How do rhizobium bacteria increase the fertility of soil?</h2></div>

Sewage sludge is used as a</h2></div>

Where is the major part of sludge pumped in secondary treatment?

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Secondary sewage treatment is mainly </h2></div>

Write two advantages of Blue-green algae in farming.</h2></div>

In the following diagram, what does the question mark represent?

Wastewater treatment generates a large quantity of sludge, which can be treated by?</h2></div>