Continuous Activated Sludge System Research Articles

Long-term nitrite-oxidizing bacteria suppression in a continuous activated sludge system exposed to frequent changes in pH and oxygen set-points.

Research has proven the adaptation of nitrite-oxidizing bacteria to unfavorable environmental conditions, and this work presents a novel concept to prevent nitrite oxidation during partial nitrification in wastewater. The approach is based on the real-time updating of mathematical models of the process to search for optimal set-points of pH and oxygen concentration in a continuous activated sludge reactor with a high sludge age (20.3 days). A heuristic optimization technique by 13 optimum set-points simultaneously maximized the degree of ammonia oxidation (α) and nitrite accumulation (β), achieving an (α+β)=190% per day. The activated sludge reactor was conducted for 780 days under three control schemes: open-loop control, fuzzy model supervisory control and phenomenological supervisory control. The phenomenological supervisory control system achieved the best results, simultaneously reaching 95% ammonium oxidation and 90% nitrite accumulation. The Haldane kinetics were analyzed using steady-state concentrations of all nitrogen species, concluding that the simultaneous maximization of α+β led to selecting set-points at the extreme values of the following ranges: pH=7.5-8.5 and DO=0.8-1.0mg O2/L, which enabled the inhibition of one nitrifier species. At the same time, the other one was relieved from inhibition. The 16sRNA assays indicated that the nitrite-oxidizing bacteria presence (genera Nitrobacter and Nitrospira) shifted from 32% to less than 8% after 280 days of continuous operation with optimal pH and oxygen set-points.

Treatment of swine wastewater in continuous activated sludge systems under different dissolved oxygen conditions: Reactor operation and evaluation using modelling

Swine wastewater was treated in two continuously aerated activated sludge (AS) systems at high (AS1: 1.7–2.6 mg/L) and low (AS2: 0.04–0.08 mg/L) dissolved oxygen (DO), and at three temperatures (10, 20, and 30 °C). Biochemical oxygen demand (BOD) removal was >94.8%. Meanwhile, total nitrogen (N) removal was significantly higher in AS2, at 64, 89, and 88%, than in AS1, at 12, 24, and 46%, for 10, 20, and 30 °C, respectively. The experimental data were considered in a simulation study using an AS model for BOD and N removal, which also included nitrite, free ammonia, free nitrous acid, and temperature. Simulations at high-DO showed that ammonium was partly oxidized into nitrate but not removed, whereas at low-DO ammonium was removed mainly through the nitrite shortcut in simultaneous nitrification–denitrification. This study demonstrates that treatment at low-DO is an effective method for removing N, and modelling a helpful tool for its optimization.

Biodegradation of Toilet Wastewaters Generated in Aircrafts

AbstractWastewater generated in buses and aircrafts are known as blue water due to the addition of blue chemical products to deodorize, sanitize and break down solid waste in tank holdings. The blue water at high concentration presents inhibitory effects on microorganisms making difficult their treatment. This research evaluates the acclimation and degradation of aircraft blue water using discontinuous and continuous bioreactors. Acclimation was conducted in batch mode. In a discontinuous biological process with an initial COD concentration of 1150 mg COD/L, the removal efficiency was higher than 80% as COD, TOC and BOD5. Total Suspended Solids in the effluent were lower than 60 mg/L. In the continuous activated sludge system an initial concentration of 850 mg COD/L of blue wastewater was applied. Here, removal efficiencies up to 72 and 88% as COD and BOD5 were obtained, respectively. For both processes, the application of punctual high concentration of blue water resulted in decrease of removal efficiency (< 40%).

Control filamentous bulking caused by chlorine-resistant Type 021N bacteria through adding a biocide CTAB

Filamentous bulking sludge due to excessive growth of filamentous bacteria is a serious operational problem in activated sludge plants. The addition of chemicals is one of widespread ways to control filamentous bulking. In this study, filamentous bulking in a continuous activated sludge system was found to be mainly caused by Eikelboom Type 021N filamentous bacteria likely due to low substrate concentration gradients. These Type 021N bacteria were found to be resistant to chlorination, maintaining cell integrity at a dosage of up to 80 mg Cl/gSS. An alternative biocidal agent, cetyltrimethyl ammonium bromide (CTAB), exhibited a much stronger biocidal effect on these filaments, which significantly improved sludge settleability. Type 021N with filamentous index of 5 was selectively killed, but floc-formers recovery their activity after CTAB termination. The study implied that CTAB might have more penetration capacity to cell wall of chlorine-resistant Type 021N bacteria. We therefore suggest the penetration property of filament cell wall should be considered or tested before the selection of biocide type in practice.

Total and stable washout of nitrite oxidizing bacteria from a nitrifying continuous activated sludge system using automatic control based on Oxygen Uptake Rate measurements

Partial nitrification (ammonium oxidation to nitrite) has gained a lot of interest among researchers in the last years because of its advantages with respect to complete nitrification (ammonium oxidation to nitrate): decrease of oxygen requirements for nitrification, reduction of COD demand and CO 2 emissions during denitrification and higher denitrification rate and lower biomass production during anoxic growth. In this study, an extremely high-strength ammonium wastewater (3000–4000 mg N L −1) was treated in a continuous pilot plant with a configuration of three reactors in series plus a settler. The system was operated under the maximum possible volumetric nitrogen loading rate, at mild temperature (around 25 °C), with high sludge retention time (around 30 d) and significant nitrifying biomass concentration (average of 1800 ± 600 mg VSS L −1). The implemented control loops transformed the system, which was operating with complete nitrification, into a continuous partial nitrification system. Nitrite oxidizing bacteria (NOB) washout was accomplished with local control loops for pH and dissolved oxygen (DO) with proper setpoints for NOB inhibition (pH = 8.3 and DO = 1.2–1.9 mg O 2 L −1) and with an inflow control loop based on Oxygen Uptake Rate (OUR) measurements, which allowed working at the maximum ammonium oxidation capacity of the pilot plant in each moment. This operational strategy maximized the difference between ammonia oxidizing bacteria (AOB) and NOB growth rates, which is the key point to achieve a fast and stable NOB washout. The results showed a stable operation of the partial nitrification system during more than 100 days and NOB washout was corroborated with fluorescence in-situ hybridization (FISH) analysis.

Biological phosphorus removal from a phosphorus-rich dairy processing wastewater

Dairy industry processing wastewaters consist mainly of dilutions of milk, milk products and cleaning solutions and, depending on the processes used, may be rich in phosphorus. In New Zealand and internationally, chemical removal of phosphorus is typically the phosphorus removal method of choice from dairy processing wastewaters. The enhanced biological phosphorus removal (EBPR) process was investigated in this study as an alternative phosphorus removal option using a continuous activated sludge system. A synthetic dairy processing wastewater was firstly subjected to fermentation in an anaerobic reactor (HRT = 12 hrs, pH = 6.5, temperature = 35 degrees C) resulting in a fermented wastewater with an average volatile fatty acid (VFA) concentration of 1055 mg COD/L. The activated sludge reactor was operated in an AO configuration with an HRT of 2.5 days and an SRT of 15 days. Stable EBPR was exhibited with 42 mg P/L removed, resulting in a final sludge phosphorus content of 4.9% mg P/mg TSS. In the anaerobic zone (HRT = 2.85 hrs) the sludge had a phosphorus content of 3.16% mg P/mg TSS and a poly-beta-hydroxyalkanoate (PHA) concentration of 86 mg COD/g TS.

Fate and effects of triclosan in activated sludge

Triclosan (TCS; 5-chloro-2-[2,4-dichloro-phenoxy]-phenol) is a widely used antimicrobial agent. To understand its fate during sewage treatment, the biodegradation and removal of TCS were determined in activated sludge. In addition, the effects of TCS on treatment processes were assessed. Fate was determined by examining the biodegradation and removal of TCS radiolabeled with 14C in the 2,4-dichlorphenoxy ring in laboratory batch mineralization experiments and bench-top continuous activated-sludge (CAS) systems. In batch experiments with unacclimated sludge, TCS was mineralized to 14CO2, but the total yield varied as a function of test concentration. Systems that were redosed with TCS exhibited more extensive and faster mineralization, indicating that adaptation was a critical factor determining the rate and extent of biodegradation. In a CAS study in which the influent level of TCS was incrementally increased from 40 microg/L to 2,000 microg/L, removal of the parent compound exceeded 98.5% and removal of total radioactivity (parent and metabolites) exceeded 85%. Between 1.5 and 4.5% of TCS in the influent was sorbed to the wasted solids, whereas >94% underwent primary biodegradation and 81 to 92% was mineralized to CO2 or incorporated in biomass. Increasing levels of TCS in the influent had no major adverse effects on any wastewater treatment process, including chemical oxygen demand, biological oxygen demand, and ammonia removal. In a subsequent experiment, a CAS system, acclimated to TCS at 35 microg/L, received two separate 4-h shock loads of 750 microg/L TCS. Neither removal of TCS nor treatment processes exhibited major adverse effects. An additional CAS study was conducted to examine the removal of a low level (10 microg/L) of TCS. Removal of parent equaled 94.7%, and biodegradation remained the dominant removal mechanism. A subsequent series of CAS experiments examined removal at four influent concentrations (7.5, 11, 20, and 50 microg/L) of TCS and demonstrated that removal of parent ranged from 98.2 to 99.3% and was independent of concentration. Although TCS removal across all experiments appeared unrelated to influent concentration, removal was significantly correlated (r2 = 0.87) with chemical oxygen demand removal, indicating that TCS removal was related to overall treatment efficiency of specific CAS units. In conclusion, the experiments show that TCS is extensively biodegraded and removed in activated-sludge systems and is unlikely to upset sewage treatment processes at levels expected in household and manufacturing wastewaters.

The fate of linear alcohol ethoxylates during activated sludge sewage treatment

Model continuous activated sludge (CAS) plants (Husmann units) were used to study the fate of two commercial, alcohol ethoxylate (AE) surfactants during aerobic sewage treatment. The surfactants were produced by the ethoxylation of an essentially linear C 12–15 alcohol (NEODOL 25™) with an average of 7 (C 12–15EO 7) or 3 (C 12–15EO 3) moles of ethylene oxide (EO). Recent analytical developments made it possible to measure levels of AE that included the free alcohol and EO 1 oligomers across the CAS system, from the influent feed, on the activated sludge, through to the effluent. Measured concentrations of AE (as C 12–15EO 0–20) in the synthetic sewage feeds to the test CAS plants lay in the range 11–13 mg/l. During stable operation at 20°C, an average of 5 μg/ l AE were present in the C 12–15EO 7 CAS plant effluent, giving a removal (bioelimination) of >99.9%. When levels of AE on the sludge, and polyethylene glycols (PEGs – an expected biodegradation intermediate) in the effluent and on the sludge were also taken into account, biodegradation was considered to be responsible for >98.7% of the observed removal. During operation at a winter temperature (10°C), an average of 26 μg/ l AE were present in the C 12–15EO 7 CAS plant effluent, giving a removal of 99.8%. Biodegradation was estimated to be responsible for >97.2% of the observed removal. During operation at 20°C, an average of 7 μg/ l AE were present in the C 12–15EO 3 CAS plant effluent, giving a removal of >99.9%. No analysis for PEG was performed in this case but the low level of AE on the sludge (0.2 mg/g dry solids) suggested that biodegradation was responsible for most of the observed removal. Neither surfactant had any adverse effect on the sewage treatment efficiencies of the CAS plants in terms of dissolved organic carbon (DOC) removal, nitrification or biomass levels.

Contribution of Pseudomonas spp. to phosphorus uptake in the anoxic zone of an anaerobic-anoxic-aerobic continuous activated sludge system

A continuously operated laboratory-scale (32 L) nitrification denitrification biological excess phosphorus removal (NDBEPR) activated sludge system (modeled on the 3-stage Phoredox configuration) was maintained for 140 d. The transition from a non-biological excess phosphorus removal (BEPR) sludge to one exhibiting a strong BEPR mechanism was monitored. Mixed liquor seed inoculum was obtained from a full-scale single aerobic activated sludge installation and subjected to conditions conducive to BEPR, i.e. increasing influent acetate (HAc) concentrations. At a sludge age of 10 d with 100% HAc feed, the system was capable of removing a maximum of ca. 40 mgPO4-P/L from the bulk liquid; P/VSS of ca. 0.27 (mgP/mgVSS); and VSS/TSS of 0.53 (mgVSS/mgTSS) in the aerobic zone was attained. Although typical BEPR phosphorus transformation patterns were routinely observed, i.e. anaerobic phosphate release and aerobic phosphate uptake, phosphate uptake in the anoxic zone was also recorded indicating the presence of denitrifying phosphorus accumulating organisms (DPAOs) in the sludge community. The microbial community was screened (using both isolation and direct methods of analysis) for the presence of Pseudomonas spp. as this genus is known to perform both polyphosphate accumulation and denitrification processes. Isolation of anoxic mixed liquor bacteria on solid media and identification using the APl 20NE system resulted in the total dominance of the Pseudomonads (> 50%). However, direct fluorescent in situ hybridizations (FISH) revealed that Pseudomonas spp. only constituted ca. 3% of the total bacterial community indicating that other bacterial genera are contributing to simultaneous polyphosphate accumulation and denitrification processes in the anoxic zones of NDBEPR systems.

How adaptation and mass transfer control the biodegradation of linear alkylbenzene sulfonate by activated sludge.

We use a nonsteady-state model to evaluate the effects of community adaptation and sorption kinetics on the fate of linear alkylbenzene sulfonate (LAS) in batch experiments conducted with activated sludge that was continuously fed different concentrations of LAS. We observed a sharp decrease in the biodegradation rate between 30 and 60 minutes and the presence of an LAS residual at the end of the batch experiments. The modeling analysis indicates that these phenomena were caused by relatively slow inter-phase mass transport of LAS. The modeling analyses also showed that the amount of LAS-degrading biomass increased when the continuous activated sludge was fed a higher LAS concentration. Although community adaptation to LAS involved accumulation of more LAS degraders, the increase was not proportional to the feed concentration of LAS, which supports the concept that LAS degraders also utilized portions of the general biochemical oxygen demand (BOD) fed to the continuous activated sludge systems.

Adaptation of the cas test system and synthetic sewage for biological nutrient removal: Part II: Design and validation of test units

A global increase in biological nutrient removal (BNR) applications in wastewater treatment and concern for potential effects of anthropogenic substances on BNR processes resulted in the adaptation of the Continuous Activated Sludge (CAS) laboratory test system (cf. guideline OECD 303A [1] or ISO 11733 [2]). In this paper two novel systems are compared to the standard CAS unit: the Behrotest KLD4 ® and a University of Cape Town system (CAS-UCT). Both are ‘single sludge’ systems with an anoxic/aerobic and an anaerobic/anoxic/aerobic configuration, respectively. They both can simulate the essential processes of full-scale BNR installations. The units where fed with a specially designed synthetic sewage, Syntho (cf. Part I of this study), or its precursor BSR3 medium. The performance of the two new units was benchmarked against the standard CAS system in terms of carbon/nitrogen/phosphorus (C/N/P) removal, as well as primary biodegradation of the surfactants linear alkylbenzene sulfonate (LAS) and glucose amide (GA). Both systems allow to easily achieve stable excess N- and P-removal. Experimental C/N/P removal data compared closely with simulations obtained with the IAWQ Activated Sludge Model No. 2 (ASM2), and with full scale BNR plants with a similar configuration. In both units the effluent concentrations of the surfactants tested were significantly reduced in comparison to the standard CAS system (up to 50% less). No adverse effects on BNR were noted for the test surfactants dosed at 400 μg/l together with an overall surfactant background concentration in the feed of ca. 20 mg/l. The proposed systems hold potential to complement the standard CAS system for situations where advanced sewage treatment plants with BNR need to be simulated in the laboratory with minimum effort.

Modelling biological treatability for meat processing effluent

Biological treatability of major agro-industries wastewaters, such as meat processing effluents can only be evaluated with specific emphasis on slowly biodegradable substrate and using a multi-component modelling approach. This paper reviews the framework of the endogenous decay model and summarizes the necessary COD fractionation and the kinetic information to be incorporated in this model as applied to a meat processing effluent. Model interpretations of the respirometric experiments are used to define the fate of slowly biodegradable COD. Behavior of this wastewater in continuous activated sludge systems is studied by model simulations based upon experimental results.

Modelling biological treatability for meat processing effluent

Biological treatability of major agro-industries wastewaters, such as meat processing effluents can only be evaluated with specific emphasis on slowly biodegradable substrate and using a multi-component modelling approach. This paper reviews the framework of the endogenous decay model and summarizes the necessary COD fractionation and the kinetic information to be incorporated in this model as applied to a meat processing effluent Model interpretations of the respiromeuic experiments are used to define the fate of slowly biodegradable COD. Behavior of this wastewater in continuous activated sludge systems is studied by model simulations based upon experimental results.

A comparison of biological nutrient removal in intermittent cyclic and continuous activated sludge systems

An assessment was made of biological phosphorus and nitrogen removal and related sludge bulking control in both intermittent cyclic and continuous activated sludge systems. Experimental results from a laboratory-scale investigation using fermented domestic sewage indicated the advantages of achieving high levels of P and N removal and good sludge-settling properties using the intermittent cyclic process, as compared to the continuous process. The modified intermittently fed and decanted system incorporating non-mixing sequences produced an effluent quality of NO 3-N < 5 mg L −1, PO 4-P < 1 mg L −1 and NH 3-N < 2 mg L −1, with a non-bulking sludge having a sludge volume index < 120 mL g −1, despite unfavourable influent characteristics (TCOD : TKN < 7, TKN > 60 mg L −1, TP-9 mg L −1 and RBCOD < 30 mg L −1). In contrast, the modified 4-stage Bardenpho process could not achieve effluent NO 3-N and PO 4-P concentrations of < 10 and < 1 mg L −1, respectively. The sludge volume index (SVI) values were always higher than 250 mL g −1 and occasionally reached as high as 422 mL g −1. The causative filaments were low F:M growers - Types 0041/0675.

Efficiency of sequencing batch reactor (SBR) in the removal of selected microorganisms from domestic sewage

The relationships between the loading factor ( L i) and BOD 5, COD, TSS, total coliform, fecal coliform and coliphage removals were explored using three laboratory SBRs receiving a medium-to-low strength domestic sewage. The SBRs were operated with different FILL/REACT ratios in a 6-h cycle. Not unexpectedly results showed that increases in L i led to decreases in the removals of BOD 5, COD, TSS and potential pathogen indicators. However, low SVIs (Sludge Volume Index) in the SBR need not necessarily mean good solids settleability. Good settleability is, however, necessary for good TSS removal as it led to better removals of total BOD 5, total COD and indicators of potential pathogens. During FILL, fermentation reactions probably occurred and this resulted in up to 86% reduction of the initial dissolved BOD 5 concentration. Degradation of waste then continued into the REACT period. This preliminary assessment showed that the SBR is, potentially, a viable option for domestic wastewater treatment, especially since the aeration time required for treatment to achieve comparable effluent quality is shorter than the conventional continuous activated sludge systems which are usually operated with about 3–8 h of aeration time.

The Treatability of Chromium Tannery Wastes

Leather processing industry wastewaters contain several conventional pollutants as well as micropollutants. Conventional pollutants can be effectively treated although considerable treatment comprised of several stages is required. However, the potential of conventional methods to remove COD and micropollutants has not been fully explored. The purpose of this paper is to investigate the extent of COD removal by conventional methods. Experimental studies are conducted using raw wastewaters of industries applying chromium tanning to cattlehides and sheepskins. Biological treatability after pretreatment by chemical coagulation is assessed using a lab-scale continuous activated sludge system. Results indicate that chemical coagulation produces wastewaters with a COD of around 2000 mg/l almost independent of raw wastewater quality. Activated sludge operated at 0.17 g BOD5/g MLSS.d loading is found to provide limited nitrification and almost complete BOD5 removal while the COD could only be reduced to around 500 mg/l. The residual COD identified using a newly developed experimental approach shows that it cannot be reduced lower than 300-550 mg/l.