Septic (Waste Water)Designs

Wastewater is any contaminated water discharged from a plumbed fixture from a dwelling, commercial premises, industrial site etc.

Wastewater is categorized into two distinct types:

  • Black water
  • Grey water

Black water is water that issues from lavatories. Kitchen wastewater is also potentially regarded as ‘black’ due to its high level of contaminants and is regarded as so by many local authorities. (Engineers Queensland treats kitchen wastewater as ‘black’ in all cases).

Grey water is water that issues from baths, showers, washbasins or laundries. Grey water irrigating is allowed within sewered areas. Laundry wastewater may be applied directly to ground. Read More

 Treatment Systems

There are two common levels of treatment:

1. Primary

Primary systems may treat the effluent either:

  • Anaerobically (without oxygen – e.g. septic tanks)
  • Via filtration (e.g. prior to grey water discharge whether pumped or not) with all discharge being piped below ground.

Note: Laundry water discharged directly to ground need not be treated. This method may be categorized into mainly two economical methods. The economy of each method over the other is dependent upon soil type (see below) and the size of the system:

All waste systems receive both black water and grey water components for anaerobic treatment in a septic tank prior to discharge.     

 Black water and grey water may be separated into two systems: (More economical in heavy (clayey) soils)

1. The black water (lavatory and kitchen waste) through a septic system.

2. The grey water via a filter and, either a pump out tank (short-term storage) or gravity fed to a garden bed(s)

Please Note: Storing of grey water for periods greater than 24 hours will render the water ‘black’ as this allows pathogens to proliferate and give rise to foul odors and, if irrigated may provide conditions conducive to the transmission of disease.

 2. Secondary

Secondary systems are proprietary systems that treat water to a much higher level than primary systems using aerobic and/or anaerobic or environmental methods, and allowing the discharge to be applied via sprinklers in heavy droplet form directly to the garden.

There are many and varied systems available on the market each requiring periodic servicing by the supplier/manufacturer. These systems all require connection to electrical supply.

 Percolation tests and Soil types

Percolation tests involve the drilling of boreholes at, or close to the placement of the ‘application area’ (see below). From these tests the permeability of the soil will be ascertained and a soil category given.

Soil categories range from the most permeable 1, through to the least permeable 6. Category 1

being gravels and sands, 2 – 4 sandy loams to clay loams and categories 5 & 6 light and

medium/heavy clays.

 Application Area(s)

The application area is the parcel of land to which the effluent will be ultimately discharged. This may be as a bed with a calculated surface area and depth, or as a trench with width, depth and length assigned.

The sizes of these areas are determined by the soil type, region (rainfall, evaporation etc) and the numbers of persons normally in residence, which in turn, is dictated to by the total number of bedrooms within the building.

It is important for the householder to note that fully enclosed rooms such as studies, media rooms, computer rooms etc. may be regarded as potential bedrooms.

 Designer’s Role

The designer’s role is to calculate the size of the area required and the means of application to the ground, whether sub-surface or aboveground.

The factors that determine the size of the system are the number of inhabitants/users, use of the building (domestic, commercial or industrial), the class of soil and the meteorological effects of the area (i.e. rainfall, evaporation etc.) and the type of system chosen (primary or secondary).


For more information or making free consultation appointment please Contact us.