Challenge

For several years, the plant suffered from struvite scale buildup in its sludge and pipes, which caused problems for the dewatering process of sludge and in biogas production. In addition to solving the problem, Herning Vand wanted to exploit the potential of recycling struvite into agricultural fertiliser and control the precipitation of phosphorus by using ferric chloride. Lastly, the WWTP in Denmark are taxed according to the amount of discharged phosphorus, therefore, a reduction in phosphorus discharge would be economically beneficial.

Solution

A solution based on controlled precipitation of struvite was designed, as well as a full-scale phosphorus compound struvite recovery plant. Magnesium chloride was added to create the precipitation process. The plant is also able to control the pH by using lye (NaOH). The solution also included drying 1-3mm pellets and store them in the biogas.

Result

The results include the reduction of internal phosphorus load at the WWTP and thereby a reduction in the use of ferric chloride, as well as a tax reduction and phosphorus recovery.  At the plant, the struvite is precipitated as a ready-to-use fertiliser and is sold to fertiliser companies. An official approval of the product as commercial fertiliser has been obtained for the struvite produced under the name PhosphorCare®.  The plant was inaugurated in 2015, and the operational savings and the expected revenue from the sale of struvite is expected to result in a payback time of 7 years.

What is struvite and Phosphorus?

• Struvite consists of phosphorus (12.6%),  magnesium (10%), and nitrogen (5.6%)
• The Danish Ministry of Environment and Food has approved struvite as a fertilizer
• Phosphorus is a scarce resource, which is excavated in mines in China, Marrocco and Russia
• The phosphor resource is expected to be completely depleted in 50-100 years
• We expect our struvite facility to produce approx. 200 tonnes of fertilizer a year