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water’s analyses

Feed water’s analyses vs. permeate’s qualities desired.

Accurate feed water data, precise product water’s qualities desired (and/or their applications), and good understanding and interpretation of these are essential towards optimising the designs of RO systems, more so than other water systems, since they are relatively more energy-consuming. Less than optimal RO system’s designs tend to cost significantly more in long-term operating costs. But the spectrum of RO feed water’s parameters is very wide, having a good grasp in evaluating them and translating them into optimal RO system’s designs requires established expertise and experiences.

Silt Density Index (SDI).
Indicator for potential fouling.

Silt in RO applications refers to the suspended particulates of all kinds
that accumulate on the membrane surface over time. They include
organic colloids, iron corrosion elements, iron hydroxide, algae and
other fine particulate matters. SDI is used to measure how quickly feed
water fouls (or plugs) a membrane filter. The tests are carried out by
constantly feeding water through a 45 mm (diameter) filter of 0.45μm
pore size at a constant pressure of 30 psi (2.07 bar) for a desired test
time (usually 15 minutes). The difference between the time required to
filter a fixed volume of water (usually 500ml) and the time required to
do so again, after the 15-minute continuous filtration process of the
same water, is then used to calculate the SDI value – which reflects the
membrane’s “% of plugging per minute”. In this case, if the SDI15 is 5, it
indicates that the membrane will be 75% (15 x 5) plugged per minute
during the filtration process. Hence, a water sample that totally blocks
the membrane filter will have a maximum SDI value of 6.7 (100/15).

SDI < 5.

Lasting membrane’s life and performance.

Though not accurate all the times, SDI is the commonly used indicator today to determine the feed water’s fouling potential of RO membrane, and if a pre-treatment process is required. Typically, most spiral wound RO membrane will require an SDI of less than 5. Otherwise, appropriate pre-treatment method must be in place to minimise membrane’s fouling occurrence. SDI’s values are also frequently used to determine the appropriate types of pre-treatment methods needed. Pre-treatment options >


SDI < 1       | Membrane may last several years without fouling.
SDI < 3       | Membrane may need cleaning every few months.
SDI 3 – 5    | Fouling may occur frequently and frequent cleaning needed.
SDI > 5       | Direct feed to RO membrane is unacceptable, pre-treatment needed.


Total Dissolved Solids (TDS).

Define feed waters. And permeates too.

Total Dissolved Solids

TDS refers to the amount of inorganic substances such as minerals, metals and salts (principally calcium, magnesium, potassium, sodium, bicarbonates, chlorides and sulfates) and some small amounts of organic matter dissolved in water, excluding H2O molecules and suspended solids. It is measured as parts per million (ppm), or as “milligrams per litre” of water with “dried mineral content” in weight collected from one litre of water. Subject to the intended application of purified water, the salinity of the source water measured by level of conductivity (as “micro siemens per centimetre” -μS/cm) is sometimes preferred, as salt content conducts electricity. Though there is convertibility between TDS and conductivity measurements, a certain conversion factor must be applied depending on the type of source water intended for purification.



A primary indicator
of permeate’s quality.

The use of RO process is primarily targeted to reduce the TDS (which may include dissolved calcium, magnesium, iron, manganese, chloride etc). Though not the only parameter to depend on, the limit of TDS in the permeate is commonly used today as one of the primary indicators of the product water’s qualities for an intended application.

Generally for potable water, the ideal TDS content in water shall not be more than 300 ppm. Otherwise, the water concerned is deemed very hard water. High TDS level in excess of 500 ppm (or mg/L) is not palatable for consumption, and affects the taste of beverage or food qualities. It will also result in excessive scaling in water pipes, water heaters, boilers and household appliances. Water that contains more than 1,000 ppm of TDS is generally unsuitable for many industrial applications. Some industries such as pharmaceutical production may require very stringent TDS limits to as low as 10 – 0.5 ppm. TDS is hence often a key permeate’s parameter that helps determine the appropriate, and optimal, designs of a RO process and system required by a targeted application.

other parameters

And a host of other parameters.

As important for RO performance.

Apart from SDI and TDS, there is a whole range of other feed water’s data that need to be examined. Subject to sources of feed water, and applications intended, other feed parameters that may have to be examined may include Total Organic Carbon (TOC), Total Suspended Solids (TSS), Turbidity, pH, chlorine content, temperature of feed water, silicon, oil and grease, Biological Oxygen Demand (BOD), Chemical Oxygen Demand (COD) etc.

These other chemical parameters, if excessive but overlooked, can undermine the performance of the RO membrane and systems, and cost considerably more as a results of membrane’s fouling, scaling and/or degradation within short periods of operations.

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