It's Not “Just Water:” Understanding What’s Really in Your Supply

In the 1980s, a popular TV commercial showed a cantankerous man driving down a two-lane road. The vehicle’s tailpipe was spewing smoke and blinding the drivers behind. One of his passengers questioned his choice of motor oil, to which the man growled, “Motor oil is motor oil.”  The announcer went on to say “motor oil definitely is not motor oil.”

The polarity of water molecules allows water to dissolve, at least partially, many minerals in the Earth’s crust. Water flows along countless pathways, which puts it in contact with many soils and geological formations.

The variable chemistry of freshwater supplies in the U.S. is briefly illustrated in the following table, which provides comparisons between two river water, two lake water and two groundwater sources.

While this table represents only a small fraction of the water supplies around the country, it very much helps to illustrate issues that must be considered when designing water-treatment systems.  These items include:

• Some areas of the country, including portions of the Midwest, have large limestone deposits just below the earth’s surface. While the principal components of limestone (calcium carbonate (CaCO₃) with usually lesser amounts of magnesium carbonate (MgCO₃)) are only slightly soluble in natural water, their presence can still add appreciable hardness and bicarbonate alkalinity to water supplies. This is reflected in the Missouri River sample. Conversely, note the two sources in the Northeastern U.S., one river water and one lake water, that have very low hardness, indicating passage along or through formations with different and very stable mineral compositions. (Consider that the nickname for New Hampshire is the Granite State.)
• As the well-water sample from Manhattan, Kansas, suggests, elevated hardness and alkalinity are common in some groundwater supplies because “once rainwater penetrates soil, it is exposed to CO₂ gas levels much greater than in the atmosphere, created by respiration of soil organisms as they convert organic food into energy and CO₂.”[3] The extra CO₂ increases dissolution of calcium and magnesium carbonate. Additional groundwater impurities may include iron and manganese and sometimes dissolved gases such as hydrogen sulfide (H2S). Supplies with high hardness may require lime/soda ash softening for primary pretreatment, which adds complexity and cost to the process.  
• Other sources may have noticeable salinity, even when not located near the ocean, for example, the groundwater supply from Dallas. In my experience, engineering firms often don’t take chloride concentrations into account when selecting metallurgy for heat exchangers such as steam surface condensers. We will examine this issue later in the series.
• Many river supplies, and especially in locations with substantial topsoil, can see huge spikes in suspended solids during heavy precipitation. The high turbidity of the Missouri River sample clearly illustrates this issue. Surges in solids concentration may cause serious difficulties in pretreatment equipment, such as clarifiers and micro- or ultrafiltration units.
• Many facilities employ cooling towers for various processes. Evaporation is the primary method of heat transfer, but this (mostly) pure water loss causes impurities in the circulating water to “cycle up” in concentration. Vigorous makeup pretreatment may be necessary to protect cooling systems from increased levels of scale- and corrosion-inducing impurities. The reader can find more details about cooling tower operation in a recently concluded Chemical Processing series.[4]
• Although not shown in Table 1, some groundwater sources, and particularly those in the Southwestern U.S., contain silica (SiO₂) in double digit (mg/L) concentrations, which, when cycled up in a cooling tower or reverse osmosis (RO) unit, can potentially form tenacious deposits.
• Potentially troublesome in some locations, e.g., bayou country along the Gulf Coast, are large organic molecules produced from decaying vegetation. These compounds can foul RO membranes and ion exchange resins.
• For the many high-purity makeup water treatment systems that have RO as a core technology, dissolved ion concentrations increase at least four-fold from the leading to the trailing elements. Like the concentration increase in cooling towers, this factor requires careful evaluation of pre- and process treatment methods to minimize scale formation. 

As will be outlined in Part 2, modern clarifier designs are much improved, but this basic illustration allows straightforward discussion of the fundamental processes.

Pretreatment systems typically have inlet screens to remove large materials that would otherwise foul or damage downstream equipment. The remaining particles cover a wide size range.

As can be seen, without assistance fine particulates can remain in suspension for very long periods.  Clarifiers were an initial answer to this difficulty. Suspended particles typically have a negative surface charge causing them to repel each other. Coagulation involves the introduction of chemicals to neutralize the negative charges and bring particles close together. Most common are inorganic compounds:

• Aluminum sulfate (alum), Al₂(SO₄)₃•18H₂O
• Sodium aluminate, Na₂Al₂O₄
• Aluminum chlorohydrate, Al₂ClH₇O₆
• Ferric chloride, FeCl₃
• Ferric sulfate, Fe₂(SO₄)₃•9H₂O

Consider one of the most well-known coagulants, alum, which when added to water reacts as follows:

This equation illustrates two important aspects of alum chemistry. Foremost, aluminum ions combine with water to form a hydroxide precipitate. The gel-like material neutralizes particle charges allowing particulates to move much closer together. Second, this reaction generates sulfuric acid that removes alkalinity from the water and lowers the pH. Alkalinity replenishment of the effluent may be needed. Iron-based coagulants react similarly. Alum functions most effectively within a 5.5-8.0 pH range, while iron salts perform over a broader pH range of 5-11. Since sodium aluminate has the opposite effect of alum on pH, it may be a better coagulant in some applications.

Organic coagulants are also available, and these compounds have a high cationic charge. Examples include polyamines and poly-diallyl-dimethylammonium chloride (DADMAC). Organic coagulants have many active sites, so low ppm concentrations may be sufficient for charge neutralization.

As Figure 5 suggests, clarifiers typically have a rapid mix zone to ensure proper coagulant-particle interaction. Water exiting the rapid mix zone enters an area of lower velocity for flocculation.  Flocculants are organic polymers of high molecular weight that act as bridging agents for the coagulated particles.