9.3 Processes for the Municipal Purification of Water 223
- Organic nitrogen: Organic nitrogen compounds
include proteins and its various breakdown
products – peptones, polypeptides, and amino
acids. These compounds greatly retard chlorine
with which they may react over several days.
Known as chloro-organic compounds, they
contribute to the odors in water. Furthermore,
they produce a series of unstable residuals.
Chloro-organic compounds, which titrate as
combined chlorine, are also believed to have no
germicidal action. Apart from all these, organic
nitrogen is undesirable because it is a fairly
good indication of recent pollution. It has
therefore been suggested that its amount in raw
waters be limited to 0.3 mg/1. The smaller the
amount of protein, the more available chlorine
is for disinfection.
(b) Hydrogen sulfide
H 2 S is frequently dissolves in underground water
and is common in waters where anaerobic decom-
position has occurred. At a pH value of 6.4 and
below, the H 2 S is completely oxidized, giving rise
to sulfuric acid and hydrochloric acid:At higher pH values, the reaction is thus(c) Iron
The precipitate resulting from the reaction of chlo-
rine and iron (i.e., Fe(OH) 2 ) in water serves two
useful purposes. First, it helps remove iron; sec-
ond, it helps produce a coagulant for the treatment
of the water. The ultimate reaction is thus:H S 4Cl 2 ++ → + 2 4H O 2 H SO 24 8HClH S Cl 22 + →+ S H O 2Table 9.3 Comparison of chlorine and other water disinfectants (From The American Chemistry Council. http://www.americanchemistry.
com/s_chlorine/sec_content.asp?CID=1133&DID=4530&CTYPEID=109. With permission) (Anonymous 2010 c)
Disinfectant Advantages Limitations
Chlorine Gas • Highly effective against most pathogens
- Provides “residual” protection required for drinking
water - Operationally, the most reliable
- Generally the most cost-effective option
- By-product formation (THMs, HAAsa)
- Special operator training needed
- Additional regulatory requirements (EPA’s Risk
Management Program) - Not effective against Cryptosporidium
Sodium
hypochlorite
- Same efficacy and residual protection as chlorine gas
- Fewer training requirements than chlorine gas
- Fewer regulations than chlorine gas
- Limited shelf life
- Same by-products as chlorine gas, plus bromate
and chlorate - Higher chemical costs than chlorine gas
- Corrosive; requires special handling
Calcium
hypochlorite - Same efficacy and residual protection as gas
- Much more stable than sodium hypochlorite, allowing
long-term storage - Fewer Safety Regulations
- Same byproducts as chlorine gas
- Higher chemical costs than chlorine gas
- Fire or explosive hazard if handled improperly
Chloramines • Reduced formation of THMs, HAAs
- More stable residual than chlorine
- Excellent secondary disinfectant
- Weaker disinfectant than chlorine
- Requires shipment and use of ammonia gas or
compounds - Toxic for kidney dialysis patients and tropical fish
Ozone • Produces no chlorinated THMs, HAAs Fewer safety
regulations
- Effective against Cryptosporidium
- Provides better taste and odor control than chlorination
- More complicated than chlorine or UV systems
- No residual protection for drinking water
- Hazardous gas requires special handling
- By-product formation (bromate, brominated
organics and ketones) - Generally higher cost than chlorine
UV • No chemical generation, storage, or handling
- Effective against Cryptosporidium
- No known by-products at levels of concern
- No residual protection for drinking water
- Less effective in turbid water
- No taste and odor control
- Generally higher cost than chlorine
Chlorine
dioxide - Effective against Cryptosporidium
- No formation of THMs, HAAs
- Provides better taste and odor control than chlorination
- By-product Formation (chlorite, chlorate)
- Requires on-site generation equipment and
handling of chemicals - Generally higher cost than chlorine
a THMs trihalomethanes, Haas haloacetic acids