How to Operate your WTP and Disinfection Systems at Reduced Flows

How to Operate your Water Treatment Plant and Disinfection Systems at Reduced or Lower Flows

The current global pandemic has resulted in significant impacts on various business sectors. The downturn from the shut-down or isolation period and social distancing measures have impacted many industry sectors, with negligible travel and holiday making hitting resorts, holiday, construction, and contractor accommodation hard.

Many remote resorts and communities need to provide their own wastewater and water treatment infrastructure, and this infrastructure relies upon visitors, guests, contractors, and fly-in residents to operate reliably.

However, this shutdown or isolation period will not last forever. When it finishes, resorts, holiday places, construction and contractor camps, etc. will need to be ready for a possible sudden influx of visitors.

To help you keep your water treatment infrastructure operating at low flows and still make good clean water, we offer the following support.

WTP Operation at Low Demand

Running a WTP at low flow requires specific management but does not require any drastic changes to the plant itself; for most plants, simply changing its operational schedule will allow it to operate reliability and make safe potable water.

To operate the plant at lower flow, the best method is to limit water making to a few long runs one or a couple of days a week.

WTP equipment will typically operate better with more consistency if it is run for closer to its design run time, which is typically between 8 and 20 hours per day.

It’s far better to run a WTP for 8 hours once a week as opposed to 1 hour every day. As such, water making should be saved up, and conducted in a few large batches.

This is especially the case if the plant includes filtration equipment, as short run times generally do not allow filters to ripen effectively, and can result in water going ‘off’ within the filter due to ineffective or infrequent backwashing.

Disinfection Maintenance

Maintenance of the disinfection residuals within water storages is generally the harder part of maintaining a potable water network at low flows. Low flows can result in water sitting still for long periods in storage. Chlorine decays over time, which can then lead to chlorine concentrations below the minimum required to maintain a safe and effective disinfection residual, which causes a public health risk.

For water storages equipped with chlorine top-up dosing and recirculation equipment, the process is simple; ensure sufficient chlorine is available to the dosing system, and monitor pH and TDS to ensure the chlorine dosing is not causing other water quality issues.

For systems that do not include a recirculation or top-up dosing system, chlorine needs to be manually dosed to ensure a sufficient residual is maintained. This can be achieved by adding chlorine directly to the storage tanks. Chlorine residuals will need to be checked daily to ensure that both sufficient chlorine is added to maintain an adequate residual, and that accidental over-dosing is not occurring.

As a rule of thumb, a dose of 85 mL of 12% sodium hypochlorite (hypochlorite is generally between 10% and 12% active chlorine) will increase the chlorine residual of 10,000L of water by 1 mg/L. Note that this assumes that any underlying chlorine demand has been met as part of your treatment process (i.e., that the process has achieved breakpoint).

If the underlying demand has not been met, the chlorine added will have little to no effect on the free chlorine residual.  At this point, a bucket test should be conducted to estimate how much chlorine is required to be added. This can be conducted by adding small amounts of chlorine to a bucket of water (preferably 20L or more; the extra volume makes chlorine addition less prone to error due to the accuracy of the measuring equipment used) and measuring the residual.

Once the correct dosage is found, it can be applied to the storage.

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