Wastewater treatment activities
Rural, urban, and suburban wastewater treatment methods.
The standard wastewater treatment options used in rural, urban, and suburban areas are described below. Composting or biological toilets and greywater systems are becoming popular alternatives, predominantly used in rural areas where no centralised wastewater treatment is available. When considering an alternative system such as a composting toilet, a special building or Resource Consent is usually required.
Septic tanks are predominantly used for the treatment of wastewater on rural properties as central wastewater treatment facilities are impractical and expensive when dwellings are far apart. Septic tanks are large concrete or steel tanks (~4,000 litres in capacity) that are buried beneath the ground, with wastewater flowing into the tank at one end and leaving the tank at the other.
Three layers form in these tanks: a floating ‘scum layer’ at the top, a ‘sludge layer’ of heavier material at the bottom, and in the middle a relatively clear water layer. This body of water contains bacteria and nutrients like nitrogen and phosphorous, but is largely free of solids. As new water enters the tank, the existing water is displaced and flows into a drain field made of perforated pipes buried in trenches filled with gravel. The water is slowly absorbed and filtered through the ground in the drain field. The size of the drain field is determined by the capacity of the ground to absorb water, with larger drain fields required where the ground is hard clay that absorbs water very slowly.
Urban and suburban
In urban and suburban areas with high population densities central plants are used to treat wastewater. Most treated wastewater is discharged into rivers, streams out to sea or onto land (some is also discharged to groundwater, estuaries, and wetlands). There are currently 320 municipal wastewater treatment plants in Aotearoa. Treatment plants commonly use a series of techniques to remove wastes from the water. Sewage generally travels through a series of screens, tanks, bioreactors, clarifiers, and ultraviolet treatments before being discharged into the environment.
The bulk of solids, such as toilet paper, are mechanically removed by screening and settlement in sedimentation tanks (primary treatment). Commonly, this material is washed and compressed and sent to landfills for disposal.
Nutrients and more solids are removed using biological treatments, including activated sludge, aerated systems, trickling filters, biological reactors, and lagoons (secondary treatment). This sludge is dewatered and the solids, which are the remaining organic material and suspended solids, are also disposed of as landfill.
Tertiary treatment is used to remove constituents of wastewater such as organic and inorganic nutrients, dissolved solids, and chemical contaminants like heavy metals. Processes involved in tertiary treatment include percolation, filtration, adsorption, biological and chemically aided removal.
Finally, some treatment plants disinfect the remaining wastewater prior to it being discharged, using chlorination, ozonation, ultraviolet radiation, or membrane filtration to kill pathogens.