Organic Overloading Research Articles

Advanced Monitoring of an Anaerobic Pilot Plant Treating High Strength Wastewaters

The operation of an industrial pilot scale treating wastewater from a fibreboard-processing factory was monitored by an advanced system. The plant, an anaerobic hybrid UASB-UAF bioreactor (Upflow Anaerobic Sludge Blanket-Upflow Anaerobic Filter), was equipped with the following measurement devices: biogas flow-meter, feed and recycling flow-meters, thermometer Pt-100, biogas analyser (CH4 and CO), Hydrogen analyser and pH-meter. Other parameters such as alkalinity, Chemical Oxygen Demand (COD) and Volatile Fatty Acids (VFA) were determined off-line. All the on-line sensor measurements were monitored, through a PLC (Programmable Logic Controller), which indicated about the plant failures, including the measuring devices (giving messages or alarms to the operator) and provided the set points for the PLC. The pilot plant was started-up at an initial Organic Loading Rate (OLR) of 2 kg COD/m3.d (Hydraulic Retention Time (HRT) 5 days and 10 kg COD/m3), this value increasing up to 10 kg COD/m3.d by decreasing HRT to 1 day. The behaviour of the bioreactor during start-up and steady state operation was studied. After that, an experiment was performed to analyse the response of the bioreactor to an organic overload. From the results, different variables were evaluated as useful control parameters. Monitoring of CO concentration did not permit the prediction of destabilisation of the bioreactor. However, H2 concentration is quite a sensitive variable, which must be analysed together with other parameters such as methane composition or gas flow-rate. Besides, alkalinity is easy to measure and provides immediate information about the state of the plant, as was shown through the off-line measurements.

Advanced monitoring of an anaerobic pilot plant treating high strength wastewaters

The operation of an industrial pilot scale treating wastewater from a fibreboard-processing factory was monitored by an advanced system. The plant, an anaerobic hybrid UASB-UAF bioreactor (Upflow Anaerobic Sludge Blanket-Upflow Anaerobic Filter), was equipped with the following measurement devices: biogas flow-meter, feed and recycling flow-meters, thermometer Pt-100, biogas analyser (CH4 and CO), Hydrogen analyser and pH-meter. Other parameters such as alkalinity, Chemical Oxygen Demand (COD) and Volatile Fatty Acids (VFA) were determined off-line. All the on-line sensor measurements were monitored, through a PLC (Programmable Logic Controller), which indicated about the plant failures, including the measuring devices (giving messages or alarms to the operator) and provided the set points for the PLC. The pilot plant was started-up at an initial Organic Loading Rate (OLR) of 2 kg COD/m3.d (Hydraulic Retention Time (HRT) 5 days and 10 kg COD/m3), this value increasing up to 10 kg COD/m3.d by decreasing HRT to 1 day. The behaviour of the bioreactor during start-up and steady state operation was studied. After that, an experiment was performed to analyse the response of the bioreactor to an organic overload. From the results, different variables were evaluated as useful control parameters. Monitoring of CO concentration did not permit the prediction of destabilisation of the bioreactor. However, H2 concentration is quite a sensitive variable, which must be analysed together with other parameters such as methane composition or gas flow-rate. Besides, alkalinity is easy to measure and provides immediate information about the state of the plant, as was shown through the off-line measurements.

Multi-Fed Upflow Anaerobic Filter: Development and Features

To overcome two of the main problems intrinsic to upflow anaerobic filters used to treat industrial waste water, clogging and poor efficiency in the upper zone, a new feeding strategy consisting of distributing the influent flow through several entrances along the system was studied. Two identical upflow anaerobic filters, a single-fed reactor (SFR) and a multi-fed reactor (MFR), were used with synthetic substrate of {approximately}10-kg chemical oxygen demand (COD)/m{sup 3} at an organic loading rate of up to 32-kg COD/m{sup 3} {center_dot} day. After more than 1 year of operation, the MFR strategy proved to be more efficient than the SFR, in terms of COD removal as well as with regard to stability against hydraulic and organic overloads. The MFR developed a more homogeneous bacteria consortium throughout the filter, therefore the specific methanogenic and nonmethanogenic activities are very similar in the bottom and the upper part of the filter. The MFR does not need a recycling flow to increase mixing. Moreover, the working volume of the MFR was higher than 85%, while only 65% of SFR remained active after 4310 days.

Evaluating composite ion exchangers (CIX) for improved stability of anaerobic biological reactors

An extensive laboratory study demonstrated that the stability of anaerobic biological processes can be improved and the process failure prevented by retaining specialty functionalized polymers inside the reactors. Such functionalized polymers, referred to as composite ion exchanger (CIX), are essentially microbeads of chelating exchanger encapsulated in fibrous, cloth-like polytetrafluoroethylene or Teflon. During the course of the investigation, severe stresses were introduced into a train of anaerobic reactors through organic and heavy-metal overloading of the feed. The reactors containing CIX successfully survived the stresses by dissipating the inhibitory hydrogen ions and dissolved heavy metals from the aqueous phase. The reactors containing no CIX always failed under stress conditions, all other conditions remaining identical. The CIXs were found to be chemically stable, durable, did not require any intermittent cleaning or regeneration, and occupied less than one percent of the reactor volume. Due to its thin-sheet like morphology, CIX can be easily introduced into or withdrawn from a biological medium without any difficulty. The proposed control technique does not warrant any external intervention and may replace the need for continuous dosage of excess alkalinity.

Fuzzy control of disturbances in a wastewater treatment process

This paper describes a real-time process control scheme to cope with the problem of input disturbances in wastewater treatment processes, based on a fuzzy inferential control system. This can detect the presence of a dangerous input conditions whenever either organic overload or inhibitory/toxic compounds are present in the raw wastewater. Based on this diagnosis, a set of fuzzy rules are implemented to divert the process flow and bring the system back into a safe state. The control system was designed and tested using a pilot plant, to which a toxic disturbance was applied. Its behaviour with and without fuzzy control are compared, and the process reliability improvement in the controlled case is demonstrated.

Neural network and on-off control of bicarbonate alkalinity in a fluidised-bed anaerobic digester

A laboratory-scale fluidised-bed anaerobic digester with a sintered glass carrier, Siran®, was operated for 8 months on a simulated baker's yeast wastewater (12,000 mg soluble COD l −1) at a loading rate of 27 kg COD m −3 d −1, giving 75% removal of soluble COD. Percentage CO 2, H 2 concentration, gas flow rate and pH were measured continuously. An on-line bicarbonate alkalinity (BA) monitor was used in experiments comparing two control strategies, adjusting digester buffering by addition of NaHCO 3 solution during organic overloads. The first, an on-off controller with a set point at the steady-state level (2700 mg CaCO 3 l −1), maintained BA concentration but resulted in levels above the upper set point. Thus, to avoid consuming excess NaHCO 3 the rate of delivery and solution strength must be carefully adjusted. The second was a controller developed from a neural network trained on BA data from an anaerobic filter operating on ice-cream processing wastewater (alkalinity around 1400 mg CaCO −1). Without re-training, despite the different steady-state BA levels and reactor type, the neural network based controller was capable of maintaining stable BA levels during overload without overshoot. Control of BA during overloads did not prevent changes in gaseous CO 2 and H 2 concentrations and gas flow rate.

Comment on “One-dimensional modeling of secondary clarifiers using a concentration and feed velocity-dependent dispersion coefficient” by R. W. Watts, S. A. Svoronos and B. Koopman, Wat. Res.30, 2112–2124 (1996)

Pulp and paper mills generate large amounts of waste organic matter that may be converted to renewable energy in form of methane. The anaerobic treatment of mill wastewater is widely accepted however, usually only applied to few selected streams. Chemical oxygen demand (COD) removal rates in full-scale reactors range between 30 and 90%, and methane yields are 0.30–0.40 m3 kg−1 COD removed. Highest COD removal rates are achieved with condensate streams from chemical pulping (75–90%) and paper mill effluents (60–80%).Numerous laboratory and pilot-scale studies have shown that, contrary to common perception, most other mill effluents are also to some extent anaerobically treatable. Even for difficult-to-digest streams such as bleaching effluents COD removal rates range between 15 and 90%, depending on the extent of dilution prior to anaerobic treatment, and the applied experimental setting. Co-digestion of different streams containing diverse substrate can level out and diminish toxicity, and may lead to a more robust microbial community. Furthermore, the microbial population has the ability to become acclimated and adapted to adverse conditions. Stress situations such as toxic shock loads or temporary organic overloading may be tolerated by an adapted community, whereas they could lead to process disturbance with an un-adapted community. Therefore, anaerobic treatment of wastewater containing elevated levels of inhibitors or toxicants should be initiated by an acclimation/adaptation period that can last between a few weeks and several months. In order to gain more insight into the underlying processes of microbial acclimation/adaptation and co-digestion, future research should focus on the relationship between wastewater composition, reactor operation and microbial community dynamics. The potential for engineering and managing the microbial resource is still largely untapped.Unlike in wastewater treatment, anaerobic digestion of mill biosludge (waste activated sludge) and primary sludge is still in its infancy. Current research is mainly focused on developing efficient pretreatment methods that enable fast hydrolysis of complex organic matter, shorter sludge residence times and as a consequence, smaller sludge digesters.Previous experimental studies indicate that the anaerobic digestibility of non-pretreated biosludge from pulp and paper mills varies widely, with volatile solids (VS) removal rates of 21–55% and specific methane yields ranging between 40 and 200 mL g−1 VS fed. Pretreatment can increase the digestibility to some extent, however in almost all reported cases, the specific methane yield of pretreated biosludge did not exceed 200 mL g−1 VS fed. Increases in specific methane yield mostly range between 0 and 90% compared to non-pretreated biosludge, whereas larger improvements were usually achieved with more difficult-to-digest biosludge. Thermal treatment and microwave treatment are two of the more effective methods. The heat required for the elevated temperatures applied in both methods may be provided from surplus heat that is often available at pulp and paper mills. Given the large variability in specific methane yield of non-pretreated biosludge, future research should focus on the links between anaerobic digestibility and sludge properties. Research should also involve mill-derived primary sludge. Although biosludge has been the main target in previous studies, primary sludge often constitutes the bulk of mill-generated sludge, and co-digestion of a mixture between both types of sludge may become practical. The few laboratory studies that have included mill primary sludge indicate that, similar to biosludge, the digestibility can range widely. Long-term studies should be conducted to explore the potential of microbial adaptation to lignocellulosic material which can constitute more than half of the organic matter in pulp and paper mill sludge.

Using biotic indices to estimate macrobenthic community perturbations in the Bay of Brest

Shallow-water macrofaunal communities have been studied in the Bay of Brest in order to estimate the impact of the main sources of urban, industrial, harbour and agricultural perturbations. Sampling stations were located on the shallow muddy grounds, between 0 and 5 m depth and in the estuaries. The analytic method of evaluation was based on the recognition of ecological groups of different sensitivity to organic matter overload. Their relative abundances allowed identification of stages of perturbation including eutrophication. The heavily polluted areas were easily identified in the northern basin, along Brest city and its harbour complex. With respect to the latter, opposite situations were shown for the northern and southern basins (the northern being eutrophicated). The double statistical analysis revealed that the first factor structuring the communities was anthropogenic perturbations, the second was the edaphic factor.

Shock load modelling in the anaerobic digestion process

A simplified mathematical model for the behaviour of anaerobic digesters under shock loading conditions is derived with a special emphasis on bicarbonate alkalinity. This study was motivated by the development of a novel bicarbonate alkalinity measuring device providing on-line information of this variable, which is particularly well suited for the early detection of organic overloading. The model includes a set of dynamic equations and an extended ion balance to describe the effect of alkali, volatile fatty acids and carbon dioxide. After the structural description, the model is calibrated using specific shock experimental data and its implications are discussed in terms of a variety of shock conditions. Lastly, the feasibility of bicarbonate dosing control is assessed through proportional-integral-derivative (PID) controller design and model simulation.

Monitoring metabolic activity of denitrification processes using gas production measurements

The aim of this study was to determine the impact of biological parameters on the overall performance of a biological denitrification process. A method for evaluating the metabolic activity of the microorganisms was developed based on measuring nitrogen production rate. The method was found to be particularly valuable in monitoring batch and continuous-flow processes and confirmed the existence of several phases in denitrification. Nitrate and organic overload imposed upon the denitrification process were found to be well reflected by the gas production rate, but did not have a significant long-term impact on the denitrification performance of the process. The influence of pH on the denitrification performance was studied. The results showed that the specific activity of adapted denitrifying microorganisms was not affected by a modification of the pH control set-point. However, it was also found that small (± 0.5 unit), short-term variations in the pH control set-point affect the performance of non-adapted denitrifying micro-organisms.

Stabilization ponds with recirculation

The first facultative pond in a series of stabilization ponds, or else the first part of a large pond, is sensitive to organic overloading creating anoxic or anaerobic conditions at the pond's surface, resulting in malodors and nuisances. Such adverse characteristics are usually manifested seasonally when climatic conditions change to lower temperatures and/or reduced solar irradiance. The design organic loadings on such ponds are therefore determined by the critical season and they are lowered accordingly. Introducing recirculation of effluent from a later pond in the series (usually from the second or third pond) back to the inlet of the first one, at a ratio of 1.0 - 2.5 (recirculated effluent) to 1 (raw sewage influent), can be most advantageous, as follows: (1) organic loadings on the first facultative pond in the series can reach 400-600 kg BOD5 per hectare per day (khd) during summer time and 300-400 khd as a yearly average, compared with a yearly average of 60-140 khd on ordinary facultative ponds, while still maintaining odor-free facultative conditions; (2) reseeding the first pond with active adapted algal biomass; (3) mixing the influent (which is often septic) with oxygen-rich recirculated effluent, thus enhancing the biological process in the first pond and suppressing septic odors, and (4) the increased inlet flow (by combining influent flow with the recirculated effluent) increases the area of solids (sludge) settling in the first pond. Altogether, the recirculation is manifested by reduced land requirements, better stability in pond operation, improved pond's performance and reduction or elimination of malodors and nuisances. Step feeding of the ponds further accentuates the effect of recirculation. Obviously, recirculation requires pumping (low head), energy, piping and connection to a power supply. The cost of operation and maintenance amount to US $0.01-0.02 per cubic metre of treated wastes. The advantages of recirculation nevertheless significantly outweigh the added costs. The paper describes the experience and data which have been gathered during the operation of 120 hectares of ponds with recirculation in the Dan Region (Greater Tel-Aviv) over a period of almost 20 years.

Preliminary Results of the Anaerobic Biotreatability of the Effluent from the Wet‐Air Oxidation of Sewage Sludge

ABSTRACTWet air oxidation and other thermal processes can be used on all sludges, whether they are raw, mixed or digested. The process sterilizes the sludge and gives a considerable reduction in volume. However, apart from the high capital costs which are associated with this technique, one of its principal disadvantages is the high strength of the (effluent) liquor.This study investigated the feasibility of treating the liquor using an anaerobic digester, which resulted in a good reduction in BOD and a reasonable reduction in COD. The liquor was still strong, but could be returned to the inlet works of a sewage‐treatment plant without causing significant organic overloading.

Substrate competition between methanogens and acetogens during the degradation of methanol in UASB reactors

To evaluate the effect of methanol concentration and inorganic carbon on the competition between acetogens and methanogens for methanol, eight upflow anaerobic sludge blanket reactors were operated continuously with different levels of sodium bicarbonate at variable methanol loadings. The results indicated that methylotrophic methanogens will predominate in a broad pH range (4.2–7.2) if either the methanol concentration in the effluent or inorganic carbon is low. Continuous steady state methane production resulted in chemical oxygen demand (COD) removals of 99.8–54% at organic loadings ranging from 7.6 to 22 g COD·l −1·d −1. On the other hand, significant acetogenesis occurred if exogenous inorganic carbon was supplied and if the methanol concentration in the effluent was greater than 1000 mg COD·l −1. The observation that acetogenesis only occurred in the presence of high methanol concentrations in the effluent confirms the previously determined 60-times-higher substrate affinity of methanogens. Steady state acetogenesis was only observed when unionized VFA, which inhibited the methanogens, were present at high levels for prolonged periods of time. Addition of moderate levels of NaHCO 3 (15 meq·l −1) were found to create such conditions since VFA accumulation exceeded the buffering capacity. Continuous acetogenesis from methanol at organic loading rates of 21 g COD·l −1·d −1 resulted in poor COD removal (16.3%). In contrast, high levels of NaHCO 3 supplementation (⩾25 meq·l −1) favured methane production since the alkalinity was strong enough to prevent toxic levels of undissociated acids even in periods when VFA accumulated as a result of organic overloading.

Stabilization ponds with recirculation

The first facultative pond in a series of stabilization ponds, or else the first part of a large pond, is sensitive to organic overloading creating anoxic or anaerobic conditions at the pond's surface, resulting in malodors and nuisances. Such adverse characteristics are usually manifested seasonally when climatic conditions change to lower temperatures and/or reduced solar irradiance. The design organic loadings on such ponds are therefore determined by the critical season and they are lowered accordingly. Introducing recirculation of effluent from a later pond in the series (usually from the second or third pond) back to the inlet of the first one, at a ratio of 1.0 – 2.5 (recirculated effluent) to 1 (raw sewage influent), can be most advantageous, as follows: (1) organic loadings on the first facultative pond in the series can reach 400–600 kg BOD5 per hectare per day (khd) during summer time and 300–400 khd as a yearly average, compared with a yearly average of 60–140 khd on ordinary facultative ponds, while still maintaining odor-free facultative conditions; (2) reseeding the first pond with active adapted algal biomass; (3) mixing the influent (which is often septic) with oxygen-rich recirculated effluent, thus enhancing the biological process in the first pond and suppressing septic odors, and (4) the increased inlet flow (by combining influent flow with the recirculated effluent) increases the area of solids (sludge) settling in the first pond. Altogether, the recirculation is manifested by reduced land requirements, better stability in pond operation, improved pond's performance and reduction or elimination of malodors and nuisances. Step feeding of the ponds further accentuates the effect of recirculation. Obviously, recirculation requires pumping (low head), energy, piping and connection to a power supply. The cost of operation and maintenance amount to US $0.01–0.02 per cubic metre of treated wastes. The advantages of recirculation nevertheless significantly outweigh the added costs. The paper describes the experience and data which have been gathered during the operation of 120 hectares of ponds with recirculation in the Dan Region (Greater Tel-Aviv) over a period of almost 20 years.

On-line automatic control system for monitoring an anaerobic fluidized-bed reactor: response to organic overload

The aim of this study was to develop an automatic system based on selected parameters collected from the biological process to control a fluidized-bed reactor. In the first part of this study, identification of parameters sensitive to overloads was carried out with vinasses as wastewater. From these results, three parameters, considered as the most realistic for the monitoring of the reactor, were selected: pH in the liquid phase, the biogas production and its concentration of dihydrogen. These were used to develop an algorithm based on expert system principle. This automatic control system calculated the flow rate of the feeding pump in order to adjust continuously the load applied to the reactor. The system was used to monitor and control several kinds of overload in an anaerobic fluidized bed reactor, of laboratory and pilot scales. The strategy used was able to operate automatically the digesters.

On-line monitoring of anaerobic digestion: application of a device for continuous measurement of bicarbonate alkalinity

Alkalinity or pH buffering capacity in anaerobic digesters is chiefly a function of bicarbonate levels, and should be within 10 to 50 mM for stable operation. A novel on-line instrument for measuring bicarbonate alkalinity has been developed, independent of titration techniques. Using a 10 litre anaerobic filter digester operating on ice-cream wastewater, on-line measurements were made of bicarbonate alkalinity, pH, gas production and % CO2 and hydrogen concentration in the biogas during periods of organic overload. The bicarbonate alkalinity monitor was shown to be an effective instrument for monitoring instability of anaerobic digestion, and a useful tool for early warning of overloading. Moreover, in digesters fed with wastewaters containing low potential alkalinity, it allows indirect on-line determination of variations in volatile fatty acid concentration. Finally, used in conjunction with a sensor that measures CO2 in the gas, it allows an indirect but accurate determination of pH in those solutions where fouling of electrodes is severe.

Development of an automatic control system for monitoring an anaerobic fluidized-bed

The start-up procedure of a laboratory fluidized-bed was monitored by an automatic control system. Twenty six days were necessary to increase the load from 1 to 35 kg COD/m3.d. The system took into account the pH of the liquid phase, the production of gas, and the concentration of hydrogen in the gas phase. These parameters were measured by on-line sensors. According to variations of these parameters, algorithm based on a principle very close to an expert system adjusted the flow-rate of the feeding pump. The automatic control system was tested in the following situations: perturbations in the process generated by large pH variations (± 0.2 to 0.7 pH unit) and organic overloads (from 50 to 100 kg COD/m3.d). After 44 hours of overloading, CH4 and CO2 concentrations in the gas phase were very useful to detect the deviation of the reactor.

The impact of four design parameters on the performance of a high‐solids anaerobic digestion of municipal solid waste for fuel gas production

Abstract The impact of four design parameters on the performance of a pilot scale high‐solids anaerobic digestion process is evaluated and discussed in this paper. Results obtained from this study indicate that the rate of methane gas production was inversely proportional to the average feedstock particle diameter. Feedstock with various C/N ratios, ranging from 8 to 125, were fed into the reactor and an optimum C/N ratio, based on biodegradable carbon and total nitrogen, was found to be about 25 to 30. Biogas production increased with an increase of organic loading rate up to a rate of 7.5 g BVS kg‐1 active reactor mass?d. Organic overload occurred at 7.6 g kg‐1 and continued up to 11.5 g kg‐1 active reactor mass at which point the volatile fatty acids accumulation in the reactor reached an inhibitory limit and caused digester failure. Mass retention times of 30, 20, and 15 days were investigated, while the organic loading rate was kept constant at about 7 g BVS kg‐1 active reactor mass. Shorter MRT's re...

A two‐phased anaerobic digestion process: Concept, process failure and maximum system loading rate

ABSTRACT: This research demonstrated the feasibility of a two‐phase anaerobic sludge digestion process. Process failure and maximum system loading capacity were also investigated. Three‐dimensional plots of chemical oxygen demand (sol), volatile fatty acids (VFA), and mixed liquor volatile suspended soils (MLVSS) profiles of both reactors during maximization and recovery periods (after failure) were made to evaluate the system feasibility. Hydrolysis‐acidification and acetogenesis‐methanogenesis dominated in the acid and methane phase reactors, respectively. Most of the reactions inside the methane‐phase reactor (M‐UASB) occurred in the lower part of the reactor (sludge bed and blanket). A combination of hydraulic and organic overloading of the M‐UASB reactor was a major cause of process failure; this was indicated by MLVSS washout, increase in total VFA concentration, reduction of the system removal efficiency, a drop in pH, an increase in total VFA/alkalinity ratio, a reduction of methane gas production, and an increase in the effluent total VFA. Because of system overloading, there was a shift in the metabolic pathway to a less favorable one, resulting in a shift of VFA producers (acidogens and acetogens) and consumers (methanogens, SRB, and NRB) ratio inside the reactor; this possibly led to a significant accumulation of carbon dioxide and hydrogen gas production. As a result, the partial pressure of hydrogen gas inside the system increased to a certain level, causing a shift in the metabolic pathway and accumulation of propionic acid. The maximum hydraulic loading rates achieved were 1.6 and 0.90 m3/m3 d, equivalent to hydraulic retention times of less than 0.63 and 1.13 days, for the M‐UASB and the whole system, respectively.

Mathematical modeling of an anaerobic butyrate degrading consortia: predicting their response to organic overloads

The steady-state and dynamic response of a butyrate using methanogenic consortia has been modeled as three sequential bacterial growth reactions. The model simulates the concentration of fermentation intermediates (acetate and hydrogen), incorporating their inhibition of butyrate oxidation and stimulation of methanogenesis. The hydrogen-producing acetogenic conversion of butyrate is modeled as a product-inhibited or reversible, modified Monod reaction, while hydrogen-using and acetoclastic methogenesis are simple Monod reactions. The growth yield coefficient for each growth reaction is variable and dependent on the Gibbs free energy of the energy reaction, which in turn depends on the time-varying concentrations of reactants and products