A Carolina Comparison

Conventional settling basin clarification squares off against DAF system

The Greenville Water System in Greenville, S.C., draws water from three sources—Table Rock Reservoir on the South Saluda River, Poinsett Reservoir on the North Saluda River and Lake Keowee. Table Rock and Poinsett Reservoirs are both located in the foothills of the Blue Ridge Mountains in northern Greenville County. The source waters from the mountain reservoirs were once so clear that they were unfiltered and only treated with chlorine for over 70 years. In the mid 1980s, the Adkins Filtration Plant, which utilized conventional settling basin clarification technology for 30 mgd design capacity, was built to use Lake Keowee as its source water. This plant was expanded in 2003 to increase capacity to 60 mgd using the same settling basin clarification technology. Increasing regulations required all surface water systems to be filtered, so in July 2000, a new state-of–the-art filter plant was placed in service to provide filtration for all the water drawn from the Table Rock and Poinsett Reservoirs. The new Table Rock and North Saluda Filtration Plant, designed at 75 mgd, uses a Clari-DAF system for clarification in its treatment scheme. 

The Adkins Filtration Plant uses coagulation, sedimentation, filtration and disinfection to treat source water. Alum is used as a coagulant, along with sodium hydroxide (caustic) for pH adjustment. Chlorine, combined with ammonia, is used for disinfection, while a zinc polyphosphate is added for corrosion control. Fluoridation is provided to promote healthy teeth.

The Table Rock and North Saluda Filtration Plant uses the flotation process instead of sedimentation for particle removal. The remaining processes and chemicals used are similar to those at the Adkins plant. The raw water quality of the two plants is very similar—low turbidity, low hardness and low alkalinity. Listed in Table 1 is the average water quality data for raw, clarified and finished water for the two-year time period from August 2003 to July 2005.

Table 1

 

Parameter

 

Adkins

TR/NS

Raw

pH

 

6.55

6.55

 

Alkalinity

mg/L

8.89

6.95

 

Temp

C

21.70

19.20

 

Hardness

mg/L

5.00

4.20

 

Turbidity

ntu

1.20

0.49

 

 

 

 

 

Clarified

Turbidity

ntu

0.37

0.20

 

Chlorine

mg/L

1.70

0.20

 

 

 

 

 

Finished

pH

 

7.10

7.45

 

Alkalinity

mg/L

10.70

8.70

 

Temp

C

21.40

19.00

 

Hardness

mg/L

5.25

4.40

 

Turbidity

ntu

0.04

0.05

 

Chlorine

mg/L

2.65

2.60

There is a significant difference in the footprint required for the processes at the two facilities. Table 2 compares them.

Table 2

 

Adkins

TR/NS

Total Plant

 

 

     Area (sq ft)

147,342

31,908

     Flow (mgd)

60*

75

* Plant can operate at higher flow if required.

The footprint for processing 60 mgd by the conventional settling basin system at Adkins is 147,342 sq ft. This results in a requirement of 2,456 sq ft per million gal of water processed; however, the existing footprint can deliver 90 mgd, resulting in a requirement of 1,637 sq ft per million gal of water processed. The total area for processing 75 mgd through the DAF system at Table Rock and North Saluda is 31,908 sq ft. This results in a requirement of 425 sq ft per million gallons of water processed.

By selecting the Clari-DAF process, the land footprint requirement for Table Rock and North Saluda was reduced by 82.7%.

The average chemical dosage for the two-year time frame of August 2003 to July 2005 is listed in Table 3.

Table 3

Parameter

Adkins

TR/NS

Diff

% Diff

Alum mg/L

11.1

11.4

(0.3)

(3)

Caustic mg/L

12.7

9.1

3.6

28

Chlorine mg/L

7.7

3.2

3.5

45

Aq Ammonia mg/L

1.4

0.6

0.8

57

Given the fact that the raw water turbidity was lower at Table Rock and North Saluda, it would be expected that the alum coagulant dosage would be higher. The lower the turbidity, the harder it is to clarify the water and, subsequently, the higher the inorganic coagulant dosage. However, the reduction at Table Rock and North Saluda is due to the fact that flotation can effectively remove smaller-diameter solids. Some of the increased caustic feed at Adkins can be attributed to the higher alum feed that depresses the pH for coagulation. The increased chlorine and ammonia feed at Adkins is due to the fact that a higher clarified combined residual is maintained through the clarification process at Adkins in order to meet CT requirements. 

Filter performance data listed in Table 4 was collected from one filter at Adkins and one filter/train at Table Rock and North Saluda (three total filters) for the August 2003 to July 2005 time frame:

Table 4

Parameter

Units

Adkins

TR/NS

Surface Area

 sq ft

1342

792

Total Volume

million gal

21.59

16.817

Average Filter Run

hrs

126

118

Average Backwash

gal

294458

269125

Unit Filter Run time Vol.

million gal per sq ft

17697

20984

Backwash (%) Range

%

0.77–2.41

0.88–2.42

Filters 1, 4 and 9 at Table Rock and North Saluda averaged significantly higher unit filter runtime volumes (18.6%) than Filter 6 at Adkins.

The average in-plant water usage rates for the same period are compared in Table 5:

Table 5

Parameter

Adkins

TR/NS

Diff

% Diff

 % In-Plant Use

9.47

3.84

5.63

59.45

There is a major difference and, therefore, an associated cost savings due to reduced in-plant water usage. The overall cost of water accelerates with higher figures due to the fact that the in-plant usage is water that has already been processed and, therefore, has an associated multiplied cost. Some of the difference is the result of shorter filter runs and additional backwash water required to wash the filter. In addition, some of the difference is associated with the sludge content of the solids removed from the Adkins conventional settling basin (less than 0.5%) versus the Clari-DAF unit solids thickening (to over 2%) for removal from the Table Rock and North Saluda flotation clarification process. The Adkins CT2 submerged sludge collector consumes 2.6 million gal of water per cycle per day.

Table 6 compares the expenses incurred and the total water production at each treatment plant for the two-year timeframe of August 2003 to July 2005:

Table 6

Parameter

Adkins

TR/NS

Diff

% Diff

Total Flow x 106 (gal) 

10,664

22,918

+12,254

+53

Treatment

Expense ($)

1,892,941

1,727,882

-165,059

-8.7

$/1000 gal

0.18

0.08

-0.10

-56

Again, there is a major reduction (56%) in cost per thousand gal at the Table Rock and North Saluda Plant.

Considering similar raw water conditions, with the capability of producing effluent water below maximum contaminant level along with the potential of efficient sludge handling, selecting the Clari-DAF system as a replacement for standard sedimentation technology has resulted in significant chemical and operating cost savings. The footprint required per million gal is one-sixth the size, the chemical consumption is lower, unit filter runtime volumes are 18% longer, the total cost per thousand gal is 56% lower and in-plant water usage is 59% lower.

Company Information

227 S. Division Street
Zelienople
PA
724.452.6300
Exterior view of the facility.
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    Exterior view of the facility.
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    The Clari-DAF system at work.