ROLES OF MICROORGS IN SEWAGE TREATMENT
Introduction:
Many of the microbes present in wastewater are beneficial. In fact, many wastewater treatment technologies are dependent on these beneficial microorganisms for remediation of wastewater so that it won't detrimentally impact the environment. One of the primary goals of biological treatment is the removal of organic material from wastewater so that excessive oxygen consumption won't become a problem when it is released to the environment. (Maier et al, 2009) Another goal of biological treatment is nitrification/denitrification. Nitrification is an aerobic process in which bacteria e.g Nitrosomonas sp, Nitrobacter sp oxidize reduced forms of nitrogen. Denitrification is an anaerobic process by which oxidized forms of nitrogen are reduced to gaseous forms, which can then escape into the atmosphere. This is important because the release of nitrogen to the aquatic environment can also cause eutrophication. (Willey et al, 2008)

Another goal of biological treatment is elimination of pathogenic microorganisms either through predation or out-competition. The oxidation/stabilization of organic sludge is also of importance in biological treatment of wastewater.
The oxidation of organic materials in the waste water before discharge into the environment can have profound effects on the maintenance of aquatic life and the aesthetic quality of waters. Biochemical oxidation reactions involve the conversion of organic material using oxygen and nutrients into carbon dioxide, water and new cells. The equation that expresses this is:
Organic material + O2 + nutrients CO2 + H2O + new cells + nutrients + energy
It can be seen from this equation that organisms use oxygen to breakdown carbon-based materials for assimilation into new cell mass and energy. (Maier et al, 2009)

There are different stages of sewage treatment and each involves different methods. These stages are;
1. Pre-treatment: this is an initial separation of materials which are coarse, grit and other substance like oils. (Willey et al, 2008)
2. Primary treatment: this is a physical process of removal of suspended solids using floatation and sedimentation techniques in basins or settling tanks.
3. Secondary treatment: this is also known as biological treatment, involving biodegrading processes of microorganisms (Maier et al, 2009). There is stabilization and breakdown of organic matter through aerobic and anaerobic means. Activated sludge systems, oxidation ponds, aerated lagoons, trickling filters, Imhoff and septic tanks etc are used in this stage. (Maier et al, 2009)
4. Tertiary treatment: this state is for final purification of the water using advanced techniques like membrane filtration etc. this is necessary depending on the final destination of the water. (Atlas, 1997)

Biological Treatment Methods, Microbes Involved And Roles
A. AEROBIC SYSTEMS
1. TRICKLING FILTER METHOD: here the wastes absorb oxygen when sprayed through the air and allowed to trickle through bedrock of synthetic media carrying a layer of microbial biofilm which decomposes organic matter in the waste stream. The media used may be stones, hard coal or polyvinyl chloride. (Maier et al, 2009)

Microorganisms found here are mainly;
Bacteria e.g. Flavobacterium, Nitrobacter hamburgensis, Achromobacter spp, Zooglea sp, Chromobacter sp, Thiobacillus denitrificans, Beggiotta alba, Sphaerotilus natans.
Protozoa e.g. Amoeba, Vorticella, Paramecium.
Fungi e.g. Fusarium sp, Trichosporon sp and Geotricum sp.
Algae e.g. Spirullina sp, Chlorella sp.

BASIC ROLES: the basic roles of this community of biofilm microorganisms are:
a. Absorbing nutrients in the sewage and eventually mineralizing them through oxidative processes. (Atlas, 1997)
b. High reduction in the BOD of the effluent.
c. Algae help in aeration due to photosynthetic activity. (Cheremisinoff, 1996)
d. Protozoa predate on the numerous microbes and pathogens in the waste water.

2. OXIDATION PONDS OR LAGOON: These are large earthen basins that are used to treat waste water via natural clarification means by bacteria and algae. The shallow basins allow for natural aeration via wind diffusion and photosynthetic activity. (Maier et al, 2009)
Microorganisms found here are mainly;
Bacteria: Pseudomonas aeruginosa, Flavobacterium sp, Thiobacillus denitrificans, Nitrobacter sp and other Pseudomonas sp.
Algae: Euglena vulgaris, Spirogyra, Ulothrix, Spirullina sp etc
Protozoa and Ciliates e.g. Paramecium, Calpidium, Amoeba etc
BASIC ROLES:
a. The bacteria (usually heterotrophic) help degrade sewage organic matter and produce mineral products and cellular materials and CO2 that support the growth of algae.
b. The algae replenish oxygen in the pond which aids in bacteria metabolism. They also produce some antibiotics that can kill some coliforms in the water.
c. The protozoa and ciliates feed on coliforms and thereby reduce the prevalence of pathogenic organisms in the water. (Cheremisinoff, 1996)

3. ACTIVATED SLUDGE SYSTEM: Here, incoming waste water is mixed with existing biological sludge containing microorganisms and the nutrients in the waste water utilized for metabolism with complete oxidation to CO2 and H2O while aeration is provided alongside for the process.
Microorganisms found here are mainly;
Bacteria: Zooglea ramigera, Escherichia coli, Pseudomonas sp, Bacillus sp, Flavobacterium sp, Achromobacter sp, Micrococcus sp, Mycobacterium sp, Enterobacter sp and other filamentous bacteria.
Protozoa: Amoeba, Paramecium, Calpidium etc and many other rotifers.
Fungi: Fusarium sp, Trichosporon sp. Geotricum sp. Etc
Algae: Spirogyra, Ulothrix, Euglena vulgaris, Spirullina sp etc
BASIC ROLES: a. The bacteria (usually heterotrophic) perform oxidation and hydrolysis reactions and help degrade sewage organic matter and produce mineral products and cellular materials and CO2 that support the growth of algae.
b. The algae replenish oxygen in the pond which aids in bacteria metabolism. They also produce some antibiotics that can kill some coliforms in the water.
c. Protozoa and rotifers feed on coliforms and thereby reduce the prevalence of pathogenic organisms in the water. They act as effluent polishers. (Cheremisinoff, 1996)
d. Nitrate reducing bacteria helps accomplish denitrification reducing the total ammonia load of the waste water.
e. Zooglea ramigera produces a polysaccharide gum which is useful in floc formation during flocculation. This binds both the microbes and suspended colloids together for easy settling and sedimentation.
f. Floc formation by Zooglea sp aid in the removal of particles, colour and odour from the water.
B. ANAEROBIC SYSTEMS
This involves the use of Imhoff tanks and septic tanks. The waste is digested in an anaerobic system with anaerobic microbes converting the organic matter to biogas i.e. methane and CO2¬. Here the products are stabilized solids, ammonia and gases from volatile solids. (Evans and Furlong, 2003)
Microorganisms found here are mainly;
Hydrolytic Bacteria and Fungi: Clostridium sp, Fusobacterium sp, Selenomonas sp, Butyrivibrio sp, Streptococcus sp, Actinomyces.
Acidogenic Bacteria: Campylobacter sp, Peptococcus sp, Clostridium sp etc (Zeikus 1980)
Acetogenic Bacteria: Syntrophomonas wolfei, Syntrophobacter wolinii, Acetobacter sp etc
Methanogenic Bacteria: Methanobacterium sp, Methanothrix sp, Methanobacter sp, Methanococcus sp, Methanosarcina sp etc. (Zeikus, 1980)
BASIC ROLES: a. Hydrolytic bacteria produce enzymes like lipases, cellulases, proteases, amylases etc which they use to convert polymers (carbohydrates, proteins, lipids etc) to smaller monomeric units of simple sugars, long-chain fatty acids and amino acids which are then metabolized.
b. Acidogenic bacteria ferment these sugars, fatty acids and amino acids to simple acids like propionate, acetate, lactate and ethanol. (Zeikus, 1980)
c. Acetogenic bacteria produce acetate by further digestion of sugars and ethanol.
d. Methanogenic bacteria produces methane (CH4) gas and CO2 from acetate, methanol.