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      Spring 2002 — Vol. 32, No. 1

Effect of lower chlorine dosage at Buffalo WWTP

by Kim N. Irvine, Gary W. Pettibone, Salmatou Bako, Jim Caruso, Roseanne Frandina, Gary Aures, Jane Ork, and Dan Bentivogli

BSA's Bird Island WWTP

The Buffalo Sewer Authority (BSA) investigated the consequences of reduced chlorine dosage and chlorine residual on its final effluent without increasing the risk of potential pathogenic contamination in the receiving water body. The study was conducted at the Bird Island Wastewater Treatment Plant (WWTP).

Web extra: Click here for Background on chlorination (opens new browser window).

Chlorine dosage rates were sequentially reduced in one of four chlorine contact chambers to determine the minimum practicable chlorine residual that still provided adequate disinfection. Factors that influenced disinfection also were examined in making recommendations regarding optimum chlorine residual levels.

At the time of study, the minimum permitted chlorine residual was 0.5 mg/L, although historical practice at the WWTP maintained a chlorine residual in the range of 1.0-1.5 mg/L.

The Bird Island Treatment Plant is the second largest WWTP in New York State and one of the twelve largest in the nation. Between July 1991 and June 1996, the mean secondary sewage flow from the plant was 154 MGD. A catchment area with 675,000 people contributes sanitary and storm flows. During storm events, flows may exceed 500 MGD. Clarified wastewater has a residence time in the chlorine contact chamber of 15 min at design flow of 360 MGD.

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Sampling and analysis

Samples were collected in chlorine contact chamber number 1. This chamber was chosen for study because it had the easiest access for sample collection. Manual grab samples for chlorine residual levels and fecal coliform levels were collected at the upper and downstream ends of the chamber, at a depth of 3 ft (Figure 1). The sample locations and depths were determined based on results of chlorine residual profiling done on May 29, 1996, before the start of the full sampling program.
Figure 1. Chlorine contact chamber number 1 at the Bird Island WWTP

Summer sampling began on June 3, 1996 and was completed on August 20 of that year. The sampling normally was done on Mondays and Tuesdays so that all bacterial analyses could be completed without requiring personnel to work weekends. Samples were collected four times each day: 8:45, 10:30, 12:30, and 14:30.

Winter sampling began on February 10, 1997 and was completed on March 25 of that same year—again, normally on Mondays and Tuesdays—but samples were collected only twice each day, 9:00 and 12:30.

Analysis for the chlorine residual was done in the field immediately upon sample collection using a Hach colorimeter and following Standard Methods. Fecal coliform analysis was done using a 5-tube MPN approach following Standard Methods.
Bird Island Treatment Plant aeration tanks

Treatment plant operators routinely measure various wastewater quality parameters and flow rates as part of SPDES permit requirements. Analysis of these data enabled us to place our sequential dosage reduction tests in the context of general plant operation. Data obtained from the BSA included monthly mean levels of chlorine residual and fecal coliforms, as well as secondary sewage flows for July 1991 through June 1996. Daily mean data from January 1, 1996 through March 31, 1997 also were obtained for the following:
Characteristic Measure
Peak flow MGD
Effluent temperature °C
Effluent pH  
Effluent BOD5 mg/L
Effluent total suspended solids, TSS mg/L
Chlorine residual mg/L
Chlorine dosage lb/day
Fecal coliforms MPN/100 ml


Bird Island Treatment Plant sludge digesters

Effects of chlorine reduction

BSA personnel noted, during the design of the chlorine dosage reduction experiment, that some seasonality in WWTP plant performance could be expected. Their observation was confirmed by examining the geometric monthly mean fecal coliform values for the final effluent (1991-1996). Because the fecal coliform levels tended to be lower in the winter, it was decided to present the results of our summer (1996) and winter (1997) sampling separately. Table 1 presents the results of our sampling.
Table 1a. Daily mean results, summer 1996
Sample date Chlorine residual (mg/L) Fecal coliform (MPN/100 ml) Flow rate (MGD)
6/3/96 0.8  692 264
6/4/96 0.92 63 190
6/10/96 1.0  24 202
6/11/96 0.95 50 210
6/17/96 1.15 47 158
6/18/96 1.0  76 225
6/25/96 1.1  28 158
6/26/96 0.95 33 147
7/1/96 1.08 32 160
7/2/96 0.82 33 146
7/8/96 0.78 110 130
7/9/96 1.22 20 154
7/15/96 0.3  12023 197
7/16/96 0.92 40 155
7/22/96 0.58 55 159
7/23/96 0.42 339 142
7/30/96 0.39 575 210
8/5/96 0.41 912 161
8/6/96 0.35 224 150
8/12/96 0.48 1660 152
8/13/96 0.48 1662 158
8/19/96 0.76 40 138
8/20/96 0.88 44 160

Table 1b. Daily mean results, winter 1997
Sample date Chlorine residual (mg/L) Fecal coliform (MPN/100 ml) Flow rate (MGD)
2/10/97 0.8  20 168
2/11/97 0.82 78 142
2/17/97 1.0  20 144
2/18/97 0.95 37 168
2/24/97 0.85 28 160
2/25/97 0.9  20 172
3/3/97 0.75 47 182
3/4/97 0.75 50 180
3/10/97 0.65 288 196
3/11/97 0.75 126 212
3/17/97 0.72 83 185
3/18/97 0.72 132 168
3/24/97 0.95 28 178
3/25/97 0.6  141 190

Notes to Tables 1a and 1b:
-Mean value for fecal coliforms is geometric mean; all others are arithmetic mean.
The high fecal coliform level observed on 7/15/96 was related to a temporary plant upset (a solids washout).
Results of all samples can be obtained from the authors. For brevity, we show only the results for the downstream end of the chlorine contact chamber.

A correlation analysis was done to investigate possible relationships between the sampled chlorine residual, the logarithm of fecal coliform levels, and secondary sewage flow (as a surrogate for flow rate through the individual contact chamber). We found a significant correlation (alpha=0.05) between fecal coliform levels in the summer and flow (0.26) and between fecal coliform levels and chlorine residual (-0.69). In the winter, we found a significant correlation (alpha=0.05) between fecal coliform levels and flow (0.54) and fecal coliforms and chlorine residual (-0.75).

Given the significant relationship between fecal coliform levels and chlorine residual for both seasons, a simple linear regression analysis was done. Table 2 and Figures 2 and 3 summarize the results. The Student's t values indicated that the slope of the regression line (b1) was significant (alpha=0.05) for each equation in Table 2.
Table 2. Regression results: chlorine residual vs. fecal coliform level
Site Sample season Equation
Down- stream Summer log F.C./100 ml = 3.5336 - 1.8767(chlorine residual, mg/L) 0.482
Down- stream Winter log F.C./100 ml = 3.2757 - 1.9115(chlorine residual, mg/L) 0.559

Regression results between fecal coliforms and chlorine residual at the downstream sample site. See Table 2 for regression equation.
Figure 2. Regression results, summer 1996

Figure 3. Regression results, winter 1997

Regression results, although significant, exhibited considerable scatter or “unexplained variation.” It appears that while chlorine residual was an important determinant, other factors also affected the level of fecal coliforms. The only other data that were available for our sample times were plant flows; the plant operator recorded this value about hourly.

Qualitative inspection of the data indicated that during storm events, when suspended solids levels and flow increased due to runoff, the fecal coliform levels also increased. It appears that storm water runoff in some way influences the effectiveness of chlorination.

Plant flow, therefore, was used as a second predictor variable in a multiple regression analysis for the summer sampling. A multiple regression with flow was not done for the winter sampling because chlorine residual and flow were significantly correlated.

The multiple regression equation for the summer data was calculated as:

log F.C./100 ml = 2.64 + 0.00521(flow, MGD) - 1.87(chlorine residual, mg/L)

Both of the predictor coefficients were significantly different from 0 (p <= 0.001) and the r² value increased from 48% to 55% (0.55).

Data from plant and contact chamber number 1

Samples for discussion purposes in this paper were collected at the downstream end of chlorine contact chamber number 1. However, samples routinely collected by WWTP operators for permit purposes represent the final effluent, and they comprise a mix of flow from all operating contact chambers. To investigate how representative data from contact chamber number 1 was of the general plant discharge, we examined mean chlorine residual and fecal coliform levels for common sample days:

  • We found general agreement between data from contact chamber number 1 and the routine plant data for both the summer and the winter, but the chlorine residual level for contact chamber number 1 generally was higher.

  • The correlation between mean chlorine residuals from contact chamber number 1 and the plant data during the summer of 1996 was 0.78; the correlation for the geometric mean fecal coliform levels was 0.61.

  • The correlation between mean chlorine residuals from contact chamber number 1 and the plant data during the winter of 1997 was 0.51; the correlation for the geometric mean fecal coliform levels was 0.1.

The lower correlation between fecal coliform levels in the winter may be related to the general limited variability in the data and the higher fecal coliform values reported for our samples collected on 2/11/97 and 3/10/97.


Variability of fecal coliform levels

The WWTP is a complex engineered system, and several factors appear to have a significant influence on fecal coliform levels. Our data for the downstream sample site indicated that there was a significant inverse relationship between chlorine residual and fecal coliform levels. It is not unexpected that as chlorine residual levels rise (to some limit), the levels of fecal coliforms will fall.

A significant positive relationship also was observed between peak flow rate and fecal coliform levels. The higher peak flow rates generally were associated with storm-generated runoff discharging to the combined sewer system in Buffalo. Fecal coliform levels may be higher with the greater flow rates because of a shorter and less efficient contact time. The contact efficiency may itself be influenced by the levels and quality of total suspended solids as well as mixing efficiency in the contact chambers.

A more detailed study of chlorine contact chamber hydraulics and the three-dimensional distribution of bacteria in the chamber would have to be done to determine whether the contact time or mixing efficiency were more important in optimal disinfection.

Together, chlorine residual and peak flow rate accounted for over half of the variability observed in the summer fecal coliform levels. However, other factors clearly need to be considered in explaining some of the remaining variability. For example, total suspended solids concentration, size distribution, and organic carbon content influence bacteria dynamics in natural systems, and these relationships may extend to engineered systems. Certainly, the physical and chemical characteristics of the flocculated suspended sediment in the combined sewage varies both through and between storms and may vary seasonally. Suspended solids concentrations in combined sewage also typically are greater than in sanitary flow.

Others have noted that variable influent conditions (including industrial spills) can be an important cause of plant upsets. The variable influent conditions of suspended sediment flocs and attached bacteria, therefore, may influence the relationships observed for plant effluent. It is generally acknowledged that there is a complex relationship between sludge floc characteristics (including the capacity to settle and the presence of filamentous organisms) and dissolved oxygen levels, nutrient levels, nature of organic substrate, sludge age, and reactor configuration. These complex and dynamic interactions may confound a fully satisfactory explanation of the variability in fecal coliform levels.

Fecal coliform levels were less variable during the winter sample period than during the summer sample period, with the coefficient of variation in the winter being 0.204 compared to the summer value of 0.267. The higher variability in the summer may be related to a greater number of storm events, general plant upsets, and periodically incomplete nitrification (resulting in some chlorine reacting with the nitrites rather than being involved with disinfection). Furthermore, the greater solubility of chlorine in colder water and lower temperatures may produce a greater control on bacterial levels in the winter.

Fecal coliform and chlorine residual levels

The SPDES permit limit for fecal coliforms in the final effluent is a 30-day geometric mean of 200 MPN/100 ml or a 7-day geometric mean of 400 MPN/100 ml. These limits are based on a minimum of 1 sample/day (although the BSA routinely takes 3 samples/day). Because we sampled only on 2 consecutive days each 7-day period, direct comparisons of our results cannot be made with the permit levels. Still, we chose to use the 400 MPN/100 ml level as a guideline in the discussion of our results for the bacteria and chlorine residual levels.

A qualitative inspection of our data for the summer (Table 1a) suggests that a chlorine residual level of at least 0.6-0.7 mg/L is required to maintain fecal coliform levels within the regulatory limit. See, in particular, sample dates 7/2, 7/8, 7/22, 8/19, 8/20. These five sample dates appear unaffected by storm events and exhibit fecal coliform levels < 400 MPN/100 ml.

Chlorine residual values < 0.6 mg/L may be associated with acceptable fecal coliform levels. See, for example, sample dates 7/23 and 8/6 with residuals of 0.35 and 0.42 mg/L. Nevertheless, higher fecal coliform levels were observed with greater frequency at these lower chlorine residual values. For example, see data for 8/5, 8/12, 8/13.

The daily fecal coliform (geometric mean) and chlorine residual (mean) values for the final effluent, as reported by the BSA for 6/2/96—8/20/96, support the results of our sampling.
Final effluent values, June 2-Aug 20, 1996
# days Cl residual Days with fecal coliform >400 MPN/100 ml
10 0.30-0.39 mg/L 50%
13 0.40-0.49 mg/L 31%
1 0.50-0.59 mg/L 0%
18 0.60-0.69 mg/L 22%
Data available from the authors but not reproduced here. In summarizing these data, no attempt was made to exclude days that potentially were affected by storm events.

The BSA sample results indicated that it may be possible to reduce the chlorine residual level to the 0.4-0.49 mg/L range, but to minimize the problems of non-compliance with fecal coliform levels, the chlorine residual should be maintained in the 0.5-0.7 mg/L range in the summer.
Bird Island Treatment Plant sludge press

During the winter sampling, the lowest chlorine residual levels were in the 0.65-0.75 mg/L range. Table 1b indicates that there were no problems with high fecal coliform levels (>400 MPN/100 ml) at any time during the winter sampling period.

Similarly, the BSA data for our sample period 2/10/97—3/25/97 (data available from authors, but not reproduced here) show that the lowest chlorine residual values were in the 0.6-0.65 mg/L range. Chlorine residual values in this range were observed on 21 days during the sample period, and no days exhibited fecal coliform levels greater than 400 MPN/100 ml. Our data and the data from the routine BSA sampling suggest that it may be possible to maintain a chlorine residual level even lower than 0.6 mg/L during the winter.

Lesson for treatment plant operation

Our study indicated that it was possible to operate at lower chlorine residuals without violating permit limits. As a result, the BSA target chlorine residual range was set at 0.5-0.7 mg/L during the summer and 0.3-0.5 mg/L during the winter.

Our study has benefited both the BSA and the environment. The reduction in chlorine dosage reduces the potentially undesirable environmental effects of chlorine by-products while maintaining acceptable disinfection. The BSA was able to reduce the sodium hypochlorite dosage by over 180,000 gal/yr. This action also saved over $65,000/yr.

Acknowledgements Funding for this study was provided by a grant from the New York State Great Lakes Protection Fund administered through the NYSDEC.

Kim N. Irvine is in the Department of Geography/Planning and with the Great Lakes Center at the State University College at Buffalo. Gary W. Pettibone is in the Department of Biology and with the Great Lakes Center at the State University College at Buffalo. Salmatou Bako is with the Department of Biology at the State University College at Buffalo. Jim Caruso is with URS Corporation in Buffalo; he was with the BSA at the time of this study. Roseanne Frandina, Gary Aures, and Jane Ork are with BSA at the Bird Island Sewage Treatment Plant. Dan Bentivogli is with R&D Engineering, Inc. in Buffalo; he was with the BSA when this study was conducted.

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