The effect of pH on the efficiency of an SBR processing piggery wastewater

To treat piggery wastewater efficiently, the hydrolysis of urea (mainly derived from swine urine) in piggery wastewater with the change of sewage pH must be considered. Using activated sludge, piggery wastewater was treated in a sequencing batch reactor (SBR), and the effects of influent pH on SBR processing efficiency, sludge settle ability, and sludge activity were investigated. The results showed that a high influent pH value contributed to the improvement of the removal rate of ammonia nitrogen and reduction of the chemical oxygen demand (COD). When the influent pH was between 9.0 and 9.5, the removal rate of ammonia nitrogen was higher than 90%, and the reduction of COD from its original value was 80%. The influent pH had a greater influence on sludge concentration and sludge activity. When the influent pH increased from 7.0 to 9.5, the sludge concentration increased from 2,350 to 3,947 mg/L in the reactor, and the activities of ammonium oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB) first increased and then decreased. When the influent pH was 9.0 and 8.0, the maximum values (0.48 g O₂/(g MLSS/day) and 0.080 g O₂/(g MLSS/day)) were reached, and the sludge settling ratio was nearly steady between 20 and 35% in each reactor.     

Activated sludge biotreatment of sulphurous waste emissions

Odours from wastewater treatment plants are comprised of a mixture of various gases, of which hydrogen sulphide (H₂S) is the main constituent. Sulphurous compounds can be degraded by microorganisms commonly found in wastewater. The use of activated sludge (AS) diffusion as a dual-role system, for the treatment of wastewater and for odour control, offers an alternative to traditional sulphurous waste gas treatment processes, such as biofilters, bioscrubbers and biotrickling filters, both in practical terms (use of existing facilities) and economically (minimal capital cost). Activated sludge diffusion avoids the common problems associated with these processes such as media plugging, excess biomass accumulation, gas short-circuiting, and moisture control and maintaining the correct biofilm thickness. The design issues to be considered when using AS diffusion for odour abatement, comprise odourous air pre-treatment,blowers and diffuser types, corrosion protection and increase in odour emission intensity. Nitrification inhibition depends on the composition and acclimation of the biomass, the concentration of H₂S and other components of the wastewater. Hydrogen sulphide removal rates of >98% were consistently achieved for loads of 3–34 mg H₂S/g MLSS/h, in two case studies, which also showed that sludge type has an impact on the ability of the sludge to degrade H₂S. Wastewater process performance measured as five-day biological oxygen demand (BOD₅), chemical oxygen demand (COD) and effluent suspended solids removal was not affected by H₂S diffusionat 5 ppm. A change in the microorganism population dynamics of anactivated sludge was observed when it was exposed to H₂S for aperiod of more than 21 days.     

Photo-Fenton process for excess sludge disintegration

For the minimization of excess sludge in activated sludge process, the photo-Fenton reaction was applied as a novel technique and its effectiveness was experimentally examined. A batch study was conducted to elucidate parameters governing the activated sludge integration by the photo-Fenton process such as the concentrations of sludge, Fe ion and hydrogen peroxide. It was found that the chemical oxidation sludge disintegration by photo-Fenton reaction could be divided into two phases. At the beginning of the photo-Fenton process the dissolved chemical oxygen demand (COD) increased. This is due to the discharge of organic compounds occurred by oxidative decomposition of the cell wall of a microbe in the sludge. The COD reached the maximum and then decreased. In this phase, mineralization of dissolved organic substances by the photo-Fenton reaction might be dominant. The occurrence of mineralization was suppressed as the initial MLSS increased. The Fe dosage enhanced the sludge disintegration by the photo-Fenton reaction as well as the H₂O₂ dosage. The soluble total organic carbon (TOC) of the solution increased monotonously. The results suggest that Fe ions might be entrapped into the activated sludge. It may be concluded that photo-Fenton reaction is one of feasible processes for disintegration of excess activated sludge.     

A rationale for the appropriate amount of inoculum in ready biodegradability tests

Several screening methods at the so-called ready biodegradability level are suitable to test poorly soluble substances. Typical for these tests is that mineralization is evaluated from monitoring oxygen uptake or carbon dioxide production. Unfortunately, they suffer from a rather low precision in the calculated percentage of mineralization caused by subtracting a too high inoculum control measurement from the response in the test system. Criteria for blank oxygen consumption, due to the metabolic activity of the inoculum, are proposed from which maximum amounts of activated sludge or secondary effluent per litre test medium can be derived to be used as an appropriate inoculum. Both for current and future standardized tests the precision of the method can be kept within acceptable margins. Inoculum material was sampled from 40 communal biological waste water treatment plants. From endogenous respiration rates it was derived that the concentration of secondary effluent in the Closed Bottle Test can be increased up to 50 mL/L but that in respirometry tests inoculated with activated sludge the appropriate concentration is 10 mg/L dry matter or below, depending of the design of the test system.List of abbreviations BOD biological oxygen demand - CBT Closed Bottle Test - C _as inoculum concentration in mg dry solids of activated sludge per litre test medium - C _ef inoculum concentration in ml secondary effluent per litre test medium - C _ss dry weight content of activated sludge (g/L) - CFU colony forming units - DO_7d dissolved oxygen concentration (mg/L) after 7 days - ISO International Organization for Standardization - NEN Dutch Organization for Standardization - O _c oxygen capacity in mg oxygen per litre vessel volume - OECD Organisation for Economic Co-operation and Development - Ox _as oxygen consumption after one week in mg oxygen per mg dry weight activated sludge - Ox _ef oxygen consumption after one week in mg oxygen per mL secondary effluent - Ox _ef [n] oxygen consumption after one week in mg oxygen per n mL secondary effluent - Ox _flask oxygen uptake in mg per litre flask volume - RBT Ready Biodegradability Test - SLR sludge loading rate in kg O₂/kg dry weight·d - ThOD theoretical oxygen demand - TPCBT Two Phase Closed Bottle Test - V _a volumes of air and water per litre vessel - V _w volume, respectively - _a concentration of oxygen in air at 20° C and 101.5 kPa - _s saturation oxygen concentration in te aqueous phase     

Substrate consumption and excess sludge reduction of activated sludge in the presence of uncouplers: a modeling approach

A mathematical model with a consideration of energy spilling is developed to describe the activated sludge in the presence of different levels of metabolic uncouplers. The consumption of substrate and oxygen via energy spilling process is modeled with a Monod term, which is dependent on substrate and inhibitor. The sensitivity of the developed model is analyzed. Three parameters, maximum specific growth rate (μ _max), energy spilling coefficient (q _max), and sludge yield coefficient (Y _H) are estimated with experimental data of different studies. The values of μ _max, q _max, and Y _H are found to be 6.72 day^-1, 5.52 day^-1, and 0.60 mg COD mg^-1 COD for 2, 4-dinitrophenol and 7.20 day^-1, 1.58 day^-1, and 0.62 mg COD mg^-1 COD for 2, 4-dichlorophenol. Substrate degradation and sludge yield could be predicted with this model. The activated sludge process in the presence of uncouplers that is described more reasonably by the new model with a consideration of energy spilling. The effects of uncouplers on substrate consumption inhibition and excess sludge reduction in activated sludge are quantified with this model.     

Nitrogen removal from wastewater with a low COD/N ratio at a low oxygen concentration

The goal of the study was to determine the effectiveness of nitrification and denitrification and the kinetics of ammonia removal from a mixture of wastewater and anaerobic sludge digester supernatant in an SBR at limited oxygen concentration. In addition, the COD removal efficiency and sludge production were assessed.In the SBR cycle alternating aerobic and anaerobic phases occurred; in the aeration phase the dissolved oxygen (DO) concentration was below 0.7 mg O₂/L. The low DO concentration did not inhibit ammonia oxidation-nitrification and the efficiency was ca. 96-98%. However, a relatively high COD concentration in the effluent was detected. The values of K_m and V_max, calculated from the Michaelis-Menten equation, were 43 mg N-NH₄/L and 15.64 mg N-NH₄/L h, respectively. Activated sludge production was almost stable (0.62-0.66 g MLVSS/g COD). A high net biomass production resulted from a low specific biomass decay rate of 0.0015 d⁻¹.     

Characteristics of granular methanogenic sludge grown on glucose in a UASB reactor

The formation of granules grown on glucose in an upflow anaerobic sludge blanket (UASB) reactor was investigated. Total granular sludge concentration retained in the UASB reactor was 34.5 g MLSS/l (30.0 g MLVSS/l) during 240 d operation on glucose minimum medium with the supplementation of 1.07 g NaHCO₃ per 1 g glucose. This realized a high-rate methanogenic fermentation of glucose of 17.6 g COD/l-reactor-d at 3.4 d⁻¹ of space velocity. The granules formed were relatively small, ranging mainly from 0.4 to 0.5 mm, had a relatively low cell density of 0.0542–0.0560 g MLVSS/ml, and had low specific gravity (0.97–1.19) due to very low ash content (11–13%). Electron microscopic analysis showed that Methanothrix spp. appeared dominant over the granules. The specific metabolic activities of bacterial trophic groups were the highest for H₂ followed by glucose, acetate, and propionate.     

Production of hydrogen from domestic wastewater in a pilot-scale microbial electrolysis cell

Addressing the need to recover energy from the treatment of domestic wastewater, a 120-L microbial electrolysis cell was operated on site in Northern England, using raw domestic wastewater to produce virtually pure hydrogen gas (100?±?6.4 %) for a period of over 3 months. The volumetric loading rate was 0.14 kg of chemical oxygen demand (COD) per cubic metre per day, just below the typical loading rates for activated sludge of 0.2–2 kg?COD?m^?3?day^?1, at an energetic cost of 2.3 kJ/g?COD, which is below the values for activated sludge 2.5–7.2 kJ/g?COD. The reactor produced an equivalent of 0.015 L?H₂?L^?1?day^?1, and recovered around 70 % of the electrical energy input with a coulombic efficiency of 55 %. Although the reactor did not reach the breakeven point of 100 % electrical energy recovery and COD removal was limited, improved hydrogen capture and reactor design could increase the performance levels substantially. Importantly, for the first time, a ‘proof of concept’ has been made, showing that this technology is capable of energy capture as hydrogen gas from low strength domestic wastewaters at ambient temperatures.     

Oxygen supply to bacterial suspensions of high cell densities by hydrogen peroxide

The supply of heterotrophically growing suspensions of Alcaligenes eutrophus PHB^?4 with oxygen formed by the continuous addition of H₂O₂ in the presence of bovine liver catalase was found to be restricted to well-defined conditions. The catalase-H₂O₂ system proved to be suitable during the growth at low cell densities equivalent to 2 g dry weight/liter. When under these conditions the oxygen concentration was held constant at 1.8 mg O₂/liter, the cells grew for 6–8 hr at a rate almost identical to that observed with conventional aeration. However, aeration with H₂O₂ for longer durations (10–20 hr) and at higher cell densities (5?20 g dry weight/liter) led invariably to cell damage and retardation of growth. The impairment of growth observed during the oxygen supply by the catalase?H₂O₂ system was traced back to the formation of gradually increasing steady-state concentrations of H₂O₂ in the medium. Possible sites of cell damage by H₂O₂ such as membrane function, excretion and function of siderophores, and synthesis of cell polymers have been studied, and the cytotoxic mechanism of low concentrations of H₂O₂ was discussed.     

Pollution control in pulp and paper industrial effluents using integrated chemical–biological treatment sequences

The main objective of the present study was to improve the quality of pulp and paper industrial wastewater of two local mills RAKTA and El-Ahlia, Alexandria, Egypt, and to bring their pollutant contents to safe discharge levels. Quality improvement was carried out using integrated chemical and biological treatment approaches after their optimization. Chemical treatment (alum, lime, and ferric chloride) was followed by oxidation using hydrogen peroxide and finally biological treatment using activated sludge (90 min for RAKTA and 60 min for El-Ahlia effluents). Chemical coagulation produced low-quality effluents, while pH adjustment during coagulation treatment did not enhance the quality of the effluents. Maximum removal of the tested pollutants was achieved using the integrated treatment and the pollutants recorded residual concentrations (RCs) of 34.67, 17.33, 0.13, and 0.43 mg/l and 15.0, 11.0, 0.0, and 0.13 mg/l for chemical oxygen demand (COD), biochemical oxygen demand (BOD₅), tannin and lignin, and silica in RAKTA and El-Ahlia effluents, respectively, all of which were below their maximum permissible limits (MPLs) for the safe discharge into water courses. Specific oxygen uptake rate (SOUR) and sludge volume index (SVI) values reflect good conditions and healthy activated sludge. Based on the previous results, optimized conditions were applied as bench scale on the raw effluents of RAKTA and El-Ahlia via the batch chemical and the biological treatment sequences proposed. For RAKTA effluents, the sequence was as follows: (1) coagulation with 375 mg/l FeCl₃, (2) oxidation with 50 mg/l hydrogen peroxide, and (3) biological treatment using activated sludge with 2,000 mg/l initial concentration and 90 min hydraulic retention time (HRT), while for El-Ahlia raw effluents, the sequence was (1) coagulation with 250 mg/l FeCl₃, (2) oxidation with 45 mg/l hydrogen peroxide, and (3) biological treatment using activated sludge with 2,000 mg/l initial concentration and 60 min HRT. In conclusion, results confirmed that the application of the proposed sequential treatments removed almost all COD, BOD₅, high molecular weight compounds, and silica from RAKTA and El-Ahlia influents and produced high-quality effluents, thus achieving the main objective of this study.     

Removal of organic pollutants and analysis of MLSS–COD removal relationship at different HRTs in a submerged membrane bioreactor

In order to investigate the influence of hydraulic retention time (HRT) on organic pollutant removal in a submerged membrane bioreactor (SMBR), a laboratory-scale experiment was conducted using domestic sewage as influent. The dissolved oxygen (DO) concentration was controlled at 2.0– during the experimental period. The experiments demonstrated that when HRT was 3, 2 and 1 h, the reduction of chemical oxygen demand (COD) was 89.3–97.2, 88.5–97.3 and 80–91.1%, and the effluent COD was 38.9–11.2, 41.6–10.8 and 63.4–, respectively. It is suggested that an HRT of 1 h could meet the normal standard of discharged domestic sewage, and an HRT of 2 h could meet that of water reclamation. In addition, we use mathematical software MATLAB to analyse the relation of mixed liquor suspended solids (MLSS) and COD removal. The results showed that the optimum MLSS concentration should be maintained at around in the SMBR. The results also showed that the COD removal was related to HRT (τ), influent concentration (S₀) and sludge loading rate for COD removal (N_S). Moreover, the high COD removal could be achieved through adjusting τ, S₀ and N_S.     

Anaerobic granule development for removal of pentachlorophenol in an upflow anaerobic sludge blanket (UASB) reactor

The granulation process using synthetic wastewater containing pentachlorophenol (PCP) in four 1.1 l laboratory scale upflow anaerobic sludge blanket (UASB) reactors was studied, and the anaerobic biotransformation of PCP during the granulation process investigated. After 110 days granular sludge was developed and up to 160 and 180 mg/l of PCP was added into the reactors R1 and R2, respectively, when they were inoculated with acclimated anaerobic sludge from an anaerobic digester of a citric acid plant. The inoculum was predominately composed of bacilli and filamentous bacteria. Granulation did not occur in reactors R3 and R4 which were inoculated with acclimated anaerobic sludge from aerobic sludge of the municipal sewage treatment plant which consisted mainly of cocci. Despite similar bacilli in the granule, the filamentous bacteria from reactor R1 were thicker than those of reactor R2. The granular sludge had a maximum diameter of 2.5 and 2.2 mm, and SMA of 1.44 and 1.32 gCOD/gTVS per day for reactors R1 and R2, respectively. Over 98% chemical oxygen demand (COD) removal rate and 99% of PCP removal rate were achieved when reactors R1 and R2 were operated at PCP and COD loading rates of 150 and 7.5 g/l per day, respectively. H₂-producing acetogens were the dominant anaerobes in the granular sludge.     

Superoxide dismutase and the oxygen enhancement of radiation lethality

Escherichia coli B were more susceptible to radiation lethality and showed a greater oxygen enhancement ratio when exposed in dilute suspension (1 × 10⁵ cells/ml) than when exposed in dense suspensions (1 × 10⁹ cells/ml). The oxygen enhancement, seen with dilute suspensions, was diminished by superoxide dismutase, catalase, mannitol, or histidine. Heat-denatured superoxide dismutase was without effect. The results are interpreted as indicating a role for O₂^? plus H₂O₂ in the oxygen enhancement of radiation lethality, and a scheme is proposed which is consistent with the observations.     

Improved biological treatment of nitrogen-deficient wastewater by incorporation of N2-fixing bacteria

The N₂-fixing bacterium, Azotobacter vinelandii, was used both in single culture and in combination with activated sludge culture for the treatment of nitrogen-deficient wastewaters as an alternative to external nitrogen supplementation. Azotobacter-supplemented activated sludge culture removed more total organic carbon (TOC), especially at low initial TN/COD (total nitrogen/chemical oxygen demand) ratios, than the Azotobacter-free culture. Up to 95% TOC removal efficiencies were obtained with synthetic media of TN/COD<4 when Azotobacter was used singly or with activated sludge. The results indicated clear advantage of using Azotobacter in the activated sludge to improve TOC removal from nitrogen-deficient wastewaters.     

Partial reversal by sodium ascorbate of hyperoxia-induced damage to HEp-2 cell cultures

Summary Hyperoxia induced cellular damage was used as an experimental model system for examining the ameliorative role of antioxidants. Multiplication of HEp-2 cells in monolayer culture was inhibited after exposure to 100% O₂ either hyperbarically at 3 atm absolute (atma) or normobarically at 1 atma for periods from 15 s to 4 h. The inhibition was characterized by a slower rate of replication for a period from 1 to 3 d after exposure than in unexposed cultures, and then massive cellular death. Less killing followed exposure to normobaric O₂ than to hyperbaric O₂, and the shorter the period of exposure to hyperoxia the less killing. Addition of 100 μg/ml of sodiuml-ascorbate to unexposed cultures enhanced growth (cell number at 6 d) almost twofold. When added ascorbate was present only during hyperoxic exposure (but not afterward), subsequent growth in air was enhanced 1.6-fold. However, when cells were exposed without added ascorbate, there was from 2 to 12-fold greater growth in air in the presence of the added ascorbate (as compared to exposed controls). This greater growth was always only a partial reversal of the lethal effect resulting from hyperoxia. Addition of 25 μg/ml catalase did not affect control or exposed cultures. Addition of ascorbate plus catalase was not as effective as ascorbate alone in promoting growth; the catalase moiety antagonized some of the growth enhancing influence of ascorbate. This suggests that extracellular H₂O₂ was not a factor in the lethal effect resulting from hyperoxia.     

Starvation is not a prerequisite for the formation of aerobic granules

Activated sludge with sludge volume index (SVI)₃₀ of 77 ml g⁻¹ and SVI₃₀ of 433 ml g⁻¹ was inoculated to start up reactors R1 and R2, respectively. In both R1 and R2, cycle time of 1 h and the influent chemical oxygen demand (COD) concentrations of 1,000 mg l⁻¹ were employed. Initial settling time of 2 min resulted in the loss of a substantial amount of biomass as wash-out and high effluent COD concentrations within the first week of operation. This implied that there was no starvation phase in each cycle of R1 and R2 during the first week of operation. However, aerobic granules with a size above 400 μm formed by day 7. Thus, it was concluded that starvation was not a prerequisite for the formation of aerobic granules. When cycle time was 1 h, the instability of aerobic granules was observed. When cycle time was prolonged to 1.5 h and granular sludge of 200 ml was used to start up reactor R3, the reactor R3 reached steady state within 1 week. SVI, size, and the morphology of granular sludge in R3 remained stable during the 47-day operation, which indicated that prolonged starvation time had positive effects on the stability of aerobic granules.     

Fermentative hydrogen production from fresh leachate in batch and continuous bioreactors

This research for the first time investigated hydrogen production from the fresh leachate originated from municipal solid wastes. We found that fermentation of the leachate generated H₂ and was very much enhanced in the presence of extra phosphate in the batch reactor. The continuous expanded granular sludge bed (EGSB) reactor started to generate H₂ at day 20 and continued to 176 days with 120 mg/l of extra phosphate present. The highest chemical oxygen demand (COD) removal efficiency (66.9%) was achieved at liquid up-flow velocity of 3.7 m/h and hydraulic retention time of 12 h. Under proposed optimal operation conditions, the mean H₂ production rate reached up to 2155 ml/(l day). We also found that over 80% liquid metabolites were acetic acid and ethanol, suggesting the ethanol-type fermentation was dominant in the bioreactor. These findings indicate that the fresh leachate can be used as the source for continuous hydrogen production.     

Performance of upflow anaerobic sludge blanket (UASB) reactor treating wastewaters containing carbon tetrachloride

The anaerobic biodegradation of carbon tetrachloride (CT) was investigated during the granulation process by reducing the hydraulic retention time, increasing the chemical oxygen demand (COD) and CT loadings in a 2l laboratory-scale upflow anaerobic sludge blanket (UASB) reactor. Anaerobic unacclimated sludge and glucose were used as seed and primary substrate, respectively. Granules were developed 4 weeks after start-up, which grew at an accelerated rate for 8 months, and then became fully grown. The effect of operational parameters such as influent CT concentrations, COD, CT loading, food to biomass ratio and specific methanogenic activity (SMA) were also considered during granulation. The granular sludge cultivated had a maximum diameter of 2.1 mm and SMA of 1.6 g COD/g total suspended solid (TSS) day. COD and CT removal efficiencies of 92 and 88% were achieved when the reactor was firstly operating at CT and COD loading rates of 17.5 mg/l day and 12.5 g/l day, respectively. This corresponds to hydraulic retention time of 0.28 day and food to biomass ratio of 0.5 g COD/g TSS day. Kinetic coefficients of maximum specific substrate utilization rate, half velocity coefficient, growth yield coefficient and decay coefficient were determined to be 2.4 × 10^–3 mg CT/TSS day^–1, 1.37 mg CT/l, 0.69 mg TSS/mg CT and 0.046 day^–1, respectively for CT biotransformation during granulation.     

Investigation on the formation and kinetics of glucose-fed aerobic granular sludge

Aerobic granular sludge was cultivated in a glass sequencing batch reactor (SBR) with glucose synthetic wastewater. The spherical shaped granules were observed on 4th day with the mean diameter of 0.1 mm. With the increase of chemical oxygen demand (COD) concentration of the influent, aerobic granules grew matured, the size of which ranged from 1.2 to 1.9 mm. The aerobic granular sludge could sustain high organic loading rate (about 4.0 g COD L⁻¹ d⁻¹), with good settling ability (settling velocity 36 m/h) and high biomass concentration (MLSS 6.7 ±0.2 g/L). Experimental data indicated that the substrate utilization and biomass growth kinetics followed Monod's kinetics model approximately. The corresponding kinetic coefficients of maximum specific substrate utilization rate (k), half velocity coefficient (K_s), growth yield coefficient (Y) and decay coefficient (K_d) were 13.2 d⁻¹, 275.8 mg/L, 0.183–0.250 mg MLSS/mg COD and 0.023–0.075 d⁻¹, respectively, which made aerobic granules have short setup period, high rate of substrate utilization and little surplus sludge.     

The effect of methanogenesis inhibition,inoculum and substrate concentration on hydrogen and carboxylic acids production from cassava wastewater

Manipueira is a carbohydrate-rich agro-industrial waste from cassava processing. It is considered well suitable for biotechnological processes, such as hydrogen and carboxylic acids production, due to the high content of easily degradable organic matter. However, the proper methanogenesis inhibition method, inoculum type, and organic loads are factors still limiting the processes. The objective in this work was to evaluate the effects of such factors on byproducts production in anaerobic reactors. Batch experiments were conducted with 2.3-L flasks during two operational phases. In the first phase (P1), inhibition of methanogens in the sludge was evaluated using acetylene (1% v/v of headspace) and heat treatment (120 °C, 1 atm for 30 min). In the second phase (P2), three inoculum types obtained from common anaerobic sludges (bovine rumen and sludges from municipal and textile industrial wastewater treatment plants) were individually assayed. P2 aimed to identify the best inoculum, based on hydrogen production ability, which was tested for three initial concentrations of manipueira in terms of chemical oxygen demand (COD) (10, 20 and 40 g O₂/L). Results of P1 indicated that either acetylene or heat treatment efficiently inhibited methanogenesis, with no methane production. However, the maximum H₂ production potential by applying heat treatment (~ 563 mL) was more than twice compared with that by acetylene treatment (~ 257 mL); and butyrate was the main carboxylic acid by-product (~ 3 g/L). In P2 experiments after sludge heat treatment, the highest hydrogen yield (1.66 ± 0.07 mol H₂/mol glucose) and caproic acid production (~ 2 g/L) were observed at 20 g O₂/L of manipueira COD, when bovine rumen was the inoculum. The primary metabolic degradation products in all P2 experiments were ethanol, acetic, butyric, propionic and caproic acids. The finding of caproic acid detection indicated that the applied conditions in manipueira anaerobic degradation favored carbon chain elongation over methanogenesis.