“ F ew, if any, coatings have been effective in preve ntin g the deterioration of concrete under highly corrosive condi- tions.” So says th e Water E n vironm ent Fed- eration’ s 1969 Man ual of Practice No. 17, Paints an d Pro tective Coatings for Wast ewater Tr eatm ent Fac ilities. “Highly corrosive conditions” refers to sulfuric acid, which is secreted by bacteria as it consum es hydrogen sulfide and other sulfur-containing chem ical s f oun d in raw sewage. Sulfuric-acid attack is responsible for billions of d ollars of damage to concrete wastewater collection an d treatment systems throughout the United States. “However , man y agencies are unaware of the signifi- cant d eteri oration occurring to t heir concrete facilities,” says John Red- n er, sewerage system m an ager for the County Sanitation Districts (CSD) of Los Angeles County. Over the past 15 years, manufac- turers have developed n umerous high-solids, fast-curing coating sys- tem s that th ey claim will resist sulfu- ric aci d in sewers an d wastewater treatment plants. Manufacturers spend mu ch time and m oney evalu- ating prot ective coatings in th e lab, but field results have varied greatly. On e agency reports nothin g but suc- ces s, while anoth er reports noth ing but failure. The on ly success ful meth od CSD has foun d for resisting sulfuric acid is to install polyvin yl- chloride liners to concrete surfaces durin g construc- tion. However, many rehabilitation Ca n coatings pr otect wastewater treatment systems? BY M ARTIN S . M CGOVERN County Sanitation Districts of Los Angeles County tests the sulfuric-acid resistance of 78 products M any coating s, such as this epoxy, have not fared well in th e aggres sive condi- tions found in wastewater treatment systems. P h o t o s b y C o u n t y S a n i t a t i o n D i s t r i c t s o f L o s A n g e l e s C o u n t y
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8/3/2019 Can Coatings Protect Waste Water Treatment Systems__tcm45-348362
spon sible for all surface preparationbefore applying the coating. Gener-
ally, the manufacturers chose either
sandb lastin g or high-pressure wa-
terblasting for surface preparation. If
too m uch aggregate was exposed for
proper application of the coating,
the manufacturer was responsible for
surface repair as well. Most surface
repairs used fast-curing con crete or a
mixture of th e coating m aterial and
a san d filler.
The coatings tested had to cure
sufficient ly so water could be add ed
to th e test tank within 48 hours of
application. Nin ety-six hou rs after
application, concentrated sulfuric
acid was added to the test tanks to
achieve a concentration of 10%. The
acid solution exposed both the d ete-
riorated an d u nd eteriorated coated
surfaces (Fig. 2).Manufacturers were not allowed
to perform an y pinho le or holiday
testing after applying th e coatings,
even th ough such testing is part of a
standard application specification.
Any coating system th at could not
be applied on such a sm all scale by
the m anufacturer without th is type
of failure was not considered a viable
system.
Evaluation
The coatin gs were evaluated for atleast a year. Unless coating failure
was observed earlier, th e acid solu-
tion was usually remo ved every 3
mo nth s to allow for a ph ysical in-
spection of th e tanks. During th e in-
spection, ph otographs were taken to
docum ent an y chan ges in th e coat-
ing’s protective characteristics or ap-
pearance. The bonding quality was
observed, and th e coating th ickness
was m easured. A cross section o f the
coating was inspected to evaluate
pin ho ling, air pockets, or any grad-
ual deterioration or reaction with
the acid. The manufacturers were
given th e opportun ity to repair any
areas damaged by th e inspection.
Successful coating systems were
tested beyond 1 year to obtain addi-
tional inform ation on long-term per-
formance.
For each coating system, the time
to failure (or comp letion of th e test)
was recorded. The coatings’ ease of
application, acid resistan ce, and
bon ding ch aracteristics were also
rated on a scale of one to four, with
a score of on e being th e best. Th e
three scores were then added to pro-
duce a cum ulative score.
Of the 78 coatin gs tested, only 27successfully com pleted 1 year of
evaluation (see table on page 55). All
of these coatings had a cumulative
score of five or less. The highest sur-
vival rate belonged to th e m ortar
system s, regardless of wheth er th e
coating resin w as a coal-tar epo xy,
epoxy, polyester, or vinyl ester. The
n ext high est survival rate belonged
to th e lin er category. The neat ep oxy
survival rate was 25%. The survival
rates for th e specialty concrete an d
urethane categories were 23% and12%, respectively. The neat coal tar,
n eat polyester, and n eat vinyl-ester
coating systems all failed. The on ly
n eat system s to survive th e evalua-
tion were the polyurea and two ou t
of 16 ureth an e system s. Th e overall
survival rate for all coating systems
was 35%.
Reference
John A. Redner, Randolph P. Hsi, Ed-ward J. Esfandi and Roger Sydney,
“ Evaluation of Protective Coatings forConcrete,” County Sanitation Districtsof Los Angeles County, Whittier, Calif.,August 1998.
Figure 1. To simulate actual condit ions, the evaluations were conducted in shal-low concrete tanks constructed by inserting two concentric, precast, reinforced-concrete manhole shafts into a freshly poured concrete base slab.
Figure 2. Coating systems were ap-plied to both the deteriorated bottomportions of the tanks and to t he unde-teriorated top portions. The appliedcoatings were then subjected to a10% solution of sulfuric acid.
