Many drivers tend to foster the development of renewable energy production in wastewater treatment plants as many expectations rely upon energy recovery from sewage sludge, for example through biogas use. This paper is focused on the assessment of grease waste (GW) as an adequate substrate for co-digestion with municipal sludge, as it has a methane potential of 479-710 LCH(4)/kg VS, as well as the evaluation of disintegration technologies as a method to optimize the co-digestion process. With this objective three different pre-treatments have been selected for evaluation: thermal hydrolysis, ultrasound and enzymatic treatment. Results have shown that co-digestion processes without pre-treatment had a maximum increment of 128% of the volumetric methane productivity when GW addition was 23% inlet (at 20 days of HRT and with an OLR of 3.0 kg COD/m(3)d), compared with conventional digestion of sewage sludge alone. Concerning the application of the selected disintegration technologies, all pre-treatments showed improvements in terms of methane yield (51.8, 89.5 and 57.6% more for thermal hydrolysis, ultrasound and enzymatic treatment, respectively, compared with non-pretreated wastes), thermal hydrolysis of GW and secondary sludge being the best configuration as it improved the solubilization of the organic matter and the hydrodynamic characteristics of digestates.

Thermal treatment applied in association with a biological system allows for a significant reduction in excess sludge production (∼50%). In general, heat treatment is described as a sludge disintegration technique. This paper offers a thorough study on the impact of heat treatment, at temperatures below 100 °C, on the solubilisation of the sludge COD and its biodegradability. Discontinuous heating experiments were performed on activated and digested sludge. At all temperatures tested the released COD for digested sludge was systematically higher than that for activated sludge (15 and 40%, respectively, at 95 °C for 40 min of contact time). For the first 30 min, a 1st order kinetic, with respect to the residual COD, was systematically found. In the range of 40–95 °C, digested sludge had a lower activation energy than activated sludge (26 kcal/mol compared to 70–160 kcal/mol). COD solubilisation is thus more positively influenced by temperature in the case of activated sludge. This may be due to the significant difference in the ratio of protein/carbohydrate in digested and activated sludge (1–5 and 0.2–0.7, respectively). The increase in the COD/TKN ratio in the solubilised fraction after thermal treatment of activated sludge suggests a preferential solubilisation of proteins over carbohydrates. Respirometric tests performed on the solubilised COD showed that whatever the sludge origin, only 40–50% of released COD is biodegradable at a conventional hydraulic retention time (i.e. 24 h). Hence, heat treatment would act more through organic matter solubilisation rather than by a biodegradability increase.

Municipal wastewater treatment is closely linked with the production of sludge. This production may be reduced by combining a sludge disintegration technique with a conventional biological reactor, such as an activated sludge. The disintegration techniques are based on mechanical, electrical, thermal, thermo-chemical, biological and oxidative treatments. This work tries to give an evaluation of the efficiency and the economics of disintegration techniques that have demonstrated their potential to reduce the excess sludge production when combined to an activated sludge plant fed with a real urban wastewater. A high ESP reduction rate is achievable (more than 40%) by using disintegration technique such as a thermal (95°C), an ozonation or a hydrogen peroxide treatment. These three disintegration techniques were studied in details in our laboratories. The experiments carried out provided the technical data in order to make an assessment of the economics of the three selected combined processes. The ozone appeared to be the most interesting route for sludge reduction but the heating route is also economically competitive with conventional sludge treatment and disposal especially if stringent constraints on the sanitary quality of the sludge are imposed.

Among the technologies aimed at reducing sludge production, the combination of thermal treatment at 95 degrees C of sludge and the activated sludge process is a promising route. The feasibility of such a combined process is demonstrated (up to 60% sludge reduction) and the impacts of operating conditions on its efficiency are presented. Major emphasis was put on understanding the complex phenomena occurring within the thermal treatment: release and biodegradability of sludge organic matter, impact on the biological activity (decay, maintenance requirements, etc.). These effects were taken into account for the development of an ASM1-based model. Comparison between the modeling approach and experimental data (continuous and batch) showed that thermal treatment had three major issues partly explaining the reduction of sludge production: (i) a low release of organics; (ii) an immediate and reversible biological inactivation associated with additional maintenance energy requirements; and (iii) a potential inert production.

Thermal, electric, mechanical or oxidative stress seem a promising way to reduce the production of excess activated sludge during biological wastewater treatment. However, the adaptation and the resistance of the sludge microbial ecosystem to stress conditions is a major question as it may definitively limit the effect of some treatments. Defence mechanisms developed by aerobic organisms, in particular, in response to oxidative stress involve various antioxidant activities and compounds such as glutathione. An HPLC method was developed for measuring reduced and total glutathione (GSH and GSHt) in perchloric acid sludge extracts. The method was sensitive, highly specific and validated for linearity, precision and recovery. Considering the extraction yield and the oxidation of GSH during extract storage, the measured GSH concentration was estimated to represent 60% of the GSH content from activated sludges. GSHt ranged from 0.32 to 3.34micromolg(-1) volatile solids and the GSH/GSHt ratio ranged from 32% to 91%. Measurements performed on sludges stressed in precise conditions selected to reach a reduction of sludge production showed a decrease of GSH and GSHt concentrations with thermal, mechanical, electric and ozone stress.

The release performances of an organic and mineral activated sludge matrix were studied for a wide range of disintegration treatments like mechanical, thermal, thermal-chemical and oxidative disintegration techniques. The maximal COD release was 35% of total COD after 24 hours contact time at 95 degrees C. A limiting value of 60% COD release was obtained for 500 and 700 bars after 10 passes. Concerning theoxidative disintegration techniques (O3 and H2O2), a limiting value of around 60-65% of TOC release was observed. Therefore, it was hypothesised that thermal and mechanical treatments allow mainly for breaking apart the micro-organisms while the oxidative treatment destroys the sludge flocs and disrupts the micro-organisms. A release effect of the mineral fraction is observed only oxidative disintegration techniques.

The action mechanisms and performances of a combined system associating activated sludge and mechanical treatment (High Pressure Homogenizer) were evaluated for urban wastewater. Discontinuous experiments showed that the energy applied at the first pass was high enough to modify the sludge particulate fraction (high COD release) but without cell lysis. The applied shear forces led to a progressive cell break up (maximal COD release 90% total COD). Continuous experiments showed less than 20% reduction in sludge production (compared to a control run under the same loading conditions Y(TSS) = 0.35 g TSS x g(-1) COD(removed)) through the application of mechanical treatment (stress frequency = 0.2 d(-1)). Recycling of mechanical treated sludge to the aeration tank induced a slight increase in effluent TSS, but the biological performance seemed to be maintained. Significant improvements in sludge settling characteristics were observed.

A combined system associating activated sludge and ozonation was evaluated for the treatment of urban wastewater. Experiments have shown that 70% reduction in sludge production can be reached (compared to a reference system running in low loaded conditions Y(obs) = 0.28 g VSS.g COD(-1)) by applying an ozone dosage of 0.05 g O3/g VSS(treated) Recycling of the ozonated sludge to the aeration tank induces a slight increase in effluent COD, but the biological treatment performance is maintained. Nitrification capabilities are not altered by the sludge reduction process and active biomass measurements revealed that autotrophic biomass seems to be less affected than the heterotrophs. Significant improvements in sludge settling characteristics are observed.

Pentoxifylline (PTX), a xanthine derivative used in the treatment of circulatory insufficiency, has been found to have protective effects in different models of sepsis. We hypothesized that this drug might either increase oxygen delivery (DO2) and/or increase tissue oxygen -extraction to meet oxygen demand in sepsis. We studied the effects of PTX on the oxygen uptake/oxygen delivery (VO2/DO2) relationship and tissue oxygen extraction when blood flow was reduced by Inducing cardiac tamponade in 14 anesthetized, ventilated and paralyzed dogs. Via a left thoractomy, a catheter waa inserted into the pericardial space for saline injection. Each dog was given a 2 mg/kg bolus of E. coli endotoxin and received 20 mlkg.h of normal saline•duririg the study. In 7 dogs, PTX was administered as a 20 mg/kg i.v. bolus, followed by a continuous infusion at 20 mglkg.h. V02 was derived from the expired gases. 002 was calculated by the product of the modüution cardiac index and arterial oxygen content. Oxygen extraction ratio (02ER) was defined as the ratio of V02/002. Dual-line regression was used to determine the critical 002 (DO2crit) in each animal. ANOVA was used for statistical analysis. PTX resulted in significant increases in V02 and DO2. Critical V02 was slightly higher in the PTX-treated than in the control group, but it did not reach statistical significance (6.3 ± 2.4 vs 5.4 ± 1.0 ml/kg.min, NS). D02crit which was 11.3± 4.9 ml/kg.min in the control group, was decreased to 9.6 ± 3.6 mlkg.min in the PTX-treated group (p = 0.05). Critical 02ER significantly increased from 50 ± 20% in the control to 68 ± 19% in the PTX-treated animals (p< 0.05). The V02/D02 dependency slope was steeper in the PTX-treated than in the control group (0.77 ± 0.31 vs 0.46 ± 0.18, p< 0.05). At D02c rit, PTX-treated group had lower venoarterial PCO2 difference (12.9 t. 4.3 vs 18.4 ± 7.4 mmHg, p < 0.05) and arteriovenous pH gradient (0.08 ± 0.02 vs 0.11 ± 0.06 U, p < 0.05) than in the control group. Thus, the addition of PTX and fluid therapy can increase 002 and global oxygen extraction capabilities when cardiac output is progressively reduced in the endotoxemic dog. The exact mechanisms remain to be defined.