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While rumen microorganisms are known to facilitate the hydrolysis of lignocellulosic substrates in anaerobic digestion (AD), it is unclear how rumen content can be used to maintain rumen microorgan...

This study investigated waste activated sludge (WAS) and food waste (FW) co-fermentation in batch assays to produce carboxylic acids. Three mixtures (50%, 70% and 90% WAS in VS basis) were studied under different conditions: with and without extra alkalinity, and with and without WAS auto-hydrolysis pre-treatment. All tests were carried out at 35°C, without pH adjustment and without external inoculum. Experimental results showed that co-fermentation yields, including volatile fatty acids and lactic acid, were always higher than WAS and FW mono-fermentation yields (ca. 100 and 80 mgCOD/gVS, respectively). Co-fermentation yields increased as the proportion of FW in the mixture increased, indicating that the improvement was primarily due to a higher FW degradation under co-fermentation conditions. The maximum co-fermentation yield was on average 480 mgCOD/gVS for the WAS/FW_50/50 mixture. The importance of pH on co-fermentation performance was evident in the experiments carried out with extra alkalinity, which showed that the proportion of WAS in the mixture should be high enough to keep the pH above 5.0. However, fermenters operational conditions should also prevent the enrichment of acetic acid consuming microorganisms. WAS auto-hydrolysis pre-treatment did not enhance co-fermentation yields but showed minor kinetic improvements. Regarding the product profile, butyric acid was enriched as the proportion of FW in the mixture increased and the concomitant pH decreased to the detriment of propionic acid. Propionic acid prevailed under neutral pH in the WAS mono-fermentation and the WAS/FW_90/10 mixture.

The implementation of anaerobic membrane bioreactor as mainstream technology would reduce the load of sidestream anaerobic digesters. This research evaluated the techno-economic implications of co-digesting sewage sludge and food waste in such wastewater treatment plants to optimise the usage of the sludge line infrastructure. Three organic loading rates (1.0, 1.5 and 2.0 kg VS m−3 d−1) and different strategies to manage the additional nutrients backload were considered. Results showed that the higher electricity revenue from co-digesting food waste offsets the additional costs of food waste acceptance infrastructure and biosolids disposal. However, the higher electricity revenue did not offset the additional costs when the nutrients backload was treated in the sidestream (partial-nitritation/anammox and struvite precipitation). Biosolids disposal was identified as the most important gross cost contributor in all the scenarios. Finally, a sensitivity analysis showed that food waste gate fee had a noticeable influence on co-digestion economic feasibility.

Anaerobic digestion is a worldwide technology to treat organic waste streams, primarily due to its capacity to produce methane as renewable energy. However, there is an increasing interest on nutrient recovery (N and P), which from both environmental and economic reasons have been identified as key feature in anaerobic digestion plants. The manuscript presents a comprehensive overview on recent advances in nutrient recovery technologies applicable for anaerobic digestion systems. The review focus on N and P recovery through the use of digestates as fertilizers, struvite precipitation and biological systems such as phycoremediation (i.e. algae cultivation) and polyphosphates accumulating organisms. Keywords: Anaerobic digestion, nutrient recovery, struvite, phycoremediation, fertilizer.

Anaerobic digestion is a worldwide technology for the treatment of organic waste streams with clear environmental benefits including generation of methane as renewable energy. However, the need to improve process feasibility of existing applications as well as to expand anaerobic digestion to a range of new substrates has raised interest on several intensifications techniques. Among them, the supplementation of inorganic and biological additives has shown good results at improving digesters performance. This manuscript presents a comprehensive review about recent advances in the utilization of inorganic and biological additives. On the one hand, reviewed inorganic additives comprise: (i) macro- (e.g. P, N and S) and micro- (e.g. Fe, Ni, Mo, Co, W and Se) nutrients supplements, (ii) ashes from waste incineration, (iii) compounds able to mitigate ammonia inhibition, and (iv) substances with high biomass immobilization capacity. Among them, iron (Fe 0 and Fe(III)) has shown particularly promising results, which have been mainly related to their action as electron donor/acceptor and cofactor of key enzymatic activities. On the other hand, reviewed biological additives include: (i) the dosage of microbial inocula with high hydrolytic or methanogenic activity (bioaugmentation), and (ii) the addition of enzymes able to facilitate particulate organic matter solubilization.

A sidestream anaerobic membrane bioreactor (AnMBR) was started up and operated under organic load oscillations in order to determine its feasibility for winery wastewater treatment. The stable operation was assured by keeping a ratio between intermediate alkalinity and total alkalinity below 0.4 and the specific organic loading rate below the methanogenic activity (0.3 kgCOD kgMLSS−1 d−1) achieving a 96.7 ± 2.7% chemical oxygen demand (COD) removal efficiency. The maximum organic loading rate (OLR) achieved was 3.4 kgCOD m−3 d−1. The biogas production varied according with the OLR that was on average 0.50 ± 0.17 m3biogas m−3digester d−1 with an 87.1 ± 3.0% of methane. The external membrane module reached a flux of 20.2 ± 8.5 LMH operating at a mixed liquor suspended solids concentration of 4.78 ± 1.9 g L−1 and a crossflow velocity of 0.64 m s−1. The crossflow velocity helped to remove the cake layer attached on the membrane that was the main contribution (>80%) to flux decline. The energy balance of the AnMBR revealed that a sidestream configuration would be a feasible option during vintage season.

A two-stage Partial Nitritation (PN)/Anammox process was carried out at lab-scale conditions to treat reject water from a municipal WWTP. PN was achieved in a granular SBR obtaining an effluent with a NH4(+)-N/NO2(-)-N molar ratio around 1.0. The microbial characterization of this reactor revealed a predominance of Betaproteobacteria, with a member of Nitrosomonas as the main autotrophic ammonium oxidizing bacterium (AOB). Nitrite oxidizing bacteria (NOB) were under the detection limit of 16S rRNA gene pyrosequencing, indicating their effective inhibition. The effluent of the PN reactor was fed to an Anammox SBR where stable operation was achieved with a NH4(+)-N:NO2(-)-N:NO3(-)-N stoichiometry of 1:1.25:0.14. The deviation to the theoretical stoichiometry could be attributed to the presence of heterotrophic biomass in the Anammox reactor (mainly members of Chlorobi and Chloroflexi). Planctomycetes accounted for 7% of the global community, being members of Brocadia (1.4% of the total abundance) the main anaerobic ammonium oxidizer detected.

Biogas upgrading using bottom ash from the incineration of municipal solid waste (MSW) is especially interesting for integrated treatment facilities as it would allow closing a complete loop in MSW management. The aim of this research was to assess the interaction between bottom ash (BA) and biogas from anaerobic digestion with the outlook of upgrading in terms of CO 2 and H 2 S removal, with particle size and bottom ash aging as the main variables. The finest fraction of fresh bottom ash (FBA) presented the best performance, with a sorption capacity of 30–50 kg CO 2 ·t −1 dry FBA. The sorption capacity of weathered bottom ash (WBA) was lower than the recorded for the fresh material. From the results obtained it can be stated that the CO 2 absorption capacity of bottom ash mainly relies in the finest fraction because of its larger surface area and higher CaO content. In what respect to H 2 S sorption capacity, WBA presented a better performance, with values ranging 3.1–5.2 kg H 2 S∙t −1 dry WBA. In addition to the energetic enhancement, the proposed technology also allows an immediate stabilization and reuse of bottom ash as the leaching of heavy metals and metalloids remained under regulatory limits.

The feasibility of coupling anaerobic digestion and struvite precipitation in the same reactor was evaluated to enhance manure anaerobic digestion methane yields through ammonia inhibition mitigation. Five different magnesium sources were tested as a struvite (ammonia sequestration agent) precursor, i.e. MgCl2, Mg(OH)2, two industrial by-products rich in MgO but with different reactivity, and a stabilizing agent. The latter was formulated in advance with the low reactivity industrial by-product and phosphoric acid. The effect of each magnesium source on anaerobic digestion as well as its struvite precipitation capacity was evaluated through biomethane potential tests. Results indicated that all magnesium sources were able to reduce ammonia concentration to different extents. However, the stabilizing agent was the unique magnesium source that did not inhibit the anaerobic digestion process. The avoidance of adding a phosphate source directly into the digester medium and the high newberyite content were the advantages of the stabilizing agent over the other magnesium sources. Finally, a series of experiments indicated that if anaerobic digestion and struvite precipitation are combined in a single reactor, stabilizing agent addition should be carried out through several small additions rather than few large additions.

Some existing anaerobic digesters treating sewage sludge have a non-used capacity. The use of this extra capacity by introducing additional wastes to conduct the co-digestion could enhance biogas production and plant economic feasibility. Fruit wastes from the food industry could be proper co-substrates due to their high biodegradability, but the harvesting seasons require the use of different kind of fruits causing many transitory conditions throughout the year. Two lab-scale continuous anaerobic digesters treating sewage sludge were operated, one as a reference reactor and the other one as a co-digester. The transitory state was evaluated when fruit waste supply was started, when the co-substrate was changed (peach, banana and apple waste) and when fruit waste supply was stopped. In the transition from mono- to co-digestion, volatile fatty acids concentration rose from 0.07 to 1.70 g L−1 due to the OLR increase, but this situation was recovered in only 5 days. The introduction of different kind of fruit wastes resulted in an alteration of alkalinity, without affecting volatile fatty acids concentration, and in an increase of methane production between 110% and 180% depending on the characteristics of the co-substrate. Finally, when co-digestion was stopped, the parameters converged, at different rates, to the values recorded in the reference digester. It could be concluded that the change of one co-substrate by another one of the same type did not lead to system instability.

The pre-treatment of the organic fraction of municipal solid waste is one of the main challenges in mechanical–biological treatment plants equipped with anaerobic digesters. The present study shows the successful revamping of the Sant Adria del Besos plant pre-treatment line, where a prototypal optical sorter, a wet-crusher, and a hydrocyclone–decanter substituted the previous pulper and decanter system. The prototypal optical sorter, which uses near-infrared spectroscopy, was able to enhance the organic matter content from 40% to 60% in a more efficient and less energy demanding way than conventional systems. The new configuration not only improved digester feedstock and performance but also led to a significant reduction of the treatment and maintenance costs. The high methane yield of the digester, between 480 and 580 CH 4 m 3 t −1 VS, together with an energy efficiency index of 2.2 kW h produced /kW h consumed , demonstrated the viability of this novel configuration with respect to conventional ones. Thus, optical sorting arises as a new alternative for mechanical–biological treatment plants that needs to be constructed or revamped.

Struvite precipitation and pig manure anaerobic digestion were coupled in the same reactor in order to mitigate the inhibitory effect of free ammonia and avoid precipitator costs. The stabilizing agent used to facilitate struvite precipitation was formulated with low-grade magnesium oxide by-product; an approach that would notably reduce struvite processing costs. The interaction between pig manure and stabilizing agent was analyzed in batch experiments, on a wide range of stabilizing agent additions from 5 to 100 kg m−3. The monitoring of the pH and ammonia removal during 24 h showed the high capacity of the stabilizing agent to remove ammonia; removal efficiencies above 80% were obtained from 40 kg m−3. However, a long-term anaerobic digester operation was required to assess the feasibility of the process and to ensure that the stabilizing agent does not introduce any harmful compound for the anaerobic biomass. In this vein, the addition of 5 and 30 kg m−3 of the stabilizing agent in a pig manure continuous digester resulted in a 25% (0.17 m3 kg−1) and a 40% (0.19 m3 kg−1) increase in methane production per mass of volatile solid, respectively, when compared with the reference digester (0.13 m3 kg−1). Moreover, the stability of the process during four hydraulic retention times guarantees that the stabilizing agent did not exert a negative effect on the consortium of microorganisms. Finally, scanning electron microscopy and X-ray diffraction analysis confirmed the presence of struvite as well as two precipitation mechanisms, struvite precipitation on the stabilizing agent surface and in the bulk solution.

Anaerobic digestion is a commercial reality for several kinds of waste. Nonetheless, anaerobic digestion of single substrates presents some drawbacks linked to substrate characteristics. Anaerobic co-digestion, the simultaneous digestion of two or more substrates, is a feasible option to overcome the drawbacks of mono-digestion and to improve plants economic feasibility. At present, since 50% of the publication has been published in the last two years, anaerobic co-digestion can be considered the most relevant topic within anaerobic digestion research. The aim of this paper is to present a review of the achievements and perspectives of anaerobic co-digestion within the period 2010-2013, which represents a continuation of the previous review made by the authors [3]. In the present review, the publications have been classified as for the main substrate, i.e., animal manures, sewage sludge and biowaste. Animal manures stand as the most reported substrate, agro-industrial waste and the organic fraction of the municipal solid waste being the most reported co-substrate. Special emphasis has been made to the effect of the co-digestion over digestate quality, since land application seems to be the best option for digestate recycling. Traditionally, anaerobic co-digestion between sewage sludge and the organic fraction of the municipal solid waste has been the most reported co-digestion mixture. However, between 2010 and 2013 the publications dealing with fats, oils and greases and algae as sludge co-substrate have increased. This is because both co-substrates can be obtained at the same wastewater treatment plant. In contrast, biowaste as a main substrate has not been as studied as manures or sewage sludge. Finally, three interdisciplinary sections have been written for addressing novelty aspects in anaerobic co-digestion, i.e., pre-treatments, microbial dynamics and modeling. However, much effort needs to be done in these later aspects to better understand and predict anaerobic co-digestion.

En esta investigación se realizó la digestión anaeróbica de residuos de alimentos preparados (RAP) en un reactor anaeróbico de mezcla completa (RAMC) de 3,5 L, con flujo discontinuo bajo condiciones mesofílicas (CM) a 37 ºC y termofílicas (CT) a 55 °C. El objetivo fue evaluar la biodegradabilidad de RAP con distintos lodos biológicos y tiempos de retención hidráulicos (TRH). Antes de la degradación, se realizó un ensayo de biodegradabilidad con diferentes proporciones de carbohidratos (C) frutas y vegetales (FV) y proteínas (P) en el que se obtuvo una mayor producción de biogás al trabajar con la proporción C:FV:P 24:71:5 en CM y CT. La misma se utilizó en el RAMC con TRH de 30, 25 y 15 d en CM y CT. Para la CM se obtuvieron remociones en ST de 59 % y 67 %, SV de 76 % y 81 % para los TRH de 30 y 25 d, con producciones de metano de 0,64 L/gSVremovido y 0,31 L/gSVremovido. Durante el TRH de 15 d se registró una inestabilidad debido a concentraciones de CO2 del 60 %. Durante la CT (TRH de 30 d) se removieron 67 % de ST, 80 % de SV y se registraron producciones de metano de 0,58 L/gSVremovido. Cuando se implementó el TRH de 25 d se incrementaron los ácidos grasos volátiles hasta 2916 mg/L y se detuvo el tratamiento. Las mejores remociones y producción de biogás se generaron durante el TRH de 30 d en CM con el lodo EFLUENTE CACHAZA.

Temperature has a great influence over the anaerobic digestion process, affecting both biochemical and physical reactions. The present study analyses the capability of the anaerobic biomass to stand different temperature operational conditions. Initially anaerobic biodegradability tests were done to evaluate the capability of mesophilic and thermophilic anaerobic biomass to adapt to different temperature conditions. Afterwards, the impact of the temperature fluctuation from mesophilic to thermophilic conditions was evaluated in a continuous stirred digester. Finally, the recovery of a volatile fatty acid inhibited digester was analysed. The results of the anaerobic biodegradability tests indicated that thermophilic biomass present in the mesophilic inoculum was developed when the inoculum was subjected to 55 °C; meanwhile, when the thermophilic biomass was subjected to 37 °C some thermophilic bacteria remained active. When a continuous stirred digester was subjected to daily temperature fluctuations between 37 °C and 45 °C during 1, 3, 6 and 12 h and between 37 °C and 55 °C during 1 h, neither the organic matter removal nor the biogas yields improved or decreased. In contrast, when the exposure time at 55 °C was 2 and 24 h, the anaerobic digestion process was adversely affected; the volatile fatty acid concentration increased and the process yields dropped. In both cases, methanogens were the most sensitive microorganisms to temperature fluctuation. The recovery stage took 22 days and it was developed in two periods: starving and feed re-establishing. Propionate was found to be a key indicator to monitor the digester recovery.

An anaerobic membrane bioreactor and aerobic granulation technologies were tested at laboratory scale to treat winery wastewater, which is characterised by a high and variable biodegradable organic load. Both technologies have already been tested for alcohol fermentation wastewaters, but there is a lack of data relating to their application to winery wastewater treatment. The anaerobic membrane bioreactor, with an external microfiltration module, was started up for 230 days, achieving a biogas production of up to 0.35 L CH 4 L −1 d −1 when 1.5 kg COD m −3 d −1 was applied. Average flux was 10.5 L m −2 h −1 (LMH), obtaining a treated effluent free of suspended solids and a chemical oxygen demand (COD) concentration lower than 100 mg COD L −1 . In contrast, the aerobic granular sequencing batch reactor coped with 15 kg COD m −3 d −1 , but effluent quality was slightly worse. Aerobic granulation was identified as a suitable technique to treat this kind of wastewater due to excellent settleability, high biomass retention and a good ability to handle high organic loads and seasonal fluctuations. However, energy generation from anaerobic digestion plays an important role, favouring anaerobic membrane bioreactor application, although it was observed to be sensitive to sudden load fluctuations, which led to a thorough pH control and alkali addition.

Two Granular Sequencing Batch Reactors were operated to perform partial nitrification of sludge reject water at different temperatures, from 25-41 degrees C. Every temperature was fixed for about a month in order to evaluate the nitritation rate, morphological features of aggregates and bacterial populations. The optimum temperature was found between 33 and 37 degrees C in terms of nitritation rate. Morphological features of granules did not show significant changes with temperature in the range between 28 and 37 degrees C; Feret diameter remained at 5.8 +/- 0.7mm and roundness was 0.76 +/- 0.02. Lower temperatures promoted the appearance of filamentous bacteria, leading to an increase of the sludge volume index (SVI) and a consequent reduction of biomass concentration. When the temperature was increased to 39 degrees C, more than the 80% of aggregates showed a diameter higher than 6mm but density decreased from 28 to 19 g VSS L(-1), resulting in an increase of the SVI from 33 to 80 mL g(-1). The establishment of 41 degrees C caused a rapid destabilization of the system and nitritation activity disappeared. Bacterial populations did not experience significant changes during the experimental period and Nitrosomonas was the dominant species at all the temperatures assayed.

Winery wastewater is characterized by a high variability of its organic content, and flexible systems such as granular sequencing batch reactors (GSBRs) are required for its treatment. Despite the high load capacity inherent to these systems, the operation with a predetermined cycle length is not suitable to withstand high COD fluctuations. In this study, a control system able to automatically control the HRT depending on the actual organic load has been designed. The relation between ORP slope and DO profiles with the COD consumption together with a proper periodic starvation time are the basis of the control strategy. GSBR operation using the designed system led to an organic matter removal up to 95% for a wide range of organic matter concentrations (1–8 g/L), optimizing cycle duration and avoiding unnecessary aeration costs. The reactor showed a high stability and a rapid response to sudden changes in the COD influent concentration. Biomass settleability remained almost constant around 95 m/h and aggregates showed a Feret diameter of 4–6 mm and sphericity up to 0.8.

BACKGROUND: Partial nitrification–Anammox is a combined promising advanced biological process for the removal of nitrogen from wastewater, which allows important savings in energy consumption, sludge production, and organic carbon. Granular biomass appears to be an interesting alternative to conventional activated sludge, mainly because of its better settling properties. This study deals with the experimental results of a comparison between a conventional and a granular sequencing batch reactor (SBR) for the partial nitrification of reject water. RESULTS: After some days of operation, 30 days in the conventional SBR (system A) and 100 days in the granular SBR (system B), partial nitrification was achieved. Granular sludge showed much better settling properties than suspended biomass, with values of sludge volumetric index (SVI10) of 130 mL g−1 in system A and 38 mL g−1 in system B. Consequently, the solids concentration within the granular reactor was three times higher than for the conventional system while the concentration of solids in the effluent was 10 times higher in the conventional SBR. Morphology, microstructure and microbial populations in both systems were also studied. CONCLUSION: A partial nitrification process was successfully achieved in both systems, obtaining an effluent with a NO2−-N/NH4+-N ratio near 1, suitable for a following Anammox process. Granular biomass, mostly formed by round particles, showed better settling properties, leading to better sludge–effluent separation as well as higher biomass retention in the reactor. The granulation process does not affect bacterial populations, since they were the same in both systems. Copyright © 2011 Society of Chemical Industry

In this study, two Membrane Biological Reactors (MBR) with submerged flat membranes, one at lab-scale conditions and the other at pilot-plant conditions, were operated at environmental temperature to treat an industrial wastewater characterised by low phenol concentrations (8–16 mg L −1 ) and high salinity (∼150–160 mS cm −1 ). During the operation of both reactors, the phenol loading rate was progressively increased and less than 1 mg phenol L −1 was detected even at very low HRTs (0.5–0.7 days). Membrane fouling was minimized by the cross flow aeration rate inside the MBRs and by intermittent permeation. Microbial community analysis of both reactors revealed that members of the genera Halomonas and Marinobacter ( gammaproteobacteria ) were major components. Growth-linked phenol degradation by pure cultures of Marinobacter isolates demonstrated that this bacterium played a major role in the removal of phenol from the bioreactors.

SBR technology is used to treat the supernatant from mesophilic anaerobic digestion of piggery wastewater. The novelty of the treatment consists in the use of a final coagulation/flocculation step inside the SBR cycle to reach the legal COD effluent standard. The pH changes introduced by the use of FeCl3 do not affect the nitrifying activity. The SBR treatment includes a strategy to the control of oxygen supply and ammonia concentrations inside the digester to favor the biological nitrogen removal over nitrite, which makes the process more economical. The influence of several of these parameters on the AOB biomass activity is studied in this paper.

In this study, a comparison of the biodegradation of adsorbed organic halogen compounds (AOX) and polychlorinated biphenyls (PCB) in thermophilic and mesophilic anaerobic digestion (seeded with waste activated sludge) at different hydraulic retention times (HRT 18, 22 and 26 days in the mesophilic digester and 8, 12, 18, 22 and 26 days in the thermophilic digester) was performed. Results obtained in this work showed an enhancement of both PCB and AOX biodegradation under thermophilic conditions. The total PCB removal efficiency was in the range of 59.4–83.5% under thermophilic conditions and 33.0–58.0% under mesophilic conditions. HRT played an important role in the digester performance since high working HRTs implied more reduction of the total PCB amount in the sludge. The total PCB content in the treated sludge under thermophilic conditions lied below the cut-off limit proposed in the 3rd draft of Directive presented to the European Commission [CEC, Working Document on Sludge (3rd Draft), Commission of the European Communities Directorate-General Environment, ENV.E.3/LM, Brussels, 27 April 2000]. Besides, a bioaccumulation of lightly chlorinated PCBs was detected in the mesophilic digester, which is in concordance with the theory that the PCBs are anaerobically biodegraded by means of a reductive dechlorination mechanism. On the other hand, the AOX removal efficiency was in the range of 40.4–50.3% for thermophilic conditions and 30.2–43.2% for mesophilic conditions. The AOX content in the treated sludge of both thermophilic and mesophilic digesters did not exceed the cut-off limit proposed in the 3rd draft [CEC, Working Document on Sludge (3rd Draft), Commission of the European Communities Directorate-General Environment, ENV.E.3/LM, Brussels, 27 April 2000]. Moreover, high HRTs promoted an improvement of the AOX removal capacity of the anaerobic digestion.

Municipal solid waste fermentation, simulating landfill conditions, was studied using five test cells operated at different temperatures. The digestion was carried out under enhanced conditions. Leachate recycle was set up in all the test cells. Depending on temperature, the digestion was complete (more than 90% biodegradable matter converted) in a period of 25 to 57 days. Optimum operating temperatures were in the range of 34 to 38°C. Two kinetic models were fitted to experimental data. Their kinetic constants were related to temperature using an Arrhenius expression. Landfill life operated under the studied conditions could be reduced to less than 2 years.

A comparative study to produce the correct influent for Anammox process from anaerobic sludge reject water (700–800 mg NH4+-N L−1) was considered here. The influent for the Anammox process must be composed of NH4+-N and NO2−-N in a ratio 1:1 and therefore only a partial nitrification of ammonium to nitrite is required. The modifications of parameters (temperature, ammonium concentration, pH and solid retention time) allows to achieve this partial nitrification with a final effluent only composed by NH4+-N and NO2−-N at the right stoichiometric ratio. The equal ratio of HCO3−/NH4+ in reject water results in a natural pH decrease when approximately 50% of NH4+ is oxidised. A Sequencing batch reactor (SBR) and a chemostat type of reactor (single-reactor high activity ammonia removal over nitrite (SHARON) process) were studied to obtain the required Anammox influent. At steady state conditions, both systems had a specific conversion rate around 40 mg NH4+-N g−1 volatile suspended solids (VSS) h−1, but in terms of absolute nitrogen removal the SBR conversion was 1.1 kg N day−1 m−3, whereas in the SHARON chemostat was 0.35 kg N day−1 m−3 due to the different hydraulic retention time (HRT) used. Both systems are compared from operational (including starvation experiments) and kinetic point of view and their advantages/disadvantages are discussed.

In many anaerobic digestion processes for the treatment of the sludge produced in wastewater treatment plants, the hydrolysis of the organic matter has been identified as the rate limiting step. This study is focused on the effect of ultrasonic pretreatment of raw sewage sludge before being fed to the mesophilic and the thermophilic anaerobic digestion. From particle size reduction, COD disintegration degree and biodegradability test, 11,000 kJ/kg TS was estimated as the optimal specific energy in ultrasonic pretreatment. Moreover, the use of pretreated sludge improved significantly the COD removal efficiency and biogas production in lab-scale anaerobic digesters when compared with the performance without pretreatment, specially under mesophilic conditions. During ultrasonic pretreatment, the diffusion of polycyclic aromatic hydrocarbons (PAH) compounds to the aqueous phase was stated by a reduction in the pretreated sludge micropollutants content. With sonication, naphthalene was better removed than without this pretreatment, particularly in the mesophilic digester. However, pyrene removal remained at same efficiency level with and without ultrasonic pretreatment.

In this study, a lab-scale sequencing batch reactor (SBR) has been tested to remove chemical oxygen demand (COD) and NH4+-N from the supernatant of anaerobic digestion of the organic fraction of municipal solid waste. This supernatant was characterized by a high ammonium concentration (1.1 g NH4+-N L−1) and an important content of slowly biodegradable and/or recalcitrant COD (4.8 g total COD L−1). Optimum SBR operating sequence was reached when working with 3 cycles per day, 30 °C, SRT 12 days and HRT 3 days. During the time sequence, two aerobic/anoxic steps were performed to avoid alkalinity restrictions. Oxygen supply and working pH range were controlled to promote the nitrification over nitrite. Under steady state conditions, COD and nitrogen removal efficiencies of more than 65% and 98%, respectively, were achieved. A closed intermittent-flow respirometer was used to characterize and model the SBR performance. The activated sludge model ASM1 was modified to describe the biological nitrogen removal over nitrite, including the inhibition of nitrification by unionized ammonia and nitrous acid concentrations, the pH dependency of both autotrophic and heterotrophic biomass, pH calculation and the oxygen supply and stripping of CO2 and NH3. Once calibrated by respirometry, the proposed model showed very good agreement between experimental and simulated data. Copyright © 2007 Society of Chemical Industry

Controlled struvite formation has been attracting increasing attention as a near mature technology to recover nutrients from wastewater. However, struvite feasibility is generally limited by the high cost of chemical reagents. With the aim to understand and control reagent use efficiency, experiments and equilibrium model simulations examined inorganic nitrogen (TAN) removal from pig manure via struvite with added magnesium and phosphate reagents. Four industrial magnesium oxide (MgO), a commercial product and three by-products from magnesite calcination, were tested with phosphate added as a highly soluble potassium salt. TAN removal extents with the MgOs ranged from 47 to 72%, with the highest grade MgO providing the greatest extent of TAN removal. However, model analysis showed that all the MgO reagents were poorly soluble (only about 40% of added magnesium actually dissolved). The model results suggested that this poor dissolution was due to kinetic limitations, not solubility constraints. A further set of additional reagents (termed stabilization agents) were prepared by pre-treating the MgO reagents with phosphoric acid, and were tested separately as a source of both magnesium and phosphate. Results showed that acid pre-treatment of moderate to highly reactive MgOs (soft to medium-burnt) primarily formed bobierrite as the stabilizing agent, whereas the pre-treatment of very low reactivity MgOs (dead-burnt) mostly formed newberyite. The newberyite stabilizing agents achieved very high TAN removal extents of about 80%, which is significant, considering that these were formed from dead-burnt/low-grade MgOs. However, the bobierrite stabilizing agents achieved a substantially lower TAN removal extent than their medium-to-high reactivity precursor MgOs. Again, model analysis showed that the bobierrite stabilizing agents were poorly soluble, due to kinetic limitations, not solubility constraints. In contrast, the model suggested that the newberyite stabilizing agents almost completely dissolved to very effectively form struvite. A mechanism was proposed by which conditions near a dissolving reagent particle surface causes unwanted struvite nucleation onto and overgrowth of the reagent particle, inhibiting further dissolution and markedly reducing reagent efficiency. The findings of the study could have implications for reagent efficiency with struvite in general, even when using other solid reagents such as magnesium hydroxide or other MgOs.

Different internal flows from a wastewater treatment plant were investigated for their usefulness as internal organic carbon source in the treatment of reject water with high ammonium loading, such...

Experiments in a lab-scale SBR were conducted to demonstrate the feasibility of using an internal carbon source (non-digested pig manure) for biological nitrogen and phosphorus removal in digested piggery wastewater. The internal C-source used for denitrification had similar effects to acetate. 99.8% of nitrogen and 97.8% of phosphate were removed in the SBR, from an initial content in the feed of 900 mg/l ammonia and 90 mg/l phosphate.

The results of the start-up of two digesters in mesophilic and thermophilic conditions, together with its steady results at several organic loading rates are described. A kinetic study is also carried out which allows one to estimate the ultimate methane production, together with the first-order kinetic constant. Operation at thermophilic temperature yields better results as it allows a more loaded reactor and the methane production is slightly higher.

Municipal solid waste is broadly generated in everyday activities and its treatment is a global challenge. Total solids (TS) and volatile solids (VS) are typical control parameters measured in biological treatments. In this study, the TS and VS were determined using the standard methods, as well as introducing some variants: (i) the drying temperature for the TS assays was 105°C, 70°C and 50°C and (ii) the VS were determined using different heating ramps from room tempature to 550°C. TS could be determined at either 105°C or 70°C, but oven residence time was tripled at 70°C, increasing from 48 to 144 h. The VS could be determined by smouldering the sample (where the sample is burnt without a flame), which avoids the release of fumes and odours in the laboratory. However, smouldering can generate undesired pyrolysis products as a consequence of carbonization, which leads to VS being underestimated. Carbonization can be avoided using slow heating ramps to prevent the oxygen limitation. Furthermore, crushing the sample cores decreased the time to reach constant weight and decreased the potential to underestimate VS.

A pilot scale mesophilic anaerobic acidogenic fermenter was fed with mixtures of vegetables and fruits shredded by a hammer mill and mixed in a stock tank, in order to produce a liquid phase suitable as RBCOD source in denitrification and EBPR processes. Different operative conditions were studied working with a HRT in the range 1–:12 days. The effluent coming from the fermenter was screw pressed, and the solid phase was recycled adopting different ratios to the fermenter, in order to define its effect on the final liquid phase composition. The variations of the VFA, lactate, methyl and ethyl alcohol concentrations, TCOD, SCOD and pH during more than one year were analysed and discussed both with reference to the fresh feed, and to the content of the fermenter. It was found that almost all the organic matter in the liquid phase inside the fermenter was represented by VFA (mainly acetate), lactate (in particular) and methyl and ethyl alcohols when HRT was longer than 6 days.

Monod, zero and first order kinetic models have been used to represent the biodegradation of domestic sewage in a combined treatment system. This consisted of one UASB reactor, 750 l working volume, followed by two serial RBC reactors, each having a working volume of 55 l. First order model is the only one that adequately represents biodegradation in both aerobic and anaerobic parts of the system. In anaerobic unit the first order kinetic constant is 0.31 h−1 in summer temperatures (around 19°C) and 0.20 h−1 in winter temperatures (around 12.5°C). In aerobic units the values are higher (around 2.0 h−1), using, in all these cases, BOD units for substrate concentration

Experiments in a continuous UCT-bench scale plant using synthetic wastewater, have been carried out to demonstrate the feasibility of using the biodegradable organic fraction of municipal solid waste (BOF-MSW) as an easily biodegradable C source for nitrogen and phosphorous removal. Yields obtained are quite good and the additional load introduced by the use of the fermentation products of the BOF-MSW can be considered negligible. In addition, the basic mass balances carried out, show that the amount of easily biodegradable C-source needed to treat the wastewater produced by a given unit of persons can be obtained from the amount of BOF-MSW generated by this unit.

This article presents the results for the treatment of raw sewage in an anaerobic-aerobic system at pilot plant level. Experiments were carried out in summer and winter, with different loads and temperatures (19°C and 12°C) to evaluate the potential of this system to be used in touristic-coastal areas. Results show that removal efficiencies are similar in both seasons because temperature balances the excess of the summer load. However, the overall yield coefficient for sludge production is reduced around, compared with reported values for conventional activated sludge processes. Nitrification takes place in Rotating Biological Contactors with good yields at hydraulic loads below 0.13 m 3 /m 2 ·d.

A comparative kinetic study of thermophilic anaerobic digestion under semi‐dry conditions was carried out, using the step‐diffusional model and the first‐order model. Data for the analysis came from the operation of a 3 m3 digester, operated at relatively overloaded conditions, using the fresh and precomposted, mechanically selected, organic fraction of municipal solid waste as substrate. A comparison was also made with other published kinetic data using these models. The results show the coherence of the step‐diffusional model, explaining some of the basic mechanisms of substrate degradation. Values of the kinetic constants involved are within the range of those previously obtained by other researchers.

This report summarises some of the discussions at the recently held international conference on biological waste treatment (Biowaste '95, 21–24 May). The conference took place in Aalborg, in the north of the Jutland peninsula (Denmark). Denmark is one of the countries in which biotreatments for household waste are more developed.

Possibilities of co-digestion of sewage sludge (SS) with other organic wastes are examined in this paper. Anaerobic co-digestion of macroalgae of the Venice lagoon (A) with SS, in wastewater treatment plants is studied in detail. This approach can contribute to the solution of the final disposal of the 50 000 m 3 of macrophytes harvested each season. These are mainly Ulva rigida and Gracilaria confervoides . In the experiments A and SS were mixed at different ratios (20–40% algae, TS basis) and fed to mesophilic (37°C) and thermophilic (55°C) digesters which operated at 11- to 15-day hydraulic retention times and 1.7–4.4 kgTVS/m 3 /day organic loading rates. It was concluded that the mesophilic co-digestion process is applicable with potentialities of around 30% of the present SS flow-rate. Thermophilic digestion is not possible, because of the inhibition of methanogens probably due to the activity of sulphate-reducers.

A kinetic model is developed to simulate the biodegradation of the organic fraction of municipal solid waste in a high-solids complete-mix continuous-flow anaerobic digestion process. A mass balance correction factor (f) is used to account for (i) the water that is incorporated into the production of biogas, and (ii) the water vapor present in the biogas. With the inclusion of the factor f, expressions are developed to determine the substrate removal efficiency based on first-order and Monod kinetic models. Simulations using first-order and Monod kinetic models are carried out to illustrate the importance of the f factor on the substrate mass removal and microorganism concentration in the high-solids anaerobic digestion process. The effect of the value of f on kinetic constants, and the error involved in considering the high-solids process as a low-solids process are discussed. Finally, experimental results from a pilot-scale high-solids complete-mix batch-fed anaerobic digestion process are used to estimate the first-order rate constant for the biodegradable fraction of municipal solid waste.

Municipal solid waste production has increased over the past years. The European Union Waste Framework Directive establishes different hierarchy levels for its management, where prevention and recycling appear as the most convenient management strategies for the organic fraction of MSW. This fraction can be either recycled by composting or by anaerobic digestion followed or not by composting. Anaerobic digestion has the advantage of producing energy instead of consuming it and it is widely described in this chapter. First, a revision of the state of the art of its implementation at full scale is carried out. Then, a detailed description of the environmental and operational factors affecting the process performance is described. Finally, methods for improving the yields of the anaerobic digestion process are considered, which includes biological and physical pre-treatment technologies together with the basis and possibilities of anaerobic co-digestion.

Results of a two-phase mesophilic (35°C) anaerobic digestion treatment of fruit and vegetable wastes, carried out at laboratory scale, are presented and discussed. They are contrasted with other results obtained with a similar waste, but digested in a one-phase system. The yields are lower in this simple two-phase system, because of the higher organic loading rate. It is concluded that this two-phase system does not seem appropriate to treat these wastes unless it is equipped with some type of control of the hydrolytic step. A one-phase system is simpler and can yield at least the same yields.

A mathematical model to represent the two-phase anaerobic digestion of a mixture of fruit and vegetable wastes is presented. The hydrolytic phase is represented by a first order model and the methanization phase by a Monod one. The first order constant and the yield coefficient are taken from other fitting studies performed with the same substrate. The values of μ max and the saturation constant are fitted from the results of a discontinuous two-phase digestion. μ max is fitted as a pH-dependent constant. The model is validated with the results obtained by a continuous two-phase digestion carried out with the same substrate

Volatile fatty acids (VFA) production by anaerobic fermentation of municipal solid wastes was studied at pilot-plant level. A plug-flow reactor (80 l total volume), with and without concentrated sludge recirculation, was employed to digest an organic fraction of the municipal solid waste (OFMSW)-based substrate. This was composed of OFMSW mechanically-selected in a full-scale plant with OFMSW collected from fruit and vegetable markets and mixed in a way to obtain a feed mixture with a proportion of total volatile solids (TVS)/total solids (TS) of 60%. The fermentation process was studied at different retention times (RT) (from 8 h to 8 days) and at very high organic loading rates (OLR) (from 22·4 to 85·2 kg VS/m3 d) at mesophilic temperature (37 ± 2°C). Results showed an increase in VFA production at higher retention times (from 11·8 g/l at 2 days to 23·1 g/l at 6 days RT) when no recirculation was used. VFA production was also high when part of the digested sludge was recirculated (19·5 g/l at 2 days of hydraulic RT and 8 days of solid RT). Acetic and butyric were the acids produced in the largest quantities.

The acidogenic fermentation of the organic fraction of municipal solid wastes (OFMSW) was studied at thermophilic temperature conditions (55 ± 2°C) in a 80 l plug-flow reactor. In particular, volatile fatty acids (VFA) production and individual acid distribution was monitored at different hydraulic retention times (HRT) (from 2 to 6 days HRT). The feed mixture was composed of OFMSW mechanically selected in a full-scale plant and OFMSW collected from a fruit and vegetables market, with TS content around 25% and a ratio of total volatile solids (TVS)/total solids (TS) of 60%. When inoculum was added to the feed mixture, the thermophilic acidogenic fermentation of the OFMSW became a feasible process for obtaining VFA. The maximum VFA concentration obtained was 19·6 g/l at 6 days retention time, although VFA production at short hydraulic retention time (8–16 h) was already important. VFA production was distributed in all individual acids, but mainly acetic and butyric acids.

The production of volatile fatty acids (VFA) by anaerobic fermentation of municipal solid wastes was studied at pilot-plant level. A plug-flow reactor (80 1 total volume) without solid or liquid recirculation was employed to digest a mixture of two types of organic fraction of the municipal solid waste (OFMSW): OFMSW mechanically selected and OFMSW coming from a market of fruit and vegetables. The acidogenic process was studied at different retention times (between 2 and 6 days) in the mesophilic (37° ± 2°C) range of temperature. The VFA concentration obtained in the first valve of the tubular reactor ranged from 9.1 to 13.4 g 1−1 and in the outlet sludge oscillated between 11.8 and 23.1 g 1−1, increasing when increasing retention time from 2 to 6 days. A mathematical model of a continuous steady state plug-flow reactor proposed in a previous paper was used to represent the VFA production obtained experimentally. The results obtained when fitting the mathematical model with the experimental results were not sufficiently good. Some modifications of the initial model were considered. The best fitting of the results was obtained when the inhibition effect of the fermentation product on microorganisms growth was taken into account. The errors of this fitting, calculated by least squares, can be considered optimal for retention times between 4 and 6 days, while at shorter retention times the inhibition effect of pH and product makes the volatile acid production lower than the values obtained experimentally.

Granular sludge formation was performed in a laboratory scale Sequencing Batch Reactor (SBR) fed with supernatant of anaerobic digestion of sewage sludge. This effluent was concentrated progressively in order to enhance biomass capacity without inhibiting it. During the first part of the study, ammonium nitrogen was converted to nitrate, so conventional nitrification took place. When a nitrogen load of 0.8 g N L−1 d−1 was treated, the effluent concentration of nitrite started to increase while the nitrate concentration decreased until it disappeared. So, partial nitrification was achieved. At the end of this study, a nitrogen load of 1.1 g N L−1 d−1 was treated obtaining an effluent with 50% ammonium and 50% nitrite. The volatile suspended solids concentration in the reactor reached 10 g VSS L−1. At this point the granule morphology was quite round and no filamentous bacteria was observed. The Feret's diameter was in the range between 1 and 6 mm with an average value of 4.5 mm. Roundness value was all the time higher than 0.7. Granule density increased during the experimental period, obtaining a final value of 7.0 g L−1.

The interest on anaerobic digestion (AD) of biowaste is increasing because of the successful implementation of separate collection of the fractions of municipal solid waste. This allows for the treatment of a relatively ‘clean’ stream with high-energy content: up to 200 m 3 biogas per tonne of biowaste collected through the door-by-door system. On the other hand, several European experiences showed how the treatment of the mechanically selected organic fraction of municipal solid waste gave a number of problems and low-energy yields (typically, lower than 100 m 3 biogas per tonne). Because of its peculiarities, AD can be a key technology in the field of the so-called ‘green economy’ for the management of the organic wastes (biowaste, sludge, livestock effluents, agro-waste, etc.) in order to recover energy and organic matter. When designing AD plants, expected biogas yield is a key input data for designers and processes; therefore, the theoretical and experimental basics for calculating the biomethane potential of organic substrates are addressed. Finally, this article reviews the main findings of past years in the field of full-scale applications for biowaste treatment and emphaszes successes and problems encountered in the application of this technology.