L-21 and L-22Trickling filter

Part-IIIHRTF

Environmental Engineering-II

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Single stageSingle stage

Two stage

Introduction• Hydraulic as well as organic loading of

high rate trickling filter is many times more than the low rate or standard rate trickling filter.

• Main defect of SRTF is it large area and high initial cost.

• Experiments conducted on trickling filters with increase in the rate of sewage flow revealed the following points

1.As the flow increases the thickness of biofilm reduced and the organic material deposited on the surface is continuously washed away with the effluent

2.Thinner biofilm is more efficient and 2.Thinner biofilm is more efficient and supplies more continuous nutrients to the aerobic bacteria.

3.Lesser oxidation of organic matter because of reduction in contact period.

4. Since large quantity of less degraded matter reaches the SST, load on SST increasesincreases

5. The sludge produced as a result of HRTF is not easily digestible.

6. The cost of construction, initial cost etc decease with increase in rate of filtration.

Modifications in SRTF

• Following modifications are done in to achieve high rate filtration.

1.Better quality of filtering media is 1.Better quality of filtering media is used so as to give more specific area.

2.Depth of filter media is reduced to 1.2 to 2.0 m so as to obtain better aeration.

3. Size of under drain is increased and their slope is also made steeper so that more effluent collected and conveyed to SST more quickly.

4. The speed of rotation arm is increased to 4. The speed of rotation arm is increased to 2 rpm for increased hydraulic loading.

5. Capacity of SST is also increased to cope up with increased quantity of flow and solids coming out of TF.

Filter mediaMedia Size

mmUnit wt. (Kg/m3)

Specific surface(m2/m3)

Void space%

Granite 25-75 1440 62 46

Granite 100 1440 47 60

Slag 50-75 1090 67 49

Plastic sheet 600 x600x1200 32-96 82-115 94-97

Red wood slats

1200x1200x500 165 46 76

Recirculation

Recirculation is defined as returning a portion of treated or partly treated sewage to the treatment process.to the treatment process.

Recirculation is needed to provide uniform hydraulic loading as well as to dilute the high strength influent

• Recirculation improves the operating results because of following reasons:-

1. It allows continuous dosing of filter irrespective of fluctuations in flow.

2. Recirculation equalizes and reduces loading thereby increasing efficiency of loading thereby increasing efficiency of the filter.

3. Recirculation provides longer contact of applied sewage with the biofilm on media, thereby seeding it with bacteria and accelerating the oxidation process

4. The effluent remains fresh all the time and also helps in reducing odours. The fly nuisance is also comparatively less.

Recirculation increases size of PST because of increased flow in PST. Same is true with SST. But volume of filter remains with SST. But volume of filter remains unchanged.

If recirculation is from SST to PST then sizes of both the tanks should be increased, If recirculation is from SST to HRTF then the size need not be too large

Advantages of HRTF

i. Construction Cost is less

ii. Operating cost is low

iii. Trouble of smell and odour is lessiii. Trouble of smell and odour is less

iv.There is absence of trickling filter flies

v. Working is flexible

Disadvantages of HRTF

1. As effluent is not highly nitrified, hence it requires large amount of dilution water for disposal.

2. Raw sewage can not be treated hence it requires PST.

Hydraulic Loading

Flow to filter

Filter surface area

m3/m2/day

Organic Loading

Kg of BOD

Volume of filter media

BOD load kg/m3/day

cubicmeter

Recirculation factor and recirculation ratio

• Refer class note book

NRC equation

• Refer class note book

PART-IV

RCC SLAB

Comparison between SRTF and HRTFCharacteristics SRTF HRTF

Depth of media 1.8 to 2.4 m 1.2 to 1.8 m

Hydraulic loading (m3/m2/d)

1 to 4 30 to 40

(m3/m2/d)

Organic loading as 5- day BOD in gm/m3/day

80 to 320 500 to 1000

Recirculation system

Usually nor provided but can be provided if hydraulic load does not exceed limit

Always provided in the range of 1:1 to 4:1

Volume of Bed 5 times 1

Interval of dosing Not more than 5 min Not more than 15 seconds

Sloughing intermittent continuous

Cost of operation more Less

Land required more less

Final effluent quality

BOD less than or equal to 20 mg/lir.Highly nitrified

BOD greater than or equal to 30 mg/lit. Not fully nitrified

Secondary sludge Highly oxidized, black colour, having fine particles

Not fully oxidized, brownish black colour containing fine particles

Trickling filter operating problems

1. Ponding:- is normally the result of:

(a) excessive organic loading without a corresponding higher recirculation rate,corresponding higher recirculation rate,

(b) use of media which is too small,

(c) clogging of underdrainsystem,

(d) non-uniform media size or breaking up of media, and

(e) trash or debris in filter voids.

• Ponding can cause odors and decrease filter efficiency.

•• PondingPonding can be eliminated by:

1. Shutting off the flow to the filter. The growths will die and tend to be flushed out when the unit is put back into service.

2. Stirring or agitating ponding area with stick, rake, etc.

3. Dousing the filter with chlorine.3. Dousing the filter with chlorine.

4. Flooding filter and keeping the media submerged for approximately 24 hours will sometimes cause the growth to slough. Growths become anaerobic and tend to release from media.

5. Spraying the surface with high pressure water hose.

2. Odors:- Since the trickling filter is an aerobic process, no serious odors should exist. If foul odors are present, anaerobic conditions are the most likely cause.Anaerobic conditions usually predominate next to the media surface.predominate next to the media surface.If the surface of the slime growth is aerobic, odors should be minimal. If odors are present, corrective action should be taken immediately or the condition could get worse.

• Some corrective measures are:

1. Try to maintain aerobic condition in the collection system and in the primary treatment units.

2. Check the ventilation of the filter for clogging and stoppages.clogging and stoppages.

3. Check the under drain system for clogging and stoppages.

4. Keep wastewater in filter; do not allow it to splash on exposed surfaces, weeds, or grass.

5. Add odor-masking agents.

6. Pre-chlorination at primary tank influent or at the dosing tank.or at the dosing tank.

7. Increase recirculation rate; this usually provides added oxygen to the filter and may increase sloughing

3. Filter Flies are a nuisance to plant personnel and nearby neighbors. These tiny, gnat-size flies are called psychoda. They are occasionally found in great numbers, preferring an alternate wet and dry environment for development. and dry environment for development. Lay their eggs in ponded water.

• The flies are most frequently found in low or standard rate filters with an intermittent dosing system.

Control can be accomplished by:

1. Flooding the filter intermittently to prevent completion of the fly life cycle. This life cycle can be as short as seven days in warm weather. Filters should be flooded for approximately 24 hours.

2. Increasing recirculation. A continuous waste flow to the filter will tend to wash fly larvae from the filter.

3. Flushing or washing the side walls of the filter by opening the flap valve at the end of the

distributor arm.

4. Keeping the plant grounds neat, clean and free from excessive weeds, plants, and grass,which are excellent breeding and grass,which are excellent breeding grounds for the flies.

5. The addition of chlorine, which is toxic to the flies and larvae.

Objective Questions1. _________ equation is used for finding

HRTF efficiency.

2. Organic loading for HRTF is ___________. (100-400/400-600/500-1000/1000-2000).(100-400/400-600/500-1000/1000-2000).

3. _____________ increases size of SST.

4. Increases recirculation can control __________, due to anaerobic conditions and also helps in __________ control.

Theory Questions

Q1. Write short note on

i. Recirculation w.r.t. HRTF

ii. Operational problems in trickling filterii. Operational problems in trickling filter

Q2. Give NRC equation for two stage HRTF

Q3. Compare SRTF and HRTF

L-23DESIGN PROBLEMS