Revisiting the Selection of Stainless Steel in Water and Wastewater Treatment Environments: Part 2

May 29, 2002

About the author: Frederick Bloetscher, Ph.D., P.E., is with Public Utility Management and Planning Services, Inc., Hollywood, Fla. Phone 954-925-3492; Fax 954-925-2692; e-mail [email protected]. Richard J. Bullock is with Weir Materials and Foundries, Ponte Vedra Beach, Fla. Phone 904-285-8039; Fax 904-285-8043; e-mail [email protected]. Robert E. Fergen, P.E., is with Hazen and Sawyer, P.C., Raleigh, N.C. Phone 919-833-7152; Fax 919-833-1828; e-mail [email protected] Gerhardt M. Witt, P.G., is with Gerhardt M. Witt & Associates, Inc., West Palm Beach, Fla. Phone 561-642-9923; Fax 561-642-3327; e-mail [email protected] Gary D. Fries, P.E., is with Boyle Engineering Corporation, Orlando, Fla. Phone 407-425-1100; Fax: 407-422-3866; e-mail [email protected]

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Bacterial occurrence is observed in natural waters, from both the surface and underground. Aerobic bacteria (Crenothrix, Gallionella) primarily are encountered in the fresh surface waters, although anaerobic sulfate-reducing bacteria and facultative strains also are encountered. Ground waters are more likely to contain anaerobic and facultative bacteria, as well as Gallionella than surface waters.3 Sulfate-reducing bacteria also are found in seawater.3

Slime-producing bacteria are ubiquitous. They are typified by the Pseudomonas genera. Members of this genera often are used to protect farm crops from fungal growth and, as a result, are to be expected in groundwater containing organics. However, these bacteria are highly adaptive. Research several years ago indicated that these bacteria would grow in almost any environment into which they were introduced. The Pseudomonas genera are facultative anaerobes that can persist in oxygen-depleted environments by breaking down complex hydrocarbons for the oxygen. In some circumstances, Pseudomonas species will use nitrogen in the absence of oxygen.6 Some Pseudomonas species are among the predominant denitrifiers, while others grow prodigiously in and on tertiary treatment devices such as reverse osmosis and electro-dialysis membranes and in sand or carbon filtration beds. Pseudomonas species are capable of producing the polysaccharide matrix (biofilm) that acts as a barrier, protecting the bacteria incorporated in the biofilm from harmful substances such as disinfectants as well as trapping corrosive, metabolic byproducts near the surface of the pipe. The fact that they are both slime-formers and acid-producers, permits them to be the backbone of a biofilm matrix.

One of the most significant Pseudomonads in drinking water, and a predominant slime-former, is Pseudomonas aeruginosa (see Figure 3). This is a waterborne genus capable of causing severe effects in compromised hosts.7 They can survive on minimal nutrients and tend to have a high negative surface charge.9 It also is an acid former that liberates metal ions from the surface.8 Pseudomonas aeruginosa is known to cause pneumonia in humans, but also is notorious for forming clumps—biofilms that resist chemical disinfectants, antibiotics and the human immune system.8

Just how bad is this organism with respect to biofilm formation? Vanhaecke, et al.9 conducted a study of permanent adhesion rates (a precursor to biofilm formation) between an electro-polished surface and rougher surfaces (400, 320 and 120 grit surfaces) at various pH values. While the electro-polished steel adhesion rate was up to 100 times slower for a biofilm than the 120 grit surfaces, in all cases permanent adhesion occurred in a matter of minutes.9 Maximum adhesion rates occurred between pH 6 and 7.5, the normal range for RO waters. The study concluded that adhesion of Pseudomonas aeruginosa to stainless steel, even electro-polished stainless steel, occurred in less than 30 seconds. Hydrophobicity was determined not to be a good indicator for projecting biofilm adhesion to the steel.9

As previously noted, the slime incorporates other detrimental bacteria. Filamentous iron-oxidizing bacteria often cited are Sphaerotilus, Crenothrix and Leptothrix, while the stalked bacteria Gallionella is found in tubercles as well. Iron bacteria, such as Gallionella species are common in aerobic environments where iron and oxygen are present in the groundwater and where ferrous minerals exist in the aquifer formation. These bacteria attach themselves to the ferrous materials and create differentially charged points on the surface, which in turn create corrosion problems. The iron bacteria then metabolize the iron that is solublized in the process, often creating ferric hydroxide to obtain the energy necessary for growth.5 Iron bacteria tend to be rust colored or cause rust colored colonies on the pipe surfaces.6 These colonies can grow on non-ferrous materials such as fiberglass column pipes. (See Figure 4 for an example of a biofilm on blue fiberglass pipe.)

Iron bacteria can cause damage to pipes due to the deposition of iron compounds, resulting in clogging or tuberculation of pipes and red water.10 Tubercles impede the penetration of biocides and corrosion inhibitors, diminishing their effectiveness.5 Within tubercles, the anode is at the bottom where access to oxygen is limited. Tubercle growth may be accelerated by the presence of sulfate-reducing bacteria in the inner area of the tubercle where conditions are anaerobic. Gallionella has been shown to be dependent on a symbiotic relationship with sulfate-reducing bacteria in their habitat for just this reason.5 The slime forming Pseudomonads also are frequently found near tubercles.5 Electrolytic concentration cells are set up on the metal surface beneath the tubercle as a result of poor adhesion and irregular encrustations of organisms in the tubercles.

Anaerobic conditions will form under deposits, in crevices and under the influence of BOD or COD independently of dissolved oxygen content. Hydrogen sulfide production reduces pH and causes corrosion in conjunction with chloride ions.3 The relevance of ennoblement of the corrosion potential is to decrease pH and oxygen at the metal surface under the biofilm, supplemented by porphyrin-type organo-heavy metal complexes or enzymes for the respiratory system of the organisms forming the biofilm. Therefore, high peroxide concentrations at the metal/biofilm interface will provide an alternative cathodic reaction in addition to oxygen reduction. The effect of ennoblement decreases the induction time for localized pitting and crevice corrosion of susceptible alloys.5 The ennobling effect of the biofilm of the corrosion potential would make the onset of pitting corrosion initiation more probable.5

Sulfate-reducing bacteria often are responsible for the hydrogen sulfide smell released when raw water is aerated. These bacteria are common where sulfate exists naturally in the aquifer and will tend to form black colonies on pipe surfaces. These bacteria are anaerobic and will live in the oxygen-depleted environments under biofilms such as those produced by Pseudomonas species and under tubercles. These bacteria produce acids as metabolic byproducts that are corrosive to ferrous materials, and often are a greater problem for low alloy stainless steel than for higher alloy stainless steel.6 Sulfate-reducing bacteria metabolism brings to the metal/solution interface several sulfur compounds of corrosive characteristics, either as a metabolic product (sulfides, bisulfides or hydrogen sulfides) or intermediate metabolic products (thiosulfates and polythionates) that are very corrosive to carbon and steel.5 Since the bottom of the film is anaerobic, sulfate-reducing bacteria can proliferate despite a measurable dissolved oxygen concentration in the moving water.

The control of microbial populations in reverse osmosis plant operations relies both on the control and management of disinfection as well as good plant design, construction and mechanical material selection.11 The ability to observe and understand microbial concentrations and biofilm growth concerns as they occur can eliminate more costly and problematic issues. Pretreatment with chlorine is detrimental to the reverse osmosis process and membranes. Where disinfection is practiced, bacterial growths are most often controlled by chlorination—continuous chlorination of 0.2–0.3 ppm normally is sufficient to deter bacterial growth unless silt or other debris in the pipe are present.3 A 5 ppm chlorine residual commonly is used to minimize growth potential in seawater sources. While chlorination is beneficial in minimizing fouling and biogrowths, chlorination displaces the corrosion potential in the ennoblement and, therefore, increases the crevice corrosion potential.3

While bacteria populations in raw ground or surface water supplies can be controlled to an extent by programs of routine disinfection (and may mitigate gross concerns regarding biofouling), they cannot eliminate the presence of organisms that are detrimental to ferrous materials.12 The use of PVC or fiberglass casings can address these concerns, but the organisms will continue to populate raw water intakes or wells. These inert materials can be used as a pump column pipe, further reducing concerns, but these materials may not be appropriate or practical in all situations.

Water Quality Testing Can Help

Obtaining representative samples of the feed water is the most critical issue regarding the design of water treatment facilities and the selection of materials. This requires sampling each feed water source. Water samples can provide valuable information of the ionic composition and concentration of the variety of microbial contaminants.5 Water quality parameters of importance in selecting steel grades include

               Total Dissolved Solids (TDS),

               Calcium hardness,

               Magnesium hardness,

               Total hardness,

               Phenolphthalein alkalinity,

               Methyl orange alkalinity,

               pH,

               Chlorides,

               Sulfates,

               Silica,

               Iron, and

               Dissolved oxygen.3

These tests are recommended for selecting the steel to be used in the treatment process. There also are samples for the treatment process itself and the raw water supply appurtenances (such as wells). For example, the City of Hollywood, Fla. has more than 25 wells in three separate wellfields displaying two completely different water qualities. In addition, another source is Broward County’s wellfield. Ideally water quality samples should be taken from each well or at a minimum, a representative set of samples from each different or suspected source should be collected. The water quality analyses should include the following parameters and tests in addition to those specifically required to predict corrosion potential as many of these analyses also will provide information about corrosion potential.

Field Tests

               Temperature

               pH

               Redox Potential (Eh)

               Conductivity

               Corrosion Rate

               Free Carbon Dioxide

               Dissolved Oxygen

               Hydrogen Sulfide

               Sand Production (Concentration)

               Silt Density Index (SDI)

Laboratory Tests

               Chlorides

               Color

               Turbidity

               Calcium

               Magnesium

               Sodium

               Total Iron

               Manganese

               Carbonate

               Bicarbonate

               Sulfate

               Nitrate

               Ionic Analyses

For a list of references, visit our website at www.waterinfocenter.com.

Part 3 will present a case study on the City of Hollywood, Fla.

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