Polishing Pellets Recharged

Phosphate pollution leads to excessive algal growth and threatens aquatic life, while stricter WFD and wastewater guidelines present a major phosphate-removal challenge. KWR is developing the Polishing Pellet, a pellet made from the ferrous sludge produced during drinking water production that can efficiently remove phosphate and arsenic. The project is focusing on the optimisation of pellet regeneration, the recovery of phosphate and the development of a washing process for widespread circular application. In collaboration with partners from the water sector, we are working on a sustainable, reusable and scalable alternative for traditional approaches to removing phosphate.

Phosphate is essential for everything that grows, flourishes and lives. However, too much phosphate can lead to excessive algal growth that displaces or smothers aquatic life. The reduction of levels of phosphate in the aquatic environment is essential to comply with the Water Framework Directive (WFD). Under the same WFD targets and increasingly strict standards for phosphate, wastewater treatment plants are also required to comply with more stringent phosphate standards. The projected requirement for phosphate removal for water authorities is approximately 910 tons P/year.

Challenge

Current phosphate-removal technologies depend on the addition of chemical flocculants (FeCl₃) or they have a relatively low adsorption capacity (as in the case of ferrous sand). In addition, the availability of these technologies depends on supplies of bulk chemicals (such as HCl) from outside the Netherlands. Supply-chain disruptions (as seen during COVID-19) have led to the threat of shortages and the disruption of the market. Furthermore, stocks of ferrous sand cover only 0.8% of the total phosphate-removal requirement.

Technology

In the TKI Polishing Pellets project, KWR has developed a pellet made from the ferrous sludge produced during drinking water production: the Polishing Pellet. Ferrous sludge is rich in iron(oxyhydr)oxide, which is known for its strong affinity with phosphate and a high phosphate adsorption capacity [1]. The Polishing Pellets reduced the phosphate concentration in WWTP effluent from 0.1-1.2 mg/L ortho-P to 0.01-0.028 mg/L ortho-P in a long-term test, which amounts to 78-99% removal. In addition, the pellets can capture substances including arsenic and hydrogen sulphide. On the basis of our current pellet knowledge, ferrous sludge from drinking water production can remove 100-150 tons of P annually. The high requirement for phosphate removal of ~910 tons P/year makes it essential to reuse the pellets multiple times for phosphate removal, or for regeneration to be possible.

Solution

In light of the WFD targets (deadline 2027) and the Dutch Urban Wastewater Directive (deadline 2039), water authorities and government authorities are facing a major phosphate-removal challenge. There are also drinking water utilities that are looking for ways to treat streams containing arsenic. Polishing Pellets can provide a solution for these challenges.

Full practical implementation requires more research in the following areas:

pellet regeneration and phosphate recovery from the regeneration stream to meet the phosphate-removal requirement;
a washing process that prevents unwanted leaching (such as organic matter and manganese) so that the pellets can be used more widely (for example in drinking water treatment/concentrate treatment);
the stability and phosphate/arsenic adsorption capacity of the pellets in practical circumstances such as WWTP effluent, RO Concentrate or surface water.

[1]When phosphate (PO₄^3-) adsorbs to the iron(oxyhydr)oxide surface, it replaces the OH group on the surface. In this case, the phosphate can form multiple bonds with one Fe atom or bond to multiple Fe atoms. Iron(oxyhydr)oxides therefore have a strong affinity with phosphate.