What substances remain in wastewater after effective secondary treatment?

- The nitrates  still exist after the second treatment of water waste and the way to remove it is:
by using the enzymes in the organisms like bacteria to gradual change the NO3 to N2 to finally be able to remove the nitrogen from it.

- The inorganic quality of the water is changed when the water is being used in a municipality; inorganic salts containing chlorides, sulfates, phosphates, potassium and sodium among those added.
-But it removes by varies methods after the secondary treatment of water like filtration, sedimentation, and activated carbon adsorption.

- It remains biological nutrient, and micropollutants, such as environmental persistent pharmaceutical pollutants.

- But the secondary treatment removing dissolved and colloidal compounds measured as biochemical oxygen demand (BOD).

- even after the primary and secondary treatment, disease-causing organisms may remain in the water.

- To disinfect and kill harmful organisms, the water spends a minimum of 15-20 min in chlorine-contact tanks mixing with sodium hypochlorite.

- The sludge produced by primary and secondary treatment is approximately 99 % water and must be concentrated to enable its further processing.

-Thickening tanks allow the sludge to collect, settle and separate from water up to 24 h. The water is then sent back to the head of a plant or to the aeration tanks for additional treatment.