Question

Define i) primary treatment, ii) secondary treatment, and iii) sludge stabilization.

Answer 1

All kind of waste water produced must be treated prior to disposal. Treatment is mainly aimed to remove BOD from waste water. Prior to BOD removal , it is necessary to remove substances which interfere with BOD removal. Therefore waste water is treated in stages.

1) Primary treatment

2)Secondary treatment

3)Tertiary treatment

PRIMARY TREATMENT OF WASTE WATER

It is a preparatory process. In this step settlebale organic solids are removed.

SCREENS - It is the first unit operation employed in treating waste water. Screens are aimed to remove large floating objects. According to the size of screen openings ,it is classified into three

1)Coarse screen

2)Medium screen

3)Fine screen

GRIT CHAMBER - Grit chamber aimed to remove grit from waste water.

SKIMMING TANK - Skimming tank aimed to remove oils & grease from waste water. It is removed by applying compressed air from bottom.

PRIMARY CLARIFIER (PRIMARY SETTLING TANK)

It is aimed to capture settlebale organic solids present in water. It follows type 3 settling (Hindered settling). It removes

1) 60%-70% of organic solids which are biodegradable

2)20%-30% BOD

SECONDARY TREATMENT OF WASTE WATER

In this step soluble BOD is removed from waste water (BOD present in colloidal & dissolved form). Soluble BOD from waste water removed by

1) Physico-Chemical treatment methods

2) Biological treatment methods

Physico-Chemical treatment methods

This method is employed when

1) Micro screening 2) Adsorption 3) Absorption 4) Chemical oxidation 5) Chemical coagulation 6) Filtration

Biological treatment methods

This method is employed when,

complex organic matter present in waste water is converted to stable & simple products with the help of micro organisms.

biological treatment further classified into AEROBIC TREATMENT & ANAEROBIC TREATMENT.

Aerobic biological treatment of waste water

1) suspended growth process ( eg Activated sludge process)

2) attached growth process ( Trickling filters,Biotowers)

SLUDGE TREATMENT

Sludge obtained from treatment plant further treated for safe disposal. Sludge treatment is mainly aimed to achieve

1)volume reduction

2)strength reduction

Sludge thickening - reducing moisture content of sludge.

Sludge digestion - anaerobic decomposition for strength reduction and BOD removal.

Sludge conditioning - improving the drainability of sludge.

Sludge composting -

Sludge incineration -complete destruction of biosoilds by heat.

SLUDGE STABILIZATION

Sludge contains pathogens, heavy metals etc..it cause odour and other problems...so it is necessary to stabilize the sludge.

sludge stabilization is a chemical or biological process that stops the fermentation of sludge. In chemical stabilization, lime is

mixed with sludge. Because of lime pH of sludge will raise. In this condition growth of pathogens controlled.

eg: heat treatment is a sludge conditioning process. but it is helpful to stabilise the sludge.