Guidance Manua pdf

 

44

• 

Prechlorination ahead of presedimentation basins is effective but increases DBP 

levels in finished waters by 10% to 25%.  

 

• 

Liquid copper product (copper sulfate, Earthtec) ahead of presedimentation 

basins is effective.   

 

• 

Powdered activated carbon (PAC) addition (> 3 mg/L) in presedimentation 

basins limits light penetration required for algae growth.  PAC is removed near 

the head of sedimentation basins, so PAC provides limited algae control at 

sedimentation weirs or in filter basins. 

 

Prechlorination treatment during periods of warm water (> 20

o 

C) could be conducted 

once every week for algae control.  Prechlorination doses are selected to give a residual 

prior to additional chlorine addition near filters.  Direct filtration and conventional WTPs in 

the metro-Phoenix region only remove 5% to 15% of the TOC.  TOC can react with 

chlorine to form DBPs (THMs and HAAs).  Therefore, delaying the point of chlorine until 

after TOC  removal (after sedimentation basins) would be advantageous.  Prechlorination 

for 24 hours increases DBP levels leaving the WTP for that period.  Prechlorination 

should not take place concurrent with or prior to PAC addition because chlorine reacts 

with the PAC and reduces its effectiveness to adsorb MIB or geosmin.  Prechlorination 

has not been observed to lyse algae cells resulting in release of MIB or geosmin. 

 

Liquid copper products (e.g., copper sulfate, Earthtec) can be fed (0.3 to 0.8 mgCu/L) at 

the head of presedimentation basins to reduce algae growth.  Copper is toxic to algae at 

low levels.  Copper addition for 12 to 24 hours once every 9 to 14 days should control 

algae growth.  Excessive copper addition can lead to growth of copper-resistant algae, 

and accumulation of copper in WTP and wastewater sludges.  Recommended copper 

doses are below action levels for the Lead and Copper Rule.  Copper addition has not 

been observed to lyse algae cells resulting in release of MIB or geosmin. 

 

5.5.2  Powdered Activated Carbon (PAC) Adsorption 

 

Geosmin is removed more efficiently by PAC than MIB.  Given that MIB concentrations 

are usually greater than geosmin concentrations selection of PAC feed doses is usually 

based upon raw water MIB concentrations.  The process of adsorption of MIB and 

geosmin by PAC takes time (> 1 hour).  Therefore, maximizing the contact time between 

PAC and water is critical.  PAC should be added and well-mixed prior to 

presedimentation basins. Chemical tracer tests should be conducted on 

presedimentation basins to assure there is no short circuiting within the basins, thus 

maximizing contact time between the PAC and water.  Based upon suspended solids 

analysis, hydraulic retention time (HRT) in the presedimentation and flocculation basins 

only “count” towards PAC contact time.  PAC is removed within the first 25% of the 

length of the sedimentation basins.  The HRT of the presedimentation basin plus 

flocculation basin should exceed one hour.  If HRTs are less than one hour, the PAC 

dose should be increased by 25%. 

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45

Not all brands of PAC remove MIB or geosmin equally.  Bid selection criteria for PAC 

suppliers should be partially performance-based.  One method for bid selection includes 

determination of an Index Value for each PAC brand based upon simple laboratory PAC 

tests.  Appendix D shows a proposed test protocol developed during this study. 

 

After selection of a PAC brand, a dose-MIB removal nomograph should be developed.  A 

dose-MIB removal plot for Norit 20B is shown in Figure 5.10.  The graphs are developed 

by conducting experiments similar to those used to develop the Index Values, but with a 

single PAC brand and two or three different initial MIB concentrations (e.g., 30, 50, 70 

ng/L).  In separate experiments the PAC dose is varied (e.g., 1, 3, 5, 8, 12, 16, 20, 25, 

30, 40 mg/L).  After the prescribed contact time the samples are syringe-filtered and 

analyzed for MIB.  Data are plotted as C/Co versus PAC dose, where C is MIB in 

finished water and Co is MIB in raw water.  For different initial MIB concentration the 

C/Co versus PAC dose should overlay, since fractional MIB removal is independent of 

initial concentration.  All the data together should be plotted using a best-fit equation 

(e.g., exponential fit  – Equation 5.5).  The best-fit equation can be used directly to 

compute a PAC dose (see below) or used to generate a dose-MIB removal nomograph 

(e.g., Figure 5-10). 

 

0

5

10

15

20

25

30

35

40

0

5

10

15

20

25

30

35

40

45

50

20B PAC dose (mg/L)

Finished Water MIB (ng/L)

90

80

70

60

50

30

40

20

10

100

 

Figure 5-10.  Dose response MIB removal using Norit 20B. 

 

 

The following equation was developed for Norit 20B with a 4-hour contact time: 

 

(

)

0927

.

0

ln

/

























−

=

raw

finished

MIB

MIB

L

mg

PACDose

 

 

Equation 5.5 

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