Organic Loading Rate Values Research Articles

Retention time and organic loading rate as anaerobic co-digestion key-factors for better digestate valorization practices: C and N dynamics in soils

The growing use of anaerobic co-digestion (AcoD) in processing organic waste has led to a significant digestate production. To effectively recycle digestate back into soils, it is crucial to understand how operational variables in the AcoD process influence the conversion of organic matter (OM). To address this, a combination of biochemical fractionation and various soil incubation tests were employed to assess the stability of OM in digestates generated from anaerobic continuous reactors fed with a food waste-hay mixture and operating at different hydraulic retention times (HRT) and organic loading rates (OLR). This study revealed that digester performance and operating parameters impacted carbon dynamics in soils. A decrease in the carbon mineralization in soils when increasing the HRT was reported (48 ± 4 % for 70 days compared to 59 ± 1 % for 42 days). Specific HRT and OLR values were found to be linked to carbon accessibility and complexity, confirming that longer HRT lead to higher OM removal and increased complexity in soluble OM, despite minor discrepancies in relative carbon distribution. Furthermore, comparable rates of nitrogen mineralization in soils were observed for all digestates, consistent with the accessibility of nitrogen from the particulate OM. Nevertheless, AcoD converted substrates with the potential to immobilize nitrogen in soils into fast-acting fertilizers. In summary, this study underscores the importance of controlling the AcoD performances to evaluate the suitability of digestates for sustainable agricultural practices.

Investigation and Optimization of Operational Conditions of Anaerobic Digestion Process for Enhanced Biogas Production Yield in a CSTR Using RSM

In this study, pilot‐scale experiments were performed on the anaerobic digestion (AD) in a semicontinuous process using a continuously stirred tank reactor to determine the effects of organic loading rate (OLR), temperature, and mixing levels on biogas production yield. Fresh cow manure was considered the input feedstock of this study due to its high production volume. Response surface methodology (RSM) was employed to design and analyze experiments to optimize OLR, temperature, and mixing intensity in biogas production. The central composite design was applied as a RSM optimization tool. Four cubic mathematical models were derived to predict the responses. The optimization study was carried out to identify the highest yields achievable when temperature and mixing factors are minimized. Temperature and OLR have been found to have a greater influence on biogas production compared to mixing intensity. The AD process is significantly influenced by temperature. Variations in temperature, specifically near 40 and 55°C, lead to an increase in the rate of biogas production. Therefore, the best temperatures for biogas production rate are near 40 and 55°C. The results of the performed optimizations suggest to adopt OLR values range of 1.6–2.5 kgVSCow manure/m3 · day, and 2.5–3.4 kgVSCow manure/m3 · day in the case of temperature ranges 20–40°C and 40–55°C, respectively.

Kinetic Study of Anaerobic Digestion of Compost Leachate from Organic Fraction of Municipal Solid Waste

The anaerobic digestion (AD) of compost leachate has been scarcely investigated and, to the best of our knowledge, no previous work has analyzed the kinetics of the process in completely stirred tank reactors (CSTR). To overcome this lack of knowledge, the present work aimed to deepen the study of the AD of compost leachate in CSTR and to identify the kinetics that can represent the process evolution under different operating conditions. In this regard, an experimental investigation was carried out on a laboratory anaerobic pilot plant that worked in semi-continuous mode under mesophilic conditions. After the start-up phase, the digester was fed with organic loading rates (OLR) between 4 and 30 gCOD/Ld. The chemical oxygen demand (COD) removal ranged between 80 and 85% for OLR values up to 20 gCOD/Ld and, then, it was observed as 54% at 30 gCOD/Ld. The deterioration of process performance was caused by an excessive generation of volatile fatty acids leading to a decrease of methane production yield from 0.32–0.36 LCH4/gCODremoved at 20 gCOD/Ld, to 0.23–0.26 LCH4/gCODremoved at 30 gCOD/Ld. Using kinetic analysis, the Monod model was shown to be quite accurate in modelling the trends of COD degradation rates for OLR values up to 20 gCOD/Ld. On the other hand, a better fit was achieved with the Haldane model at 30 gCOD/Ld. The conducted modelling allowed to identify the kinetic parameters for each model. The detected results could help in the management and design of the digesters for the treatment of compost leachate.

Performance of an anaerobic biofilm reactor through the application of different operational conditions

Inadequate management of Municipal Solid Waste (MSW) and the energy crisis due to dependence on fossil fuels (carbon and hydrocarbons) are growing problems in Latin American countries such as Mexico. These problems are caused by various factors, including the lack of infrastructure and the limited development of technologies focused on addressing these areas. In Mexico, between 37.55 and 43.84 million tons of MSW are generated annually, while 5,896 PJ of fossil fuels are produced in the same period, contributing strongly to environmental pollution due to inadequate management and procurement-use processes respectively. In order to mitigate these problems, it is necessary to propose dual-purpose strategies, such as anaerobic digestion, that can help in the treatment of the organic fraction of MSW (OFMSW) and at the same time producing biogas as a renewable energy source. For these reasons, the aim of this work was to evaluate the performance of an anaerobic biofilm reactor through increases in the Organic Loading Rate (OLR) using the liquid fraction of the OFMSW as a substrate for biogas production. An anaerobic biofilm reactor called the Anaerobic Hybrid Reactor (AHR) was used to carry out this stud. The performance of the AHR in four stages applying different OLR values from 5 to 21 gCOD/L.d was analyzed. Anaerobic biofilm formation, pH, %COD and %solids removal, alkalinity, biogas production, and methane yield were evaluated. After 119 days of reactor operation, 93.45% colonization of the inverse fluidized bed, 85% total and soluble COD and removals greater than 80% for TS and VS, alkalinity less than 0.3, around 20 L of biogas per day with methane yields of 0.31 LCH4 at STP/gCODrem were obtained. The use of high OLR allows a larger volume of OFMSW liquid fraction to be treated producing a greater quantity of biogas with an excellent methane yield, thus demonstrating the high efficiency of the AHR.

Kinetic study of thermophilic anaerobic digestion of sugarcane vinasse in a single-stage continuous stirred tank reactor

Vinasse, the liquid waste from the bottom of a bioethanol distillation column, is a potential feedstock for biogas production due to its high organic content. Vinasse is discharged at a high temperature, i.e., 60–80 °C. Therefore, thermophilic anaerobic digestion is more efficient to directly treat the vinasse at a high organic loading rate (OLR). This study investigated the performance of thermophilic anaerobic digestion on sugarcane vinasse inoculated with acclimated digested cow manure (DCM). The experiment was run at varied OLRs (1,2; 1.8 and 2.4 gCOD.L−1.day−1). The process was carried out in a continuous stirred tank reactor (CSTR) with controlled pH (7 ± 0.5) and oxidation-reduction potential/ORP (−500 ± 10 mV). During the 111 days of the experiment, we monitored the volatile fatty acids (VFA), total chemical oxygen demand (tCOD), and soluble chemical oxygen demand (sCOD), as well as methane and biogas production rates, to evaluate the reactor performance. The experimental data were fitted on the Anaerobic Digestion Model 1 (ADM 1) using the AQUASIM® software. The data were categorized in three process stages observed in this study: stabilization, enrichment, and steady-state. The kinetic parameters, i.e., the coefficient of affinity (KS,ac, KS,pro) and the specific growth rate (km,ac and km, pro), were obtained from the data fitting. The good fit of ADM 1 to the data highlighted the flexibility of ADM 1 to represent a dynamic system of a thermophilic anaerobic digestion system with simultaneous changes in both process variables and microbial composition. The data fitting gave consistently low Ks values in the lower OLR of 0.2 gCOD. L−1.day−1 up to the higher OLR of 2.4 gCOD. L−1.day−1. The Ks values, which were less than 1/1000 of the specific growth rate constants (km), indicated that the combined effects of the thermophilic condition and vinasse composition increased the microbial-substrate affinity (showed by KS) more essentially than the microbial growth rate (addressed by km). In addition, inhibitory effects were not observed with all OLR values tested in this study.

Effect of the organic loading rate on the PHA-storing microbiome in sequencing batch reactors operated with uncoupled carbon and nitrogen feeding

Over the last years, in a search for sustainable and biodegradable alternatives to petrol-based plastics, biotechnological applications turned to the potentialities of mixed microbial cultures (MMC) for producing polyhydroxyalkanoates (PHAs). Under a feast and famine regime, an uncoupled carbon (C) and nitrogen (N)-feeding strategy may be adopted by dosing the C-source at the beginning of the feast and the N-source at the beginning of the famine in order to stimulate a PHA storage response and microbial growth. Even though this strategy has been already successfully applied for the PHA production, very few information is to date available regarding the MMC operating in these systems and the influence of Organic Loading Rate (OLR) on their selection and enrichment. To fill the gap, this study investigated the effect of the OLR on the selection of PHA-accumulating microorganisms in a sequencing batch reactor (SBR) operated with an uncoupled C and N feeding strategy. The SBR cycle length was set at 12 h and four OLRs values (4.25, 8.50, 12.75 and 18 gCOD L−1 d−1) were tested by changing the concentration of the feeding solution, made of a synthetic mixture of acetic (85% of the overall COD) and propionic (15%) acids. The PHA-storage yield increased by increasing the OLR (up to 0.69 COD/COD at 12.75 gCOD L−1 d−1) but significantly decreased (0.27 COD/COD) at 18 gCOD L−1 d−1 concomitantly with a longer feast phase and a lower PHA content in the biomass at the end of the feast phase. The selective pressure induced by the applied OLRs strongly influenced the microbiome composition revealing a high content of putative PHA-storing bacteria, such as Rhodobacter, Thauera and Paracoccus, in the SBR operated at OLRs 4.25, 8.50 and 12.75 g COD L−1 d−1 (up to 97.4% of total reads) and a low content (5.4%) in the SBR at 18 g COD L−1 d−1where the predominance of genus Nitrinicola was instead observed.

Performance Evaluation of Pressurized Anaerobic Digestion (PDA) of Raw Compost Leachate

Anaerobic digestion (AD) represents an advantageous solution for the treatment and valorization of organic waste and wastewater. To be suitable for energy purposes, biogas generated in AD must be subjected to proper upgrading treatments aimed at the removal of carbon dioxide and other undesirable gases. Pressurized anaerobic digestion (PDA) has gained increasing interest in recent years, as it allows the generation of a high-quality biogas with a low CO2 content. However, high pressures can cause some negative impacts on the AD process, which could be accentuated by feedstock characteristics. Until now, few studies have focused on the application of PAD to the treatment of real waste. The present work investigated, for the first time, the performance of the pressurized anaerobic digestion of raw compost leachate. The study was conducted in a lab-scale pressurized CSTR reactor, working in semi-continuous mode. Operating pressures from the atmospheric value to 4 bar were tested at organic loading rate (OLR) values of 20 and 30 kgCOD/m3d. In response to the rise in operating pressure, for both OLR values tested, a decrease of CO2 content in biogas was observed, whereas the CH4 fraction increased to values around 75% at 4 bar. Despite this positive effect, the pressure growth caused a decline in COD removal from 88 to 62% in tests with OLR = 20 kgCOD/m3d. At OLR = 30 kgCOD/m3d, an overload condition was observed, which induced abatements of about 56%, regardless of the applied pressure. With both OLR values, biogas productions and specific methane yields decreased largely when the pressure was brought from atmospheric value to just 1 bar. The values went from 0.33 to 0.27 LCH4/gCODremoved at 20 kgCOD/m3d, and from 0.27 to 0.18 LCH4/gCODremoved at 30 kgCOD/m3d. Therefore, as the pressure increased, although there was an enhanced biogas quality, the overall amount of methane was lowered. The pressured conditions did not cause substantial modification in the characteristics of digestates.

Dual-stage pulse-feed operation enhanced methanation of lipidic waste during co-digestion using acclimatized consortia

Methanation was improved during the dual-stage pulse-feed anaerobic co-digestion of extracted lipid (EL) from food waste leachate using acclimatized consortia by gradually increasing the organic loading rates (OLRs) in the subsequent phases. The utilization of major saturated and unsaturated long chain fatty acids (LCFAs) reached 78% and 98% in the acidogenic fermenters with consecutive organic loading, respectively. The acclimatized consortium induced the LCFA degradation and interconversion of short-medium chain fatty acids (SCFA–MCFAs) in the fermenters, even under highest OLR values. The highest methane yield (0.46 g g−1 VSinitial) was achieved in the third phase (61% EL loading of total injected volatile solid) with the complete utilization of LCFAs and SCFA–MCFAs in the digester. The assessment of the microbial community revealed that acidogenic Intestinimonas and Megasphaera, and acetogenic Levilinea were predominant in the fermenters due to their preeminent activities. The predominance of acetogenic Syntrophomonas (26%) indicated that it played a major role in β-oxidation, and its syntrophy with mixotrophic methanogens, such as Methanosarcina (93%), facilitated methanation through the complete degradation of EL in the digesters. The use of pulse-feed operation in dual-stage anaerobic co-digestion improved the methanation through complete utilization of EL under varied OLRs mediated by the acclimatized consortia.

Removal of polar micropollutants from domestic wastewater using a methanogenic – aerobic moving bed biofilm reactor system

A lab-scale system consisting of a methanogenic and an aerobic moving bed biofilm reactor (MBBR) was operated for seven months to investigate major pollutants’ and polar organic micropollutants’ removal from domestic wastewater under three different organic loading rates (OLRs). For all OLR values tested, the system achieved efficient COD (>84%) and NH4-N removal (>96%); both reactors contributed to the decrease of organic loading, while the aerobic bioreactor was exclusively responsible for the removal of ammonium nitrogen. The increase of OLR from 0.20 ± 0.10 to 2.10 ± 0.20 kg COD m−3 d−1 resulted to increase of the detected volatile fatty acids in the anaerobic reactor and increase of the produced biogas to 172.7 ± 25.3 mL L reactor−1 d−1. All target micropollutants were partially removed during different experimental phases; their removal ranged between 31% (5-methyl-1H-benzotriazole, 5TTR) and 97% (2-hydroxybenzothiazole, OH-BTH). Except benzotriazole (BTR) and OH-BTH, both reactors contributed significantly to the removal of target micropollutants, while the addition of extra aerobic MBBR as a final polishing step enhanced only OH-BTH removal. Use of experimental and literature data showed that the increase of organic loading enhanced the removal of three out of five target micropollutants, while the key factor responsible for this effect was COD concentration in influent wastewater. The modelling of micropollutants’ removal efficiency using biodegradation kinetics calculated in batch experiments showed satisfactory correlation between predicted and measured values for most target compounds.

Thermophilic two-phase anaerobic digestion using an innovative fixed-bed reactor for enhanced organic matter removal and bioenergy recovery from sugarcane vinasse

This study considered the application of anaerobic digestion (AD) with phase separation combined with the use of an anaerobic structured-bed reactor (ASTBR) as the methanogenic phase for the treatment of sugarcane vinasse, a high-strength wastewater resulting from ethanol production. Two combined thermophilic acidogenic-methanogenic systems formed by one single acidogenic reactor followed by two methanogenic reactors operated in parallel were compared, namely, a conventional UASB reactor and an upflow ASTBR reactor. Increasing organic loading rate (OLR) conditions (15–30kgCODm−3d−1) were applied to the methanogenic reactors. The results highlighted the feasibility of applying the ASTBR to vinasse, indicating a global COD removal higher than 80%. The ASTBR exhibited a stable long-term operation (240days), even for OLR values as high as 30kgCODm−3d−1. The application of similar conditions to the UASB reactor indicated severe performance losses, leading to the accumulation of acids for every increase in the OLR. An energetic potential of 181.5MJ for each cubic meter of vinasse was estimated from both hydrogen and methane. The provision of bicarbonate alkalinity proved to be a key factor in obtaining stable performance, offsetting the limitations of relatively low hydraulic retention times (<24h).

Biomethane production from vinasse in upflow anaerobic sludge blanket reactors inoculated with granular sludge.

The main objective of this study was to evaluate the anaerobic conversion of vinasse into biomethane with gradual increase in organic loading rate (OLR) in two upflow anaerobic sludge blanket (UASB) reactors, R1 and R2, with volumes of 40.5 and 21.5 L in the mesophilic temperature range. The UASB reactors were operated for 230 days with a hydraulic detection time (HDT) of 2.8 d (R1) and 2.8–1.8 d (R2). The OLR values applied in the reactors were 0.2–7.5 g totalCOD (L d)−1 in R1 and 0.2–11.5 g totalCOD (L d)−1 in R2. The average total chemical oxygen demand (totalCOD) removal efficiencies ranged from 49% to 82% and the average conversion efficiencies of the removed totalCOD into methane were 48–58% in R1 and 39–65% in R2. The effluent recirculation was used for an OLR above 6 g totalCOD (L d)−1 in R1 and 8 g totalCOD (L d)−1 in R2 and was able to maintain the pH of the influent in R1 and R2 in the range from 6.5 to 6.8. However, this caused a decrease for 53–39% in the conversion efficiency of the removed totalCOD into methane in R2 because of the increase in the recalcitrant COD in the influent. The largest methane yield values were 0.181 and 0.185 (L) CH4 (g totalCOD removed)−1 in R1 and R2, respectively. These values were attained after 140 days of operation with an OLR of 5.0–7.5 g totalCOD (L d)−1 and totalCOD removal efficiencies around 70 and 80%.

Wineries wastewater treatment by constructed wetlands: a review.

The application of wetland systems for the treatment of wineries wastewater started in the early 1990s in the USA followed a few years later by France, Italy, Germany and Spain. Various studies demonstrated the efficiency of constructed wetlands (CWs) as a low cost, low maintenance and energy-saving technology for the treatment of wineries wastewater. Several of these experiences have also shown lessons to be learnt, such as some limits in the tolerance of the horizontal subsurface flow and vertical subsurface flow classic CWs to the strength of the wineries wastewater, especially in the first stage for the multistage systems. This paper is presenting an overview of all the reported experiences at worldwide level during the last 15 years, giving particular attention and provision of details to those systems that have proven to get reliable and constant performances in the long-term period and that have been designed and realized as optimized solutions for the application of CW technology to this particular kind of wastewater. The organic loading rates (OLRs) applied to the examined 13 CW systems ranged from about 30 up to about 5,000 gCOD/m² d (COD: chemical oxygen demand), with the 80th percentile of the reported values being below 297 gCOD/m² d and the median at 164 gCOD/m² d; the highest OLR values have in all cases been measured during the peak season (vintage) and often have been linked to lower surface removal rates (SRRs) in comparison to the other periods of the year. With such OLRs the SRRs have ranged from a minimum of 15 up to 4,700 gCOD/m² d, with the 80th percentile of the reported values being below 308 gCOD/m² d and the median at 112 gCOD/m² d.

Performance evaluation and kinetic modeling of the start-up of a UASB reactor treating municipal wastewater at low temperature

A kinetic modeling-based study was carried out to evaluate the start-up performance of a 10-L up-flow anaerobic sludge blanket (UASB) reactor treating municipal wastewater under different organic and hydraulic loading conditions. The reactor was operated for 105 days (around 4 months) below 20 °C and with three different hydraulic retention times of 24, 12 and 5 h. Imposed volumetric organic loading rates (OLR) ranged from 0.57 (±0.05) to 11.78 (±0.85) kg TCOD/m(3)-day. Although relatively high incoming volumetric OLR values were employed to the system, the UASB reactor demonstrated a favorable performance on the anaerobic treatability of municipal wastewater, and no process failure was recorded in the start-up stage. On the basis of experimental results, the modified Stover-Kincannon model was successfully applied to define the start-up kinetics with a very high value of the correlation coefficient (R = 0.9729). Maximum substrate utilization rate constant and saturation constant of the modified Stover-Kincannon model were determined as U (max) = 1.996 g/L-day and K (B) = 1.536 g/L-day, respectively.

Treatment of wastewater from red and tropical fruit wine production by zeolite anaerobic fluidized bed reactor

A study of the anaerobic treatment of wastewaters derived from red (RWWW) and tropical fruit wine (TFWWW) production was carried out in four laboratory-scale fluidized bed reactors with natural zeolite as bacterial support. These reactors operated at mesophilic temperature (35°C). Reactors R1 and R2 contained Chilean natural zeolite, while reactors R3 and R4 used Cuban natural zeolite as microorganism support. In addition, reactors R1 and R3 processed RWWW, while reactors R2 and R4 used TFWWW as substrate. The biomass concentration attached to zeolites in the four reactors studied was found to be in the range of 44–46 g volatile solids (VS)/L after 90 days of operation time. Both types of zeolites can be used indistinctly in the fluidized bed reactors achieving more than 80%–86% chemical oxygen demand (COD) removals for organic loading rates (OLR) of up to at least 20 g COD/L d. pH values remained within the optimal range for anaerobic microorganisms for OLR values of up to 20 and 22 g COD/L d for RWWW and TFWWW, respectively. Toxicity and inhibition levels were observed at an OLR of 20 g COD/L d in reactors R1 and R3 while processing RWWW, whereas the aforementioned inhibitory phenomena were not observed at an OLR of 24 g COD/L d in R2 and R4, treating TFWWW as a consequence of the lower phenolic compound content present in this substrate. The volatile fatty acid (VFA) levels were always lower in reactors processing TFWWW (R2 and R4) and these values (< 400 mg/L, as acetic acid) were lower than the suggested limits for digester failure. The specific methanogenic activity (SMA) was twice as high in reactors R2 and R4 than in R1 and R3 after 120 days of operation when all reactors operated at an OLR of 20 g COD/L d.

Performance of an immersed membrane bioreactor (IMBR)

Due to stringent environmental regulations and standards on wastewater reuse and disposal, conventional treatment processes may not be able to cope with the new stringencies. Therefore, alternative processes need to be implemented in the wastewater treatment industry, provided that their technical and economic feasibilities are taken into consideration. The technical feasibility of a treatment process is determined by investigating the performance of that process under different operational conditions. In this study, the performance of immersed membrane bioreactor (IMBR) in treating synthetic municipal wastewater was investigated using laboratory-scale experimental setup at different mixed liquor suspended solid (MLSS) concentrations of 3,000, 5,000, 10,000, and 15,000 mg/l and organic loading rates (OLR) ranging between 0.11 and 1.19 kg COD/kg MLSS/day. The results showed that CODremoval efficiency that was affected by the MLSS concentration was in the range of 80 to 98%. Increasing the MLSS concentration was found to increase the COD removal efficiency. With respect to the cumulative permeate flux, the results showed that it was ranging between 27 and 36 l/m2/hr. The hydraulic retention time, which was affected by the permeate flux was in the rage of 12 to 15 hours. Sludge production was found to be on the average of 0.26 mg VSS per mg COD, which is 35% less than that reported for conventional activated sludge processes. The organicshock loading investigation showed that the IMBR process was not significantly affected by increasing the OLR values. Phenol and chromium were found to have short-term detrimental effect on the performance of the processand the COD removal efficiency was found to resume to its original values whence the toxicants were ceased. Ingeneral, the results of the investigation revealed that immersed membrane bioreactors are potential alternative wastewater treatment processes, particularly, when the reuse of wastewater is considered as a vital option.

Continuous methanogenesis of black liquor of pulp and paper mills in an anaerobic baffled reactor using an immobilized cell system

AbstractAn anaerobic baffled reactor together with an immobilized cell system has been proposed for methanogenesis of the black liquor from pulp and paper mills in a continuous system. A maximum chemical oxygen demand reduction of 50%, and biogas generation of 10 L d−1, having methane content of 66% (v/v) at an organic loading rate (OLR) of 7 kg m−3 d−1 with hydraulic retention time of 2 days, were recorded. OLR values higher than 7 kg m−3 d−1 were toxic to methanogenesis and destabilized the reactor system. Copyright © 2006 Society of Chemical Industry

Posttreatment of upflow anaerobic sludge blanket-treated industrial wastewater by a rotating biological contactor.

The performance of a rotating biological contactor (RBC) for posttreatment of the slaughterhouse effluent from an upflow anaerobic sludge blanket (UASB) reactor was investigated in this study. The 280-L, six-stage RBC pilot plant was operated at different organic loading rates (OLRs) and biodisk speeds. The overall removal efficiencies for soluble biochemical oxygen demand (SBOD), total biochemical oxygen demand (TBOD), and total chemical oxygen demand (TCOD) decreased with increasing OLRs. Disk rotational speed did not have a significant effect on performance in the range studied. The results showed that satisfactory posttreatment to meet regulatory requirements for agricultural purposes (effluent biochemical oxygen demand of 100 mg/L) can be achieved at an OLR of 5.3 +/- 2.9 g SBOD x m(-2) x d(-1), with an SBOD removal efficiency of 85 +/- 3%. Most of the organics were removed in the first three stages, with minimal contribution from the remaining stages of the RBC reactor. There was a decrease in SBOD removal efficiency to 74 +/- 3% at an OLR value of 17.8 +/- 2.1 g SBOD x m(-2) x d(-1). The results for elimination capacity indicated a linear relationship with first-stage OLRs without any signs of limitation at the range of loading rates investigated in this study. However, average first-stage elimination capacity rates of 4.8 and 3.8 g x m(-2) x d(-1) at OLR values of 17.8 and 11 g SBOD x m(-2) x d(-1) were relatively lower than previous studies of RBC performance using domestic or industrial wastewater without anaerobic pretreatment. It was suggested that the lower elimination capacity rates were due to the fact that a smaller fraction of UASB effluent was biodegradable as reflected in TBOD/TCOD ratios of 0.47 +/- 0.04.

The effect of organic loading rate on the performance of UASB reactor treating slaughterhouse effluent

Organic loading rate (OLR) is an important parameter significantly affecting microbial ecology and characteristics of UASB systems. In this study, UASB performance was evaluated in a 1000 1 reactor receiving feed from a traditional medium-size slaughterhouse. The initial seed for granules formed earlier was from a mesophilic municipal anaerobic digester sludge with a VSS content of 29 g l −1. The temperature of influent was adjusted by an inline thermostat around 33 °C. The reactor was started with an OLR of 5 kg SCOD m −3 d −1 with gradual increase to 10 kg SCOD m −3 d −1 over a 2-week period. Examination of VSS data showed that on the average 89.3±11.3% of bioparticle mass was present at the lower 30% of the reactor height. Under steady state conditions, experiments were conducted at OLRs of between 13 and 39 kg SCOD m −3 d −1 and hydraulic retention times (HRT) of 2–7 h. Removal efficiencies in the range of 75–90% were achieved at feed SCOD concentrations of 3000–4500 mg l −1. A reduction in removal efficiency to as low as 67% could have been related to a combined effect of high OLR and low HRT. Up to 300 l of methane were produced per kilogram of SCOD removed at OLR values of less than 30 kg COD m −3 d −1 but methane production rate seemed to decline to below 200 at higher OLR values. No sign of cell washout was observed at high OLRs and sludge loading rates (SLR) of up to 2.7 kg SCOD kg −1 VSS d −1. Elimination capacity of the reactor consistently increased from 9 to 25 kg SCOD m −3 d −1 corresponding to 1–2 kg SCOD kg −1 VSS d −1. Solids retention time (SRT) calculations for the reactor indicated a range of 3.3 days at high upflow velocity of 1 m h −1 to 60.3 days at low upflow velocity of 0.33 m h −1 s during different phases of the study.

Anaerobic treatment of landfill leachate by sulfate reduction

The present study was conducted to investigate the effectiveness of the sulphate-reduction pathway in the anaerobic treatment of landfill leachate. The effects of several COD/SO4 ratios (keeping COD constant) and loadings on anaerobic filter performance were studied and compared with the results from anaerobic filters which followed the methanogenic pathway. Results indicated that the treatability of leachate by sulphate reducing bacteria (SRB) was dependent upon the leachate strength. With high strength leachate (COD = 15,000 mg/L) from the Keele Valley Landfill, it was found that at lower COD/SO4 ratios (< or = 1.6) toxic conditions developed in the system that were more inhibitory to the SRB than to the methane producing bacteria (MPB). As the COD/SO4 ratio increased, methanogenesis predominated. No predominance of SRB occurred at any COD/SO4 ratio with high strength leachate. The highest COD removal achieved was about 70% of which 20% was accomplished by the SRB at a COD/SO4 ratio of 1.6 and an organic loading rate (OLR) of 4 kg COD/m3.d. With low strength leachate (COD = 1500-3300 mg/L) from the Brock West Landfill, and a COD/SO4 ratio < or = 1, SRB became predominant. In these anaerobic filters in which SRB were predominant, the SRB reduced the COD as well as the MPB could. Sulphide inhibition did not take place at any loading in units treating low strength leachate. Consequently, both SRB and MPB should function at COD/SO4 ratios between 1 and 3. About 60% COD removal was achieved at a loading of 2.8 kg COD/m3.d and a COD/SO4 ratio of 1.0. However at a loading of 6 kg COD/m3.d only 27% COD removal was achieved, all of it through the sulphate-reduction pathway. These OLR values are comparable to those applied in systems where methanogenesis was dominant. It was also observed that once the methanogens were established in the units, it was not possible to displace them completely. However, where methanogenesis had not been previously established, it was found that sulphate-reduction could be the sole pathway for COD removal. From this study, it can be concluded that there is no advantage to the sulphate-reduction pathway in the anaerobic treatment of landfill leachate. The other options for increasing the loadings, i.e. the use of high surface/volume filter media (to achieve higher biomass concentrations) or high rate systems are likely to be more successful.

Hydrogen monitoring in anaerobic sludge bed reactors at various hydraulic regimes and loading rates

ABSTRACT: The dissolved hydrogen concentration was monitored in four laboratory‐scale upflow sludge bed reactors. The upflow velocity (υUP) differed in each reactor, providing different hydraulic regimes. Five pseudo‐steady‐state conditions were applied to the four reactors. The dissolved hydrogen concentration increased proportionally with the organic loading rates (OLR) of the reactors, to attain values as high as 20 μM, while still remaining independent of the hydraulic regime. The experimental data yielded low hydrogen mass‐transfer rates (kLa) and high ratios of the measured dissolved hydrogen concentration value to the equilibrium value calculated from the hydrogen partial pressure in the gas phase (sursaturation factor): from 0.04 to 0.46 h−1 and from 19 to 102, respectively. Also, the increase in the υUP did not affect the hydrogen kLa nor the sursaturation factor. The kLa values showed a reasonable and positive correlation with the OLR values and the gasflow rates, however no correlation was found between the sursaturation actor values and the latter parameters. This indicates that dissolved hydrogen concentration can not be predicted from hydrogen gaseous measurements in such reactors.