WATER TREATMENT

FILTRATION
Filtration Principle Key Feature

- Physically traps suspended particles too large to pass through filter pores.
1. Mechanical Straining - Works like a sieve, removing larger impurities such as silt and debris.
- Most effective in the top layers of the filter media.

- Particles settle out of water while moving slowly through filter pores.
2. Sedimentation - Helps in removal of fine suspended solids by gravity within the media.
- Occurs in the voids or spaces between filter particles.

- Microorganisms form a biological layer (schmutzdecke) on the filter surface.

3. Biological Action - Degrades organic matter and destroys pathogenic bacteria.
- Common in slow sand filtration systems.

- Involves electrical charges attracting and binding fine particles and dissolved impurities.
- Filter media particles may carry surface charges that attract oppositely charged
4. Electrolytic Action
contaminants.
- Enhances removal of colloidal and very fine particles.
The Slow Sand Filter
The basic elements of a slow sand filter are shown Essential features of the slow sand filtration
diagrammatically but each plant will have site-specific process are:
variations of detail to minimize cost, taking advantage of
local conditions. A - A storage capacity above the sand bed to
provide the necessary head to produce the
design flow through the bed under the worst head
loss conditions.
B - A bed of media which achieves the filtration
and other effects, including biological
purification, which a slow sand filter is known to
provide in upgrading the quality of an influent
water.
C - A system of underdrains to allow
unobstructed passage of treated water and to
support the filter medium so that a uniform rate
of filtration is maintained over the whole area of
the filter.
D - Filter regulation and control devices to
maintain the water level over the bed during
operation at the design filter rate and allow
adjustment of water level during filter cleaning
and re-introduction to operation after cleaning.
The Slow Sand Filter
PROCESS
• Initial water storage above the sand allows flocculation and sedimentation of large particles

• Multiple processes occur in the sand bed: straining, filtration, and adsorption (primarily in first 40-60 cm)

• Sunlight exposure during storage has a bactericidal effect

• Micro- and macro-organisms settle out with solids during storage

• The 'schmutzdecke' (surface layer) is where main biological activity happens

• Bacteria, algae, protozoa, and rotifers thrive in the schmutzdecke layer

• Organisms break down organic components of raw water

• The end result is improved water quality across physical, chemical, and bacteriological aspects
OUTPUT

• Provides greatest improvement in water quality of any conventional water treatment process

• Removes 98-99.5% or more of bacteria

• Reduces E. coli by a factor of 1000

• Achieves even greater virus removal than bacterial removal

• Most efficient process for removing parasites (helminths and protozoa)

• May contain some E. coli and viruses during early phase of a filter run

• Produces safer effluent than conventional chemical treatment/rapid sand filtration processes

• Delivers significant community health benefits due to high degree of water quality improvement

• Chlorination remains desirable if continuous and effective application can be assured

PRE-TREATMENT

• Highly turbid raw water will quickly clog a slow sand filter

• Frequent cleaning is problematic since it's not an automatic process

• Ideal cleaning frequency should be no more than once every two or three months

• Cleaning disrupts biological action and reduces water quality for 1-2 days after restart

• Very short filter runs are inadvisable due to these disruptions

• Pre-treatment becomes necessary when average raw water turbidity exceeds 25 NTU

• Pre-treatment should be considered even for lower turbidity levels

• Benefits of pre-treatment include reduced cleaning frequency and potential cost-effectiveness

• Various pre-treatment processes are available before final polishing in slow sand filters
ADVANTAGE
Advantage Key Details
Simplicity • No automatic backwashing needed
• Simple system design
• Minimal equipment requirements
• Simple control and measurement devices
• Materials available in most developing countries
• Limited professional supervision needed during construction
Village-level Operation and Maintenance
• Least demanding water treatment process for villages
(VLOM)
• No chemical preparation or dosing required
• No desludging requirements
• Simple operation requiring minimal training
• Robust equipment with minimal breakdown risk
• Continuous operator attendance not essential
• Simple manual cleaning (removing 2–3 cm surface layer)
• Cleaning needed only every 2–3 months (if influent <20 NTU)
High Effluent Quality • Produces high quality without chemical treatment
• Matches sophisticated treatment processes
• Typically achieves turbidity of 1 NTU (crystal clear)
• Generally pathogen-free (no fecal coliforms)
• Can improve color, total iron, and organic matter
• Unmatched efficiency in upgrading turbid surface waters
Low Cost • Uses local materials and simple equipment
• Reduces/eliminates need for imported items
• Involves local skills and community participation
• No chemical costs
• Negligible energy requirements
• More economical than rapid-gravity filters (up to 8000 m³/day)
• Affordable water production for low-income communities
Economy of Water • No water wasted on desludging sedimentation tanks
• No water needed for backwashing (unlike rapid-gravity filters)
DISADVANTAGE
Disadvantage Details How It Can Be Overcome

Use SSF in rural/provincial areas where land is more available and

SSF plants require more land than rapid-gravity plants,
Land Use affordable. For peri-urban areas, the long-term benefits may justify
which can be a challenge near cities.
the land requirement.

Use local villagers for short-term cleaning activities; permanent

Requires regular (but not continuous) attention,
Labour Intensive crews can be assigned in larger plants. Leverage low labour costs in
especially during cleaning.
developing countries.

Raw water with turbidity >25 NTU shortens filter run; Install simple prefilters based on known turbidity ranges; use
High Turbidity Problem
>150 NTU requires complex pretreatment. pretreatment when needed to maintain SSF performance.

Train local operators for consistent 24-hour operation or practice

Filters must operate continuously; shutdown degrades
Intermittent Operation declining-rate filtration overnight. Use elevated storage to maintain
effluent quality.
flow during off-hours.

Large Volume of Graded Requires 50–100 times more sand than rapid filters, Use locally available builder-grade sand or alternative media like
Sand creating sourcing and cost issues in some areas. burnt rice husk. Avoid strict specifications if not essential.

Algae may grow under tropical conditions, potentially Consider using filter covers in problematic areas. Monitor growth
Algal Growth
causing blockages and taste/odor issues. and intervene only if serious issues occur.

Recognize that SSF provides superior baseline quality. Use

Need for Final
Chlorination is still needed for guaranteed safety. chlorination as a precaution, especially where reliability of rural
Chlorination
chlorination is low.
DESIGN CRITERIA FOR SSF (GUIDELINE)
DESIGN
Design SSF for a projected city population of 25000 with average water demand rate 125 lpcd.
RAPID SAND FILTER (RSF)
 KEY POINTS
Aspect Key Points
Filtration Mechanisms - Rapid filtration involves straining, sedimentation, adsorption, and minimal biological activity.
- Straining is minimal due to larger pores.
Sedimentation - Occurs mainly at the top of sand grains.
- Forms caps on particles due to laminar flow.
Adsorption - Most effective in rapid filtration.
- Driven by Van der Waals forces and electrostatic interactions.
- Affected by mineral content in water.
Electrostatic Interactions - Clean sand has a slight negative charge.
- Charge interactions vary with water chemistry.
- Minerals reduce range of electrostatic forces.
Flow Rate & Contact Time - Water in rapid filters passes through in minutes vs. hours in slow filters.
- Less time for biological processes.
Biological & Biochemical Action - Rapid filters lack sufficient biological activity.
- Organic matter is not degraded effectively.
Backwashing - Used to remove accumulated deposits.
- Ensures partial restoration of filter capacity.
Need for Further Treatment - Rapid filters alone do not ensure bacteriologically safe water.
- Requires post-treatment like chlorination or slow sand filtration.
Head Loss Buildup - Caused by accumulation of solids in pores.
- Reduces pore volume and increases resistance to water flow.
Filter Run Termination - Should end when head loss and filtrate quality reach set limits.
- Prevents breakthrough of suspended solids.
DESIGN CRITERIA
BACKWASH
SPECIAL NOTES (RSF)

Backwashing with Water Air Washing (with Compressed Air)

•Water type: Only filtered water should be •Used when water alone can't remove dirt
used for backwashing. lodged in the filter media.
•Volume: Should not exceed 2% of the total •Purpose: Enhances scrubbing using less water.
treated water. •Air delivery options:
•Pressure: Wash water should be applied at a • Through underdrains before water is
pressure of 5 meters head. introduced.
•Flow rate: • Or through separate piping system
• 600 liters/minute/m² of filter surface between gravel and sand.
area. •Air rate: 0.60–0.80 m³/min/m² of filter area at
• Causes 60 cm rise per minute in filter 35 kg/cm² pressure.
tank. •Air wash duration: Approximately 5 minutes.
•Duration: Typically 10 minutes. •Follow-up water wash rate: 400–600
•Storage: Wash water tank should support liters/minute/m².
washing two filter tanks for 5–6 minutes each.
SPECIAL NOTES (RSF)
Aspect Advantages Disadvantages

- Removes turbidity effectively after - Less effective in removing bacteria and

Treatment Efficiency
coagulation and flocculation viruses without additional disinfection

- High filtration rate (4,000–12,000 - Requires frequent backwashing due to

Filtration Rate
L/m²/hr) clogging
- Needs space for associated treatment
- Requires smaller land area compared
Area Requirement units (e.g., backwash systems,
to slow sand filters
sedimentation)
- Requires skilled operators for
Operation & Control - Easier to control and automate
monitoring and maintenance
- Suitable for meeting high water - Backwashing consumes significant
Water Demand
demand in urban areas amounts of clean water

- May not be suitable for communities

Startup Time - Short startup time
with intermittent water supply

- Higher capital and operational costs

Cost - Lower land cost due to compact design
than slow sand filters
- Needs mechanical systems and
Backwashing - Quick and efficient cleaning
uninterrupted power supply
PRESSURE FILTER
 ATTRIBUTES
Aspect Details
Type Rapid sand filters enclosed in closed steel cylindrical tanks
Water Flow Water passes under pressure (pumping head: 30–70 m)
Orientation Vertical or horizontal
Vertical Unit Dimensions Diameter: 0.4–2.5 m; Height: 2–3 m
Horizontal Unit Dimensions Diameter: 1.5–2.8 m; Length: 2.5–7.5 m
Inspection Feature Manholes/inspection windows at the top
Metal Thickness Wall: 8–10 mm; Top & Bottom: 10–12 mm (depending on pressure)
Corrosion Protection Inner lining: Fiber Reinforced Plastic (FRP); Outer: Coated against corrosion

Filter Media Same sand, gravel, bed thickness, and underdrain system as rapid sand filters

Operation Similar to rapid sand filters; raw coagulated water enters directly without pre-treatment
Coagulant Alum, kept in a pressure container connected to influent line
Backwashing Same method as rapid sand filters; automatic options available
Air Blower Specification Air/Water ratio of 10; must move air upward sufficiently
Filtration Rate 120–300 m/day of filter area (higher than rapid sand filters)
Efficiency Less efficient than rapid sand filters in bacterial, turbidity, and color removal
Head Loss Similar to rapid sand filters
Not suitable for public water supplies; ideal for small schemes, private estates, swimming
Suitability
pools, industrial plants, iron removal, and softening applications
COGULATION
Phytoplankton are micr
oscopic plant-like
organisms that drift in
aquatic environments,
including oceans, lakes,
and rivers.
MOST Common and
universal coagulant
Requires presence of
alkalinity in water to
form the floc
Hydrated ferrous sulphate (copperas)

First oxidize to ferric sulphate and

ferric chloride

Ratio of chlorine to copperas 1:7.8

 Coagulant Use Case Advantages Disadvantages Cost

General-purpose coagulant Produces large volume of

Aluminium Inexpensive, forms strong
for turbidity, color, and sludge, reduces alkalinity, 💲 (Low)
Sulphate (Alum) flocs, widely available.
pathogens. ineffective in very low pH.

Effective for colored, acidic Removes color, works in Needs chlorine, low solubility
Chlorinated 💲💲
waters and industrial acidic pH, good for raw in cold water, requires
Copperas (Moderate)
effluents. colored waters. oxidation step.

Requires oxidation step, not

Ferrous Sulphate Effective for highly alkaline Produces dense flocs, low
suitable for soft colored 💲 (Low)
+ Lime waters; iron removal. cost of iron salt.
waters, high pH needed.

Used where organic color and Produces slurry-like sludge,

Magnesium Removes color, Fe, and Mn 💲💲
some metals like Mn and Fe harder to manage, less
Carbonate + Lime effectively. (Moderate)
are present. common in practice.
DISINFECTION
• ✅ Main Requirement: Drinking water must be free from microorganisms that can cause disease or

illness.

• ⚙️ Pre-Treatment Processes: Storage, sedimentation, coagulation and flocculation, and filtration help

reduce bacterial content—but only partially.

• ❗ Limitations: These methods alone cannot guarantee bacteriologically safe water.

• 🛡️ Disinfection : The process of destroying or inactivating harmful microorganisms in water.

• 🔒 Final Barrier: Disinfection is considered the last protective step in water treatment.

• 🧪 Standalone Use: If other treatments are unavailable, disinfection may be used as the sole method

to control bacterial contamination in drinking water.

PROPERTIES
Overview of Chlorination Advantages of Chlorine as a Disinfectant
• Most widely used and effective method of • ✅ Broad-spectrum germicidal potency.
disinfection. • ✅ Good residual effect—persists in water systems
• Economical, but must be administered and can be measured and monitored.
continuously and safely. • ✅ Simple, reliable, low-cost dosing equipment.
• Chlorine is available in gas and powder forms • ✅ Feasible at village level using appropriate
(e.g., sodium hypochlorite / bleaching powder). technology feeders.
• Requires special equipment for gas injection and • ✅ Easily available, even in remote areas of
careful handling due to its corrosive nature. developing countries.
• ✅ Highly economical and cost-effective.
Chlorine Properties
•Chemical symbol: Cl
•Atomic weight: 35.45
•Melting point: -101.5°C
•Boiling point: 34.5°C
•Greenish-yellow gas, 2.5 times heavier than air.
•As a liquid, it’s amber-colored, oily, and 15 times heavier than water.
•Liquefied at 35 kg/cm² pressure after drying and compressing.
•Solubility in water:
• 4.61 volumes per 1 volume water at 0°C
• 2.26 volumes per 1 volume water at 20°C
•The resulting solution is called chlorine water, which is unstable and decomposes rapidly in sunlight.
Hazards of Chlorine

• Has a pungent odor; causes lung and tissue damage if

Concept of Active Chlorine
inhaled.
• Active chlorine = % of molecular chlorine by
• Even trace amounts in the air can be lethal.
weight in a compound.
• 1 part in 10,000 parts of air causes severe coughing.
• Example: 10% active chlorine = 10 g of
• Non-combustible but supports combustion.
chlorine gas in 100 ml of water.
• Chemically active and corrosive in presence of

moisture.
NUMERICAL

• A water supply system treats 2 MLD water. The required chlorine dosage is 2 mg/L. If the bleaching powder
used contains 32% available chlorine, how many kilograms of bleaching powder are needed per day?
•Breakpoint chlorination is the process of adding chlorine to water until the chlorine demand is fully

satisfied, and free residual chlorine begins to appear.

•Used to ensure complete disinfection by both killing bacteria and oxidizing organic matter.
•Line A (Ideal/Pure Water): All
applied chlorine results in
residual chlorine; no chlorine
demand.
•Line B (Natural Water):
Chlorine first reacts with
ammonia to form chloramines.
• Further chlorine oxidizes
chloramines → free
chlorine residual
appears.
• The "breakpoint" is the
point where further
addition of chlorine
results in free residual
chlorine.
• Initial Stage: Chlorine reacts with ammonia and organic

matter.

• Middle Stage: Chloramines are formed and then oxidized.

• Breakpoint: All chlorine demand is satisfied; additional

chlorine appears as free chlorine residual.

• Beyond Breakpoint: Any added chlorine remains in the

water as free chlorine, ensuring effective disinfection.

HARDNESS

• Water hardness is the traditional measure of the capacity of water to

react with soap, hard water requiring considerably more soap to
produce a lather. Hard water often produces a noticeable deposit of
precipitate (e.g. insoluble metals, soaps or salts) in containers,
including “bathtub ring”. I

• Water containing calcium carbonate at concentrations below 60 mg/l

is generally considered as soft; 60–120 mg/l, moderately hard; 120–
180 mg/l, hard; and more than 180 mg/l, very hard
Ion Source Type Common Natural Sources Remarks

Calcium (Ca²⁺) Sedimentary rocks, soils Limestone, chalk, gypsum Major contributor to hardness

Dolomite, serpentine, igneous

Magnesium (Mg²⁺) Sedimentary rocks, soils Major contributor to hardness
rocks

Bauxite, feldspar, clay Minor contributor; usually in

Aluminium (Al³⁺) Soils, some rocks
minerals acidic conditions

Barium (Ba²⁺) Sedimentary rocks, soils Barite, witherite Minor contributor

Minor contributor; also

Iron (Fe²⁺/Fe³⁺) Soils, rocks, seepage Hematite, magnetite, siderite
affects water color/taste

Minor contributor; similar

Manganese (Mn²⁺) Soils, rocks Pyrolusite, rhodochrosite
effects as iron

Strontium (Sr²⁺) Sedimentary rocks Celestine, strontianite Minor contributor

Trace amounts; minor

Zinc (Zn²⁺) Rocks, seepage Sphalerite, smithsonite
contributor
Aspect Calcium Magnesium

Essential for bone health, muscle Supports muscle and nerve

Role in Health
function, nerves function, energy production

Health Consequences of Can lead to osteoporosis, muscle Can cause muscle cramps, mental
Deficiency spasms, fatigue disorders, fatigue

Set by national/international Set by national/international

Recommended Daily Intake
guidelines guidelines

Varies among individuals based on Varies among individuals based on

Intake Variability
age, diet, etc. age, diet, etc.
 • CH = A
• NCH = TH-CH
1. TH > A • NCH = TH-A

• CH= TH
• NCH=0
2. TH =<A
The analysis of water from a well showed the following results in mg/lit;

Ca= 65, Mg=51, Na=101.5, K= 21.5,

HCO3 = 248, SO4 = 221.8, Cl = 79.2

Find the total hardness, carbonate hardness and non-carbonate hardness.

Hardness due to M++ as CaCo3 = hardness as M++ (mg/lit) x eq. wt. of CaCo3 / eq. wt. of M++

Total hardness = [ 65x 50/20 +51 x 50/12.2] mg/lit as CaCo3.

= 371.52 mg/lit as CaCo3

Alkalinity due to B– as CaCo3 = alkalinity as B– x eq. wt. of CaCo3 / eq. wt. of B–

= 248 x 50/61
In this case.
= 203.28 mg/lit
If, TH > A

then, CH = A

NCH = TH-CH

= 371.52-203.28

= 168 mg/lit
SOFTENING
SOFTENING
SOFTENING
SOFTENING