Development of a sustainable land treatment system for erosion prone slopes

As municipal wastewater discharges to land become increasingly common, so does the pressure to utilise hill country. This country is often at risk of some degree of erosion. Land based wastewater treatment systems have tended to focus on the hydraulic properties but in hill country slope, geology and erosion must also be considered.

While many hydrological, nutrient and soil-landscape mapping models exist, few enable the assessment of irrigation on sloping land. This paper describes the process CPG recently developed to:

• Assess the suitability of an erodible hill country site for use in the land-based application and treatment of municipal wastewater under hardwood plantation forest;
• To identify Irrigation Management Units (IMUs); and
• To develop appropriate hydraulic loading rates for the IMUs.

A case study is presented which demonstrates how a land treatment system was identified as an option for an inland community with a small permanent population and an aged reticulation system. The town has an existing discharge to a river in a phosphorus sensitive environment. The land treatment site is disadvantaged in that the topography is typically steep, creating access and erosion issues. For the life time of the land treatment system the owners will have an established hardwood plantation on the site. Notwithstanding the stabilising effect of the plantation forest, the addition of wastewater has the potential to exacerbate erosion on the site if not managed conservatively, with corresponding damage to infrastructure, reduction in treatment capacity and export of sediment and nutrients from the sites.

In the design process it was identified that a fit for purpose model was needed. The iterative process used to develop this model and the resultant IMUs for this proposed wastewater discharge is described.