Kinetics of multicomponent substrate removal by activated sludge

A kinetic model for multicomponent substrate removal by activated sludge has been presented. The model is based on the linear removal concept which is a special case of the broader Monod equation. Both the simultaneous and sequential removals have been considered in the kinetic model. It maintains the “order of reaction” analogical to chemical reaction kinetics, which in this case simulates random and gradual diminuation of individual components with time, and is not limited to the integers only. The model has been verified experimentally and it has been found that the variation coefficients of the substrate removal kinetics constants were 3–5 times lower than those of the chemical reaction kinetics constants, calculated from the same data.     

Effluent variability estimation for complete-mix activated sludge treatment systems

A frequency transform technique has been employed for development of the mathematical models describing the wastewater influent variability removal for completely-mixed biological treatment plants. The influent variability was simulated by pulse input step function harmonic variations, and random variations. Experimental and theoretical results were compared for random, pulse and step function inputs. Observed influent-effluent variation characteristics correlated well with the theoretical relationship.     

Phosphorus removal in the activated sludge process

The results from this research suggest that both calcium phosphate precipitation and enhanced biological uptake play a role in phosphorus removal in the activated sludge process when a non-nitrifying, anaerobic-aerobic system is used to treat a low calcium wastewater. The primary removal mechanism was found to be biological uptake, as calcium phosphate precipitation accounted for only 15–27% of the total phosphorus removed. Calcium phosphate precipitation in the aerobic unit was enhanced because of the pH increase in that reactor. This was the result of low CO₂ production (indicated by low specific oxygen uptake values) and intense aeration which caused excessive CO₂ stripping in the aerobic unit     

Soluble phosphate removal in the activated sludge process

This study is intended to be a full scale process feasibility study which will translate experimental results into a form which can be used by practicing engineers as part of the overall effort to combat eutrophication of our waters. Data collected to date show the process to be a reliable method for precipitating phosphorus from domestic wastewater while at the same time realizing enhanced removal of BOD. Detailed operating procedures and cost data are being developed and will be part of the final report.     

Biooxidation in a continuous activated sludge process

The paper presents a critical examination of the biochemical oxidation rate theory in the activated sludge process based on many years of lab, pilot and full scale experiments. A new critical equation relating the overall biochemical reaction rate on the activated sludge load is introduced. The concept introduced is shown to be valid for conventional activated sludge loading range for single and multistage process.     

The activated sludge process—3 single sludge denitrification

The general aerobic bi-substrate active-site death-regeneration activated sludge model including nitrification of Dold et al. (Prog. Wat. Technol.12, 47–77, 1980) is extended to include the kinetic behaviour of the denitrification process in single sludge systems. The extension requires a change in the value of only one of the kinetic constants (K′_mp) in the expression for the particulate substrate utilization rate when the environment becomes anoxic. The extended model simulates very closely the response of the multi-reactor nitrification-denitrification process configurations under both constant and cyclic flow and load conditions. Under constant flow and load conditions, the denitrification response predicted can be reduced to that approximated by a zero order reaction dN/dt = ?KX_a with two rates in the primary and one in the secondary anoxic reactor respectively.     

Cell replication and biomass in the activated sludge process

Because of the inherent difficulty in enumerating cell numbers in a flocculent suspension, deoxyribonucleic acid (DNA) was chosen as an indicator of cell numbers in the activated sludge process. A direct relationship between the concentration of DNA in the sludge and plate count was found with a dispersed growth of mixed culture microorganisms. Therefore, it was assumed that a direct relationship existed between cell numbers and DNA concentration in the sludge in a flocculent suspension of mixed culture microorganisms as found in activated sludge.An increase in DNA was therefore assumed to be an indication of cell replication. An increase in biomass before an increase in DNA, indicated a storage of substrate in some form and not replication of organisms. The average increase in the weight per cell was determined by dividing the weight of biomass just prior to an increase in DNA by the initial weight of organisms present. Storage was a function of the loading rate to which the organisms were acclimated. A sludge acclimated to a loading rate of 2.0 per day increased 270 per cent in biomass before replication. Sludges acclimated to lower loading rates showed an extended time lag before cell replication occurred, while higher loading rates maintained the sludges in a more active state having shorter time lags before replication occurred. Increased frequency of feeding also resulted in less time before replication occurred.Replication commenced as long as 4 h after the external substrate was exhausted and the maximum biomass was reached in the F/M = 0.4 system. Thus, in the contact stabilization process, cell replication would be expected in the stabilization tank with only substrate storage taking place in the contact tank, due to the low loading rates used.     

Evolution of microbial communities in the activated sludge process

In a series of bench scale activated sludge experiments, the evolution of sludge microbial communities was studied. The different communities required 2–3 months to reach functional optimum as measured by parameters such as substrate removal efficiency, effluent suspended solids and sludge volume index. Nevertheless, a period of at least 4 months, corresponding to 10 mean cell residence times, was necessary before full nitrification and minimum endogenous respiration were reached. Inoculation with wastewater sludge enhanced the evolvement of the microbial community, but was not essential. Activity parameters such as invertase and ATP-content, as well as the behaviour of the numerically dominant species, suggest that a microbial community evolves to a climax pattern rather than to a distinct type of climax state.     

Equilibrium of bod materials in the activated sludge process

A mathematical model based on the equilibrium of BOD materials in the activated sludge process was developed. When the excess sludge is negligibly small and the BOD concentration of the influent is given, the BOD concentration of the activated sludge or mixed liquor and the efficiency of BOD removal are determined only by the aeration period. Moreover, it was verified mathematically that the performance of the activated sludge process became unstable with decrease of the return sludge ratio and the aeration period.     

Fractionating soluble microbial products in the activated sludge process

Soluble microbial products (SMP) are the pool of organic compounds originating from microbial growth and decay, and are usually the major component of the soluble organic matters in effluents from biological treatment processes. In this work, SMP in activated sludge were characterized, fractionized, and quantified using integrated chemical analysis and mathematical approach. The utilization-associated products (UAP) in SMP, produced in the substrate-utilization process, were found to be carbonaceous compounds with a molecular weight (MW) lower than 290 kDa which were quantified separately from biomass-associated products (BAP). The BAP were mainly cellular macromolecules with an MW in a range of 290-5000 kDa, and for the first time were further classified into the growth-associated BAP (GBAP) with an MW of 1000 kDa, which were produced in the microbial growth phase, and the endogeny-associated BAP (EBAP) with an MW of 4500 kDa, which were generated in the endogenous phase. Experimental and modeling results reveal that the UAP could be utilized by the activated sludge and that the BAP would accumulate in the system. The GBAP and EBAP had different formation rates from the hydrolysis of extracellular polymeric substances and distinct biodegradation kinetics. This study provides better understanding of SMP formation mechanisms and becomes useful for subsequent effluent treatment.     

The influence of sludge age on heavy metal removal in the activated sludge process

In a laboratory simulation of the activated sludge process ten heavy metals were added continuously to the system which was allowed to equilibrate at six sludge ages between 3 and 18d. Cobalt, manganese and molybdenum removals were poor and were unaffected by changes in the sludge age. The highest removal efficiencies for the other metals occurred at the 15d sludge age. Chromium (trivalent) and cadmium had the highest removal efficiencies, typically greater than 50%. The behaviour of the majority of the metals which were removed to a significant extent was related to one of the parameters influenced by sludge age, i.e. mixed liquor suspended solids, effluent suspended solids or effluent chemical oxygen demand. The metals which were poorly removed showed little affinity for the activated sludge, while most metals exhibited maximum specific uptake by the mixed liquor at a sludge age of 9–12d. However, the affinity of silver for the mixed liquor continued to increase as the sludge age increased to 18d. An affinity series, based on an arbitrary measure of the specific accumulation of metals by the mixed liquor, indicated that chromium, cadmium and silver were most readily adsorbed by the activated sludge.     

The effects of organotin on the activated sludge process

Organotin compounds which find increasing use in marine antifouling paints may be present in the discharge from dry dock operations. This investigation was aimed at determining the effect of such wastewater when discharged to a municipal activated sludge treatment plant. Experiments were conducted using a Warburg respirometer and continuous flow bench-scale activated sludge systems. The results showed that unacclimated biological cultures can be inhibited by tributyl tin oxide (TBTO^™) concentrations as low as 25 μgl⁻¹. However, TBTO doses of over 8000 μgl⁻¹ can be tolerated by a well acclimated culture. Continuous loading of up to 1000 μgl⁻¹ TBTO had no effect on organic removal in activated sludge systems. However, an adverse effect on sludge settleability was noticed at 100 μgl⁻¹ TBTO. Shock loadings of 500 and 1000 μgl⁻¹ TBTO had no effect on soluble organic removal but resulted in impaired settling and higher effluent suspended solids. The LC₅₀ of TBTO to the fathead minnow was estimated at 45–200 μgl⁻¹. The toxicity was reduced considerably by activated sludge treatment.     

Redox measurement of treatability by the activated sludge process

The change in redox potential with time following the addition of a substrate to an aerated activated sludge suspension follows a well-defined pattern characteristic of the substrate. The shape of the trace indicates the relative ease or difficulty with which the sludge can purify a substrate or waste material. Successive additions causing a sludge to adapt result in a change in the shape of the trace.     

Optimal aeration control in a nitrifying activated sludge process

An important tool to minimise energy consumption in activated sludge processes is to control the aeration system. Aeration is a costly process and the dissolved oxygen level will determine the efficiency of the operation as well as the treatment results. What aeration control should achieve is closely linked to how the effluent criteria are defined. This paper explores how the aeration process should be controlled to meet the effluent discharge limits in an energy efficient manner in countries where the effluent nitrogen criterion is defined as average values over long time frames, such as months or years. Simulations have been performed using a simplified Benchmark Simulation Model No. 1 to investigate the effect of different levels of suppressing the variations of the effluent ammonium concentration. Optimisation is performed where the manipulated variable for aeration (the oxygen transfer coefficient, K_La) is minimised with the constraint that the average daily flow-proportional ammonium concentration in the effluent should reach a desired level. The optimisation results are compared with constant dissolved oxygen concentrations and supervisory ammonium control with different controller settings. The results demonstrate and explain how and why energy consumption can be optimised by tolerating the ammonium concentration to vary around a given average value. In these simulations, the optimal oxygen peak-to-peak amplitude range between 0.7 and 1.8 mg/l depending on the influent variation and ammonium level in the effluent. These variations can be achieved with a slow ammonium feedback controller. The air flow requirements can be reduced by 1-4% compared to constant dissolved oxygen set-points. Optimal control of aeration requires up to 14% less energy than needed for fast feedback control of effluent ammonium.     

Possible cause of excess sludge reduction in an oxic-settling-anaerobic activated sludge process (OSA process)

Modification of a conventional activated sludge process by inserting a sludge holding tank in a sludge return line forms an oxic-settling-anaerobic (OSA) process that may provide a cost-effective way to reduce excess sludge production in activated sludge processes. In this paper we systematically evaluate the following possible scenarios that may explain the reduction of excess sludge in the OSA process: (i). energy uncoupling, (ii). domination of slow growers, (iii). soluble microbial products (SMPs) effect and (iv). sludge decay in the sludge holding tank under a low oxidation-reduction potential (ORP) condition. Results show that only the final scenario may reasonably explain this reduction. It has also been found that the sludge decay process in the sludge holding tank may involve the reduction of the cell mass.     

Nitrification process in activated sludge with suspended marble particles

The process of nitrification in activated sludge was investigated. As the solid support for nitrifiers' growth a suspension of marble particles has been used. The results proved the possibility of successful nitrification of 100 mg l^?1 NH₄-N simultaneously with the removal of 600 mg l^?1 COD.     

Stochastic modeling of an industrial activated sludge process

The purpose of this paper is to develop a Box-Jenkins time series transfer function model of the effluent COD using the influent COD, from a sewage treatment process. The data is from an industrial waste treatment plant encompassing a 14-month period.The work was done in 3 steps: (1) Box-Jenkins ARIMA (autoregressive integrated moving average) modeling of the influent COD; (2) ARIMA modeling of the effluent COD; and (3) development of a transfer function model for the waste treatment process.It was found that single difference models best described influent and effluent COD. For the transfer function model, the influent COD had an insignificant effect on the effluent COD.     

Feed forward control of an activated sludge process

The control of an activated sludge process is an inherently difficult problem. The non-linearty of the process, the wide range of operating conditions, and the lack of a really suitable control variable all contribute to this difficulty. A computer simulation of a perfectly mixed aeration tank was used to show that feed forward control is the most suitable control method for a wide range of operating conditions. In particular, proportional control with measurement of the substrate flow rate and manipulation of the rate of return sludge was as effective as much more complex combinations of feed forward and feed back controllers. There was some indication that on—off control would be highly effective.     

Purification of high-salinity wastewater by activated sludge process

The objective of the present work was the enhancement of the efficiency of microbiological processes in purification of high-salinity oil-field brine by adding the powdered activated carbon (PAC) to the bioreactor and by diluting the brine with surface water. The investigation was carried out in a laboratory setup. Results show that the nature of pollutants and the high salinity (about 29 g/l) of oil-field brine had an unfavourable effect on the activated sludge process. Higher hydraulic loads (above 2.5 d⁻¹) increased wash-out of the activated sludge from the bioreactor, so that an increased concentration of the suspended matter in the settler was observed. The addition of PAC to the bioreactor, combined with the dilution of oil-field brine with surface water, produced a substantial improvement in the sludge volume index. Also, under the given conditions, it was possible to maintain the given concentration of the activated carbon in the bioreactor. The procedure involving PAC and dilution with surface water resulted in an increased rate of pollutant degradation. The positive effect of PAC on the process of oil-field brine purification with the activated sludge was explained by the possibility of immobilization of microorganisms on the activated carbon surface in the form of a biofilm.