The Douglas Drinking Water Management Plan Report 2020-2021 documents the performance of Douglas Shire Council as a Drinking Water Service Provider (SPID 558) during the financial year 2020-2021.

Council is operating under an approved Drinking Water Quality Management Plan (DWQMP) to ensure consistent supply of safe drinking water in order to protect public health and as required under the Water Supply (Safety and Reliability) Act 2008.

The report provides a mechanism to report publicly on the performance in managing drinking water services and drinking water quality.

Raw water is filtered to 200 microns through four pre-filters. The pre-filters provide a duty/ standby operation and are designed to provide raw water flow to meet 100% of the treatment plant design capacity.

During high turbidity events, water is additionally passed through a hydrocyclone to remove sand. During normal operations, the hydrocyclone is bypassed.

The pre-filters are cleaned by automated backwashing using raw water (typically every 60 minutes depending on raw water turbidity).

As no chemicals are used, backwash water is discharged directly to the water course. The pre-filters are removed and inspected to assess serviceability every 12 months. It is possible to bypass the pre-filters at Mossman.

The Mossman treatment plant uses five racks of 52 Koch polysulfone ultrafiltration membranes per rack. The membranes have a nominal size cut-off of 100,000 Daltons.

The ultra filtration process is fully automated and includes its own main control panel which operates UF inlet/outlet valves, recirculation pumps, backwash supply pumps, a Chemical Enhanced Backwash (CEB), and Clean In Place (CIP) system including chemical dosing equipment.

Critical process equipment is installed with a duty/standby capacity to minimise disruptions to the water treatment process.

Operation and monitoring of the ultra filtration treatment plants is via a PLC/SCADA system with a Citec user interface for process operation, monitoring and alarming functions.

The cleaning of the cartridges is via an automated backwashing sequence that utilises water only backwashing (typical frequency of 60 minutes) and chemically enhanced backwashing (CEB’s) with a minimum ratio of one CEB to twelve backwashes in total.

The frequency of backwashing and the ratio of CEB’s to water only backwashes may be varied and is determined by the operator by observing trending values of the Trans-Membrane Pressure (TMP’s) in relation to production flow set points and raw water inflow turbidity.

Currently, CEBs are programmed automatically in SCADA to occur one in every 12 backwash cycles. This changes operationally to maintain membrane performance. Chemically Enhanced Backwashing uses a Caustic/ Chlorine cleaning solution which
is introduced to the membrane cartridges at a pre-determined concentration, typically pH 10 and concentration of chlorine at 60 mg/L. The cartridges are allowed to soak in this solution for 400 seconds. Effectiveness of the backwashing sequence is continually monitored and all associated parameters are recorded for reporting and operational planning purposes. CEB backwash water is directed to the sewer.

Following backwashes, the membranes are rinsed prior to coming back into service.

A clean in place (CIP) utilises a heated cleaning solution of either citric acid pH 4 solution caustic (max pH 12) or a combined caustic/chlorine solution (pH 12, 200 mg/L). These are used to remove both organic and inorganic fouling. A CIP is typically undertaken on each rack once per month.

At the completion of a CIP the UF rack is backwashed, rinsed and tested to ensure all traces of chemicals are removed prior to placing the UF rack back into service. Testing is performed on the UF rack by means of sampling the retentate and permeate header water and conducting in house lab testing for pH and free chlorine levels.

Test limit results for free chlorine < 0.1 mg/L and a pH result equivalent to the raw water pH value (typically 6.5 to 7.5) must be achieved prior to placing the UF rack back into service.

Pre-filter and UF water only backwashes are recycled through backwash recovery plant and fed back into the head of works to recover water and reduce water intake from Rex Creek. The backwash recovery plant is a direct filtration plant that utilises 3-5 mg/L aluminum chlorohydrate (ACH) for coagulation, pressurised sand filtration to typically < 1 NTU and disinfection by UV, specified at 2 log Cryptosporidium reduction at 55% UVT (34.8 mJ/cm2).

By utilising the backwash recovery plant, approximately 1 ML of water gets recovered daily.

Membrane integrity is evaluated every 24 hours (of elapsed production time) by undertaking an automated pressure decay test (PDT). The pressure decay test measures whether there are any breaches of the membrane greater than three microns in size.

Membrane integrity is considered as a critical control point for managing the protozoa risk, and the CCP procedure, relevant to all schemes, is presented in a later section. If the UF rack fails the integrity check it is immediately and automatically taken off line for inspection and repair.

Trending data and outcomes of the integrity check cycle are monitored to pre-determine UF cartridge maintenance/repair intervals allowing UF racks to be removed from service and repaired to avoid unexpected shutdowns on account of integrity check failure.
Nonetheless, there is sufficient production capacity of treated water that under normal demand 2 racks can remain offline until repaired.

The permeate turbidity is monitored using individual filter rack permeate turbidity meters capable of 0.001 NTU resolution.

Where the permeate turbidity exceeds 0.15 NTU on any rack for over 15 minutes, the affected UF rack is taken offline and undergoes a PDT to determine if there is a breach of the membranes, as per the relevant CCP procedure.

The ultrafiltration racks can be bypassed – but this would not be used except in emergency situations. Permeate is directed to the clear water reservoirs.

All disinfection points are operated as per the Chlorination (Primary and Re-dosing) CCP procedure.
Disinfection is achieved through gas chlorination using 2 by 920 kg chlorine gas drums as duty standby, with automated change over operation.

The system uses a vacuum chlorine gas draw off injector disinfection system to chlorinate the Mossman 1.8 ML and Port Douglas 5 ML reservoirs.

Current operation uses a recirculation system on each of the treated water reservoirs with a set point control mode of operation.

The target, action and critical limits for chlorine are stated in the CCP. The two critical limits ensure effective disinfection (low side) and prevent exceedances of the chemical health guideline value (high side).

It is possible to bypass chlorination. This is not used under normal operation.

You have probably seen it many times and thought to yourself, “what a waste of good clean water!” Flushing can be frustrating to residents in places with low water supplies or those who pay high premiums to keep their taps flowing.  While it may seem like a waste of treated water, flushing of water lines is essential to keeping your water safe and maintaining the integrity of a pipe network.  If you see a hydrant running, it isn’t the Water Team just wasting water.  The team is actually working hard to make sure you have safe drinking water.  So, no matter how you feel about it, you shouldn’t attempt to turn off the hydrant at any time as this may cause more issues with the water quality and could affect public health.

If the water could be kept and reused, it would be.  While it would be an ideal outcome to transport the wasted water to a park or put it to good use elsewhere, it is currently too costly to implement, and diverting the excess is the only viable solution.  In the meantime, there are various initiatives being explored for diverting the flushed water to garden beds in the different locations where flushing occurs.  There are some garden irrigation systems currently operating in Daintree Township to ‘flush’ at set times to draw water through to assist in water quality at the end of water lines.  Other locations are also being reviewed to maximise the use of the flushed water, and where practical, will be rolled out as and when budget is allocated.

The answer is simple:  Do nothing.  Please do not touch any water infrastructure, even if you think there is a problem, as you may create a bigger problem in doing so.  If you are concerned that there is a major issue, such as a large water main break, please contact staff on 07 4099 9444 to report what you see.