Organic waste biorefineries: Looking towards implementation.

The concept of biorefinery expands the possibilities to extract value from organic matter in form of either bespoke crops or organic waste. The viability of biorefinery schemes depends on the recovery of higher-value chemicals with potential for a wide distribution and an untapped marketability. The feasibility of biorefining organic waste is enhanced by the fact that the biorefinery will typically receive a waste management fee for accepting organic waste. The development and implementation of waste biorefinery concepts can open up a wide array of possibilities to shift waste management towards higher sustainability. However, barriers encompassing environmental, technical, economic, logistic, social and legislative aspects need to be overcome. For instance, waste biorefineries are likely to be complex systems due to the variability, heterogeneity and low purity of waste materials as opposed to dedicated biomasses. This article discusses the drivers that can make the biorefinery concept applicable to waste management and the possibilities for its development to full scale. Technological, strategic and market constraints affect the successful implementations of these systems. Fluctuations in waste characteristics, the level of contamination in the organic waste fraction, the proximity of the organic waste resource, the markets for the biorefinery products, the potential for integration with other industrial processes and disposal of final residues are all critical aspects requiring detailed analysis. Furthermore, interventions from policy makers are necessary to foster sustainable bio-based solutions for waste management.

Exploring dynamic membrane as an alternative for conventional membrane for the treatment of old landfill leachate.

This study compares the performance of a lab-scale pre-anoxic and post-aerobic submerged dynamic membrane bioreactor (DMBR) with similar studies on conventional membrane bioreactors (MBRs) for the treatment of old landfill leachate (LFL) while presenting a strategy to achieve stable DMBR operation. The results suggested that DMBR performed similar, or in some cases, better than MBRs. Like conventional MBRs treating LFL, DMBR can also accommodate large variations in operating parameters including influent feed composition and loading rates and thus, it can guarantee long term stable bioreactor operation (total nitrogen removal up to 98%) with acceptable effluent quality (Turbidity < 10 NTU). The results also demonstrated that gradual increment in influent LFL concentration was found to be effective for a stable DMBR operation however, it significantly deteriorated dynamic membrane (DM) filtration performance (p < 10E-7), resulting in higher fouling rate and deteriorated effluent quality. Nonetheless, poor DM performance and higher fouling rate were effectively controlled by using lower mesh porosity (52 µm instead of 200 µm) and increase in DM effective filtration area.

Microalgae-bacteria gas exchange in wastewater: how mixotrophy may reduce the oxygen supply for bacteria.

Microalgae-bacteria consortia application to wastewater treatment is considered as a potential and cheap strategy towards a self-sustaining oxygen-carbon dioxide gas exchange. However, microalgae can also carry out mixotrophy, thus reducing the net oxygen production, due to consumption of organic substrates. In this work, respirometric tests were used to quantify the oxygen reduction in the presence of biodegradable COD (chemical oxygen demand), which resulted up to 70%, depending on the biodegradability of the carbon substrate. The implication of mixotrophic metabolism on nutrient removal in urban wastewater was also measured by co-cultivating C. protothecoides with bacteria from activated sludge. To better understand the contribution of different populations, ad hoc experiments under controlled conditions were designed to quantify the nutrient consumption of bacteria and microalgae. Microalgae and bacteria were cultivated together and separately, with and without external bubbling, so to better ascertain the specific role of gas production and nutrient removal. Results showed that microalgae can remove up to 100 and 85% of P and N respectively, but the contribution on COD consumption may affect the net O2 supply to heterotrophic bacteria. However, a mutual COD consumption by microalgae and bacteria was proved by both experimental growth curves and mass balance application, based on stoichiometry experimentally adjusted.

Application of anaerobic dynamic membrane bioreactor (AnDMBR) for the successful enrichment of Anammox bacteria using mixed anaerobic and aerobic seed sludge.

This study investigated a novel bioreactor configuration coupled with a side-stream dynamic membrane (DM) for Anammox enrichment as an alternative for conventional membrane. Bioreactor was fed with synthetic feed and seeded with a mix of anaerobic and aerobic sludge. In situ mechanical cleaning was employed for DM cleaning. DM development and performance was analysed over two polyamide-nylon meshes (200 and 52 µm). Solid-liquid separation of 52 µm mesh outperformed 200 µm with an average effluent turbidity of 2.4 ±â€¯0.1 NTU. The system was operated at a maximum nitrogen loading rate of 696 mg-N L-1 d-1 and achieved a maximum nitrogen removal rate of 611.6 mg-N L-1 d-1. At steady state, the average ammonium, nitrite and total nitrogen removal efficiencies were 87 ±â€¯0.6%, 98.5 ±â€¯0.15% and 87.5 ±â€¯0.56% respectively. Digital realtime PCRSequence analysis showed that Planctomycetales belonging to ascertained Anammox-specific genera progressively increased their presence in the reactor consistently with its nitrogen removal performance.

Assessment of dynamic membrane filtration for biological treatment of old landfill leachate.

This study investigated the behaviour of dynamic membrane (DM) filtration for the treatment of stabilised landfill leachate in a bench-scale pre-anoxic and aerobic submerged dynamic membrane bioreactor (DMBR). Four meshes with different openings (10, 52, 85 and 200 µm) were tested to support the development of DM. Differences were observed among the meshes in supporting the development of the cake layer constituting the DM. The treatment of landfill leachate had an impact on sludge characteristics resulting in deteriorated filtration performance of the DM. Effluent turbidity was often higher than 100 NTU for larger mesh pore size (85 and 200 µm). Low effluent turbidity was achieved with meshes with 10 and 52 µm (13 ±â€¯2 and 26 ±â€¯4 NTU, respectively) although at membrane fluxes lower than 10 L m- 2 h-1. The bioreactor exhibited a moderate organics removal of 50-60% and an ammonia oxidation between 80 and 90%. Incomplete nitrification was observed due to increased concentrations of free ammonia and free nitrous acid, with nitrite effluent concentrations up to 1062 mgNO2--N L-1. Due to the large presence of refractory organic matter in landfill leachate, denitrification was limited resulting in a total nitrogen removal of approximately 20%.

Effect of filtration flux on the development and operation of a dynamic membrane for anaerobic wastewater treatment.

Dynamic membrane represents a cost effective alternative to conventional membranes by employing fouling as a means of solid-liquid separation. This study evaluated the effects of initial flux on both development rate of dynamic membrane and bioreactor performance during two consecutive experiments. The dynamic membrane was developed over a 200 µm mesh and the reactor was operated under anaerobic conditions. It was found that the effect of an initial higher applied flux on dynamic membrane development was more pronounced than mixed liquor suspended solid concentration inside the bioreactor. The development of the dynamic membrane was therefore positively associated with the applied flux. The rapid development of the dynamic membrane during the second experimental run at high initial fluxes and lower MLSS concentrations also affected the performance of the bioreactor in terms of more efficient COD removal and biogas production. A major shortcoming of applying higher initial applied flux was the formation of a denser and robust dynamic membrane layer that was resistant to applied hydraulic shear to control desired permeability and thus represented an obstacle in maintaining a long term operation with sustainable flux at lower transmembrane pressure (TMP).

Model-based analysis of the effect of different operating conditions on fouling mechanisms in a membrane bioreactor.

This study proposes a model-based evaluation of the effect of different operating conditions with and without pre-denitrification treatment and applying three different solids retention times on the fouling mechanisms involved in membrane bioreactors (MBRs). A total of 11 fouling models obtained from literature were used to fit the transmembrane pressure variations measured in a pilot-scale MBR treating real wastewater for more than 1 year. The results showed that all the models represent reasonable descriptions of the fouling processes in the MBR tested. The model-based analysis confirmed that membrane fouling started by pore blocking (complete blocking model) and by a reduction of the pore diameter (standard blocking) while cake filtration became the dominant fouling mechanism over long-term operation. However, the different fouling mechanisms occurred almost simultaneously making it rather difficult to identify each one. The membrane "history" (i.e. age, lifespan, etc.) seems the most important factor affecting the fouling mechanism more than the applied operating conditions. Nonlinear regression of the most complex models (combined models) evaluated in this study sometimes demonstrated unreliable parameter estimates suggesting that the four basic fouling models (complete, standard, intermediate blocking and cake filtration) contain enough details to represent a reasonable description of the main fouling processes occurring in MBRs.

Modelling wastewater treatment in a submerged anaerobic membrane bioreactor.

Mathematical modelling has been widely applied to membrane bioreactor (MBRs) processes. However, to date, very few studies have reported on the application of the anaerobic digestion model N.1 (ADM1) to anaerobic membrane processes. The aim of this study was to evaluate the applicability of the ADM1 to a submerged anaerobic MBR (SAMBR) treating simulated industrial wastewater composed of cheese whey and sucrose. This study demonstrated that the biological processes involved in SAMBRs can be modelled by using the ADM1. Moreover, the results showed that very few modifications of the parameters describing the ADM1 were required to reasonably fit the experimental data. In particular, adaptation to the specific conditions of the coefficients describing the wastewater characterisation and the reduction of the hydrolysis rate of particulate carbohydrate (khyd,ch) from 0.25 d(-1) (as suggested by the ADM1 for high-rate mesophilic reactors) to 0.13 d(-1) were required to fit the experimental data.

Partial nitrification for nitrogen removal from sanitary landfill leachate.

Biological nitrogen removal using nitrite as a shortcut has recently been proposed for the treatment of high strength landfill leachate. The aim of this study was to assess the application of the SHARON (Single reactor High activity Ammonium Removal Over Nitrite) process for the partial nitrification of leachate generated in old landfills. Particular attention was given to the start-up phase of the process. This study demonstrated that partial nitrification can be obtained when treating raw leachate after biomass acclimation. Only a fraction (50-70%) of the ammonia present in the leachate can be oxidised due to a limited amount of alkalinity available. Stable nitritation was obtained by applying a hydraulic retention time (HRT) of 4-5 d, which is higher than the values proposed for the effluent of anaerobic digesters. This higher HRT could probably be allowed by the high concentration of free ammonia present in the leachate, which could severely inhibit the growth of nitrite-oxidising bacteria.

Evaluation of aeration pretreatment to prepare an inoculum for the two-stage hydrogen and methane production process.

This study evaluates the effect of aeration pretreatment to prepare an inoculum for H2 and CH4 production in a two-stage process. Moreover, the biochemical hydrogen potential and biochemical methane potential of waste from the food industry in a two-stage process was assessed. The results confirmed the possibility of using an aerobic stress for selecting a hydrogen-producing inoculum. The inoculum was fairly stable since no hydrogenotrophic-methanogenic activity was observed in 25 days. The yields measured using glucose as substrate were of approximately 160 and 280 N mL(H2) g(COD⁻¹) of glucose for hydrogen and methane, respectively, which are in agreement with other studies using heat-shock for the pretreatment of the inoculum. When waste of the food industry (wheat milling) was used as substrate, a lower H2 yield was achieved by the aerobically-pretreated inoculum if compared to heat-shock; however, when combined with methane production in a two-stage process, much higher CH4 yield was achieved.

Development and permeability of a dynamic membrane for anaerobic wastewater treatment.

Dynamic membranes (DMs) have recently been proposed as an alternative to microfiltration and ultrafiltration in membrane bioreactors (MBRs) in order to contain capital and management costs. This study aims to develop an anaerobic dynamic MBR for wastewater treatment by using a large pore-sized mesh. The study demonstrated that a DM can be developed by using a mesh of 200µm pore-size and applying low cross flow velocity. The bench-scale reactor achieved COD removal efficiency between 65% and 92% and proved to be able to remove approximately 99% of the mixed liquor suspended solids, maintaining a solids retention time well above 200d. A significant quantity of biogas was produced by the external dynamic membrane module and was released with the effluent stream. The flux-step experiment, designed to estimate the critical flux in ultrafiltration MBR, can also be used for monitoring the development and stability of DMs.

Enhanced methane production from rice straw co-digested with anaerobic sludge from pulp and paper mill treatment process.

Rice straw is a widely available lignocellulosic waste with potential for energy recovery through anaerobic digestion. Lignin slows the hydrolysis phase, resulting in low methane recovery and long digestion periods. Although pretreatment is effective, it often requires high energy inputs or chemicals that are not feasible for farm-scale systems. This study investigates a unique co-digestion strategy to improve methane yields and reduce digestion times for farm-scale systems. By adding both piggery wastewater and paper mill sludge, specific methane yields in laboratory-scale digesters reached the theoretical value for rice straw (i.e. 330LNCH4/kgVS) over the 92-day period. Accelerated hydrolysis of the straw was directly related to the quantity of sludge added. The most stable digester, with sufficient buffering capacity and nutrients, contained equal parts of straw, wastewater and sludge. This approach is feasible for farm-scale applications since it requires no additional energy inputs or changes to existing infrastructure for dry systems.

Innovative two-stage anaerobic process for effective codigestion of cheese whey and cattle manure.

The valorisation of agroindustrial waste through anaerobic digestion represents a significant opportunity for refuse treatment and renewable energy production. This study aimed to improve the codigestion of cheese whey (CW) and cattle manure (CM) by an innovative two-stage process, based on concentric acidogenic and methanogenic phases, designed for enhancing performance and reducing footprint. The optimum CW to CM ratio was evaluated under batch conditions. Thereafter, codigestion was implemented under continuous-flow conditions comparing one- and two-stage processes. The results demonstrated that the addition of CM in codigestion with CW greatly improved the anaerobic process. The highest methane yield was obtained co-treating the two substrates at equal ratio by using the innovative two-stage process. The proposed system reached the maximum value of 258 mL(CH4) g(gv(-1), which was more than twice the value obtained by the one-stage process and 10% higher than the value obtained by the two-stage one.

Decolourisation of textile wastewater in a submerged anaerobic membrane bioreactor.

Azo dye decolourisation can be easily achieved by biological reduction under anaerobic conditions. The aim of this study was to evaluate the applicability of submerged anaerobic membrane bioreactors (SAMBRs) for the decolourisation of dyeing wastewater containing azo dyes. The reactive orange 16 was used as model of an azo dye. The results demonstrated that very high decolourisation (higher than 99%) can be achieved by SAMBRs. Although decolourisation was not significantly influenced by the azo dye concentrations up to 3.2 g L(-1), methane production was greatly inhibited (up to 80-85%). Since volatile fatty acids accumulated in the treatment system with the azo dye concentration increase, methanogenes seem to be the most sensitive microbial populations of the anaerobic ecological community. The results demonstrated that anaerobic process combined with membrane filtration can deal with highly concentrated wastewaters that result from stream separation of industrial discharges.

Stabilisation of biodried municipal solid waste fine fraction in landfill bioreactor.

The biodrying process of solid waste is a pre-treatment for the bio-stabilisation of the municipal solid waste. This study aims to investigate the fate of the municipal solid waste fine fraction (MSWFF) resulting from a biodrying treatment when disposed in landfills that are operated as bioreactors. Biodried MSWFF was apparently stable due to its low moisture content that slows down the microbial activity. The lab-scale anaerobic bioreactors demonstrated that a proper moisture content leads to a complete biodegradation of the organic matter contained in the biodried MSWFF. Using a pilot-scale landfill bioreactor (LBR), MSWFF stabilisation was achieved, suggesting that the leachate recirculation could be an effective approach to accomplish the anaerobic biodegradation and biostabilisation of biodried MSWFF after landfilling. The biostabilisation of the material resulting from the LBR treatment was confirmed using anaerobic and aerobic stability indices. All anaerobic and aerobic indices showed a stability increase of approximately 80% of the MSWFF after treatment in the LBR. The similar values of OD7 and BMP stability indices well agree with the relationship between the aerobic and anaerobic indices reported in literature.

Wastewater treatment in a submerged anaerobic membrane bioreactor.

Although most membrane bioreactors are used under aerobic conditions, over the last few years there has been increased interest in their application for anaerobic processes. This paper presents the results obtained when a bench-scale submerged anaerobic membrane bioreactor was used for the treatment of wastewaters generated in the agro-food industry. The reactor was fed with synthetic wastewater consisting of cheese whey and sucrose, and volumetric organic loading rates (OLRs) ranging from 1.5 to 13 kgCOD/(m(3)*d) were applied. Under the operating conditions studied, the maximum applicable OLR was between 6 and 10 gCOD/(g*L), which fell within the ranges of the high-rate anaerobic wastewater treatment systems, while high concentrations of volatile fatty acids were produced at higher OLR rates. With an OLR of 1.5-10 gCOD/(g*L), the reactor showed 94% COD removal, whereas this value dropped to 33% with the highest applied OLR of 13 gCOD/(g*L). The study therefore confirms that membrane bioreactors can be used for anaerobic wastewater treatment.

Textile wastewater treatment in a bench-scale anaerobic-biofilm anoxic-aerobic membrane bioreactor combined with nanofiltration.

This study evaluated the treatability of textile wastewaters in a bench-scale experimental system, comprising an anaerobic biofilter, an anoxic reactor and an aerobic membrane bioreactor (MBR). The MBR effluent was thereafter treated by a nanofiltration (NF) membrane. The proposed system was demonstrated to be effective in the treatment of the textile wastewater under the operating conditions applied in the study. The MBR system achieved a good COD (90-95%) removal; due to the presence of the anaerobic biofilter, also effective color removal was obtained (70%). The addition of the NF membrane allowed the further improvement in COD (50-80%), color (70-90%) and salt removal (60-70% as conductivity). In particular the NF treatment allowed the almost complete removal of the residual color and a reduction of the conductivity such as to achieve water quality suitable for reuse.

Monitoring the biochemical hydrogen and methane potential of the two-stage dark-fermentative process.

A two-step process has been recently proposed whereby the products of biological hydrogen production processes are used as substrates for biological methane production. The aim of the present study is to evaluate a simple bench-scale batch procedure for measuring the biochemical hydrogen and methane potential of organic substances as a two-step simulated process. Glucose fermentation showed an hydrogen and methane recovery (measured as the ratio of electron equivalents recovered as hydrogen and methane and electron equivalents of the initial substrate added) from the initial substrate of 13.3% and 75.5%, respectively, that approximates mass balance closure. On the contrary, gas recoveries ranging from 61% to 75% were measured from wastes originating from the food-industry. Moreover, the results demonstrate that the substrate origins significantly influence the ratio of H(2) and CH(4) recovery.

Effect of the organic loading rate on biogas composition in continuous fermentative hydrogen production.

Some systems did not select for hydrogen-producing microorganisms and an unexpected growth of hydrogenotrophic methanogens was observed, although the reactors were operated under well-defined operating conditions that could result in biohydrogen production. The aim of this study was to evaluate the effect of the organic loading rate (OLR) on the hydrogen and methane composition of the biogas produced in dark fermentative processes. The study was carried out using an upflow anaerobic sludge blanket (UASB) reactor in order to evaluate the OLR effect in systems with sludge retention. During continuous operation, the UASB reactor showed the slow development of methanogenic activity, related to the applied OLR. The results demonstrate that operating an UASB reactor at pH 5.5 is not enough to prevent the acclimation of methanogens to the acidic pH and therefore long-term biohydrogen production cannot be achieved. Moreover, this study demonstrates that OLR also has an effect on the biogas composition, where the higher the OLR the greater the biogas H2 content.

Artificial intelligence control of a sequencing batch reactor for nitrogen removal via nitrite from landfill leachate.

Leachate generated in old landfills is a high-strength wastewater, which is particularly difficult to treat owing to its low biochemical oxygen demand/total Kjeldahl nitrogen ratio. This paper seeks to demonstrate that reliable leachate treatment by means of sequencing batch reactors (SBRs) is indeed possible by means of the application of a smart control system. This study assesses the results of a computer-controlled bench-scale SBR treating raw sanitary landfill leachate to achieve nitrogen removal through the nitrite shortcut. Significant improvements have been obtained by introducing a fuzzy inferential system based on simple process measurements (i.e. dissolved oxygen, oxidation-reduction potential and pH). The paper analyzes the results of a test period of over 280 consecutive days of operation, during which the fuzzy control system correctly recognized over 97% of the SBR phase transitions and provided smart adjustments of the process operating conditions in terms of phase length and external COD addition. In spite of time-varying process conditions, the application of fuzzy logic provided stable nitrogen removal via nitrite through continuous adjustments of the main process parameters and resulted in a decreased hydraulic retention time, an increased loading rate, a saving in the external COD addition and considerable aeration energy conservation.