Wastewater EXAM Flashcards
Typical standards (BOD5, TSS, P, N)
BOD5: 150 to 40mg/L
TSS: 150 to 400 mg/L
P: 5 to 15 mg/L
N: 40 to 80 mg/L
UK standards
BOD5, TSS
UK standards
BOD5: 20 mg/L
TSS: 30 mg/L
DWF equation
DWF = LP + I + E
L: per capita consumption; P: population;
I: infiltration, E: industrial and commercial
effluent contribution.
treat 3DWF+ holding tanks for a further 3DWF;
excess of 6DWF overflows untreated to the
receiving water.
If discharge is to sensitive areas (e.g. beach),
then use 6DWF(treatment) + 6DWF(holding tanks)
DWF can be approximated by an equivalent
to 225L/head.day
If discharge to sensitive areas (DWF)
If discharge is to sensitive areas (e.g. beach), then use 6DWF(treatment) + 6DWF(holding tanks)
Peaking factor for domestic wastewater flowrates
Peak hourly flowrate = Average flowrate
* Peaking factor
(Use Chart)
Basis for BOD test
If sufficient oxygen is available total decomposition of the biodegradable organic waste through 3 distinct activities:
1. Portion of the waste is oxidised to produce energy
2. Some of the energy is used to produce new cells
3. When all organic matter is used up, the new cells consume their own cell tissue
Difference between in-line and off-line equalisation basin.
Inline : significant flow and BOD5 dampening (20% BOD5 reduction)
Offline : Less effective dampening but pumping costs are much less.
What happens when flow is too high in an equalisation basin?
Wash out microorganisms in secondary treatment
What happens if flow is too low in equalisation basin?
Lack of nutrients
reduction in microorganism population
WW flows into equalisation basin to …
maintain the flow rate out of the basin constant or between prescribed limits to protect the subsequent processes.
It reduced the effect of toxic shocks on the biological processes.
Example flow sheet for lowland river or reservoir
RPC FCA PAC
Reservoir
Pre ozone
Coag
Flocculation
Clarifier
ASG filter
Post-Ozone
Activated carbon
Chlorination
Example flow sheet for an upland catchment reservoir
RCR FFS RMC
Reservoir
Coag
Rapid mix
Flocculator
Flotation
Saturator
Rapid gravity filter
Manganese contractors
Contact tank
Example flowsheet for a groundwater
POA C CMP
Pump
Ozone Contactor
Activated Carbon
(Cl2 added)
Contact Tank
Membranes
(Phosphoric acid added)
What are reasons for screening?
Its objective is to remove large objects such as rags, paper, plastics, metals, and
the like. These objects, if not removed, may damage the pumping and sludge
removal equipment, and block valves, nozzles, channels, pipelines etc.
Consists of parallel bars, rods/wires, wire mesh, or perforated plate with circular or
rectangular shape
What are reasons for grit removal?
*To protect moving mechanical equipment and pumps from unnecessary
wear and abrasion
*To prevent clogging in pipes, heavy deposits in channels
*To prevent cementing effects on the bottom of sludge digesters and primary
sedimentation tanks
*To reduce accumulation of inert material in aeration basins and sludge
digesters which would result in loss of usable volume
Ways to handle screenings
- Traditionally screenings have been collected on a hopper, transferred to
a skip, and disposed of to landfill.
– Fine screens produce greater volumes of screenings
– Untreated screenings contain large amounts of water
– Screenings are classified as hazardous waste and landfill disposal is
therefore expensive
– A significant proportion of screenings are non-biodegradable
Some form of post screening treatment may therefore be desirable.
- This is known collectively as screenings handling.
- Screenings handling may be divided into a number of possible sub processes:
– Washing, to remove hazardous material for separate treatment
– Maceration, to facilitate compaction
– Dewatering, to reduce volume and weight
– Compaction, to reduce volume
Advantages of aerated grit chamber
- Can be used for chemical addition, mixing, and flocculation ahead of primary
treatment - Fresh wastewater, thus reduce odours and remove BOD5
- Minimal headloss
- Grease removal by providing a skimming device
- Remove some organic matter by air supply
- Remove any desired size by controlling the air supply
Disadvantages to Aerated Grit chamber
- Volatile organic compound (VOC) and odour emission
- Due to a health risk, covers may be required or nonaerated type grit chambers
may be used. - High maintenance
- High energy costs
What are the 4 types of settling?
- Discrete particle (free settling)
- Flocculent settling
- Hindered or zone settling
- Compression settling
Impact of excessive nutrients
- Algal blooms
- Low levels of dissolved oxygen
- kills fish
- turbidity
- plant and animal population shift
Describe the attached growth processes:
- The microorganisms are attached to a surface
over which they grow (ex. rock, plastic, wood) - Recycling of settled biomass is not required.
Describe the suspended growth processes
- The microorganisms are suspended in the WW
- Recycling of settled biomass is required.
Trickling filter problems
- ponding
- weed growth
- flies
plastic media
Higher specific organic loading than mineral media
More susceptible to cold weather and drying out
good distribution and recirculation - wetting
Advantages of constructed wet lands
٭ Less expensive to build than other treatment options
٭ Simple construction, operation and maintenance
٭ Low operation and maintenance costs
٭ High ability to tolerate fluctuations in flow and inlet quality
٭ High process stability (buffering effect)
٭ Sludge produced only by the primary treatment stage
٭ High pathogen removal – good water reuse and recycling options
٭ Optimal aesthetic appearance
Definition of constructed wet land
Constructed wetlands can be defined as engineered water saturated areas in
which the natural removal processes for the water pollutants are reproduced
and enhanced in order to optimise the purification performances.
Applications of CWL
- Secondary treatment
- Tertiary treatment
- Landfill leachate
- Diffuse pollution treatment
- Sludge de watering and mineralisation
What is flotation
Air bubbles: Air bubbles are added or caused to form
in one of the following methods:
*Injection of air while the liquid is under pressure, followed
by release of the pressure (dissolved-air flotation DAF)
*Aeration at atmospheric pressure (air flotation)
*Saturation with air at atmospheric pressure, followed by
application of a vacuum to the liquid (vacuum flotation)
*Decarbonation and electrolysis
What is DAF
Dissolved air flotation
Application of DAF
- Colored Waters - resemble iced tea
- High TOC and natural organic matter (NOM) content
- Low to medium turbidity
- High algae content causing filter problems
- High Fe and Mn seasonal content
What is coagulation/Flocculation?
*The purpose of coagulation/flocculation is to produce particles, by means of
aggregation, that can be removed by inexpensive particle-separation procedures such
as gravity sedimentation and filtration.
*Aggregation of particulate material is a two-step process: coagulation followed by
flocculation.
What is the significance of the time of year in the activated sludge process?
Temperature hence reaction rates
What is the significance of the residence time of the substrate in the activated sludge process?
Time for reactions
What is the significance of the Materials of construction in the activated sludge process?
No significance
What is the significance of the sludge age in the activated sludge process?
So that the biomass is in the log growth phase
What is the significance of the mixed liquor suspended growth solids conc in the activated sludge process?
Sufficient for load Eg BOD flow
What is the significance of the pH in the activated sludge process?
Extreme pH promotes ASP bacteria growth
What is the significance of the aluminium conc in the activated sludge process?
Not significant
What is the significance of the Low BOD/COD conc in the activated sludge process?
Low biodegradability or toxic inhibition
What are two of the main factors for activated sludge process
Oxygen: required for oxidation of organic matter and NH3
Nutrients N and P are required for bacterial growth
Which impeller should be used for rapid mixing?
Radial
What is the electrical motor efficiency
0.8
When designing an aerated grit chamber, what width/depth ratio should be used?
Between 1:1 and 1:5
When designing an aerated grit chamber, what air supply ratio should be used?
Between 4.6 L/ s . m and 12.4 L/ s. m
When designing an aerated grit chamber, what ratio should be used to estimate the quantity of grit at peak flow?
between 3.46 * 10^-6 and 200 * 10^-6
What are methods of primary treatment
Bar screens
Grit removal
Settling Tank
Methods of secondary treatment
Aeration
Sedimentation
Trickling filter
Methods of sludge treatment
Dewatering
Anaerobic digestion
Describe free discrete settling
*Occurs in settling of a suspension of low solids
concentration <500 mg/L.
- Particles settle as individual entities,
*No significant interaction with neighbouring particles
- e.g. grit and sand particles from wastewater
No interaction between particles as they settle
Describe Flocculent settling
*Occurs in dilute suspension of particles that coalesce, or
flocculate,
*Particles coalesce or flocculate during settling so that their mass
increases and settling velocity increases as they fall.
*Occurs in primary and secondary sedimentation tank treatments.
*Also occurs during removal of chemical flocs in settling tanks.
Interaction between particles as they settle
Describe hindered settling
*Occurs in suspensions of intermediate concentration (500 to
2000 mg/L)
*Interparticle forces hinder the settling of neighbouring particles.
*The particles tend to remain in fixed positions with respect to
each other, and the mass of particles settles as a unit.
*A distinct “solid-liquid interface” develops at the top of the settling
mass with clarified liquid above and particle mass below.
*Occurs in secondary settling facilities used in conjunction with biological treatment facilities
Describe Compression settling
*Occurs in solutions containing sufficiently concentrated
particles that they form a structure (gel-like).
*Settling occurs by compression of the structure (water
gets squeezed out) by the weight of additional solids
settling from above.
*Usually occurs in the lower layers of a deep sludge mass
(e.g. bottom of a deep secondary settling tanks; sludge thickening
facilities).
Why is disinfection important?
- Pathogens cause disease
- Pathogens are common in the environment,
especially in human and animal faeces - Acute effects of digestive upsets
- Disease can lead to dehydration and death
- A major factor in public health
