In case sludge contains iron it is red in colour. In case it is lime sludge, it is grayish brown in colour. This sludge also has an objectionable odour.

A recessed plate filter press yields highest cake solid formation while handling sludge disposal. However, it has low suspended solids in filtrate. The capital cost in this treatment method is high.

Packed bed reactors are not used for oxidation reaction. Oxidation is carried out in a venturi reactor, tank in series and stirred tank reactors. Oxidation of Iron, Oxidation of manganese, dechlorination of SO₂ and ozone reactions are carried out in se reactors.

Coarse screens remove coarse materials. The size of coarse screen is 6mm-25mm. These coarse screens maybe manual or mechanically driven.

Screens are in use from intake structure where y prevent floating matter enters into pumping units, and fine and coarse screens are in use to treat waste water to prevent entry of floating wastes and coarse solids into treatment.

The maximum temperature up to which sludge can be heated in order to carry out conditioning is 60 degree. By heating sludge soluble BOD increases which are contained in supernatant. This sludge preheating method enhances dewatering performance.

Pumping facilities may be employed to gain sufficient head for wastewater to flow through treatment works to point of final disposal. Pumping is also generally required for recirculation of all or part of flow around certain units within plant. Pumping facilities are classified as influent, effluent, or recirculation stations and perform a critical function.

The maximum approach velocity for a mechanical screen would be in range 0.6-1m/s. The minimum approach velocity would be in range 0.3-0.5m/s. These mechanical screens reduce manual labour.

Humans deposit about 70 g per capita per day of solids into wastewater. With ‘garbage grinders’, this can reach 100g per day. The impurities present in wastewater must eir be transformed into innocuous end-products or be effectively separated from effluent stream.

In open reactors diffusion and dispersion contribute to solute fluxes into and out of reactor. An example of an open reactor is ground water with contaminant plume. In this case, plume flows along with ground water by diffusion.