How to optimize hollow-fiber submerged membrane bioreactors.

Membrane fouling is linked to reversible or irreversible accumulation of macromolecules and solids on membrane surfaces and to the irreversible adsorption inside pores. If reversible accumulation can be controlled by filtering in subcritical conditions, then adsorption could also be minimized by reducing the soluble organic matter [extracellular polymeric substances, soluble microbial products (SMP)]. This research shows how the choice of operating parameters related to biological reaction (solid retention time and the organic loading rate) can influence the process rate and the by-product (SMP) production. It also illustrates how suspension characteristics and membrane aeration can influence membrane fouling control according to the hollow fiber configuration and to the different scales of observation. The investigations were based on the definition of different fouling level and fine-tuning of a model to better understand the effects of operating parameters on membrane bioreactor filtration.

Sorption study of an acid dye from an aqueous solutions using modified clays.

The removal of the pollutant Supranol Yellow 4GL (S.Y.4GL) was studied by using different clays: clay exchanged with sodium (BNa+) and hydroxyaluminic polycation pillared clays in the presence or absence of non-ionic surfactant. While decomposing the surfactant at 500 degrees C, the surface of the clay changed significantly. The study of the behaviour of the three clays with respect to coloring solutions, allowed to determine the equilibrium time and the rate-determining step of the dye S.Y.4GL adsorption. Two simplified kinetic models, were tested to investigate the adsorption mechanisms in terms of pseudo-first order and pseudo-second order equations. Besides, the adsorption capacity data were fitted to Langmuir and Freundlich equations as well. A better fixation was obtained with an acidic pH. The effect of temperature on the adsorption of dye has been also studied and the thermodynamic parameters DeltaG degrees , DeltaH degrees and DeltaS degrees were determined.

Start up of an anaerobic inverse turbulent bed reactor fed with wine distillery wastewater using pre-colonised bioparticles.

The long start-up period of fluidized bed biofilm reactors is a serious obstacle for their wide installation in the anaerobic treatment of industrial wastewater. This paper presents the results of an anaerobic inverse turbulent bioreactor treating distillery wastewater during 117 days of operation at a laboratory scale. The pre-colonized bioparticles for this work were obtained from a similar reactor processing the same wastewater and which had a start-up period of 3 months. The system attained carbon removal efficiency rates between 70 and 92%, at an organic loading rate of 30.6 kg m(-3) d(-1) (chemical oxygen demand) with a hydraulic retention time of 11.1 h. The results obtained showed that the start-up period of this kind of reactors can be reduced by 3 using pre-colonized bioparticles.

Transportation of reclaimed wastewater through a long pipe: inhibition of sulphide production by nitrite from the secondary treatment.

The agricultural reuse of reclaimed wastewater has become a necessity in places with water shortages. Frequently, this involves the operation of long transportation pipelines, like in the South Tenerife reuse system, whose main element is a completely filled 61 km long gravity pipe in cast iron. Sulphide generation, which could contribute to pipe corrosion, is a usual process taking place during transportation if anaerobic conditions prevail. In the Wastewater Treatment Plant of Santa Cruz (Tenerife, Spain) a partial nitrification process was achieved by increasing the mean residence time through the aeration step at low dissolved oxygen concentration. Such conditions, combined with the right temperature and a free ammonia concentration above 1 mg l(-1), inhibited nitratation and favoured nitritation, which led to concentrations of NO2-N above 8 mg l(-1) in the secondary effluent. During the transportation, nitrite inhibited the appearance of anaerobic conditions, and, consequently, no sulphide generation occurred. At the same time, a nitrite reduction process took place with a first order kinetics and a rate coefficient of 0.052 h(-1)' at 25 degrees C. A parallel behaviour between the nitrite depletion and the oxidation-reduction potential evolution along the pipeline was also observed.

Equilibrium and kinetics of color removal from dye solutions with bentonite and polyaluminum hydroxide.

Wastewater from the textile industry contains soluble dyes that are toxic and particularly difficult to remove. A promising low-cost treatment, however, is use of polyaluminum hydroxide associated with bentonite. At suitable conditions, this process is able to efficiently remove color from solutions containing mixtures of soluble acid azo dyes and produce easily settleable sludge. The removal mechanism, which is believed to involve adsorption or precipitation and weak pH variations, is not well understood. With the overall reaction being second order, two elementary first-order reactions could be assumed. The equilibrium removal is a decreasing function of the temperature. However, this effect is weak and decreases when the dye concentration increases. At usual values of concentration in textile wastewater, this effect can be neglected. The resulting solid compound is particularly resistant to mechanical stress. Moreover, color was significantly released at pH greater than 8. Sodium ions have no influence on the compound stability, which reinforces the assumption of the involvement of an adsorption process.

Anaerobic digestion of dairy wastewater by inverse fluidization: the inverse fluidized bed and the inverse turbulent bed reactors.

This paper describes the application of the inverse fluidization technology to the anaerobic digestion of dairy wastewater. Two reactors were investigated: the inverse fluidized bed reactor and the inverse turbulent reactor. In these reactors, a granular floating solid is expanded by a down-flow current of effluent or an up-flow current of gas, respectively. The carrier particles (Extendospheres) were chosen for their large specific surface area (20,000 m2m(-3)) and their low energy requirements for fluidization (gas velocity of 1.5 mm s(-1), 5.4 m h(-1)). Organic load was increased stepwise by reducing hydraulic retention time from more than 60 days to 3 days, while maintaining constant the feed COD concentration. Both reactors achieved more than 90% of COD removal, at an organic loading rate of 10-12 kgCOD m(-3) d(-1), respectively. The performances observed were similar or even higher than that of other previously tested fluidized bed technologies treating the same wastewater. It was found that the main advantages of this system are: low energy requirement, because of the low fluidization velocities required; there is no need of a settling device, because solids accumulate at the bottom of the reactor, so they can be easily drawn out and particles with high-biomass content can be easily recovered. Lipid phosphate concentration has been revealed as a good method for biomass estimation in biofilms since it only includes living biomass.

Oxidising and disinfecting by hydrogen peroxide produced in a two-electrode cell.

Hydrogen peroxide was produced by direct current electrolysis using two electrodes only, a carbon felt cathode and a dimensional stabilised anode (titanium coated with RuO2), without adding any chemical. The required oxygen was supplied by water oxidation and by transfer from the atmosphere. The intensity should be maintained under a maximum value to avoid peroxide reduction. High peroxide production rate and concentration were then reached. Electroperoxidation partially removed dissolved organic carbon (DOC) contained in solutions of phenol, salicylic acid, benzoic acid and humic acids. The DOC removal in effluent of municipal sewage plant corresponded to a breakage of the double bonds. Real effluents were significantly disinfected owing to the direct effect of electric current and the indirect effect of peroxide. Moreover, a remnant effect was ensured.

Ammonia removal in a deep reclaimed wastewater reservoir before agricultural reuse.

An experimental study has been carried out to determine the ammonia removal in a deep reclaimed wastewater reservoir of 50,000 m3. The reservoir is part of the Reuse System of Reclaimed Wastewater of South Tenerife (Spain). The study has been conducted under batch mode operation, in three different periods (winter, spring-summer and winter-spring), with an average storage time of 85 days. Vertical profiles of temperature, dissolved oxygen and pH were determined together with NH3-N, NO2-N and NO3-N concentrations at different points and different depths in the reservoir. Maximum removal efficiency was obtained during the winter periods, in absence of stratification and with good mixing conditions throughout the water column. During these periods, nitrification took place in some extension, favoring the ammonia removal. Average NH3-N concentrations in the reservoir were calculated and apparent first-order rate constants were determined for different stratification conditions. Although ammonia nitrogen could be used as nutrient in the agricultural reuse, its removal from reclaimed wastewater could be useful in order to diminish the chlorine needs for disinfection.

How partial nitrification could improve reclaimed wastewater transport in long pipes.

Reclaimed wastewater transport is studied in a concrete-lined cast iron pipe, where a nitrification-denitrification process occurs. The pipe is part of the Reuse System of Reclaimed Wastewater of South Tenerife (Spain), 0.6 m in diameter and 61 km long. In order to improve wastewater quality, at 10 km from the inlet there is injection of fresh water saturated in dissolved oxygen (DO), after which a fast nitrification process usually appears (less than two hours of space time). The amount of oxidized nitrogen compounds produced varies between 0.8 and 4.4 mg/l NOx(-)-N. When DO has disappeared, a denitrification process begins. The removal of nitrite is complete at the end of the pipe, whereas the nitrate does not disappear completely, leaving a concentration of about 0.4-0.5 mg/l. For a COD/NOx(-)-N ratio higher than 5, a first order nitrification rate in NOx(-)-N has resulted, with the constant k20 = 0.079 h-1, for a NOx(-)-N concentration range of 0.8-4.4 mg/l. Finally the following temperature dependency for the first order denitrification rate constant has been found: k = k20 x 1 x 15T-20. Although nitrogen could be used as nutrient in the agricultural reuse, its removal from reclaimed wastewater could be useful in order to diminish the chlorine needs for reclaimed wastewater disinfection.

Influence of biomass accumulation on bed expansion characteristics of a down-flow anaerobic fluidized-bed reactor.

This article describes the bed expansion characteristics of a down-flow anaerobic fluidized bed reactor treating a synthetic wastewater. Experiments were carried out in a 0.08 m diameter and 1 m length PVC column. The carrier used was ground perlite (an expanded volcanic rock). Particles characteristics were 0.968 mm in diameter, specific density of 213 kg x m-3 and Umf (minimal fluidization velocity): 2.3 m x h-1. Experimental data of terminal velocities and bed expansion parameters at several biofilm thicknesses were compared to different models predicting the bed expansion of up-flow and down-flow fluidized beds. Measured bed porosities at different liquid superficial velocities for the different biofilm thicknesses were in agreement with the Richardson-Zaki model, when Ut (particle terminal velocity) and n (expansion coefficient) were calculated by linear regression of the experimental data. Terminal velocities of particles at different biofilm thicknesses calculated from experimental bed expansion data, were found to be much smaller than those obtained when Cd (drag coefficient) is determined from the standard drag curve (Lapple and Sheperd, 1940) or with others' correlations (Karamanev and Nikolov, 1992a,b). This difference could be explained by the fact that free-rising particles do not obey Newton's law for free-settling, as proposed by Karamanev and Nikolov (1992a,b) and Karamanev et al. (1996). In the present study, the same free-rising behavior was observed for all particles (densities between 213 and 490 kg x m-3).