Bioleaching using an iron-oxidizing bacterium, Acidithiobacillus ferrooxidans, and its biogenic flocculants was evaluated to improve the dewaterability of chemically enhanced primary treatment (CEPT) sewage sludge. CEPT sludge in flasks was inoculated with A. ferrooxidans culture, medium-free cells and the cell-free culture filtrate with and without the energy substance Fe2+, and periodically the sludge samples were analysed for the dewaterability. This investigation proves that bioleaching effectively improved the sludge dewaterability as evidenced from drastic reduction in capillary suction time (≤20 seconds) and specific resistance to filtration (≥90%); however, it requires an adaptability period of 1–2 days. On the other hand, the biogenic flocculant produced by A. ferrooxidans greatly decreased the time-to-filtration and facilitated the dewaterability within 4 h. Results indicate that rapid dewatering of CEPT sludge by biogenic flocculants provides an opportunity to replace the synthetic organic polymer for dewatering.

INTRODUCTION

Chemically enhanced primary treatment (CEPT) is an advanced primary treatment process to remove pollutants from wastewater using chemical coagulants and flocculants. In the CEPT process, inorganic coagulants such as ferric chloride or polyaluminum chloride, followed by anionic organic polymer are added as flocculants (Poon & Chu 1999; Wang et al. 2009). CEPT as a wastewater treatment process has the advantages of efficient reduction of biochemical oxygen demand and phosphate (Jiang & Graham 1998), low energy requirement (De Feo et al. 2008), and easy to operate and maintain (Jordao & Volschan 2004). The CEPT process has been considered as a cost-effective method for the treatment of municipal wastewater for those cities that cannot afford to have secondary wastewater treatment. The CEPT plant located in Stonecutters Island, Hong Kong, is the world's largest CEPT plant with a treatment capacity of 1.7 million cubic metres of sewage per day. This plant uses ferric chloride and polymer to facilitate faster settlement of suspended solids. The settled sludge is then removed from the sewage, resulting in a clear supernatant for discharge to the sea. About 1.4 million cubic metres of sewage are treated every day, which generates about 600 tonnes of sewage sludge.

Despite the advantages of the CEPT process, it generates a huge volume of odorous sludge with water content over 95%. The CEPT sludge has a higher amount of nitrogen, phosphorus and heavy metals than conventional biologically treated sludges (Chu et al. 1998). The water content of the sludge should be removed or reduced to facilitate the disposal or recycling of the sludge. Dewatering of the sewage sludge is an expensive process covering approximately 50% of the total operation cost of the wastewater treatment process due to the requirement of pre-dewatering sludge conditioning. In order to improve the sewage sludge dewatering, various physical (Xu et al. 2005; Lee 2006), chemical (Zhai et al. 2012; Niu et al. 2013), physicochemical (Mohapatra et al. 2010) and biological (Molla & Fakhru'l-Razi 2012; Murugesan et al. 2014a, b) conditioning methods have been reported in the literature. Currently, commercial synthetic polymer-based flocculants are being used as sludge conditioners in sewage treatment works due to their dewaterability. However, they are more expensive and also cause secondary pollutions. Besides, some polyacrylamide polymeric derivatives are recalcitrant and some of the degradation residues released are carcinogenic compounds and/or strongly odorous (Chang et al. 2005). Therefore, intensive researches have been undertaken to develop biopolymer flocculants as an alternative conditioning agent to improve sludge dewatering (More et al. 2012). Although some microbial biopolymers are effective, they require metal cations to facilitate flocculation, and in addition, production and separation of biopolymers require additional cost.

The bioleaching technique, a successful strategy in biohydrometallurgy mediated by iron- or sulphur-oxidizing bacteria or their consortium, has been reported as a potential method for metal bioleaching from sewage sludge (Wong et al. 2002). The iron-oxidizing bacterium Acidithiobacillus ferrooxidans or its products are involved directly or indirectly in the bioleaching process as shown in the following equations. 
formula
Our recent report on treatment of anaerobically digested sludge proves that the A. ferrooxidans mediated bioleaching process is an effective strategy to improve the sludge dewatering (Murugesan et al. 2014a, b). Acidithiobacillus ferrooxidans effectively bio-oxidizes the energy substrate ferrous iron and produces ferric iron (Kurade et al. 2014). The biogenic ferric iron effectively coagulates and flocculates the sludge particles, and improve the dewaterability of the sludge. In addition, A. ferrooxidans also produces extracellular polymeric substances (Gehrke et al. 1998; Zeng et al. 2010), which might be synergistically involved in sludge flocculation. The properties of sewage sludge from conventional treatment and CEPT processes vary (Chu et al. 1998); thus the bioacidification, flocculation and dewatering might also vary. Therefore, the objective of this investigation was to use the different fractions of A. ferrooxidans culture as sludge conditioning agents with and without ferrous iron supplementation to elucidate their role in improving the dewaterability of CEPT sludge.

MATERIAL AND METHODS

Sludge sample

The CEPT sludge was collected from Stonecutters Island Sewage Treatment Works, Hong Kong, and immediately transported to the laboratory and stored at 4 °C until used in the experiments. Selected initial physicochemical and dewaterability properties of the sludge are given in Table 1.

Table 1

Selected properties of CEPT sludge used in this study

ParameterValue
pH 6.2 
ORP (mV) −97 
CST (s) 122.7 
SRF (m kg−17.19 × 1013 
Total solids (%) 2.71 
Organic matter (%) 52.5 
ParameterValue
pH 6.2 
ORP (mV) −97 
CST (s) 122.7 
SRF (m kg−17.19 × 1013 
Total solids (%) 2.71 
Organic matter (%) 52.5 

Preparation of biogenic flocculants

The iron-oxidizing bacterium A. ferrooxidans ANYL-1, previously isolated from anaerobically digested sludge (Gu & Wong 2004a, b), was routinely maintained in a modified 9 K medium (Silverman & Lundgren 1959) at pH 2.5, with 44.2 g L−1 FeSO4.7H2O as the energy source. After 3 days of incubation, the whole culture (∼108 cells mL−1), filtrate obtained after filtering through a 0.22 μm filter membrane, and the isolated cells were used for sludge conditioning

Sludge treatment

Sludge treatment was carried out in 500 mL Erlenmeyer flasks with 270 mL CEPT sludge and 30 mL of A. ferrooxidans culture or cells or the filtrate that was separated from 30 mL culture. The cells were washed and resuspended in 30 mL 9 K medium and used as inoculum. The treatment was evaluated with and without supplementing Fe2+ as the energy substrate. All the flasks were incubated at 30 °C at 180 rpm on a rotary shaking incubator. All the treatments were carried out in duplicate. Periodically, sludge samples (40–50 mL) were collected from the control and treatment flasks to evaluate the dewaterability by capillary suction time (CST), time to filtration (TTF) and specific resistance to filtration (SRF).

Analytical methods

Sludge pH and oxidation–reduction potential (ORP) were measured using an Orion 920A pH meter. Sludge dewaterability was determined by measuring the CST and SRF of the sludge. CST was measured using a capillary suction timer (Triton Electronics Type 304M, UK) with 18 mm reservoir (holding capacity: 6 mL) resting on standard CST filter paper (7 × 9 cm). SRF was determined using 30 mL of well-mixed sludge sample by filtration in a Buchner funnel fitted with a filter paper disc (Advantech No. 1) and using 0.07 MPa suction pressure. The TTF was estimated by recording the collection of each 5 mL of the filtrate until complete filtration; the SRF was calculated according to the method of Lo et al. (2001). Sludge samples were centrifuged and filtered through 0.45 μm membrane filter, and the residual ferrous iron contents were determined through spectrophotometric method using 1,10-phenanthrene (APHA 2005).

Statistical analysis

All the experiments were conducted in duplicate. The data presented are the means and standard deviations of the duplicate samples. Statistical analysis was carried out with IBM SPSS software version 21.0. The variations between different treatments (Dunnett multiple comparisons test) were considered statistically significant at a confidence interval of P < 0.05.

RESULT AND DISCUSSION

Sludge properties

CEPT sludge is a primary sludge consisting of the flocculated colloidal particles. The collected CEPT sludge had the initial properties of pH 6.2, ORP −97 mV, total solids 2.71%, CST 122.7 s, and SRF 7.19 × 1013 m kg−1 (Table 1). The high CST and SRF values of the CEPT sludge indicates the poor filterability and dewaterability. Thus, an effective sludge conditioning is very essential to improve the dewaterability. The CST values of the sludge should be below 20 s to achieve an effective dewaterability. In this study, sludge treatment was performed using whole A. ferrooxidans and its fractionated components, including isolated cells and cell-free filtrate. To determine the effect of energy substrate of A. ferrooxidans, Fe2+ was amended to the sludge on the basis of the Fe2+/sludge solid ratio, 0.05:1, that had been optimized in our preliminary studies.

Sludge bioacidification

Under the employed conditions, the initial pH of the control sludge gradually increased from 6.2 to 7.8 during the 6-day incubation (Figure 1(a)). A similar trend was also observed in the sludge treated with A. ferrooxidans cells without Fe2+. On the other hand, treatment with A. ferrooxidans cells supplemented with Fe2+ showed a slight reduction in pH. During the first 2 days of incubation, the initial pH of sludge decreased from 6.2 to 5.5. After 2 days, the sludge pH rapidly dropped and reached 3.5 and 2.3 after 4 and 6 days of incubation, respectively. Similarly, with Fe2+ addition, CEPT sludge treated with A. ferrooxidans culture showed the maximum acidification compared with other treatments (Figure 1(a)). The pH profile of treatments indicates that CEPT sludge possessed high buffering capacity. In contrast, under the similar bioacidification treatment conditions, the pH of the activated sludge and anaerobically digested sludge rapidly decreased to strong acidic condition (Liu et al. 2012; Murugesan et al. 2014a, b).
Figure 1

Changes in CST (a) and SRF (b) of CEPT sludge treated with whole A. ferrooxidans culture, and its cells only and cell-free culture filtrate.

Figure 1

Changes in CST (a) and SRF (b) of CEPT sludge treated with whole A. ferrooxidans culture, and its cells only and cell-free culture filtrate.

Figure 1(b) shows the changes in ORP profile of the CEPT sludge during the sludge treatment with A. ferrooxidans. Similar to the pH profile, the ORP of control and A. ferrooxidans cells without Fe2+ showed the same trend, whereas a significant (P < 0.05) increase in ORP was observed in the treatments with the addition of whole culture and filtrate only, indicating the role of iron-flocculant from A. ferrooxidans culture, especially in the liquid phase. It can be seen that a rapid increase in ORP was observed after 2 days of incubation, which corroborated well with a reduction in pH (Figure 1(a)).

The sludge acidification under iron-oxidizing bacterial treatment is due to biooxidation of Fe2+ to Fe3+ and further hydrolysis of Fe3+ which releases protons (Wong & Gu 2004, 2008). Thus, the sludge becomes strongly acidified in Fe2+-supplemented treatments, which could increase the in situ generation of iron-flocculant and facilitate the dewaterability. On the other hand, the sludge treated with whole culture or filtrate without Fe2+-supplement showed slow acidification as it depends on the ferric iron-flocculant from the A. ferrooxidans culture. Figure 2 reveals the residual Fe2+ concentrations in sludge filtrate. During the first day of incubation, the Fe2+ concentration in Fe2+-supplemented treatments decreased about 41–55% which might be due to the rapid abiotic oxidation of Fe2+ at pH above 4.0. But after 2 days of incubation, when the pH decreased to below 4.0, the Fe2+ concentrations rapidly decreased. This clearly showed the in situ biooxidation of Fe2+ by the production of iron-flocculant from A. ferrooxidans (Figure 2). During the initial treatment, the pH reduction was very poor, which might be due to acclimatization of A. ferrooxidans cells to the sludge, and the presence of inhibitory organic compounds in the sludge (Gu & Wong 2004a, b). However, as the treatment progresses, within 2 days, the organic acids might have been consumed by heterotrophic microbes (Gu & Wong 2007; Zhou et al. 2013), facilitating the biooxidation of Fe2+.
Figure 2

Changes in the residual ferrous iron concentration during bioleaching of sludge using whole A. ferrooxidans culture, and its cells only and cell-free culture filtrate.

Figure 2

Changes in the residual ferrous iron concentration during bioleaching of sludge using whole A. ferrooxidans culture, and its cells only and cell-free culture filtrate.

Sludge dewaterability

Sludge dewatering is essential to reduce the volume of sludge to be disposed and the cost of transportation to the disposal site. CST and SRF are the commonly accepted parameters to determine the sludge dewaterability in laboratory studies (APHA 2005; Sawalha & Scholz 2010). It has been reported that the CST value of sludge should be below 20 s for efficient dewaterability. The CST of the control sludge was 122.7 s and this value did not decrease during the 6-day incubation, indicating that the dewaterability of CEPT sludge was quite difficult without effective pre-conditioning (Figure 3(a)). Sludge treatment with isolated cells of A. ferrooxidans without Fe2+ showed no reduction of CST, while the same treatment with Fe2+ addition reduced the sludge CST to 72.5 s within 0.0416 day (4 h) of treatment, indicating that the energy substrate added to the sludge improved the dewaterability. However, this final CST is not suitable to achieve a good filterability and required a prolonged bioleaching process. On the other hand, when the sludge was treated with A. ferrooxidans culture or its filtrate, a significant (P < 0.05) reduction of CST was observed within 4 h of the treatment. The initial CST (122.7 s) dropped to the lowest values of 20, 25 and 21 s in the treatments that received A. ferrooxidans culture with Fe2+, culture without Fe2+, and filtrate without Fe2+, respectively (Figure 3(a)). Further increase in bioleaching time, increased the CST of these three treatments but to a much small extent for the treatment with A. ferrooxidans culture with Fe2+. The CST increase might be due to the deflocculation of the flocculated sludge. This study indicates that biogenic iron-flocculant existing in the filtrate produced by A. ferrooxidans exerted a significant flocculation and CST reduction for CEPT sludge without the need for Fe2+ and prolonged bioleaching.
Figure 3

Changes in CST (a) and SRF (b) of CEPT sludge treated with whole A. ferrooxidans culture, and its cells only and cell-free culture filtrate.

Figure 3

Changes in CST (a) and SRF (b) of CEPT sludge treated with whole A. ferrooxidans culture, and its cells only and cell-free culture filtrate.

The assessment of dewaterability by Buchner funnel filtration revealed that the TTF of the sludge treated with A. ferrooxidans culture or its filtrate significantly improved within 4 h of incubation as compared to the control (Table 2). The determination of SRF indicated that dewaterability profiles of control sludge and treated sludge differed significantly (Figure 3(b)). The initial SRF value of the sludge decreased about 35% within 4 h in the treatment with A. ferrooxidans cells with Fe2+, whereas 83–93% reduction was observed within 4 h with whole culture, and with its filtrate with and without Fe2+. This once again confirmed the presence of flocculants in the culture of AF and clearly indicates the advantage of using iron-oxidizing bacteria for sludge conditioning to improve the CEPT sludge dewaterability. Our previous study also reported that iron-oxidizing bacteria showed faster bioacidification and metal leaching than sulfur-oxidizing bacteria (Chan et al. 2003). This is the first report on A. ferrooxidans-mediated sludge conditioning and improvement of the dewaterability for CEPT sludge. Since, the CEPT process uses ferric chloride as the main coagulant for settling of sewage, the use of A. ferrooxidans to produce biogenic ferric ions as a replacement of its chemical form is a promising and economically feasible strategy.

Table 2

Filterability of CEPT sludge: TTF of CEPT sludge treated with A. ferrooxidans culture, cells and filtrate

TreatmentsTime-to-filtration (s) of control and treated sludge on different days
00.04161246
Sludge 1,424 1,408 1,733 2,925 6,630 3,621 
Sludge + AF culture ND 199 194 944 224 188 
Sludge + AF culture + Fe2+ ND 100 52 40 105 69 
Sludge + AF cells ND 1,800 2,040 4,620 3,420 2,881 
Sludge + AF cells + Fe2+ ND 665 155 584 140 62 
Sludge + AF filtrate ND 113 215 742 255 178 
TreatmentsTime-to-filtration (s) of control and treated sludge on different days
00.04161246
Sludge 1,424 1,408 1,733 2,925 6,630 3,621 
Sludge + AF culture ND 199 194 944 224 188 
Sludge + AF culture + Fe2+ ND 100 52 40 105 69 
Sludge + AF cells ND 1,800 2,040 4,620 3,420 2,881 
Sludge + AF cells + Fe2+ ND 665 155 584 140 62 
Sludge + AF filtrate ND 113 215 742 255 178 

Time-to-filter for 15 mL of sludge from various treatments. Data are the mean of duplicates samples.

CONCLUSIONS

This study proves the efficiency of A. ferrooxidans culture and culture filtrate for the improvement of CEPT sludge dewaterability. Although bioleaching is effective, it requires substantial time to achieve the dewaterability; therefore, the culture filtrate of the A. ferrooxidans can be used as a flocculant without the addition of Fe2+ to achieve rapid flocculation and dewatering of CEPT sludge. This strategy could be a potential alternative method to the organic polymers commonly used in sludge dewatering.

ACKNOWLEDGEMENT

This work was financially supported by Innovation and Technology Support Programme (Project No. ITS/297/11), Innovation and Technology Commission, Hong Kong SAR Government.

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