Corrosion protection in concrete sanitary sewers

A method for protecting concrete surfaces of sanitary sewers includes the steps of providing a concrete surface in a sanitary sewer environment; and coating the concrete surface with magnesium hydroxide or magnesium oxide.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to concrete sewers that are prone to 
corrosion and more particularly to concrete sewers that are prone to 
sulfide corrosion. 
2. Description of the Prior Art 
It has been found that a two step biological process corrodes collection 
system infrastructures, including concrete sewers and manholes. This is 
known as "sulfide corrosion", and is increasingly deteriorating today's 
public works infrastructure. 
Sulphide corrosion has detrimental effects on the collection system 
infrastructure ranging from more frequent odor complaints or potentially 
lethal atmospheres to failure of collection system pipes and manholes. 
In the first step, usually occurring in slow moving sewage below the water 
line where anaerobic conditions can exist, sulfur reducing anaerobic 
bacteria, primarily belonging to the genus Desulfovibrio, reduce sulfate 
ions to sulfide ions. In addition, sulfide can be produced by the 
bacterial decomposition of protein, and through the decomposition of other 
organosulfur compounds. However, it is generally recognized that the 
predominant mechanism for sulfide generation in sewer collection systems 
is sulfate reduction. Through chemical equilibria, some of the sulfide 
ions form hydrogen sulfide gas molecules and escape out of the liquid 
sewage into the headspace atmosphere of the sewer pipe. 
In the second step, a different group of sulfur bacteria, primarily 
belonging to the genus Thiobacillus, establish colonies in the concrete 
pipe, and through an oxidation process, convert the atmospheric hydrogen 
sulfide to sulfuric acid with the liberation of free protons and a drop of 
Ph. The resulting acid attacks the concrete, causing the ultimate 
destruction of the pipe. It is believed that the acid reacts with the lime 
in the concrete converting it into a soft putty-like gypsum. 
History 
Trunk sewers, especially the large diameter lines in the lower reaches of a 
tributary system, are, for the most part, reinforced concrete pipe. These 
large sewers generally range in size from 54 inches in diameter up to 144 
inches in diameter. In Los Angeles, for example, the oldest of these 
sewers have been in service for approximately 65 years. At the time these 
sewers were being designed there were concerns of sulfide corrosion. 
To guard against possible sulfide corrosion, the earliest of the large 
sewers were constructed with vitrified clay liner plates installed on the 
interior sides and crowns. Vitrified clay, which is used to construct 
small diameter pipe, is unaffected by sulfuric acid. However, hydrogen 
sulfide gas and sulfuric acid penetrated between the joints in the tiles 
and destroyed grouting and cementing materials. By the late 1930's, the 
practice of using tile liners was discontinued. 
Notwithstanding the problems with the tile liners, it was believed that 
major damage to the structural steel and concrete could be avoided by 
designing sewers to have sufficient water velocities so that natural 
aeration forces would minimize the growth of the anaerobic slime layers on 
the submerged pipe walls where the Desulfovibrio bacteria grow. These 
natural aeration forces would also help oxidize any sulfide in the water 
that did form, prior to its being released as hydrogen sulfide gas. 
In the early 1950's concrete pipe manufacturers began to market pipes 
internally lined with plastic to protect against sulfide corrosion. 
However, at that time there was little data to document how well these 
plastic liners would remain securely bonded to the concrete and provide 
effective protection. The cost of the lined pipe was expensive when 
compared to that of regular, unlined pipe. Consequently, during the 1950's 
and the 1960's, unprotected reinforced concrete pipe continued to be used. 
By the mid-1960's sulfide generation was increasing, especially at 
locations such as pumping plant force mains where depletion of available 
oxygen occurs. 
Research in the late 1960's devised an empirical formula to predict sulfide 
generation rates and resulting concrete corrosion rates. See report 
entitled "Sulfide Occurrence and Control In Sewage Collections Systems" 
which was published in 1983. 
In the early to mid-1970's, thorough inspections of concrete sewer lines, 
for example in Los Angeles, were made in areas where sulfide generation 
was known to be occurring. Depths of corrosion along the interior crowns 
of the corresponding sewers were measured. The actual corrosion which was 
found very closely matched that predicted by the aforementioned formula. 
Based on the rates of corrosion observed, it then appeared that the 
remaining structural lives of most of these sewer pipes ranged from at 
least several decades for the oldest of the sewers, up to hundreds of 
years for most of the post-World War II sewers. These results were very 
encouraging, for the normal design life of a major sanitary sewer is 
assumed to be 100 years. 
In the early 1980's, a second thorough inspection of these same sewers were 
made, and the results were unbelievable: in less than one decade, many of 
these sewers had experienced significant corrosion to the point where the 
reinforcing steel was exposed and corroding. 
The rate of corrosion had definitely increased and was no longer 
predictable with the existing empirical formula. The causes of the 
increased rate of corrosion in the late 1970's and 1980's are not 
completely understood, but it appears that at least two different factors 
may have played important roles. First, the institution of limitations on 
the strength and toxicity of industrial waste waters that could be 
discharged to the sewers beginning in 1975 and the institution of the U.S. 
Environmental Protection Agency's Categorical Pretreatment Program for 
industrial waste discharges in 1983 resulted in significant reduction in 
discharges of heavy metals to the sewers. These heavy metals played an 
important role in binding sulfide and preventing the release of hydrogen 
sulfide to the sewer headspaces and had an inhibitory effect on the 
Desulfovibrio bacteria. Second, detergent manufacturers employed new 
formulations for surfactants and brighteners using sulfonated compounds 
(e.g., linear alkylbenzenesulfonates and derivatives of amsonic acid). 
Some of these organsulfur compounds may be easily biodegraded into 
sulfide. 
Sulfide and Corrosion Control in Sewers 
In the past few years attempts have been made to control the sulfide 
corrosion problem by attempting to reduce the growth of Desulvovibrio 
bacteria or to chemically bind up the sulfide which is generated. Research 
in West Germany, show that the control level for sewer headspace hydrogen 
sulfide to significantly reduce corrosion is between 1.0 and 3.0 parts per 
million. This correlates to being able to obtain sufficient control of 
sulfate reduction to keep the dissolved sulfide concentration in the waste 
water below 0.1 mg/l. This has proven to be extremely difficult and costly 
with the conventional methods to chemical control available. 
Ferrous and ferric chloride (iron) and liquid caustic soda (sodium 
hydroxide pH 13-14) are currently being routinely added to selected trunk 
sewers at a cost of over $3 million per year to attempt to control sulfide 
generation and corrosion. Iron is added continuously to bind up sulfide as 
a nonsoluable iron sulfide precipitate. 
The caustic soda is added at a semi-weekly frequency to provide a 30 
minute, high Ph, shock dose to the Desulvovibrio bacteria. This controls 
sewer corrosion by neutralizing the sulfuric acid already formed by the 
bacteria, inactivating and destroying these bacteria, and limiting the 
formulation of new colonies to prevent the production of acid. 
The effectiveness of this treatment program is evaluated by monitoring the 
concentrations of hydrogen sulfide in the headspaces of the sewers being 
treated. To date, only modest reductions (50%-60%) have resulted from 
these treatments, even though significant (75%-95%) dissolved sulfide 
reductions have been obtained in the waste water. Measurement taken of the 
surface pH on the crowns of the treated sewers have not changed 
substantially from their typical acidic values varying between a pH of 1 
to 3. 
A recent development involves a spray application of a caustic solution, 
e.g., caustic soda, to the sewer crown. The caustic spray process appears 
to control micorocrobial formulation of acid on the crown of unprotected 
reinforced concrete sewer pipe. It is estimated that the operation and 
maintenance cost to use caustic spray is $0.03 per inch diameter per 
linear foot of sewer. This compares quite favorably to a sewer 
rehabilitation cost of $11.00 per inch diameter per linear foot. 
Use of caustic soda, however, has several important deficiencies. First, 
caustic soda is only temporarily effective in halting the progression of 
crown corrosion. Testing shows that acid producing bacteria are capable of 
re-establishing themselves in a very short time. The effect of caustic 
soda spraying is limited to about 60 days. 
Caustic soda is a hazardous chemical and is known for its ability to 
dissolve human flesh. Even a small splash of caustic soda can cause 
permanent blindness. 
When spraying sewer crowns, large above ground hose reels are filled and 
pressurized with caustic soda. This equipment is often located in 
residential areas where automobile and pedestrian traffic are common. 
Traffic accidents, spills, ruptured hoses, valve and pump failures, or 
operator error represent an unreasonable risk to the safety of both field 
crews and the public. 
The economics of this treatment are subject to frequent variations in the 
cost and availability of caustic soda. This makes budgeting difficult with 
chemical costs fluctuating as much as 400% within a one year period. 
Last, the treatment must be applied 5 or 6 times per year. This requires a 
large specially trained group of field technicians to routinely transport, 
pump, and spray hazardous chemicals in densely populated areas. The long 
term risks associated with this process may outweigh the benefits. 
Rehabilitation 
Recently, large sums of money, in Houston, Phoenix, Atlanta and Los Angeles 
for example, have been expended to rehabilitate or replace many miles of 
18" to 144" diameter sewer which have been excessively corroded. All 
replacement sewers are reinforced concrete with polyvinyl chloride liners 
cast in place to protect the sewer headspace. 
Sliplining of large diameter sewers without diversion of flow presents 
unique logistic problems regarding control of odors emanating from 
insertion pits. To provide odor control for ongoing sewer rehabilitation 
projects, odor control scrubbers are required. 
There are still many miles of sewers for which repair or replacement is 
currently under design in sanitation districts throughout the country. The 
estimated cost is in the hundreds of millions. There are also many 
additional miles of sewers which have suffered moderate sulfide corrosion 
damage, but if the corrosion process is not controlled and continues at 
its current rate, these sewers will also need to be repaired and replaced 
in the next 10 years. 
The potential for hydrogen sulfide (H.sub.2 S) generation is expected to 
increase as more municipalities adopt water conservation programs that 
include the installation of low-flow plumbing devices. Reduced flows 
entering collection systems from these water-conserving fixtures is the 
primary cause. As a result of these reduced flows, collection systems may 
experience longer retention times in pipes, wet wells, and force mains; 
increased damming caused by settled solids and grease; and less dissolved 
oxygen (DO) caused by increased biochemical oxygen demand (BOD). 
SUMMARY OF THE INVENTION 
It has been found that the corrosion problems described herein may be 
eliminated or largely diminished by applying magnesium hydroxide and/or 
magnesium oxide to a concrete surface. 
Other features and advantages of the present invention will become apparent 
from the following description of the invention which refers to the 
accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION 
Due to sulfide corrosion, it has been found that a decrease in surface pH 
of a concrete sewer of only 2 points, lowers the life expectancy of a 
collection system crown by a factor of 10, FIG. 1. A ph of the concrete 
surface above 4 is required to maintain a concrete corrosion rate under an 
acceptable 0.03"/Year, FIG. 1. At pH 2, concrete corrosion is about a 
quarter of an inch per year. 
In order to control sulfide corrosion, it has been found that applying a 
layer of magnesium hydroxide (Mg(OH).sub.2), and/or magnesium oxide (MgO), 
preferably in the form of a slurry, to concrete surfaces of sanitary 
sewers prevents corrosion caused by acid. Magnesium hydroxide and/or a 
magnesium oxide slurry forms a thick, adherent coating of acid 
neutralizing, relatively insoluble, highly alkaline material sufficient to 
substantially reduce bacterial density, neutralize acid and discourage 
further corrosion. Once applied to a concrete sewer surface, the magnesium 
hydroxide and/or a magnesium oxide raises surface pH on contact and 
maintains the ph of the concrete surface above 4 for long periods of time 
after treatment. 
Magnesium hydroxide and/or magnesium oxide are superior to other chemicals, 
including, for example, caustic soda, lime and soda ash for preventing 
corrosion of concrete sewers in a variety of ways. One eighth inch of 
magnesium hydroxide, for example, has 100 times less solubility as lime 
and provides five (5) times the neutralization protection of 200 ml/sqft 
of 25% caustic soda. More importantly it is much safer. Some other 
advantages include: 
1. Magnesium hydroxide is an insoluble slurry that adheres in a thick layer 
to unprepared surfaces providing protection that lasts longer than 60 
days. It is expected that magnesium hydroxide will provide pH protection 
for over one year. Caustic soda is a soluble solution and cannot be 
applied in a thick layer. Caustic soda quickly dissipates permitting acid 
producing bacteria to return in only 60 days. 
2. Magnesium hydroxide has two OH ions and provides higher neutralizing 
capacity per gram mole. Caustic soda has only one OH ion. 
3. Magnesium hydroxide provides more insoluble hydroxyl ions. Therefore, 
magnesium hydroxide tends to stay in place rather than rinsing away with 
splashing water. Hydroxyl ions in caustic soda are dissociated. 
4. Magnesium hydroxide produces a safe soluble reactant with very little 
sludge. Sludge from neutralization of acid by caustic soda is gelatinous 
and contributes to sludge. 
5. Magnesium hydroxide requires no placarding or special handling and 
presents no chemical hazard to the environment, users, or the public. 
Caustic soda is hazardous requires D.O.T. truck placards. 
6. Magnesium hydroxide adds little mechanical loading to corroded 
structures. 
7. Magnesium hydroxide is white allowing easy inspection ensuring complete 
coverage. Caustic soda is a clear liquid which is difficult to see on the 
treated surface. 
8. Magnesium hydroxide is soft, preventing egg shelling and blockage of 
sewers. 
9. Magnesium hydroxide may be pumped long distances. 
10. Magnesium hydroxide passes through small diameter spray nozzles. 
11. Magnesium hydroxide has the lowest annualized installed cost versus 
other surface treatments. 
12. Magnesium hydroxide has sufficient pH to kill or disable acid producing 
bacteria. 
13. Sanitation districts have used dusted lime in the past to control crown 
corrosion however C0.sub.2 levels in the headspace quickly carbonate lime 
rendering it ineffective. Further, past practice has taught that sludge 
generated from lime treatment is high in volume and weight often 
generating eight (8) times as much sludge as the amount of lime added 
eliminating the material cost advantage of lime. 
FIG. 2 shows the results of a representative crown spraying field trial 
comparing caustic soda and magnesium hydroxide. It can be seen that the pH 
of a concrete surface 6 sprayed with magnesium hydroxide, having 
approximately 50% solids content, 450 ml/Sqft 50% Thioguard Mg(OH).sub.2) 
maintained a surface pH above the corrosion threshold (pH 4) for almost 
one year. Additional testing indicates that magnesium hydroxide will 
maintain the surface pH above the corrosion threshold for over one year. 
In contrast, it has been found that the pH of a concrete surface 10 
sprayed with a 25% solution of caustic soda 200 ml/sqft 25% NaOH, dipped 
below the corrosion threshold 8 (pH 4) only after about sixty (60) days. 
Magnesium hydroxide and/or magnesium oxide rely on two phenomena to be 
effective. First they have a pH near 10.5, which while safe to humans is 
just above the tolerance of common acid producing bacteria to kill or 
disable them. Small amounts of lime (calcium hydroxide) can be added to 
magnesium hydroxide and/or magnesium oxide slurry to increase the pH and 
enhance the slurry's ability to kill bacteria. It is anticipated that 
other biocides or hardening agents such as sodium silicate, sodium 
bisulfate, magnesium sulfate, magnesium chloride, phosphates, or other 
materials intended to impart mechanical strength, may be added to further 
enhance its performance. 
Secondly, as the bacteria re-establishes, alkalinity provided by the 
magnesium hydroxide and/or magnesium oxide neutralize acids produced by 
the bacteria producing a soluble, tightly bonded sulfate and prevents 
rapid re-establishment of bacteria. This prevents low pH necessary for the 
really aggressive acidifiers. The magnesium hydroxide and/or magnesium 
hydroxide slurry is sacrificial and protects the cement which bonds 
concrete. 
A magnesium hydroxide and/or magnesium oxide slurry can be prepared by 
adding caustic calcined magnesium oxide (MgO), preferably in a dry powder 
form, to water. The magnesia can be obtained from any of the known 
suppliers including, Premier Services Corporation, King of Prussia, Pa. 
Premier Services sells magnesia in dry powder form under the trademark 
MAGOX.RTM.. 
When magnesium oxide is added to water it undergoes hydration and converts 
to magnesium hydroxide. The rate of this reaction can be varied depending 
upon the surface area of the MgO, starting water temperature, vessel 
configuration, and agitation. Either a slowly hydrating MgO, or a fully 
hydrated Mg(OH).sub.2 slurry may be applied to the concrete surface. 
A magnesium hydroxide slurry can also be purchased by any of the known 
suppliers, including Premier Services who sells magnesium hydroxide slurry 
under the trademark AQUAMAG.RTM.. 
In a preferred embodiment, a specially hydrated and formulated slurry, 
marketed by Premier Services Corporation under the trademark 
THIOGUARD.TM., is used in sanitary sewers as an acid acceptor. 
THIOGUARD.TM. offers a safe, economic alternative reagent for acid 
neutralization and water treatment and has been found to be particularly 
effective in extending the useful life of concrete sewer crowns and 
manholes by neutralizing harmful sulfuric acid. 
THIOGUARD.TM. is an off-white slurry composed predominantly of agglomerated 
magnesium hydroxide particles and is made from hydrated calcined natural 
magnesite or precipitated from sea water, bitterns, or brines. Table I, 
below, depicts a typical chemical analysis of THIOGUARD on a loss free 
basis. 
TABLE I 
______________________________________ 
Chemical Analysis, Wt % 
(loss free basis) 
Typical Maximum Minimum 
______________________________________ 
MgO 93-98 98.5 92.0 
CaO .5-2.5 3.5 -- 
R.sub.2 O.sub.3 
.5-1.5 -- -- 
Insolubles .5-3.0 -- -- 
Viscosity, cps 500-10,000 -- -- 
Density, lb/gal 
11.8 -- -- 
% Solids by Wt % 
50 55 45 
______________________________________ 
The component R.sub.2 O.sub.3 refers to natural impurities such as Al.sub.2 
O.sub.3 and Fe.sub.2 O.sub.3 that are indigenous to ore bodies. The 
insolubles include, for example, SiO.sub.2, MgCO.sub.3 and CaCO.sub.3. 
In a preferred embodiment, the magnesium hydroxide or magnesium oxide in 
the form of a slurry is sprayed on the inside crown portion of a sewer 
pipe from the water line up. Preferably, the spray delivery system is 
similar to that used to apply a caustic solution to the inside of a sewer 
line. 
It should be realized by those skilled in the art that the magnesium 
hydroxide and/or magnesium oxide can be applied to any other concrete 
surface that is subject to sulfide corrosion or the like, e.g., a manhole, 
or by any method in any form, e.g., dry powder form or the like. 
Referring now to FIG. 3, the basic spray system 10 consists of a spray head 
assembly 12 fitted with two or three fan type airless spray nozzles 14 
arranged to provide full coverage of the surface to be treated 16. The 
nozzles 14 are mounted on a collapsible spray head float 18. A supply 
tanker 20 delivers the magnesium hydroxide to a chemical pump 22, such as 
a pneumatic or hydraulic powered GRACO 10:1, and pumps the magnesium 
hydroxide through a high pressure hose 24 mounted on a hose reel 25. 
The spray head float 18 is pulled through the sewer 26 between manholes 28 
and 30, for example, using a cable 32 and one or more electrically driven 
cable winch 34 by which the travel speed of the float 18 is controlled. 
Operators up and downstream communicate by radio to monitor the hose and 
spray head float 18 progress. The correct spray head float speed is 
determined by the rate of flow of magnesium hydroxide to the nozzles 14. 
The spray head 12 is constructed of a 12 inch section of 4 inch diameter 
PVC pipe with end caps. One end is fitted with a quick disconnect caustic 
feed nipple, not shown. Preferably, there are three 316 stainless steel, 
clog-free, whirl type nozzles 14 with a full cone, 90.degree. angle spray 
pattern. The nozzles 14 are mounted diagonally across the top of the spray 
head 12 at a 45.degree. angle to the horizontal axis at equal distances 
apart to achieve full coverage of the sewer crown area 16 above the sewage 
surface. The nozzles 14 can spray up to 2.4 gallons per minute at 40 psi. 
The spray head float 18 consists of three 4 inch diameter, 60 inch long PVC 
tubes connected in parallel by two adjustable arms on each side, not 
shown. The adjustable arms allow the outside tubes to be moved away from, 
or closer to, the center tube to accommodate different size sewers flowing 
at various depths. The float 18 can be pulled forward or backward, which 
gives the spray operation maximum flexibility. It also enables the crew to 
remove the flow from the sewer if an emergency occurs. 
The pulling equipment consists of two identical electrical cable winches 34 
(one positioned at each manhole) and are used to facilitate the spray 
operation. The cable winch frame is made of lightweight aluminum for ease 
of handling. One of the two winches 34 is used to pull and control the 
speed of the float 18. The second winch is connected to the float 18 for 
emergency purposes. Preferably, each winch 34 has a 2,500 foot length of 
1/8 inch diameter, stainless steel cable to allow for treatment of more 
than one sewer section without moving the float from the sewer. 
The power source for the equipment is provided by two portable generators, 
one rated at 3.3 kilowatts and the other at 6.5 kilowatts, not shown. The 
6.5 kilowatts generator is used to provide power to the pump motor, one 
cable winch in the motor operator for the hose reel. The 3.3 kilowatt 
generator is used to power the pulling equipment at the other end of the 
sewer section being treated. 
The viscosity of the magnesium hydroxide and/or magnesium oxide slurry can 
be varied to provide the optimum sprayability and pumping characteristics 
and achieve different degrees of surface adhesion to the concrete. 
Preferably, the slurry should have a viscosity to allow pumping while 
enhancing adhesion and discouraging runoff. It has been found that 
viscosity's ranging between 500 and 5000 centipoise (cps), preferably 2000 
cps, provide the widest range of application. 
The viscosity and the properties of the slurry can be varied by any of the 
known methods including changes in the solids to water ratio, or by the 
use of polymers to enhance or alter these properties as desired for 
differing field conditions or equipment configurations, e.g., increasing 
or decreasing the water content or by adding in more magnesia powder. 
It is recommended that once applied to a concrete surface, the slurry 
should include at least 30%, preferably at least 50%, by weight magnesium 
oxide or magnesium hydroxide. For best results, the magnesium hydroxide 
slurry should be applied to the concrete surface to result in a layer 
approximately 0.0625 to 0.25 inches thick. 
A related chemistry for this application addresses varying water level. 
Magnesium oxide mixed with sodium silicate produces a slurry which, when 
dried, yields a hard alkaline material composite of unhydrated magnesium 
oxide encapsulated in sodium silicate. Acid produced by surface bacteria 
is neutralized by the sodium silicate. As the sodium silicate dissolves, 
magnesium oxide is exposed which dehydrates the bacteria and also 
neutralizes. 
Although the present invention has been described in relation to particular 
embodiments thereof, many other variations and modifications and other 
uses will become apparent to those skilled in the art. It is preferred, 
therefore, that the present invention be limited not by the specific 
disclosure herein, but only by the appended claims.