Suspended Growth Systems Research Articles

Treatment of organic synthesis wastewater using anaerobic packed bed and aerobic suspended growth bioreactors

The performance of an anaerobic mesophilic packed bed reactor, with a mixture of GAC and tezontle, followed by an aerobic suspended growth system was studied for the treatment of organic chemical wastewater with a high COD concentration (22-29 g/L). The testing of the anaerobic-aerobic system was conducted in an experimental set-up for almost 2.5 years. Different operational conditions were evaluated. The anaerobic reactor showed performance stability and COD removals higher than 80% were obtained with loads up to 16.6 kg x m(-3) x d(-1). The acclimation of the aerobic biomass to the substrate in the anaerobic effluent was very quick and COD removals higher than 94% were obtained even at high organic loads. The combined anaerobic-aerobic system allowed total COD removals higher than 99.5% and the accomplishment of the discharge requirements of 200 mgCOD/L when the anaerobic reactor was operated with loads of 8-11 kg x m(-3)x d(-1) and the aerobic reactor with 0.33 kg x kg(-1) x d(-1), being the total HRT of 4.4. The average TKN removal in the anaerobic-aerobic system was 97%, the average for the anaerobic reactor being 52% and that one for the aerobic system being 94%.

Robust Optimal Design of Activated Sludge Bioreactors

This paper proposes an algorithm for a robust optimal design of the biological reactor and secondary settling facilities in suspended growth nitrogen and phosphorus removal systems. Robust optimization includes uncertainty in the decision-making procedure and seeks a solution that remains “close” to optimal for all potential operation scenarios. It thus differs fundamentally from the deterministic and stochastic approaches, where uncertainty is ignored or a solution based on either the most likely scenario or the average performance over all potential scenarios is produced. The robust optimization of a suspended growth system is a multiobjective optimization problem concerned with minimization of the global costs and variability of the system’s performance around the optimal. The proposed robust optimization approach uses the ASM3 model, making use of its performance prediction capabilities to produce a powerful tool for designing activated sludge systems. The algorithm was applied to the design of the bi...

Fixed film phosphorus removal – flexible enough?

While biological phosphorus removal (BPR) has been practised for 30 years, up to recently it has been restricted mainly to activated sludge processes, with the corresponding need for large basin volumes. Yet, research with biofilm reactors showed that the principle of alternate anaerobic and aerated conditions was applicable to fixed bacteria by changing the conditions in time rather than in space. Attached growth enhanced biological phosphorus removal (EBPR) systems are attractive because of their compactness and capability to retain high biomass levels. However, the phosphorus extraction depends on backwashes to enhance the phosphorus-rich attached biomass, and correct control of unsteady effluent quality created by frequently modified process conditions. Accordingly, EBPR remains a challenging task in terms of combining nitrogen and phosphorus removal using attached growth systems. Nevertheless, a combination of activated sludge and biofilm carriers, in the integrated fixed-film activated sludge system, provides treatment opportunities not readily available using suspended growth systems. Current practice is only at the beginning of exploiting the full potential of this combination, but the first full-scale results show that compact tankage and low nutrient results based on biological principles are possible.

Introducing a New Media for Fixed-Film Treatment in Decentralized Wastewater Systems

Both naturally occurring and artificial media have been used to treat wastewater in small community settings. Systems using naturally occurring media include sand filters and peatbased biofilters. Artificial media include open cell foam, textiles, and plastics. All the above operate as fixed-film systems, in that attached growth microorganisms are encouraged to colonize and reproduce in the media. Fixed film systems appear to produce a more consistent effluent under peaked sewage inputs and periods of no use (vacation homes, weekend retreats) compared to suspended growth systems in this flow range. Naturally occurring media have advantages in that naturally occurring fauna and flora may already be present in the media. Start up is therefore shortened because colonists are already present in the media and just need to multiply to handle the waste renovation process. A disadvantage of naturally occurring media is that they must be mined. Extraction of these natural resources is sometimes criticized because they formed over thousands of years. They will eventually be exhausted and will not be replaced. Quanics. Inc. has patented a product that combines advantages of both artificial and naturally occurring media. The Bio-COIR™ filter uses coir, the recycled husks of coconut in a biological fixed film filter. The material is a waste product of agricultural operations in developing countries. The product has successfully passed NSF Standard 40 certification. Systems in the ground for two years show virtually no change in media properties except a slightly darker color, giving the expectation that the media will continue to function for a comparable period to peat (8 to 16 years between media replacement).

Evaluation of kinetic parameters of a sulfur–limestone autotrophic denitrification biofilm process

In this study, four kinetic parameters of autotrophic denitrifiers in fixed-bed sulfur–limestone autotrophic denitrification (SLAD) columns were evaluated. The curve-matching method was used by conducting 22 non-steady-state tests for estimation of half-velocity constant, K s and maximum specific substrate utilization rate, k. To estimate the bacteria yield coefficient, Y and the decay coefficient, k d, two short term batch tests (before and after the starvation of the autotrophic denitrifiers) were conducted using a fixed-bed SLAD column where the biofilm was fully penetrated by nitrate-N. It was found that K s =0.398 mg NO 3 −–N/l, k=0.15 d −1, k d=0.09–0.12 d −1, and Y=0.85–1.11 g VSS/g NO 3 −–N. Our results are consistent with those obtained from SLAD biofilm processes, but different from those obtained from suspended-growth systems with thiosulfate or sulfur powders as the S source. The method developed in this study might be useful for estimation of four Monod-type kinetic parameters in other biofilm processes. However, cautions must be given when the estimated parameters are used because the measurements of the biomass and the biofilm thickness could be further improved, and the assumption of sulfur being a non-limiting substrate needs to be proved.

Photosynthetic suspended-growth systems in aquaculture

Standardized evaluation and rating of biofilters for aquaculture should be assessed in the context of the economic efficiency of ecological services (waste assimilation, nutrient recycling, and internal food production) provided by earthen ponds, and the availability and cost of land, water, and electrical energy resources required to support particular classes of production systems. In photosynthetic suspended-growth systems, water quality control is achieved by a combination of natural and mechanical processes. Natural processes include photosynthesis of oxygen, algal nutrient uptake, coupled nitrification–denitrification, and organic matter oxidation; mechanical processes include aeration and water circulation. Ammonia is controlled by a combination of phytoplankton uptake, nitrification, and immobilization by bacteria. Unlike biofilters for recirculating aquaculture systems, unit processes are combined and are an integral part of the culture unit. The important design and operational considerations for photosynthetic suspended-growth systems include temperature effects, aeration and mixing, quantity and quality of loaded organic matter, and fish water quality tolerance limits. The principle advantages of photosynthetic suspended-growth systems are lower capital costs relative to other recirculating aquaculture systems and increased control over stock management relative to conventional static ponds. The main disadvantage is the relatively low degree of control over water quality and phytoplankton density, metabolism, and community composition relative to other recirculating aquaculture systems. Examples of photosynthetic suspended-growth systems include semi-intensive ponds, intensively aerated outdoor lined ponds, combined intensive–extensive ponds, partitioned aquaculture systems, greenwater tanks, greenwater tanks with solids removal, and greenwater recirculating aquaculture systems.

Biological filters in aquaculture: Trends and research directions for freshwater and marine applications

Factors such as limitations in water quality and quantity, cost of land, limitations on water discharges, environmental impacts and diseases, are driving the aquaculture industry toward more intensive practices. This will force producers to adopt environmentally friendlier technologies. Recirculating systems, with a biofilter as the most prominent characteristic, treat internally the water contaminated with dissolved organics and ammonia and reduce the amount of water use and discharge from aquaculture operations. This paper reviews the implications of the changing use of recirculating aquaculture systems (RAS) on biofiltration research for freshwater and marine operations. Demand for cost effective biofilters will increase with the expansion of recirculating systems, both as a complement and replacement of traditional ponds. For freshwater aquaculture, emphasis should be placed in cost competitiveness, low head operations, intensification of ponds with RAS biofiltration and the evaluation of suspended growth systems. In the marine systems, an increase in demand of oligotrophic and ultraoligotrophic systems is expected, particularly in the nursery systems. Sizing and cost efficiency of biofilters for nursery operations should be addressed. Problems in marine biofilter acclimation appear to justify the development of new acclimation procedures. Biosecurity concerns, land cost and storm threats will drive nursery systems inland, where saltwater supply and disposal will force an increased water reuse. Denitrification strategies will need to be redefined and optimized for the marine nursery environment.

Cometabolic degradation of TCE in enriched nitrifying batch systems

The aim of this study was to evaluate the effect of the trace pollutant trichloroethylene (TCE) on the nitrification process and to assess its cometabolic degradation. Nitrification was accomplished in batch suspended growth systems containing an enriched nitrifier culture. The presence of TCE resulted in both the inhibition of specific oxygen uptake rate (SOUR) and specific ammonium utilization rate (qNH 4–N). In both SOUR and qNH 4–N a 50% decrease was observed in a TCE concentration range of 1000–2000 ppb. TCE was cometabolically degraded by this enriched nitrifier culture. The cometabolic degradation of TCE was found to be dependent on initial TCE concentration. The results may be applicable in the treatment of TCE containing industrial wastewaters and contaminated groundwaters and soils.

Comparison between biological and chemical treatment of wastewater containing nitrogen and phosphorus

The present work compared chemical and biological treatment methods to achieve the most efficient treatment for the reduction or elimination of phosphorus and nitrogen from mixed industrial-domestic wastewaters. Batch chemical precipitation by ferric chloride and aluminum sulfate (alum) and a continuous biological suspended growth system were investigated as well as the optimum operating conditions. Concerning chemical treatment, Alum generally achieved a higher removal efficiency percentage for the investigated pollutants compared with FeCl(3) at their optimum pH and dose, especially with chemical oxygen demand (COD). FeCl(3) treatment achieved success only with phosphorus removal, while none of the COD, 5-day biochemical oxygen demand (BOD(5)), total nitrogen (TN) and N-NH(3) achieved acceptable treatment and remained above the maximum permissible limits (MPL). Thus, for such wastewaters, alum is more efficient than FeCl(3). Biological treatment exhibited higher efficiencies, particularly towards nitrogen. TN removal increased by increasing the flow rate to 30-60 l/day. N-NH(3) removal was effective at the slowest flow rate and decreased with increasing flow rate, while an opposite trend was recorded for N-NO(3). At all flow rates, phosphorus levels were below the accepted MPL for discharging into natural systems. Moreover, there was a general trend for the proposed biological treatment to achieve a high removal efficiency for BOD(5) and COD, bringing them to acceptable levels to be released into watercourses safely, especially at the slowest flow rates. Thus, integration between the proposed chemical and biological treatment is highly recommended, producing high-quality effluents acceptable by the environmental law.

Diversity matters: Dynamic simulation of distributed bacterial states in suspended growth biological wastewater treatment systems

A MATLAB-based program was developed to simulate the distributions of states and behaviors of microbial storage product-accumulating bacteria in suspended growth systems. Currently available computer simulators of these systems predict dynamic behaviors by numerically solving differential biokinetic equations using average, or "lumped" system states (e.g., average microbial storage products concentrations). However, individual bacterial states are expected to diverge from average values, in part because individuals can have different hydrodynamic histories in terms of their residence times in upstream completely mixed flow reactors. The distributed state simulation program presented in this paper (DisSimulator 1.0) tracks individual bacteria as they move through a completely mixed reactor system. The program was evaluated for competition between polyphosphate-accumulating organisms (PAOs) and non-polyphosphate-accumulating heterotrophs in an enhanced biological phosphorus removal (EBPR) system for treatment of domestic wastewater. For identical systems and process conditions, simulations accounting for distributed states predicted larger anaerobic and aerobic solids residence time requirements for successful EBPR than did simulations using the lumped approach. One reason for this was that distributed simulations predicted large numbers of the PAOs were essentially inactive due to depleted or maximized storage product contents, while the lumped simulations predicted homogenous, 100% active PAO populations. Characteristic state profile shapes developed rapidly and were stable as total population numbers changed. Lumped state assumptions were demonstrated to produce large errors in predictions of EBPR system performance, and so consideration of distributed states may improve the accuracy of microbial storage products-based process simulations in systems with completely mixed hydrodynamics.

The influence of settling time on the formation of aerobic granules

Aerobic granular sludge, without the addition of carrier material, has only been reported in one suspended growth system, the Sequencing Batch Reactor (SBR) operated with short fill and settling periods. Recent studies have demonstrated that extracellular polysaccharides increased with the formation of aerobic granules, and that the shear force may stimulate production of these polysaccharides. In the study described herein, two SBRs were operated with the same shear force (air flow rate 275 L h(-1)) and two different settling times (2 and 10 min). Only the reactor with 2 min settling formed completely granular sludge, although granules were present in both reactors. Community analysis using 16S rRNA PCR products and DGGE showed that the communities diverged quickly after reactor start-up. For samples taken at steady-state, the granular population was more stable and less diverse than the flocculent reactor. EPS extraction of samples using cation exchange resin yielded similar values for aerobic granular sludge and previously reported anaerobic granules. While differences in the protein and TOC content between the flocculent and granular reactors increased appreciably as the sludge became more granular, the protein to polysaccharide ratio was relatively constant. The experiment confirmed previous theories that short settling times in SBRs select for granular sludge. The settling time results in granular sludge having a higher EPS protein content and a less diverse but more stable population.

Enhanced biological phosphorus removal in RBC with SBR modification

The rotating biological contactor (RBC) system was operationally modified with a sequencing batch reactor to achieve biological phosphorus removal from a weak domestic sewage along with nitrogen removal. This study utilized three RBC units, of which two units were the main units to remove phosphorus and NH4N and the third RBC unit was used as the storage of wastewater for its minimal effect to the PAO activities in the anaerobic stage during the operation. It was noticed that the biofilm thickness in RBC must be controlled to be less than 1.8 mm in order to achieve more than 70% of P removal with about 60% of N removal. With a settled sewage representing 200 mg/L of COD and 5 mg/L of P, the predicted P content in biofilm was more than 3% and the effluent P concentration was about 1 mg/L. The %P content in biofilm decreased with an increase of influent COD/TP ratios. The COD requirement for anaerobic P release was similar to reported values for the suspended growth system, however, the overall requirement increased with thicker biofilm.

Treatment of complex chemical wastewater in a sequencing batch reactor (SBR) with an aerobic suspended growth configuration

A sequencing batch reactor (SBR) has certain advantages over conventional activated sludge processes (ASP) for the treatment of complex wastewater. The performance of a sequencing batch reactor in treating complex chemical effluents was investigated with a suspended biomass configuration and operating under aerobic conditions. A total sequence of 24 h (15 min: filling phase; 23 h: reaction phase (aeration with recirculation); 30 min: setting; and 15 min: withdrawal) was employed and studied with various organic loading rates (1.0 kg COD/m 3/day: 1.7 kg COD/m 3/day and 3.5 kg COD/m 3/day). The SBR performance was assessed by means of carbon removal and operational parameters such as pH, oxidation–reduction potential (ORP), sludge volume (SV), sludge volume index (SVI), suspended solids (SS) and volatile suspended solids (VSS), which were monitored during reactor operation. The SBR showed relatively more efficient performance over conventional suspended growth systems. About 8% sulphate was removed under anoxic conditions at all the studied organic loading rates. Enhanced performance with SBR over conventional ASP may be due to enforced short term unsteady state conditions coupled with periodic exposure of the microorganisms (physiological state) to defined process conditions which facilitate the required metabolic conditions for treating complex chemical effluents.

The effect of intermittent feeding on aerobic granule structure

Self-immobilized biofilms, or aerobic granules without the addition of carrier material, have only been reported in one suspended growth system, the Sequencing Batch Reactor (SBR) with a very short fill time (dump fill). The SBR utilizes intermittent feeding which creates a period of high load followed by starvation (often referred to as feast-famine). In this experiment, three identical SBRs were operated with different feeding conditions to determine the role of feast-famine on granule formation. All three SBRs were operated with a total volumetric load of 2.4 kg/m3 x d. The 90 minute Fill phase was altered for each reactor, providing an increasing time of Aerated Fill. A dump fill condition was applied for one reactor, while the other two reactors were aerated for different times during Fill, resulting in a smaller COD load at the beginning of each React phase. Aerobic granules formed in all reactors, but the structural properties and content of filamentous organisms were clearly dependent on a high feast condition. Only the reactor with dump fill formed compact, stable granules. It is concluded that intermittent feeding associated with the SBR affects the selection and growth of filamentous organisms and has a critical role in granule structure and composition.

Phase distribution of hexachlorobenzene in a suspended-growth culture amended with a polysorbate surfactant.

The effect of a non-ionic surfactant on the phase distribution of hexachlorobenzene (HCB) in a suspended-growth culture system was assessed. Tween 60, a polyoxyethylene sorbitan stearate ester surfactant, and an azide-inactivated, mixed, methanogenic, HCB-dechlorinating culture were used in this work. The sorption of HCB on the biomass as well as sorption and aggregation/precipitation of Tween 60 were experimentally quantified. The values of the HCB and Tween 60 distribution parameters were determined and the phase distribution of HCB in the presence of surfactant and biomass was described quantitatively. Both the HCB and surfactant distribution are highly dependent on the total amount of surfactant present in the system. At low initial surfactant concentrations, most of the HCB is associated with the biomass. As the surfactant concentration increases, the effect of surfactant sorption and precipitation diminishes and the higher pool of surfactant micelles shifts the distribution of HCB from the solid phase to the solution phase. The HCB phase distribution in the presence of the surfactant may have a significant effect on HCB bioavailability for reductive dechlorination. The quantitative description of the HCB phase distribution presented here was subsequently used as the basis for the development of a model that describes the bioavailability and reductive dechlorination of HCB in a surfactant/biomass system.

Nitrification of high-strength ammonium wastewater by a fluidized-bed reactor

A laboratory-scale fluidized-bed reactor with an external aeration loop was used for nitrification of high-strength ammonium wastewater (up to 500 mg NH4-N/L). The results demonstrated that the system is capable of handling ammonium removal rates of up to 2.5 kg NH4-N/m3 x d, while removal efficiencies were as high as 98% and independent of the applied ammonium loading rates. Ammonium loading rates higher than 2.5 kg NH4-N/m3 x d resulted in decreased ammonium removal efficiency. The data show that near complete ammonium removal occurred at DO concentrations as low as 0.3-0.5 mg/L. However, the nitrite-nitrogen fraction in the effluent increased from 3.5% to 23.2% when the DO dropped from 1.0 mg/L to approximately 0.4 mg/L, respectively. The high specific removal rates in this system are one order of magnitude higher than that of suspended-growth systems. This can reduce the supplementary reactor volumes required for nitrification to less than 10% of that needed in conventional activated sludge systems. These results clearly indicate the potential economic gains that could be achieved through implementation of this technology.

EMMC process for combined removal of organics, nitrogen and an odor producing substance

In order to improve the process performance regarding the removal of organics, nitrogen, and an odor-causing compound (sulfide) contained in domestic wastewater, an entrapped-mixed-microbial cell (EMMC) with and without humic substances for both fixed and moving carrier reactors and conventional suspended growth culture (i.e. conventional activated sludge process) were investigated simultaneously. Both synthetic (simulated to the organics concentration of general domestic sewage) and actual domestic wastewater were investigated under operational conditions of 12 h of hydraulic retention time (HRT) with 1 h of aeration and 1 h of non-aeration, and 6 h of HRT with continuous aeration, at a room temperature of 25±2 °C. It was found that entrapping humic substances in the EMMC carriers had no impact on the removal of organics, nitrogen, and the odor-producing compound. Additionally, the performance of the EMMC moving carrier system for the removal of these pollutants is similar to that of the EMMC fixed carrier system. In general, the EMMC associated systems which provide high solids retention time achieve a better removal of chemical oxygen demand (COD), nitrogen, and the odor-producing substance than the suspended growth system for both HRTs of 6 h (continuous aeration) and 12 h (1 h of aeration and 1 h of non-aeration). Both the fixed and moving carrier EMMC processes, therefore, have the potential for improvement or replacement of the existing conventional activated sludge process with regard to improving the effluent qualities (such as COD, nitrogen and odor-producing compound) for reuse/disposal.

Comparison of suspended growth and hybrid systems for nitrogen removal in ammonium bisulfite pulp mill wastewater

A series of bench‐scale nitrification/denitrification tests were carried out with both suspended growth and hybrid bioreactors. The hybrid reactor was fined with plastic (polyethylene) media to evaluate the effects of biofilm. Two types of reactor configurations were tested; 4‐compartment and 6‐compartment modes. The experiments were initiated with a half‐strength pulp and paper wastewater and its strength increased stepwise to the raw wastewater. Solid retention time was fixed at 10 days after a start‐up period while hydraulic retention time was extensively varied from 3.5 to OS days. The results from each type of reactor were compared in terms of nitrification/denitrification efficiency and stability. Experimental results demonstrate that the hybrid system showed greater stability in nitrification and higher denitrification efficiency than the suspended growth system. In the hybrid system, attached volatile solids formed 61 — 72% of total volatile solids in the reactor and the amount of attached volatile solids insignificantly varied with the organic loading rate (037 ‐ 2.76 kg COD m‐3 d‐1) after initial biomass attachment. Under the conditions tested (0.1 ‐ 2.8 kg COD m3 d‐1), organic loading rate insignificantly influenced the nitrification. Better performance was obtained in denitrification when the anoxic zone was better isolated from the aerobic compartments (6‐compartment mode). Overall, the hybrid system with fixed‐film growth had better resistance to upset caused by transients such as changes in influent composition or hydraulic retention time.

A detailed analysis of the mechanisms controlling the acceleration of 2,4-DCP monooxygenation in the two-tank suspended growth process.

The mechanisms underlying the observed acceleration of monooxygenation reactions in two-tank accelerator/aerator suspended growth system are evaluated in detail. The accelerator tank is characterized by a very high electron flow through reduced nicotinamide adenine dinucleotide (NADH + H+), particularly when the retention-time ratio is small. Only a small fraction of the electron flow was diverted to oxygenation reactions, and the major sinks of NADH + H+ were respiration and biomass synthesis. The main producer of NADH + H+ is oxidation of acetate, a rapidly degraded electron-donor substrate. The half-maximum-rate concentration for oxygen used in respiration was 0.03 mg/L, while the half-maximum-rate concentration for oxygen used as a cosubstrate in monooxygenation was 0.18 mg/L. Thus, monooxygenations were more sensitive to oxygen limitation than was respiration. The NADH + H+ concentration had a direct effect on the monooxygenation kinetics. The rate coefficients for both monooxygenation reactions were directly proportional to the specific growth rate in the accelerator, which supports that the accelerator tank caused an up-regulation of the monooxygenase content. Because the rate coefficients in the aerator tank were much larger than in the one-tank system, even though the specific growth rates were nearly the same, monooxygenases may have carried over from the accelerator tank to the aerator tank. Its higher concentration of 2,4-dichlorophenol (2,4-DCP) and the higher specific growth rate were the main reasons why the accelerator had faster kinetics for 2,4-DCP utilization than did the aerator tank. The apparently higher levels of monooxygenase in both tanks of the two-tank system also appears be a primary reason why its performance was substantially superior to that of the one-tank system in terms of 2,4-DCP removal.

Comparison of the filtration characteristics between attached and suspended growth microorganisms in submerged membrane bioreactor

An attached growth bioreactor was designed to minimize the effect of suspended microorganisms on membrane fouling in submerged membrane bioreactor. Comparison of mixed liquor from attached and suspended growth systems was made to elucidate major factors giving rise to different filtration characteristics. Unexpectedly, the rate of membrane fouling of the attached growth system was about 7 times higher than that of the suspended growth system despite similar characteristics of soluble fraction from the two reactors. Filtration performance proved to depend on the concentration of mixed liquor suspended solids (MLSS). Better filtration performance with suspended growth was explained by the formation of dynamic membranes with suspended solids. A series of analyses such as hydraulic resistance, specific cake resistance, scanning electron microscope, and atomic force microscope were carried out to elucidate the different filtration characteristics of the two systems.