Some readers have recently asked whether they should be concerned about their systems ability to handle peak water-use events. Examples of large water-use events provided were large parties over a weekend, additional guests staying for a period of a week or more, and vacation rentals of the home.

Each of these examples may or may not require some type of flow equalization depending on system size, configuration and operation. As we are all aware, our systems are designed based on an estimated daily flow. For a three-bedroom residence estimated flow is around 450 gpd. Generally, design numbers give a little cushion to allow for variation in use between residences based on how the occupants use water. 

We all should realize water is not used uniformly during the day. Most of the load or peak flows from a typical residence occur during a couple of short periods during the day. Usually in the morning (think breakfast, getting ready for work or school) and in the evening (dinner, dishwashing, showers). This variability in flow if large enough can create problems with hydraulic overloads in gravity-fed systems, pushing solids through the system and creating plugging in the soil treatment area. 

In pressure systems or systems with dosing tanks and pumps, the flow is somewhat regulated. That’s because flow goes into some type of dosing tank and is pumped periodically through the day. But if the system is set up to dose on demand rather than being regulated by a timer, high flow can still create similar problems.

Daily peaks 

In a conventional septic tank, gravity flow to drop box, then to sewage treatment trenches is designed to handle typical daily peaks by sizing the septic tank to provide two days of retention time. For the three-bedroom house mentioned earlier, a septic tank should be at least 900 gallons. Most codes provide for a minimum-sized septic tank of 1,000 gallons. During the morning and evening peaks up to 100 gallons may be delivered to the system in an hour. 

Some of the flow is attenuated and spread out just by running through the plumbing and the septic tank so it all does not hit the sewage trenches immediately. Assuming all the trenches are not full to overflowing, the flow is spread over the trench. Bottom line is these types of hydraulic loads are designed into the system.

One concern with the scenario above: During peak flows there may be a tendency to push additional solids through the tank to the sewage treatment area. This has led to most states now requiring installation of effluent screens at the outlet of the septic tank. This prevents larger solids or materials that should not be in the tank to begin with from moving downstream. While this is not a cure-all it does help maintain infiltration capacity in the soil. The effluent filter will need to be cleaned periodically to keep from totally plugging, causing sewage to back up into the house. If there is an effluent filter, the tank should have a high-water alarm to alert the homeowner that the effluent screen is plugged.

The types of events or activities described by the readers do not fit within the normal use patterns of a residence and require additional design and installation consideration based on the frequency of events and how average daily sewage flow may be increased and building in the ability to handle especially large events. 

Party time

As indicated above, moving from gravity flow to low pressure trenches, at-grade or mound systems will spread the flow out more uniformly in both space (soil treatment area) and time throughout the day. Increasing dose tank capacity and using a timer to spread flow throughout the day will address occasional large parties and periodic increased use from guests. (However, people have told us in certain areas of the country they really know how to party, and water-use events of several thousand gallons a day are not unusual.) 

For very large parties and other situations where the house is periodically filled to overflowing, such as happens in a vacation rental property, an additional component should be added and an increase in septic tank capacity and the size of the soil treatment area may be necessary.

A flow equalization tank can be added. This is sometimes called a surge tank. The purpose of the tank is to provide storage of effluent and it uses timed dosing to deliver effluent to the rest of the system over a period of several days. It is like any other dosing tank where it has a submerged pump and provides a storage volume based on estimated peak daily volumes and some additional storage space to allow time to service the pump in case of a problem.

Typically, the flow equalization or surge tank is designed to hold at least twice the normal daily flow and then dose it over the course of more than a single day. The advantage, of course, is that through use of a timer, excessive flow can be spread out over times when the residence is not occupied or at least during times of less flow.