KNCV Crystal growth prize for phosphate recovery from wastewater

Published on
December 6, 2021

Ricardo Cunha, former ETE and Wetsus PhD scientist, was awarded the KNCV Piet Bennema Prize for Crystal Growth 2021 for his thesis ‘Anaerobic calcium phosphate bio granulation’. Every three years, the prize is awarded to a young PhD scientist that has published high-level scientific research in the field of crystal growth. ‘I was really surprised and honored to receive this award’, Cunha says. Cunha’s thesis showed scientific depth and a clear practical application to recover phosphate from concentrated wastewater.

Preserve reserves

Phosphate is, among others, present in toilet waste, so called black water. Since phosphate is an essential and limiting resource, recovery and reuse are crucial to preserve the world’s reserves, while reducing pollution. Cunha studied the process of microbial induced calcium phosphate crystal growth during black water treatment using an anaerobic process in a so-called UASB (Upflow Anaerobic Sludge Blanket) reactor. He carried out his experiments at Wetsus with support from DESAH, which operated a demonstration plant in Sneek, where about 200 households are using source-separated sanitation: special toilets, that use much less water for flushing, resulting in a highly concentrated stream (i.e., black water). The black water was collected at the demonstration plant in Sneek and processed in UASB reactors at Wetsus.

Boost crystal formation

By changing reactor conditions, he managed to improve the formation of granules containing calcium phosphate crystals. ‘I focused on improving the microbial induced formation of calcium phosphate granules in the sludge bed and their concentration at the bottom of the reactor, where harvesting is easier’, he explains. ‘Since calcium was not present in sufficient amounts in the black water, I supplied extra to boost crystal formation.’ During his experiments, Cunha also noticed that the biofilm that coated the calcium phosphate crystal granules created a slightly higher pH on the inside of the granules. ‘The difference was not really large: 7.3 outside and 7.8 inside, on average’, Cunha says. ‘But the slight increase of pH in the granules enhanced inner crystal growth.’  

Sustainable replacement

In addition to enhance the formation of calcium phosphate during anaerobic black water treatment, Cunha also looked into their application as fertilizers. Normally, phosphate rock is used as or to produce fertilizer, and Cunha wanted to see if these recovered granules could be a suitable and more sustainable replacement. ‘The granules showed a better phosphate release than powdered apatite, which is found in the traditional Phosphate rock. Also, there were fewer heavy metals and radioactive materials present’, Cunha explains. ‘However, a major concern was the
presence of pharmaceutical residues in the granules, illustrating the risks of using waste as a resource.’  

PhD Thesis: Anaerobic calcium phosphate bio granulation, 214 pages ISBN 978-94-6343-503-1