Wastewater volume ​

Choosing a calculation method ​

Daily wastewater volume is often a critical input to water balance calculations. Trench provides two ways for you to calculate wastewater volume for your assessment.

1. Number of bedrooms ​

• Trench's default settings are that each bedroom generates 100 L/day of septic tank effluent, and 300 L/day of combined wastewater. The number of bedrooms determines the daily volume you want. Only this latter figure gets reported, not the number of bedrooms. Click Options... to change the default settings.

2. Independent estimate ​

• Method 2 is available if the 'number of bedrooms' method is not applicable. Enter any two combinations of wastewater streams. Also, use Method 2 for sites where only one wastewater stream is generated. For example, if (treated) sullage is to be disposed of off-site, enter zero for sullage; if a chemical or composting toilet is to be used, enter zero for septic tank effluent, and so on.

It is common (and good) practice to be conservative in estimating wastewater volumes. However, wastewater volumes used for permanently-occupied dwellings may be excessively conservative if applied to rarely-used dwellings. For these, you might consider reducing the wastewater volume but specifying conditions (e.g. larger back-up areas) to allow for a future increase in volume.

Suggested figures for wastewater volumes ​

Typical minimum wastewater flow allowances in units of L/day/person

This Table is adapted from: Draft Australian/New Zealand Standard for Comment, March 1996. On-site domestic wastewater management. (Revision of Australian Standard 1547 1994 and part revision of New Zealand Standard 4610 1982.)