United Nations Industrial Development Organization
INTRODUCTION TO THE TREATMENT
OF TANNERY EFFLUENTS
What every tanner should know about effluent treatment
Part V
Compiled by J. Buljan, I. Kral
5. Occupational Safety and Health
(OSH), [Common] Effluent Treatment
Plant ([C]ETP) Costing and Management
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Monitoring
Occupational safety and health at work
CETP collection network
CETP costing, cost distribution
Flow-chart of the fully-fledged treatment
Poisoning effects
of hydrogen sulphide gas (H2S)
Mixing of liming and tanning streams results in:
• Obnoxious smell typical for poorly managed tanneries
• Much worse: lethally poisonous gas hydrogen sulphide (H2S),
the most frequent killer in tannery accidents (mainly in
inadequately ventilated spaces, pits and channels)
Hydrogen sulphide (H2S):
• extremely toxic and irritating gas
• strong odour of rotten eggs
• paradoxically, it is felt only at lower concentrations.
This explains the numerous accidents despite the well known
fact about its toxicity.
Poisoning effects
of hydrogen sulphide gas (H2S)
Impact on unprotected
person
Exposure [ppm]
Time
0.03
No time limit
No effect
0.08 – 2.0
No time limit
(Mal)odour threshold
10
Up to 8 hours
No effect
15
Up to 15 minutes
Threshold limit value
15 - 200
Up to 15 minutes
Headache, nausea, general
weakness, pain in legs
200 - 500
1 minute
Irritation of nose & throat,
vertigo, blurring of vision, loss
of consciousness lasting a few
minutes
500 - 900
1 minute
Profound coma, convulsions,
disorientation after recovery
> 900
1 minute
Instant coma and death
ppm = parts per million
Recent changes
of the recommended chronic
exposure limits to H2S
In view of new toxicological data, some countries have
started reducing the recommended chronic exposure limits
to hydrogen sulphide.
Threshold Limit Value (TLV-TWA8)
8 hour time weighted average
Short Term Exposure Limit (TLV-STEL)
From 10 ppm
→ to 1 ppm
From 15 ppm
→ to 5 ppm
Instruments
measuring H2S presence
 Fixed
 Portable
 Personal detectors
Due to nearly permanent life threatening
hazard:
• H2S meters have to be permanently positioned at
critical points and/or carried by staff
• Entering pits only in presence of skilled
supervisor, the worker equipped with safety
harness.
Otherwise,
general and specific measures
concerning equipment and
chemicals also apply to ETPs
(protection from moving parts,
fences, storing and handling of
chemicals etc.)
Structure of the average total
treatment costs
in selected CETPs in India in 2005
Miscellaneo
us
6%
Other
costs (e.g.
R&D)
3%
Power
38%
Loan
repayments Depreciation
10%
on
investments
15%
Chemicals
5%
Salaries
& labour
11%
Repair &
maintenance
10%
Sludge
dewatering
2%
The highest component is power; sludge handling and disposal
costs comparatively low.
If the reverse osmosis (RO) stage for desalination is installed,
the cost of energy escalates even further.
Distribution of average total costs
in Italian CETPs
Repair &
maintenance
5%
Methane
gas
1%
Salaries &
labour
Sludge
dewatering
14%
34%
Chemicals
17%
Power
11%
Admin.,
depreciation,
finance
11%
Sewage
network
Miscellaneous
6%
1%
In industrialized countries the largest cost component is
dewatering and safe disposal of solids (sludge).
Computation and distribution
of (C)ETP running costs
• In an individual tannery it is quite straightforward.
• Computation, and in particular, distribution of CETP running
(operation) costs among individual tanneries is quite complex.
The key parameters typically used for
computation/distribution of CETP costs
• Water consumption, m3/day
• Production capacity, tonnes of wet salted hides/day
• Actual pollution load in terms of COD (even better COD
destroyed!), suspended solids, sulphide and chromium
• Production output, m2/day of wet blue, crust or finished leather
In developing countries
Typically based solely on effluent volume and
type of tanning
In Italy
Very sophisticated, primarily based on pollution
load parameters
Computation & distribution of
running costs in one CETP in India
• Electromagnetic Flow Metering System installed in
each operational tannery monitors the quantity of the
effluent discharged.
• The actual expenditure for the month is divided by the
total flow to arrive at the unit cost for the month.
• O & M costs of the CETP plant are covered by
monthly contributions by the tannery members
according to the quantity of effluent discharged.
Management
Nowadays it is taken for granted that in a
well managed tannery there is a person of
appropriate background and seniority
dealing only with increasingly complex
environmental issues and the ETP in
particular; in some countries it is a legal
requirement.
Example of the CETP
management set up
General summary on treatment
of tannery effluents
• Treatment of tannery effluents is now a well established
technology.
• Modular type common effluent treatment plants CETPs to
service traditional tannery clusters or newly set leather
industry zones considered optimum solution.
• ETPs for individual tanneries require a separate organizational
unit lead by a well qualified person, preferably an
environmental engineer.
• Similarly, a CETP is run by an autonomous establishment –
company supervised by a board comprising representatives of
tanners association(s), EPA, NGO, residents etc.
• Investment and operation costs vary a lot; as a rule the largest
components is the biological treatment.
Main outstanding
issues – challenges (I)
High TDS (salinity) content, unaffected by
treatment, especially where:
– raw material input are hides and skins preserved by
salting,
– no scope for mixing with domestic sewage,
– discharge into sea is not feasible,
– relocation of the tanning industry to the sea side not
feasible.
Desalination of treated effluent by reverse osmosis
(RO) is energy intensive and thus very expensive.
Main outstanding
issues – challenges (II)
Utilization or safe disposal of sludge
At the moment this is the toughest problem in
industrialized countries, increasingly difficult in
developing countries too. A genuine
breakthrough badly needed (modified
gasification, vitrification, biodegradation?).
Fully-fledged treatment system
Submersible pump
Dosing pump
SERVICE WATER
Pump
CATALYST
MnSO4
POLYELECTROLYTE
LIME
ALUM
Motor
Ball valve
Gate valve
Agitator
Drain
Drain
Drain
Drain
MIXED EFFLUENT
PRIMARY
SETTLING TANK
AUTOMATIC
COARSE SCREEN
FINE SCREEN
EQUALIZATION TANK WITH PUMPING STATION
(WITH EJECTORS FOR MIXING AND AERATION)
INLET PUMPING STATION
AIR
AIR
COAGULATION FLOCCULATION
TANK
TANK
SECONDARY
SETTLING TANK
END FLOW MEASURING
WITH FLOW SUMMARISING
AIR
BLOWER
FLOW MEASUREMENT
CHANNEL
AERATION TANK 1
AERATION TANK 2
(DIFFUSED AERATION SYSTEM)
SLUDGE DRYING
BEDS (optional)
G
SECONDARY SLUDGE
PUMPING STATION
CHAMBER
FILTER PRESS
PRIMARY SLUDGE
PUMPING STATION
SLUDGE
THICKENER
TREATED
EFFLUENT
FOR DISCHARGE