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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 synthesis.

(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:

sunlight
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.

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