The effect of operational conditions on the hydrodynamic characteristics of the sludge bed in UASB reactors.

This work aims to evaluate the hydrodynamic properties of the sludge bed of Upflow Anaerobic Sludge Blanket (UASB) reactors based on its settleability and expansion characteristics. The methodologies used for the evaluation of the settleability of aerobic activated sludge, and for the expansibility of a sludge bed of Expanded Granular Sludge Bed reactors and Fluidised Bed Reactors were adapted and applied to the particular characteristics of the sludge of UASB reactors. An easy-to-build experimental set-up was developed to assess the parameters necessary for the equations of settleability and of expansibility. The results obtained from the sludges of seven differently operated reactors show that, for the treatment of low strength wastewater, settleability increased and expansibility decreased at decreased hydraulic retention time, from 6 to 1 h, and/or increased influent concentrations, from 136 to approximately 800 mg chemical oxygen demand/L. The results also show that it is useless to design an UASB reactor with a longer hydraulic retention time to cope with hydraulic shock loads, as a more expansible sludge will develop at such condition.

The effect of operational conditions on the sludge specific methanogenic activity and sludge biodegradability.

The effects of hydraulic retention time (HRT) and influent COD concentration (CODInf) on Specific Methanogenic Activity (SMA) and the biodegradability of an anaerobic sludge need to be elucidated because of the discordant results available in literature. This information is important for the operation of anaerobic reactors and design of the sludge post-treatment unit. For this study, sludge samples obtained from eight pilot-scale Upflow Anaerobic Sludge Blanket (UASB) reactors were tested. The reactors were fed with municipal wastewater and operated with different sets of HRT and influent concentrations until the steady state was established. The results show that at a lower HRT, sludge with relatively higher SMA develops. A slight trend of declining SMA at increasing CODInf was found for reactors operated at longer HRTs; however, further experiments are necessary for more definitive conclusions. The sludge from reactors operated at longer HRTs and with lower CODInf resulted in lower biodegradability. Results also showed that it is ineffective to design a UASB reactor with a longer HRT to cope with organic shock loads.

The effects of hydraulic and organic shock loads on the robustness of upflow anaerobic sludge blanket reactors treating sewage.

In this investigation, the robustness and stability of UASB reactors was evaluated on the basis of four indicators: (i) COD removal efficiency; (ii) effluent variability; (iii) pH stability; and (iv) recovery time. The experiments were carried out using six pilot-scale UASB reactors fed with domestic sewage and operated under different operational conditions. After establishment of a "steady-state", organic and hydraulic shock loads (six times the loading rate during six hours) were imposed. The results show that the UASB reactors are robust systems with regards to COD removal efficiency and pH stability when exposed to shock loads. However, this reactor cannot attenuate the imposed fluctuation in the influent COD. A secondary treatment unit is needed to retain the expelled sludge occurring as a result of a hydraulic shock load, or prior to the shock, a sufficient amount of sludge needs to be discharged from the reactor.

Integrated energy production and reduction of the environmental impact at alcohol distillery plants.

In Brazil cane is being been grown at large scale to produce alcohol as an automotive fuel. Alcohol is the sole product, but there is generation of a large quantity of gaseous (CO2), liquid (vinasse) and solid (bagasse) by-products, which currently have very little or even negative value. By using steam turbines fuelled with bagasse combustion, electric power can be generated at a rate of 1 MWh per m3 of produced alcohol. Anaerobic digestion can be applied to vinasse to produce enough biogas for 0.5 MWh per m3 of alcohol, bringing total electric power production from subproducts to 1.5 MWh per m3 of alcohol. These operations are presently implemented at some distilleries at full scale. It has been shown at bench scale that by applying anaerobic digestion also to bagasse and burning the non-biodegradable residual, the power output can be increased to 2.25 MWh per m3 of alcohol, but the economic feasibility of this option depends on the maximum loading rate of the bagasse digester and the energy price. At the current alcohol production level of 13 x 10(6) m3/year, the power generation potential is 2.2 GW, which represents 4% of the power demand in Brazil. The digested waste water contains about 70% of the nutrient demand of the cane fields, which can be recycled. A preliminary economic evaluation shows that productive use of the subproducts of alcohol distilleries is economically feasible if the price is more than US$30 per MWHh, which is the current sales price in Brazil. Another important advantage of the rational use of by-products is that the generation of electric power has the potential to reduce the emission of CO2 to the atmosphere by 0.8-1.2t per m3 of alcohol compared to generation using natural gas.

The effect of operational conditions on the performance of UASB reactors for domestic wastewater treatment.

In this investigation, the performance of Upflow Anaerobic Sludge Blanket (UASB) reactors treating municipal wastewater was evaluated on the basis of: (i) COD removal efficiency, (ii) effluent variability, and (iii) pH stability. The experiments were performed using 8 pilot-scale UASB reactors (120 L) from which some of them were operated with different influent COD (CODInf ranging from 92 to 816 mg/L) and some at different hydraulic retention time (HRT ranging from 1 to 6 h). The results show that decreasing the CODInf, or lowering the HRT, leads to decreased efficiencies and increased effluent variability. During this experiment, the reactors could treat efficiently sewage with concentration as low as 200 mg COD/L. They could also be operated satisfactorily at an HRT as low as 2 hours, without problems of operational stability. The maximum COD removal efficiency can be achieved at CODInf exceeding 300 mg/L and HRT of 6h.

Post-treatment of anaerobic effluents in constructed wetland systems.

This papers describes the behaviour of wetlands as a post-treatment unit for anaerobically treated sewage for the removal of organic matter, suspended solids, nutrients (nitrogen and phosphorus) and faecal coliforms. Raw sewage was treated in a UASB reactor with a retention time of 5 h and the effluent was used in four units of wetlands with coarse sand as the medium and operated with different hydraulic loads. Three of the units had emerging macrophytes (Juncus sp), whereas the fourth one was operated as a control unit without plants. During the 12 months of operation, the organic material removal efficiency (measured as COD) was in the range of 79 to 85%, whereas suspended solids removal varied from 48 to 71%. Faecal coliform removal was very high (99.99%); phosphorus was also efficiently removed (average efficiency of 90% for the lowest hydraulic load), but nitrogen removal was only partial (45 to 70% for ammonia and 47 to 70% for TKN). The experimental results clearly show the technical feasibility of using wetlands for treatment of municipal sewage after a pre-treatment in the UASB reactor.

Modelling and estimation of physical parameters in a sludge drying system.

In this paper is presented the study of a Sludge Drying System used to kill pathogenic organisms living in sludge. The system is modeled and the physical parameters thermal capacity, thermal resistance and thermal time constant are estimated using conventional estimation methods.

The UASB reactor as an alternative for the septic tank for on-site sewage treatment.

Although septic tanks are amply used for on site sewage treatment, these units have serious drawbacks: the removal efficiency of organic material and suspended solids is low, the units are costly and occupy a large area and operational cost is high due to the need for periodic desludging. In this paper an innovative variant of the UASB reactor is proposed as an alternative for the septic tank. This alternative has several important advantages in comparison with the conventional septic tank: (1) Although the volume of the UASB reactor was about 4 times smaller than the septic tank, its effluent quality was superior, even though small sludge particles were present, (2) desludging of the UASB reactor is unnecessary and even counterproductive, as the sludge mass guarantees proper performance, (3) the UASB reactor is easily transportable (compact and light) and therefore can be produced in series, strongly reducing construction costs and (4) since the concentration of colloids in the UASB effluent is much smaller than in the ST effluent, it is expected that the infiltration of the effluent will be much less problematic.

Anaerobic-aerobic sewage treatment using the combination UASB-SBR activated sludge.

The performance of a pilot scale sewage treatment system composed of an upflow anaerobic sludge blanket (UASB) digester and a sequencing batch activated sludge reactor (SBR) is described. The system constitutes a simple, economic, and compact treatment option and is able to produce consistently a final effluent quality that is equal or better than that of a conventional activated sludge plant. The entire treatment system had a retention time of only 9h and an average operational temperature of 25 degrees C. Due to the efficient anaerobic pre treatment the aerobic sludge production was low and could easily be accommodated for stabilisation in the UASB reactor. The system exhibited excellent operational stability with full nitrification for aerobic sludge ages longer than 9 days. At shorter sludge ages there was excessive wash out of sludge particles and the sludge mass could not be maintained in the SBR reactor. Sludge settleability was good throughout the experimental investigation period of one year. The reduction of the reactor volume and oxygen consumption was more than 50% compared to conventional activated sludge. The anaerobic excess sludge had a high concentration and good stability so that its dewatering and final disposal was a relatively minor problem.