Figure shows the annual number of journal and patent publications in 2001-2021 on wastewater treatment using biological methods. Specific biological processes were characterized by the CAS employed concepts. Since each concept is associated with one specific publication, we can estimate the number of publications where this concept/process is mentioned. Figure shows the most frequently reported concepts related to biological processes. Among them the concepts associated with wastewater treatment sludge make up a greater fraction with 3575 publications associated with "wastewater treatment sludge" while 750 documents refer to "secondary wastewater treatment sludge", and 625 documents refer to "municipal wastewater treatment sludge". "Anaerobic wastewater treatment" is found in 800 publications (20%). "Wastewater denitrification" occurs in 400 publications, only 10% of the total concepts. "Dephosphorization wastewater treatment" is the least common concept with only 250 publications (~6%). Biological wastewater treatment concepts connected to struvite production are summarized in Fig. . "Anaerobic wastewater treatment" occurs more often for struvite production (78% vs 20% for wastewater treatment in general). It confirms the efficiency of struvite precipitation from the liquid phase of anaerobic digestates. The contributions of "dephosphorization wastewater treatment" and "wastewater denitrification" concepts are also more significant and amount to 20% and 17.5%, respectively indicating the recovery of nitrogen and phosphorus in the form of struvite.