Nutrient recycling from wastewater – literature study: an overview of technologies, costs and environmental impact
Recycling of nutrients from wastewater in Sweden today takes place almost exclusively by spreading sludge on arable land. But what are the alternative options for recycling nutrients from wastewater streams?
Dewatered digested sludge contains in principle all phosphorus that is separated at a municipal wastewater treatment plant (WWTP). However, only about 15% of the nitrogen content in incoming wastewater ends up in the dewatered digested sludge and can be recovered by sludge spreading on arable land (Jönsson 2019). Since only about a third of all produced sludge is spread on arable land in Sweden, only a few percent of the nitrogen present in incoming wastewater is recovered today. Possible requirements for phosphorus recovery may also mean that nitrogen recovery probably decreases further as the most common methods recover phosphorus from ash after sludge incineration or recycle phosphorus via biochar during sludge pyrolysis (von Bahr & Kärrman 2019).
Phosphorus is a finite resource in a high-quality concentrated form and therefore there is a great focus right now on the development and introduction of methods for recycling phosphorus from wastewater. According to a recent study (Jönsson 2019), however, recycling nitrogen from wastewater is at least as important as recycling phosphorus. This is due to the fact that conventional nitrogen fertiliser production generates high greenhouse gas emissions, but also because estimated reserves of phosphorus are high (266 years) in comparison with reserves of natural gas used as a raw material for nitrogen fertiliser production today (53 years). Access to sulphur is also limited with reserves for 60 years and potassium for 93 years. There is also potential for recycling sulphur and potassium from wastewater. However, the incentives are lower than for the recycling of nitrogen and phosphorus.
The aim of the project is to make a knowledge compilation of methods for recycling nitrogen, sulphur and potassium from wastewater and assess the feasibility of these techniques for recycling at Swedish municipal WWTPs based on practical compatibility with other process steps, technical maturity, cost and environmental impact. The project will also investigate which new nutrient recovery techniques can be combined, so that more plant nutrients can be extracted at the same time.
The project shall
- Increase knowledge of methods for nutrient recovery from wastewater among Sweden Water and Wastewater Association´s members, consultants and researchers.
- Assess costs for nitrogen recovery from highly concentrated sidestreams and from the mainstream of wastewater with different recycling methods.
- Assessment of positive effects for wastewater treatment that nitrogen recycling creates and assessment of environmental impact.
- Update the state of knowledge regarding recycling of phosphorus, sulphur and potassium.