Magnetic separation and characterization of vivianite from digested sewage sludge

Abstract

To prevent eutrophication of surface water, phosphate needs to be removed from sewage. Iron (Fe) dosing is commonly used to achieve this goal either as the main strategy or in support of biological removal. Vivianite (Fe(II) 3 (PO 4 ) 2 *8H 2 O) plays a crucial role in capturing the phosphate, and if enough iron is present in the sludge after anaerobic digestion, 70 to 90% of total phosphorus (P) can be bound in vivianite. Based on its paramagnetism and inspired by technologies used in the mining industry, a magnetic separation procedure has been developed. Two digested sludges from sewage treatment plants using Chemical Phosphorus Removal were processed with a lab-scale Jones magnetic separator with an emphasis on the characterization of the recovered vivianite and the P-rich caustic solution. The recovered fractions were analyzed with various analytical techniques (e.g., ICP-OES, TG-DSC-MS, XRD and Mössbauer spectroscopy). The magnetic separation showed a concentration factor for phosphorus and iron of 2-3. The separated fractions consist of 52% to 62% of vivianite, 20% of organic matter, less than 10% of quartz and a small quantity of siderite. More than 80% of the P in the recovered vivianite mixture can be released and thus recovered via an alkaline treatment while the resulting iron oxide has the potential to be reused. Moreover, the trace elements in the P-rich caustic solution meet the future legislation for recovered phosphorus salts and are comparable to the usual content in Phosphate rock. The efficiency of the magnetic separation and the advantages of its implementation in WWTP are also discussed in this paper.

Phosphorus Recovery From Waste Activated Sludge by Sponge Iron Seeded Crystallization of Vivianite and Process Optimization With Response Surface Methodology

“…The reason for the change in vivianite crystal morphology was possibly due to Fe 2+ adsorbed onto the surface of sponge iron and IHO through static electricity, hydrogen bonding, chemical bonding and hydrophobicity (Rothe et al, 2016). Moreover, because sponge iron itself had strong magnetic properties, the vivianite produced with sponge iron as a seed crystal was also magnetic and could be easily adsorbed by neodymium magnet, and then be separated from the sludge (Prot et al, 2019). Fig.5 shows the crystal size of different seed crystals.…”

The SPPD-WRF Framework: A Novel and Holistic Methodology for Strategical Planning and Process Design of Water Resource Factories

“…The mass efficiency of process designs can be assessed with the results of the mass and energy balances conducted in step 3. For example, if two different processes recover P but process (A) integrates struvite crystallization [96] while process (B) integrates the magnetic extraction of vivianite [97], it is useful to assess the recovery rate of the influent P of each process. Furthermore, if resource recovery technologies include not only the extraction but also on-site refinement of a recovered product, KPIs originally developed for the chemical or pharmaceutical process assessment could be applied to assess the performance of a particular recovery technology in comparison to an alternative one.…”

“…The additional treatment step leads on the one hand to additional operational costs for energy consumption [44], maintenance costs to prevent membrane fouling [104], and waste management costs for retentate handling [105]; on the other hand, it may generate steady economic benefits in the form of revenues from water customers [106]. Furthermore, P recovery requires additional investment costs but may also lead to lower waste management costs due to improved sludge dewaterability and benefits from recovered product marketing [97].…”

Mechanistic Understanding of Beneficial Aspects of Iron Salts and Waterworks Sludge Uses in Urban Wastewater System

“…Due to the magnetic properties of vivianite, this opens up opportunities for its targeted recovery from the sludge via magnetic separation. This approach has recently gained significant interest and has shown significant economic and practical potential (Prot et al, 2019).…”

“…This requires the understanding of Al and P interactions and characterization of Al-P mineral precipitates in WWTP, which would facilitate in P recovery using appropriate separation techniques. In this context, Prot et al (2019) reported P can be recovered using magnetic separation process from digested sludge, rich in Fe-P mineral precipitates (e.g. vivianite).…”