Stabilization . Anaerobic digestion – energy recovery ; The aim of the proposed workpackage is the evaluation of the impact of addition of two co-substrates (rich-fat material OFMSW) on anaerobic mesophilic digestion of sewage sludge. The proposed option will allow intensify the process, through increase degradation degree of treated substances and improve biogas production. Additionally, the performance of digester we will monitored by measuring concentration of LCFAs (long chain fatty acids) - lipid degradation products, which may inhibit anaerobic microbial activity.


  • Addition of two waste into the sewage sludge digester improves process efficiency
  • Co-digestion of 3-component mixtures increases VS removal up to 29%
  • While, methane yield increased up to and 82% relative to the control digester
  • Effectiveness of the process depends on OLR and the content of waste in the feedstock

Executive summaries of WP1 results

The aim of this Work Package was to study the efficiency of the anaerobic digestion of a waste mixture consisting of sewage sludge (Ss), grease trap sludge (GTS) and organic fraction of municipal waste (OFMSW). The scope was based on the results of previous projects supported by the Polish Ministry of Science and Higher Education, Grant Nos. R14002006/2009 and BS/PB-401-301/13. We assumed that the proposed option will allow to intensify the process, through increased degradation degree of treated substances and improved biogas production and methane content. This main objective was realised into two tasks. In Figure 1 was shown main assumptions in this Work Package as well as relationship between tasks. The main goal of TASK 1 was to assess the impact of the addition of grease trap sludge (GTS) and organic fraction of municipal waste (OFMSW) on methane yield of sewage sludge. As shown in Fig.1. based on results TASK 1, selection of the composition of mixtures for semi-continuous co-digestion was made. In second TASK the efficiency of the semi-continues anaerobic co-digestion was investigated on the basis of biogas production and volatile solids reduction. Additionally, during semi-continuous experiment, the emphasis was also put on the fate of long chain fatty acid during anaerobic co-digestion. This resulted from the fact that these compounds are the key intermediates generated during anaerobic digestion of food waste or fat rich material. Furthermore, mentioned above compounds may negatively affect the anaerobic process.

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The tasks shown in Fig.1. have been preceded by an analysis of substrates for co-digestion. OFMSW was selected due to the complementary characteristics of this waste to sewage sludge, such as: content of macro- and micronutrients, C/N ratio, content of biodegradable organic matter. While the choice of fat rich materials (grease trap sludge – GTS) was based on their high organic matter content as well as high energy potential (Grosser and Neczaj, 2016). In turn sewage sludge is an interesting substrate to co-digest due to its high alkalinity and low concentration of potential compounds inhibiting the process (Wickham et al., 2016). The achievement of milestone M1.1.1 resulted in the following conclusions: “Significant differences between the substrates were observed. The grease trap sludge was characterized by a high total solid concentration compared to sewage sludge and organic fraction of municipal solid waste. However, both the OFMSW and GTS were characterized by very high values of VS/TS, indicating that these wastes contained more digestible organic matter than the sewage sludge. However, for all co-substrates, the VS/TS ratio was higher than 0.7, hence they were easily biodegradable. The pH of both samples was lower than that of sewage sludge. The concentration of nutrients and metals (include heavy metals) in the OFMSW were lower than in the other samples. The C/N ratio of OFMSW ranged from 28.8 to 29.8. While, in the sewage sludge, this ratio was a wide range of 9.05 to 21.15, and 21.5±0.03 on average for GTW”.

TASKS 1.1. Biochemical methane potential (BMP) assay for raw materials and co-digestion mixtures To achieve this task the following activities were performed: batch experiments were carry out to determine the biochemical methane potential (BMP) of sewage sludge, grease trap sludge (GTS) and organic fraction of municipal waste (OFMSW) alone as well as their mixtures for the following seven different ratios (on volatile solids basis) 6:3:1, 5:3:2, 4:3:3, 3:3:4, 2:3:5, 1:3:6 and 7:3:0 respectively. Batch experiments were performed under mesophilic (370C) conditions. The content of GTS in the co-digestion mixtures (30% on VS basis) was determined based on previous research (unpublished) as well as literature analysis (Girault et al., 2012, Davidsson et al., 2008; Wan et al., 2011 and. Silvestre et al., 2011). Achievements The achievement of milestone M1.1.2 resulted in the following conclusions: “The optimum mixing ratio of sewage sludge with grease trap sludge and organic fraction of municipal waste was 4:3:3. For this mixture methane yield was 327 LCH4/kg VS, which is higher by approximately 130% than that of sewage sludge alone”.

TASK 1.2. Semi-continuous anaerobic co-digestion of selected mixtures To achieve this task the following activities were performed: The anaerobic digestion was carried out at mesophilic conditions (37oC) in two glass reactors filled with 6 litres of working liquid (see Fig.1.). The reactors were constantly mixed (180 rpm) using mechanical stirrers and their temperatures were controlled by a thermostatically regulated water bath. The produced biogas was collected by a gas collector (PVC tube filled with water acidified to pH 3). The reported experiment was divided into four stages which were implemented consecutively as shown in Fig.1. The reactors during stage I, II and III were operated in a draw-and-fill mode with a retention time of 20 days. The hydraulic retention time (HRT) was set to 20 d, which is within operating range values reported in literature (spanning from 15–25 days) (Hartmann and Ahring, 2006). While, during the stage IV, hydraulic retention time (HRT) parameter was progressively decreased from 20 days to 12 days (data obtained in this period was used for the statistical analysis). After this, in the second period, it was decided to continue the experiment with a HRT of 15 days (chosen based on the results from the first period) to check, how this modification impacts performance of anaerobic co-digestion (AcD). This HRT was chosen for the second stage, due to the fact that the reduction of methane yield and VS removal was not lower than 10% in comparison to the best results obtained in this study.


According to Borowski et al. (2015) as well as Wan et al. (2011), VFAs: alkalinity ratio higher than 0.4 indicates a lack of good balance in the microbial population between acidogenic and methanogenic bacteria, whereas higher than 0.8 significant instability of the digester. At the beginning of the process, the ratio ranged from 0.8 to 4.5. After the 27 day of the process VFA: alkalinity ratio remained below critical value of 0.4 (0.28±0.03 on average), it indicates that stable working parameters of the bioreactor were achieved in this stage ( M1.2.1. ). The addition of grease trap sludge does not significantly affect the average value of this indicator. In the second stage the ratio values varied between 0.24 and 0.34 (0.29±0.01 on average). While, in third stage this indicator was in the range of 0.15 to 0.31 (0.23±0.03 on average). It proves that the system had adequate buffering capacity and consequently the conditions were favorable for anaerobic digestion ( M1.2.2 ). While in stage IV, the ratio of VFAs to alkalinity was below critical values for anaerobic digestion, namely 0.4, which indicates process stability. However, in the second period (Stage IV) this ratio was significantly higher than in the first period, it ranged from 0.18 to 0.29 (av. 0.24±0.03), but still it was lower than inhibition thresholds.

The achievement of Stages I - III resulted in the following conclusions: “ Co-digestion of SS, GTS and OFMSW provided significant benefits for biogas production and VS removal. It was found that anaerobic treatment of SS and GTS at a ratio of 30% resulted in increased of Y M of approximately 52% compared to digestion of SS alone. Moreover, the addition of OFMSW as a next co-substrate significantly improved the efficiency of the sewage sludge AD process by enhancing average Y M up to 82%. Additionally, a significant increase of VS removal was observed during the experiment, average of about 29.5% compared to control sample. The addition of biowaste had significantly affect the composition of biogas. While, the concentration of almost all LCFAs which are the key intermediates of degradation of fat rich materials was below the inhibition threshold. Despite the fact that the oleic acid concentration was higher than the critical level reported by some Authors, a negative impact on the performance of anaerobic digestion has not occurred”.

While, the achievement of Stage IV resulted in the following conclusions: Although the best results were obtained at a HRT of 20 days, results showed it is also possible to carry out the co-digestion of the studied wastes at a much lower HRT with good results and without negative impact on process stability. Moreover, it was found that concentration of LCFAs in effluents increased with the reduction of hydraulic retention time, but this phenomenon did not significantly influence the performance and stability of the process. Furthermore, only, in the case of oleic acid a higher concentration than the inhibition thresholds which were demonstrated by some authors was noted. However, in the Authors’ opinion, successively reduced HRT associated with gradually increased OLR, allowed for the adaptation of methanogenic microorganisms to increasing concentrations of LCFAs. Also, Fernández et al. (2005) as well as Broughton et al. (1998), Alves et al. (2001) reported a feasibility for acclimation of bacterial consortium to different concentration of long chain fatty acids”.