Basics of waste water treatment

Question 1

Why wastewater treatment? (4)

Answer 1

• social responsibility
• license to operate requirement -> main motivation companies
• reduce water intake (increase water reuse)
• finance (lower taxes, less water use) -> main motivation companies

Question 2

structural approach water (5)

Answer 2

• remediation at source (check production process as well!)
• recycling of waste products
• valorising (= giving value) to waste products (circular approach)
• apply end-of-pipe solutions
• reusing effluent

Question 3

effluent

Answer 3

wastewater discharge to manicupality by industrial or commercial premises (either treated or untreated)

Question 4

influent

Answer 4

untreated or contaminated wastewater flowing into WWTP

Question 5

sewage

Answer 5

wastewater discharged to manicupality by community of people

Question 6

which substances are removed and why? (6)

Answer 6

1. oxygen demanding substances (COD, BOD, ammonium, nitrate): compounds that take out oxygen from surface water -> causes fish to die
2. eutrophying substances (phosphate, nitrite): enrichment of minerals and nutrients in water (from e.g. sewage or manure) -> causes explosive algea growth
3. persistant substances (POP): components resistant to environmental degradationj -> causes accumulation in foodchain
4. cyanide and (heavy) metals: toxic for aquatic life -> causes fish to die
5. suspended matter and colour (components that float in water)
6. thermal pollution
   - high temperature: lower O2 sollubility in water
   - high temperature: grwoth of anaerobic bacteria (bad smell)

Question 7

diversity in waste water: four characteristics of water stream

Answer 7

1.Components
- soluble or insoluble
- biodegradable or persistant
- organic or inorganic
- nitrogen and phosphates, etc

2.Concentration
- very diluted
- very concentrated

3.Condtions
- pH
- temperature
- salts

4.flowrate
- < 1 m3/h
- < 2000 m3/h

Question 8

BAT

Answer 8

best available technique
can be found in BREF documents

Question 9

Set up WWTP

Answer 9

influent in ->

1.pretreatment: solid removal

2.buffer: equalization of inflow peaks

3.biological treatment: treatment making use of bacteria

4.sludge/water seperation: bacteria are removed from water

5.effluent polishing: possible remove lost traces of bacteria / meet final requirements

-> effluent out

Question 10

WWTP - solid removal

Answer 10

purpose: make water suitable for biologic treatment, which can only handle dissolved components

methods: screens, settler, filter, flotation

Question 11

WWTP - buffer

Answer 11

purpose: equalize peaks - realize consistent volume and quality of WW
- makes controls easier
- protects against: hydraulic peaks (m3/h) and composition peaks = protects bacteria (pH, N/P concentration, composition)
- allows for smaller ETP

types: tanks
- in-line: everything flows through
- off-line: part of waste water flows in at peak moments (can also be a spill tank)

rule of thumb: residence time of 8 hours before completely filled (depends in production process though!)

Question 12

WWTP - spill tank

Answer 12

what: storage capacity (tank) placed *parallel to ETP *
- water is slowly added back to ETP system so to not disturb the balances in the ETP

purpose: to cope with influent peaks or calamities (large discharge from bad production, maintenance or failure in production process or ETP

Question 13

WWTP - biological treatment

Answer 13

biological treatment: treatment making use of bacteria
* bacteria form sludge
* bacteria growth control important

types:
- anaerobic: withouth oxygen -> production of biogas
- araerobic: with oxygen (areated)

Question 14

WWTP - sludge/water treatment

Answer 14

sludge/water seperation: bacteria are removed from water
* part of sludge is recycle back to biological treatment tank
* part of sludge is discharged

methods: settlers, membranes, inclined screens, lamella separators, flotation

Question 15

Conditions which influnce WWTP design (6)

Answer 15

1.effluent requirements and future expectations
- e.g. discharge requirements became more stringent

2.characteristics of wastewater and future expectations
- note: sometimes production process uses biocides
- e.g. production process was changed but WWTP not yet

3.footprint: available space (compact building often more expensive)
4.possibilities sludge processing (buy and discharge sludge is expensive)
5.robustness
- redudancy (extra optional elements for e.g. failure)
- simplicity
- automation
- control

6.budget
- OPEX
- CAPEX

Question 16

Ideal vs Realistic WWTP

Answer 16

Ideal WWTP has:
- pack back time of 1-3 years
- no chemical dosing required
- produces no sludge
- does not need man-power
- meets discharge requirements 24/7

Realistic WWTP has:
- discharge of sludge or other raw materials
- costs money
- biologic processes are dynamic: there is no set equilibrium, biology pushes system to right
-> control is difficult, needs expensive professionals
-> discharge requirements are not always made

Question 17

Common problems WWTP

Answer 17

• no fixed equilibrium -> unpredictable
• unpredictable process requires high awareness and alertness
• often insufficient knowledge and experience of operator with biological treatment

Question 18

Methodology working with biological WWTP (5 elements)

Answer 18

1. get a feeling for it through looking (macroscopic and microscopic)
2. determine rate of change in treatment process (sludge retention time)
3. determine critical parameters and analyse them frequently (sampling)
4. consultation between production team and treatment team is very important
5. build knowledge by determining cause of disruptions