Pre-Treatment of Water – Environmental Engineering

Water from streams and dams is not pumped directly to the basins. There are some intermediate processes, through which the water is passed and treated and called pre-treatment of water.

1. Screening

2. Microstainer

3. Raw water storage

4. Pre-chlorination

5. Aeration

6. Algal Control

7. Preliminary settling

All of these processes are not required at every plant and only some of them may be required at a particular plant. Each of them performs a particular function, hence they are provided only when needed otherwise they can be omitted.

Screening is carried out for the removal of heavy suspended solids like plants, trees, stones. Pebbles etc. from the water.

• It is generally adopted for surface cleaning.
• Screens are generally provided in front of the pumps or the intake work, so as to exclude the large size particles.
• Screening is done with the help of the screens which are generally of two types:

• Coarse screens are in the form of bars of diameter 10 to 25 mm having spacing of 20 to 100 mm center to center in between them.
• These screens are sometimes placed in front of the fine screens.
• These screens are placed at an inclination of 45⁰ to 60⁰ or “3 to 6 Vertical : 1 Horizontal” as it helps in affective cleaning of the screens with the help of raking mechanism and it also helps in increasing the efficiency of screening process.

• Fine screens are in the form of wire mess of size 10 mm.
• These screens are normally fitted immediately after coarse screens. In case of raw water is provided, the fine screens are provided at the outlet from the storage reservoir.
• Under normal treatment conditions fine screens are avoided as they get frequently chocked, requiring its frequent cleaning which in turn increases its operational cost hence it is recommended to use coarse screen instead of fine screen and to remove the fine suspended particles in following sedimentation and filtration process.

Microstrainers are revolving drums of stainless steel wire fabric or other material having fine mesh.

• These are useful in pretreatment of water by screening the stored water, which do not contain a large amount of suspended matter, but contain plankton, algae and other microscopic size particles.
• When microstrainers used in the treatment of water system, they lighten the load on the filter i.e. reduce the load on filter.
• Water stored in lake or a large storage reservoir water is ideal for microstrainers.
• The ideal placement for a microstrainer is before the rapid gravity or slow sand filters whose output is increased by as much as 50%.

Raw water storage has been regarded as an almost essential “first line of defense” against the transmission of water borne diseases.

• Prolonged storage of raw water sometimes causes growth in large numbers of various, forms of algae, which increases difficulties in treatment.
• It is recommended that storage provided purely to improve water quality and it should be equivalent to 1 to 2 weeks of average water demand. This is sufficient to reduce pathogenic bacteria and river algae.

Pre-chlorination is adopted when the quality of the water is poor with respect to organic matters and micro-organisms present in it.

• Pre-chlorination is always succeeded by post-chlorination but the reverse is not.
• Dose of chlorine fairly used is generally 2-5 mg/l.
• When the water stored for longer duration in settling basins, chlorination oxidizes and precipitates iron and manganese.
• It also kills algae and bacteria, reduces colour and slime formation and assists in settlement.
• If excessive silt is present in suspension, then pre-chlorination is not so effective, because silt absorbs chlorine without settling. Hence, in case of heavily turbid water, it is not recommended.
• Pre chlorination is very effective in case of clean water with high ammonia content.

Aeration is the process of providing oxygen from the atmosphere to raw water for some beneficial changes.

Aeration is carried out for the treatment of water which is devoided in oxygen.

The raw water is brought in intimate contact with air so as to allow the absorption of oxygen in it, required for the removal of following dissolved impurities:

• It removes undesirable gases like CO₂ (responsible for corrosive properties of water), H₂S (cause bad taste and odour) etc. from water.
• It removes volatile liquids like phenol, humic acid etc. from the water.
• It removes dissolved organic matter from the water.
• It removes dissolved minerals like iron and manganese from the water.

a. Gravity Aerator or Tray Tower or Trickling bed

b. Spray Towers or nozzles

c. Air Diffusers

d. Cascade Aerators

• Aeration is not very effective in removal of taste and odour caused by non-volatile substances like oil and gases.
• Also not very effective in removal of taste and odour caused by chemical such as due to individual waste.
• Iron and manganese can be precipitated only when organic matter is not present in the water.
• There is always the possibility of air-borne contamination.

• Algae are small organisms classified as plants and proliferate in rivers and reservoirs.
• The building of an impounding reservoir on a stream encourage growth, mostly in the upper layers of water.
• Fairly alkaline water containing more concentration of nitrates and phosphates promote to algae growth.
• Algae tend to float and are not easy to remove by means of settling basins.
• The best way to kill algae is by pre-chlorination with a dose of 1 mg/l.
• If the organisms are more, a heavier dose of copper sulphate (2 mg/l) or chlorine (3-5 mg/l) may be used and again this high dose has to be removed before water is allowed to use.
• Strainers are commonly used to remove algae.
• Microstrainers are effective if water is silt free and have been found to reduce the algae problems to extent of 80% to 90%.

• They are included in the treatment of water system to reduce the silt load on basins, which are difficult to clean.
• The requirement of pre-settlement basins are there where water reaching main settling basins has suspended solids concentration more than 1000 mg/l by dry weight.
• Although 3 hours detention time is more than sufficient, pre-settlement basins with 1 hour detention period have been found to be effective.