INTRODUCTIONINTRODUCTION TOTO TTHE THE TREATMENTREATMENTOF TANNERY EFFLUENTSOF TANNERY EFFLUENTS

What every tanner every tanner should know about effluent treatment

Part IV

Compiled by JJ. . BuljanBuljan, I. Kral, I. Kral

United Nations Industrial Development Organization

4. 4. Biological (secondary) Biological (secondary) treatmenttreatment

• Activated sludge process – principle, parameters

• Aeration devices• Oxidation ditch• Flow-chart of the biological (secondary)

treatment• Flow-chart of the fully-fledged treatment

Activated sludge process Activated sludge process

AERATION TANK

CLARIFIER

Influent Effluent

Sludgerecirculation

pumpSludge return

Excess of sludge

Q

Air

R

Q+R

Activated sludge process Activated sludge process • Total Influent Volume, Q: Volume of the treated effluent [m3/day]

• Tank Volume, V: Aeration tank volume [m3]

• Organic Loading, F: The total BOD5 applied [kg/day]

F = (f * Q)/1000where f – the BOD5 of the influent [mg/l]

• Mixed Liquor Suspended Solids, M:

M = (MLSS * V)/1000where MLSS - is the concentration of SS in Mixed Liquor in the aeration tank [mg/l]

• Loading factor, F/M: BOD5 kg per day per kg of Mixed Liquor Suspended

Solids (MLSS) in the aeration tank [mg/l].

• Hydraulic retention time, T:

T = (V/Q) * 24where V – aeration tank volume [m3]

Typical food to microorganisms Typical food to microorganisms (F/M) and oxygen to food ratios(F/M) and oxygen to food ratios

Process

Extended aeration

Conventional

High rate

Food to microorganisms

F/M

0.1

0.3

1.0

Oxygen to food

kg O2/kg BOD

2.0

1.2

0.8

Other operational parametersOther operational parameters

• Dissolved Oxygen, DO: The content of molecular oxygen in the aeration tank [mg O2/l].Usually DO = 0.5-2.0 mg O2/l.

 

• pH:The optimum pH range for aerobic process is 7.0-7.5

• Temperature:Affects the metabolic and growth rates of the organisms.Optimum: 10-30oC.

• Nutrients, desirable ratio:

BOD : N : P = 100 : 5 : 1• Sludge Volume Index, SVI:Volume of 1 g of activated sludge after settling the aerated mixed liquor in a 1000 ml

graduated cylinder or Imhoff cone for 30 min.

Desired value < 100 ml/g.

lmglml

solidsSuspended

volumesludgeSettledSVI /

/1000

Aeration devicesAeration devices

1. Surface aeratorsa) Radial flow, low speed, 20-60 rpmb) Axial flow, high speed, 300-1200 rpmc) Brush rotor (oxidation ditch) 

2. Submerged turbines 3. Diffused air

a) Bubblers – porous and nonporous diffusersb) Tubularc) Jets (developed from Venturi ejectors)

There are many types and subtypes of aerators, models being continuously improved. Here is an arbitrary classification of types frequently installed in tannery ETPs.

ClassificationClassificationof mechanical aeratorsof mechanical aerators

Surface (floating) aeratorsSurface (floating) aerators

Surface (floating) aeratorsSurface (floating) aerators

Submersed aeratorSubmersed aerator

Submersed Submersed aeratoraerator

Bubble bottom diffusersBubble bottom diffusers

Oxidation ditchOxidation ditch

Oxidation ditchOxidation ditch

Bottom diffusers in the Bottom diffusers in the oxidation ditchoxidation ditch

Submersed turbine aeratorsSubmersed turbine aeratorsin the oxidation ditchin the oxidation ditch

Oxidation ditchesOxidation ditches

Purification efficiency of the treatment Purification efficiency of the treatment stages referred to raw effluentstages referred to raw effluent

Parameter

Physical-chemical Biological

ReductionApprox.

valueReduction*

Approx. value

% mg/l % mg/l

SS 80 - 90 300 - 600 95 - 98 20 - 50BOD5 50 - 65 750 - 1500 97- 99 20 - 30

COD 50 - 65 1500 - 3000 87 - 93 250 - 400

TKN 40 - 50 250 - 300 60 - 70 100 - 200

Cr 92 - 97 5 - 10 99 - 99.5 < 1

S2- 80 - 90 10 - 20 99 - 99.5 < 1TDS, mainly Cl- and (SO4)2- No reduction

* Approximately at the load level of 0.30 kg BOD5/kg MLSS, oxygen requirement of 2.0 kg O2/kg BOD5 and MLSS 3300 g/m3.

FLOW MEASUREMENTCHANNEL

END FLOW MEASURINGWITH FLOW SUMMARISING

TREATEDEFFLUENT

FOR DISCHARGE

CHAMBERFILTER PRESS

GSLUDGE

THICKENER

SLUDGE DRYINGBEDS (optional)

SECONDARY SLUDGEPUMPING STATION

Biological treatment stageBiological treatment stage Simplified flow-chart

SECONDARY SETTLING TANK

BLOWER AIR AIR

AIR

Submersible pump

Pump

Motor

Gate valve

PRIMARY TREATEDEFFLUENT

AERATION TANK 1 AERATION TANK 2(DIFFUSED AERATION SYSTEM)

MIXED EFFLUENT

SERVICE WATER

DrainDrain Drain Drain

CATALYSTMnSO4

ALUM LIME POLYELECTROLYTE

EQUALIZATION TANK WITH PUMPING STATION(WITH EJECTORS FOR MIXING AND AERATION)

COAGULATIONTANK

FLOCCULATIONTANK

PRIMARYSETTLING TANK

FLOW MEASUREMENTCHANNEL

END FLOW MEASURINGWITH FLOW SUMMARISING

TREATEDEFFLUENT

FOR DISCHARGE

CHAMBERFILTER PRESS

G

SLUDGETHICKENERPRIMARY SLUDGE

PUMPING STATION

INLET PUMPING STATION

FINE SCREEN

AUTOMATICCOARSE SCREEN

SECONDARYSETTLING TANK

AERATION TANK 1 AERATION TANK 2(DIFFUSED AERATION SYSTEM)

BLOWERAIR AIR

AIR

SECONDARY SLUDGEPUMPING STATION

Submersible pump

Dosing pump

Motor

Ball valve

Gate valve

Agitator

Pump

Fully-fledged treatment systemFully-fledged treatment system

SLUDGE DRYINGBEDS (optional)