely restrict water usage for up to 24 hours.
- PROPERTY OWNER SAVES MONEY!: The engineer will usually want to inspect the drainfield before the septic tank is pumped so that the normal operating level can be observed. In some cases, the engineer can visually determine that a drainfield is failed (operating with an excessive amount of water in it) without having to run the adequacy test, saving the property owner money. If the tank and/or the drainfield are pumped prior to the engineer’s inspection, the liquid levels will be artificially low, leading the engineer to believe an adequacy test is justified. Consequently, he/she will proceed with the test on a system that is certain to fail to meet the necessary absorption requirements.
- PUMPING THE SEPTIC TANK PREMATURELY OR PUMPING LIQUID OUT OF THE DRAINFIELD CAN REDUCE THE CHANCE OF A SYSTEM PASSING THE ADEQUACY TEST: A drainfield fails progressively over its life of operation (see Failed Septic System?). This progressive failure causes the liquid level in the field to rise into the uncontaminated drainrock, where the liquid can be absorbed into surrounding soils. This rising liquid level also increases the pressure below the water line, increasing the absorption rate. This progressive failure occurs until the system becomes so full that it eventually backs-up into the residence or overflows onto the ground. When a drainfield is pumped, the liquid level is lowered to a level where the drainrock and soil interface is more contaminated and the absorption rate is less. In short, the level in the drainfield will have been lowered into the failed portion of the system. If the engineer is not aware of this condition, he/she will not introduce enough water into the drainfield to get it above the normal operating level, and into the cleaner portion of the system where water can be more easily absorbed. This same condition occurs when a septic tank is pumped in the week prior to an adequacy test. During the time the septic tank is re-filling (after being pumped), no water will reach the drainfield. This will allow for the liquid level in the drainfield to drop down into the failed region. In order to maximize the chance of passing an adequacy test, water should be added to a drainfield that is operating at its normal liquid level, so that it can be filled into the uncontaminated portion of the system.
Septic Adequacy Test
The septic system adequacy test is performed as follows:
- The liquid level in the drainfield monitoring tube/s is measured
- A hose and flow meter are connected to an outside faucet
- The water is turned on and introduced into a drainfield clean-out or monitoring tube
- The amount of water introduced and the rise in the liquid level are recorded throughout the filling period. This information is used to calculate the gallons per inch of rise.
- Once the required amount of water has been introduced, the flow is stopped.
- The drop in the liquid level is monitored for as long as 24 hours and the total absorption rate calculated.
The engineer then evaluates the data and determines whether the system meets municipal absorption requirements. The MOA considers the data from the adequacy test to be valid for up to two (2) years. Consequently, they will issue a Certificate of On-site Systems Approval based upon the data for up to two (2) years after the testing date. If the system is deemed to be failed, an upgrade is usually the recommended course of action.