Many wastewater-treatment plants rely on iron-based coagulation to remove phosphorus from water. Following treatment with iron-based coagulants, most iron present in the sewage sludge is in the form of an iron-phosphate mineral known as vivianite. A patented technology called ViviMag is said to be the first in the world that can recover vivianite directly from sewage sludge when upstream chemical phosphorus-removal with iron is applied. Originally developed at Dutch research institutions Westus (Leeuwarden; www.westus.nl) and Delft University of Technology (www.tudelft.nl), ViviMag is now being launched commercially by Haskoning B.V. (Amersfoort, the Netherlands; www.haskoning.com).
“In anaerobic conditions, such as during the anaerobic digestion of sludge, iron (III) is reduced to iron (II), enabling the formation of vivianite, a paramagnetic iron phosphate mineral. As vivianite is being naturally formed in anaerobic digestion, its paramagnetic properties allow it to be separated efficiently using adapted magnetic separation equipment derived from the mining industry,” says Paul Roeleveld, a leading professional in sludge treatment and phosphorus recovery at Haskoning. The heart of the technology is a vertically pulsating high-gradient magnetic separator (VPHGMS).
The recovery of vivianite will be increasingly valuable for wastewater treatment sites as European operators adapt to the E.U. Urban Wastewater Treatment Directive, which requires lower phosphate concentrations in effluent streams. In order to meet the new standard, operators will be increasing their dosage rate of iron coagulants, also expanding the volume of recoverable vivianite content in sludge. Recovered vivianite can potentially be used in fertilizer products or in precursor materials for lithium-iron-phosphate (LFP) batteries.
So far, the ViviMag technology has seen several small pilot-scale demonstrations in Germany, Denmark and the Netherlands, processing around 1 m3/h of both digested and undigested sludge. Significant scaleup is underway for the first full-scale implementation of the technology at the Nieuwveer wastewater treatment plant operated by the Brabantse Delta water authority in Breda, the Netherlands. “In this demonstration project, 9 m3/h of digested sludge will be treated to recover 60% of the phosphorus, referring to the incoming wastewater. This demonstration project is part of the LIFE-project Phos4EU and is co-funded by the European Union and UppWater,” notes Roeleveld.