Abstract
The presence of soluble iron and phosphorus in wastewater sludge can lead to vivianite scaling. This problem is not often reported in literature, most likely due to the difficult identification and quantification of this mineral. It is usually present as a hard and blue deposit that can also be brown or black depending on its composition and location. From samples and information gathered in 14 wastewater treatment plants worldwide, it became clear that vivianite scaling is common and can cause operational issues. Vivianite scaling mainly occurred in 3 zones, for which formation hypotheses were discussed. Firstly, iron reduction seems to be the trigger for scaling in anaerobic zones like sludge pipes, mainly after sludge thickening. Secondly, pH increase was evaluated to be the major cause for the formation of a mixed scaling (a majority of oxidized vivianite with some iron hydroxides) around dewatering centrifuges of undigested sludge. Thirdly, the temperature dependence of vivianite solubility appears to be the driver for vivianite deposition in heat exchanger around mesophilic digesters (37 °C), while higher temperatures potentially aggravate the phenomenon, for instance in thermophilic digesters. Mitigation solutions like the use of buffer tanks or steam injections are discussed. Finally, best practices for safe mixing of sludges with each other are proposed, since poor admixing can contribute to scaling aggravation. The relevance of this study lays in the occurrence of ironphosphate scaling, while the use of iron coagulants will probably increase in the future to meet more stringent phosphorus discharge limits.
Highlights
Wastewater treatment greatly developed in the last decades
We reviewed the information available in literature for cases of wastewater treatment plants (WWTP) experiencing vivianite scaling
It needs to be noted that struvite scaling is more likely to happen in WWTPs using Enhanced Biological Phosphorus Removal (EBPR) (Parsons and Doyle 2002) while vivianite scaling should preferentially happen in WWTPs dosing iron to remove phosphorus
Summary
Wastewater treatment greatly developed in the last decades. In 2014, about 95% of the European population (EU 28) was connected to a wastewater collection system, which accounts for around 517 million people (European commission 2017). Rence of struvite scaling in WWTPs is long time recognised in literature (Rawn et al, 1939; Doyle et al, 2002) and is a plague. It can cause pipe diameter reduction, increasing the required pumping energy and eventually pipe replacement amongst other problems (Doyle and Parsons, 2002). Struvite scaling is predominantly reported in the dewatering units after digestion in WWTPs using Enhanced Biological Phosphorus Removal (EBPR), as phosphorus is released and solubilized during sludge digestion. The understanding and prevention of vivianite scaling is a relevant topic due to the current lack of information, and to the expected increased use of iron salts in the future. The possible formation mechanisms were discussed to evaluate several scaling mitigation strategies
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