When dealing with existing properties with homes, outbuildings and other improvements, it is often challenging to find a location for a replacement system. Historically, parcels were developed without setting aside sufficient room for a second septic system. This issue can arise just due to the age and usage of the system or the desire to replace or add to the existing home.

When sufficient space for a conventional full-size system does not exist, some options allow a reduced footprint or replacement of the old system. Be sure to check with your permitting authority to determine if these options are allowable under your regulations. In some cases, these solutions avoid the requirement for a holding tank, a cluster system or surface discharging of the effluent:

ADVANCED PRETREATMENT

Conventional sizing of soil treatment areas is based on the quality of septic tank effluent. Septic tanks do a good job of removing about 50% of the solids in raw wastewater but do not significantly remove pathogens or nutrients. The remaining levels of organic and inorganic material are used to size the soil treatment system based on the long-term acceptance rate of the soil after a biomat develops.

Numerous technologies have been developed that reduce the levels of solids by 90% or more. These advanced technologies utilize oxygen to break down organics, convert nitrogen and reduce pathogens. Some also include a step to reduce total nitrogen concentration. With the reduction in solids, the effluent can be applied to soil at a higher rate due to the reduction in oxygen demand. Care must be taken not to load the soil too high, as even with cleaner effluent, the soil can only take so much effluent. If there is a significant reduction in pathogens, a reduction in the vertical separation to the limiting condition may also be allowed. 

These technologies can sometimes be retrofitted into the existing system or be utilized as part of a complete system replacement. The owner must understand that additional operation and maintenance will be required. This system should be regulated under an operating permit to ensure operation and maintenance are occurring and that the system is performing as designed.  

TIME DOSING WITH FLOW EQUALIZATION

It can occur that the peak design flow for the home or business and the available area for a new system do not match. When in this situation, you can back-calculate how much wastewater the system can handle and then incorporate time dosing with flow equalization to ensure the system is not overloaded. Remember with this approach that an STA cannot be loaded at the peak design flow daily and expect it to last long term. Therefore, when setting up a time dosing system with a limited area, the system should be set up not to dose more than approximately 70% of what the system was designed for.  For example:

a. Peak design flow for the home: 600 gpd

b. Back calculated flow the STA can handle: 450 gpd

c. Time dosed amount to STA (70%): 315 gpd

With this option, the owner must understand that if peak events occur, such as large family gatherings, they may have to pump out the septic tank and flow equalization tank to avoid overloading the system. These systems should also be regulated under an operating permit to confirm flows and monitor ponding in the STA. 

A flow equalization tank, or surge tank, is a specialized dosing tank that provides effluent storage and uses timed dosing to allow more uniform delivery to a subsequent component over several days. In flow-equalization configurations, the dosing tank capacity is determined by the minimum volume required to submerge the pump, a surge volume equal to the flow generated during the designated storage period, and the reserve volume above the alarm activation level.

In this case, the tank is typically designed to hold at least twice the average daily flow of the facility and dose it over more than one day, as specified in the design. The flow from a surge or flow equalization tank is again controlled by a timer that controls pump operation according to fixed on (dose) and off (rest) cycles. In this case, effluent delivery may be spread out over several days.

BUILD THE NEW SYSTEM WHERE THE OLD SYSTEM WAS LOCATED

This option is typically the least desirable due to the common requirement to remove the old system, which adds considerable cost and complexity to the installation. Not surprisingly, it is typically the last resort. The most common approach with this option is to remove the existing system and backfill the area with clean C33 sand.

In this case, the existing groundwater condition is essential because if a high-water table condition exists, the sand could create an area of low resistance and take on extra groundwater.  This could also occur if surface water is not directed away from the STA. The STA should always be mounded so most of the rainfall is directed off the system. In addition, berms, drainage and other stormwater management may be needed. 

Occasionally, a new aboveground system (such as a mound) may be built over a portion of an existing in-ground system without removal. There is a risk that the existing system could create a barrier to subsurface water movement. Looking at the site, the likely direction of groundwater flow and the percentage of the new system over the old are all valid considerations. The topsoil should be carefully evaluated to determine if it needs removal or the best scarification technique to ensure an appropriate transition from the sand to the underlying soil. 

These systems are also best operated under an operating permit to confirm the system’s performance.