Experimental evaluation of the effect of thermal heat and pressure pre-treatment on primary sludge anaerobic digestion

Mwidu, Aaron

Experimental evaluation of the effect of thermal heat and pressure pre-treatment on primary sludge anaerobic digestion

Thesis / Dissertation

2025

Publisher

University of Cape Town

Department

Department of Civil Engineering

Faculty

Faculty of Engineering and the Built Environment

Abstract

Studies have proved that anaerobic digesters have been traditionally used for stabilization of sewage sludge before disposal. Methane and carbon dioxide are produced as by products bacterial synthesis of organic material in an oxygen deficient environment (Nopharatana et al., 1998). More recently development of the thermal hydrolysis process pre-treatment of sewage sludge has shown great economic and financial potential in terms of; an increased sludge degradability implying more biogas production, improved digestion rate, stable and highly efficient digester, improved dewaterability of sludge and finally pathogen free cake as a final product which can be used for soil conditioning. The generation of more biogas and class A biosolids would fit into the sustainable developments goals and is in line with the changing paradigms that involve resource recovery from waste. The research questions posed in this investigation is: How does thermal heat and pressure (THP) pre-treatment affect primary sludge input characteristics for anaerobic digestion (AD) process in completely mixed AD systems? The overarching aim of this study is to evaluate the effect of THP, with a reduced temperature and pressure (of 114°C and pressure 1.7 bars) on primary sludge (PS) degradability under Mesophilic (37°C) anaerobic digestion (AD) conditions. The specific objectives are: (i) comparison of selected anaerobic digester performance parameters for pre-treated and untreated primary sludges over the selected varying sludge retention times (SRTs) of 8, 10, 12, 15 and 30 days, (ii) determination of the unbiodegradable particulate fraction (fPS'up) for both the pre-treated and untreated primary sludge and their hydrolysis rates, (iii) complete elemental characterization stoichiometry and weak acid base chemistry for both primary sludges and (iv) graphical comparison of measured data and anaerobic digestion model-predicted results. The effect of thermal heat and pressure (THP) pre-treatment on primary sludge degradability was evaluated. Primary sludge was heated at 114°C at a pressure of 1.7 bars in a pressure cooker (model number 17995) for 1 hour and anaerobically digested. The degradability of the pre-treated primary sludge fed to the digester (Digester 2; D2) was compared to an identical control digester (Digester 1: D1) fed the same sludge without heat pre-treatment. Both digesters were operated for 30, 15, 12, 10 and 8 days SRT. The percentage of Chemical Oxygen Demand (COD) removal for D1 ranged from 17% a to 54%, while that of D2 ranged from 16% to 62% for 8 to 30 day SRT respectively, gas production per liter influent for D1 ranged from 1.94 liter gas/liter influent to 19.08 liter gas/liter influent while that for D2 ranged from 1.81 liter gas/liter influent to 20.01 liter gas/liter influent, methane gas composition for both D1 and D2 remains the same for all the sludge ages varying between 50%-69% for methane and 34% to 45% for carbon dioxide. This signifies stability in both digesters. The effluent volatile fatty acid (VFA) concentration is very low (less than 0.08 g COD/L) in concentration and notably below that measured in the influent for both digesters except for the 15-day SRT D2 which was taken as an outlier. The low effluent VFA concentration signifies stability of both digesters. A Ripley ratio of less than 0.4 in both digesters indicates stable conditions, (Jiang et al. 2012). The effluent H2CO3*Alk., and pH for both digesters D1 and D2 is similar ranging from 1167 to 1990 mg/ L CaCO3 and 1145 to 2016 mg/ L CaCO3 respectively at pH ranging from 6.77 to 7.16 and 6.83 to 7.08 for D1 and D2 respectively. These additional results qualitatively supported that the heat pre-treatment increased the degradability of the primary sludge. To quantify the increase in degradability by heat pre-treatment, the steady state anaerobic digestion (AD) model of Sötemann et al. (2005) was calibrated to the D2 and control D1 results. However, because the values of the measured methane production results were more consistent than the measured effluent COD concentration results, the AD was recast in terms of the methane gas production instead of the effluent COD concentration as the basis for calibration. With statistical curve fitting techniques, the unbiodegradable particulate organic (UPO) fraction of the primary sludge (fPS'up) and saturation (Contois) kinetics specific hydrolysis rate constants were determined for the control D1 AD to be 0.35, KM = 1.595 gCOD/(gCOD.d) and KS = 2.607 gCOD/L. While the KM and KS yield a faster hydrolysis rate than observed by Sötemann et al. (2005), the UPO fraction was closely the same (0.36). For the pre-heated sludge AD D2, the UPO fraction was 0.22 and saturation (Contois) kinetics specific hydrolysis rate constants were KM =1.474 gCOD/ (gCOD.d) and KS = 0.971 gCOD/L. From this it was concluded that the degradability of primary sludge PS was increased in two ways: (1) by converting some (~1/3rd) of the Unbiodegradable Particulate Organics (UPO) to Biodegradable Particulate Organics (BPO) and (2) increased hydrolysis rate of the BPO. The AD model of Sötemann et al. (2005) also comprises a stoichiometric section that is sequential to the part which calculates sludge breakdown (i.e., BPO utilized) via hydrolysis kinetics. This stoichiometric section is used to calculate the changes in H2CO3* Alk., FSA, partial pressure of the biogas and digester pH from the concentration and composition of the BPO utilized. Using this stoichiometric part in reverse, and assuming 100% carbon flux mass balance over the digesters, the composition of the used BPO was determined from the differences in the measured concentrations between influent and effluent. For the control digester D1, the BPO composition was found to be C1H0.647O0.065N0.063 and for D2 with heat pre-treatment it was C1H0.541O0.087N0.067. While these BPO compositions were similar for the D1 and D2 digesters, both are marked different to the BPO compositions previously observed by (Ekama, 2009) e.g. C1.0H2.0O0.571N0.052 by (Sötemann et al., 2005). The different BPO compositions than previously obtained is expected because the composition of the organics depends on the pollutants sources of the wastewater and could vary on a daily basis (Gaszynski, 2021). This research provides evidence that with THP pre-treatment the degradability of primary sludge is improved by increasing both the biodegradable fraction and the hydrolysis rate. However, because this is the first study to provide such evidence, it is recommended the further research be done which should include a method devised for validating the UPO composition (in this case it was set to be the same as observed in literature studies; (Gaszynski, 2021; Ekama et al., 2005). Such a method may involve batch AD of the sludge in question at a long solids retention time (>60d; All organics eventually degrade at very long sludge ages at but sludge ages of above 60d are deemed too high for considered design of AD systems (Sötemann et al., 2005)). Elemental analysis of influent and effluent sludges so that the calculated composition of the BPO is not dependent on the assumed composition of the UPO.

Keywords

Thermal heat

Thermal hydrolysis

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