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.

Recovery of phosphates from ferrous and/or ferric residues as a result of chemical precipitation was investigated. The product of biological reduction of sulphates--sulphides and free hydrogen sulphide--can react with iron ions forming ferrous sulphide which are less soluble then ferrous phosphates, releasing phosphates into solution. Laboratory scale experiments have confirmed the possibility of phosphates recovery. It was found that strict anaerobic conditions are required and that the process is COD/S ratio dependent.

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Abstract:The inter-reservoir enrichment phenomenon was exploited to curtail the reservoir eutrophication process. The Plawnowice reservoir (South Poland -Upper Silesia Region) has an area of 225 ha, volume of 29 mln m 3 , and a depth of 15 meters. According to the monitoring results in the years 1993-1998 the reservoir was qualifi ed as hypereutrophic. Beginning in December 2003 a bottom pipe for hypolimnetic withdrawal was installed. In the period 2004-2010 a negative phosphorous balance was achieved. The discharge load of total phosphorous was in the beginning twice as high as the infl owing. During the fi rst eight years with an infl ow of 75 Mg P, the removed load of total phosphorus was 103 Mg P. In effect the net balance was 28 Mg P. The load, in respect to the surface area, of 2.2 to 3.3 gP/m 2 per year, was reduced to a negative load of -0.48 to -3.3 gP/m 2 . The hypolimnetic maximum concentration of orthophosphates equal to 1.254 mg P-PO 4 /dm 3 in 2004, was reduced to 0.236 mg P-PO 4 /dm 3 in 2011. The respective factors and rate of eutrophication curtailing, including changes of phosphorus compounds have been discussed. Also changes of pH and visibility of the Secchi disc are presented. It was concluded that the presented method of hypolimnetic withdrawal is a lasting and effective process.

Activated sludge systems designed for enhanced nutrient removal are based on the principle of altering anaerobic and aerobic conditions for growth of microorganisms with a high capacity of phosphorus accumulation. Most often, filamentous bacteria constitute a component of the activated sludge microflora. The filamentous microorganisms are responsible for foam formation and activated sludge bulking. The results obtained confirm unanimously that the filamentous bacteria have the ability of phosphorus uptake and accumulation as polyphosphates. Hydrodynamic disintegration of the foam microorganisms results in the transfer of phosphorus and metal cations and ammonium-nitrogen into the liquid phase. It was demonstrated that the disintegration of foam permits the removal of a portion of the nutrients in the form of struvite. Water Environ. Res., 80, 617 (2008).