Influences of mechanical pre-treatment on the non-biological treatment of municipal wastewater by forward osmosis

Municipal wastewater treatment commonly involves mechanical, biological and chemical treatment steps as state-of-the-art technologies for protecting the environment from adverse effects. The biological treatment step consumes the most energy and can create greenhouse gases. This study investigates municipal wastewater treatment without the biological treatment step, including the effects of different pre-treatment configurations, e.g., direct membrane filtration before forward osmosis.

Forward osmosis was tested using raw wastewater and wastewater subjected to different types of mechanical pre-treatment, e.g., microsieving and microfiltration permeation, as a potential technology for municipal wastewater treatment. Forward osmosis was performed using thin-film-composite, Aquaporin Inside(TM) and HTI membranes with NaCl as the draw solution.

Both types of forward osmosis membranes were tested in parallel for the different types of pre-treated feed and evaluated in terms of water flux and solute rejection, i.e., biochemical oxygen demand and total and soluble phosphorus contents. The Aquaporin and HTI membranes achieved a stable water flux with rejection rates of more than 96% for biochemical oxygen demand and total and soluble phosphorus, regardless of the type of mechanical pre-treated wastewater considered.

This result indicates that forward osmosis membranes can tolerate exposure to municipal waste water and that the permeate can fulfil the Swedish discharge limits for small- and medium-sized wastewater treatment plants.