COASTAL on-site wastewater
ACH Consulting were engaged to provide an on-site wastewater treatment system for a new 3 bedroom dwelling to be constructed on a site which presented several challenges. The site was situation 70m from the foreshore with a net site area of 764m2. On-site wastewater treatment systems are a point source for nitrogen discharges into natural waters. In the case of the foreshore area these can lead to red-tide algal blooms. The goals of this design were to provide adequate wastewater treatment for a three bedroom dwelling while minimizing nitrogen impacts to the natural waters, because at the end of the day, what is the point of having a fantastic coastal view if you can't fish or swim in the water.
For this project the lack of space, the need for maximum nitrogen reduction and minimal pathogens led to the decision to provide tertiary treatment and UV sterilisation with the Advantex AX20. The effluent disposal system utilised shallow pressurized dispersal systems (SPDS) which research indicates provides substantial nitrogen removal beyond that provided by the advanced treatment. The SPDS has an additional advantage of using a considerably smaller footprint than a traditional effluent disposal field.
The SPDS system is pressured and time-dosed within the root zone of the ground. The use of the SPDS system in concert with advanced treatment provides nitrogen and pathogen removal and is effective close to tidal tributaries where shallow groundwater exists, where the elevation of the water table is expected to increase as a result of sea level rise and where sites have limited space.
Note: These systems are assessed by Auckland Council on a case by case basis and require a Resource Consent.