Laboratory-scale Activated Sludge Unit Research Articles

Co-treatment of acid mine drainage with municipal wastewater: performance evaluation

Co-treatment of acid mine drainage (AMD) with municipal wastewater (MWW) using the activated sludge process is a novel treatment technology offering potential savings over alternative systems in materials, proprietary chemicals and energy inputs. The impacts of AMD on laboratory-scale activated sludge units (plug-flow and sequencing batch reactors) treating synthetic MWW were investigated. Synthetic AMD containing Al, Cu, Fe, Mn, Pb, Zn and SO4 at a range of concentrations and pH values was formulated to simulate three possible co-treatment processes, i.e., (1) adding raw AMD to the activated sludge aeration tank, (2) pre-treating AMD prior to adding to the aeration tank by mixing with digested sludge and (3) pre-treating AMD by mixing with screened MWW. Continuous AMD loading to the activated sludge reactors during co-treatment did not cause a significant decrease in chemical oxygen demand (COD), 5-day biochemical oxygen demand, or total organic carbon removal; average COD removal rates ranged from 87-93%. Enhanced phosphate removal was observed in reactors loaded with Fe- and Al-rich AMD, with final effluent TP concentrations<2 mg/L. Removal rates for dissolved Al, Cu, Fe and Pb were 52-84%, 47-61%, 74-86% and 100%, respectively, in both systems. Manganese and Zn removal were strongly linked to acidity; removal from net-acidic AMD was <10% for both metals, whereas removal from circum-neutral AMD averaged 93-95% for Mn and 58-90% for Zn. Pre-mixing with screened MWW was the best process option in terms of AMD neutralization and metal removal. However, significant MWW alkalinity was consumed, suggesting an alkali supplement may be necessary.

Inhibitory effects of non-ionic surfactants on the microbial activity of activated sludge system

The purpose of the present work was to study the influence of a non-ionic surfactant, a nonylphenol with four ethoxylated units (NP4EO), on the activity of the microbial population present in a laboratory-scale activated sludge unit. Traditional control methods for this type of unit were used (measurement of suspended solids and chemical oxygen demand) as well as specific techniques for the measurement of bacterial activity (dehydrogenase activity and specific oxygen uptake rate) and the results were compared. It was shown that the Specific Oxygen Uptake Rate (SOUR) is the simplest and quickest way to carry out routine control of activated sludge activity, while measuring dehydrogenase activity provides more complete control. The results obtained indicated that there was no inhibition of microbial activity at a concentration of 5 ppm of NP4EO, which was not the case with a concentration of 10 ppm.

Simultaneous biodegradation and adsorption of textile dyestuff in an activated sludge unit

Decolorization of textile dyestuff Everzol Turquoise Blue G by simultaneous biodegradation and adsorption was investigated using the white-rot fungus Coriolus versicolor and an activated sludge culture in a laboratory scale activated sludge unit. The fungus was used in the form of a biofilm on surfaces of inclined plates immersed in the aeration tank. Wood ash was used as adsorbent and environmental conditions were adjusted to T=25 °C and pH=5. Box–Wilson experimental design was employed to evaluate the effects of major process variables, dyestuff and adsorbent concentrations, and sludge retention time on decolorization efficiency. Response function coefficients were determined by regression analysis of experimental data and predictions were found to be in good agreement with the experimental results. The highest colour removal efficiency was found to be E=82% at dyestuff and adsorbent concentrations of D 0=200 mg l −1, A=150 mg l −1 and a sludge age of θ c=20 days, respectively.

Effect of chromium(VI) on the biomass yield of activated sludge

Effect of 1–25 mg/l of influent Cr(VI) concentrations on true biomass yield ( Y t) and decay coefficient ( k d) of activated sludge units was investigated in this study. To this purpose, completely mixed, once-through laboratory scale activated sludge units with acclimated cultures were operated. After obtaining dilute-out curves for each Cr(VI) concentration, the values of Y t and k d were determined. The results revealed that presence of Cr(VI) caused stimulatory effects on biomass yield and maintenance up to the Cr(VI) concentration of 25 mg/l. Lower Cr(VI) concentrations exerted higher stimulation of the system. Extended oxidation due to the presence of Cr(VI) and/or increased synthesis of cellular material due to the stimulatory effects of Cr(VI) were thought to be the mechanisms of the observed stimulation.

The Response of Activated Sludge Process to Hazardous Organic Wastes

The response of a laboratory-scale activated sludge unit to diethyl phthalate, dimethyl phthalate, 2,4-dinitrophenol, 2,4-dimethylphenol and 4-nitrophenol was examined. The system tolerated phthalate esters well and the acclimation was quick. The response of the system to stepwise increasing concentrations of other hazardous compounds was observed as increasing effluent COD and solids losses from the system due to changing settling characteristics of the sludge. It was concluded that: (1) Diethyl phthalate and dimethyl phthalate do not affect activated sludge process performance, and their removal rates (measured in terms of COD) are high (95 percent or better). (2) High concentrations (in excess of 900 mg/L) of phthalate esters are tolerated well by activated sludge, and acclimation to each step increase requires just a few days. (3) Acclimation to 2,4-dinitrophenol is slower, and approximately 8 mg/L of this compound is enough to upset system performance temporarily. (4) Activated sludge is also a viable process (in terms of COD removal) for wastes containing 2,4-dimethylphenol in addition to phthalate esters and 2,4-dinitrophenol. (5) Activated sludge may not tolerate hazardous organic wastes well if the wastewater composition is complex. (6) Acclimation of activated sludge process to a particular organic compound (4-nitrophenol in this study) will improve the response of the system to a shock load by the same compound.

Treatment of Cr(VI)-containing wastewater by addition of powdered activated carbon to the activated sludge process

The effect of powdered activated carbon (PAC) on the removal of hexavalent chromium [Cr(VI)] and chemical oxygen demand (COD) and on oxygen uptake rate (OUR) by activated sludge microorganisms was investigated in batch and continuous flow reactors. The initial adsorption rate of Cr(VI) by PAC was greater than that by activated sludge flocs, and the equilibrium data could be described well by the Freundlich isotherm equation. The addition of PAC was an effective method to allow continuous operation of the biological process in the presence of Cr(VI) and increases in oxygen uptake rate as well as Cr(VI) and COD removal efficiencies were evident. The average COD and Cr(VI) removal efficiencies with PAC addition were 96 and 41%, respectively, while those without PAC addition were 85 and 9%, respectively. The kinetic constants of biological oxidation were determined on the two laboratory scale activated sludge units, one with PAC addition and the other without PAC addition, fed with synthetic wastewater containing Cr(VI). The addition of PAC to the aeration basin increased overall microorganism growth rate and biological removal rate of the substrate.

An assessment of excess biological phosphorus removal during activated sludge treatment

Abstract Modified laboratory scale activated sludge units were designed and constructed specifically for excess biological phosphorus removal. Once operational several modifications were executed to enhance the removal of phosphorus, these included the improvement of anaerobic conditions; augmentations with acetate and Acinetobacter /Aeromdnas species; increases in sludge age and temperature. The major factor found to influence phosphorus removal was temperature, a 1°C increase resulted in a 2.6% increase in phosphorus removal in the 12–25°C temperature range. During periods of high removal experiments were undertaken to identify the mechanism involved.

The effects of sludge age and metal concentration on copper equilibrium in the activated sludge process

Abstract This research investigated the general effects and equilibrium behavior of copper in the activated sludge process under a range of organic loadings and influent copper concentrations. This was achieved in a two‐phase experiment using continuous‐flow laboratory scale activated sludge units.

The Effect of Aeration Basin Configuration on Activated Sludge Bulking at Low Organic Loading

Continuous-flow laboratory-scale activated sludge units were operated on domestic sewage at low F/M. In CSTR units at the F/M range of 0.05–0.25 g COD removed/g TMLVSS, day bulking did not occur with a weak (BOD5 = 139 mg/L) sewage feed and TMLSS = 1.5 g/L. Supplementation of sewage by blending with raw sludge produced a stronger sewage (BOD5 = 315 mg/L) which caused TMLSS to increase to 3.5 g/L. Bulking occurred in CSTR, 2, 4, 8, and 16 compartment units. An aeration basin with initial compartment l/32nd of aeration basin volume prevented but did not cure bulking. An aeration basin with initial compartments 1/74th of aeration basin volume, prevented and cured bulking. Anoxia in an aeration basin with two initial compartments each 1/64th of the total aeration basin volume did not aid in curing bulking. The SVI of sludges at low F/M (0.3–0.35 g COD removed/g TMLVSS, day) and high TMLSS (3.5 g/L) is related to conditions in the initial compartment rather than those in the remainder of the aeration basin. Initial compartment soluble COD, first-compartment F/M, and initial compartment size are important; floc loading, dispersion number and total number of aeration basin compartments are not important.

Stalked bacteria in activated sludge

A laboratory-scale activated sludge unit was fed continuously with a simulated industrial wastewater, consisting of a dilute solution of inorganic salts, at a rate giving a mean retention time of about 3 days. The system produced a well-settling sludge which on examination by electron microscopy was found to contain considerable numbers of stalked bacteria. These were identified as Caulobacter, which have the ability to attach to surfaces and other organisms by means of a prosthecal ‘holdfast’ and to flourish in waters with a low content of organic nutrients, and whose occurrence in activated sludge has not apparently been previously recorded. Conditions advantageous to Caulobacter generally prevail in activated sludge systems when these operate in growth phases tending to produce well-settling sludge. Since their ‘holdfast’ gives Caulobacter the ability to initiate and enlarge microbial clusters by attachment, it is suggested that Caulobacter contribute to microbial floc-formation in activated sludge.

Response of dissolved oxygen to changes in influent organic loading to activated sludge systems

The major objective of this investigation was to examine the response of the activated sludge process to transient organic loadings and to evaluate the applicability of dissolved oxygen (DO) concentration and oxygen uptake rate as process control variables. A modified laboratory-scale activated sludge unit was used to continuously measure the ambient dissolved oxygen level. The change of the dissolved oxygen concentration responded to variations in the influent composition of the wastewater. First, the system was maintained at steady-state and system variables were measured for soluble TOC, MLVSS and oxygen uptake rate. The organic shock loading was induced by increasing or decreasing the baseline feeding of TOC concentration. As soon as a transient loading occurred the DO levels in the system were continuously monitored with the DO analyzer. A simplified material balance equation for dissolved oxygen in the system was developed and the dynamic behavior of oxygen uptake rate was examined. It was demonstrated that under transient conditions the changes in the dissolved oxygen concentration reflected the variations in the exogenous respiration rate of the biomass in the system due to fluctuations in the influent waste characteristics. Based upon the rapid response to the shock loading and the correlation of the change in the DO level to the magnitude of the shock loading, the two variables (DO concentration and oxygen uptake rate) generated from this biological monitor showed potential as a control variable for the activated sludge process.

Relationship between the observed cell yield coefficient and mean cell residence time in the completely mixed activated sludge process

A laboratory scale activated sludge unit was operated under highly controlled conditions. Hydraulic detention time was maintained constant and between 6 and 6.5 h. Sludge wasting rate was the only process parameter varied. By varying sludge wasting rate the mean cell residence was varied between 2 and 18 days. Within the span of 2–18 days only a minimal change in effluent COD was observed. Sludge production, however, was significantly altered. The data obtained were found to be described by both linear equation and an exponential equation.

Activated Sludge ATP: Effects of Environmental Stress

A series of experiments were performed with laboratory scale activated sludge units to determine the effects of various environmental conditions on the ATP content of activated sludge mixed liquor. ATP was determined by the firefly luciferin-luciferase reaction. The effect of abrupt temperature changes up to about 37°C was small. Anoxia reduced the ATP content by about 40%, but an immediate return to the original level was noted when sludge was reaerated. Complex patterns of ATP response were found for substrate (milk solids) additions and for additions of various inorganic toxicants. ATP responses to toxic heavy metals are rapid and sensitive, and ATP appears to be a good measure of sludge viability and activity.

Compartmentalization of Aeration Tanks

It has been shown theoretically that an activated sludge process in which longitudinal mixing in the aeration tank is suppressed should produce a better quality effluent than one in which the contents of the aeration tank are well mixed. The results of a laboratory investigation to confirm this theoretical prediction are presented. The experimental studies were conducted using a laboratory-scale activated sludge unit. Longitudinal mixing in the aeration vessel was controlled by dividing the vessel into compartments. Tracer experiments were used to determine the level of mixing under different operating conditions. The biological experiments were conducted using samples of activated sludge and a skim milk solution as the feed medium. The results of the tracer experiments showed good correlation with the previously presented mathematical model. The results of the biological experiments showed that the mathematical model provides a reasonable prediction of sludge growth in the aeration vessel but that the predictions of effluent quality from the model are not as accurate as might be desired.

Laboratory scale activated sludge unit.

Laboratory scale activated sludge unit.