Good enough to drink – even during extreme weather

Making water drinkable is an exacting process at the best of times, but extreme wet weather events add significant challenges. With recent experience helping clients through the last two (very wet) summers, Simmonds & Bristow have the experience and expertise to help businesses maintain through the impacts.

We saw these impacts play out on water quality in flood-affected water catchments, storage reservoirs, in the performance of water treatment processes or the integrity of distribution systems.

Last month, we published a blog to help operators understand how to get their sewage treatment plant (STP) ready for more heavy rain (click here). This month we are focused on helping you understand what is required to get your water treatment plant (WTP) ready and reliable in the event of a third La Nina.

To understand flood impacts on WTPs it helps to understand the complexity of creating drinking water from catchment to cup.

Drinking water “production” – let’s break it down

As rain falls and runs over the land, stormwater collects a wide variety of contaminants ranging from pesticides to fertilisers, soils (suspended solids), litter, grease, oil, nutrients and metals and increasing water turbidity. Without proper management, these contaminants end up in waterways, which in turn leads to pollution. Flood conditions dramatically increase the level of pollutants flowing into catchments. WTPs must be ready to respond to these conditions to ensure their treated water adheres to the stringent Australian Drinking Water Quality Guidelines. The basic water treatment processes include coagulation, flocculation, sedimentation, filtration and lastly, disinfection of water. For example:

Coagulation

To remove any dirt or other suspected material, raw water is pushed through a flash mixer. Various chemicals, including Aluminium Sulphate (alum), might be added. Mixers stir the water and the chemicals, making any particles ‘coagulate’ or change from a fluid to a thickened mass. High turbidity waters regularly occur so coagulation is an important part of the treatment process. The water must have a pH level of between 6.8 and 7.2 for the next stage of the treatment process to work, so sometimes caustic soda is added to achieve the correct pH level.

Flocculation

After coagulation, the particles stick together and form ‘floc’. Flocculation is the clumping together of suspended particles that have been destabilised through the coagulation process. When they clump together they form heavier particles.

Sedimentation

The water and ‘floc’ move into larger sedimentation basins that might resemble swimming pools. Once many particles are bound together, they form a sludge that becomes heavy enough to sink to the bottom of the sedimentation basins. The sludge is scraped or vacuumed out regularly, and is transported to a waste sludge pond or concentrator.

Filtration

After the sludge is removed, the water flows to filtration’ chambers where it might undergo a divet filtration or Dissolved Air Filtration (DAF) process. As chemically flocculated water enters the filtration chambers, a pressurised stream of water saturated with air is injected, causing a large formation of very fine bubbles to rise up from the floor. These bubbles or oxygen molecules become attached to floc particles and float them to the surface. The air-floc particles accumulate to form a floating sludge blanket. The floating sludge is removed to the waste sludge pool at regular intervals, leaving behind clear or ‘clarified’ water. Clarified water is passed through filters to remove any remaining floc. The water now appears as clean as it does when it comes out of our taps.

The water must then undergo two more processes before it is ready to drink…

Disinfection and pH correction

Any water leaving a WTP is disinfected with chlorine to kill microorganisms, bacteria and any viruses that may be present in the water. Alkalie is added to correct the pH and balance the water for human consumption so that it is not too acidic, and also so that it doesn’t corrode or rust the pipes and/or concrete infrastructure as it travels out to homes. It is mixed into the water as it flows to the storage reservoir. Fluoride is also added as a protective dental health measure, an initiative of the Government. The treated water is then pumped to reservoirs, chloramine is added (a chemical compound of chlorine and ammonia) to ensure that any harmful microorganisms are destroyed and do not regrow in the pipes.

How up to date is your Drinking Water Quality Management Plan? (DWQMP)

The challenge for WTPs is ensuring that their plant and operation team have contingencies in place to respond to stormwater and floodwater ingress and the dramatic increase in debris and pollutants. Some “headlines” for you to consider that could reduce the severity of impact from these events, and lay the pathway for more straightforward follow up action:

• Do you have a Drinking Water Quality Management Plan (DWQMP) in place?
• Is it up to date?
• If you have an up-to-date DWQMP has a contingency plan been documented?
• If you have an DWQMP and a contingency plan have they been reviewed and updated since the last extreme weather event?
• If all the above details are complete and up to date, has your operations team read and understood their responsibilities?

If you need help or advice regarding wet weather preparation for your WTP, or require review of your DWQMP, contact Simmonds & Bristow to talk about how our tune-up services can help. Call on 1800 620 690 or email [email protected].

WTP Supervisors and Operators will find this PDF handy!