Recycling Treated Municipal Wastewater for Industrial Water Use

Section 3: Recycled Wastewater System Components and Costs
Recycling Treated Municipal Wastewater for Industrial Water Use
Ammonia can induce corrosion in copper-based alloys. Ammonia is present at high concentrations in the
treated wastewater effluent of plants without advanced secondary treatment processes. Dissolved gases
and certain metals with high oxidation states also promote corrosion. For example, heavy metals,
particularly copper, can plate out on mild steel, causing severe pitting. Corrosion also may occur when
acidic conditions develop in the cooling water.
The moist environment in a cooling tower is conducive to biological growth. Microorganisms can
significantly reduce the heat transfer efficiency, reduce water flow, and in some cases generate corrosive
by-products [California State Water Resources Control Board, 1980]. Recycled wastewater used in
cooling systems may require treatment to control the nutrients, ammonia and phosphorus, and/or organic
matter which promote the growth of slime-forming organisms. Organic matter is measured by surrogate
parameters such as carbonaceous biochemical oxygen demand (CBOD) or total organic carbon (TOC).
Sulfide-producing bacteria and sulfate-reducing bacteria are the most common corrosion-causing
organisms in cooling systems using recycled wastewater. These anaerobic sulfide producers occur
beneath deposits and cause pitting corrosion that is most severe on mild and stainless steels. Serious
corrosion is caused by thiobaccillus bacteria, an acid-producer that converts sulfides to sulfuric acid.
Similarly, nitrifying bacteria can convert ammonia to nitric acid, thus causing pH depression, which
increases corrosion on most metals.
Boiler Feed Water
Boiler feed water has very stringent water quality requirements that typically requires a treatment system
even with potable water supplies. Boiler feed water must be treated to remove hardness. Calcium and
magnesium salts are the principal contributors to scale formation and deposits in boilers. Excessive
alkalinity contributes to foaming and results in deposits in heater, reheater, and turbine units. Bicarbonate
alkalinity, under the influence of boiler heat, may lead to the release of carbon dioxide, which is a source
of corrosion in steam-using equipment. Silica and aluminum form a hard scale on heat-exchanger
surfaces, while high concentrations of potassium and sodium can cause excessive foaming in the boiler.
Process Water
The suitability of recycled wastewater for use in industrial
processes depends on the particular use and is highly variable. For
example, the electronics industry requires a very high water
quality for washing circuit boards and other electronic
components. On the other hand, the tanning industry can use
relatively low-quality water. Requirements for textiles, pulp and
paper, and metal fabricating are intermediate. The constituents of
concern for the pulp and paper industry are discussed in more
detail to provide an example of the variety of water quality
parameters that must be considered for any process water use.
Use of recycled wastewater in the paper and pulp industry is a
function of the grade of paper produced. The higher the quality of
the paper, the more sensitive it is to water quality. Impurities
found in water, particularly certain metal ions and color bodies,
can cause the paper to change color with age. Biological growth
can cause clogging of equipment and odors and can affect the
texture and uniformity of the paper. Corrosion and scaling of equipment may result from the presence of
silica, aluminum, and hardness. Discoloration of paper may occur due to iron, manganese, or microorganisms.
Suspended solids may decrease the brightness of the paper.
42 Metropolitan Council Environmental Services