Waste water treatment: Trickling filter

Biological TreatmentTrickling Filter Trickling filter at Al Aweer WWTP... Trickling Filters• Whereas activated sludge is a suspended growth process, a trickling filter is an attached gro

Biological Treatment

Trickling Filter

Trickling filter at Al Aweer WWTP

Trickling Filters

• Whereas activated sludge is a suspended

growth process, a trickling filter is an

attached growth process

• Wastewater trickles over

medium, bacteria grow on

medium, creating biofilm

• Biofilms are the means of

treatment in trickling filters

• The packing of a trickling filter

could be rock (old) or plastic

(recently used) material

• Substrate, O2 and nutrients

diffuse across a stagnant

boundary layer Making the

biodegradation reaction diffusion

limited

Filter medium

Anaerobic

Aerobic

Biological layer Liquidfilm

Wastewater flow Air

Trickling Filters versus Activated Sludge

Advantages

• Less energy needed

• Simpler operation

• Less operation and maintenance

Disadvantages

• Poorer effluent quality

• Sensitive to low temperature

• Produces odors

• Nitrogen removal is difficult

Types of Trickling Filters

Low rate or standard rate

• Rock filters: 1-3 m deep

• Loading: 2-20 lb BOD/1000 ft2.day

High rate

• Rock or plastic filters: 1-2 m deep

• Loading: 20-60 lb BOD/1000 ft2.day

Super rate

• Synthetic packing

• Loading: 50- 380 lb BOD/1000 ft2.day

Raw water

(

Q

) Primary settling

Trickling filter

Recirculated flow (QR )

Secondary settling )water (QTreated

Raw

water

Treated water

One-stage Trickling Filter

Primary

settling

Trickling filter

Recirculated flow (QR )

Intermediate

settling tank

Treated water (Q )

Trickling filter

Recirculated flow (QR )

Final settling tank

Two-stage Trickling Filter

Design Parameters

BOD loading (lb/1000 ft3/day) 30-90

Recirculation ratio (R) 0.5-3.0

filter of

volume

day lb

BOD settled

loading

A

Q Q HL

loading Hydraulic = = + R

Q

Q

Typical loading for trickling filter with

a 5-7 ft depth of stone or slag media

Parameters needed for the design of a

trickling filter system include BOD

loading, hydraulic loading and

recirculation ratio These parameters are

defined as follows

:

Primary settling

Trickling filter

Recirculated flow (QR )

Q

) 20

(

20( 1 035 ) −

T E E

BOD Removal Efficiency

A one-stage trickling filter has a primary clarifier (55-ft dia, 7-ft depth, single peripheral weir), trickling filter (85-ft dia, 7 ft stone filled bed), and final settling tank (50 ft dia, 7 ft side water depth single peripheral weir) R=0.5, Q= 1.38 mgd, and average BOD = 180 mg/l

• For the primary clarifier determine overflow rate, detention time and weir loading

• For the trickling filter determine the BOD and hydraulic loading

• What is the effluent BOD at 16 oC

Example

Primary settling

Trickling filter

Recirculated flow (QR )

Secondary settling )water (QTreated

For the trickling filter:

A= 3.14* (85)2/4= 5660 ft2

V= 5660*7= 39,600 ft3

HL= (1.38 +0.5*1.38)*106/ 5660 = 366 gallon/ft2/day

BOD load: Assume 35% BOD removed by the primary clarifier, then settled BOD is 0.65*180= 117 mg/l

For the primary clarifier:

Overflow rate= vo=Q/As = 1,380,000/2370= 589 gpd/ft2

Detention time= t= V/Q= (0.124 mil gal/ 1.38 mgd) *24hr/d = 2.2 hr Weir loading= Q/Lweir= (1,380,000 g/d)/(3.14*55)=8000 gpd/ft

Solution

Using Fig 11-19 with BOD load =34 and R =0.5, we find the BOD removal efficiency is 78% at 20oC Now using

) 20

(

20( 1 035 ) −

E

We find the efficiency at 16 oC = 68% Thus, the effluent BOD will be

117 (100-68)/100= 37 mg/l

filter of

volume

day lb

BOD settled

loading

day ft

cu BOD

lb ft

cu thousand

l mg mgd

/ 1000

/

34 6

39

34 8

* / 117

* 38

1

=

=