Granular activated carbon for removal of organic matter and turbidity from secondary wastewater.

A range of commercial granular activated carbon (GAC) media have been assessed as pretreatment technologies for a downstream microfiltration (MF) process. Media were assessed on the basis of reduction in both organic matter and turbidity, since these are known to cause fouling in MF membranes. Isotherm adsorption analysis through jar testing with supplementary column trials revealed a wide variation between the different adsorbent materials with regard to organics removal and adsorption kinetics. Comparison with previous work using powdered activated carbon (PAC) revealed that for organic removal above 60% the use of GAC media incurs a significantly lower carbon usage rate than PAC. All GACs tested achieved a minimum of 80% turbidity removal. This combination of turbidity and organic removal suggests that GAC would be expected to provide a significant reduction in fouling of a downstream MF process with improved product water quality.

NDMA formation in secondary wastewater effluent.

Concern over prospective levels of N-nitrosodimethylamine (NDMA) in waters has increased in recent years due to its disinfection byproduct formation potential from chloramination. It has been mooted that this is promoted by organic precursors from municipal wastewaters, such that there is a more significant risk of excessive levels in water reuse applications. Experiments conducted on chloramination and chlorination of secondary wastewater have confirmed that that significant NDMA formation arises only from chloramination, with its concentration varying with test conditions used. A full factor analysis revealed all parameters studied (temperature, pH, monochloramine dose and contact time), both individually and synergistically, to have a statistically significant impact on NDMA formation with contact time being the most important. At raw water temperatures below 10 °C, the NDMA concentration can be minimised to below the 10 ng L(-1) threshold by not exceeding a monochloramine dose of 2 mg L(-1) as Cl(2). However, at higher water temperatures other measures are required to suppress NDMA formation, such as reducing the contact time (which could prove impractical in most applications) or maintaining a pH below 6. Further trials are required to fully develop the operating envelope to ensure NDMA concentrations do not exceed the 10 ng L(-1) threshold, or else to identify effective pretreatment methods for removing the NDMA precursors.

Screening optimisation for indirect potable reuse.

An automatic backflush pre-filter used for pre-treatment for secondary wastewater re-use was evaluated and optimised at two different mesh sizes over an 18 month period. The filter was initially run with a 500 microm rating mesh size, as recommended by the supplier of the downstream membrane filtration process, and then at 100 microm to investigate any change in water quality produced and associated improved membrane performance. With the 500 microm mesh in place, the filter fouling rate was low and a backflush was initiated every 3.5 h. For the 100 microm mesh the fouling rate was extremely rapid. Fouling was found to be caused by reverse side blockage of the pre-filter due to biofilm growth, and not by improved solids capture; there was no improvement in water quality with the smaller mesh size, since particle unloading from the biofilm took place. The pre-filter fouling rate was found to be related to turbidity. At a turbidity of 5 NTU the filter backflushed around 200 times per day, while at 10 NTU this increased to over 300 times. Further analysis enabled the backflush water volume to be decreased by reducing the backflush duration and increasing the backflush cycle time (i.e. the time between backflushes).

Precoagulation-microfiltration for wastewater reuse.

A range of coagulant chemicals and doses, up to 2 mg/L, were trialled on a microfiltration-based indirect potable reuse (IPR) pilot plant to evaluate their impact on membrane reversible and irreversible fouling. Jar tests revealed these doses to have negligible impact on organic matter removal, whilst scoping pilot trials showed them to have a positive impact on fouling rates. Initial trials carried out over a 6-h period suggested that ferric sulphate was the most promising of the coagulants tested with regards to irreversible fouling. Extended five-day trials using ferric sulphate at 0.5 mg/L were conducted at fluxes of 40-50 l/(m(2)h) (LMH). Operation at 50 LMH without coagulant resulted in rapid fouling and a subsequent shortening of the chemical cleaning interval. The addition of the ferric coagulant resulted in a reduction in both reversible and irreversible fouling to those levels experienced at 40 LMH, enabling sustainable operation. The use of low levels of coagulant thus enables the pilot plant to operate at a 25% increased flux, equating to a 20% reduction in membrane area and overall savings of >0.1 p per m(3) for a seven year membrane life.