Phytomanagement of Treated Municipal Wastewater

In New Zealand, the land application of Treated Municipal Wastewater (TMW) is the preferred option over discharge into waterways or the ocean, where it can exacerbate eutrophication and / or toxic algal blooms. Compared to direct discharge into water, Irrigation of TMW onto land reduces the contaminants that enter waterways and therefore has positive effects on the water quality. The root-zones of plants remove nutrients contained in the TMW, mitigate pathogens, and break down or immobilise contaminants that would otherwise degrade water bodies. TMW can reduce or eliminate the need for mineral fertilisers such as superphosphate, which contain elevated concentrations of toxic cadmium, fluorine and uranium that can accumulate in soil. In many countries, including NZ, TMW is used to irrigate pasture, crops and forestry.

The application of TMW to land also carries risks that need to be mitigated for a successful operation. There are numerous examples of where land application of TMW has been discontinued because of environmental degradation. Excessive rates of TMW application to land can result in unacceptable nutrient leaching, runoff, soil instability and erosion, as well as accumulation of some components, such as sodium, in the topsoil. High sodium concentrations can reduce plant growth through salinity and sodicity as well as degrade soil structure through the dispersion of clays. The nature of the risks of the land application of TMW and therefore the design of a successful system is dependent on the quality of the TMW and the local environment. Therefore, every system needs to be specifically designed.

Potential for land application of TMW on Banks Peninsula

The successful application of TMW to land on Banks Peninsula requires particular attention to soil quality. Soils of the lowland areas of the peninsula where TMW could potentially be applied are mostly derived from loess with a relatively high clay content. They are often imperfectly drained and may contain a fragipan (an layer of impermeable soil). These soils present a higher risk of infiltration problems compared to free-draining soils and consequently an improperly designed TMW application system may be susceptible to surface runoff and erosion.

The Christchurch City Council seeks to reduce the direct disposal of TMW into Akaroa harbour. Several small communities now have their wastewater irrigated onto woodlots. There is now an on-going program of options analysis for alternatives to harbour disposal for the settlement of Duvauchelle. Potentially, some of the effluent produced in Akaroa could also be land-applied. Duvauchelle produces some 27600 m³ of wastewater per year (based on 2016 data provided), which is currently discharged directly into the harbour through one long harbour outfall.

In 2014, the Christchurch City Council (CCC) approached Lincoln University regarding the possibility of irrigating TMW from Duvauchelle onto the local golf course. In subsequent discussions with stakeholders during public open days in 2015 and 2016, this brief was expanded to include cut-and-carry pasture as well as NZ native vegetation. While there are numerous examples of successful irrigation onto cut-and-carry pasture in NZ and elsewhere, there is a shortage of information on how native species will interact with TMW. Potentially, TMW could be irrigated onto NZ native vegetation, with a view to increasing the production of valuable native products or the creation of zones of ecological value. Manuka (Leptospermum scoparium) is an obvious candidate species because of its associated high-value honey and essential oils. Moreover, mānuka has been shown to kill soil-borne pathogens and reduce nitrate leaching.

Other potential valuable native species are kanuka (Kunzea robusta) for essential oil production, horopito (Pseudowintera colorata), which produces antifungal compounds, harakeke (Phormium tenax) for fibre production, and a whole suite of species, including kapuka (Griselinia littoralis) that may be a nutritious supplement due to tannins and trace elements.

It is unclear whether TMW would confer the same growth benefits to native vegetation as to pasture. Many NZ-native species, such as mānuka, are adapted to low-fertility soils and it may not respond well to the addition of high concentrations of plant macronutrients. Some NZ native plants respond positively to N (200 kg/ha equiv.), but Leptospermum scoparium did not. Biosolids improve the growth of Grisilinea littoralis and Kunzea robusta, but not Dodonaea viscosa.

A native ecosystem receiving TMW would likely remain unharvested or have only a small fraction of the biomass removed. Therefore, unlike a cut-and-carry pasture receiving TMW, there would be no significant removal of nutrients or contaminants from the system. It is likely that nitrate leaching and phosphorous accumulation in the soil would therefore be greater.

We aimed to determine the suitability of soils from the Duvauchelle golf course and Takamatua peninsula to receive treated municipal wastewater from the Duvauchelle Wastewater Treatment Plant. Specifically, we sought to determine whether irrigation rates of up to and in excess of 1000 mm per year would result in ponding, excess nitrate leaching, accumulation or depletion of elements in soil, changes in pasture growth and quality, change in the survival and growth of NZ native vegetation.

The major outcomes of this study were:

• Following an initial assessment of the soils where the TMW would be applied, a lysimeter trial was set up at Lincoln University in December 2014. This trial comprised 18 50 cm x 70 cm lysimeters containing intact soil cores from the golf course at Duvauchelle (12 lysimeters) and an area between Takamatua and Akaroa (6 lysimeters). The soils from Duvauchelle and Takamatua were Barry’s soil and a Pawson silt loam, respectively.
• From December 2014 until April 2015, these lysimeters were irrigated with 10 mm per day, resulting in all lysimeters draining approximately equal volumes. On the 22nd of April, treatments started with municipal wastewater from Duvauchelle. Treatments comprised a control (Duvauchelle, Akaroa), 440 mm/yr (Duvauchelle), 825 mm/yr (Duvauchelle, Takamatua) and 1650 mm/yr (Duvauchelle). These treatments continued until the 3rd of October 2016. The lysimeters were then deconstructed and analysed.
• All lysimeters drained freely and there was no ponding. Nitrogen leaching was negligible in all treatments, although mineral nitrogen accumulated in the soil profile of the 1650 mm/yr treatment. It is unlikely that phosphorus, potassium, sulphur, calcium and magnesium will cause problems with either fertility or environmental quality in a system irrigated with TMW.
• Sodium-induced degradation of soil structure is a major concern when using TMW as irrigation water. Sodium accumulated in the soil columns in all the TMW treatments. The rate of accumulation was not proportional to the TMW application rate, indicating that sodium was moving down through the soil profile and leaching. The sodium accumulation ratio of the TMW was 15, indicating that in the long term (>10 years) at a moderate irrigation rate (<1000 mm) the soil may need to amended with gypsum, lime or dolomite to maintain soil structure.
• Pasture growth in the lysimeters was significantly enhanced by the TMW throughout the entire experiment. There were no signs of toxicity.
• A field trial comprising 11 native species, namely Leptospermum scoparium, Kunzea robusta, Olearia paniculata, Pseudopanax arboreus, Coprosma robusta, Podocarpus cunninghamii, Griselinia littoralis, Pittosporum eugenioides, Cordyline australis, Phormium tenax, Phormium colensoi was established on ca. 1000 m² of land near Pipers Valley Road. Trees irrigated with TMW grew better than or the same as unirrigated trees. There were no signs of toxicity. The plants with the greatest positive response to TMW were Leptospermum scoparium, Olearia paniculata, Coprosma robusta, Podocarpus cunninghamii, Cordyline australis, and Phormium tenax. The field trial will continue until at least June 2018.
• The use of TMW to produce valuable biomass such as cut-and-carry pasture, grazed pasture, or valuable native products such as manuka honey or essential oils constitutes the beneficial reuse of a valuable resource that is less environmentally damaging than disposal into the sea.
• It is recommended that the effluent be applied at a rate of 500 – 800 mm per year and that the soil is periodically monitored for aggregate stability. Gypsum, dolomite, or lime may need to be added periodically. A successfully designed system requires a hydrological and geotechnical assessment of the area to be irrigated.

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