CalPrex® – Post-digestion P-Recovery


The challenge

Recovery of limited resource

Nutrients Cycle

Motivation of phosphorus recovery in sewage treatment plants:

  • Reduction of the biological backload through filtrate / centrate
  • Struvite problem before and after digestion
  • Improved dewaterability => reduction of disposal costs
  • Reduction of polymeric flocculants during dewatering
  • Improvement of the N-to-P ratio in biosolids that are land applied
  • Production of P-rich fertilizer

CNP’s Solution

CalPrex® Pre-digestion high-rate P-Recovery from anaerobic acid digesters

CNP CYCLES CalPrex Flow Diagram

This is how CalPrex® works:

A mesophilic anaerobic pre-acidification prior to digestion allows a good return of the phosphate. In the centrate, which contains soluble phosphates, the precipitation of the phosphate as brushite takes place under slightly acidic conditions and addition of Ca (OH)2.

After the brushite removal the reacted centrate is recombined with cake and sent to the methane digester or, if available, to a hydrolysis process.

Benefits

Highly-efficient P-Recovery and digester protection

Benefits of CalPrex® process:

  • Reduction of the P load in the dewatered biosolids effluent, which is recycled to the headworks
  • Struvite control pre and post digestion and dewatering efficiency
  • Brushite formed in acid conditions is a valuable high-grade fertilizer
  • Improvement of the N-to-P ratio in biosolids that are land applied
  • Low processing volume and high total P recovery (>50%)