An apparatus for the biological treatment of saltwater-based domestic wastewater, the apparatus having an inlet for the introduction of the saltwater-based domestic wastewater, an outlet for the removal of treated wastewater, means for monitoring the salinity level of the saltwater-based domestic wastewater entering or within the apparatus and means for controlling the salinity level of the saltwater-based domestic wastewater entering or within the apparatus such that fluctuations in the salinity level of the saltwater-based domestic wastewater are reduced and the biological treatment of the saltwater-based domestic wastewater is thereby maintained.