Newsletter and Technical Publications
<International Source Book On Environmentally Sound Technologies
for Wastewater and Stormwater Management>
2.3 The role of micro-organisms
As can be seen in Section 2 (2.2) micro-organisms, such as
bacteria, play an important role in the natural cycling of materials and
particularly in the decomposition of organic wastes. The role of micro-organisms
is elaborated further here because they are also important in the treatment of
wastewater. What is waste for humans and higher vertebrates becomes a useful
food substrate for the micro-organisms. In both natural and engineered treatment
systems micro-organisms such as bacteria, fungi, protozoa, and crustaceans play
an essential role in the conversion of organic waste to more stable less
polluting substances. They form what is termed a 'food chain'. For example
inorganic and organic substances in wastes are consumed by bacteria, fungi and
algae. These are in turn consumed by protozoa and nematodes (some fungi however
trap nematodes) and the latter by rotifers.
In a natural water body, e.g. river or lake, the number and type
of micro-organisms depends on the degree of pollution. The general effect of
pollution appears to be a reduction in species numbers. For example in a badly
polluted lake, there are fewer species but in larger numbers, while in a healthy
lake there can be many species present but in lower numbers.
Micro-organisms are always present in the environment and given
the right conditions of food availability, temperature and other environmental
factors, they grow and multiply. Figure 2.7 shows a generalised pattern of
growth of micro-organisms.
Figure 2.7: Generalised representation of growth of micro-organisms
Micro-organisms require cellular building blocks, such as
(carbon) C, (hydrogen) H, (oxygen) O, (nitrogen) N, (phosphorus) P, and minerals
for growth. These can be obtained through consuming organic substances
containing these elements, or from inorganic materials, such as carbon dioxide,
water, nitrate and phosphate. Micro-organisms also require energy. They obtain
this through respiration. In this process organic carbon is oxidised to release
its energy. Oxygen or other hydrogen acceptors is needed for the respiration
process. Algae and photosynthetic bacteria can also utilise energy from
sunlight, while certain types of bacteria can utilise energy from chemical
reactions not involving respiration. The building blocks and energy are used to
synthesise more cells for growth and also for reproduction.
In the treatment of wastewater three types of overall processes
are distinguished to represent the conversion of organic wastes by
micro-organisms. The classification is based on whether the environment where
the process takes place is aerobic, anaerobic or photosynthetic. Under aerobic
conditions (in the presence of oxygen), micro-organisms utilise oxygen to
oxidise organic substances to obtain energy for maintenance, mobility and the
synthesis of cellular material. Under anaerobic conditions (in the absence of
oxygen) the micro-organisms utilise nitrates, sulphates and other hydrogen
acceptors to obtain energy for the synthesis of cellular material from organic
substances. Photosynthetic organisms use carbon dioxide as a carbon source,
inorganic nutrients as sources of phosphate and nitrogen and utilise light
energy to drive the conversion process.
Micro-organisms also produce waste products, some of which are
desirable and some undesirable. Gases such as carbon dioxide and nitrogen are
desirable, since they can be easily separated and do not produce pollution.
Gases such as hydrogen sulphide and mercaptans, although easily separated
require treatment for odour. Micro-organisms' cellular materials are organic in
nature and can also cause pollution. It would be desirable if the cellular
materials have undergone self oxidation (endogeneous respiration utilising own
body cells) to produce non-biodegradable materials that are relatively stable.
Self-oxidation is achieved when there is no substrate/food available.
The microbiological conversion reactions of organic waste into
cellular material can be empirically represented as shown below.
(i) Conversion under aerobic conditions (see diagram below):
Under aerobic conditions ammonia is further oxidised to nitrate.
Phosphorus and sulphur contained in the organic substances are oxidised to
phosphate and sulphate. These can be further utilised by the micro-organisms for
(ii) Conversion under anaerobic conditions (see diagram below):
Methane (CH4) is a useful gaseous by-product of anaerobic
conversion, because it can be combusted to produce heat/energy. On the other
hand if it is released to the atmosphere without being combusted, it contributes
to the greenhouse gas effect.
(iii) Conversion under photosynthetic conditions:
aCO2 + rH2O + tNH3
Cw Hx Oy Nz + bO2
As shown by the conversion reactions (the utilisation of organic
wastes for food by micro-organisms) the product is mainly the cellular material
of the micro-organisms i.e. more organisms are produced. The growth yield is the
weight of micro-organisms produced per unit weight of organic substances
consumed by the micro-organisms. The growth yield depends on the type of
substrate and environmental conditions. The smaller the value of the growth
yield the better it is for waste treatment, because less sludge is produced
which requires disposal. Its value is usually between 0.2 and 0.5 for aerobic
conversion, while the corresponding value for anaerobic conversion is smaller.