Testing

Corrosion Indicators

Every treatment plant should have a corrosion control plan for its distribution system.  This system may be as simple as long-term monitoring of the water to determine if water is corrosive, or it can include a complex array of chemicals or equipment.  Here, we will consider methods used to monitor the stability of water.

The most common indicators of corrosion in the distribution system are red water complaints and leaks.  If the incidence of these problems increases in a certain area of the distribution system, then some sort of corrosion control may need to be undertaken.  Red water is usually caused by tuberculation and iron bacteria while leaks are caused by the pitting below tubercles.  However, the operator should be aware of other possible causes of these problems.  High iron concentrations in the source water can cause red water problems while leaks can be caused by corrosive soil acting on the outside of the pipes as well as by corrosive water acting on the inside of the pipes. 

During routine maintenance of the distribution system, the operator should watch out for signs of corrosion and scale.  When pipes are removed and replaced, the old pipes should be visually examined for signs of tubercles, pitting, or uniform corrosion, and for excessive scaling.



Long-Term Testing

More active forms of corrosion monitoring include coupons and tests for flow, dissolved oxygen, and heavy metals.  These tests will determine whether the treated water is corrosive over a span of a few months (in the case of coupons), weeks (for flow tests), or immediately. 

Coupon

Coupons, like the one shown above, are small pieces of the same type of metal used in the distribution system piping.  These coupons are inserted into pipes at various locations in the distribution system and are left in place for about three months to give adequate time for corrosion to occur.  By weighing the coupon before and after the test period, the amount of metal lost from the coupon due to corrosion can be determined.  This is a simple method of corrosion monitoring which is widely used in many distribution systems. 

Flow monitoring can also be used to detect corrosion.  A new piece of pipe is placed in service and the flow of water through the pipe is measured over time.  If the flow becomes lower after a few weeks, then either tubercles or scale have formed on the inside of the pipe, decreasing the area available to carry water. 



Short-Term Testing

Dissolved oxygen and toxic heavy metals in the distribution system can be used as indicators of corrosion over a much shorter time frame.  There are also a range of tests done at the water treatment plant to determine whether water is stable. 

Dissolved oxygen is tested at various points in the distribution system at the same time.  If the dissolved oxygen concentration becomes lower further from the treatment plant, then the oxygen is probably being used up by corrosion.  However, the operator should be aware of the possibility that D.O. is being used to oxidize organic matter. 

Toxic heavy metals, such as copper and lead, are tested at the consumer's tap.  High concentrations of these metals in the water indicate corrosion in the distribution system, although in a few cases the metals may have originated in the source water. 

Finally, water can be tested directly to determine whether it is stable.  Both the Langelier Index and the Marble Test are laboratory tests which can determine the degree of calcium carbonate saturation in the water at the treatment plant.  Water which is just saturated with calcium carbonate or which is slightly supersaturated with calcium carbonate is considered stable and safe to release into the distribution system.