Environmental Engineering Lecture 6

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

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 WATER Screening  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 Design the size of an equalization tank to balance flow rates from a municipal wastewater 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 WATER Sedimentation  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:  m3/day/m2 Detention time:  2-8 hours Weir overflow rate:  m3/day/m

Sedimentation tank design  Stokes law for settling velocity

Sedimentation tank design  Stokes law for settling velocity

Sedimentation tank design

Surface overflow rate, Same for circular tanks Surface area

Sedimentation tank design example

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