MERCER02-14
7
products may be a concern with chlorination (e.g., chlorinated hydrocarbons and tri-
halomethanes) and they should bemonitored for environmental safety.
Liquid sodium hypochlorite has proven to be effective, is easily available and the purchase cost
is relatively low. However, safe transport and handling, and on-site storage of large volumes
should be taken into consideration.
Electrochlorination uses a direct current to create an electrolytic reaction to transform salts in
seawater into sodium hypochlorite; thus, no liquid chlorine is stored on-site. Both systems may
need supplementary brine to be effective in low salinity (typically less than 10 ppt) and
freshwater. The initial startup costs, power consumption, and system complexity (which may
require more training) of electrochlorination systems should be weighed against the constant
influx of liquid chemicals, the storage footprint, and the safe handling of a liquid chlorination
system.
Both experimental studies and actual full-scale operational applications have demonstrated that
chlorine residual doses as low as 1.0 ppm can prevent settlement and growth of invertebrates
(Venkatesan and Murthy 2008, Surry Nuclear Power Station personal communication).
However, an intermittent or pulse chlorination dose of 1.2 ppm at a frequency of 0.5 to 2 hours
was sufficient to control the settlement and growth of higher organisms and slime formation. As
described in the case studies below (Section 8), a pulsed approach of dosing the cooling water
systemwith relatively low doses of sodium hypochlorite at less than 3.0 ppm for 2 hours every
24 hours can meet EPA discharge requirements and be effective at controlling
G. franciscana
and other fouling growth. However, some applications of pulse chlorination to control the
growth of
G. franciscana
have not proven successful (Entergy Sabine Power Station, personal
communication).
Examples:
A. EvoquaWater Technologies, Chloropac
Chloropac systems fromEvoqua Water Technologies are designed to help keep intake and
cooling water systems free of biofouling, and help maintain the efficiency of heat transfer.
Chloropac systems are designed andmanufactured in fully-assembled, multi-skid packages. The
Chloropac electrolytic cell assembly consists of two concentric titanium tubes between which
salt water (seawater) flows. By passing electric current through the salt water, Chloropac
converts sodium chloride into sodium hypochlorite. Chloropac does not require back-flushing or
periodic cleaning. The cell has been specifically designed to operatewith turbulent high velocity
flows over the total electrode surface, preventing precipitation of magnesium and calcium
hydroxides, and keeping the cell continuously clean of calcareous deposits.
B. NALCO, CoolingWater Treatment Biocontrol
NALCO provides oxidizing biocides to control cooling water biofouling, including open
recirculating cooling towers, once-through cooling systems, closed loop cooling systems, and
cooling ponds. A combination biocide approach is currently used to control the growth of
G.
franciscana
and other macrofouling organisms at another power plant on the Chesapeake Bay.
The combination includes injection of liquid sodium hypochlorite and the NALCO patented
Acti-Brom (sodium bromide, described in Section 4.1.1.2.) that may reduce the amount of
oxidizing biocide required to control fouling in coolingwater.
