Environmental EngineeringLecture 6

Sources of Drinking Water Rivers: upland and lowland Lakes and reservoirs Groundwater aquifers Sea water (Desalination)

WATER TREATMENT PROCESSES

WATER TREATMENT PROCESSES

Selection of Treatment Process

PRE-TREATMENT OF WATER

Steps required before standard treatment processes

Screening Storage, equalization Aeration Chemical pre-treatment: pre-chlorination

PRE-TREATMENT OF WATERScreening

Coarse Screening typically inclined bars of 25 mm diameter and 100 mm spacing prevent large floating material Raking is facilitated by the inclination of the bars. Velocities are usually limited to about 0.5 mls through the screens

Fine screens fine screens are fitted after the coarse screens if storage is not

provided, If there is storage then fine screens are placed at the outlet of the

storage tanks. typically mesh with openings about 6 mm diameter circular drum type or a traveling belt

Micro screening. the mesh openings :range from 20 to 40 µm. used only as the main (physical) treatment process for relatively

uncontaminated waters.

PRE-TREATMENT OF WATER Storage and Equalization

Storage Serve as a safety line in the event of pollution. Also serve as reservoirs in time of low supply Should be equivalent to 7 to 10 days of the average water

demand This period is good for settling and adequate to reduce most

pathogens by exposure to daylight Storage time of about 12h is commonly used to reduce

pumping costs and balance the demand

Equalization Provide an 'equal' (or consistent) flow to the plant

Equalization Example

hour flow (m3/s) flow (m3/h)

cumulative flow (m3)

equalized flow

(m3/s)equalized flow (m3/h)

cumulative flow (m3)

required storage

(m3)1 0.13 468 468 0.139 500 500 322 0.12 432 900 0.139 500 1000 1003 0.11 396 1296 0.139 500 1500 2044 0.10 360 1656 0.139 500 2000 3445 0.08 288 1944 0.139 500 2500 5566 0.06 216 2160 0.139 500 3000 8407 0.08 288 2448 0.139 500 3500 10528 0.10 360 2808 0.139 500 4000 11929 0.12 432 3240 0.139 500 4500 126010 0.14 504 3744 0.139 500 5000 125611 0.16 576 4320 0.139 500 5500 118012 0.18 648 4968 0.139 500 6000 103213 0.20 720 5688 0.139 500 6500 81214 0.19 684 6372 0.139 500 7000 62815 0.18 648 7020 0.139 500 7500 48016 0.17 612 7632 0.139 500 8000 36817 0.16 576 8208 0.139 500 8500 29218 0.15 540 8748 0.139 500 9000 25219 0.16 576 9324 0.139 500 9500 17620 0.17 612 9936 0.139 500 10000 6421 0.18 648 10584 0.139 500 10500 -8422 0.16 576 11160 0.139 500 11000 -16023 0.13 468 11628 0.139 500 11500 -12824 0.10 360 11988 0.139 500 12000 12

0.139

Design the size of an equalization tank to balance flow rates from a municipalwastewater as given in columns (1) and (2)

Aeration is the supply of oxygen from the atmosphere to water to effect beneficial changes in the quality of the water.

It is a common treatment process for groundwater and less common for surface waters. Aeration is used :1. To release excess H2S gas which may cause undesirable tastes and

odors.2. To release excess CO2 which may have corrosive tendencies on

concrete materials.3. To increase the O2 content of water in the presence of undesirable

tastes due to algae (fishy smell), 4. To increase the O2 content of water which may have negative taste,

color and stain properties due to the presence of iron and manganese in solution. The addition of oxygen assists the precipitation of iron and manganese.

Aeration can be a simple mechanical process of spraying water into the air and allowing it to fall over a series of cascades (waterfalls), while absorbing or desorbing (stripping) oxygen in its journey.

PRE-TREATMENT OF WATER Aeration

Chemical pre-treatment is used to remove undesirable properties of water (bacteria, algae or excess color) is a more expensive process than chemical post-treatment.

Pre-chlorination is used on low turbidity water with a high coliform count.

The chlorine is injected into the water stream and over the period that it stays in the settling tanks,

it oxidizes and precipitates iron and manganese. It also causes pathogenic kill and reduces color. Doses as much as 5 mg/l are used, Water authorities tend to use pre-chlorination at times of

the year when the surface water supply is likely to be polluted from agricultural or industrial sources or when excess organic matter is transported

PRE-TREATMENT OF WATER Pre-chlorination

Standard treatment is the set of unit processes that reduce color, turbidity and particulate impurities to acceptable levels

Standard treatment consists of the following unit processes:

Sedimentation Coagulation and flocculation Sedimentation of flocculent particles Filtration

PRIMARY-TREATMENT OF WATER

PRIMARY-TREATMENT OF WATERSedimentation

Sedimentation by definition is the solid-liquid separation using gravity settling to remove suspended solids' (Reynolds, 1982).

In water treatment, sedimentation processes used are: Type L: to settle out discrete non-flocculent particles in a

dilute suspension. Type n: to settle out flocculent panicles in a dilute

suspension

Type L Settling tanks are of two types: Rectangular

length width ratio of 2 and a depth of the order of 1.5 to 6m. Circular

Dimensions typically are 10 to 50 m in diameter and 2.5 to 6m in depth

Rectangular Sedimentation Tank

Circular Sedimentation Tank

Sedimentation tank design

Key parameters and typical values in the design of settling tanks: Surface overflow rate:

20-35 m3/day/m2 Detention time:

2-8 hours Weir overflow rate:

150-300 m3/day/m

Sedimentation tank design Stokes law for settling velocity

Sedimentation tank design Stokes law for settling velocity

Sedimentation tank design

Sedimentation tank design

Surface overflow rate,Same for circular tanks

Surface area

Sedimentation tank designexample

Sedimentation tank designexample

Sedimentation tank designexample

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