3 water treatment processes

Water Treatment Processes

Operation units of water treatment depend on :

Water resource quality

Application

4 stages of water/wastewater treatment process:

preliminary, primary, secondary & tertiary (advanced) treatments

Conventional Wastewater Treatment Process Pretreatment involves:

Screening Grit Removal Oil separation Flow equalization

Disinfection can use:

Chlorine compounds Bromine Chloride Ozone UV Radiation

Chemical Treatment is used in conjunction with the physical and chemical processes:

Chemical precipitation Adsorption

Sludge Treatment and Disposal involves: grinding, degritting, blending, thickening, stabilization, conditioning, disinfection, dewatering, heat drying, thermal reduction, ultimate disposal

Aim: to protect the operation of wastewater treatment plant from big suspended or floating matters carried in with the wastewater that can interfere, clog, & damage equipments and piping.

How: screen coarse solids (gravel, plastics, rags) out and substances dissolved in raw water

Unit operations involved: Screening

Pre-sedimentation

Pre-aeration

Preliminary Treatment Process

Specific aims:

- Remove objects such as solids, plastics, paper, rags, metals.

Screening devices: - Coarse screen (clear opening of 0.25 in or larger)

- Fine screen (clear opening of 0.06 to 0.25 in)

- Very fine screen (clear opening of 0.01 to 0.06 in)

The bars may be cleaned manually or mechanically.

Attention: as debris collected on bars, it blocks the channel.

Preliminary Treatment ProcessScreening

bar rack bar screen

In addition to the screening classification based on the screen size, other design consideration is

proposed.

Grit is defined as particles larger than 0.21 mm with specific gravity > 2.65 (US. EPA, 1987). Equipment design was traditionally based on tremoval of 95% of these particles. However, modern design is capable to remove 75%of 0.15 mm material.

Specific aims: - Remove grits (sand, gravel) & other heavy solid materials that are

heavier than the organic biodegradable solids in the wastewater. This includes eggshells, seeds, food waste.

- This can prevent unnecessary abrasion, grit deposition in pipelines, accumulation of grit in anaerobic digesters & aeration basins.

There are several types of grit removal systems. Factors such as grit quantity, grit characteristics, head loss requirements, removal efficiency, organic content, and cost are considered when selecting a system.

Preliminary Treatment ProcessGrit removal

Grit removal systems:

a. Vortex grit chamber

b. Centrifugal separator

c. Aerated grit chamber

d. Horizontal flow grit chambers (velocity-controlled

channel)

Vortex grit chamber

Centrifugal separator

Aerated grit chamber

Horizontal flow grit chamber

Vortex grit chamber

Remove high percentage of fine grit, up to 73% of 140-mesh size

The system has a consistent removal efficiency over a wide flow range

However, modification to the design is difficult, paddles tend to collect rags,

require deep excavation due to its depth

Centrifugal separator

Remove high percentage of fine grit, up to 73% of 140-mesh

size

Commonly used for grit washing

As fluid spirals inward, centrifugal force pushes the grit toward the walls. The accumulated grit slides spirally down & then discharge into outlet point.

The separation efficiency is influenced by:

- particle size

- specific gravity of particles

- differential pressure across the cone. For sludge less than 1.5% solids, a pressure drop of 10-13 psi has been suggested

- diameter of the cone

Aerated grit chamber

Tank with slopping bottom & hopper

Air is introduced along one side to create a perpendicular spiral velocity pattern

Injected air lowers specific gravity of fluid resulting in better grit settlement

Heavier particles drop to the bottom side, while lighter particles are suspended &

carried out of the tank

However, harmful volatile organics & odors may be released, require more power &

labor to maintain the aeration system

FOG (Fats, Oils, Grease) Pollution

FOG (Fats, Oils, Grease) Pollution = Foam Pollution

Horizontal flow grit chamber

The oldest type of grit removal system

Grit is removed by maintaining a constant upstream velocity of 0.3 m/s by controlling

weirs or rectangular control sections such

as Parshall flumes.

However, sometimes it is difficult to maintain the velocity (1 ft/s) over a wide

range of flows

Grit washing

Remove excess organic materials from the grit & return it to process stream

Grit disposal

Specific aims:

To reduce the load on the coagulation/flocculation basin and on the sedimentation chamber

To reduce the amount of coagulant chemical required to treat the water

To homogenize water quality entering the plant/system

Preliminary Treatment ProcessPre-sedimentation

How? By precipitating the water for certain period

Presedimentation devices:

- Settling ponds

- Concrete basins

Presedimentation can also be called plain sedimentation

River water suspended solids can be reduced from 500 to 200 ppm

Specific aim:

- Remove volatile compounds

- Promote flotation of fat, grease, soap

- Reduce the amount of Fe2+ and Mn2+

- Freshen wastewater

How? By passing the air through the water

A skimmer is employed to

remove the floated substances

Preliminary Treatment ProcessPreaeration

Removal mechanisms:

Stripping off dissolved gases

CO2(aq) CO2(g)

H2S(aq) H2S(g)

NH3(aq) NH3(g)

Oxidation of metals

4Fe2++O2+10H2O 4Fe(OH3)(s)+8H+

2Mn2++O2+2H2O 2MnO2(s)+4H+

Primary Treatment Process

Aim: remove suspended solids by sedimentation

How: by allowing the water in a tank for certain period to obtain clear water escapes over a V-notch weir.

Equipment: settling tank or clarifiers (circular or rectangular), typically 3-5 m deep with detention time is up to 8 h. The tank operate as plug-flow reactor & turbulence is kept to a minimum.

Types of Settling

Type I: Discrete particle settling - Particles settle individually

without interaction with neighboring particles.

Type II: Flocculent Particles Flocculation causes the particles to increase in mass and settle at a faster rate.

Type III: Hindered or Zone settling The mass of particles tends to settle as a unit with individual particles remaining in fixed

positions with respect to each other.

Type IV: Compression The concentration of particles is so high that sedimentation can only occur through compaction of the

structure.

Advantage: - Low energy consumption. - Easy maintenance.

Disadvantage: - Treatment capacity is small. - Fails to satisfy the requirements for environmental

quality associated with particulates/colloidal matter.

Chemically Enhanced Primary Treatment (CEPT)

CEPT

The use of chemical treatment to obtain additional solid removal in a sedimentation process.

Coagulation is the destabilization of colloids by neutralizing the forces that keep them apart. Cationic coagulants provide positive electric charge to reduce the negative charge (zeta potential) of the colloids. Care must be taken not to overdose the coagulant as this can re-stabilize the colloid complex.

Flocculation is the action of polymers to form bridges between the flocs & bind the particles into large agglomerates or clumps

The chemistry of coagulation/flocculation process:

Most particles dissolved in water have a negative charge. As a result, they stay dispersed and dissolved or colloidal in the water

Positively charged coagulant attracts the negatively charged particles due to electricity. As a result, the particles no longer repel each other

Naturally, the neutrally charged particles attract each other due to van der Waals force. As a result, the particles drift toward each other and join together into a group (= floc)

After coagulation/flocculation, gravity sedimentation is performed to separate flocs from the water prior to filtration stage

In addition to remove turbidity, coagulation/flocculation process removes bacteria suspended in the water as well as remove color from the water

Turbidity and color are more common in surface water than in groundwater

How is in practical field?

1. Coagulant is added in a flash mixer

The mixture is mixed quickly and violently

Mixing speed: 100-150 rpm

Mixing time: 30-60 s

- Less than 30 s : not properly mixed

- More than 60 s : the mixer blades will shear the newly forming flocs back into small particles

2. After flash mixing, coagulation occurs.

Two types of coagulants: primary coagulants & coagulant aids

Primary coagulant neutralizes the electrical charges of the fine particles, bringing the particles together to create a very small (pinpoint) floc that is invisible.

Examples of primary coagulants:

- metallic salts such as aluminium sulfate (alum), ferric sulfate, ferric chloride

- polymer: poly aluminium chloride (PAC)

3. A process of gentle (10-25 rpm) mixing brings the particles formed by coagulation into with each other so the particle size increases from submicroscopic floc to visible suspended particles

How is in practical field?

The flocculation basin has a number of compartments

with decreasing mixing speeds as the water advanced

through the basin

How long? 1 h

The floc will then settled out

in the sedimentation basin,

with remaining floc being removed

in a filter.

Coagulant aids add density to slow-settling flocs so they will not break up during the mixing and settling process. Consequently, the chemicals are generally used to reduce flocculation time

Examples of coagulant aids: Ca(OH)2 (lime), CaO, CaCO3, bentonite (clay)

Al2(SO4)3 + 3Ca(OH)2 2Al(OH)3 + 3CaSO4

alum lime floc

In contrast to the primary coagulants, coagulant aids are not always required. It will be used during emergency treatment of water which has not been adequately treated in the flocculation & sedimentation basins.

How to choose the coagulant?

Alkalinity

- Importance factor to select metal salt coagulant because these

materials need some alkalinity to provide OH to drive the

hydrolysis reactions that allow the coagulants to function.

- The degree of alkalinity: > 50 ppm

Low alkalinity (< 50 ppm): acidic metal salts may be precluded

& high basicity coagulant such as PAC is used. Optionally, add

artificial alkalinity (NaOH, Ca(OH)2, Na2CO3)

- Very low alkalinity : add artificial alkalinity supplement.

In such a case, it might be useful to try a combination of acidic and basic coagulants such as PAC or alum together with sodium aluminate

pH

- If the pH is > 8.5 and Dissolved Organic Carbon

(DOC), often referred to as colour, has to be removed, a

highly acidic coagulant that will drive the pH down to 7

need to be considered. Soda ash may be necessary to bring

the Langlier Stability Index back to zero after such treatment

- In acidic condition, care should be taken to ensure that the

chemical reactions occur as desired. Ferric salts often

performed well in acidic conditions. Water with a colour

will coagulate better at low pH (4-6) with alum.

Turbidity

- Turbidity can be classified as being anionically charged silica particles

- Inorganic coagulant

Temperature

- The higher the temperature, the faster the reaction, and more effective is the coagulation.

- Almost all coagulants work well at 10 < ToC < 25

- Wet season temperature will slow down the reaction rate which can be helped by an extended detention time. Mostly, it is naturally provided due to lower water demand.

- It is unusual for a water plant to heat the raw water to a minimum of 8oC in winter to maintain adequate finished water quality. The non-sulphated polyhydroxy aluminium chloride choice does not appear to be as temperature sensitive.

Four zones in sedimentation basins: Inlet zone

Settling zone

Sludge zone

Outlet zone

Zones can be seen most easily in a rectangular sedimentation basin.

However the four zones can still be found within the clarifier.

1. Inlet zone - The incoming flow must be evenly distributed

across the width of the basin to prevent short-circuiting

- Short-circuiting is a problematic circumstance in which water bypasses the normal flow path through the basin & reaches the outlet in less than the normal detention time

- The velocity of the incoming flow must be controlled to prevent short-circuiting, i.e. < 0.5 ft/s

Two types of inlets are

- stilling wall or perforated baffle wall

- channel

Baffle

helps to

evenly

distribute

the

incoming

water

2. Settling zone

- After passing through the inlet zone, water enters the settling zone where the velocity is greatly reduced

- Floc settling is occur in this zone

- Tube settlers and lamella plates may be introduced in the settling zone

Water flows up along slanted tubes (plates)

Floc settles out in the tubes & drifts back down into the lower portion of the basin.

Clarified water passes through the tubes & flows out of the basin

- Tube settlers and lamella plates increase the settling speed & thus, the settling efficiency

- Each tube or plate acts as a miniature sedimentation basin

- The tube & plate are very useful where the site area is limited

a. Circular solid contact clarifier

b. Parallel inclined plates in a circular clarifier

c. Tube settler in a rectangular clarifier

TUBE SETTLER

LAMELLA SETTLER

Tube settlers VS lamella settlers

Advantages of plate settler. 1. Plates made of SS have longer life. 2. Plates can

be removed from the slots for cleaning and packing them back into grooves. 3.

Plates can be ordered as per design and delivered by the manufacturing

company for ready to fit in. 4. Needs less storage space at site during installation.

5. FRP plates are easily available in the market. 6. FRP plates are comparable in

price against Tube settler made of UPVC. 7. FRP plates have higher thickness

than Tube settler. 8. FRP plates can be easily resized at site during installation

Tube settlers are based on different theory that states that settling is dependent on

settling area rather than detention time. We keep the angle of inclination of Tubes

between 45 degree and 60 degree. since above 60 degree angle will cause them to

be ineffective and below 45 degree angle will cause choking problems.

Advantages of tube settlers over plate settlers are numerous. 1. Tube settlers are

self supporting block, plates need specific grooves with fixing mechanism. 2. Tube

settler can be easily fitted into a circular/square/rectangular tank, plates are

suitable for rectangular/square tank only. 3. Tube settler made of UV stabilised

PVC are more economical than plates. 4. mechanical strength of TS block is more

than plates. 5. TS block allows foot traffic. 6. Choking chances are less as

compared to plates. 7. Efficiency is more than plates. 8. Provides higher settling

area compared to plates

3. Sludge zone

- Velocity in this zone should be very slow to prevent resuspension of sludge

- The bottom shape should slope toward the drains to facilitate sludge removal

- A drain at the bottom allows the sludge to be easily removed from the basin

- In some plants, sludge removal is achieved continuously using automated equipment

- The sludge built up on the bottom may become septic (decay anaerobically odour problems & may float to the top of the water and become scum)

4. Outlet zone - outlet zone controls the water

flowing out of the basin - the zone is design to prevent short-

circuiting of water in the basin

- the outlet can be used to control the water level in the basin by setting up a baffle in front of the effluent

- the baffle prevents floating materials from escaping the sedimentation basin and clogging the filter

Improper coagulation related to coagulant may result from:

Using old chemicals

Using the wrong coagulant

Using the wrong concentration of coagulant

Primary treatment can remove up to 60% of solids, 30% demand for oxygen, and perhaps 20%

of phosphorus. If the removal is adequate & the

water dilution factor is such that the adverse effects

are acceptable, then a primary treatment plant is

sufficient wastewater treatment. After addition of

alum, the effluent BOD can be reduced to about 50

mg/L, and this BOD level may be able to meet required effluent standard.

Testing & control of coagulation and flocculation

The sludge is primarily composed of water The solid part is mainly excess coagulant Sludge is typically dried before it is trucked away. The

drying process is known as dewatering or thickening Alum sludge is difficult to thicken FeCl3, CaO & polymers may be added to improve sludge

dewatering process by acting as coagulant. Treating the sludge to aid in thickening is called as conditioning the sludge

Once the sludge has been conditioned, it may be thickened in a lagoon or drying bed

After a few months, gravity & evaporation will have reduced the

sludge to a 30-50% solid state

The sludge can then be covered with soil and left on site OR may

be trucked to a landfill off-site

Lagoons

In this topic we will learn the following:

How does secondary treatment processes fit into the water treatment process?

How does the secondary treatment processes clean water?

What types of reactors are used?

How are sludge treated?

Secondary Treatment Process

Why? Water leaving the sedimentation tank sometimes still contains a high demand for oxygen due to dissolved biodegradable organics.

Aim: remove the demand for oxygen (or BOD).

In most cases, secondary treatment involves the biological treatment processes. Thus, the performance of the secondary treatment plants is measured the demand for oxygen in term of BOD.

How? by performing aerobic biological treatment to metabolize organic matters (from food processing, beverage, dairy, fermentation, & certain pharmaceutical industries) to inorganic end-products such as CO2, NH3 & H2O.

Secondary Treatment Process.Biological

General speaking, in wastewater, N is found in the form of ammonia. When secondary treatment is used, a great deal of NH3 is discharged in the effluent. It also contains phosphorous (P). In addition, N & P are ingredients in fertilizers.

When excess amount of N & P are discharged, plant growth in the receiving waters may be accelerated

Algae growth may be stimulated causing blooms which are toxic to aquatic life as well as unpleasing odour.

Nitrogen removal by nitrification/denitrification processes:

aerobic NH3

> NO3- (nitrification)

anoxic NO3

- > N2 (denitrification)

Phosphorous removal by chemical process: alum, sodium aluminate, ferric chloride, ferric sulfate,

lime, etc.

Secondary treatment systems are classified as:

1. Fixed-film reactors (trickling filters, rotating biological contactor)

Principally, biomass grows on media & the water passes over its surface that suspended solid & dissolved organic matter are used as food for production of new cells, while other is oxidized to CO2, NH3 & H2O.

The tower is filled with support

media such as rocks or various

plastic shapes

Oxygen is supplied by natural

air flow or forced by blower

Scrapped off the biofilm periodically

Advantages

Simplicity of operation

Good quality (80-90% BOD5 removal) for 2-stage efficiency could reach 95%

Moderate operating costs (lower than activated sludge)

Low sludge yield

Disadvantages

High capital costs

Clogging of distributors or beds

Snail, mosquito and insect problems

Typical loading rate:

STANDARD-RATE FILTER

Media (rock): 6-8 ft depth, growth sloughs periodically

Hydraulic loading: 25-100 gal/day/sq ft.

BOD loading: 5-25 lbs BOD/day/1000 cu ft

HIGH-RATE FILTER

Media (rock): 3-5 ft depth, growth sloughs periodically


BOD loading: 25-100 lbs BOD/

Media (synthetic): 15-30 ft depth, growth sloughs periodically


BOD loading: 50-300 lbs BOD/day/1000 cu ft day/1000 cu ft

Rotating Biological Contactor:

Is a fixed media filter which microorganisms are growth on a series of large discs. The discs are supported on a single shaft rotated slowly through the wastewater, which provides food required to grow the microorganisms

Oxygen is supplied naturally when the film is out of the water and from the liquid when submerged

Produce high quality effluent

Rotating Biological Contactors are covered for several reasons relating to climatic conditions:

Protect biological slime growths from freezing

Prevent intense rains from washing off some of the slime growth

Stop exposure of media to direct sunlight to prevent growth of algae

Avoid exposure of media to sunlight which may cause the media to become brittle

Advantages

Short contact periods

Handles a wide range of flows

Easily separates biomass from waste stream

Low operating costs

Short retention time

Low sludge production

Excellent process control

Disadvantages

Need for covering units installed in cold climate to protect against freezing

Shaft bearings and mechanical drive units require frequent maintenance

Since typical effluent characteristics do not meet todays strict effluent limitations, many systems have converted to activated sludge

Secondary treatment systems are classified as:

2. Suspended growth reactors

The water is mixed with biomass in the form of activated sludge. Aeration device is employed for the survival of aerobic organisms

The microorganisms come into contact with dissolved organics & rapidly adsorbed on their surface. In time, the microorganisms use the carbon & release CO2, H2O, and stable compounds.

Following the process, microorganisms separated by sedimentation exist on the bottom without additional food become hungry waiting for more dissolved organic matter the microorganisms are said to be activated hence, the term activated sludge.

Can be operated in a smaller space than trickling filter to treat the same amount of water. However, fixed-film systems are more able to cope with drastic changes in the amount of biological matter.

A schematic of an activated sludge process

Advantages

Flexible, can adapt to minor pH, organic and temperature changes

Small area required

Degree of nitrification is controllable

Relatively minor odor problems

Disadvantages

High operating costs (skilled labor, electricity, etc.)

Generates solids requiring sludge disposal

Some process alternatives are sensitive to shock loads and metallic or other poisons

Requires continuous air supply

Biological solids removed during secondary sedimentation, called secondary or biological sludge, are normally combined with primary sludge for sludge processing.

Sludge treatments:

Anaerobic digestion (generates CH4, heat the tank & run engines)

Aerobic digestion

Composting (mixed with carbon source such as sawdust, straw or wood chips)

Group work

Tertiary Treatment Process

Tertiary treatment may be defined as any treatment

process in which unit operations are added to the flow

scheme following conventional secondary treatment, i.e.

Addition of a filter for suspended solid removal (simple)

Nutrients removal

Addition of unit processes for disease-causing microorganisms removal

Problems associated with nutrients presence in wastewater:

accelerate the eutrophication

stimulate the growth of algae such as in The Tualatin River (US) which has proved sensitive to algae bloom (phosphorus is allowed only 0.07ppm)

aesthetic problems & nuisance

depleting D.O. concentration in receiving waters

toxicity towards aquatic life

increasing chlorine demand

presenting a public health hazard

affecting the suitability of wastewater for reuse

Algae blooms

(irritate, fishy smells)

Red algal bloom

Gulf Coast, Mexico

Red algal bloom

Bondi beach, Australia 2012

Filtration

Aim: to remove suspended particles & other impurities (bacteria, plankton, cysts) left in water prior to be discharged to the receiving environment

How? By passing the water through a medium such as sand. As the water passes through the filter, floc & impurities get stuck in the sand & the clean water goes through

Location in the treatment process: after sedimentation stage

Up to 99.5% of the suspended solids in the water can be removed, including flocs & microorganisms

Depend on the presence of flocculation & sedimentation, the filtration can be divided into three groups: conventional, direct & in-line filtration

Conventional filtration: the most common method of filtration where filtration follows coagulation/flocculation and sedimentation. This type of filtration results in flexible & reliable performance, especially when the source water is very turbid

Direct filtration: if filtration follows coagulation/ flocculation without sedimentation. This method can be used when raw water has low turbidity

In-line filtration: if operating the filter without coagulation/ flocculation & sedimentation

How does filtration clean water mechanisms Straining: passing the water through a filter in which the pores

are smaller than the particles to be removed

Flocs cannot fit through the gap between sand particles & thus, the flocs are captured

The water is able to flow through the sand, leaving the floc particles behind

In filtration, adsorption involves particles becoming attracted to and sticking to sand particles

Very small particles can be removed from water

Adsorption: gathering of gas or dissolved solids onto the surface of another material

Biological action: breaking down the particles in water by biological processes. This involves decomposition of organic particles by algae, plankton & bacteria

The biological action is an important part of filtration in slow sand filtration

In most other filters, the water passes through the filter too quickly for much biological action to occur

What filter media used in filtration?

Desirable characteristics for filter media: Good hydraulic characteristics (permeable)

Does not react with substances in the water (inert & easy to clean)

Hard & durable

Free of impurities

Insoluble in water

What filter media used in filtration?

Granular filter classification based on its media:

Single medium filter: sand or crushed antrachite coal

Dual media filter: sand & crushed antracite coal

Multimedia filter: sand, crushed antracite coal & garnet

The media have varying density as well as varying the pore size

Sand can medium or coarse

Anthracite coal is a very light coal

Garnet is a very dense sand

Other media: activated carbon & diatomaceous earth

The former is used in association with tastes, odours & organic substances

The latter is used primary for removal of Giardia cysts & Cryptosporidium oocysts in potable water treatment

Media arrangement

The gravel at the bottom of the filter is not part of the media, merely provides a support between the media & the underdrain to allow an even flow of water during filtering & backwashing

The largest particles or floc are removed by the anthracite coal

The medium particles size are removed by the sand

The smallest particles are removed by the garnet

The media are arranged so that the water moves through media with progressively smaller pores

Since the particles are filtered out at various depths of media, the filter does not clog quickly

Filter classification

Based on the velocity of water being filtrated: Rapid filter standard: 2-5 & high: 5-15 gpm/sqft

Slow filter 0.05-0.15 gpm/sqft

Based on the operational methods: Gravity filter

Pressure filter

Slow sand filtration (in-line filtration)

Constitute a simple, efficient design and may be constructed using local resources

An old technology: entails a porous bed of graded sand fortified by an underlying layer of gravel. Raw water enters the filter bed

and undergoes purification

Mechanisms for removing impurities: adsorption, straining & biological processes

Principal application: low turbidity water (

Construction

If the tank is being

fed by pumps:

situated at a higher

elevation than the

filter to allow for a

generous operating

range

Inlet to the treated

water tank should be

slightly more elevated

than top of filter sand;

prevents filter from

being accidentally

drained if treated

water tank is emptied

Filtration rate is expressed as m3/m2/h or m/h

Ideal filtration rate: 0.2 m/h

Maximum recommended rate: 0.3 m/h

Method of operation

Slow sand filters work through the formation of a gelatinous layer (biofilm) called Schmutzdecke in the top 0.5-2 cm of the fine sand

layer. Usually the film will be formed in few days of operation &

ripens within 2-3 weeks (depend on the turbidity level)

The Schmutzdecke is a biologically active layer that initiates the process of breaking down pathogens into inorganic, innocuous

molecules 90-99% bacterial reduction

Also works as a fine, mechanical filter by trapping particles and then metabolized by bacteria, fungi & protozoa

Note: the Schmutzdecke is only one example of a biologically active

zone. Further biological activity occurs below the Schmutzdecke (at

a depth up to ~ 0.4-0.5m)

Slow sand filters are very efficient in microorganism removal

from water

The Schmutzdecke layer explanation

The filter loses the performance as the Schmutzdecke layer grows and thereby reduces the water flow rate

How to refurbish the filter?

- scrapped off (dont forget to replace the removed sand)

- wet harrowing: lowering the water level to just above the

Schmutzdecke layer, stirring the sand & running the water to

waste

Process limitation: land availability, higher initial cost (3x higher than rapid sand filter), difficult to maintain the operation during

cold season, certain types of algae may interfere with filter that

result in being premature choking

Rapid Sand Filtration

Provides greater filtration rate up to 40 times than slow sand filter (due to coagulation/ flocculation & sedimentation) & ability to

clean automatically using backwashing technique

Rapid filters are used primarily to remove turbidity in large water treatment plants

Mechanisms for removing impurities:

- adsorption

- straining

The tank is constructed of concrete or other corrosion-resistant materials

Rapid sand filters can be rapid gravity filter or pressure filter

Coarse sand is the most commonly used

The sand size is larger than in slow sand filtration. Generally it is 0.4-0.6 mm in diameter

Filtration rate designed to operate 4 12 m/h

Filter aid may be added to strength the floc and prevent its breakup. Generally the filter aid is polymer because it creates strong bonds with the floc

There should be over 90% reduction in turbidity

Check whether there is a sand in the clear water, indicating underdrain problems which may need to be replaced

Pressure Sand Filter

Extensively used to remove iron & manganese removal

The water is first aerated to oxidise the iron & manganese, then pumped through the filter to remove suspended materials

The tank may be vertical or

horizontal, depending on

the space available

Cracking of the filter bed can

occur quite easily

How are the filters cleaned?

Clogged filter beds are cleaned by backwashing with water or combination water air

Filtration is started slowly after a backwash to prevent breakthrough of suspended material

What is breakthrough? cracking the filter media and/or separation of the media from the filter wall

How? caused by running the filter at an excessive filtration rate or running the filter too long between the backwashing

Effect? the water flowing through the filter untreated which in turn results in a sudden high turbidity in the effluent water

Gravel separation can be caused by

The backwash valve opening too quickly; as a result, the supporting

gravel is forced to the top of the filter

Filter underdrain being plugged, causing uneven distribution of the

backwash water. When this happens,

a boil occurs from the increased

velocity in the filter. The filter media

will start washing into the filter

underdrain system and be removed

from the filter

If gravel displacement is occur, the media must be removed from the filter and rebuilt by placing each medium in its proper place

Disposal of filter backwash water:

Stored in a backwash tank & allowed to settle

The supernatant is pumped back to the treatment plant

The settled material is treated in solid-handling process of the plant

Check whether there is a sand in the clear water, indicating underdrain problems which may need to be replaced

Diatomaceous Earth Filter

Commonly used for the treatment of swimming pool water, winery products

Slow Sand Filter Rapid Sand Filter Pressure Filter Diatomaceous earth filter

(Diatomite filter)

Filtration rate

(GPM/ft2)

0.015-0.15 2-3 2-3 1-2

Pros Reliable. Minimum

operation and maintenance

requirements. Usually does

not require chemical

pretreatment. Conserves

water by no backwashing

required. Less trouble with

sludge disposal

Relatively small and

compact

Lower installation and

operation costs in small

filtration plants

Small size. Efficiency. Ease of

operation. Relatively low

cost. Produces high clarity

water. Usually does not

require chemical pretreatment

Cons Large land area

required. Need to manually

clean filters

Requires chemical

pretreatment. Doesn't

remove pathogens as well

as slow sand filters.

Sludge disposal problems

Less reliable than gravity

filters. Filter bed cannot

be observed during

operation

Sludge disposal problems. High

head loss. Potential decreased

reliability. High maintenance

and repair costs

Filter Media Sand Sand OR sand &

anthracite coal OR sand &

anthracite coal & garnet

Sand OR sand &

anthracite coal OR sand &

anthracite coal & garnet

Diatomaceous earth

Gravity or

Pressure?

Gravity Gravity Pressure Pressure, gravity, or vacuum

Filtration

Mechanism

Biological action, straining,

and adsorption.

Primarily adsorption. Also

some straining

Primarily adsorption. Also

some straining

Primarily straining

Cleaning

Method

Manually removing the top 2

inches of sand.

Backwashing Backwashing Backwashing

Common

Applications

Small groundwater systems. Most commonly used type

of filter for surface water

treatment.

Iron and manganese

removal in small

groundwater systems.

Beverage and food industries

and swimming pools. Smaller

systems.

Operating Parameters

Head loss is an important parameter to determine whether the filter should be backwashed

Head loss is a loss of pressure (also known as head) by water flowing through the filter. When water flows through a clogged filter, friction causes the water to loose energy so that the water leaving the filter is under less pressure than the inlet water

A piezometer connected to the filter above the media is employed to measure the difference in the head

The head loss is higher with more particles blocked within the bed the filtration efficiency is decreased

Headloss filter pada kondisi bersih dapat ditentukan dengan persamaan Carmen Kozeny

Uniform size Filter

75,11

150'

reN

f

w

wsp

re

vdN

(1)

(2)

(3)

p

sf

dg

vLfh

3

2' )1(

Non uniform size filter

ijp

ijijsf

d

xf

g

vLh

'

3

2)1(

where:

hf = Head loss

L = filter depth

= porosity

vs = superficial velocity

g = gravity

dp = particle diameter

= particle shape faktor x = weight fraction

w = density of water w = viscosity of water

(4)

Selain persamaan Carmen Konzeny, juga dapat digunakan persamaan Rose Uniform size Filter

Non Uniform Filter

p

sDf

dge

VLCh

4

2067,1

( 6 )

( 7 )

( 8 )

Factors affecting the filtration efficiency: Influent characteristics

Types of previous treatment processes (conventional, direct or in-line filtration)

Types of filter used

Water volumetric rate

GAC Use in Water Treatment

Flash mixing Flocculation

Sedimentation Filtration

Carbon treated water

GAC adsorbers

Sump

Sludge

Raw water

Untuk parameter regenerasi perlu dibuat breakthrough curve

FILTRATION, DISINFECTION & STORAGE:

FILTRATION. The water passes through filters, some made of

layers of sand & charcoal that help remove smaller particles

DISINFECTION is a process design to

destroy microorganisms to a safe level

by adding a certain disinfectant

STORAGE. Water is placed in a

closed tank or reservoir for

disinfection to take pace. The water

then flows through pipes to home

and business in the community WATER FROM

SEDIMENTATION TANK

Tertiary Treatment ProcessDisinfection

Disinfection VS sterilisation ? - Disinfection (by chlorine) does not destroy all pathogens. - Sterilisation is a process design for complete destruction of all living

microorganisms

There is no need to destroy all microorganisms completely as it is unnecessary & extremely costly - Meet fecal coliform limits for effluent discharge

Inactivation is achieved by altering or destroying essential structures or functions within the microorganisms

Inactivation processes include denaturation of proteins, nucleic acids, and lipids

3 common disinfection processes in large scale: Chlorination

UV light radiation

Ozonation

Most common method

Advantages:

- low cost

- effective

Disadvantages: chlorine residue could be harmful to environment

Chlorination

Chlorine may be added in the form of chlorine gas (Cl2), hypochlorite (OCl) or chlorine dioxide (ClO2)

All types of chlorine will kill bacteria & some viruses but only chlorine dioxide effectively kill Cryptosporidium, Giardia, protozoans & some viruses

ClO2 is the most powerful substance: - kill viruses that other systems may not kill

- convert organic matters to CO2 and H2O

- remove sulfide compounds & phenolic tastes and odours

- trihalomethanes would not be formed

So why isnt chlorine dioxide used in all systems?

ClO2 must be generated on site which is a very costly process requiring a great deal of technical expertise

ClO2 is highly combustible

When chlorine is added to water, a variety of reactions take place

Cl2 + H2O HOCl + HCl

HOCl (hypochlorous acid) ionisation to OCl (hypochlorite ion) is occurred that depends on pH

HOCl H+ + OCl

HOCl is a weak acid & is not dissociated at pH < 6

At pH of 7.3 (depending on T), would be 50% HOCl & 50% OCl

Chlorination chemistry

% distribution HOCl vs pH

HOCl has 40-80 times greater disinfection ability than OCl

Thus, the higher the pH the greater OCl concentration and the more chlorine required to achieve disinfection

This is an important reaction to understand because HOCl is the most effective form of free chlorine residual, meaning that it is chlorine available to kill microorganisms in water

Leave a residual to give protection against further contamination. This is achieved by ensuring a free residue 0.20.5 ppm of chlorine in the disinfected water after a contact time of 30 min. Longer time during cold season

The residual will inhibit any subsequent growth of organisms within the water distribution system

To achieve good result, water turbidity should be < 5 NTU. It can be tolerated up to 20 NTU. At high level of turbidiy, some organisms may survive as well as leaving a strong chlorine taste

How to add chlorine to a water supply? - dosing a continuous flow of a 1% solution of chlorine - adding chlorine tablets or powder directly to water tank (for emergency chlorination)

How about hypochlorite compounds? It could be - sodium hypochlorite (NaOCl) - calcium hypochlorite (Ca(OCl)2)

Reaction with water: NaOCl + H2O HOCl + NaOH

When UV radiation penetrates the cell wall of an organism, it damages genetic structure of bacteria, viruses and other pathogens & prevents the cell from reproducing

Advantages: no chemicals used water taste is more natural

Disadvantages: high maintenance for UV-lamp

UV light radiation

Ozone oxidises most pathogenic microorganisms with less contact time and concentration than other disinfectants

Ozone (O3) is generated on-site at water treatment facilities by passing dry oxygen or air through a system of high voltage electrodes

3O2 2O3

Ozonation

Advantages:

- safer than chlorination - more effective than chlorine in destroying viruses & bacteria

- fewer disinfection by-products

- ozonation elevates the dissolved oxygen concentration

Disadvantage:

- high cost due to more complex technology - cannot maintain a residual in the distribution system, so ozone

disinfection should be coupled with a secondary disinfectant such as chlorine

- ozone is corrosive, thus, require corrosion-resistant material

3 water treatment processes

Documents

accumulation of grit

grit characteristics

grit deposition

accumulated grit slides

types of grit removal

high percentage of fine

screen size

coarse screen clear