Abstract
In the present study we attempted to develop a reactor system to recover phosphorus by struvite precipitation, and which can be installed anywhere in the field without access to a laboratory. A reactor was developed that can run fully automated and recover up to 93% of total phosphorus (total P). Turbidity and conductivity signals were investigated as automation proxies for magnesium dosage, thus making laboratory phosphate measurements to determine the exact magnesium dosage unnecessary. Conductivity is highly influenced by the dosing parameters (molarity and pump speed) and turbidity is affected by particle size distribution issues. Algorithms based on both conductivity and turbidity signals were not able to detect the precipitation endpoint in real time. However it proved possible to identify the endpoint retrospectively from the conductivity signal, and thereafter to dose an algorithm-calculated volume of urine to use up the excess magnesium dosed.Keywords: struvite precipitation, human urine, turbidity, conductivity, automation, magnesium dose, VUNA
Highlights
Worldwide, 2.5 billion people lack access to improved sanitation, leading to waterborne diseases (UNICEF and WHO, 2012)
The measured pH was around 8.9, which is in the ideal range for effective struvite precipitation (Munch and Barr, 2001)
Measured phosphate recovery with the used cotton fibre filter was up to 93% total phosphorus (total P)
Summary
2.5 billion people lack access to improved sanitation, leading to waterborne diseases (UNICEF and WHO, 2012). One way to extract nutrients from stored urine is by adding soluble magnesium to recover phosphate by precipitating struvite (magnesium ammonium phosphate hexahydrate; MgNH4PO4∙6H2O) (Tilley et al, 2008). This can be done in industrial processes such as OSTARA or Multiform Harvest. An important factor for struvite recovery in a reactor is the magnesium source itself, as it has an effect on operating costs (Etter et al, 2011). No known method exists, to the authors’ knowledge, to dose magnesium accurately in a struvite reactor without prior phosphate measurement of the urine
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