WP4 – Nutrients
Wastewater contains both phosphorus (P) and nitrogen (N) in significant amounts, substances of paramount importance in intensive farming.
Today, nitrogen is removed from sewage based on aerobic biological processes in all Danish wastewater treatment plants. Hereby about 70-80% of the incoming nitrogen is being released to the atmosphere as free nitrogen gas. There are no broadly applied processes that recover all nitrogen from the wastewater.
As part of VARGA, an Anammox plant with a disposal capacity of 350,000 PE will be constructed, which is expected to result in a more stable nitrogen removal without the use of carbon source. Hereby the overall need for carbon for nitrogen removal will be further minimized and the remaining/gained amount of carbon could be converted into energy in the digester. Anammox process is a prerequisite for increased carbon-harvest in the raw wastewater (WP 1). To ensure complete nitrogen turnover to free nitrogen, the emission of a nitrous oxide will be monitored (WP 2).
Today phosphorus is recovered at some Danish treatment plants when the wastewater is treated. However, only a limited proportion of the phosphorus (about 30%) can be collected and recycled. The majority of the phosphorus is originated from sludge, and it is recycled when it is spread on fields. In Copenhagen, BIOFOS burns all sludge from the three treatment plants, and the ash from it contains a large proportion of Danish wastewater’s phosphorus.
There will probably be legal requirements for recycling phosphorus from the sludge within the foreseeable future. The government’s resource strategy aims at 80% recycling of phosphorus from wastewater already in 2018. In the VARGA project, it is planned to investigate the recycling of phosphorus from ash using electrodialysis separation (EDS). In 2013, DTU developed the appropriate technology to recover phosphorus from ash. EDS technology offers potential for a scalable process that can extract phosphorus without the addition of large amounts of acid. In VARGA, the construction of an EDS plant is expected to be able to process approx. 1/3 of BIOFOS annual ash production. The plant will be modular so that it can be easily expanded afterwards.
Prior to an upscale of the EDS plant, experiments will be carried out in pilot scale and laboratory trials to clarify elements of the final design of the EDS plant. The plant will consist of one or more reactors with subsequent dewatering of the ash residue.