Parameters and limitations of GDM

Water quality

Increased levels of TOC in raw water lead to the stabilization of flux at lower values. However, flux stabilization was observed with all waters tested even for waters with TOC of over 15 mg/L as long as water was aerobic. Decline of dissolved oxygen values to <1mg/L results in a continuous decline of flux.
Laboratory experiments with river water containing kaolin showed that the system can be operated at stable flux conditions when raw water turbidity does not exceed 300 NTU. Pilot plant experiments in South Africa with highly turbid waters showed that turbidity values of up to 100 NTU did not influence the stable flux significantly while turbidity peaks of 600-1000 NTU resulted in a decline of flux.

Membrane cut-off and flux

Membranes serve as absolute physical barriers and the retention of protozoa, bacteria and viruses depends on the pore size or cut-off of the membrane. In general, most microfiltration (MF) and UF membranes retain protozoa, tight MF membranes (pore size < 0.22 µm) provide complete removal of bacteria, and tight UF membranes (cut-off < 100 kDa) retain both bacteria and viruses. Permeability also affects the selection of a membrane. In general, membranes with lower cut-offs have lower permeability which reduces capacity of the system.
However, in GDMD technology stable flux does not depend on the membrane cut-off for flat sheet membranes. The same flux values were observed for MF and UF membranes although initial flux was an order of magnitude higher for MF membranes.

Retention of bacteria during long term filtration

Flow cytometry - a novel highly reliable and fast method to measure total cell count in water samples was used to examine structural integrity of membranes on a long term. The study showed that after 30 days of operation without flushing and cleaning, membranes remained intact and no bacteria added to the feed were detected in the permeate.

Total cell counts in feed (a) and permeate (b) by flow cytometry. Cell counts in permeate are lower than detection limit.