February 15, 2006
The Effect of Wastewater Cations on Activated Sludge Characteristics: Effects of Aluminum and Iron in Floc
By Park, Chul; Muller, Christopher D; Abu-Orf, Mohammad M; Novak, John T
Wastewater samples collected from seven wastewater treatment plants (WWTPs) were characterized to assess the impacts of wastewater cations on the activated sludge process. The cations included in this study were sodium (Na+), potassium, ammonium, calcium, magnesium, aluminum (Al), and iron (Fe). Among the selected cations, Al and Fe were of most interest to this study because their role in bioflocculation has not been extensively studied and remains largely unknown. The data showed that WWTPs contained highly varying concentrations of Na+, Al, and Fe in the wastewater and that these cations were responsible for differences between WWTPs as to sludge dewatering rates and effluent quality. In general, a high influent Na+ concentration caused poor sludge dewatering and effluent characteristics. However, when sufficient Al and Fe were present in floe, the deleterious effects of Na+ were offset. The data associated with Al further revealed that waste activated sludge with low Al contained high concentrations of soluble and colloidal biopolymer (protein + polysaccharide), resulting in a high effluent chemical oxygen demand, high conditioning chemical requirements, and poor sludge dewatering properties. These results suggest that Al will improve activated sludge effluent quality by scavenging organic compounds from solution and binding them to floc. Water Environ. Res., 78, 31 (2006).
KEYWORDS: activated sludge, floc, cations, aluminum, iron, biopolymer, protein, effluent, dewatering, conditioning.
The activated sludge process is a commonly used biological process for wastewater treatment. Bioflocculation, the aggregation of microorganisms and microbial product into floe, is a key operating mechanism in this process. If flocculation is poor, solid/ liquid separation will be ineffective and a poor quality of effluent will result. In spite of its widespread use and long history of process operation, failures in meeting regulatory effluent requirements are often reported and researchers are still trying to better understand the mechanism of bioflocculation.
To better understand microbial flocculation, the floe structure needs to be better defined. Although highly hydrated and heterogeneous, activated sludge floe consists mainly of microorganisms, exocellular biopolymer or extracellular polymeric substances (EPS), cations, and debris (Bruus et al., 1992; Higgins and Novak, 1997a). It is thought that biopolymers originate from microbial growth, decay, and wastewater influent, providing an important matrix in which microorganisms can be aggregated (Dignac et al., 1998; Urbain et al, 1993). Because the majority of exocelluar biopolymers are negatively charged, cations then become an important structural component as a binding agent within the biopolymeric network (Bruus et al., 1992; Higgins and Novak, 1997a; Urbain et al., 1993).
Because of the critical role cations play in bioflocculation, a great deal of research has been conducted to study the effect of cations on activated sludge characteristics. Historically, most of this research dealt with monovalent and divalent cations in activated sludge. Several studies showed the importance of calcium (Ca^sup 2+^) in floe structure by removing Ca^sup 2+^ from biological floes using a cation exchange resin (Keiding and Nielsen, 1997), ethylenediamine tetraacetic acid (Kakii et al., 1985), and ethyleneglycol terra-acetic acid (Bruus et al., 1992), and by observing deterioration in settling and dewatering properties. Magnesium (Mg^sup 2+^) is also known to be an important cation for floe stability. Sobeck and Higgins (2002) reported that activated sludge fed with either Mg^sup 2+^ or Ca^sup 2+^ improved in a similar manner with regard to settling and dewatering. In the same study, the authors examined three different cation induced-floe models and concluded that the divalent cation bridging, in which both Ca^sup 2+^ and Mg^sup 2+^ equally bind negatively charged biopolymers, best explains the role of divalent cations in floe structure.
Unlike divalent cations, high concentrations of monovalent cations, especially sodium (Na+), were shown to be detrimental to activated sludge properties (Bruus et al, 1992; Higgins and Novak, 1997b; Novak et al., 1998). Muithy et al. (1998) reported that wastewater treatment plants (WWTPs) treating industrial wastewater where sodium hydroxide was used for pH control produced sludges with poor settling and dewatering properties. It was theorized that ion exchange within floes results in displacement of divalent cations by monovalents, making floes weak and sensitive to physical stresses. Examining the effect of mono and divalent cations on the floe properties, Higgins and Novak (1997b and 1997c) proposed that the ratio of monovalent to divalent cations (MfD), on a charge equivalent basis, could be used to determine if the cation content is likely to cause sludge problems. According to the authors, sludges with an M/D ratio greater than 2 would encounter problems with settling and dewatering due to the weak floe structure induced by the high concentration of monovalent cations.
Aluminum (Al) and iron (Fe) salts are widely used in WWTPs as coagulating and phosphorous-removing agents. However, little is known regarding their influent source, chemical species in the influent wastewater or wastewater biomass, and impact on bioflocculation, especially for Al. Although the role of Al and Fe in floe structure has not been extensively studied, these multivalent cations are often found at high concentrations in activated sludge. A study by Kakii et al. (1985) showed that both Al and Fe were more abundant than Ca^sup 2+^ and Mg^sup 2+^ in the biomass of the activated sludge systems they investigated. Wiln et al. (2003) also found relatively high concentrations of Fe and Al from several activated sludges. Because Al and Fe have a higher charge valence than mono and divalent cations, their contribution to floe stability could be of great importance. Additionally, unique properties of these cations in aqueous environments suggest that their role in flocculation would be physiochemically different from Ca^sup 2+^ and Mg^sup 2+^.
Table 1-Process descriptions of seven WWTPs.
It was shown previously that the reduction of Fe either by the Fereducing bacterium Shewanella alga (Caccavo et al., 1996) or by the addition of sulfide (Nielsen and Keiding, 1998) led to a significant increase in turbidity in the solution of activated sludge, suggesting that oxidized Fe is an important component of the floe structure. According to Rasmussen and Nielsen (1996), approximately 70 to 90% of Fe was present as ferric Fe in the fresh activated sludges they investigated. However, they also admitted difficulties in identifying the species or chemical forms of Fe (III) in activated sludge. Later, Nielsen and Keiding (1998) hypothesized that either the direct reaction of ferric Fe with extracellular polymeric substances (EPS) or ferric hydroxides precipitates are involved in flocculation. Recent studies showed that Fe (III) has an affinity for activated sludge protein. Murthy et al. (2000) and Abu-Orf et al. (2001) demonstrated that Fe (III) salts selectively coagulated protein present in sludge from autothermal thermophilic aerobic digesters. Later, Novak et al. (2003) postulated that the large release of protein from floe during anaerobic digestion is due to iron reduction and the subsequent loss of binding between protein and ferric Fe. Protein is the most abundant organic compound in activated sludge EPS (Frlund et al., 1996). Therefore, an understanding of the relationship between iron and protein could significantly improve our knowledge of floe structure.
Compared to Fe, Al does not undergo oxidation-reduction and there is little information regarding its role in biological flocculation. Investigations dealing with Al in sludge are often found in the field of drinking water treatment because Al salts are commonly used as coagulants for removing natural organic matter in water treatment plants. With regard to the interaction between organic matter and Al, two studies showed that polysaccharides found in lake and river water were preferentially coagulated by aluminum polychlorosulfate (Masion et al., 2000; Vilge-Ritter et al., 1999). The earlier of these studies also tested alum, but a high selectivity for polysaccharide was not found for alum and the reason why two species of Al worked differently was not provided. Lu et al. (1999) showed that humic substances collected from natural sources such as swamp water and surface soil were well removed by alum. These studies imply that, although the nature of Al in wastewater and commercial Al chemicals used in water treatment might be considerably different, there is a likelihood that Al may play a significant role in binding of exocelluar biopolymers in biological floes.
An examination of the literature revealed that poor bioflocculation due to improper cation composition might cause settling and dewatering problems. Simultaneously, unflocculated materials might be washed out of the system causing deterioration ineffluent quality. To assess the overall impact of cations on the activated sludge systems, this study was undertaken using samples collected from seven WWTPs. Both solution and floe cations were quantified and various activated sludge parameters were characterized. The data were used to determine if various cations, especially Al and Fe in floe, can account for differences between effluent characteristics of WWTPs and sludges they produce. The hypothesis of this study was that both divalent and trivalent cations positively affect the floe structure and activated sludge process performance and that each has a unique role in floe formation, sludge properties, and effluent quality.
Methods and Materials
Experimental Approach. Wastewater samples were collected from seven different WWTPs. These plants are listed alphabetically from A to G, and the first and second samplings are designated with "1" and "2" for some facilities. The process description and general background for each facility are included in Table 1. Aluminum was not added to any of these plants, while plant E had Fe added to the influent to the aeration basin. Each sampling set consisted of activated sludge influent (primary effluent), mixed liquor, waste activated sludge (WAS), and secondary effluent. For all the samples, cations in solution were measured and dried WAS was used for analysis of Al and Fe in floe. In this study, the term "soluble", or "solution", refers to be the material that can be filtered through a 0.45-m filter. Prior to filtering, all the WAS samples were centrifuged at 9 460 g for 15 minutes and the centrate was filtered through 1.5-m, 0.45-m, 30 000-Dalton (30-k), and 1 000-Dalton (1-k) filters. Ultrafiltration was performed at 60 psi through Amicon (Amicon, Massachusetts) YM30 (30 k) and YMl (1 k) partly hydrophilic membranes. Unlike monovalent and divalent cations, soluble Al and Fe were not detected in most of the aeration basin influents, indicating that they are primarily associated with solids or materials greater than 0.45 m in the wastewater. Therefore, Al and Fe were designated as "floe" cations in this study. The sludge settling and dewatering properties were measured using mixed liquor and WAS, respectively. Both shear and conditioning tests were also conducted using WAS. Soluble chemical oxygen demand (COD) and suspended solids in secondary effluent were analyzed during the study. Also, soluble protein and polysaccharide were measured using the solution phase of WAS and secondary effluent.
Table 2-Summary of cation composition in 10 waste activated sludges.
Analytical Methods. Solution cations Na+, potassium (K+), ammonium (NH^sub 4^^sup +^), Ca^sup 2+^, and Mg^sup 2+^ were analyzed using a Dionex (Sunnyvale, California) Ion Chromatograph. Total Al and Fe in dry sludge were measured using U.S. Environmental Protection Agency (1996) Method 305OB. Waste activated sludge was dried at 105 C and a sample of the dried sludge underwent acid digestion. The Al and Fe in the acid-digested samples were quantified using atomic absorption spectroscopy (AAS). Aluminum and Fe in the influent were also measured using AAS.
Total solids (TS), total suspended solids (TSS), mixed liquor suspended solids (MLSS), total volatile solids (TVS), and volatile suspended solids (VSS) were measured according to Standard Methods (APHA et al., 1995). The sludge volume index and soluble COD were measured based on Methods 271OD and 522OC, respectively (APHA et al., 1995). The protein concentration was determined by the Hartree (1972) modification of the Lowry et al. (1951) method using bovine serum albumin as the standard. Polysaccharide was measured by the Dubois et al. (1956) method using glucose as the standard.
Table 3-Characteristics of 10 waste activated sludges.
Both ferric chloride and alum at concentrations of 100g/L and ahigh charge cationic polymer, BC650 (Stockhausen), at 0.5% (w/w) were used as sludge conditioners in this study. Optimum doses of each conditioner were determined using a capillary suction time (CST) test by obtaining the lowest CST reading. Specific resistance to filtration (SRF) and CST were used as measures of the sludge dewatering rate according to the method described by Christensen and Dick (1985) and Method 271OG (APHA et al, 1995), respectively. The shear test used the same procedure as that of Higgins and Novak (1997b). A sludge sample was placed in a baffled container and stirred with a flat paddle mixer at G = 500/s. For each test, a 500- mL sample of WAS was placed into a 2-L baffled cylinder and, once mixing began, CST values were measured every 2 minutes for 8 minutes. The increase in CST with mixing time indicates the degree of sensitivity to shear.
Results and Discussion
A summary of the cation composition for the 10 waste activated sludges is provided in Table 2. Sodium, K+, NH^sub 4^^sup +^, Ca^sup 2+^, and Mg^sup 2+^ were measured in WAS solution and concentrations of these cations on an equivalent basis were used to determine the M/ D ratio for each WAS. Dewatering properties and optimized chemical conditioning doses for WAS are summarized in Table 3. Protein and polysaccharide in WAS and the effluent, along with the effluent COD, are also included in Table 3.
Figure 1-Change in CST as a result of shearing time for (a) WAS from plant C, (b) WAS from plant G, and (c) other WAS (G = 500/s).
Potassium Release in Activated Sludge. The data in Table 3 show that the WAS dewatering rates from plants C and G, described by CST and SRF, are substantially different from those of other plants. The floe strength of these sludges was evaluated and the data in Figure 1 show that WAS from plants C and G were sensitive to shear as indicated by a large increase in CST with mixing. The other sludges were more shear-resistant. The M/D ratios did not account for the poor dewatering rates and weak floe strength of the WAS from plants C and G. As shown in Table 2, the M/D ratios of these sludges were 2.3 for WAS from plant C and 3.4 for WAS from plant G. These values are above the optimum of 2 recommended by Higgins and Novak (1997b), but are comparable to some of the other sludges that were not shear sensitive. The only noticeable difference in cation content for these sludges was that an abnormally high K+ concentration was found in the WAS compared to its concentration in the plant influent. The K+ concentrations across each facility are shown in Table 4. The K+ was approximately 4 times greater in the WAS for plants C and G compared to their influent K+. Because K+ is present at high concentrations in bacterial cytoplasm (Bakker, 1993), it was speculated that some stress to microbes might have been present in these two facilities that resulted in K+ being released into activated sludge solution.
Table 4-The K+ concentrations across a WWTP.
Bott and Love (2002) have shown that when spikes of electrophilic toxic chemicals are added to activated sludges, bacteria release K+ as part of a specific protective stress response, causing sludge deflocculation. Rasmussen et al. (1994) observed K+ release during short-term anaerobic storage of activated sludge and hypothesized that the release took place because the strict aerobes lost their ability to retain internal K+ under anaerobic conditions. High NH^sub 4^^sup +^ levels in WAS from plants C and G indicate that nitrification in these facilities was poor; this could also be due to selective toxicity in these plants. Both the high K+ and NH^sub 4^^sup +^ would contribute to poor flocculation.
Although the data are limited, Figure 2 illustrates that the ratio of K+ in WAS to that in influent produces a useful indicator of poor dewatering for WAS from plants C and G. The exact cause of the unique K+ response in plants C and G was not determined in this study, although it was presumed from these results that sludge characteristics associated with K+ release across the treatment plant might be unique to these sludges. As a result, data points representing plants C and G are noted separately in several figures.
Effects of Sodium and M/D on Biopolymer Binding in Activated Sludge Floe. The M/D ratio has been suggested as a useful indicator of the potential for cations to affect biological flocculation (Higgins and Novak, 1997b). The conceptual basis of M/D was that monovalent cations displace divalent cations from floe by ion exchange, thereby reducing the efficiency of biopolymer binding and resulting in poor flocculation. Table 2 shows that a wide range of M/ D ratios were obtained from the WAS. It can also be seen from Table 2 that there was much more variation in monovalent cations than divalent cations and, among the monovalents, the difference between facilities was the most conspicuous by the variation in Na+. In effect, Na+ was the cation that accounted for the difference of M/D ratios between these WWTPs. This observation was supported by the data in Figure 3 showing that M/D was mainly determined by Na+, except for WAS from plant E.
Figure 2-Effect of K+ release on the SRF of WAS.
Figure 3-Relationship between WAS soluble Na+ and MJD.
To determine if Na+ or M/D were related to sludge and effluent properties from the WWTPs, effluent protein and polysaccharide were compared to the M/D in the WAS. Biopolymer in the solution phase and effluent can be used to characterize the extent of biopolymer binding in activated sludge floe because biopolymer will remain in the solution if flocculation is poor.
As Table 3 shows, large amounts of protein and polysaccharide were found in the WAS solution and effluent from plant B. Moreover, the soluble effluent COD in this plant was much higher than for any other plants. These data suggest that flocculation in plant B was poor and a substantial fraction of organic matter was washed out in the effluent. Plant B is a facility that receives wastewater from a meat processing industry as a major influent \source. This plant was shown to have the highest influent Na+ and the lowest floe Fe and Al of any of the plants. The highest K+ was also found. Accordingly, the M/D ratios for WAS from plants Bl and B2 were 18.4 and 13.7, respectively. The high concentrations of solution biopolymer coupled with high monovalent cations in the influent are consistent with prior findings by Higgins and Novak (1997b). The data in Figure 4 also indicate that biopolymer in the effluent is correlated with M/ D. In general, as the M/D ratio increased, more biopolymer was found in the effluent.
Effects of Aluminum and Iron on Effluent Quality. Although M/D is a useful indicator of the effects of solution cations on effluent and sludge properties, this ratio by itself could not explain all the differences in effluent and sludge characteristics seen at the plants. Specific examples were the wastewater sampled from plants A and E. The high Na+ in plant A (plant E is discussed in a later section) resulted in an M/D ratio of 4.1 for plant Al and 7.1 for plant A2. Although both M/D ratios are much higher than optimum, protein and polysaccharide concentrations found in the effluent are comparable to those from plants with lower M/D ratios. An important difference between the sludges from plant A and plant B is that the Fe and Al content of plant A was much greater than from plant B, and this was thought to account for the different sludge and effluent characteristics.
The concentrations of floe Al and Fe in seven different WWTPs are listed in Table 2. Widely varying amounts of both Al and Fe were found, even for WAS taken from the same WWTP at different times. Maeda and Azumi (1982) and Kakii et al. (1985) characterized activated sludge from a single WWTP over a year and found seasonal variations in Al and Fe in their mixed liquor sludge samples. It was also observed in these two studies that both the metals and ash content increased during the summer but decreased during the winter. However, the ratio of total metal to ash content in the MLSS was constant throughout the year. These investigators postulated that the summer increase was not because of actual increase in the inflow of these materials, but was more likely due to a decrease in the volatile fraction in the mixed liquor. They attributed the low VSS and high ash content in summer to autooxidation of biomass at the higher summer temperatures.
In this study, three WWTPs (A, B, and F) were sampled twice. The sampling time, the percentage of ash in WAS, and the sum and ratio of Al and Fe are listed in Table 5. Consistent with the studies by Maeda and Azumi (1982) and Kakii et al. (1985), both the metal concentrations and percentage of ash were greater in WAS samples collected during the warmer period. The mass of metals in sludge is usually expressed on a total solids basis (Table 2). If increased Al and Fe were not really due to their increased inflow, the concentrations of these inorganic cations might be better expressed as a fraction of the ash content in the mixed liquor. Therefore, units of milligram of cations per gram of ash were used to express Al and Fe concentrations in the floe. The data in Figure 5 show the effect of Al and Fe on effluent biopolymer using both units of milligram per unit of ash and units of milligram per unit of total solids. As Figure 5 shows, units on an ash basis better describe the relationship between Al + Fe and effluent biopolymer.
Figure 4-Effect of M/D on soluble biopolymer in the effluent. Open circles are for systems with large K+ increases through the plant. The trend line was drawn manually to fit the data.
Table 5-Variations in floc cations and ash content in the WAS at different times.
Figure 5-Comparison of the sum of floe Al and Fe with effluent biopolymer. The units of floe cations were based on (a) 1 g total solids and (b) 1 g ash. Open circles are for systems with K+ increases through the plant. The regression lines were obtained without using the K+ release data.
Figure 6-Molecular weight fractions of biopolymer in WAS from plants A and B.
Even more importantly, Figure 5 illustrates that as the sum of Al and Fe increased, solution biopolymer decreased in the plant effluent, suggesting that higher Al and Fe in activated sludge can improve bioflocculation. The striking difference in sludge properties between plant A and plant B further supported this. As Figure 6 shows, more than 90% of solution protein and polysaccharide in WAS from plants Bl and B2 were found in the size less than 30-k Dalton, and there was also a large amount of biopolymer in the size less than 1-k Dalton. The high concentrations of biopolymer and the large fraction of small-sized organic matter in solution indicate that biopolymer binding and polymerization in these wastewater sludges were extremely poor.
It has been speculated that both Fe and Al might have better flocculating capability than divalent cations. Keiding and Nielsen (1997) predicted that when sludge is deficient of Fe and Al, many organic compounds would remain unflocculated and wash out of the system. This is clearly shown in Figure 5. A similar trend was also found between the sum of Al and Fe and effluent COD, although the correlation was better with effluent biopolymer. The effects of these metal ions on effluent quality could be of specific importance in wastewater treatment in Europe and Asia, where effluent COD is often used for design and effluent regulations or for plants where water reuse is to be used or considered.
Effects of Aluminum on Biological Flocculation. Among the seven facilities sampled in this study, plant E showed unique characteristics regarding its cation composition. Although high concentrations of Na+, K+, and NH^sub 4^^sup +^ were found, the highest Mg^sup 2+^ and Ca^sup 2+^ were also present at the same facility, resulting in a M/D ratio of approximately 0.8. At this M/ D ratio, the sludge properties were expected to be among the best of the sludges. However, conditioning and dewatering data shown in Table 3 reveal that WAS from plant E dewatered poorly and large amounts of conditioning chemicals were required. The high level of Fe in this sludge was the highest of any of the sludges, although this did not result in good sludge dewatering properties.
Protein, polysaccharide, and their sum in WAS solution are shown with floe Al in Figure 7. The data clearly illustrate that both the protein and polysaccharide decreased in solution as the Al concentration increased in biomass. This data strongly indicate that Al is an excellent collector of negatively charged organic matter because as floe Al increased, biopolymer in solution decreased. The poor sludge characteristics for WAS from plant E were likely due to the low Al. This suggests that Al is necessary for good flocculation in activated sludge, regardless of the M/D ratio and floe Fe concentration.
Figure 7-Floe Al versus WAS solution (a) protein, (b) polysaccharide, and (c) sum of protein and polysaccharide. Open circles are for systems with large K+ increases through the plant. The regression lines were obtained without using the K+ release data.
The data in Figure 7 also illustrate that once the Al level reached approximately 30 to 40 mg/g ash, additional Al did not seem to reduce the concentration of solution biopolymer. These data suggest there may be an optimum Al content for good flocculation in activated sludge. Because the pH of these wastewaters is near neutral, Al is expected to be present as aluminum hydroxide. Although Al in this form is known to function as an effective coagulant, it is not clear how Al in activated sludge floes coagulates biopolymer during the flocculation process. Clearly, this area requires further study.
Figure 8-Comparison of optimum alum dose with (a) WAS polysaccharide and (b) WAS protein. Open circles are for systems with large K+ increases through the plant.
Alum conditioning data shown in Figure 8 indicate that selective binding between Al and polysaccharide might occur. In Figure 8, optimum alum conditioning doses for 10 different activated sludge samples are compared with protein and polysaccharide passing a 1.5- m filter. Although an interaction between alum and protein cannot be excluded, the data in Figure 8 indicate that the optimal alum demand for conditioning was primarily influenced by solution polysaccharide. In earlier studies for drinking water treatment, polysaccharide found in natural water was selectively removed by aluminum polychlorosulfate (Masion et al., 2000; Vilge-Ritter et al., 1999). Although the environmental conditions between natural water and wastewater sludges are considerably different, these observations support the idea that there is a preferential binding in activated sludge between polysaccharide and aluminum.
The Combined Effects of Aluminum and Sodium on Activated Sludge Characteristics. Because this study was undertaken using samples collected from the field, the effects of various cations on activated sludge characteristics are not expected to be independent; more likely, they will reflect the composite cation concentrations. The results discussed previously clearly pointed out that floe Al and solution Na+ are important in determining differences in sludge characteristics between the seven WWTPs. Therefore, an attempt was made to describe the combined effect of Al and Na+ on sludge properties.
Figure 9-Relationship between optimum FeCl^sub 3^ dose and the ratio of Al/Na. Open circles are for systems with large K+ increases through the plant. The regression line was obtained without using the K+ release data.
The ratio of aluminum to sodium (Al/Na) was compared with the optimum ferric chloride (FeCIs) concentration needed for sludge conditioning to determine if this ratio can account for differences in sludge characteristics. It was expected that the higher value of this ratio would lead to more favorable slu\dge properties. The data in Figure 9 clearly show that optimum Fe (III) conditioning dose for activated sludges was well predicted by the ratio of Al/Na (R^sup 2^ = 0.95). The correlation coefficients for optimum FeCl^sub 3^ doses with floe Al and Na+ were 0.85 and 0.82, respectively. Although further study is required to assess the specific role of Al in bioflocculation, these data suggest that the ratio of Al to Na+ can be usefully applied to predict sludge characteristics. These results also show that floe models that only deal with mono and divalent cations are insufficient to describe the floe structure and the role of cations in flocculation.
Figure 10-Relationship between optimum FeCl^sub 3^ dose and WAS solution biopolymer. Open circles are for systems with large K+ increases through the plant.
Figure 11-Impact of colloidal biopolymer on (a) CST and (b) optimum cationic polymer dose. The analysis of molecular weight size separation was not performed for WAS from plant E so CST and polymer dose data for WAS from plant E are not shown in this figure.
The Impact of Colloidal Solids on Sludge Conditioning and Dewatering. Alum conditioning data (Figure 8) showed that chemical conditioning of sludges was affected by colloidal or solution-phase biopolymer. This observation is more clearly illustrated from the optimum FeCI^sub 3^ conditioning data presented in Figure 10. As the data show, the optimum FeCI^sub 3^ conditioning doses for WAS were determined by solution biopolymer, where solution biopolymer is the material less than 0.45 m.
In contrast to the FeCIs and alum data, optimum cationic polymer doses, SRF, and CST were more closely associated with a specific size range of biopolymer, that is, the biopolymer between 1.5 m and 30-k Dalton. The relationship between colloidal biopolymer and both CST and the optimum cationic polymer dose is shown in Figure 11. These data showing that colloidal biopolymer determines conditioning and dewatering properties are in good agreement with studies showing that smaller particles in sludge determine sludge dewaterability (Bivins and Novak, 2001; Karr and Keinath, 1978; Novak et al., 1988). Because colloidal biopolymer in WAS can result from poor flocculation due to a lack of floe Al or from excessive influent Na+, the control of cations can benefit both sludge dewatering operations and effluent quality.
Implications. This study showed that insufficient Al and Fe, especially Al, in activated sludge floe resulted in poor binding of biopolymer, which led to poor effluent quality, high-conditioning chemical requirements, and poor sludge dewatering rates. The aluminum- and iron-deficient wastewater sludges were from facilities with a high industrial wastewater input (Tables 1 and 2). Activated sludges from industrial facilities often exhibit poor flocculation compared to domestic wastewater systems. Grady et al. (1999) attributed this partly to the absence of influent wastewater EPS. Based on the results obtained in this study, it appears that the limitation is more likely divalent cations, especially Al. An outgrowth of this observation is that for the WWTPs with low influent Al and Fe, addition of these metal ions should lead to significant improvement in plant operation. Floe Al at or greater than 30 mg/g ash (approximately 10 mg/g total solids; Figure 7) or the ratio of Al/Na greater than 10 (mg/g ash)/(meq/L) (Figure 9) can be targeted to improve flocculation and sludge dewatering characteristics.
Results obtained from a field study illustrated that various wastewater sludge characteristics were strongly influenced by influent wastewater cation content. The study postulated that monovalent and divalent cations as well as the floe trivalent cations, Al and Fe, have significant impacts on activated sludge characteristics. The following conclusions are drawn from this study:
* Wastewaters with high influent Na+ and low floe Al and Fe had poor effluent quality, as evidenced by a high effluent COD.
* Effluent biopolymer (protein + polysaccharide) and effluent COD vary directly with the sum of Al and Fe in activated sludge floe. These effluent parameters decreased as the floe Al + Fe increased.
* Increases in K+ by greater than 3-fold across an activated sludge facility were associated with poor flocculation and dewatering rates of WAS. This suggests that such changes could be useful for indicating plant upsets.
* An Al level of 10 mg/g total solids or 30 mg/g ash is required for good flocculation and a high quality effluent.
* The ratio of AIfNa appeared to be an important cation parameter for describing the effect of cations on effluent and sludge properties.
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Credits. This work was funded by US Filter/Veolia Environment, North American Technology Center, Vineland, New Jersey.
Authors. Chul Park and Chris D. Muller are Ph.D. candidates in the Department of Civil and Environmental Engineering at Virginia Polytechnic Institute & State University, Blacksburg, Virginia. Mohammad Abu-Orf is director-Biosolids R & D Program, US Filter/ Veolia Environment, North American Technology Center. John T. Novak is a professor of civil and environmental engineering with the Virginia Polytechnic Institute & State University. Correspondence should be addressed to John T. Novak, Department of Civil & Environmental Engineering, Virginia Polytechnic Institute & State University, Blacksburg, VA 24061; e-mail: [email protected]
Submitted for publication January 21, 2003; revised manuscript submitted June 28, 2004; accepted for publication September 29, 2004.
The deadline to submit Discussions of this paper is April 15, 2006.
Copyright Water Environment Federation Jan 2006