High nutrient concentrations from sewage treatment are threatening the Hartenbos estuary, a highly sensitive ecosystem in Mossel Bay, South Africa. We assessed how different infrastructure options can improve the wastewater treatment and unlock additional benefits.
The results show that hybrid infrastructure, a combination of grey and nature-based infrastructure (NBI), and natural alternatives, such as artificial wetlands offer cost-effective wastewater services. These options also have the potential to provide additional co-benefits, such as additional water supply by diverting water post-treatment and tourism opportunities for the Mossel Bay municipality through biodiversity benefits resulting from the wetland.
This assessment shows that hybrid and natural alternatives are not just cost-competitive when it comes to the cost of treatment, but also outperform grey alternatives in terms of treatment efficiency.
Hartenbos estuary is located in Mossel Bay, a municipality in South Africa’s Western Cape Province. Every year, about 85,000 tourists visit Mossel Bay, and the lagoon is one of the coastal town’s attractions. However, water quality issues in the estuary strain the health and attractiveness of the ecosystem and are expected to worsen as population and tourist numbers grow.
Sewage from Mossel Bay is treated in a wastewater treatment plant in Hartenbos, and the cleaned effluent flows into the lagoon. Even though the plant complies with local standards, the discharged water contains elevated loads of nitrogen and phosphorus. The resulting high nutrient concentrations in the estuary regularly lead to algae growth and fish kills due to a lack of oxygen, which has the potential to also affect offshore fish yields as spawns go from the estuary into the open sea.
We used the Sustainable Asset Valuation (SAVi) methodology to analyze three different wastewater treatment options for improving the water quality in the Hartenbos estuary:
We compared these infrastructure options to a business-as-usual scenario where no upgrade of the current treatment plant takes place.
Comparing the Costs for Treating Wastewater
The SAVi assessments shows that the hybrid infrastructure solution (Organica) is the cheapest option for improving the wastewater treatment (USD 0.27 per m3), followed by the constructed wetland (USD 0.34 per m3) and a grey infrastructure upgrade (USD 0.34 per m3) (see Figure 1) over 35 years.
Trade-offs Between Nutrient Removal Efficiencies of Different Technologies
The assessment illustrates how planners and policy-makers need to carefully consider the benefits and trade-offs of the different infrastructure options. The hybrid Organica plant is the most effective option for removing nitrogen from the wastewater and reducing the nitrogen loads in the sensitive estuary, in comparison to the grey and nature-based alternatives (see Figure 2). In contrast, the constructed wetland is the most effective solution for removing phosphorus from the water (see Figure 3).
Water Recycling Protects the Estuary and Supports Climate Adaptation
Climate change is impacting the water-scarce Western Cape province by leading to decreased rainfall, rising temperatures and increasingly extreme weather events. Water scarcity is threatening agricultural production and rural livelihoods in the region.
The SAVi assessment highlights that recycling treated wastewater for irrigation offers great benefits for climate adaptation and the agricultural sector. Reusing 50% of the wastewater by 2060 would cover the water demand of 330 additional hectares of agricultural land, which would reduce water insecurity, increase agricultural productivity, and create additional jobs. The cumulative benefits from water recycling amount to more than USD 29 million over the next 40 years; it should be noted that detailed water quality analysis for water exports and the cost of establishing additional infrastructure for water distribution are not considered in this assessment, as this would require a more detailed analysis.
Harvesting treated water before it reaches the estuary also benefits the sensitive ecosystem by diverting part of the nutrient-rich effluent. Our assessment shows that water harvesting reduces the nutrient loadings reaching the estuary by almost 50%.
The Added Benefits of NBI for the Local Economy
The integrated cost-benefit analysis indicates that the additional water recycling is a key factor for reducing nutrient flows to the estuary, thereby making all three water treatment investments economically viable as only shadow cost of nutrients is considered. In the scenario without water harvesting, only the Organica (hybrid) treatment option generates a sufficient amount of avoided costs to generate net benefits at system level.
While all three infrastructure options deliver a net benefit for society, the hybrid Organica plant and the wetland are particularly beneficial because they provide larger added benefits and avoided costs (see Table 1 and Figure 4).
One of the added benefits of the constructed wetland is that it serves as a habitat for birds and other animals, as well as a source of tourism revenue. In fact, the positive effect on tourism demonstrates the attractiveness of investing in a wetland from a societal perspective.
Our financial model shows that integrating water harvesting into the calculations of the sustainable internal rate of return and the sustainable net present value of each option considerably improves their profitability.
The assessment results provide a strong incentive to swiftly expand the capacities to reuse water as this is key for protecting the estuary. Policy-makers should prepare to amend water use licenses in parallel to upgrading the wastewater treatment infrastructure. This policy change would ensure that water harvesting can begin as soon as the improved treatment infrastructure is in place.
This assessment was developed in collaboration with the Western Cape Government and Mossel Bay Municipality. The Western Cape Government aims to promote infrastructure investments that deliver cost-effective services while contributing to climate adaptation, sustainable livelihoods, and a healthy environment The assessment can help public authorities better consider the economic, environmental and social outcomes of infrastructure procurement decisions and to make the case for investing in nature-based infrastructure.
This customized valuation of different sewage treatment options helps policy-makers in South Africa make informed decisions about infrastructure investments. It highlights the cost-effective wastewater treatment by hybrid infrastructure and the large societal benefits of NBI. We see how investing in nature can support the tourism sector, help farmers adapt to a changing climate, and protect the sensitive Hartenbos estuary. Realizing this potential will require stakeholders to recognize the greater benefits to be achieved by working together instead of focusing only on their own mandates. — Catherine Bill, Western Cape Government
You can find more information about the methodology and results of the assessment in this technical report.
This article has been republished in ten languages at the Water Science Platform.