Activated Sludge Plant Research Articles

Municipal wastewater sludge rheology: Impact of temperature, solid content, and settling solids.

The analysis of the rheological properties of the sludge produced in biological purification plants is paramount for the design of equipment (pumping, mixing, conditioning, aeration, filtration, etc.) and the management of these plants. In particular, solid content in the sludge plays a significant role in the rheological quantities. In this study, an investigation of the sludge rheology produced in different sectors of an activated sludge plant (aerated sludge, recirculation sludge, and thickened sludge) has been performed. Specifically, for each sludge sample, various physical-chemical parameters have been analyzed, and rheological tests have been performed at different temperatures (10°C, 20°C, 30°C, and 40°C ± 0.1). The sludge was considered as a non-Newtonian Bingham fluid. Viscosity and critical stress were determined, and their dependence at different temperatures, on some physical parameters, such as solid content and settling solids, has been studied. Results evidenced that viscosity and critical stress increase with increasing solid concentration. Moreover, at the same concentration, as the temperature increases, the viscosity decreases, whereas the critical stress decreases until temperature values of about 30°C and then increases again. PRACTITIONER POINTS: An investigation of the sludge rheology produced in different sectors of an activated sludge plant has been performed. For each sludge sample, various physical-chemical parameters have been analyzed, and rheological tests have been performed at different temperatures. Viscosity and critical stress were determined, and their dependence at different temperatures, on some physical parameters, has been studied. Results evidenced that viscosity and critical stress increase with increasing solid concentration.

Assessing the environmental impacts and greenhouse gas emissions from the common municipal wastewater treatment systems

This study measured the environmental impacts from three same-size wastewater treatment systems, specifically activated sludge, a constructed wetland, and a high rate algal pond. Detailed data inventories were employed using SimaPro 9 software to calculate the entire consequences by ReCiPe 2016 and Greenhouse Gas Protocol method. The environmental outcomes caused by substance emissions and resource extraction are presented in several impact categories at the endpoint level. For a better comparison, the single score tool was applied to aggregate all factors into three areas of protection: human health, ecosystem, and resource shortage. Results showed that concrete and steel are the main contributors to the construction phase, while electricity is responsible for the operation stage. The single score calculation indicates that the proportion of construction activities could be equal to or even higher than the operation stage for a small capacity plant. The total environmental impact of the conventional system was 2.3-fold and 3-fold higher than that of constructed wetland and high rate algal pond, respectively. High rate algal pond has the best environmental performance when generating the least burdens and greenhouse gas emissions of 0.72 kg CO2 equivalent per m3. Constructed wetland produces 5.69 kg CO2, higher than an algal pond but much lower than activated sludge plant, emitting 11.42 kg CO2 per m3.

Design Criteria Assessment for the Aeration Tank in the Activated Sludge Process. (Dept. C)

In Egypt, the use of activated sludge process for treating domestic wastewater has been started recently. Several of activated sludge plants have been constructed in cities and villages. The kinetic coefficients used in sizing of the aeration tanks for these plants were taken the same as in the developed countries although the climatic conditions, load variation, presence of industrial wastewater and wastewater characteristics in Egypt differ than that in developed countries. A continuous flow activated sludge pilot plant was operated in El-Mansoura university sanitary engineering laboratory to determine the kinetic coefficients for El-Mansoura city and Meet-Mazah village wastewater. The results indicated that the aeration tank design volume greatly influenced by wastewater kinetic coefficients and model used in the design. The aeration tank design volumes according to Lawrence and mccartys model and Kincannon and Stover model are almost equal. On the other hand the design volumes according to Eckenfelder model Eckenfelder modified model are very high compared to other models. In all cases, it is recommended to run a pilot plant to determine the wastewater kinetics for accurate sizing of aeration tanks.

Continuous aerobic granular sludge plants: Better settling versus diffusion limitation

The application of aerobic granular sludge in continuous wastewater treatment plants is receiving increased attention. The introduction of better settling sludge in existing installations is expected to increase the plant’s treatment capacity by allowing a higher biomass concentration in the reactor. A lot of recent research has therefore focused on how to get stable granules in a continuous flow plant, which make up the majority of current infrastructure. Still, the effect of aerobic granular sludge on the treatment capacity and energy requirements has not thoroughly been studied so far and is not that straightforward. While it is clear that the introduction of aerobic granular sludge will result in better settling, it will also bring about a higher diffusion limitation – which is often overlooked. This study scrutinized the effect of better settling versus diffusion limitation in a typical continuous activated sludge plant (predenitrification type) through a simulation study. When only considering the improved settling velocity, the treatment capacity increased by 40% compared to conventional activated sludge. However, diffusion limitation could almost totally counteract the positive effect of better settling for a non-improved continuous system. The continuous system could be improved for aerobic granular sludge by increasing the aerobic reactor volume fraction, while lowering the oxygen set-point to benefit from simultaneous nitrification–denitrification. The optimization led to a 20% improvement in treatment capacity and a 10% reduced energy consumption compared to a conventional activated sludge system. Overall, the performance of continuous wastewater treatment plants could indeed benefit from the excellent settling properties of aerobic granular sludge, as long as process operation was improved in order to take into account diffusion limitation.

Dynamic resilience for biological wastewater treatment processes: Interpreting data for process management and the potential for knowledge discovery

Climate change, population growth and increasing regulation are causing wastewater treatment plants to become increasingly stressed, especially in countries like the UK, where many of these systems date back to the early part of the 20th century. Understanding resilience dynamics for these ageing wastewater assets represents a fundamental step in classifying multi-dimensional water stressors toward preventing severe pollution incidents. This paper explores the potential of a novel dynamic resilience approach to assess and predict the dynamic resilience of biological wastewater treatment based on the separation of stressor events (cause) and process stress (effect) to consider the deviation from reference conditions. The approach presented provides a fundamental link between (1) conventional activated sludge modelling methodologies, (2) actual biological wastewater process instrument data (potential for knowledge discovery) and (3) the characterisation of dynamic resilience in wastewater treatment processes. Results first present the dynamic resilience approach by modelling simulated shock flow conditions on an activated sludge plant, then incorporates ten years of wastewater process instrument data to demonstrate the actual dynamic resilience. The aim is to represent the “dynamic resilience” as self-ordering windows, a visual knowledge base (three dimensional, heat map), which operational staff can easily interpret. The outcomes presented suggest that such an approach is feasible and has the potential for real-time identification of conditions that result in pollution incidents based on actual historical process instrument data (knowledge discovery). Also, the methods presented could be extended to develop an improved understanding of wastewater system resilience under a range of future stressor scenarios.

Nitrous oxide emission from full-scale municipal aerobic granular sludge

The nitrous oxides emission was measured over 7 months in the full-scale aerobic granular sludge plant in Dinxperlo, the Netherlands. Nitrous oxide concentrations were measured in the bulk liquid and the off-gas of the Nereda® reactor. Combined with the batch wise operation of the reactor, this gave a high information density and a better insight into N2O emission in general. The average emission factor was 0.33% based on the total nitrogen concentration in the influent. The yearly average emission factor was estimated to be between 0.25% and 0.30%. The average emission factor is comparable to continuous activated sludge plants, using flocculent sludge, and it is low compared to other sequencing batch systems. The variability in the emission factor increased when the reactor temperature was below 14 °C, showing higher emission factors during the winter period. A change in the process control in the winter period reduced the variability, reducing the emission factors to a level comparable to the summer period. Different process control might be necessary at high and low temperatures to obtain a consistently low nitrous oxide emission. Rainy weather conditions lowered the emission factor, also in the dry weather flow batches following the rainy weather batches. This was attributed to the first flush from the sewer at the start of rainy weather conditions, resulting in a temporarily increased sludge loading.

Heterotrophic enzymatic biotransformations of organic micropollutants in activated sludge.

While heterotrophic microorganisms constitute the major fraction of activated sludge biomass, the role of heterotrophs in the biotransformation of organic micropollutants (OMPs) has not been fully elucidated. Yet, such knowledge is essential, particularly when conceiving novel wastewater treatment plants based on a two-stage process including an A-stage under heterotrophic conditions and a B-stage based on anammox activity. Biotransformation of OMPs in activated sludge is thought to mostly occur cometabolically thanks to the action of low specificity enzymes involved in the metabolism of the primary substrates. For a better understanding of the process, it is important to determine such enzymatic activities and the underlying mechanisms involved in OMPs biotransformation. This task has proven to be difficult due to the lack of information about the enzymatic processes and the complexity of the biological systems present in activated sludge. In this paper, a continuous aerobic heterotrophic reactor following 20 OMPs at environmental concentrations was operated to (i) assess the potential of heterotrophs during the cometabolic biotransformation of OMPs, (ii) identify biotransformation reactions catalyzed by aerobic heterotrophs and (iii) predict possible heterotrophic enzymatic activities responsible for such biotransformations. Contradicting previous reports on the dominant role of nitrifiers in OMPs removal during activated sludge treatment, the heterotrophic population proved its capacity to biotransform the OMPs to extents equivalent to reported values in nitrifying activated sludge plants. Besides, 12 transformation products potentially formed through the activity of several enzymes present in heterotrophs, including monooxygenases, dioxygenases, hydrolases and transferases, were identified.

Influence of road salt thawing peaks on the inflow composition and activated sludge properties in municipal wastewater treatment.

Operational data over 2 years from three large Austrian wastewater treatment plants (WWTPs) with design capacities of 4 million, 950,000 and 110,000 population equivalent (PE) were examined. Salt peaks, due to thawing road salt were detected and quantified by electrical conductivity, temperature and chloride measurement in the inflow of the WWTPs. Daily NaCl inflow loads up to 1,147 t/d and PE-specific loads of 0.26-0.5 kg NaCl/(PE · y) were found. To mimic the plants' behaviour in a controlled environment, NaCl was dosed into the inflow of a laboratory-scale activated sludge plant. The influence of salt peaks on important activated sludge parameters such as sludge volume index, settling velocity and floc size were investigated. Influent and effluent were sampled extensively to calculate removal rates. Respiration measurements were performed to quantify activated sludge activity. Particle size distributions of the activated sludge floc sizes were measured using laser diffraction particle sizing and showed a decrease of the floc size by approximately two-thirds. The floc structure was examined and documented using light microscopy. At salt concentrations below 1 g/L, increased respiration was found for autotrophic biomass, and between 1 and 3 g NaCl/L respiration was inhibited by up to 30%.

Predictive control of activated sludge plants to supply nitrogen for optimal crop growth

We report the preliminary results of a feasibility study in which we investigate the possibility to operate a common class of activated sludge plants to produce effluent wastewater of varying quality for crop irrigation. Firstly, a nitrogen reference trajectory is computed as solution to a higher-level optimal control problem that aims at maximizing plant biomass in a crop growth system. We then study how to control the treatment plant with a zero-offset predictive control strategy designed to operate the treatment process to supply nitrogen according to this optimal planning for crop growth. We show how an ad hoc tuning of the predictive controller allows to define alternative policies that favour the manipulation of different forms of nitrogen in the treatment plant. We show that the designed controllers are only partially capable to operate the treatment plant to meet the nitrogen demand, when subjected to typical municipal wastewater influent conditions. However, zero-offset can be achieved under constant influent conditions. We analyse the performance of the controller in terms of tracking accuracy and operational energy.

Design and evaluation of advanced automatic control strategies in a total nitrogen removal activated sludge plant

AbstractAdvanced control strategies are designed for an activated sludge‐based wastewater treatment plant (WWTP) consisting of two anoxic, three aerobic reactors and secondary settler by considering the influent data from an existing WWTP. Four different control strategies (PI/FPI/MPC/FLC) are designed and a systematic procedure is developed to implement these control strategies. The plant performance is evaluated using operational cost index (OCI) and effluent quality index (EQI) and the controller performance is assessed based on integral of squared error. The EQI and OCI for open loop are reported as 4469 and 12 743 respectively. Whereas, when advanced controllers are implemented, a significant decrease of 19.89% and 5.24% in EQI and OCI is observed for MPC and a decrease of 20.9% and 4.63% in EQI and OCI is observed for fuzzy controller. The outcomes of this research help the wastewater utilities to adopt advanced control strategies for enhanced operation and decision‐making.

Optimizing Treatment of Cesspool Wastewater at an Activated Sludge Plant

The purpose of this work was to determine the optimal percentage of wastewater from cesspool in the mixture of wastes subjected to treatment processes, which will not have a negative impact on the functioning of the collective treatment plant. The study was carried out over a period of two years, with 48 samples of wastewater flowing in from the sewage network and delivered with the slurry tanker collected and subjected to physical and chemical analysis. The analysis included: Biochemical Oxygen Demand (BOD5), Chemical Oxygen Demand (COD), and Total Nitrogen (TN). In addition, the study defined the daily balance of the amount of inflowing and transported wastewater. Based on the analysis carried out, it was found that the unit loads of BOD5, COD and TN in the mixture of wastewater subjected to the treatment process will be at the level of loads assumed in the project, when the share of supplied wastewater, i.e., from cesspool, will be at the level of 5% of the total amount of wastewater. Considering that in the analysed period the total average daily amount of wastewater subjected to the treatment process was 253.5 m3·d−1, the optimal amount of wastewater delivered should be 12.7 m3 in each day of the week.

Removal of pharmaceutical metabolites in wastewater ozonation including their fate in different post-treatments

Advanced treatment technologies for the removal of pharmaceuticals and other organic micropollutants in WWTPs primarily target the removal of parent compounds. Nevertheless, the removal of metabolites originating from human- or microbial metabolism during biological treatment needs comparable consideration, as some of them might be present in high concentrations and contribute to toxicity.This study was conducted to elucidate the removal of human and microbial metabolites of pharmaceuticals as a function of the specific ozone dose. Ozonation was performed on four sites with two pilot- and two full-scale plants operated downstream of conventional activated sludge plants. The ozone reactivity of all metabolites (expressed as the ozone dose to remove 90% of the compound/decadic ozone dose) was lower than those of their parent compounds. The decadic ozone dose was 1.0, 1.3 and 1.1 mg O3/mg DOC for Epoxy-carbamazepine, Di-OH-carbamazepine and N-Desmethyl tramadol, respectively.20–40% of the remaining metabolites were removed in a polishing sand/BAC-filter (biological activated carbon). Similar removal was observed for Epoxy-carbamazepine, Di-OH-carbamazepine and Hydroxy-diclofenac in a constructed wetland. However, the sand/anthracite filter had no effect. All four metabolites were removed in a GAC (granulated activated carbon) filter.

Closing the loop in a constructed wetland for the improvement of metal removal: the use of Phragmites australis biomass harvested from the system as biosorbent

Among the numerous clean-up techniques for water treatment, sorption methods are widely used for the removal of trace metals. Phragmites australis is a macrophyte commonly used in constructed wetlands for water purification, and in the last decades, its use as biosorbent has attracted increasing attention. In view of a circularly economy approach, this study investigated improvement of trace metal removal by recycling the biomass of P. australis colonizing a constructed wetland, which operates as post-treatment of effluent wastewater from an activated sludge plant serving the textile industrial district of Prato (Italy). After the annual mowing of the reed plants, the biomass was dried and blended to derive a sustainable and eco-friendly biosorbent and its sorption capacity for Fe, Cu, and Zn was investigated comparing the batch system with the easier-to-handle column technique. The possibility of regeneration and reuse of the biosorbent was also evaluated. The biomaterial showed an interesting sorption capacity for Cu, Fe, and Zn, both in batch and in column experiments, especially for Fe ions. The immobilization of the biosorbent in column filters induced some improvement in the removal efficiency, and, in addition, this operation mode has the advantage of being much more suitable for practical applications than the batch process.

25 years of the AMTREAT® process treating high‐strength ammonia liquors

AbstractOver the last 30 years, a number of biological treatment processes have emerged for treating high‐strength ammonia liquors arising from the dewatering of anaerobic digestate. The AMTREAT® process was the first process to become established and recognised as a viable and reliable process to treat these high‐strength ammonia liquors. The process is a high‐rate activated sludge plant supplied by ACWA Services Ltd. The first full‐scale plant was installed in 1995, treating centrate produced when dewatering anaerobically digested sludge. The process is a compact, high‐rate ammonia treatment system, which has a relatively small footprint compared to conventional activated sludge treatment processes. Originally designed to fully nitrify dewatering ammonia liquors by controlling the environmental conditions of the treatment reactor, the process has evolved first to include alkalinity recovery and subsequently to offer total nitrogen removal. This paper discusses the development of the AMTREAT® process from its initial development in 1992, to the modern day third generation plants which achieve typical achieve ammonia removal exceeding 99% and total nitrogen removal in excess of 80%.

Potential of aerobic membrane bioreactor for recycling and reuse of domestic wastewater for irrigation

This paper explored the positive attributes of aerobic membrane bioreactor for recycling and reuse of domestic wastewater for irrigation. For this purpose, performances of two different full-scale treatment plants, one designed as aerobic membrane bioreactor (AMBR) equipped with a hollow fiber membrane with a nominal pore size of 0.03 μm, and the other, as a conventional nitrogen removal activated sludge plant with pre-denitrification (A2O) were evaluated and compared. The effluent of the A2O system included an average of 109 mg L−1 chemical oxygen demand (COD), 32 mg L−1 total suspended solids (TSS) and 17 mg L−1 of total nitrogen. The AMBR system was able to reduce the effluent COD level to 24 mg L−1, obviously with lower TSS and turbidity. The effluent quality of the AMBR was in compliance with irrigation water standards; its nitrogen content should be evaluated as an added nutritional value for most plants. Considering the additional profit of the recovered water, the investment and operational costs of A2O and AMBR processes were compatible. The results underlined the merit of AMBR process, especially in low populated areas also requiring resources for irrigation.

Dried aerobic heterotrophic bacteria from treatment of food and beverage effluents: Screening of correlations between operation parameters and microbial protein quality

Consortia of aerobic heterotrophic bacteria (AHB) have potential as sustainable microbial protein (MP) source in animal feed. A systematic screening of the nutritional value and safety of AHB biomass from full-scale activated sludge plants from 25 companies in the food sector was performed. The variable protein content (21–49%) was positively correlated with biomass-specific nitrogen loading rate and negatively with sludge retention time (SRT). Compared to the essential amino acid profile of soybean meal protein, AHB displayed an overall surplus of threonine and valine, and deficits in cysteine, histidine, lysine and phenylalanine. Histidine was positively correlated with bCOD/PO43− in the influent and valine, isoleucine and threonine with SRT. Most AHB samples were safe apropos heavy metals, polycyclic aromatic hydrocarbons and antibiotics. Some pesticides exceeded regulatory limits, necessitating mitigation. This work highlighted that the food sector can provide high-quality MP, while retrofitting existing activated sludge plants towards high-rate processes can increase AHB quality and productivity.

Microbial community similarity and dissimilarity inside and across full-scale activated sludge processes for simultaneous nitrification and denitrification.

Simultaneous nitrification and denitrification under low dissolved oxygen conditions is an energy-saving modification of the activated sludge process to achieve efficient nitrogen removal. Geographically distinct full-scale treatment plants are excellent platforms to address the links of microbial community with operating parameters. Mixed liquor samples were collected from a sequencing batch reactor plant, oxidation ditch plant, and step-feed activated sludge plant. Next-Generation Sequencing of the samples showed that the microbial communities were similar at the phylum level among the plants, being dominated by Proteobacteria. Microbial composition of functional groups was similar between the react fill and react phases of the sequencing batch reactors, among four sequencing batch reactors, and among four oxidation ditches. Nitrospira was the only identified genus of autotropic nitrifying bacteria with a relative abundance of 2.2-2.5% in the oxidation ditches and 0.4-0.7% at the other plants. Heterotrophic nitrifying-aerobic denitrifying bacteria were dominated by Dechloromonas with a relative abundance of 0.4-1.0%. Microbial community composition and nitrogen removal mechanisms were related to overall level and local zonation of dissolved oxygen, mixed liquor suspended solids concentration, nitrogen and organic loadings, and solids retention time. Low dissolved oxygen and low organic and nitrogen loadings favored growth of Nitrospira.

On the observability of activated sludge plants

In this work, the full-state observability properties of a class of biological wastewater treatment plants are analysed. Specifically, the five biological reactors and the secondary settler in the Benchmark Simulation Model no. 1 are studied. For the task, we represented the activated sludge plant as a dynamical system consisting of 145 states, 8 controls, 14 disturbances and 12 outputs and as a complex network to study its observability properties from a structural and a classical point of view. By analysing the topology of the network, we show how the system is not observable in the structural sense and thus how it is also not observable in the classical sense for all possible realisations of its parameters. As this is also true for a linearisation commonly used in the literature, we analysed a reduced-order system that, based on such linearisation, does not consider the state variables corresponding to dissolved oxygen and alkalinity in the upper-layers of the settler. We show how this system configuration is only observable in a structural sense.

Adsorption kinetics of colour removal from palm oil mill effluent using wastewater sludge carbon in column studies

Treated palm oil mill effluents (POME) is of great concern as it still has colour from its dissolved organics which may pollute receiving water bodies. In this study, the removal of colour from treated palm oil mill effluent were investigated through adsorption studies using carbon derived from wastewater sludge (WSC). Sludge from activated sludge plants were dried and processed to produce WSC. In this study, three different bed depths of WSC were used: 5 cm, 10 cm, and 15 cm. For each bed depth, the flowrate was varied at three different values: 100 mL/hr, 50 mL/hr and 25 mL/hr. It was found that at bed depth of 5 cm, the breakthrough curves were occurred at 360 min, 150 min and 15 min for flowrates of 25, 50 and 100 mL/hr respectively. It was observed that at a particular depth the exhaustion time for column reduced as flow rate increases. Kinetic models, Adams-Bohart and Yoon-Nelson were used to analyze the performance of the adsorption. It was found that rate constant for Adams Bohart model decreased with the increase in bed depth. Adsorption capacity obtained from Adams-Bohart model ranged from 2676.19 mg/L up to 8938.78 mg/L. The maximum adsorption capacity increases with smaller bed depth. For Yoon-Nelson model, the rate constant decreases with increase in bed depth. The required time for 50% breakthrough obtained from the models ranged from 17.01 to 104.17 minutes for all three bed depths. The reduction of colour was found to be effective at all bed depths. The experimental data was best described by both models as with higher values of correlation coefficient (R2).

The Phylogeny, Biodiversity, and Ecology of the Chloroflexi in Activated Sludge.

It is now clear that several of the filamentous bacteria in activated sludge wastewater treatment plants globally, are members of the phylum Chloroflexi. They appear to be more commonly found in treatment plants designed to remove nitrogen (N) and phosphorus (P), most of which operate at long sludge ages and expose the biomass to anaerobic conditions. The Chloroflexi seem to play an important beneficial role in providing the filamentous scaffolding around which flocs are formed, to feed on the debris from lysed bacterial cells, to ferment carbohydrates and to degrade other complex polymeric organic compounds to low molecular weight substrates to support their growth and that of other bacterial populations. A few commonly extend beyond the floc surface, while others can align in bundles, which may facilitate interfloc bridging and hence generate a bulking sludge. Although several recent papers have examined the phylogeny and in situ physiology of Chloroflexi in activated sludge plants in Denmark, this review takes a wider look at what we now know about these filaments, especially their global distribution in activated sludge plants, and what their functional roles there might be. It also attempts to outline why such information might provide us with clues as to how their population levels may be manipulated, and the main research questions that need addressing to achieve these outcomes.