Water purification and wastewater treatment generate sludge, which must be adequately handled to prevent detrimental effects to the environment and public health. In this study, we examined the influence of the application of settled sludge from a drinking water treatment plant (SDWTP) on the anaerobic digestion (AD) of the thickened primary sludge from a municipal wastewater treatment plant (SWWTP) which uses chemically assisted primary treatment (CAPT). On both plants the primary coagulant is ferric chloride. The study was performed at laboratory scale using specific methanogenic activity (SMA) tests, in which mixtures of SWWTPSDWTP with the ratios 100:00, 80:20, 75:25, 70:30 and 00:100 were evaluated. Methane detection was also performed by gas chromatography for a period of 30 days. Our results show that all evaluated ratios that incorporate SDWTP, produce an inhibitory effect on the production of methane. The reduction in methane production ranged from 26% for the smallest concentration of SDWTP (20%) to more than 70% for concentrations higher than 25%. The results indicated that the hydrolytic stage was significantly affected, with the hydrolysis constant Kh also reduced by approximately 70% (0.24–0.26 day−1 for the different ratios compared with 0.34 day−1 for the SWWTP alone). This finding demonstrates that the best mixtures to be considered for anaerobic co-digestion must contain a fraction of SDWTP below 20%.

INTRODUCTION

Several technologies are employed for water treatment for human consumption. Owing to the flexibility in the treatment of raw water with different levels of turbidity, the conventional methods of coagulation, flocculation, sedimentation, filtration, disinfection and pH adjustment are prevalent (Kawamura 2000; WHO 2011). In Colombia, approximately 63% of drinking water treatment plants (DWTPs) utilize this technology and a significant fraction of these plants employ iron chloride (FeCl3) as the primary coagulant (MDE 2000).

The sludge generated in a DWTP is primarily composed of the material in the raw water (organic or inorganic materials suspended or dissolved in the water). After being treated with chemical processes, such as oxidation, coagulation and flocculation, these materials are efficiently separated from the water via physical barriers. Sedimentation and filtration are the most common processes for the removal of particulates (turbidity and suspended solids) (WHO 2011). According to Torres et al. (2012), up to 90% of the total solids (TS) can be removed by sedimentation.

Sludge from a wastewater treatment plant (WWTP) concentrates part of the pollutants removed from wastewater. Although their quantity and quality are highly variable and dependent on the characteristics of the wastewater and the treatment technology, they generally contain large quantities of organic matter. As a result, anaerobic digestion (AD) processes, which are viable treatment options for reducing environmental impacts, constitute a source of renewable energy due to their production of methane (Appels et al. 2008).

In the process of AD, macronutrients (nitrogen, phosphorous and potassium) and micronutrients are both essential to the growth of microorganisms. The absence of any type of nutrient can render the treatment unfeasible. Depending on their concentration, these components can either stimulate or inhibit the process (Feng et al. 2010). Iron is one of the trace elements involved in energetic metabolism, such as cytochrome and ferredoxin in methanogenic organisms, as well as some enzymes. According to the properties of the substrate, iron concentrations between 1 and 200 mg L−1 stimulate AD (Schattauer et al. 2011). Other authors, such as Liu et al. (2011) and Ofverstrom et al. (2011), remarked that the addition of iron in concentrations above 300 mg L−1 may generate an inhibitory effect on AD reflected in a significant decrease in the quality and quantity of biogas.

Regarding the digestion of mixtures of SWWTP with SDWTP, Manzochi et al. (2006) and Carvalho (2011) employed SWWTP:SDWTP ratios between 80:20 and 40:60 and discovered that the highest methane production was obtained for lower concentrations of SDWTP (20% and 30%, respectively), with values of approximately 0.37 gCODCH4 gTVS−1 in the latter case. Lesteur et al. (2010) suggest analyses on a case-by-case basis to prevent inhibition problems, as adequate proportions for the co-digestion of the residues are dependent on the characteristics of the substrates and the inoculum.

Considering that the evaluated sludges in this study were obtained from WWTP and DWTP that use ferric chloride in their treatment processes, the potential for the joint anaerobic co-digestion of both types of sludge was evaluated using specific methanogenic activity (SMA) tests. These test results may be beneficial if the joint treatment can be performed with existing WWTP sludge, without requiring large economic investment. In addition they may provide treatment alternatives for this type of waste, which is increasing in the cities, especially in developing countries where the management for this type of sludge is incipient.

MATERIALS AND METHODS

Experimental development

Substrate (thickened primary WWTP and DWTP sludge) and inoculum

The SDWTP was obtained from a conventional DWTP, in which the process of coagulation is performed with ferric chloride and the flocculation–sedimentation process occurs in high-rate sludge-blanket clarifiers (Pérez et al. 2012). For the AD tests, the SDWTP was dehydrated through centrifugation to obtain a concentration of total volatile solids (TVS) that is similar to the concentration in the SWWTP obtained from a WWTP that employs ferric chloride in the CAPT technique, in which the primary sludge is thickened and subsequently undergoes AD.

The substrates were characterized based on the guidelines of APHA (2012), in terms of the pH (Unit), chemical oxygen demand (COD) (g L−1), TS, TVS (g L−1) and lipids (g L−1). The sludge used as an inoculum for the SMA tests was obtained from one of the anaerobic digesters in the same municipal WWTP from which the SWWTP was obtained. This sludge was characterized in terms of the physico-chemical variables of the corresponding substrates.

Evaluation of the incorporation of SDWTP on the AD of SWWTP for methane production

The SMA tests were performed at an altitude of 1,001 meters above sea level (m.a.s.l.) at an average environmental temperature of 23°C. The temperature of the experiments was set to 30 ± 0.5°C, in batch reactors with a capacity of 120 mL and each reactor by triplicate. The reactors were incubated in a Brinder incubator and intermittently agitated by hand for 30 days. The quantification of the methane in the biogas was conducted using gas chromatography on a GC2014 chromatograph. To guarantee the optimal conditions for the anaerobic degradation of the substrate, the pH was set to values near 7.0 unit with a NaOH solution at 0.01 mol/L, and a solution of macronutrients and micronutrients was used (Aquino et al. 2007).

A substrate/inoculum (S/I) ratio of 2 gTVSsubstrate per 1 gTVSinoculum was employed for all tests (Raposo et al. 2006) with a constant inoculant concentration of 2 gTVS L−1 (Aquino et al. 2007). The substrate ratios SWWTP:SDWTP were T1:100:00; T2:80:20; T3:75:25; T4:70:30 and T5:00:100. At the beginning and end of the tests, the iron (Fe+3) levels, the pH and the volatile fatty acids (VFA) in each reactor were determined.

To quantify the incidence of inhibitory phenomena, the inhibition percentage – %I, was determined based on Equations (1) and (2). 
formula
1
 
formula
2
where %SMA is the ratio of the SMA of each evaluated concentration to the SMA of the SWWTP and %I is the inhibition percentage that corresponds to each evaluated SWWTP:SDWTP ratio.
Considering that the limiting stage for the AD of solid residues and sludge is the hydrolysis (Aldin 2010) the influence of the incorporation of SDWTP on the hydrolysis of the process by the application of a first-order kinetic model was evaluated (Liew et al. 2012). Using Equations (3) and (4), Equation (3) converts the chromatographic data into concentrations (Mmol L−1) and Equation (4), which is used to determine the hydrolysis constant: 
formula
3
where M(t) is the methane concentration at time t (Mmol L−1), C is the methane concentration (mg L−1) at time t, and MW is the molecular weight of methane (g moL−1). 
formula
4
where ln is the natural logarithm, t is time (d), MU is the methane production at the end of the experiment (Mmol L−1), M is the remaining production at a given time (M = MUM(t)) and Kh is the hydrolysis–acidogenesis constant (day−1).

The model was validated based on the determination of the correlation coefficient (R2); the validation was performed using Microsoft Office Excel 2007.

RESULTS AND DISCUSSION

Characterization of the substrate (thickened primary WWTP and DWTP sludge) and inoculum

Table 1 shows the results of the physico-chemical analyses of the substrates (SWWTP and SDWTP) and the inoculum.

Table 1

Physico-chemical characterization of the substrates and the inoculum

Parametern Units SWWTP SDWTPa Inoculum 
pH Units 6.3 ± 0.2 5.3 ± 0.1 7.2 ± 0.1 
COD g L−1 40.2 ± 1.7 8.21 ± 0.39 67.2 ± 2.7 
TVS g L−1 35.7 ± 1.1 31.3 ± 1.2 15.6 ± 2.7 
TS g L−1 108.2 ± 2.5 401.3 ± 6.0 46.6 ± 2.6 
TVS/TS – 0.33 ± 0.01 0.08 ± 0.02 0.33 ± 0.05 
Lipid g L−1 1.2 ± 0.2 ND ND 
Parametern Units SWWTP SDWTPa Inoculum 
pH Units 6.3 ± 0.2 5.3 ± 0.1 7.2 ± 0.1 
COD g L−1 40.2 ± 1.7 8.21 ± 0.39 67.2 ± 2.7 
TVS g L−1 35.7 ± 1.1 31.3 ± 1.2 15.6 ± 2.7 
TS g L−1 108.2 ± 2.5 401.3 ± 6.0 46.6 ± 2.6 
TVS/TS – 0.33 ± 0.01 0.08 ± 0.02 0.33 ± 0.05 
Lipid g L−1 1.2 ± 0.2 ND ND 

DWTP centrifuge sludge; ND: not determined; n: number of samples: 5.

As the substrates presented an acidic pH, they needed to be conditioned to prevent the inhibitory effect on AD. As expected, the SWWTP presented a larger concentration of organic matter than the SDWTP due to the predominantly inert composition (particularly clay and sands) of the latter (WHO 2011).

The inoculum presents values that are typical of anaerobic sludge from CAPT plants (Raposo et al. 2006). The low ratio TVS/TS is likely a consequence of the characteristics of the raw wastewater, which is diluted due to the predominance of combined sewage systems.

Evaluation of the incorporation of SDWTP on the AD of SWWTP for methane production

Table 2 shows that both the SWWTP and the SDWTP presented high iron concentrations, given that both treatment plants employ ferric chloride (CAPT and conventional treatment, respectively). Although iron concentrations in the SWWTP fall within the range of 300–600 mg Fe+3 L−1, in which an inhibitory effect on AD may exist (Liu et al. 2011), much higher values were obtained for all evaluated treatments with SDWTP. This may have stimulated the acidification processes reflected in the pH and VFA measured after the tests, which were higher than the maximum values recommended (1,500 mg L−1) for the adequate development of the AD (Vereda et al. 2006). This situation arises as the iron reduces the alkalinity, forming precipitates as it reacts with the carbonates present in the medium (Mamais et al. 1994). As a result, a reduced iron concentration is observed after the tests.

Table 2

Parameters measured initially and after the process

  Fe+3 (mg L−1)
 
pH (units)
 
VFA (mg L−1)
 
Tin (SWWTP:SDWTP) Before Final Before Final Before Final 
T1 (100:00) 370 ± 50.1 222 ± 25.3 7.0 ± 0.1 6.7 ± 0.1 270 ± 72.3 1,050 ± 45.3 
T2 (80:20) 1,076 ± 120 694 ± 30.5 7.0 ± 0.1 6.5 ± 0.2 260 ± 56.5 1,550 ± 44.5 
T3 (75:25) 1,144 ± 142.3 950 ± 52.5 7.1 ± 0.2 6.5 ± 0.1 270 ± 45.8 1,550 ± 39 
T4 (70:30) 1,222 ± 122.5 939 ± 85.3 7.1 ± 0.1 6.5 ± 0.1 230 ± 57.5 1,850 ± 45.2 
T5 (00:100) 1,182 ± 130 974 ± 45.2 7.0 ± 0.1 6.7 ± 0.1 260 ± 55.8 2,150 ± 85.6 
  Fe+3 (mg L−1)
 
pH (units)
 
VFA (mg L−1)
 
Tin (SWWTP:SDWTP) Before Final Before Final Before Final 
T1 (100:00) 370 ± 50.1 222 ± 25.3 7.0 ± 0.1 6.7 ± 0.1 270 ± 72.3 1,050 ± 45.3 
T2 (80:20) 1,076 ± 120 694 ± 30.5 7.0 ± 0.1 6.5 ± 0.2 260 ± 56.5 1,550 ± 44.5 
T3 (75:25) 1,144 ± 142.3 950 ± 52.5 7.1 ± 0.2 6.5 ± 0.1 270 ± 45.8 1,550 ± 39 
T4 (70:30) 1,222 ± 122.5 939 ± 85.3 7.1 ± 0.1 6.5 ± 0.1 230 ± 57.5 1,850 ± 45.2 
T5 (00:100) 1,182 ± 130 974 ± 45.2 7.0 ± 0.1 6.7 ± 0.1 260 ± 55.8 2,150 ± 85.6 

n: number of samples: 3.

In addition, Horban & van den Berg (1979) indicate that concentrations above 600 mgFe+3 L−1, in addition to promoting the accumulation of VFA, reduce the capacity for the assimilation of important nutrients in AD, such as phosphorus.

Figure 1 shows the behaviour of the SMA for each of the evaluated treatments. The values obtained with a SWWTP:SDWTP ratio of less than 20% exceed those reported by other authors such as Carvalho (2011) (0.37gCOD gTVS−1). However, it was evident that an inverse proportionality between the SMA and the addition of SDWTP and a negative effect from the lowest proportion of incorporated SDWTP was observed. The statistical analysis of the SMA results indicates significant differences (p < 0.05), especially between the ratios 75:25 and 70:30 compared with the ratios 100:00 and 80:20. This finding indicates that the SWWTP:SDWTP ratios, which may be considered as candidates for a possible joint AD, should be less than 80:20 for all evaluated sludge ratios. However, other factors, such as the control of the buffer capacity, must be considered.

Figure 1

Specific methanogenic activity for each evaluated treatment.

Figure 1

Specific methanogenic activity for each evaluated treatment.

Figure 2 shows the behaviour of the inhibition percentage and that even the smallest evaluated SWWTP:SDWTP ratio (80:20) causes a 26% inhibition and a maximum inhibition of 70% for ratios above 75:25. These results are similar to the results presented by Carvalho (2011), who discovered an inhibition percentage of 27% for SDWTP (with ferric chloride) concentration below 30%.

Figure 2

Inhibition percentage index for each evaluated treatment.

Figure 2

Inhibition percentage index for each evaluated treatment.

Regarding the analysis of the hydrolysis stage of the process, Figure 3 shows an extensive duration of the lagphase of the SWWTP (approximately 9 days). This duration may be associated with the particular characteristics of this sludge; according to Esposito et al. (2012) and Hidalgo & Martín (2014), substrates with high lipid concentrations may destabilize the metabolic process of the methanogenic microbial consortia. In the case of the SWWTP, the lipids are obtained from the raw wastewater.

Figure 3

Methane production for each evaluated treatment.

Figure 3

Methane production for each evaluated treatment.

The comparison of the performance of the different SWWTP:SDWTP ratios with the performance of the SWWTP alone, show a negative effect of the incorporation of the SDWTP, as the methane production is significantly decreased for the same period of time (169,000 ± 76,500 mg CH4 L−1 for T1, compared with productions below 3,500 ± 120 mg CH4 L−1 for all other treatments). This result provides evidence of the inhibition phenomena associated with the low buffer capacity and the Fe+3 levels caused by the incorporation of the SDWTP. This behaviour is confirmed in Table 3, which lists the corresponding hydrolysis constants for each evaluated ratio.

Table 3

Hydrolytic-acidogenesis constants for each evaluated ratio

Ti (SWWTP:SDWTPKh (day−1R2 
T1 (100:00) 0.34 ± 0.05 0.89 
T2 (80:20) 0.26 ± 0.04 0.88 
T3 (75:25) 0.25 ± 0.05 0.85 
T4 (70:30) 0.24 ± 0.03 0.85 
T5 (00:100) – – 
Ti (SWWTP:SDWTPKh (day−1R2 
T1 (100:00) 0.34 ± 0.05 0.89 
T2 (80:20) 0.26 ± 0.04 0.88 
T3 (75:25) 0.25 ± 0.05 0.85 
T4 (70:30) 0.24 ± 0.03 0.85 
T5 (00:100) – – 

The Kh values for T1 are typical of the AD processes in wastewater sludge and solid residues (Aldin 2010). For all evaluated ratios, the values of Kh were considerably reduced, which verifies the inhibitory effect at this stage of the process. According to Muddo & Kumar (2013), the highest enzymatic activity occurs during the hydrolytic phase; therefore, this stage is susceptible to alterations due to the excess of metals, such as iron. This activity delays the onset of methane production. This behaviour indicates the need for longer solid retention times (SRTs) and larger reactor volumes (Aldin 2010), which creates complications due to the presence of sands and clays in the SDWTP (Carvalho 2011).

CONCLUSIONS

  • In addition to the high iron concentration due to the use of ferric chloride as a coagulant during the treatment, low pH levels and bicarbonate alkalinity may generate inhibitory phenomena on the microorganisms that participate in AD processes.

  • Among the evaluated SWWTP:SDWTP ratios, the 80:20 ratio generated the lowest inhibitory effect on the methane production, which indicates that anaerobic co-digestion may occur for SDWTP fractions below 20% for the sludge mixtures in this study. To guarantee the optimal conditions for the anaerobic process, the system's buffer capacity must be controlled to minimize acidification; the inhibitory processes caused by the accumulation of VFA and high iron concentration levels must also be controlled.

  • The analysis of the hydrolytic stage using the first-order kinetic model confirmed the incidence of inhibitory processes, which are caused by the incorporation of DWTP sludge. This finding is evidenced by the lag phase preceding the onset of the significant production of methane (from 9 to 18 days), the amount of methane produced, and the reduction in the hydrolysis constant in all evaluated ratios. Each type of sludge and inoculum and the appropriate mixture ratio should be defined to minimize the inhibitory processes.

ACKNOWLEDGEMENTS

The authors acknowledge technical and financial support from the Environmental Laboratory of the Institute for Marine and Coastal Research (INVEMAR) and the Research Group Study and Control of Environmental Pollution (ECCA) of the Universidad del Valle.

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