Phosphorus recovery from sewage sludge by nanofiltration in diafiltration mode

•Phosphorus was recovered from pretreated sewage sludge with nanofiltration.•Heavy metals were retained while phosphorus permeated to a high degree.•The separation selectivity increased with decreasing pressure and pH value.•A significantly higher phosphorus yield was reached by applying diafiltration.

As mineral resources of phosphorus are limited, the use of alternative phosphorus sources such as wastewater becomes an increasingly important option. In this study, the recovery of phosphorus from pretreated sewage sludge is examined, using nanofiltration for the removal of heavy metals from the product stream. The membranes DL, NF270, AS and Duracid were tested in a bench scale filtration unit at pH values of 0.5 and 1.5. The dependence of flux and separation efficiency on transmembrane pressure (TMP) and on permeate recovery was studied. Thanks to the ion-selectivity of nanofiltration membranes, multivalent metal cations were rejected effectively, while neutral and negatively charged phosphorus compounds could be collected in the permeate. It was shown that heavy metals and metals had retention values higher than 0.94, while a retention lower than 0.3 was reached for phosphorus. The retention of phosphorus was found to increase at higher TMPs and with decreasing pH value. The phosphorus yield could be improved significantly if diafiltration was applied. By doing so, a maximum phosphorus yield of 83.7% could be reached at a permeate recovery of 90%.