The first step of anaerobic digestion, the hydrolysis, is regarded as the rate-limiting step in the degradation of complex organic compounds, such as waste-activated sludge (WAS). The aim of lab-scale experiments was to pre-hydrolyze the sludge by means of low intensive alkaline sludge conditioning before applying hydrodynamic disintegration, as the pre-treatment procedure. Application of both processes as a hybrid disintegration sludge technology resulted in a higher organic matter release (soluble chemical oxygen demand (SCOD)) to the liquid sludge phase compared with the effects of processes conducted separately. The total SCOD after alkalization at 9 pH (pH in the range of 8.96–9.10, SCOD = 600 mg O2/L) and after hydrodynamic (SCOD = 1450 mg O2/L) disintegration equaled to 2050 mg/L. However, due to the synergistic effect, the obtained SCOD value amounted to 2800 mg/L, which constitutes an additional chemical oxygen demand (COD) dissolution of about 35 %. Similarly, the synergistic effect after alkalization at 10 pH was also obtained. The applied hybrid pre-hydrolysis technology resulted in a disintegration degree of 28–35 %. The experiments aimed at selection of the most appropriate procedures in terms of optimal sludge digestion results, including high organic matter degradation (removal) and high biogas production. The analyzed soft hybrid technology influenced the effectiveness of mesophilic/thermophilic anaerobic digestion in a positive way and ensured the sludge minimization. The adopted pre-treatment technology (alkalization + hydrodynamic cavitation) resulted in 22–27 % higher biogas production and 13–28 % higher biogas yield. After two stages of anaerobic digestion (mesophilic conditions (MAD) + thermophilic anaerobic digestion (TAD)), the highest total solids (TS) reduction amounted to 45.6 % and was received for the following sample at 7 days MAD + 17 days TAD. About 7 % higher TS reduction was noticed compared with the sample after 9 days MAD + 15 days TAD. Similar results were obtained for volatile solids (VS) reduction after two-stage anaerobic digestion. The highest decrease of VS was obtained when the first stage, the mesophilic digestion which lasted 7 days, was followed by thermophilic digestion for 17 days.
The formation of toluene in municipal anaerobic primary and secondary sludge digestion processes was investigated. Experiments were carried out in a large laboratory‐scale reactor using sludge from a primary settling tank of a municipal treatment plant. It was found that toluene was produced in the supernatant in relatively large concentrations for almost all cases tested. The concentration of toluene varied and was found to depend on the stage of the anaerobic process. During the acidity phase, which is the first stage of anaerobic digestion, an increase of toluene concentration was observed, while in the transition period, from the acidity phase to methanogenesis, the toluene concentration decreased. It was concluded that biosynthesis of toluene occurs in the acidogenic phase, while biodegradation was prevalent in the methanogenic stage. Depending on the type of experiments, an increase of toluene from a base value of approximately 200 μg/L up to 20 000 and 42 000 μg/L was measured in the first stage of anaerobic digestion. In the subsequent methane‐production stage of digestion, the estimated rate of toluene decrease (biodegradation) varied from 400 to 900 μg/L‐d.
Disintegration by hydrodynamic cavitation has a positive effect on the degree and rate of sludge anaerobic digestion. By applying hydrodynamic disintegration the lysis of cells occurs in minutes instead of days. The intracellular and extracellular components are set free and are immediately available for biological degradation which leads to an improvement of the subsequent anaerobic process. Hydrodynamic disintegration of the activated sludge results in organic matter and a polymer transfer from the solid phase to the liquid phase, and an increase in COD value of 284 mg•ℓ-1 was observed, i.e. from 42 mg•ℓ-1 to 326 mg•ℓ-1. In addition the degree of disintegration changed from 14% after 15 min disintegration to 54% after 90 min of disintegration. A disruption of bacterial cells by hydrodynamic cavitation has a positive effect on the degree and rate of excess sludge anaerobic digestion. The cells of the activated sludge microorganisms rupture and addition to the digestion process leads to increased biogas production. The hydrodynamic disintegration of activated sludge leads to a higher degree of degradation and higher biogas production. Adding the disintegrated sludge (10%, 20% and 30% of volume) to fermentation processes resulted in an improvement in biogas production of about 22%, 95% and 131% respectively.
Volatile organic compounds have been measured at two relatively large sewage treatment plants. Quantitative estimation of benzene, toluene, m.p-xylene, o-xylene and isopropylbenzene have been made for raw sewage, sewage after primary treatment and after biological treatment. Also measurements of 14 different volatile organic compounds in the ambient air, close to screens, and the air above (0.5 m above) aeration tanks have been done. Tests on air stripping of added volatile organic compounds to clean water have been performed in parallel in the laboratory. The removal of examined VOCs in full scale treatment plants was very much below the expected level. In the low loaded activated sludge process the removal was between 2 and 56%, depending on the compound considered. The behavior of volatile organic compounds in laboratory tests was very much different. The concentration of VOCs in the air of rooms where bar racks have been installed was found to be very high. The concentration of toluene in the ambient air could be as high as 460 μg/m3.
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