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 drain field to go anaerobic (without air).
Your POWTS performs best in aerobic conditions. In aerobic (with air) conditions, a lot more bacteria 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 drain field. When these additional solids end up in the drain field, it will accelerate the failure rate of the drain field. In other words, the drain field will fail much quicker than a POWTS that does not experience frequent peak flows.
All septic tanks designed from about 1978 to the 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 washing machines. They typically will use about 1/3 of the amount of water per load as standard top-load washing machines.