A. Noyola

Analysis of microbial communities developed on the fouling layers of a membrane-coupled anaerobic bioreactor applied to wastewater treatment.

The structure of the biofouling layers formed on a pilot-scale membrane-coupled upflow anaerobic sludge blanket bioreactor (UASB) used to treat urban wastewater was analyzed by scanning electron microscopy and electron-dispersive X-ray microanalysis. For comparison, control samples of the membranes were fed either UASB effluent or raw wastewater in a laboratory-scale experiment. Microbial diversity in the fouling materials was analyzed by temperature gradient gel electrophoresis (TGGE) combined with sequence analysis of partial 16S rRNA. Significant differences in structure of the Bacteria communities were observed amongst the different fouling layers analyzed in the UASB membranes, particularly following a chemical cleaning step (NaClO), while the Archaea communities retained more similarity in all samples. The main Bacteria populations identified were evolutively close to Firmicutes (42.3%) and Alphaproteobacteria (30.8%), while Archaea were mostly affiliated to the Methanosarcinales and Methanospirillaceae. Sphingomonadaceae-related bacteria and methanogenic Archaea were persistently found as components of biofouling, regardless of chemical cleaning.

Effect of trace metals on the anaerobic degradation of volatile fatty acids in molasses stillage

A laboratory UASB reactor was fed with cane molasses stillage at organic loadings from 5 to 21.5 kg COD/m3 d. With an organic load of 17.4 kg COD/m3 d, an accumulation of VFA, principally propionic acid, was observed due to little bioavailability or lack of trace metals (Fe, Ni, Co and Mo). Associated to this, the performance of the UASB reactor was low (44% COD removal efficiency), with an alkalinity ratio above 0.4. The addition of Fe (100 mg/l), Ni (15 mg/l), Co (10 mg/l) and Mo (0.2 mg/l) to the influent reduced significantly the level of propionic acid (5291mg/l to 251 mg/l) and acetic acid (1100 mg/l to 158 mg/l). The COD removal efficiency increased from 44% to 58%, the biogas production from 10.7 to 14.8 l/d (NTP) and 0.085 to 0.32 g CH4-COD/g SSV d for specific sludge methanogenic activity with propionic acid as substrate. These improved results were obtained with high COD (68.9 g/l) and organic load (21.5 kg COD/m3 d).

Two-phase (acidogenic-methanogenic) anaerobic thermophilic/mesophilic digestion system for producing Class A biosolids from municipal sludge.

Two different arrangements of two-phase anaerobic sludge systems were operated treating a mixture of primary and secondary sludge. Two steady state periods were evaluated: the first acidogenic thermophilic phase was operated at hydraulic retention times of 3 and 2 days and the second methanogenic (mesophilic and thermophilic) phases at 13 and 10 days. The two-phase systems had an efficient removal of pathogens and parasites, achieving values lower than those specified for Class A biosolids, according to the Mexican Standard NOM-004-SEMARNAT-2002. The first thermophilic phase achieved almost complete destruction of pathogens and parasites by itself. During the second steady state period, volatile fatty acids accumulated in the second methanogenic phases (HRT of 10 days and an organic load of 3 kg VS/m(3)d) indicating that the systems were overloaded, mainly the mesophilic digester. In this case, the accumulation of propionic acid may be related to a deficiency of micronutrients. The results show that the two-phase thermophilic/mesophilic anaerobic sludge digestion may be considered as an adequate option for the production of Class A biosolids.

Changes in Physical Properties of a Compost Biofilter Treating Hydrogen Sulfide

Abstract A technique is presented that can be used to estimate the changes in physical structure in a natural biofilter packing medium, such as compost, over time. The technique applies information from tracer studies, grain size distribution, and pressure drop analysis to a model that estimates the number of channels, average channel diameter, number of particles, and specific surface area of the medium. Important operational factors, such as moisture content, pressure drop, and sulfate accumulation also were evaluated both in a conventionally operated biofilter and in one operated with periodic compost mixing. In the conventionally operated laboratory-scale compost biofilter, hydrogen sulfide (H2S) removal efficiency decreased from 100% to ∼ 90% over 206 days of operation. In a similar system, operated with compost mixing, the H2S removal efficiency was maintained near 100%. Variations in media moisture conditions and specific surface area can explain the results observed in this study. Under conventional operation, drying near the inlet disintegrated the compost particles, producing a large number of particles and flow channels and increasing the specific surface area. At the top of the column, where moisture was added, particle size increased and specific surface area decreased. In the column with media mixing, moisture content, particle size, and specific surface area remained homogeneous.

Dextran blue colorant as a reliable tracer in submerged filters

Abstract A reliable method to evaluate residence time distribution in submerged filters is presented. This method is useful even when using a porous packing. The tracer employed is Dextran blue, which is better detected in the u.v.-zone (220 nm). The advantage of this colorant is that it avoids the tailing phenomenon, allowing accurate mean retention time evaluation. The tailing phenomenon is explained using the diffusion theory.

Anaerobic sewage treatment: state of the art, constraints and challenges

Abstract The interest in high-rate anaerobic (pre-)treatment of sewage using UASB reactors is steadily growing since its introduction in the mid-1980s. Today there are hundreds of full-scale plants in operation in various parts of the tropical world, notably in Latin America and India. The main advantage of UASB technology is the very low or even zero energy demand, leading to an up to tenfold drop in operational costs compared to activated sludge. This paper presents a literature review focussing on current design criteria and post-treatment options, alongside discussing the centralized and decentralized approach. The current limitations and constraints regarding temperature, nutrients, pathogen removal, odour nuisance, operational constrictions and methane emissions are also presented and discussed. Further, recent challenges in relation to energy recovery from biogas, sludge and scum are discussed, alongside with advances related to recovery of dissolved methane and sludge management. Finally, the paper provides some outlooks for upcoming developments.

Biofouling and pollutant removal during long-term operation of an anaerobic membrane bioreactor treating municipal wastewater

Two different sludge retention times (SRTs) were tested in order to assess the impact on membrane fouling and effluent quality in an anaerobic membrane bioreactor (AnMBR). Two up-flow anaerobic sludge bed (UASB) reactors (1 l volume) coupled to external tubular ultrafiltration membranes (filtration area = 81 cm2) were operated at a hydraulic retention time of 3 h and two different SRTs (100 and 60 days). The transmembrane pressure (TMP), flux (J) and relevant parameters to assess water quality were measured. Effluents from UASB reactors were filtered for 500 h without intermediate cleaning. The permeate met Mexican standards for wastewater reclamation in both tested conditions. Abrupt and periodical changes in the TMP and J were noticed during the experimental period. A fouling layer collapse and compression hypothesis was set forth in order to explain these changes. An autopsy performed on biofouled membranes indicated that deposited mass was mainly composed of volatile solids (85%) and the rest related to mineral matter, with the presence of inorganic salts containing Ca, Mg, Fe, P and Si. Biomass in the fouling layer was estimated at 0.27% based on the DNA/biomass ratio for the bacterial biofilm. No clear difference in membrane fouling was detected under the two SRTs applied to the systems. However, when operated over 500 h, repetitive sudden TMP and flux changes occurred later in system A (SRT of 100 days) than in system B (SRT of 60 days) suggesting a stronger fouling layer structure in the former.

Selected dyes for residence time distribution evaluation in bioreactors

A selection of dyes for tracer studies in bioreactors, specially for wastewater treatment, is presented. Substances that showed no adsorption on air or biomass, stability in time, good solubility and no color change between pH 6.5 to 8.5, were: bromocresol green, bromophenol blue, dextran blue, eosin Y and mordant violet. Consequently they seem to be adequate for common biochemical engineering processes. In addition, dyes that showed some limitations, but may be employed in special cases, were: bromophenol red and phenol red (color change between pH 5.0 to 6.8 and 6.8 to 8.4 respectively) and methylene violet Bernsthen (low spectrophotometric response).

Tracer Studies in a Laboratory and Pilot Scale UASB Reactor

Residence time distribution studies on laboratory and pilot scale upflow anaerobic sludge blanket reactors (UASB) were carried out in order to measure and evaluate the influence, on the reactors hydraulic performance, of the biogas collection device position (gas-liquid-solid separator), of the presence of granular sludge and of the arrangement of the influent distribution system within the reactor. The main purpose of the pilot scale experiment was to assess the influence of the biogas collection device on the reactor hydraulic performance accurately. The results show that if the biogas collector system is located in a lower position (nearer to the sludge bed), the dead volume fraction decreases and a better performance of the settler could be obtained. In addition, it can be appreciated that the mixed flow pattern has more dead volume fraction than a plug flow hydraulic pattern.

Anaerobic digestion Of a Petrochemical Wastewater using the UASB process

Anaerobic digestion of effluent from a petrochemical plant producing terephthalic acid has been tested using two Upflow Anaerobic Sludge Blanket (USAB) reactors. The reactors were seeded with two different inocula: one from an anaerobic stabilization pond receiving wasted sludge from the aerobic treatment plant of the petrochemical industry (reactor A); the other was an anaerobically adapted activated sludge from a municipal wastewater treatment plant (reactor B). At the beginning of the experiment, reactor A attained higher COD removal efficiencies and biogas production, but both reactors reached the same performances after 7 mo operation. The efficiencies in COD removal were low. At a 3 d hydraulic retention time (HRT), reactor A was loaded at 2.6 kg COD/m3 d and reactor B at 2.2 kg COD/m3 d. COD removals were 46.4 and 43.9% for reactor A and B, respectively. In view of these results, the UASB reactor does not appear as the most suitable treatment process for this kind of effluent.