Bioremediation

Sewage water treatment uses a variety of microorganisms present in sewage, such as bacteria, fungi, protozoa, algae, nematodes, amoebae and viruses. The important bacteria mentioned are coliforms, streptococci, clostridia, micrococci, Proteus, Pseudomonas and lactobacilli.

Their role is to break down and stabilise organic matter in sewage during biological treatment. In the activated sludge process, aerobic microbes oxidise organic matter to CO₂ and H₂O and mineralise complex substances into inorganic forms. In trickling filters and oxidation ponds, microbes digest the organic load and reduce BOD. Thus, microorganisms help in degradation of sewage pollutants and treatment of wastewater before discharge.

Biological Oxygen Demand (BOD) is a measure of the amount of dissolved oxygen required by microorganisms to decompose the oxidisable organic matter present in sewage.

- A higher BOD means more oxidisable organic matter, so the sewage is strong.
- A lower BOD means less organic load, so the sewage is weak.
- When strong sewage enters water, microorganisms use up dissolved oxygen quickly, causing oxygen depletion, which can kill fish and other aquatic animals.

So, BOD represents the condition of sewage water by showing how much organic pollution it contains.

A toxic substance gets biomagnified when it is non-biodegradable or persists for a long time in the environment.

- It enters the food chain from soil or water.
- Small organisms absorb it first.
- When larger organisms eat many smaller contaminated organisms, the toxin accumulates in their bodies.
- At each higher trophic level, its concentration increases.

Thus, the toxic substance becomes more concentrated in top consumers, including humans. This increase in concentration along the food chain is called biomagnification.

Xenobiotic compounds are synthetic compounds or foreign chemical substances not naturally present in the environment, such as many pesticides and industrial chemicals.

These compounds affect soil productivity because many of them are persistent and non-biodegradable. They remain in soil for a long time, may be toxic to useful soil microorganisms, and can reduce microbial activity and nutrient cycling. As a result, the fertility and productivity of soil decrease.

Sewage wastewater treatment involves both aerobic and anaerobic decomposition.

### Aerobic decomposition
- It occurs in the presence of oxygen.
- In the activated sludge process, compressed air is pumped into sewage.
- Aerobic microbes break down organic matter into CO₂ and H₂O and mineralise it into substances like nitrates, phosphates and sulphates.
- In trickling filters and oxidation ponds, aerobic microbes also digest organic matter.

### Anaerobic decomposition
- It occurs in the absence of oxygen.
- The sludge collected after primary treatment is sent to a sludge digester.
- Anaerobic microbes decompose the organic matter into soluble substances and gases such as methane (60–70%), CO₂ (20–30%), H₂S, H₂ and N₂.
- This biogas can be used as fuel.

So, aerobic treatment mainly reduces organic matter in the presence of oxygen, while anaerobic treatment digests sludge without oxygen and produces biogas.

The different types of solid wastes produced are:

- Residential wastes: food waste, paper, cardboard, plastics, glass, metals, ashes, etc.
- Industrial wastes: housekeeping waste, packaging waste, construction and demolition materials, hazardous waste, ashes, etc.
- Commercial wastes: paper, cardboard, plastics, wood, food waste, glass, metals, etc.
- Construction and demolition wastes: wood, steel, concrete, dirt, etc.
- Municipal service wastes: street sweeping waste, landscape and tree trimming waste, sludge, etc.
- Process/manufacturing wastes: scrap materials, off-specification products, slag, tailings, etc.
- Agricultural wastes: spoiled food wastes, agricultural wastes, hazardous wastes like pesticides.

So, solid wastes come from households, industries, commercial areas, construction sites, municipal services and agriculture.

Different microorganisms help in composting in different ways:

- Bacteria: They form about 80–90% of the microorganisms in compost. They carry out rapid decomposition of organic matter. At higher temperatures, bacteria like Thermus are found.
- Actinomycetes: These filamentous bacteria degrade lignin, cellulose, chitin and proteins. They help break down woody stems, bark, newspaper and cardboard.
- Fungi: They decompose many complex polymeric waste materials and are present during all phases of composting, mostly in the outer layer.
- Protozoans: They act as secondary consumers and feed on bacteria and fungi in the compost matrix.
- Insects, mites, ants, earthworms and other worms also help in the composting process.

Thus, composting depends on a succession of microorganisms that decompose waste and convert it into humus and useful compost.

Pesticides are harmful for non-target organisms because:

- Many pesticides, especially organochlorines and organometallics, are persistent and non-biodegradable.
- They can remain in soil and water for a long time.
- Because of their lipid solubility, they accumulate in the bodies of organisms other than the target pests.
- They enter the food chain and undergo biomagnification, so their concentration becomes higher at higher trophic levels.
- This can harm birds, fish, animals and even humans.

Therefore, pesticides may damage non-target species directly and also through long-term accumulation in the ecosystem.