Biological Treatment of Municipal Wastewater
L.M. Chu
Department of Biology
Treatment of municipal wastewater usually involves biological processes such as the
activated sludge system in the secondary stage after preliminary screening which
remove coarse particles and primary sedimentation which settles out suspended solids.
These secondary treatment steps are generally considered environmental
biotechnologies which harness natural self-purification capacity in enclosed reactors
for the biodegradation of organic matter and bioconversion of soluble nutrients in the
wastewater. This talk attempts to introduce the secondary biological treatment of
wastewater from an environmental biotechnology viewpoint using activated sludge
process as an example. The aim of wastewater treatment, the operational requirement
of the bioreactor, factors affecting treatment efficiency and the current development
for biological nutrient removal will be discussed. Municipal facilities for wastewater
treatment in Hong Kong will also be illustrated.
Activated sludge process is a suspended growth system in which free living organisms
are exploited to breakdown the organic matter in the wastewater. Another type, which
is quite common overseas for smaller communities, is the trickling filters which
utilize surface attached organisms for wastewater purification. In the activated sludge
process, the wastewater is treated aerobically by a microbial consortium dominated by
heterotrophic bacteria which are flocculated in the mixed liquor of the aeration tank to
form discrete clumps of microorganisms. Protozoa and rotifers which enhance floc
formation and effluent clarification are also present in significant numbers.
Aeration is one of the key operational designs which contributes to the efficient
degradation of organic matter (BOD removal). This is done by bottom air blowers
such as porous diffusers, or surface aerators, which oxygenate the mixed liquor for
aerobic degradation. Other operational parameters which affect process efficiency
include mixed liquor suspended solids, organic loading, food to microorganisms ratio,
sludge residence time and sludge recycle ratio.
Effluent quality depends not only on the treatment process in the aeration tank, but
also on the separation of the flocs from the treated effluent by a final sedimentation
tank. The latter is dependent on floc settleability which is primarily affected by the
density of the flocs, as in the case of sludge bulking caused by filamentous bacteria
such as Sphaerotilus natans which interferes with the performance of many sewage
works both locally and overseas.
Another operational problem commonly
encountered is bacterial foaming which is caused by the excessive growth of
filamentous actinomycetes such as Nocardia amarae. The control measures for these
problems will be discussed.
Although activated sludge system is an aerobic process, modern wastewater treatment
plants which employ activated sludge system usually have anaerobic unit(s) or zone(s)
which allow biological denitrification to take place for nitrogen removal. This
anoxic-oxic (A/O) system is essential as conventional activated sludge process is not
effective in removing nutrients, which inevitably results in the eutrophication of the
water bodies receiving the effluent. Sewage treatment plants in Hong Kong all use
M-5-1
activated sludge process with up to 5 alternate aeration zones (modified Bardenpho
process).
References
Grady, C.P.L., Daigger, G.T. & Lim, H.C. (1999) Biological Wastewater Treatment.
2nd Ed. Marcel Dekker, New York. (Chapters 10, 11, 19 & 20)
Gray, N.F. (1989) Biology of Wastewater Treatment. Oxford Science Publications,
Oxford. (Chapters 2 & 4)
Bitton, G. (1994) Wastewater Microbiology. Wiley-Liss, New York. (Chapters 8, 9
& 10)
Forster, C.F. and Wase, D.A.J. (1987) Environmental Biotechnology.
Horwood, Chichester. (Chapters 1, 2 & 4)
Ellis
Rehm, H.J. and Reed, G. (1986) Biotechnology. Vol. 8 Microbial Degradations.
VCH, Weinheim. (Chapters 2 & 3)
Questions for revision
1. Why is the aeration tank of a sewage works considered a bioreactor?
2. What is common between biological wastewater treatment systems and natural
purification process?
3. What are the factors affecting the quality of the effluent discharged from an
activated sludge process?
4. Discuss the operational problems commonly encountered in activated sludge
treatment plants.
5. Aerobic treatment system is not necessarily a solely aerobic process. Discuss.
6. How is nitrogen removed from the wastewater in an activated sludge plant?
M-5-2