Water Treatment Engineering - Chapter 3: Conventional Water Treatment Design

Screening

Aeration

Coagulation

Flocculation

Sedimentation

Filtration

• Separation technique to remove suspended and colloidal impurities.
• Water passes through fine granular medium (sand, anthracite, garnet, activated carbon).
• Removes small flocs, fine suspended impurities, and bacteria that remain after clarification.
• Water fills pores between particles, impurities are trapped by clogging or attachment to sand.
• Removes color, odor, turbidity, and some pathogenic bacteria.
• Reduces bacterial content by 98-99%.
• Effluent after coagulation needs further treatment to meet drinking water standards.
• Filtration removes remaining flocs/suspended solids from previous treatments.
• Reduces load on disinfection, increasing its efficiency.
• During water passage, suspended and colloidal matter is removed and chemical characteristics change.
• Bacterial numbers are also reduced.

Theory of Filtration

• Actions during filtration through granular media:
  • Mechanical straining
  • Sedimentation
  • Biological action
  • Electrolytic action

Mechanical Straining

• Suspended particles larger than voids are trapped.
• Most particles removed in upper layers.
• Arrested particles and flocs form a mat on top, aiding straining.

Sedimentation

• Filters remove particles smaller than void sizes.
• Voids act as tiny coagulation-sedimentation tanks.
• Colloidal matter in voids attracts finer particles.
• Fine particles settle in voids.
• Water flow is laminar, velocity changes due to media grains, causing sedimentation in low-velocity zones.

Biological Action

• Microorganisms and bacteria reside on filter media, forming coatings over sand grains.
• Organisms use organic impurities as food, converting them into harmless compounds.
• Harmless compounds form a layer called Schmutzdeck or dirty skin.
• This layer acts as a fine mesh, absorbing and straining impurities.
• Process is known as biological metabolism.

Electrolytic Action

• Friction between medium and suspended solids causes ionization.
• Suspended matter and sand particles ionize with opposite charges.
• These neutralize each other, changing water's chemical character.

Types of Filters

• Classified based on:
  • Filtration rate: Slow sand filter & Rapid sand filter (High-rate)
  • Driving force: Gravity or pressure filter
  • Direction of flow: Down flow filters and Up-flow filter

Slow Sand Filters

• First used in 1804 in Scotland and subsequently in London.
• Consists of concrete rectangular basin containing graded sand supported on gravel and stones.
• Consist of uniform sand media with no provision for backwashing.
• Removal mechanisms: straining, adsorption, and biological action.
• Advantage: low maintenance and simplicity of operation.
• Most removal takes place in the top portion of the bed due to low hydraulic loading.
• When the bed becomes clogged, the top 2–3 cm is scraped off and replaced with new media.
• Not common in larger plants due to high labor/area requirements and unsuitability for turbid waters (>10 NTU).

Essential Parts of Filter Unit

• Enclosure tank
• Filter media
• Base material
• Underdrainage system
• Inlet & outlet arrangement
• Appurtenances

Enclosure Tank

• Open water tight rectangular tank, made of masonry or concrete
• Depth: 2.5 to 3.50 m
• Surface area: 30 to 2000 m^2
• The bed slope is kept at about 1 in 100 towards the central drain

Filter Media – Sand

• A bed of graded sand - Most important part of the filter
• Sand thickness: Normally 1 m (0.9 - 1.10 m)
• Preferably rounded with effective diameter of 0.2-0.4 mm (Effective size).
• Uniformity co-efficient: 2 to 3 (Normal value, 2.5)
• Finer the sand, better bacterial efficiency, but filtration rate is low
• Should not contain more than 2% of Ca & Mg as carbonates
• After immersion in HCI for 24 h, should not loose weight by more than 5%
• Free from clay, lime, vegetable matter, organic impurities
• Supported by graded gravel (0.3-0.40 m deep)
• Water percolates through the bed and gets filtered by Mechanical straining, sedimentation, adsorption, oxidation and bacterial action
• Rate of filtration: 0.1-0.4 m^3/hour/m^2

Vital Layer

• Surface covered with slimy growth known as 'Schmutzdecke', vital layer or zoological layer or biological layer
• It is slimy gelatinous, consists of algae, plankton, diatoms and bacteria
• Formation is known as RIPENING of the filter, which may take several days to fully develop (2-3 cm)
• Removes organic matter, holds back bacteria, oxidizes ammonical nitrogen to nitrates and yield a bacteria free water
• Until the vital layer is fully formed, filter only worked as a mechanical strainer & so for the first few days filter water run to the waste

Base Material - Gravel

• The base material is gravel, which supports the sand
• Gravel thickness- 30 to 75 mm, with gravels of different sizes, placed in 3 to 4 layers
• Thickness of each layer is around 15 to 20 cm
• The coarsest gravel is placed in the bottom most layer and the finest layer is used in the top most layer
• The size of gravel in each layer should be as :
• Bottom most layer: 40 - 65 mm
• Intermediate layers: 20-40 mm & 6-20 mm
• Top most layer: 3-6 mm

Under Drainage System

• Lateral drain - 750 – 1000mm diameter earthenware pipes or perforated pipes
• Spacing - 2 to 3 m

Inlet and Outlet Arrangements

• An inlet chamber is constructed for admitting the water from clarifier without disturbing the sand layers of the filter and to distribute it uniformly over the filter bed
• A 'filtered water well' is also constructed on the outlet side to collect the filtered water from the main-under drain
• Inlets and outlets are generally governed by automatic valves

Appurtenances

• Vertical air pipes
• Loss of head through filter media- depth of water 1.5 m above sand media
• Adjustable telescopic tube
• Filter head- 0.10 to 0.15 m for fresh clean water, i.e ., difference in water level b/n filter basin & outlet chamber and 0.7 to 1.20 m during cleaning

Operation of Slow Sand Filter

• Water from plain sedimentation tank (non-coagulated) is allowed into inlet chamber for uniform distribution over filter bed
• Depth of water on filter media is kept equal to thickness of sand
• Water percolates through filter media and Gravel layer and gets purified
• Water gets collected in the under drainage system
• Slow sand filter works on a combination of straining and microbiological action

Limitations

• Rate of filtration: 100 to 200 L/h/m^2 of filter area
• Filtration applicable for non-coagulated water
• Only plain sedimentation prior to filtering

Cleaning of Slow Sand Filter

• Done by scrapping and removing the top 1.5 to 3 cm of sand layer.
• Cleaning is repeated until the sand depth is reduced to about 40 cm.
• The interval between two successive cleanings depends upon Nature of impurities and Size of filter media.
• This interval normal ranges between one to three months

Rapid Sand Filter (RSF)

• In the middle of the twentieth century, rapid sand filters completely took over from slow sand filters, except in rural areas.
• In the latter case, slow sand filtration is commonly used without any prior coagulation.
• The popularity of rapid sand filtration is because of the enhanced filtration rate of 5–20 m/h compared to 0.1– 0.2 m/h for slow sand filters (50–100×).
• Rapid sand filtration is commonly used to filter coagulated water and thus produces high-quality water filtration.
• Coarse sand is used as the filter media.
• Filtration rates are 30 to 40 times greater than that of slow sand filtration.
• This is achieved by maintaining 2–3 m of head above the media.
• In addition to straining, the removal mechanisms include sedimentation, adhesion, and adsorption.
• Because of the increased rate of filtration, filter runs on RSF range from 20 h to 60 h.
• Cleaning of the bed is achieved by agitating it either mechanically or with compressed air and washing water upwards through the bed to the surface and out through the troughs.
• RSFs' of gravity type - most commonly used in Water Supply Plants
• RSF differs from SSF in the following aspects
• Effective size & uniformity coefficient of sand
• Rate of filtration & filtration head
• Method of cleaning & frequency of cleaning
• Pre-treatment

Essential Features

1. Enclosure Tank
   • Open water-tight rectangular tank in masonry or concrete
   • Depth ofTank - 2.5 to 3.5 m
   • Surface area - 10-80 m^2 for each unit
   • Length to Breadth ratio - 1.25 to 1.35
   • Number of Units - Morrell &Wallace equation N = 1.22 \sqrt{Q}
   • Where, N is the number of filter units and Q is plant capacity in MLD
2. Filter media - Graded sand
   • Sand grain size distribution is selected to optimize the passage of water, while minimizing the passage of particulate matter
   • RSF uses sand coarser than SSF
   • Effective size: 0.35 - 0.6 mm, normal value 0.45 mm
   • Uniformity coefficient: 1.3 - 1.7, normally 1.5
   • Void space increases due to increase in effective size & decrease in Uniformity Coefficient
   • This increases rate of filtration
3. Base material : Graded gravel
   • Garnet (6 – 8 cm) layer: to check gravel upsets due to localized high velocity during back wash
   • Total depth: 0.60 - 0.90 m
   • Five to six layers - each 0.15 m thick
   • Grade size - 2-6 mm, 6-12 mm, 12-20 mm and 20-40 mm
4. Under-drainage system (UDS)
   • UDS in RSF serves two purposes
   • Collects filtered water uniformly over the area of gravel bed
   • Uniform distribution of back wash water without disturbing the gravel bed and filter media

Rapid Sand Filter - Operation

• The working and back washing of rapid sand filter is regulated by operating SixValves,Viz .,
  • Valve 1 - Inlet for RawWater
  • Valve 2 - To drain dirty water collected in wash water trough
  • Valve 3 - To regulate Initial run after backwash
  • Valve 4 - To treated water storage reservoir
  • Valve 5 - Air compressorValve
  • Valve 6 - To regulate elevated wash water tank for backwash
• The water from clariflocculator enters the filter unit by regulatingValve #1
• The filtered water collected in the manifold is collected by opening theValve # 4
• During Filtration, Valves 1 and 4 are kept open and otherValves are in closed position

Backwashing

• RSF are cleaned by passing air and water backwards through the sand. This operation is known as Backwashing
• Stored filtered water in elevated wash water tank is used for the backwash
• The filtration is stopped and the water level is maintained above the surface of the filter bed by closingValve 1 andValve 4 (Inlet and Filtered water storage tank)
• Air valve and Wash water Valve (Valve 5 & 6) are kept open, wash water and compressed air is forced upwards from under drainage system through gravel and filter bed
• The backwash flow rate has to be great enough to expand and agitate the filter media and suspend the floc in the water for removal
• If it is too high, media will be washed from the filter into the troughs and out of the filter
• Air valve (# 5) is closed
• The dirty water of washings flows into wash water troughs and joins wash water gutter by openingValve #2
• After completion of backwash,Valves 2 & 6 is closed andValves 1 & 3 are kept open
• After backwash, filtered water is not collected for a few minutes and sent to drain by operatingValve #3
• Finally,Valve 3 is closed andValve 4 is kept open to get clear filtered water

Filter Ripening

• After the backwash operation, the filter needs to be rested to allow the media and other particulate matter to settle down.
• If filter ripening is not done, filtered water will be of poor quality and turbidity levels may exceed the regulatory limits.

Appurtenances

• Wash water troughs
  • Provided @ the top of the filter to collect BWW
  • It emerges out from sand & conveys toWW Drain
  • Troughs are used & runs across the length of the tank
  • Bottom of trough is placed at least 5cm above the top level of sand
  • This prevents entry of sand during BW
  • Spacing of trough: 1.5 to 2 m
• After backwashing, the sand settle back into place. The larger particles settle first, resulting in fine sand layer on top and coarse sand layer on the bottom
• After the air scour cycle, clean backwash water is forced upwards through the filter bed continuing the filter bed expansion and carrying the particles in suspension into backwash troughs suspended above the filter surface
• Water requirement: 2 to 5% of total amount of water treated
• Frequency of cleaning: 24 to 48 h
• Back wash duration : 15 minutes

Difference b/n SSF and RSF

Pressure Filters

• Based on position of installation, Pressure filters are classified into two types
  • Horizontal pressure filters &
  • Vertical pressure filters

Constructional details

• Vertical pressure filter consists of steel cylinder

• Diameter varies b/n 1.5 - 3.0 m

• Height varies b/n 2.5 - 8.0 m

• Inspection windows are provided @ top

• Requires frequent cleaning

• Rate of filtration - 2.5 times higher than RSF

• Rate of filtration - 6000 to 15000 L/hr/sq. m

• Applicability - Industrial plants & swimming pools

• Pressure filter is type of rapid sand filter in closed water tight cylinder through which the water passes through the sand bed under pressure.

• All the operation of the filter is similar to rapid gravity filter; expect that the coagulated water is directly applied to the filter without mixing and flocculation.

• These filters are used for industrial plants but these are not economical on large scale.

• Backwash is carried by reversing the flow with values.

• The rate of flow is 120 to 300 m^3/m^2/day.

Example

• Six slow sand filter beds are used to treat a maximum flow of 15 ML/d with a filtration rate of 5.0 m^3/m^2·d. What should be the size of the rectangular filter box of length twice that of width? Also, assume that one unit out of six is kept as standby.

Solution:

• Given:
  • Q=15.0 ML/d
  • V_f = 5.0 m/d
  • L = 2B
  • V_f = ?
• filter surface required
• AF = \frac{Q}{VF} = \frac{15 \frac{ML}{d}}{5.0 \frac{m}{d}} = \frac{15 \frac{1000 m^3}{d}}{5.0 \frac{m}{d}} = 3000 m^2
• Since one unit is to be kept standby, 5 units should provide the required surface.
• B = \frac{A_F}{2 \cdot 5} = \frac{3000 m^2}{10} = 17.3 = 17.5 m
• Six filter units each measuring 35 m × 17.5 m will meet the requirements.