Application of Direct Filtration to MIEX® Treated River Murray Water
Dissolved natural organic matter (NOM) has been shown to interfere with the coagulation process, particularly with alum (aluminium sulphate) which is the most widely used coagulant in potable water treatment. The additional coagulant demand of the NOM can often be a significant contribution to the overall coagulant dose required for effective turbidity control in conventional water treatment.
The MIEX® DOC (Magnetic Ion Exchange Resin) process, jointly developed by SA Water, CSIRO and Orica Australia Pty. Ltd. has been shown to be efficient for the continuous removal of NOM from water supplies. By applying this technology upstream of the conventional treatment train, numerous benefits can be achieved, including a reduction in the coagulant demand of up to 75 percent. With the MIEX® DOC process, the primary function of the downstream coagulation process is turbidity control.
Extensive laboratory studies by the Australian Water Quality Centre (AWQC) have demonstrated that with MIEX® treated water from a wide variety of sources, alum doses of less than 20 mg/L are sufficient to obtain an acceptable treated water turbidity. At these relatively low doses, direct filtration may become a viable option. In direct filtration, the coagulant is rapid mixed with the MIEX® treated water and applied directly to the filters. If a tank is provided for extended flocculation time, the process is termed contact filtration.
Direct filtration is normally practiced with waters of very low turbidity (< 10 NTU) and generally low DOC.
The proposed Mount Pleasant water treatment plant in the Adelaide Hills will incorporate MIEX® treatment technology. The designated water source is the River Murray, via the Mannum-Adelaide pipeline. The water quality is generally poor, with turbidity averaging 55 NTU (5 year average) with frequent spikes above 150 NTU. With the highly turbid nature of this water, direct filtration becomes a significant challenge as studies have shown that direct filtration is feasible only when low coagulant doses (< 20 mg/L) are applied.
Laboratory scale simulations of the direct filtration process demonstrated that its application to MIEX® treated River Murray water may be feasible. The purpose of this study was to assess whether MIEX® pretreatment of River Murray water would allow successful application of alum in direct filtration mode on a pilot scale. Acceptable operation was
defined as achieving the product water turbidity goal of 0.3 NTU or less. Other coagulants including several polymers were also examined.