Chemical Reactor Engineering

The activated sludge process (2005-present)

Background and motivation

The activated sludge process is widely used in wastewater treatment plants to reduce effluent levels in contaminated wastewaters originating from both the municipal and industrial sectors. It uses naturally occurring micro-organisms to remove pollutants, such as organic matter and nitrogen compounds, from wastewaters (sewage). The process generally consists of two units: an aerated biological reactor, in which bacteria are used to degrade pollutants, and a settling unit (or clarifier), in which the activated sludge settles to the bottom of the unit. The settling of the sludge, which contains most of the bacteria, at the bottom of the settling unit clarifies the treated wastewater, allowing it to be separated from the bacteria. The water is then discharged whilst most of the activated sludge, along with mixed liquor, is recycled from the bottom of the clarifier into the biological reactor. The settling process concentrates any microorganisms that are not discharged with the water. The activated sludge that is not recycled is disposed of using standard methods.

A key feature of the process is that large quantities of air are bubbled through the wastewater in open aeration tanks. The oxygen contained in the air is required by the bacteria, and other microorganisms present in the system, to live, grow and multiply.

The Curds model (2005-present)

We have investigated a model for the treatment of wastewater in the activated sludge process due to Curds (1971). The biochemical model assumes that the incoming sewage is broken down by two types of bacteria, sludge bacteria and sewage bacteria, and two types of ciliated protozoa, free-swimming ciliates and ciliates attached to sludge flocs. The recycling process is assumed to concentrate the sludge bacteria and the attached and crawling protozoa. The wastewater reactor is assumed to be well mixed, so the mathematical formulation for this process can be represented by a continuously stirred tank reactor with recycle.

Curds (1971) analysed the wastewater process by solving the governing equations numerically to determine the stable steady-states of the system for one particular set of parameter values. The single well-mixed reactor, as well as the three and five reactor cascades were analysed. Curds showed that the behaviour of the model was similar to those observed in full-scale and experimental activated-sludge plants.

Latter Jianqiang and Ray (2000) analysed the well-mixed two-reactor system by solving the governing equations numerically. They mainly focussed on the use of natural oscillations to improve the efficiency in the system. By natural oscillations, it is meant that the process parameters are chosen so that the steady input of sewage into the first reactor generates self-sustained oscillations in its output, which then forces the second reactor. The attraction of this method is that it uses no external energy to generate the oscillations. These authors showed that by implementing such a strategy, the performance of the cascade can be improved.

Our analysis combines steady-state analysis with path-following techniques. We have shown that such methods enable the dependence of the system efficiency upon the residence time for a single reactor with recycle [Watt et al 2006] and two reactors with recycle [Sidhu et al 2006, Sidhu et al 2009] to be readily obtained. For the cascade the total residence time in the two reactors was fixed, and the residence time in the first reactor was then treated as the primary bifurcation parameter. The optimal performance of the single reactor is used as a benchmark for comparison with performance of a cascade. For sufficiently low total residence times, an optimised single reactor was found to outperform a cascade. At sufficiently high total residence times, an optimised cascade outperforms an optimised single reactor. In some cases the improvement in the cascade performance may be small, however it was seen that to achieve the same level of efficiency as in the cascade, a single reactor would have to be operated for a significantly larger residence time.

  1. C.R. Curds. Computer Simulations of microbial population dynamics in the activated sludge process. Water Research, 5: 1049-1066, 1971.
  2. S. Jianqiang and A.K. Ray. Performance improvement of activated sludge wastewater treatment by nonlinear natural oscillations. Chem. Eng. Technol, 23, 1115-1122, 2000.

The activated sludge model number 1 (2006-present)

The activated sludge model number 1 (ASM1) was developed by Henze et al (1987). This model is an internationally accepted standard for activated sludge modelling. It describes nitrogen and chemical oxygen demand within suspended-growth treatment processes, including mechanisms for nitrification and denitrification. The model has been found to give a good description of the activated sludge process provided that the wastewater has been characterised in detail and is of domestic or municipal, but not industrial, in origin. The ASM1 model includes eight processes that are fundamental to the activated sludge process. These are: aerobic and anoxic growth of heterotrophic biomass, death of heterotrophic biomass, aerobic growth of autotrophic biomass, decay of autotrophic biomass, ammonification of soluble organic nitrogen and hydrolysis of both entrapped particulate organic matter and entrapped organic nitrogen.

The ASM1 model contains 13 differential equations. However, four equations, pertaining to inert soluble organic material, particulate inert organic material, non-biodegradable particulate products arising from biomass decay and alkalinity, uncouple from the remaining nine equations, and therefore do not affect the dynamics of the system.

We have analysed the ASM1 in a single reactor without recycle using continuation methods to determine the steady-state behaviour of the system (Sidhu & Nelson; 2007, Nelson & Sidhu; 2009). In particular, we determine bifurcation values of the residence time, corresponding to branch points, that are crucial in determining the performance of the plant. The first branch point marks a transition at which the washout solution becomes unstable. Thus the residence time must be higher than that at this branch point. Although both the total chemical oxygen demand and the concentration of biodegradable soluble substrate both decrease with increasing residence time, the second branch point marks a transition above which improvement in reactor performance with increasing residence time are much more marginal. If the reactor performance is to be significantly improved over that obtained at the second branch point then either very large residence times or a different reactor configuration, such as a cascade, must be used.

In (Nelson & Sidhu; 2009) the value of the residence time at which the second branch branch points occurs was found as a function of the oxygen transfer coefficient.

  1. M. Henze, L. Grady Jnr, C.P. Gujer, W.G.V.R. Marais and T. Matsua. (1987). A general model for single-sludge wastewater treatment systems. Water Research, 21, 505-515.

The activated sludge process: published papers

    Refereed journal papers

  1. S.D. Watt, H.S. Sidhu, and M.I. Nelson. Analysis of a model for the treatment of wastewater by the activated sludge process. The Anziam Journal, 47: C1-C18, 2006.
  2. M.I. Nelson and H.S. Sidhu. Analysis of the activated sludge model (number 1). Applied Mathematics Letters, 22, 629-635, 2009.

  3. H.S. Sidhu, S.D. Watt, and M.I. Nelson. Performance comparison between a two-reactor cascade and a single tank in an Activated Sludge Wastewater Treatment Process. International Journal of Environment and Waste Management, 3(3/4), 214-225 2009.

  4. Refereed conference proceedings

  5. H.S. Sidhu, S.D. Watt and M.I. Nelson. Activated sludge wastewater treatment process: Performance comparison between a two-reactor cascade and a single tank. In Proceedings of the 34th Australasian Chemical Engineering Conference, CHEMECA 2006, 6 pages (on CDROM). 2006. ISBN 0-86869-110-0.
  6. H.S. Sidhu and M.I. Nelson. Mathematical analysis of the activated sludge process for domestic wastewater treatment. In Proceedings of the 35th Australasian Chemical Engineering Conference, CHEMECA 2007, pages 712-720 (on CDROM), Engineers Australia, 2007. ISBN 0-858-25844-7.

Highlights of research

  1. Applied continuation methods to the Curds model in a single reactor and to a two reactor cascade with recycle.
  2. Applied continuation methods to the ASM1 model without recycle in a single reactor. Showed that the steady-state diagram contains two branch points. The branch point at the lower residence time marks a transition at which the washout solution becomes unstable. The branch point at the higher residence time marks a transition above which the improvement in performance with increasing residence time dramatically slows down (Sidhu & Nelson; 2007, Nelson & Sidhu; 2009).


Dr H.S. Sidhu. 2005-Present
Dr S.D. Watt. 2005-Present

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Page Created: 11th January 2010.
Last Updated: 12th January 2010.