Table 12.1 Scoping the options: conventional systems – benefits and limitations
| Type of System | Brief description and possible benefits | System and site limitations |
|---|---|---|
| On-site: basic treatment system | Wastewater is treated and then discharged within the
property boundaries. Treatment is usually by a simple septic tank system
followed by some dispersal system such as sub-surface trenches or mound. This is the lowest-cost on-site option. If well designed, can be reliable with minimal maintenance and operational requirements |
These systems are likely to be inappropriate on properties with the following limiting factors:
It can be an expensive option in urban areas, with costs of $4,000–$7,000 per standard home, and $50 to $100 annual maintenance costs |
| On-site: high quality treatment | Wastewater is treated and then discharged within the
property boundaries. Treatment may be by an active aerated system or
multi-chamber septic tank, followed by a sand-filter system. The treated wastewater is of higher quality and can be dispersed by sub-surface irrigation, and is therefore better suited to sites with poorly draining soils. Irrigation effects can be beneficial |
These systems are likely to be inappropriate on properties with the following limiting factors:
It may cost $8,000–$13,000 per standard home and up to $150/year operating and annual costs. |
| Cluster | The wastewater from a collection of local houses, or
other activities, is reticulated to a nearby treatment plant where it
is treated and then returned to land, usually within the site area set
aside for treatment and ecosystem re-entry. Cost sharing can mean lower cost per connection while maintaining a high quality of treatment. Water recycling is made easier; loading to centralised system is reduced. |
This is best suited for a housing development specifically
designed for a cluster system. It requires a local area of suitable
land for the treatment plant and re-entry of the treated effluent to
the ecosystem. Adequate soil types, groundwater conditions and topography are required. An appropriate management and servicing structure is required. Costs are variable, and will depend on design, site and number of connections. |
| Centralised | All wastewater is collected at the source and then transported (through sewer pipes) to a central site for treatment and final return to the ecosystem. This may be the lowest-cost option (although full environmental costs are often not factored in). Management and control are very easily centralised. | This is not appropriate in sparsely populated areas (eg, rural areas) due to cost. Because such systems can involve very large wastewater volumes, there may be site limitations in providing a sustainable ecosystem re-entry technique. Costs per property are usually less than on-site options. |
