Oxygen Uptake Rate Values Research Articles

How high salt shock affects performance and membrane fouling characteristics of a halophilic membrane bioreactor used for treating hypersaline wastewater

In the present study, the effect of short-term salt shocks (13% and 20%) on the performance of a halophilic MBR bioreactor used to treat a hypersaline (5% salt) synthetic wastewater was considered. 13% and 20% salt shocks resulted in a transient and permanent decrease in chemical oxygen demand removal efficiency, respectively which could be correlated with soluble microbial products (SMP) concentration and specific oxygen uptake rate values of the halophilic population. DNA leakage tests suggested that both 13% and 20% short-term salt shocks resulted in some cell structural damage. During both 13% and 20% salt shocks mixed liquor SMP, extracellular polymeric substances (EPS), zeta potential and endogenous respiration increased while relative hydrophobicity, EPSp/EPSc and exogenous respiration decreased; in both cases, however, the pre-shock values for these parameters were restored after the removal of the salt shock. 13% salt shock resulted in a transient increase in the membrane fouling rate and a permanent rise in total membrane resistance (Rt). On the other hand, both membrane fouling rate and Rt increased during 20% salt shock. Membrane fouling rate initially reduced after the 20% salt shock removal but after 5 days a “TMP jump” occurred. The latter was caused by the higher steady state SMPc and SMPp concentrations after removal of 20% salt shock compared to pre-shock values. This might have either resulted in a decrease in critical flux or an increase in local flux above critical flux in some parts of the membrane. The contribution of cake layer resistance to overall membrane resistance increased after the 13% and 20% salt shocks. The findings of the present study reveal the robustness of halophilic MBRs against salt shocks in the treatment of hypersaline wastewater. However, in cases of very high salt shocks, appropriate membrane fouling reduction strategies should be carried out during its operation.

Effect of dissolved oxygen concentration on activated sludge bacterial community and oxygen uptake rate in a SBR using co-produced oxygen from a PEM hydrogen electrolyser

A laboratory-scale sequential batch reactor (SBR) was operated for 5 months on artificial wastewater using co-produced oxygen from a hydrogen proton exchange membrane (PEM) electrolyser. High chemical oxygen demand (COD) removal (>80–85 %) was achieved consistently using a Food to Microorganism (F/M) ratio as high as 1. The effect of dissolved oxygen (DO) concentration, ranging from 1 to 8 mg/L, on COD removal and oxygen uptake rate (OUR) was evaluated. DO concentration was shown to influence bacteria respiration as OUR values increased across the DO range studied. OUR increased linearly from 4.24 to 9.23 mgO2/L/h/g of MLSS with DO concentration of 1 to 8 mg/L respectively, however no effect on COD removal was observed. Microorganism communities were characterized, and significant variation in bacteria communities distribution was observed when increasing the DO concentration along with a decrease in bacteria diversity. The dominant phylum observed during this study were Proteobacteria, Bacteroidetes, and Candidatus Sacharibacteria. Aeration energy expenditures were estimated to be two times lower than for a similar air SBR when keeping DO concentration below 5 mg/L.

Oxygen Uptake Rate as an Indicator of the Substrates Utilized by Candidatus Accumulibacter

Candidatus Accumulibacter belongs to phosphate-accumulating organisms (PAOs) which exhibit a cyclic metabolism and are capable of intracellular polyphosphate accumulation and their hydrolysis under feast-famine anaerobic-aerobic cycling. In consortia of activated sludge microorganisms, these bacteria are responsible for enhanced biological phosphorus removal (EBPR). The spectrum of the substrates used by Ca. Accumulibacter remains insufficiently studied. It was investigated by measuring the oxygen uptake rates (OUR) of Ca. Accumulibacter-enriched culture supplemented with 17 different organic substrates. The highest oxygen uptake rate values were observed in the presence of tryptone, volatile fatty acids (acetate, propionate, and butyrate), succinate, pyruvate, and amino acids (aspartate and glutamate). Phosphate dynamics in the medium under shifts from anaerobic to aerobic cultivation in batch experiments were studied for these compounds (except for tryptone). All tested substrates were shown to cause phosphate cycling (release in the anaerobic phase and uptake in the aerobic one), with OURs for the substrates correlating with the number of phosphates consumed during the aerobic phase. It was concluded that OUR may be used as an indicator of the monosubstrates used by Ca. Accumulibacter in the anaerobic/aerobic cycle. The possible pathways for substrate transport and metabolism by Ca. Accumulibacter are discussed using stoichiometric data and the results of metagenomic analysis.

Immobilization of mixed cells by Flaxseeds (Linum usitatissimum) extract as new nonconventional biocarrier for biodegradation of sodium dodecyl sulfate

Flaxseeds as a new nonconventional biocarrier was used for the first time to immobilize mixed bacterial cells for biodegradation of sodium dodecyl sulfate (SDS) in actual wastewaters. The results revealed that SDS elimination efficiency in domestic wastewater (DWW) and household detergents manufacturing wastewater (HWW) were 98.8% and 94.5%, respectively. The immobilized cells were repeatedly used for 60 batches without notable loss of their bioactivity, mechanical and chemical stability. Seven kinetic models were applied to depict the interrelation between cells growth and substrate consumption which were; Blackman, Monod, Haldane, Teissier, Moser, Han-Levenspiel, and Contois Models. Teissier model fit well the experimental data with lowest sum of squares error (SSE) of 2.41 × 10−14 indicating exponential growth of biocatalyst inside the biocarrier. Endogenous rate constant (Kd), and yield coefficient (Y) were found to be 0.0039 h−1, and 0.44 h−1, respectively. Values of first order rate constant (K1), effective diffusivity (De), mass transfer coefficient, effectiveness factor (η), and Thiele modulus (Φ) were found to be 1.55 × 10−5 s−1, 6.44×10−7 cm2/s, 2.14×10−6 cm/s, 0.88, and 0.49, respectively. Values of oxygen uptake rate (OUR), volumetric oxygen mass transfer coefficient (KLa) and oxygen transfer rate (OTR) were 0.0013 mg O2/L. s, 0.0222 s−1 and 0.00111 mg O2/L. s, respectively. Experimental and predicted results of (OUR) were in a good agreement with determination coefficient (R2) of 0.94.

Agronomic valorization of sewage sludge: The potential of thermal drying to achieve sanitation and biological stability

This work aims to contribute to the discussion of using a waste (sewage sludge) to obtain a valuable product (organic-rich fertilizer), based on a circular economy perspective. The composition of sewage sludge (SS) was considered, highlighting important characteristics as raw material for fertilizer production (e.g., macronutrients, microbiological contamination, phytotoxicity). Particularly, the biological stability of SS was evaluated measuring the oxygen uptake rate (OUR) using the Oxitop® method, and the phytotoxicity using germination tests. The potential of thermal drying to stabilize the SS was evaluated at two temperatures (100 and 130°C). The results showed that the organic matter (OM), N, and P levels are within the range of interest for agronomic valorization, while the content of some potentially toxic metals (e.g., Zn and Cu) may exceed the legal limits. The phytotoxicity assessment showed that raw SS samples negatively impacted the germination phase. Concerning the biological stability, results showed that raw anaerobic digested samples were characterized by OUR values above 50 mmol O2/kg OM.h (the limit for stable SS). Although thermal treatment of SS at 100 and 130°C eliminated pathogens and significantly stabilized SS to OUR values below 50 mmol O2/kg OM.h, it was not enough to reach the limit established for a stable fertilizer (25 mmol O2/kg OM.h). Globally, SS has potential for soil application, within the scope of circular economy, but it is essential to control its microbiological contamination and stability.

Online monitoring of the cell-specific oxygen uptake rate with an in situ combi-sensor.

In a biotechnological process, standard monitored process variables are pH, partial oxygen pressure (pO2), and temperature. These process variables are important, but they do not give any information about the metabolic activity of the cell. The ISICOM is an in situ combi-sensor that is measuring the cell-specific oxygen uptake rate (qOUR) online. This variable allows a qualitative judgement of metabolic cell activity. The measuring principle of the ISICOM is based on a volume element enclosed into a small measuring chamber. Inside the measuring chamber, the pO2 and the scattered light is measured. Within a defined measuring interval, the chamber closes, and the oxygen supply for the cells is interrupted. The decreasing oxygen concentration is recorded by the pO2 optode. This measuring principle, known as the dynamic method, determines the oxygen uptake rate (OUR). Together with the scattered light signal, the cell concentration is estimated and the qOUR is available online. The design of the ISICOM is focused on functionality, sterility, long-term stability, and response time behavior so the sensor can be used in bioprocesses. With the ISICOM, measurement of online and in situ measurement of the OUR is possible. The OUR and qOUR online measurement of an animal cell batch cultivation is demonstrated, with maximum values of OUR = 2.5 mmol L-1 h-1 and a qOUR = 9.5 pmol cell-1 day-1. Information about limitation of the primary and secondary substrate is derived by the monitoring of the metabolic cell activity of bacteria and yeast cultivation processes. This sensor contributes to a higher process understanding by offering an online view on to the cell behavior. In the sense of process analytical technology (PAT), this important information is needed for bioprocesses to realize a knowledge base process control.

Effect of Oxygen Supply on Surfactin Production and Sporulation in Submerged Culture of Bacillus subtilis Y9

Fermentation parameters for surfactin production and sporulation in a submerged culture of Bacillus subtilis Y9 with various oxygen transfer rates in 5 L jar fermenters were investigated. The oxygen-uptake rate (OUR) was positively correlated with volumetric surfactin productivity. When OUR value increased from 0 to 250 s−1, productivity increased up to 45 mg/L·h; however, no further increase was observed at OUR values above 255 s−1. The volumetric mass transfer coefficient KLa increased with increasing agitation speed. However, a reduction in surfactin production was observed at the highest agitation speed of 500 rpm. Productivity sharply decreased after spore appearance, and remained low until the end of the culture. A mesh-type sparger was installed to generate microsized air bubbles. When the system was operated at 400 rpm with the mesh-type sparger, KLa was higher than that at 500 rpm with an original sparger. Under agitation at 400 rpm with the mesh-type sparger, productivity was maintained above 42.3 mg/L·h until 24 h, resulting in the highest surfactin concentration of 875 mg/L. Thus, a mesh-type sparger promotes KLa, leading to an increase in productivity.

A simplified implementation of the stationary liquid mass balance method for on‐line OUR monitoring in animal cell cultures

AbstractBACKGROUNDCompared with other methods, the stationary liquid mass balance method for oxygen uptake rate (OUR) determination offers advantages in terms of estimation accuracy and reduction of stress. However, the need for sophisticated instrumentation, like mass flow controllers and gas analysers, has historically limited wider implementation of such a method.In this paper, a new simplified method based on inexpensive valves for the continuous estimation of OUR in animal cell cultures is evaluated. The determination of OUR values is based on accurate operation of the dissolved oxygen (DO) control loop and monitoring of its internal variables.RESULTSThe method developed was tested empirically in 2 L bioreactor HEK293 batch cultures. OUR profiles obtained by a dynamic method, global mass balance method and the developed simplified method were monitored and compared. The results show how OUR profile obtained with the proposed method better follows the off‐line cell density determination. The OUR estimation frequency was also increased, improving the method capabilities and applications.The theoretical rationale of the method was extended to the sensitivity analysis which was analytically and numerically approached.CONCLUSIONSThe results showed the proposed method to be not only cheap, but also a reliable alternative to monitor the metabolic activity in bioreactors in many biotechnological processes, being a useful tool for high cell density culture strategies implementation based on OUR monitoring. © 2017 Society of Chemical Industry

Optimization of Herbicidin A Production in Submerged Culture of Streptomyces scopuliridis M40.

Herbicidin A is a potent herbicide against dicotyledonous plants as well as an antibiotic against phytopathogens. In this study, fermentation parameters for herbicidin A production in submerged culture of Streptomyces scopuliridis M40 were investigated. The herbicidin A concentration varied with the C/N ratio. High C/N ratios (>4) resulted in a herbicidin A production of more than 900 mg/l, whereas maximally 600 mg/l was obtained at ratios between 1 and 3.5. In 5-L batch fermentation, there was a positive correlation between the oxygen uptake rate (OUR) and herbicidin A production. Once the OUR increased, the substrate consumption rate increased, leading to an increase in volumetric productivity. Mechanical shear force affected the hyphal morphology and OUR. When the medium value of hyphal size ranged from 150 to 180 μm, high volumetric production of herbicidin A was obtained with OUR values >137 mg O2/l·h. The highest herbicidin A concentration of 956.6 mg/l was obtained at 500 rpm, and coincided with the highest relative abundance of hyphae of 100-200 μm length and the highest OUR during cultivation. Based on a constant impeller tip speed, which affects hyphal morphology, herbicidin A production was successfully scaled up from a 5-L jar to a 500-L pilot vessel.

A concept development of an early warning system for toxic sewage detection

The article presents a concept of an EWS (early warning system) for toxic sewage detection upstream from Wroclaw Wastewater Treatment Plant. It includes design and construction of three toxicity warning stations, situated in municipal sewage system. Early detection of toxic contaminants provides time for corrective actions. Wastewater toxicity level is based on activated sludge OUR (Oxygen Uptake Rate) measurement. The database of normal level of activated sludge OUR is collected during the measurement initial phase. Hence, if toxic event occurs, device will measure decreased OUR value and WWTP operators will be alarmed. Additionally, flow measurement is carried out. It allows to estimate the volume of a toxic stream. Development of the early warning system requires solving minimum six different scientific and technical issues and is a subject for a complex research project.

Assessing the performance of an MBR operated at high biomass concentrations

Reducing the footprint requirements of membrane bioreactors (MBR)s can both decrease the surface area needs for new wastewater treatment plants (WWTP)s, and increase the treatment capacities of existing WWTPs at a given surface area. In addition, it may promote the development of movable/portable containerized MBRs for a diverse range of wastewater treatment applications. Applications may include the provision of municipal/industrial wastewater treatment in remote areas without sewerage, and the provision of sanitation services under challenging site-specific conditions such as after the occurrence of a human-made or a natural disaster. The reduction of the footprint requirements of MBRs is constrained by the maximum amount of biomass that can be accommodated in the aerobic basin. The biomass concentration is mainly limited by the extremely low oxygen transfer efficiency (OTE) experienced by conventional aeration bubble diffuser systems at mixed liquor total suspended solids (MLSS) concentrations higher than 20 g L−1. Another potential limitation for the operation of MBRs at such high MLSS concentrations is the reduction on the membrane permeability due to excessive fouling. A pilot MBR with a treatment capacity of one m3 d−1 was installed at the research hall facilities at the Harnaschpolder wastewater treatment plant in Delft, The Netherlands. The MBR was operated at MLSS concentrations of up to 28 g L−1 at sludge retention times (SRT)s ranging from 30 to 35 days. The MBR was provided with a Speece cone concentrated oxygen delivery system to overcome the oxygen transfer limitations of conventional bubble diffuser aeration systems at high MLSS concentrations. The MBR performance was evaluated by monitoring the influent and effluent water quality, the membrane permeability, the sludge filterability, the dissolved oxygen (DO) concentration, and the oxygen uptake rate (OUR). The Speece cone proved to be effective in delivering enough oxygen to maintain DO concentrations in the MBR of approximately 2 mg L−1 at MLSS concentrations of up to 22 g L−1. OUR values above 200 mg L−1 h−1 were observed at 14 g L−1 MLSS and higher than 300 mg L−1 h−1 at 22 g L−1 MLSS. The MBR exhibited chemical oxygen demand (COD) removal efficiencies of up to 99% even at a hydraulic retention time (HRT) as low as 3.7 h. A reduction in permeability from 33 to 11 lmh bar−1 was observed when the MLSS concentrations increased from 18.7 to 27.8 g L−1. Sludge filterability values expressed as the added resistance (ΔR20) fell in the range of “poor filterability” for all the evaluated operational conditions; however, a lower filtration resistance in the range of “moderate filterability” at approximately 23 g L−1 MLSS was noticed. The experimental results suggest that at the evaluated experimental conditions the existent limitations on poor oxygen transfer and low permeability when operating a MBR at high MLSS concentrations can be overcome; therefore, the footprint requirements of MBR systems may be further reduced.

Effect of flow rate on growth and oxygen consumption of biofilm in gravity sewer.

The function of sewer as reactors must rely on the biofilm in it. In this paper, the formation, structure, oxygen transfer, and activity of the biofilm under different hydraulic conditions were studied by the microelectrode technology, oxygen uptake rate (OUR) technology, and 454 high-throughput pyrosequencing technology. Results showed that when the wall-shear stresses were 1.12, 1.29, and 1.45Pa, the porosity of the steady-state biofilm were 69.1, 64.4, and 55.1%, respectively. The maximum values of OUR were 0.033, 0.027, and 0.022mg/(L*s), respectively, and the COD removal efficiency in the sewers reached 40, 35, and 32%, respectively. The research findings had an important significance on how to improve the treatment efficiency of the sewers. Fig. a Graphical Abstract.

Preparation and Structure of Poly(vinyl alcohol)/Polyacrylate Elastomer Composite Hydrogels and Their Application in Wastewater Treatment by Immobilizing with Microorganisms

Poly(vinyl alcohol) (PVA)/powdered ethyl acrylate rubber (ACM) composite hydrogels were prepared through in situ chemical cross-linking. The addition of ACM could accelerate the cross-linking reaction of PVA, and increase pore nucleus, resulting in a decrease of pore size and formation of a homogeneous and compact porous network structure. Under proper content of ACM, the shear elastic modulus (G′) and network density (νe) of the composite hydrogels can be improved, leading to an enhancement of the mechanical property and hydraulic shock resistance. The PVA/1.5 wt % ACM hydrogel presented the best long-term hydraulic stability, and the toughening mechanism was studied. The values of oxygen uptake rate and chemical oxygen demand removal rate of PVA/ACM composite hydrogels embedded with microorganism fluctuated with the addition of ACM, and a high microorganism activity can be retained.

Application of a new model based on oxygen balance to determine the oxygen uptake rate in mammalian cell chemostat cultures

The application of dynamic methods for determining the oxygen uptake rate (OUR) in continuous mammalian cell cultures frequently ignores contributions to the oxygen balance such as the oxygen content in the culture medium inlet and outlet and the oxygen transfer between the culture and the headspace through the culture surface. We develop a mathematical model that allows OUR determination in mammalian cell chemostat cultures through a dynamic method, incorporating these neglected variables, as well as the application and validation of this model at two culture temperatures, 37°C and 33°C.The proposed model was compared with a model that only includes the OUR term (typical model). The standard error of the proposed model was less than that of the typical model, making the proposed model more accurate. The results showed that at 37°C, the results significantly differed depending on which model was used. At 33 and 37°C, the specific oxygen uptake rate (qO2) values obtained with the proposed model were within the range usually reported for mammalian cells. It was also shown that the OUR value was underestimated if the oxygen transfer through the headspace-medium interface was not considered under mild hypothermia conditions. The model could be applied for developing strategies based on OUR monitoring and controlling for recombinant protein production under conditions of mild hypothermia.

Impact of carrier filling ratio on oxygen uptake & transfer rate, volumetric oxygen transfer coefficient and energy saving potential in a lab-scale MBBR

This study investigates the effect of bio-carriers on the oxygen uptake rate (OUR) in the effluent of the reactor with an aim to provide optimized filling ratio for the practical operation of a moving bed biofilm reactor. The experimental work was carried out in seven different phases with respect to the filling ratio. In each phase, oxygen transfer efficiency and volumetric oxygen mass transfer coefficient (kla) were measured by varying the carrier filling ratio (10–70%). The effect of filling ratios, mixing intensity, hydraulic retention time and kla on OUR were observed. The value of OUR increased 2–3 times with the augmentation of carrier filling ratio but it decreased when more carriers were added inside the reactor. Secondly, efforts were also made to find ways to reduce aeration energy. Savings of energy consumption (up to 35%) were noted by providing intermittent aeration at an average oxygen transfer rate of 6mg-O2L−1h−1. Still, further scope exists to conserve more energy if the oxygen transfer rate is maintained at 4mg-O2L−1h−1.

Structure and property of polyvinyl alcohol/precipitated silica composite hydrogels for microorganism immobilization

Polyvinyl alcohol (PVA) hydrogel filled with precipitated silica (PSi) was prepared by chemical crosslinking in the saturated boric acid solution. The structure and property of the composite hydrogel was studied. It was found that PSi could accelerate the crosslinking reaction of PVA, and the intermolecular bonding of PVA/PSi composite was confirmed. Effect of PSi on reactive kinetics of PVA hydrogel was investigated. The tensile strength of PVA hydrogel increased with PSi content. Proper content of PSi could also enhance the permeability and swelling property, resulting in the improvement of the capillary water absorption capacity of PVA hydrogel. By addition of PSi, many large pores formed in the hydrogel so as to provide channels for microbial metabolites. When being applied in waste water treatment, the value of oxygen uptake rate (OUR) and COD removal rate of the PVA composite hydrogel immobilized with activated sludge increased significantly, indicating that the microbial activity of PVA immobilized beads could be enhanced.

Performance of membrane bioreactor in presence of flocculants

This study focused on evaluating the effects of flocculant addition on the performance of membrane bioreactor (MBR). Two different flocculants, aluminum sulfate (alum) and polyaluminum chloride (PACl), were tested and the performance of MBR system in terms of COD removal, membrane fouling, and sludge properties was investigated. Addition of both flocculants resulted in significant reduction of membrane fouling rate and polyaluminum chloride was found more effective than alum. The sustainable filtration time at optimum dosage of alum and PACl was, respectively, 2.5 and 4.2 times more than that in control MBR which had no flocculant. Additionally, sludge oxygen uptake rate (OUR) improved in the presence of both flocculants and average OUR values obtained from flocculant-added membrane bioreactors was about 1.5 times more than that in control MBR. The results also revealed that there were no significant difference in COD removal of flocculant-added MBRs and control MBR and the treatment in all the three MBRs led to high COD removal efficiencies.

Effect of sawdust addition on composting of separated raw and anaerobically digested pig manure

Manures need the addition of carbon-rich bulking agents to conserve N during composting, which increases the cost of the composting process. The recommended proportion of manure/sawdust, based on a carbon (C):nitrogen (N) ratio, is approximately 3:2. Two composting experiments were conducted to determine the impact of varying the proportion of sawdust to either separated raw, or separated anaerobically digested pig manures. To determine stability and maturity of the final compost, oxygen uptake rate (OUR) and germination index (GI) tests were conducted. For both experiments, three treatments were employed: manure-only (Treatment A), manure/sawdust mixed 4:1, fresh weight (Treatment B), and manure/sawdust mixed 3:2, fresh weight (Treatment C). The mixtures were composted in tumblers for 56 days with regular turning. The composting material was tested over the study duration for temperature, pH, water content, organic matter, C:N ratio and bulk density. For both Treatments B and C, the GI indicated low levels of phytotoxicity, and OUR values were lower than the recommended Irish threshold of 13 mmol O2 kg OM−1 h−1, indicating that a high quality compost was produced. The proportion of sawdust to separated manure used can be reduced to make a cost saving, while still producing a stable end-product: 60% less sawdust is required to compost at a manure-to-sawdust ratio of 4:1 compared to the previously recommended ratio of 3:2.

Removal of bisphenol-A from spiked synthetic effluents using an immersed membrane activated sludge process

With the recent emergence of estrogenic contaminants as reclaimed wastewater quality issue, our study has investigated the removal of bisphenol-A (BPA) using a submerged membrane bioreactor. Experiments were performed at initial BPA concentrations ranging from 1.0 to 15mg/L. A very low sludge mineralization occurred in the bioreactor (with a VSS/TSS ratio around 0.8–0.9), irrespective of the variation of biomass concentration and a long retention time of solids. The specific oxygen uptake rate values recorded did not reveal any significant change in the microbial respiratory activity in the absence (2.00mgO2/gVSSh) and in the presence (1.4mgO2/gVSSh) of BPA in the synthetic effluent. The performance of the bioreactor in terms of COD removal remained constant and reached 99% irrespective of the hydraulic retention time imposed (3–6h), whereas 66–99% of COD removal was recorded while using conventional activated sludge system. The limit of the toxicity (21.6g BPA/m3/d) determined for the bioreactor membrane system was higher than that recorded for the conventional activated sludge (2.5g BPA/m3/d). BPA was effectively removed in the membrane bioreactor (abatement ⩾98% was reached) due to adsorption and biodegradation/transformation processes. Residual BPA concentrations recorded in the mixed liquor remained low (0.017–0.004mg/L), when the raw wastewater contained a concentration of 1.0mg BPA/L. This indicates that BPA was not accumulated in the membrane bioreactor, but bacteria oxidized it. Likewise, the membrane bioreactor was found to be effective in removing ammonia–nitrogen (>99% of N–NH4 removed), whereas 61% of phosphate (P–PO4) could be simultaneously removed. The membrane bioreactor was a total barrier for total suspended solid and turbidity was effectively removed (residual turbidity was lower than 1.0 NTU).

Biodegradation potential of bulking agents used in sludge bio-drying and their contribution to bio-generated heat

Straw and sawdust are commonly used bulking agents in sludge composting or bio-drying. It is important to determine if they contribute to the biodegradable volatile solids pool. A sludge bio-drying process was performed in this study using straw, sawdust and their combination as the bulking agents. The results revealed that straw has substantial biodegradation potential in the aerobic process and sawdust has poor capacity to be degraded. The temperature profile and bio-drying efficiency were highest in the trial that straw was added, as indicated by a moisture removal ratio and VS loss ratio of 62.3 and 31.0%, respectively. In separate aerobic incubation tests, straw obtained the highest oxygen uptake rate (OUR) of 2.14 and 4.75 mg O 2 g −1VS h −1 at 35 °C and 50 °C, respectively, while the highest OUR values of sludge were 12.1 and 5.68 mg O 2 g −1VS h −1 at 35 °C and 50 °C and those of sawdust were 0.286 and 0.332 mg O 2 g −1VS h −1, respectively. The distribution of biochemical fractions revealed that soluble fractions in hot water and hot neutral detergent were the main substrates directly attacked by microorganisms, which accounted for the initial OUR peak. The cellulose-like fraction in straw was transformed to soluble fractions, resulting in an increased duration of aerobic respiration. Based on the potential VS degradation rate, no bio-generated heat was contributed by sawdust, while that contribution by straw was about 41.7% and the ratio of sludge/straw was 5:1 (w/w, wet basis).