Sludge Growth Research Articles

Sludge production in membrane bioreactors under different conditions

With membrane bioreactors, the production of surplus sludge is lower than with conventional activated sludge systems, a fact that has been confirmed in a large number of analyses. There is, however, no consensus about the dimension of the reactions and their respective causes. In order to examine these, at the University of Hanover a pilot plant with a capacity of 220 l was run for one year without any extraction of surplus sludge. The plant was started with 2 g MLSS/l; after one year, this value had risen to approximately 18 g MLSS/l. In order to be able to set the plant for different sludge loads (0.04 to 0.2 kg COD/(kg MLSS · d)), the wastewater was artificially stocked up. The emerging result was that in contrast to conventional systems the sludge growth was lower, but still continuously existing. Then, comparisons with theoretical approaches were run – among others with the ASM1-Model – which confirmed the findings. One possible reason could be the different biocoenoses, which was assumed to be the cause after several microscopic examinations had been run.

A kinetic model for the prediction of sludge formation in a membrane bioreactor

A long-term experiment with a pilot membrane bioreactor (MBR) system to treat urban wastewater was carried out. Based upon theoretical inference and experimental data analysis, the kinetic properties of the MBR were comprehensively studied. An equation to calculate the sludge concentration in the MBR was successfully derived from the material balance of substrate and biomass, and expressed as X= Y× SRT 1+K d × SRT C i −C e HRT + C i −C sup SRT Consequently, the kinetic constants including the theoretical yield factor ( Y), the decay coefficient ( K d) and the declining growth constant ( K) were calculated as Y=0.56 mg VSS/mg COD, K d=0.08 day −1 and K=0.0026 day −1 using the operational data of the MBR. This equation was then satisfactorily employed to predict the trend of activated sludge growth during the experiment.

Determination of biotreatability of a kraft pulp bleaching wastewater based on conventional and spectrophotometric parameters

The biological treatability of a pulp bleaching wastewater from a conventional Kraft pulp mill was investigated in batch activated sludge reactors. Substrate removal was followed in terms of the parameters COD and BOD5 and by the measurement of spectrophotometric changes. These measurements provided information on the specific pollutant groups that were removed during biodegradation. The nonbiodegradable fraction of these wastes was assessed as 30–40% of the initial wastewater COD. Sludge growth was negligibly small under the present substrate to bio‐mass ratios. Negligible changes in chloride levels during aeration were indicative that no deha‐logenation of chlorinated compounds occurred.

Optimal operating policies in activated sludge treatment plants

A great number of industries, such as textiles, tanneries, chemicals, food processing, paper, among others, generate liquid waste (wastewater) which requires treatment prior to disposal. Whenever organic compounds must be removed by microorganisms, the most suitable treatment is activated sludge. During the treatment, there is an increase in the mass of activated microorganism, also known as sludge, which needs to be removed. The daily operation in industrial wastewater treatment plants is subject to disturbances in the flowrate as well as in the concentrations of the effluent streams. The operation of these plants usually requires manipulation of the inflow of the aeration tank, sludge recycle flowrate and sludge discharge flowrate. The purpose of this paper is to develop an optimization model to minimize the sludge discharge by manipulating the feed stream distribution in the aeration tank. The resulting model is a Nonlinear Programming (NLP) problem that generates the best operational strategy for the aeration tank feed streams so as to minimize the sludge growth with a high substrate removal. Results show that reductions of up to 30% can be achieved with the proposed strategy.

Effects of copper and zinc on the activated sludge bacteria growth kinetics

A simple experimental set-up has been proposed to study the influence of Cu(II) and Zn(II) on the activated sludge growth kinetics by following the rate of change on biomass concentration during batch growth experiments. Data on biomass were fitted to a sigmoidal equation providing us the main biokinetics parameters ( Y xs , and μ m). Inocula seeded to the system was obtained from a wastewater treatment plant operating at 14 days cell residence time. A synthetic feed solution containing 2000 mg/l gelatin (corresponding to 1630 mg/l COD) served as source of carbon. Different concentrations of Cu(II) and Zn(II) (1, 5, 10 and 20 mg/l) were introduced singly in the reactors keeping all environmental parameters constant (pH, T a, basic nutrients). The combined effects of Cu(II) and Zn(II) were determined by mixing these metallic ions (5/5, 5/10, and 10/5 mg/l of Cu(II)/Zn(II) respectively). Experimental data showed that Zn(II) was less toxic than Cu(II), as expressed by a slight stimulating effect for 1 mg/l Zn(II). Moreover, biokinetic parameters were not adversely affected by the presence of Cu(II) up to a concentration of 5 mg/l. However, a concentration of 10 mg/l and higher, caused serious upsets in the system. Combined effects of copper and zinc on the activated sludge growth kinetics, indicated that these two heavy metals acted neither synergistically nor antagonistically. The sigmoidal model very well fits the experimental data and could be used in a simulation study.

Methanogenic fermentation and growth of granular methanogenic sludge on a methanol-propionate mixture

The effect of methanol on the methanogenic fermentation of propionate and the growth of granular methanogenic sludge on a methanol-propionate mixture was studied. Degradation of propionate was markedly inhibited by the addition of methanol (0.5 to 8.6 g/ l) to the synthetic propionate basal wastewater. When the methanol was completely consumed by methanol-degrading bacteria, propionate degradation was restored. Upflow anaerobic sludge blanket (UASB) reactors were operated at a volumetric loading rate of 7.3 g propionate/ l·d with and without addition of methanol at a space velocity of 2.2 d −1. In a run of propionate basal wastewater, a propionate removal efficiency of more than 96% could be maintained, while efficiency decreased gradually to 25% with increasing methanol concentrations up to 8.6 g/ l, giving methanol removal efficiencies of more than 85%. In all runs, granular growth of methanogenic sludge occurred successfully, and larger-sized granules were obtained on methanol-propionate mixtures rather than on propionate basal wastewater. Microscopic observation and identification of corrinoid showed that Methanosarcina spp. was the predominant species of methanol-degrading bacteria, indicating that methanol was directly converted to methane. The results indicated that spontaneous growth of granules occurred on methanol-propionate mixtures and that methanol inhibition was due to the toxicity of methanol to propionate-degrading bacteria.

Nickel and copper removal at the San Jose/Santa Clara water pollution control plant

The fate of particulate and dissolved nickel (Ni) and copper (Cu) was determined at various stages in a 7.3 m3/s (167 mgd) tertiary treatment plant consisting of primary sedimentation, off‐line primary effluent equalization, first‐stage (secondary) activated sludge, second‐stage (nitrification) activated sludge, prefilter chloramination, filtration, disinfection and dechlorination. At micrograms/liter levels, particulate Cu and Ni removals were higher than dissolved Cu and Ni removals. Only secondary activated sludge removed the dissolved Cu. No other process removed either dissolved Cu or Ni. The dissolved Cu concentration in the second stage nitrification activated sludge plant effluent was higher than that in the first stage secondary activated sludge plant effluent. Prefilter chloramination solubilized particulate Cu and decreased the overall Cu removal by filtration. The dissolved Cu removed by the activated sludge systems varied seasonally; the higher removals seemed to be associated with increased activated sludge growth.

Anaerobic bioprocessing of organic wastes

Anaerobic digestion of dissolved, suspended and solid organics has rapidly evolved in the last decades but nevertheless still faces several scientific unknowns. In this review, some fundamentals of bacterial conversions and adhesion are addressed initially. It is argued in the light of ΔG-values of reactions, and in view of the minimum energy quantum per mol, that anaerobic syntrophs must have special survival strategies in order to support their existence: redistributing the available energy between the partners, reduced end-product fermentation reactions and special cell-to-cell physiological interactions. In terms of kinetics, it appears that both reaction rates and residual substrate thresholds are strongly related to minimum ΔG-values. These new fundamental insights open perspectives for efficient design and operation of anaerobic bioprocesses. Subsequently, an overview is given of the current anaerobic biotechnology. For treating wastewaters, a novel and high performance new system has been introduced during the last decade; the upflow anaerobic sludge blanket system (UASB). This reactor concept requires anaerobic consortia to grow in a dense and eco-physiologically well-organized way. The microbial principles of such granular sludge growth are presented. Using a thermodynamic approach, the formation of different types of aggregates is explained. The application of this bioprocess in worldwide wastewater treatment is indicated. Due to the long retention times of the active biomass, the UASB is also suitable for the development of bacterial consortia capable of degrading xenobiotics. Operating granular sludge reactors at high upflow velocities (5-6 m/h) in expanded granular sludge bed (EGSB) systems enlarges the application field to very low strength wastewaters (chemical oxygen demand < 1 g/l) and psychrophilic temperatures (10°C). For the treatment of organic suspensions, there is currently a tendency to evolve from the conventional mesophilic continuously stirred tank system to the thermophilic configuration, as the latter permits higher conversion rates and easier sanitation. Integration of ultrafiltration in anaerobic slurry digestion facilitates operation at higher volumetric loading rates and at shorter residence times. With respect to organic solids, the recent trend in society towards source separated collection of biowaste has opened a broad range of new application areas for solid state anaerobic fermentation.

Competition of methanogenic and sulfidogenic bacteria

The competition of acetotrophic methanogenic bacteria (AMB) and acetotrophic sulfidogenic bacteria (ASRB) was studied by assessing growth rates, activities and acetate and sulfate affinities at different pH levels and sulfide concentrations in batch reactors. Both anaerobic granular sludge and suspended anaerobic sludge were tested. The results indicate that at pH-levels below 6.9 AMB will outcompete ASRB, whereas above a pH of 7.7, ASRB will win the competition. If ASRB and AMB are present in granular sludge growth will be found in a wider pH-range than if they are present as suspended sludge. The affinities for acetate and the sulfide toxicity are dependent on the sludge form as well. In granular sludge the acetate affinities of ASRB and AMB are comparable, whereas in suspended sludge ASRB show a lower affinity than AMB. With respect to sulfide toxicity, the results indicate that above pH 7 sulfide inhibition in granular sludge is caused by the total sulfide concentration, while in suspended sludge the free H2S-concentration determines the toxicity. At high pH-levels growth is stronger inhibited than the activity.

Competition of methanogenic and sulfidogenic bacteria

The competition of acetotrophic methanogenic bacteria (AMB) and acetotrophic sulfidogenic bacteria (ASRB) was studied by assessing growth rates, activities and acetate and sulfate affinities at different pH levels and sulfide concentrations in batch reactors. Both anaerobic granular sludge and suspended anaerobic sludge were tested. The results indicate that at pH-levels below 6.9 AMB will outcompete ASRB, whereas above a pH of 7.7, ASRB will win the competition. If ASRB and AMB are present in granular sludge growth will be found in a wider pH-range than if they are present as suspended sludge. The affinities for acetate and the sulfide toxicity are dependent on the sludge form as well. In granular sludge the acetate affinities of ASRB and AMB are comparable, whereas in suspended sludge ASRB show a lower affinity than AMB. With respect to sulfide toxicity, the results indicate that above pH 7 sulfide inhibition in granular sludge is caused by the total sulfide concentration, while in suspended sludge the free H2S-concentration determines the toxicity. At high pH-levels growth is stronger inhibited than the activity.

Granular sludge growth under different reactor liquid surface tensions in lab-scale upflow anaerobic sludge blanket reactors treating wastewater from sugar-beet processing

The formation of anaerobic granular sludge on wastewater from sugar-beet processing was examined in upflow anaerobic sludge blanket reactors. Two strategies were investigated: addition of high-energy substrate, i.e. sugars, and varying the reactor liquid surface tension. When there were insufficient amounts of sugars i.e. less than 7% of the chemical O2 demand of the influent, no granulation was observed; moreover lowering the reactor liquid surface tension below 48 mN/m was found to increase biomass wash-out. On the other hand, when there were sufficient sugars, granular sludge growth occurred; moreover operating the reactor at a low reactor liquid surface tension reduced biomass wash-out and increased granular yield.

Anaerobic digestion and wastewater treatment systems

Upflow Anaerobic Sludge Bed (UASB) wastewater (pre-)treatment systems represent a proven sustainable technology for a wide range of very different industrial effluents, including those containing toxic/inhibitory compounds. The process is also feasible for treatment of domestic wastewater with temperatures as low as 14-16 degrees C and likely even lower. Compared to conventional aerobic treatment systems the anaerobic treatment process merely offers advantages. This especially is true for the rate of start-up. The available insight in anaerobic sludge immobilization (i.e. granulation) and growth of granular anaerobic sludge in many respects suffices for practice. In anaerobic treatment the immobilization of balanced microbial communities is essential, because the concentration of intermediates then can be kept sufficiently low. So far ignored factors like the death and decay rate of organisms are of eminent importance for the quality of immobilized anaerobic sludge. Taking these factors into account, it can be shown that there does not exist any need for 'phase separation' when treating non- or slightly acidified wastewaters. Phase separation even is detrimental in case the acidogenic organisms are not removed from the effluent of the acidogenic reactor, because they deteriorate the settleability of granular sludge and also negatively affect the formation and growth of granular sludge. The growing insight in the role of factors like nutrients and trace elements, the effect of metabolic intermediates and end products opens excellent prospects for process control, e.g. for the anaerobic treatment of wastewaters containing mainly methanol. Anaerobic wastewater treatment can also profitably be applied in the thermophilic and psychrophilic temperature range. Moreover, thermophilic anaerobic sludge can be used under mesophilic conditions. The Expanded Granular Sludge Bed (EGSB) system particularly offers big practical potentials, e.g. for very low strength wastewaters (COD << 1 g/l) and at temperatures as low as 10 degrees C. In EGSB-systems virtually all the retained sludge is employed, while compared to UASB-systems also a substantially bigger fraction of the immobilized organisms (inside the granules) participates in the process, because an extraordinary high substrate affinity prevails in these systems. It looks necessary to reconsider theories for mass transfer in immobilized anaerobic biomass. Instead of phasing the digestion process, staging of the anaerobic reactors should be applied. In this way mixing up of the sludge can be significantly reduced and a plug flow is promoted. A staged process will provide a higher treatment efficiency and a higher process stability. This especially applies for thermophilic systems.

Organic and ammonium nitrogen and oxygen in relation to granular sludge growth in lab-scale UASB reactors

Laboratory studies of in-reactor granular-sludge yield (Ygran) were carried out to investigate the effect of substrates rich in proteins. Both lab-scale upflow anaerobic sludge blanket (UASB) and shake-flask systems were used to monitor the behaviour of the sludge. Influent based on molasses with a COD to N ratio of 100:2.5-3.0 gave good Ygran; increasing substitution of the carbohydrate COD by protein COD resulted in a deterioration of the sludge characteristics. The negative effects appear to be related to the protein rather than to the NH4+ formed. Of the various environmental parameters examined, the surface tension of the mixed liquor was the main deviating factor. At a proper range of oxygenation i.e., by means of monitoring of both the input COD to oxygen ratio (gCOD gO2−1) and the daily oxygen loading rate (mgO2 gVSS−1.d−1), the UASB reactors treating the nitrogen-rich wastewater showed enhanced Ygran with only a minor reduction in methanogenesis. A range of working conditions in which this principle can be applied to decrease fluffy growth to the advantage of granular increase is proposed.

Evaporator Condensate and Caustic Extraction Liquor from a Pulp Factory Treated with an Anaerobic Process

Borregaard Industries Ltd, Norway, is a Ca-bisulphite based pulp manufacturer which has an interest in an anaerobic wastewater treatment process. At this pulp factory a combination of sulphite evaporator (SEC) and caustic extraction liquor (CEL) would eliminate the costs for neutralization chemicals in an anaerobic treatment process. The anaerobic process was also suitable because of the high concentration of organic compounds in CEL, the biogas production and the low sludge growth. Today a full-scale plant treating SEC in combination with CEL is in operation with a capacity to treat 60 tons of COD/d. All CEL qualities are treated anaerobically even those containing peroxide. The highest guaranteed AOX concentration in the CEL to be treated is 25 mg AOX/I. The experience from a 2-year pilot plant study has influenced the design of the full-scale plant. An adaption of the sludge to the CEL has to be performed due the toxic compounds in the CEL. The incoming wastewater to the full-scale plant has a composition of 50% COD from the SEC and 50% COD from the CEL. With an organic load of 40 tons COD/d, a SEC/CEL ratio of 51/49, the COD reduction is 65% and the gas yield 0.20 m3 CH4/kg COD added. This full-scale plant is unique due to the treatment of different CEL qualities and to the high combination of CEL.

Anaerobic digestion of palm oil mill effluent using an up-flow anaerobic sludge blanket reactor

Anaerobic treatment of palm oil mill effluent (POME) was studied using a 16-litre laboratory scale up-flow anaerobic sludge blanket reactor (UASB) run over a range of influent concentrations from 5.1 to 42.5 g Chemical Oxygen Demand (COD) per litre at a constant hydraulic retention time of four days. Methane production, volatile fatty acid conversion, net sludge growth and Chemical Oxygen Demand reduction were monitored. Over 96% Chemical Oxygen Demand was removed at loadings up to 10.6 g COD l −1 day −1. At the highest influent concentration reactor instability was observed. Up to this point the results indicated that the UASB could treat POME more effectively than other systems reviewed.

Determination of the Maintenance Requirements of Activated Sludge

Maintenance energy plays an important role both in basic kinetic studies and in process development. Numerous studies have been devoted to the maintenance concept in various microbial fields but very few in biological wastewater treatment. Using a fermenter coupled to a mass spectrometer, we investigated the influence of the ratio So/Xo (ratio between initial substrate concentration and initial biomass concentration) on the observed sludge growth yield of an oxic-settling anaerobic (OSA) system. By measuring the substrate removed, the oxygen consumed and the carbon dioxide produced, we were able to estimate the substrate fraction used for maintenance purposes. The results indicate that at a high So/Xo ratio, a greater proportion of the substrate is devoted to maintenance thus significantly decreasing the observed growth yield. These findings are of particular importance in view of the cost associated with the disposal of excess sludge in aerobic wastewater treatment processes.

Phosphorus requirement in high-rate anaerobic wastewater treatment

In this study the effect of phosphate limitation on reactor performance and on methanogenic activity was examined. Laboratory scale UASB reactors were fed with gelatin or volatile fatty acids as COD substrate while the phosphorus content of the substrate was varied. Phosphate uptake was investigated using the granular sludge which was developed in the phosphorus-deficient reactors. Phosphorus deficiency reduces methanogenic activity in UASB reactors to 50% of the control. This reduction was found to be reversible by dosage of phosphate. In batch experiments a complete recovery of the maximal methanogenic activity was observed by a dosage of 5 mg PO 4-PI −1. At low concentrations (0.1 g PO 4-Pl −1) phosphate dosage is also profitable. When the applied sludge loading rate was below the methanogenic activity of the sludge under phosphorus-limiting conditions, the treatment efficiency was not affected. The sludge growth however was slightly reduced. Overdosage of phosphate, which in practice is related to high effluent phosphate concentrations, was found to be unprofitable.

Acidogenesis in Relation to In-Reactor Granule Yield

A systematic study of in-reactor granule yield data was set up in the laboratory. Methanogenic granular sludge growth in fed batch shake flasks for different substrates appeared to decrease with the energy content of the substrate and required the presence of a mixing force. In UASB reactors, granular cell yields were found to be quite variable suggesting the involvement of complex microbiological interactions. The factor foremost in influencing the build-up in the reactor of granular sludge was the presence of high-energy carbohydrates; pre-acidified influents affected in-reactor granular sludge yield very negatively. The influence of sulphate reduction and sulphide concentrations was found not to be of primary importance. The ionic strength of the medium also had no strong influence. Granule formation proceeded somewhat better at low than at nigh strength buffer capacity. High levels of protein, however, reduced the in-reactor granular sludge build-up strongly. Measurement of the acid production by granular and fluffy sludges revealed that the former rapidly produce volatile acids, particularly propionate, while the latter do not. Measurement of the pH in the granule indicated that the outside layer of the granule is rich in acidogens. Calculations of proton and hydrogen fluxes in the granule support the concept that acidogens, able to ferment high energy carbohydrates efficiently to cells and exo-cellular binding materials, might be of primary importance for in-reactor granular growth.

Pilot plant investigations of the treatability of ethylene glycol by activated sludge

Abstract Activated sludge pilot plant studies were carried out in order to investigate the treatability of ethylene glycol, an organic chemical commonly used as a ‐coolant in the pharmaceutical industry. Four completely mixed, single‐stage, activated sludge units, with a capacity of 80‐litres were operated in parallel. The effect of various concentrations of the test chemical on the sludge settleability, the sludge growth, and the plant performance in terms of BOD and COD removal were examined during the study. It was concluded that a concentration of 0.1% ethylene glycol in the raw wastewater of the full‐scale plant was treatable, as higher concentrations appeared to cause a deterioration in the floc structure and to enhance Nocardia scum formation on the surface of the aeration basin.

固定床式嫌気性リアクターにおける汚泥増殖と空隙率（高濃度有機性廃水での運転例）

固定床式嫌気性リアクターにおける汚泥増殖と空隙率（高濃度有機性廃水での運転例）