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7. Wastewater and stormwater disposal (Topic e)

Disposal of wastewater and stormwater should preferably be considered only when reuse options are not feasible. Ultimate disposal of wastewater is either onto land or water (river, lake, ocean).

7.1 Land-based disposal of wastewater

Disposal onto land takes the form of effluent from on-site and off-site treatment systems being allowed to percolate through the ground. For a septic tank, for example, this occurs through the soakage of overflow from the septic tank in a leach drain (Section 2 (4.1.4)). Disposal onto land generally pollutes groundwater, and may reach surface water when groundwater eventually discharges into surface water. The impact of BOD and nutrients in the wastewater on the surface water has been attenuated by soil processes and is therefore not as severe as direct disposal into surface water. Disposal from an off-site treatment plant for groundwater recharge to control encroachment of sea water in coastal areas is a form of reuse.

Injection of wastewater into a deep confined aquifer via a borehole is a possibility. Only treated wastewater with very low content of suspended and colloidal solids can be injected into a deep aquifer to prevent blockage of the pore spaces surrounding the borehole. The long-term effect of deep well injection is still unclear and the method is not generally recommended.

In New Zealand the use of treated wastewater is considered to be a disposal method as opposed to a reuse method. This keeps wastewater from being discharged into bodies of water (rives, streams, groundwater and the ocean). Nutrients (nitrogen and phosphorus) are taken up by the crop thus protecting the groundwater. This system has many advantages as follows:

• Nutrients act as a fertiliser thus reducing amounts from traditional types of fertilisers
• Reduces surface water and groundwater pollution potential
• Culturally exceptional to indigenous NZ people
• More likely to get approval by regulating bodies
• Environmentally better.

7.2 Wastewater disposal to water environments

Disposal into a lake, stream or ocean needs to take into account the ability of the receiving water to assimilate wastewater. The natural purification capacity of the environment is limited (Section 2 (2.2)). Even when wastewater is disposed to the ocean, the area surrounding the outfall can be sufficiently polluted and the pollutants (including pathogens) can be washed towards the beaches. The minimum water quality standard for disposal to a water environment is BOD < 20 mg/L and SS < 30 mg/L. This standard is generally achieved by secondary treatment processes (lagooning or activated sludge treatment). This standard was initially developed for wastewater discharge into rivers, assuming that an eight fold dilution by river water takes place. A class 1 river therefore can maintain a BOD of less than < 3 mg/L (Section 1). Such dilution is not always achieved in arid or semi arid areas.

Nutrients (nitrogen and phosphorus) promote the growth of algae in the receiving water. In lakes and sensitive water environments the removal of nutrients may be required. Furthermore if the wastewater contains high levels of heavy metals and toxic chemicals, these may have to be removed before wastewater disposal. Over the years the requirement for disposal into water environments have become stricter as the impact of pollutants is better appreciated. It can be expected that this trend towards more stringent discharge requirements will continue (See Western Europe and North America Regional Overviews).

7.3 Stormwater disposal

Ultimate disposal for stormwater is onto land (by infiltration to groundwater) and to water environments (river, lake, ocean). These have been covered as part of stormwater treatment (4.3) and reuse (6.3), because they utilise infiltration as a general technique. Techniques for reuse are those that delay its ultimate flow to water environments to improve flow management and hence reduce the frequency and extent of flooding. At the same time these techniques also generally remove pollutants (particulates and oils) prior to the water reaching a river, lake or the sea, while creating amenities such as wetlands, waterfowl habitats and water-based passive and active recreational facilities.

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