The disposal of waste sludge generated from wastewater treatment plants (WWTPs) is a growing problem worldwide, and contributes to over 50% of the operating costs of current WWTPs. In this study, temperature-phased anaerobic digestion (TPAD) and conventional (single-stage) mesophilic anaerobic digestion were investigated in order to determine the most beneficial process for the intended digester facility to be constructed for the fermented primary and secondary sludge from a Bardenpho type biological nutrient removal plant. This was accomplished by considering several operational control parameters for three different sludge retention times. This study has shown that the TPAD system had significant improvement in biogas and methane production; solids and organic removal; pathogen reduction and dewaterability over the conventional digestion. In terms of overall volatile sulphur compounds formation normalized per volatile solids added, no significant effects were observed between TPAD and control digester.

 dewaterability, fecal coliform, methane production, solids removal, temperature-phased anaerobic digestion (TPAD)

Currently, there are various methods of waste sludge disposal, although methods that are also capable of producing energy and/or usable products during the stabilization of this high-strength waste stream are preferential. An example of a viable method of sludge disposal is the process of anaerobic digestion (AD), which not only produces energy in the form of methane gas, but also produces nutrient-rich fertilizer. Moreover, AD reduces the level of pathogens present in sludge, decreases overall sludge volume, and diminishes odors commonly related to putrescible solids.

The temperature-phased anaerobic digestion (TPAD) system treats organic waste using two digesters in series: the first digester contains an inoculum involved with hydrolytic/acidogenic chemical pathways, while the second digester contains the acetogenic/methanogenic processes. TPAD systems are able to operate at a higher organic loading rate, have increased deactivation of pathogens, and are less sensitive to changes in influent sludge characteristics than single-stage AD (Azbar et al. 2001; Vandenburgh & Ellis 2002). Although single-stage AD is less expensive in regards to operational costs, the previously mentioned advantages of TPAD may yield a more stable and cost effective digestion process over time.

In this study, as one of the advanced digestion methods, TPAD was investigated in order to improve the performance of a conventional AD utilizing a mixture of fermented primary and biological nutrient removal (BNR) treatment plant sludge. Although there are several TPAD studies in the literature, these studies mostly report on performance enhancement for sludge from activated sludge systems achieving carbon removal only, rather than BNR plants. Furthermore, its advantages and disadvantages over time on several control parameters for different sludge retention times (SRT) and land application of digestate were extensively investigated.

The first stage of TPAD was conducted at thermophilic conditions (55 ± 2 °C) with an SRT of 2 days and an effective reactor volume of 250 mL, followed by the second stage digester under mesophilic conditions (35 ± 2 °C) with a 500 mL effective volume. The control digester was operated as a single-stage mesophilic digester with an effective volume of 750 mL. Both digester systems were operated for 20, 14 and 7 day SRTs to observe the effects of organic loading on the control parameters.

Feed sludge was obtained from the Kelowna wastewater treatment plant (WWTP) (BC, Canada) and thermophilic and mesophilic inocula were originally obtained from full-scale digesters at Annacis Island and Penticton WWTPs (BC, Canada), respectively. The bench-scale anaerobic digesters were semi-continuously fed (once a day feeding) with a mixture of thickened waste activated sludge and fermented primary sludge, with a volume ratio of 67:33, respectively, to simulate the similar conditions in the Kelowna WWTP. The mixed digester feed was characterized regularly and the average values are reported in Table 1.

Table 1

Feed characterization

 Feed sludge
pH 5.6 (0.1; 6)a 
Alkalinity, mg/L CaCO3 765.5 (111; 6) 
tCOD, mg/L 53,512 (3,156; 20) 
sCOD, mg/L 4,348 (528; 16) 
TS,% by wt. 4.2 (0.18; 20) 
VS,% by wt. 3.5 (0.18; 20) 
Ammonia, mg N/L 448.6 (181.8; 6) 
Tot. VFA, mg/L 1,639.1 (277.8; 12) 
 Feed sludge
pH 5.6 (0.1; 6)a 
Alkalinity, mg/L CaCO3 765.5 (111; 6) 
tCOD, mg/L 53,512 (3,156; 20) 
sCOD, mg/L 4,348 (528; 16) 
TS,% by wt. 4.2 (0.18; 20) 
VS,% by wt. 3.5 (0.18; 20) 
Ammonia, mg N/L 448.6 (181.8; 6) 
Tot. VFA, mg/L 1,639.1 (277.8; 12) 

aArithmetic mean of replicates (standard deviation; number of replicates).

tCOD: total chemical oxygen demand, sCOD: soluble chemical oxygen demand, TS: total solids, VS volatile solids, Tot. VFA: summation of acetic, propionic and butyric acids.

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In order to see the effects of TPAD on digestion performance, several measurements were performed involving both the headspace biogas and the effluent sludge. In addition to the basic operational parameters, such as pH, alkalinity, ammonia (not shown), daily biogas volume and composition, volatile fatty acids, total (TS) and volatile (VS) solids were also measured. The predominant sulphur compounds found in biogas (hydrogen sulfide, methyl mercaptan, ethyl mercaptan, dimethyl disulfide, carbon disulfide, n-propyl mercaptan, ethyl sulfide, dimethyl disulfide) were also analyzed. In the effluent sludge, fecal coliforms were monitored by the membrane filter technique. The effect of TPAD on sludge dewaterability was also evaluated using capillary suction time (CST) analysis. All chemical and biological analyses were conducted according to Standard Methods (APHA-AWWA-WEF, 2005).

Biogas production of each digester at standard temperature and pressure (STP: 0 °C and 1 atm) and the specific methane production were plotted in Figures 1 and 2, respectively. The results show that the TPAD improved the biogas yield and methane production from waste sludge for the SRTs of 20 and 14 days. TPAD has achieved 350 mL/kg VSfed and 315 mL/kg VSfed methane formation during 20 d and 14 d SRTs, resulting in 37% and 43% higher methane production than the control, respectively. However, the introduction of highly hydrolyzed sludge to the methane phase digester during the 7 d SRT period increased the soluble organic load and led to the accumulation of short-chain VFAs. During the SRT of 7 d, total VFA in methane phase TPAD increased up to 2,400 mg/L within 20 days, and eventually reached up to 4,400 mg/L. As a consequence of the high VFA concentration, biogas production and methane formation decreased and inhibition was observed in the TPADs within 30 days.
Figure 1
Figure 1. Daily biogas production at different SRTs (C: Control digester, TPAD-A: Acid phase digester, TPAD-M: Methane phase digester).
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Daily biogas production at different SRTs (C: Control digester, TPAD-A: Acid phase digester, TPAD-M: Methane phase digester).

Figure 1
Figure 1. Daily biogas production at different SRTs (C: Control digester, TPAD-A: Acid phase digester, TPAD-M: Methane phase digester).
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Daily biogas production at different SRTs (C: Control digester, TPAD-A: Acid phase digester, TPAD-M: Methane phase digester).

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Figure 2
Figure 2. Specific methane production at different SRTs (TPAD: Temperature phased anaerobic digestion system).
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Specific methane production at different SRTs (TPAD: Temperature phased anaerobic digestion system).

Figure 2
Figure 2. Specific methane production at different SRTs (TPAD: Temperature phased anaerobic digestion system).
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Specific methane production at different SRTs (TPAD: Temperature phased anaerobic digestion system).

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Average solid concentrations in digester feed and effluent streams were plotted in Figure 3. As the results indicate, TPAD has provided 47% and 44% TS removal during 20 d and 14 d SRTs, resulting in 15% and 14% higher solids reduction than the control, respectively. As a result of the inhibition, caused by high VFA concentrations, TS removal deteriorated at the SRT of 7 d.
Figure 3
Figure 3. Average solids percentages of feed and digester effluents.
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Average solids percentages of feed and digester effluents.

Figure 3
Figure 3. Average solids percentages of feed and digester effluents.
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Average solids percentages of feed and digester effluents.

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Total volatile sulphur compounds (VSC) concentrations in the headspace biogas is represented in Figure 4. At all SRTs, acid phase TPAD generated significantly higher VSCs (∼2,000–4,500 ppm) than the methane phase TPAD (∼250–1,500 ppm). Furthermore, emissions of VSCs from all of the digestion systems increased with decreasing SRT. The highest VSC concentrations were observed at the shortest SRT of 7 d, which provided the lowest methane production with a poor organic transformation.
Figure 4
Figure 4. Average VSCs concentration in the headspace biogas.
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Average VSCs concentration in the headspace biogas.

Figure 4
Figure 4. Average VSCs concentration in the headspace biogas.
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Average VSCs concentration in the headspace biogas.

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In order to understand the feasibility of land application of the digestate, fecal coliforms were enumerated as the indicator organism of pathogenic contamination (Figure 5). In the effluent of TPAD, no fecal contamination was detected within the detection limits of the membrane filter technique, while over 45,000 colony forming units (CFU)/g dry solids were enumerated in the single-stage digestion effluent. For all SRTs, TPAD achieved more than 4 log inactivation over the control and met the Class A biosolids fecal coliform requirements.
Figure 5
Figure 5. Average fecal coliform counts in digester system effluents.
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Average fecal coliform counts in digester system effluents.

Figure 5
Figure 5. Average fecal coliform counts in digester system effluents.
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Average fecal coliform counts in digester system effluents.

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Sludge dewatering is one of the most costly processes in sludge handling in terms of energy usage and chemical conditioning requirement. For all SRT values, TPAD significantly enhanced the dewaterability (Figure 6). During 20 d and 14 d STRs, compared to the control, TPAD achieved by more than 30% improvement in dewaterability.
Figure 6
Figure 6. Average CST values of digester system effluents.
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Average CST values of digester system effluents.

Figure 6
Figure 6. Average CST values of digester system effluents.
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Average CST values of digester system effluents.

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For both the SRTs of 20 d and 14 d, TPAD achieved a significant improvement on solids reduction, dewaterability, pathogen removal as well as methane production. In terms of overall VSC formation normalized per VS added, no discernible differences were observed between TPAD and the control digester. At the final SRT of 7 d, due to the inhibition of the methane phase TPAD, the organic transformation could not be achieved successfully and all quality parameters deteriorated.

The authors would like to thank the City of Kelowna and BC Ministry of Environment for supporting this project. This work has been carried out under the sponsorships of the Natural Science and Engineering Council of Canada (NSERC) Strategic Project Grant (#396519-10) and the financial support of the Scientific and Technological Research Council of Turkey (TUBITAK).

APHA-AWWA-WEF
2005
Standard methods for the examination of water and wastewater
, 21st ed.
American Public Health Association
,
Washington DC, USA
.
© IWA Publishing 2016