Journal of Undergraduate Research
Volume 7, Issue 1 - September/October 2005

The Effects of Chlorine and Sunlight on Algae in the Kanapaha Water Reclamation Facility Chlorine Contact Basins

Christina Akly

ABSTRACT

In order to grow, algae need sunlight and a surface that can provide nutrients and is almost permanently wet. With this in mind, algae growth in the chlorine contact basins (CCBs) at the Kanapaha Water Reclamation Facility (KWRF) was proposed to be controlled by covering the basins, which would prevent the exposure of wastewater to sunlight. Two factors were directly altered by covering the CCBs, solar radiation was decreased and chlorine residual increased. Controlling these factors was expected to decrease the algal growth in the CCBs. Two different studies were performed in order to account for these changes: a pilot study and a full-scale study. The pilot study did not show any significant influence of solar radiation in the opaque and transparent basins. It did show, however, that chlorophyll α was immediately degraded after the addition of chlorine. From the full-scale study, 6 out of the 9 samples analyzed showed a difference in chlorophyll α concentration. The average difference was 21.6% lower for the covered basin. Despite this decrease, the data did not provide statistical evidence to prove that the difference was significant. Similarly, there was no significant difference in total suspended solids (TSS) concentration, and therefore the suggestion that algae formation would be lowered in the covered basin could not be supported. Even though statistical tests resulted in poor evidence, trends on increased solar radiation and chlorine concentration were found to have some influence in the chlorophyll α concentrations.

INTRODUCTION

The Kanapaha Water Reclamation Facility (KWRF) is an advanced wastewater treatment plant operated by Gainesville Regional Utilities (GRU) that serves the northwest and southwest areas of Gainesville, Florida. KWRF was opened in 1977 and has a current permitted capacity of 14.9 million gallons per day (mgd). The plant treats the wastewater to drinking water standards and most of the water effluent is used for irrigation, reuse, and injection into groundwater. This plant consists of the following processes: preliminary treatment, sludge thickening facility, anoxic basins, aeration basins, secondary clarifiers, a mixed liquor splitter box, Severn-Trent filters, an RAS/WAS (return activated sludge and waste activated sludge) pumping station, a post aeration basin, and chlorine contact basins. These processes are shown in an aerial view in Figure 1, and a process flow diagram is presented in Figure 2.

Figure 1. Aerial View of the Kanapaha Water Reclamation Facility (2004)

Figure 2. Process Flow Diagram of Kanapaha Water Reclamation Facility

In order to achieve the high standard goals that KWRF has set, disinfection is an important part of the treatment process. The disinfection process is carried out in two chlorine contact basins (CCBs) that are opened to the atmosphere and exposed to sunlight. Preliminary investigations carried out by a team of the Integrated Product and Process Design (IPPD) have indicated that sunlight has a number of adverse effects on the disinfection efficiency in the CCBs. One of these adverse effects is the algal formation in these basins. The presence of algae in these basins was visually confirmed by the IPPD team after an inspection inside one of the CCBs that was not operating at the time.¹ By looking at the clarifiers, significant algal formation can be observed, as shown in Figure 3 below, which is a picture of one of the actual clarifiers. Chlorination does not seem to completely remove this algal growth, since algae is still found in the CCBs.

Figure 3. Algae Formation in the Clarifiers at KWRF

Most algae are phototropic organisms that can fabricate their own food materials through photosynthesis by using sunlight, water,and carbon dioxide. Most algae contain chlorophyll α, a molecule that absorbs the light to enable photosynthesis. Chlorophyll α absorbs light in the blue (450 nm) and red (650 nm) wavelengths, as shown by the two peaks in Figure 4, and emits light in the green wavelength.²

Figure 4. Absorption spectrum of chlorophyll α
(Steer, James. Structure and Reactions of Chlorophyll)

Some of the negative consequences of algal formation in the CCBs are:

• Formation of disinfection byproducts (DBPs) from chemical releases of algae during chlorination
• Formation of solids at the bottom of the basins that can interfere with the chlorine efficiency.

In addition to the formation of algae, other aspects of concern in KWRF are the DBPs and haloacetic acids (HAAs) formation related to chlorination. The form of chlorine used for the disinfection process is chlorine gas, which is known to react with naturally occurring organic matter in wastewater to form DBPs. Similarly, the loss of chlorine residual in the CCBs is a potential problem that directly affects the cost of disinfection. These aspects were investigated by Heather Fitzpatrick, a graduate student in the department of Environmental Engineering Sciences for her thesis project.

The study was conducted at the KWRF, and it consisted in two parts: a pilot study and a full scale study on the CCBs. For the pilot study, two chlorine contact basins of 10.12 ft³ capacity each were built. One of them was covered with a UV-transmitting clear acrylic cover that allowed the penetration of both visible and UV radiation, to resemble the actual conditions in the treatment plant. The other basin was covered with a dark plastic top to prevent from direct sunlight irradiation. These two basins are referred to as transparent and opaque, respectively. The pilot study was set up in the area between the clarifiers and the filters as indicated in Figure 2. The influent water for the pilot basins was coming from the clarifiers’ effluent, and the experimental basins and instrumentation were set up as shown in Figure 5. Chlorine, in the form sodium hypochlorite (NaOCl, household bleach), was added steadily to the basins at different concentrations, hydraulic retention times (HRT) and wastewater flow rates. Chlorine concentrations ranged from 6 to 16 mg/L. Sulfuric acid was added in some occasions to keep the pH around neutral or lower, since it was expected to increase because of the basic form of chlorine used. The maximum retention time applied was 3.81 hours, while the average HRT used for most of the runs was 2.75 hours.

Figure 5. Pilot Study Schematics

The full-scale study consisted of covering one of the actual CCBs in KWRF with a tarp to prevent solar radiation while leaving the other CCB open to the atmosphere in order to assess changes influenced by sunlight.

This paper presents a study on the interactions of algae with different factors including solar radiation and chlorination, as well as its effects in the chlorine dosing concentration. Similarly, other factors such as total suspended solids (TSS) are correlated to chlorophyll α concentrations to assess the presence of algae in the CCBs.

HYPOTHESIS

1. The formation of chlorophyll α is expected to be favored by the presence of higher solar radiation in direct contact with the wastewater. This would correspond to a higher chlorophyll α concentration in the uncovered basin as compared to the covered one.
   
2. The concentration of chlorine is expected to decay by UV irradiation as described by Equation 1. Therefore, lower chlorine concentrations are anticipated for the uncovered basins than for the covered CCBs.
   
3. Chlorination is expected to reduce the formation of algae, so at higher chlorine concentrations, the formation of algae is expected to be lower.
   
4. Algae concentrations and total suspended solids are expected to have a positive correlation.

METHODOLOGY

Because of its importance in the photosynthetic process that algae undergo, sunlight can help predict the productivity of algae. Sunlight was measured as solar radiation. The solar radiation and terrestrial radiation can be divided in different wavelength ranges: ultraviolet (.2-.39 μm), visible (.39-.78 μm), near-infrared (.78-4.0 μm) and infrared (4.0-100.0 μm). Most of the solar radiation is contained in the region from 0.3 μm to 3.0 μm. The global horizontal solar radiation is part of the solar radiation, and it is composed by two components of sunlight falling together on a horizontal surface, i.e., the component of sunlight and the diffuse component of skylight. Global horizontal radiation was measured by a Black and White Pyranometer (Model 8-48) and ultraviolet radiation by a Total Ultraviolet Radiometer (Model TUVR), respectively, both manufactured by the Eppley Laboratory, and they are shown in Figures 6 and 7. The pyranometer captures energy in the wavelength range between 0.285 to 2.8 μm while the radiometer is limited to the wavelength interval from 0.295 to 0.385 μm. The measurements collected by these instruments were recorded by a data logger every five minutes in units of mW/cm².

Figure 6. Black & White Pyranometer

Figure 7. Total UV Radiometer

Sample Collection and Preservation

For the case of the pilot study, samples were collected at 3 different locations during 8 days. The sampling locations are labeled in Figure 5 with number 1 representing the effluent coming from the clarifier (5INT), number 2 the effluent from the transparent CCB (5TRU), and location 3 the effluent from the opaque CCB (5TRC). The samples were taken at 3 different times of the day: 9:00 am, 12:00 pm, and 2:00 pm to allow enough detention time between sampling. The collected samples were preserved in a cooler with enough ice to keep the samples cold until the time of analysis in the lab.

During the full-scale study, two samples were collected from the filter effluent, samples labeled Filter 1 and Filter 2 (locations 1&2, Fig. 2), one sample was collected at the post aeration effluent, 5PA, (location 3, Fig.2), one at the influent of the chlorine contact basins, right after chlorination, 58S, (location 4, Fig.2), and two samples were collected at the CCBs effluent, one from each basin, the covered, 53N (location 5, Fig.2) and the uncovered one, 53S (location 6, Fig.2). The samples were collected for 3 days at the same times as the pilot study and preserved in a similar manner.

The parameters analyzed in the field were chlorine residual, pH, water temperature, conductivity, and dissolved oxygen. The other parameters such as chlorophyll α concentration, THMs, and HAAs were analyzed in the lab. The total suspended solids were analyzed by the KWRF lab.

Sample Analyses

As mention above, field sampling analysis included the chlorine residual measurement, total and free chlorine residual, which was analyzed with a Portable Datalogger Spectrophotomer, Model DR/2010 (UF # 4910AA 151134), manufactured by Hach Company. The other parameters measured were obtained using different portable meters calibrated in advance in the lab.

Algae samples were analyzed using the Method 445.0, a standard method proposed by the Environmental Protection Agency (EPA) for the determination of chlorophyll α in marine and freshwater algae. A detailed description of this method is attached in Appendix A. Method 445.0 uses the principle of fluorescence, a physical property that allows certain atoms and molecules to absorb light energy at one wavelength when they are in an excitation state, and instantaneously re-emit light energy at another wavelength when they return to their ground state, since the chlorophyll α molecule has this ability.

The samples of water obtained from the wastewater treatment plant were slowly filtered through a glass fiber filter of 47mm manufactured by Whatman Company. The filters, together with a 90% acetone solution used to extract the pigment from the phytoplankton, were then placed in a mechanical tissue grinder where they were macerated. The resulting filter slurry was allowed to steep for a minimum of 2 hours, and in most of the cases for 24 hours in the dark at 4°C to ensure complete extraction of the chlorophyll α. The samples were later centrifuged for 15 minutes and the clear solution was immediately placed in a glass cuvette to measure the fluorescence in a fluorometer. The fluorometer used was the Laboratory Fluorometer Model TD-700 (UF # 4910AA) manufactured by Turner Designs.

The fluorometer was previously calibrated by using the Multi-Optional Mode-Direct Concentration calibration procedure, which provides the actual unknown concentration directly without using any conversions. The calibration curve obtained showed linearity in the range of 0 to 45 μg/L of chlorophyll α. Assuming this linearity range, the samples were diluted so that all of them fell within these values. To assess the quality of the method, a quality control test was run. This test consisted of four samples obtained at 9:00 am from the clarifier’s effluent (location 1, Fig. 5). According to the Method Detection Limit (MDL) performed in these samples, the lowest detected chlorophyll α concentration corresponded to a value of 0.029 μg/L. Adopting this value as the lowest possible accurate chlorophyll α concentration, all of the samples included for the analysis were only considered valid if they were above the lowest detection value.

RESULTS AND DISCUSSION

Pilot Scale Study

The pilot study results showed that the concentration of chlorophyll α dramatically decreased from the influent to the pilot CCBs after the chlorine was added. The concentrations of chlorophyll α obtained at location 1 in Figure 5, which corresponds to the clarifier’s effluent, were very high as compared to the concentrations obtained from locations 2 and 3 (Figure 5) that represent the CCBs effluent. As shown in Figure 8, samples 5TRU and 5TRC (transparent and opaque basins, respectively) were 74% to 98% lower in chlorophyll α concentration than the 5INT samples.

Figure 8. Chlorophyll a oncentrations for all samples taken during the pilot study

It is possible to infer that chlorine adversely interacted with chlorophyll α to create such decay in concentration. These results were further supported by the concentrations obtained for the TSS samples taken at the same times and locations. A minimum of 25% and a maximum of 97% of TSS reduction were recorded in these basins.

The results from this study concord with trends from previous findings on the effects of chlorination and algal formation. For example, Cadwell reported that an algal removal of 64% was accomplished in a 14-hour detention chlorine-contact pond when the applied chlorine dose was 12 mg/L.³ Similarly; Bowen also observed reductions of TSS as a result of chlorination. Except during winter periods of unusually high chlorine demand, when a residual of approximately 2 mg/L was maintained in the effluent of the Peterborough, New Hampshire oxidation ponds system, an average reduction of 53% of TSS between the point of chlorination and the point of discharge from the contact basin was obtained.⁴

Even though the decrease of chlorophyll α after chlorination in the basins was anticipated, the percent reduction was unexpectedly high. Further studies were carried out to investigate this phenomenon more in depth. Laboratory experiments were run to observe the chlorophyll α reduction after chlorination as a function of time and investigate whether the decrease was the result of the interaction of chlorine bleaching the chlorophyll α thereby producing such a low fluorescence reading. Standard chlorophyll α solutions at different concentrations (8.64, 21.6, 43.2 and 64.8 μg/L) were exposed to a chlorine dose of 8 mg/L. The results obtained from this experiment are shown in Figure 9.

Figure 9. Fluorescence Reading for Chlorophyll a concentration versus time measured after chlorine addition

The chlorophyll α concentration decayed immediately after the first minute of the chlorine addition and the concentration became then steady. Figure 9 shows this steady state to continue after 10 minutes of contact with chlorine. The same steady state concentrations were recorded for samples taken every hour for the next 5 hours, and after 24 hours. The results indicate that there is an instantaneous reaction occurring between chlorophyll α and chlorine. For a constant chlorine dose of 8 mg/L, there seems to be a higher chlorine demand for higher concentrations of chlorophyll than for the lower ones. The chlorine demand was estimated to average 0.56 mg of Cl2/mg of chlorophyll α for the higher chlorophyll α standards corresponding to 64.8 μg/L and 43.2 μg/L. For the lower concentrations of chlorophyll α (8.64 and 21.6 μg/L), the chlorine demand was 0.16 mg of Cl₂/μg of chlorophyll α. Because only one dose of chlorine was tested, it cannot be concluded that there is a specific value for chlorine demand by chlorophyll α. However, it can be suggested that higher concentrations of chlorophyll α will require higher chlorine doses. On the other hand, there should be an optimum chlorine demand to reduce chlorophyll α concentrations since these values do not completely decay to zero.

Despite the fact that the pilot study data showed good evidence to confirm the decay of chlorophyll α after the chlorine addition, it is not possible to infer anything about actual algal formation.

The results obtained confirm possible effects of chlorination on chlorophyll α; however, the pilot study did not provide enough evidence to compare the extent to which the sunlight irradiation affected the algal formation in the basins. The transparent and opaque basins did not show significant difference in chlorophyll α concentration, and the concentrations were too low for comparison. In many instances the opaque basin resulted in higher concentrations than the transparent basin which was not expected. This phenomenon can be explained by chlorine having such a high effect on the decrease of the chlorophyll α concentration that causes the retention time in the basins, either exposed to the solar radiation or not, not to have much effect for new algal growth. The concentration of chlorophyll α was not high enough after chlorination to account for any productivity that might be attributed to the solar radiation.

Full-Scale Study

The full-scale study results for the chlorophyll α concentrations provided a sequential description of the algal formation throughout the different processes in the treatment plant. Figure 10 shows the average chlorophyll α concentration at different locations of the wastewater treatment plant for the full-scale study. The concentrations from one sampling day to another may vary slightly, but they presented the same trend, with few exceptions discussed later in the section. As observed in the figure below, the water coming from the clarifiers has a high chlorophyll α concentration (Filter 1 and Filter 2). The 5PA samples, representing the post aeration basin effluent, are significantly lower in chlorophyll α concentration. It can be concluded from these samples that the filters perform optimally in removing most of the algae because the chlorophyll α concentration in the water after passing through the filters decreased dramatically. When the water leaves the post aeration basin to enter the CCBs, chlorine is injected into the water flow in pipes that are underground. Samples 58S, taken right after the chlorine addition at the inlet of the CCBs are even lower in chlorophyll α. This decrease supports the hypothesis that chlorination significantly reduces chlorophyll α concentration in very short periods of time, demonstrated earlier by the pilot study. Samples 53S and 53N represent the effluents from the uncovered and covered CCBs, respectively. The chlorophyll α concentrations slightly increased from the influent to the effluent of the CCBs. Chlorophyll α concentration was found to increase in both covered and uncovered basins. However, the chlorophyll α found in the covered basin turned out to be lower than the one found in the uncovered one, as expected.

Figure 10. Average Chlorophyll α concentration versus time of day for all samples taken during the 3 days of
full-scale study.

Out of the 9 samples collected from the CCBs effluent during the full-scale study, 3 of them did not follow the expected trend, chlorophyll α concentrations from the covered basin were higher than the uncovered basin. Figure 11 shows more clearly the difference in the two CCB effluent samples (53S-Uncovered vs. 53N-Covered). Two of the samples that were higher in the covered basin and accounted for the largest difference in the samples were obtained at 9:00 am. As observed in Figure 11, the average of the 3 samples taken at 9:00 am is lower in chlorophyll α concentration for the covered basin than for the uncovered.

Figure 11. Average Chlorophyll α concentration versus time of day for samples taken during the 3 days of full-scale study. Same as Figure 10 excluding the filter samples to give a better view in the lower concentration differences.

The mean chlorophyll α concentration found in the uncovered CCB was 0.264 μg/L as compared to 0.207 μg/L of the covered CCB (Fig.12). The lower chlorophyll α concentrations in the covered CCB can be attributed to two main factors: the decrease in solar radiation reaching the basin and the increase in chlorine residual.

Figure 12. Average chlorophyll α concentration for the covered and uncovered chlorine contact basins at KWRF.

Covering the basin with the tarp reduced the direct penetration of solar irradiation to the basin. The lower availability of light might have diminished productivity of algae in the cover basin as compared to the uncovered one. The difference in chlorophyll α concentrations from the covered basin to the uncovered as they relate to the UV radiation and global horizontal solar radiation are presented in Figures 13 and 14, respectively. Higher differences in chlorophyll α positively correlate to higher solar radiation, so it can be inferred that the lack of solar radiation influenced the decrease of chlorophyll α formation in the covered basin.

Figure 13. Difference in chlorophyll α concentrations (Uncovered CCB – Covered CCB) versus the average UV radiation.

Figure 14. Difference in chlorophyll α concentrations (Uncovered CCB – Covered CCB) versus the average global horizontal solar radiation.

Preventing the basin from solar radiation did not only decrease the algal formation, but it also increased the chlorine residual as predicted by equation 1. This hypothesis was tested and proved by Heather Fitzpatrick during the analysis of the chlorine samples for her thesis project.⁵ As shown in Figure 15, the difference in the chlorine residual of the covered basin as compared to the uncovered basin increases as the UV radiation increases. This shows that the chlorine residual in the covered basin is larger than in the uncovered one.

Figure 15. Difference in chlorine residual concentration (Covered CCB – Uncovered CCB) versus the average UV radiation.

The availability of more chlorine residual in the covered basin might have impeded the growth of algae in the CCB. Since the chlorophyll α concentration is expected to be lower in the covered basin, the difference between covered and uncovered basins should be negative. As presented in Figure 16, this appears to be the case for most samples. Similarly, chlorine residual being higher for the cover basin has a positive difference when compared to the uncovered basin. It can be observed from Figure 16 that most of the data points cluster on the second quadrant of the graph, meaning that negative differences in chlorophyll α (lower chlorophyll α for the covered basin) relate to positive differences of chlorine residual (higher chlorine for the covered basin). Therefore, it can be suggested that lower chlorophyll α concentrations related to higher chlorine residual.

Figure 16. Difference in chlorine residual concentration (Covered CCB – Uncovered CCB) versus the difference in chlorophyll a concentration (Cov-Unc).

Even though the percent difference in the average chlorophyll α concentration of the covered basin was 21.6% lower than the uncovered basin (Fig.12) and possible reasons for this occurrence have been addressed above, a significance test, the Mann-Whitney test, performed on the data did not provide enough evidence to say that this difference was significant (α = 0.10)

Similar to the chlorophyll α analysis, TSS samples were examined in the intent to prove the hypothesis that algal formation would be decreased by covering the CCB. The average TSS concentrations found in the different basins for the full scale study are presented in Figure 17, and these are 0.356 mg/L and 0.367 mg/L for covered and uncovered basin, respectively. This difference is very small, and as it would be expected from such a small difference, no significant evidence resulted from this analysis. Using a t-test for this data resulted in insufficient evidence to demonstrate that the TSS concentration in the covered basin was lower than in the uncovered basin.

Figure 17. Average total suspended solids (TSS) concentration in the covered and uncovered chlorine contact basins at KWRF.

CONCLUSION

As demonstrated by the pilot study, chlorine influences chlorophyll α concentrations. It was observed that this reaction takes place in a very short period of time. The actual chemistry involved in this interaction has not been well defined. However, it is possible to confirm with certainty that adding chlorine to wastewater decreases chlorophyll α concentrations dramatically. Even though chlorophyll α is used as a measure of algae productivity, it is not possible to say that the reduction in chlorophyll α directly relates to a decrease in algal formation because of the bleaching effect of chlorine on the chlorophyll α that resulted in very low fluorescence readings. The resulting TSS reduction in the pilot CCBs could suggest that chlorination did influence algal formation; however, not specific correlation between chlorophyll α and TSS reduction was found as it was expected at the beginning of the study.

The full scale study provided a thorough description of the chlorophyll α concentrations throughout different processes in the treatment plant. From these results it is possible to infer that as the wastewater moves from the clarifiers through the filters, a significant amount of algae is removed, and this concentration is further reduced when the chlorine is injected to the wastewater. After chlorination occurs, the retention time in the CCBs allows for additional algal development.

The extent to which either sunlight or chlorine residual influences this algae growth in the CCBs could not be significantly assessed. Statistically significant evidence was not found in order to predict whether a decrease in sunlight or an increase in chlorine residual would reduce the algal formation in the CCBs. Therefore, it is not possible to say that covering the CCBs at the KWRF would significantly reduce the algal formation. It has been suggested however, that chlorination would have an effect in algae growth, so it might be possible that keeping significantly higher chlorine residuals would decrease the concentration of algae. Additional studies would be necessary in order to further prove actual interactions of chlorine and algae, as well as the influence of sunlight in this algal formation.

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REFERENCES

1. Integrated Product and Process Design Team. "The effect of chlorine residual on disinfection by-product formation in the Kanapaha Water Reclamation Facility chlorine contact basin". University of Florida (2001-2002).
   
2. Kirk, John T.O. Light and photosynthesis in aquatic ecosystems. Cambridge University Press (1983).
   
3. Caldwell, DAHL. "Sewage oxidation ponds". Sewage Works Journal 18:3 (1946)
   
4. Bowen, ESP.. "Performance evaluation of existing lagoons, Peterborough, New Hampshire". U.S. Environmental Protection Agency, Cincinnati, Ohio. EPA-600/2-77-085 (1977)
   
5. Fitzpatrick, Heather. Covering chlorine contact basins at the Kanapaha Water Reclamation Facility: effects on chlorine residual, disinfection effectiveness, and disinfection by-product formation. M.S. Thesis. University of Florida. Gainesville, Florida (2004).

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