There are very few things that you could do that are more detrimental to your Private Onsite Wastewater Treatment System (POWTS). It is one of the quickest ways to cause your POWTS to fail.
When people do all their laundry in one day, the high (peak) flow will cause the drainfield to go anaerobic (without air). Your POWTS performs best in aerobic conditions. In aerobic (with air) conditions there are a lot more bacteria that are working to digest the organics (among other things) in the wastewater.
The peak flows that flow into the septic tank from the home cause the settled solids to re-suspend themselves in the wastewater. During re-suspension these solids get pushed out of the septic tank and end up in your piping network or drainfield. When these additional solids end up in the drainfield, it will accelerate the failure rate of the drainfield. In other words, the drainfield will fail much quicker than a POWTS that does not experience frequent peaks flows.
All septic tanks designed from about 1978 to present were designed to allow for a 24-hour settling period for any solids or suspended solids entering the tank. By creating excessive peak flows the solids settling cycle will be short-circuited and cause most suspended solids to leave the septic tank too soon.
To help minimize the peak flow effects generated from the laundry process, homeowners should consider purchasing water conserving, low flow front load wash machines. They typically will use about 1/3 of the amount of water, per load, as the standard top load wash machines.