Influence of Source Characteristics, Chemicals, and Flocculation on Chemically Enhanced Primary Treatment

By Neupane, Dilli R Riffat, Rumana; Murthy, Sudhir N; Peric, Marija R; Wilson, Thomas E

ABSTRACT: The overall objective of this research was to investigate various methods and parameters to increase the efficiency of chemically enhanced primary treatment (CEPT). The performance of CEPT was evaluated based on its efficiency of removal of nonsettleable solids (NSS). Some of the source characteristics that influenced NSS concentration included influent total suspended solids, influent turbidity, and influent total chemical oxygen demand. A higher concentration of the influent constituents led to a higher NSS concentration, suggesting that NSS represented a somewhat fixed fraction or percent of these influent constituents. The specific particle surface area (SPSA) was found to correlate with percent NSS in the effluent. A higher SPSA is a result of smaller- sized nonsettleable colloidal particles, thus leading to an increase in percent NSS. In summary, there are several parameters that affect NSS, which could be used to control NSS to improve CEPT, as demonstrated by this study. Water Environ. Res., 80, 331 (2008).

KEYWORDS: chemically enhanced primary treatment, nonsettleable solids, particle surface area, mixing, flocculation.

doi:10.2175/106143007X221355

Introduction

The design of conventional settling devices used for primary treatment at wastewater treatment plants has traditionally been based on generalized hydraulic loading criteria. These generalized loadings do not always take into account the nature and settling characteristics of suspended material in influent wastewater streams (Andoh and Smisson, 1996) or the chemicals added during treatment. It is necessary to study source characteristics of the wastewater to design an optimized settling environment. Increased efficiency of primary treatment has dual benefits, as follows:

(1) It reduces the load for downstream processes; and

(2) It enhances the rate of secondary treatment, because smaller, easily biodegradable particles are available after primary treatment (Odegaard, 1998).

Performance of primary clarification depends, to a great extent, on influent characteristics of wastewater. Influent characteristics include total suspended solids (TSS), turbidity (TU), biochemical oxygen demand, chemical oxygen demand (COD), particle size distribution, and septicity. Characterizing incoming wastewater can provide a vast array of benefits, such as feedback for chemical dosing, analysis and prevention of operational inefficiencies, establishing the trends of seasonal variations, providing the benchmark on operational performance of the plant itself, and providing parameters for the comparison of wastewater quality with that of other plants in the region or country.

Chemically enhanced primary treatment (CEPT) refers to the process that uses chemicals for coagulation, flocculation, and precipitation of paniculate/dissolved solids in the wastewater as a primary step in clarification. This process was first used around 1840 in Paris, France (Nieuwenhuijzen, 2002). In recent years, CEPT has been used at various wastewater treatment plants for phosphate removal, clarification of wastewater, reduction in sludge volume, and increase in surface overflow rates (SORs). Numerous studies have been conducted on the use of chemical coagulation for the treatment of drinking water (Amirtharajah and O’Melia, 1990; Clark et al., 1994; Faust and AIy, 1998; Hanson and Cleasby, 1990; Johnson and Amirtharajah, 1983). However, research on the application of chemical coagulation for wastewater clarification is scarce. The selection of chemicals for CEPT depends on the primary objective of using them. The dose of chemical coagulant and method of dosing need to be optimized for better clarification. Parker et al. (2001) presented a review of research work related to CEPT. In that review, not much information was available towards the process of application of chemicals (i.e., how rapid or slow mixing intensities and settling times can be optimized, what is the proper time/place to add a particular chemical, and whether or not a delay in adding a second chemical after the first improves the results). Much of that information was available for water treatment, but not for wastewater treatment. In light of this observation, the need for a study that covers wastewater characterization and the effects of wastewater characteristics in performance of CEPT was realized.

Nonsettleable solids (NSS) were selected as a solids characteristic mainly affected by CEPT. Settleable solids are removed using the normal clarification process. Therefore, CEPT can be used to control the NSS to improve primary treatment efficiency. Nonsettleable solids are defined as the TSS remaining in a sample after a specific settling period. Because the testing in this study was a laboratory exercise, idealized conditions were used to determine NSS using jar tests. Standard Methods (APHA et al., 1998) has defined NSS with a 30-minute settling time. Standard Methods does not specify any flocculation intensity or time.

Septicity of sludge in wastewater can reduce primary clarification significantly, by causing the sludge to float and flow over the weirs (Hetherington et al., 1999). Septicity can be characterized by the specific particle surface area (SPSA), oxidation-reduction potential (ORP), and concentration of sulfides, among other factors. The SPSA is defined as the geometric surface area of particles per gram of dry solids. Wastewaters received by large plants with extensive sewer networks are likely to contain smaller-sized solids, as a result of breakup with age and transportation of the wastewater. This is especially the case where there are ancillary components, such as pumping stations, which create hydraulic regimes of high turbulence and shear. Smaller particles tend to be less readily settleable and have a greater proportion of associated pollutants, such as heavy metals and polycyclic aromatic compounds (Andoh and Smisson, 1996). Theoretically, smaller particles have a larger specific surface area. Hence, SPSA was used as a parameter to characterize the influent wastewater and observe its effect on settling.

Objectives

The overall objective of this research was to understand the effect of primary clarification process parameters, influent source characteristics, and chemical type and dose, on NSS and CEPT performance. The specific objectives were to

(1) Evaluate the influence of rapid mixing and flocculation process parameters on CEPT.

(2) Characterize the primary influent. Study influent characteristics and observe the correlation among them and with NSS.

(3) Evaluate chemical type and dosages. Two types of coagulants (ferric chloride and ferrous chloride) were evaluated. Anionic polymer was selected as the flocculant.

Methodology

This study was conducted at the Blue Plains Advanced Wastewater Treatment Plant, operated by the D.C. Water and Sewer Authority (DCWASA), located in Washington, D.C. The study was carried out from October 2004 to July 2005. Influent characteristics were defined with the following parameters: temperature, pH, sulfides, ORP, SPSA, turbidity, TSS, COD, phosphates, and iron. These parameters were measured in the influent wastewater. All parameters, except SPSA, were analyzed based on Standard Methods (APHA et al., 1998). The SPSA was determined by a dye adsorption method developed by Sorensen and Wakeman (1996) using Rhodamine B as a dye. Linear correlation analysis of influent and effluent parameters, such as NSS, was carried out. Parameters with significant correlation values (r^sup 2^ larger than or equal to 0.7) were selected and plotted. T-tests were carried out to verify the statistical significance of the results presented here.

The degree of primary clarification was measured in terms of effluent TSS and NSS during laboratory tests. Effluent TSS was determined by jar tests. The NSS is the residual or effluent TSS measured after a predefined flocculation and settling period. Residual TSS by jar tests was determined based on Standard Method 2540 F (APHA et al., 1998), with added aspects on flocculation and chemical doses. A Phipps and Bird-700 Standard Jar Tester (Fisher Scientific, Pittsburgh, Pennsylvania) was used with 2-L square beakers. The jar-testing routine consisted of a period of rapid mixing (coagulation) at a high intensity for a short period of time (1 to 2 minutes), followed by slow mixing (flocculation) at a lower intensity for a longer period of time (20 to 30 minutes). At the end of flocculation, the mixture was allowed to settle for a specific period of time (20 to 30 minutes). Specific mixing and settling parameters are described for each experiment in the following section.

Mixing intensity was measured in terms of root mean square velocity gradient, G. The following equation was used to calculate G for mechanical mixing:

Where

G = root mean square velocity gradient (per second),

P = power applied (watts),

[mu] = dynamic viscosity (N . s/m^sup 2^), and

V = flocculator volume (m^sup 3^).

To evaluate the effect of mixing intensity, three types of mixers were used. The first was the mixer provided with the jar tester. The second was a high-capacity mixer (L1U10 Lightnin Mixer, Cole- Parmer, Vemon Hills, Illinois), which was calibrated for G against revolutions-per-minute values. It was used to provide mixing intensities ranging from a G value of 125 to 2500 seconds^sup -1^. The third was a kitchen/food mixer, with an estimated G value of 10 000 seconds^sup -1^. The G value of the kitchen/food mixer was estimated using information from Polasek (1979) and Rasmussen (1984). Chemical dosing strategy was also defined using the jar tests. Selection of polymers, determination of optimum dosage, and determination of optimum mixing/flocculating intensity and optimization of duration for settling were investigated by the jar tests.

Septicity of incoming wastewater was measured in terms of the presence of sulfides, ORP, SPSA. All of these parameters were used simultaneously, because reliability of one or the other was not known in advance.

A procedure developed by Sorensen and Wakeman (1996) was slightly modified and followed for measuring the SPSA. The procedure estimates the surface area of particles on the basis of extent of adsorption of a dye (Rhodamine, in this case) onto the surface of particles in the wastewater. For calibration, a curve with Rhodamine B solution in wastewater filtrate obtained using fluorometer was selected. At the time of measuring particle surface area, 2 mL of Rhodamine B dye solution of 1000[mu]g/L concentration was mixed in 2 L of wastewater sample, resulting in a 100-[mu]g/L solution. After allowing the mixture to stabilize for 15 minutes, 10 mL of well- mixed sample was filtered through a 0.45-[mu]m membrane filter. The turbidity of the filtrate was measured, and concentration of Rhodamine B remaining in the filtrate was estimated using the calibration curve. Total surface area of particles present in wastewater sample was calculated using the amount of Rhodamine B absorbed by the sample, as described by Sorensen and Wakeman (1996). The concentration of TSS of the sample was measured, and the SPSA was calculated in units of square meters per gram TSS.

Results and Discussion

This section is divided into three parts. First, the influence of the clarification process on NSS and CEPT is presented. second, the influence of influent source characteristics on NSS and CEPT is discussed. Finally, the influence of chemical type and dosage is presented.

Influence of Clarification Process Parameters. Effect of Rapid Mixing Intensity and Duration. To investigate the effect of mixing intensity, a range of different G values, from 100 to 770 seconds^sup -1^, with constant duration of 60 seconds, was used during rapid mixing with ferric coagulant in wastewater. Residual turbidity, NSS, total phosphates (TP) and orthophosphates (OP) were measured after 30 minutes of tapered flocculation followed by 30 minutes settling. The initial TSS was 200 mg/L. The results are presented in Figure 1. Results from f-tests at a significance level, alpha, of 0.05 showed that there was no significant change in NSS, turbidity, total phosphates and orthophosphates within this range of G. Deterioration of removals by particle breakup was not observed within this range. Particle breakup was observed at very high- intensity mixing (Figure 2), where TSS, turbidity and total phosphate removals were lower with a high intensity of mixing (G [asymptotically =] 10 000 seconds^sup -1^). The initial TSS was 165 mg/L. The difference between clarification at medium-intensity mixing and very high-intensity mixing was found to be statistically significant, when t-tests were carried out at significance level, alpha, of 0.05. The results indicated that very high-intensity mixing with iron did not improve clarification of wastewater, but only caused improvement of orthophosphate removal, which is known to be affected by rapid mixing (Szabo et al., 2006). When a constant G- t of 12 000 and variable G was applied, to observe the effect of various mixing intensities on removals, no significant differences were observed on solids removals at different velocity gradient (G) values. There was no apparent change in clarification resulting from an increase in the duration of mixing.

Previous researchers (Odegaard, 1998) have stated that reaction time with iron salts was extremely short and hence a very intense mixing at the time of dosing was necessary. This claim was based on theoretical reasoning or experiments in drinking-water treatment. Based on the observations of this study, it can be accepted that reaction time was short, because allowing a prolonged reaction time did not show further improvement. However, a very high intensity of mixing also did not improve clarification of wastewater.

Effect of Slow Mixing Intensity and Duration. A series of experiments with different flocculation or slow mixing times, ranging from O to 30 minutes, was conducted, to observe the effect of the duration of slow mixing on clarification. The initial TSS was 243 mg/L. Figure 3 illustrates the variation of NSS with flocculation times. The minimum flocculation time for solids removal without chemicals was approximately 20 minutes and with chemicals was approximately 10 minutes. This was similar to the results observed by Parker et al. (2000) under field conditions.

Different flocculation speeds were investigated. The results of constant speed flocculation were compared with tapered flocculation. Tapered flocculation consists of a flocculation routine, where the flocculation intensity is gradually reduced from a higher value at the beginning to a lower value at the end of flocculation. This helps to prevent breakup of the larger floes, especially towards the end of flocculation. Tapered flocculation was observed to improve clarification. Hence, tapered flocculation with 10 minutes at 50 r/ min (G = 42 seconds^sup -1^), 10 minutes at 40 r/min (G = 32 seconds^sup -1^), and 10 minutes at 30 r/min (G = 22 seconds^sup – 1^) was adopted for the jar tests.

Effect of Settling Time. The effect of settling time on clarification was investigated, with and without the use of polymers. Settling times ranging from 5 to 30 minutes were used. The initial TSS was 149 mg/L. Figure 4 illustrates the variation of NSS and turbidity remaining, with the settling times. With 5 minutes of settling time, there was a difference of 12% in NSS for samples with and without polymer. With 20 minutes of settling and beyond, the difference reduced to 1%. Similar results were observed with remaining turbidity. This experiment showed that longer settling time was required when polymer was not used, to achieve the same degree of clarification as that with polymers. When polymer was used, larger floes were formed, which settled more quickly. Merely increasing the settling time did not produce better clarification. On the other hand, when only iron was used, smaller floes were formed, and additional clarification was achieved by increasing the settling time.

Influence of Source Characteristics on Clarification. The influent parameters studied were temperature, pH, turbidity, sulfide (total and soluble), phosphates, ORP, SPSA, zeta potential, sludge volume index, TSS, COD, and total iron. Influent parameters, namely turbidity, TSS, total COD (TCOD), and total iron content, were found to significantly affect residual effluent TSS or NSS. Other influent parameters, such as temperature, pH, ORP, sulfides, and phosphates, did not show significant correlation with NSS. Protocol for the jar test comprised mixing (with or without chemicals) at 100 r/min for 1 minute, tapered flocculation for 30 minutes, and settling for 30 minutes. Tapered flocculation with 10 minutes at 50 r/min (G = 42 seconds^sup -1^), 10 minutes at 40 r/min (G = 32 seconds^sup -1^), and 10 minutes at 30 r/min (G = 22 seconds^sup -1^) was adopted for the jar tests.

Figures 5a, b, and c demonstrate the variation of NSS with influent turbidity, TSS, and TCOD, respectively, with and without chemical addition. The practical application of this observation is that NSS can be predicted measuring TCOD or TSS in the influent. The benefits of chemical addition are clearly observed in these figures. A significantly lower NSS can be achieved by chemical addition in primary treatment. Furthermore, the correlations between influent characteristics and NSS deteriorate with chemical addition, suggesting the overwhelming role of chemicals in changing influent solids characteristics and affecting NSS.

An increase in soluble COD of influent resulted in increased consumption of ferric chloride for precipitation. Increased soluble COD indicated that more of the organic matter was in the form of smaller particles, in colloidal and dissolved form. Smaller particles do not settle very well. This resulted in lower TSS, turbidity, and TCOD removals. This was observed previously by Neupane (2005).

The SPSA was correlated with soluble COD (SCOD), as illustrated in Figure 6. This relation can be explained by the fact that whenever particles are finer in the influent (shown by a higher SPSA), the soluble part of COD increases. This relationship is useful, because measuring particle surface area is faster than measuring COD. When the TSS value of wastewater is already known, the SPSA can be estimated in a short period of time, using the method described previously. Then, using the relation shown by Figure 6, the SCOD of the wastewater can be estimated.

Linear correlation coefficients among selected influent and effluent parameters (Table 1) show that there is a good correlation between the major influent parameters (TSS, turbidity, COD, and total iron) with NSS when no chemical is applied for primary treatment. These correlations gradually disappear at higher chemical doses (Table 1, parts a, b, and c). This demonstrates the benefits of using chemicals. When chemicals are not applied, stronger influent results in higher NSS; however, when chemical dose is increased, this direct relation between influent and effluent decreases, meaning that chemicals help lower the NSS, even with stronger influent. Significant correlation was observed between sulfide (total or soluble) and COD (total or soluble) in the influent. When the wastewater becomes more septic, more sulfides are produced from organic sulfates. At the same time, because of septic conditions, particle breakup and hydrolysis takes place, which causes more paniculate matter to dissolve; hence, more of the paniculate COD gets converted to SCOD.

Total sulfide in the influent varied from 0.1 to 9.0 mg/L (0.1 to 9.0 ppm) during this study. It was observed that, when total sulfide in the influent was high, soluble sulfide was also high. When sulfide was measured in the supernatant sample after completion of the jar test, it was not detected (detection limit 0.1 mg/L [0.1 ppm]). The reason for that may be that most of the paniculate sulfide settles down during the jar test and most of the soluble sulfide either reacts with ferric chloride or escapes as hydrogen sulfide gas during the turbulent mixing and flocculation of the sample. In the field condition, the turbulence during mixing and flocculation is expected to be comparatively low; hence, all of the sulfide may not be removed by CEPT.

The influent zeta potential of the sample did not correlate with NSS. Other parameters that did not show correlation with NSS include sludge volume index (SVI), pH, and ORP. Interestingly, an increase in total iron (iron in source water as opposed to chemical addition) in the influent correlated with NSS.

A good correlation was observed between SPSA and percent NSS, as shown in Table 2. In Table 1, the units of SPSA (in square meters per gram TSS) were normalized, whereas NSS is in milligrams per liter. However, when the units of both parameters were normalized, a correlation coefficient of 0.856 was obtained.

Chemical Type and Dosages. Comparison of Ferrous and Ferric Coagulants. A set of experiments was performed to understand the effect of mixing two different iron salts, at different proportions, on clarification. Ferric chloride (Fe^sup 3+^) and ferrous chloride (Fe^sup 2+^) were used. Both of the salts used were reagent-grade products from Fisher Scientific. Three jars of wastewater samples were dosed with the following: (1) 5 mg/L (5 ppm) Fe^sup 3+^ only, (2) 2.5 mg/L (2.5 ppm) Fe^sup 3+^ and 2.5 mg/L (2.5 ppm) Fe^sup 2+^, and (3) 5 mg/L (5 ppm) Fe^sup 2+^. They were rapidly mixed for 1 minute at G = 100 seconds^sup -1^, tapered flocculation for 30 minutes, and settled for 30 minutes. The initial TSS was 165 mg/L. Figure 7 presents percent removals of TSS, turbidity, total phosphates, and orthophosphates, at various Fe^sup 3+^ and Fe^sup 2+^ doses. Ferric chloride showed the best performance in removing TSS, turbidity, and phosphorus. Paired t-test results also showed that Fe^sup 3+^ produced significantly better results than Fe^sup 2+^ coagulant. Surprisingly, ferrous chloride also produced fairly good solids removal and CEPT performance. Further work is needed to understand the relative roles of ferric versus ferrous salts on CEPT performance. The efficiency of ferrous chloride relative to ferric chloride was diminished for removal of soluble phosphates compared with the other parameters investigated in Figure 7.

Effect of Chemicals. The relationship between NSS and iron dose with and without polymer is illustrated in Figure 8. The initial TSS was 171 mg/L. Contrary to the widespread belief that polymers improve removals significantly, this study did not observe increased removals by polymers. This disagreement can be explained in the following way. Many of the previous research works have suggested that application of polymers will allow higher SOR values, and the results of this study conform to that statement. In this research, it was observed that polymers do not improve NSS, but rather help improve settling rates of settleable solids. These settleable solids could also include the previously nonsettleable fraction that was converted to settleable solids using iron coagulant. Therefore, iron is the chemical that reduces NSS, while polymer improves the settling rates of settleable solids.

Conclusions

Based on the results of this research, we can formulate the following conclusions:

* Flocculation time affects NSS concentration, whereas rapid mixing does not play an important role in CEPT. An increase in flocculation time improves CEPT performance. A minimum flocculation time of 20 minutes is required for optimized clarification without chemicals. A minimum flocculation time of 10 minutes is required for optimized CEPT performance.

* Influent parameters, such as turbidity, TSS, and TCOD, were found to affect NSS in primary clarification. This relationship could be useful in predicting NSS for existing or new primary clarification facilities.

* The SPSA appeared to be one of the major parameters that was related to other influent parameters, such as soluble COD. The SPSA was observed to be useful not only in prediction of the degree of clarification or effluent NSS, but also in determination of the ferric dose required to produce a certain degree of clarification.

* Iron was determined to strongly affect NSS. A chemical-dosing strategy should be based on one of the aggregate influent properties- turbidity, TSS, soluble COD, SPSA, or any other measure of degree of fineness of particles in the influent. A higher dose of ferric chloride was required when the influent was stronger (in terms of turbidity, TSS, or soluble COD) and was more septic (in terms of SPSA, sulfides, or orthophosphates).

* Polymer did not appear to affect NSS. An iron-addition-control philosophy could be based on influent TSS load (both concentration and flow are important), while polymer addition should be based on flow (more polymer is needed at higher flows than at lower flows, to improve settling rates) and not necessarily on TSS concentration or load.

Credits

The authors gratefully acknowledge the financial support provided by the D.C. Water and Sewer Authority (Washington, D.C.) and the use of the laboratory facilities at Blue Plains Advanced Wastewater Treatment Plant (Washington, D.C.) for this research.

Submitted for publication October 26, 2006; revised manuscript submitted September 12, 2007; accepted for publication September 21, 2007.

The deadline to submit Discussions of this paper is July 15, 2008.

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Dilli R. Neupane1, Rumana Riffat2*, Sudhir N. Murthy3, Marija R. Peric1, Thomas E. Wilson4 1 Metcalf and Eddy, Laurel, Maryland.

2 Civil and Environmental Engineering Department, George Washington University, Washington, D.C.

3 Department of Wastewater Treatment, D.C. Water and Sewer Authority, Washington, D.C.

4 EarthTech, Alexandria, Virginia.

* Civil and Environmental Engineering Department, 801, 22nd St. NW. Room 633, George Washington University, Washington, D.C. 20052; e-mail: riffat@gwu.edu.

Copyright Water Environment Federation Apr 2008

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