How to decide which system is best for me?
ENGAGE A DESIGNER
The first step is to talk to an on-site wastewater treatment system designer in your local area. They will independently assess your property and recommend the best treatment system options for you. In addition, they are able to assist with any consents required by your local council.
UNDERSTANDING ON-SITE WASTEWATER TREATMENT SYSTEMS
Several types of treatment systems are available. Each has advantages and disadvantages that may or may not suit your situation.
The main types of ‘secondary’ treatment systems used in New Zealand include:
- Aerobic treatment systems (such as aerated treatment, biological filter, rotating disk and membrane systems)
- Packed bed filter systems (including intermittent sand filters, recirculating sand filters and recirculating textile filters) are a modern version of the traditional septic tank
- Alternative treatment units (such as vermiculture and wetland systems)
AEROBIC TREATMENT SYSTEMS
Wastewater is treated in a three-stage process:
- First stage - Primary settling tank
- Second stage - Biological reactor
- Third stage - Clarifier
There are two categories of Aerobic treatment systems
- Submerged Aerated Fixed Film (SAFF)
This system is often seen as the ideal solution for treating domestic wastewater.
Wastewater is treated by micro-organisms (bacteria). Inside the aerobic treatment systems, bacteria form on a bacterial carrier or media (e.g. Oxybee media in a honeycomb shape) and consume the micro-pollutants.
SAFF technology is also very tolerant to variations in the wastewater volumes to be treated (commonly seen in holiday homes). Through a recirculation process, sludge is returned to the primary settling tank (first compartment within the treatment station) ensuring the bacteria are fed continuously and never stop developing. This allows sustainable operation of your device, even when there is no one in the house.
- Sequencing Batch Reactor (SBR)
Micro-organisms (bacteria) are used to treat wastewater. The difference between SBR and SAFF systems is that the bacteria move freely within the effluent to which they bind to degrade the waste water pollution. This system does not support reduced or variable loading and is therefore not suitable for small domestic applications.
PACKED BED FILTER SYSTEMS
These systems treat the wastewater by filtration and operate as a two-stage process.
- First stage - Pre-treatment is through a primary settling or septic tank
- Second stage - Filtration treatment via a natural or artificial media
There are three categories of packed bed filter systems:
- Traditional systems
Domestic wastewater is treated in the ground or by a soil reconstituted with a silica sand bed. The traditional septic tank systems do not consume electrical energy in the treatment process. However, the footprints of these systems are large and the sand needs to be replaced between 10 and 20 years. The discharge pipe from a traditional system is often deep and may require pumping to the disposal field.
- Compact filters
A Compact filter is similar to the traditional septic tank system. However, treatment is achieved through an innovative filter media (either Xylit, coconut chip, rock wool, Zeolite or pine bark). The footprint is substantially less than a septic tank system.
- Planted filter or wetland
This treatment process is a combination of sand with plant support (such as reeds) which promotes percolation of pre-treated wastewater.