Wastewater Flashcards
What are three main harmful effects of untreated wastewater is discharged to inland receiving waters?
- High content of C, N and P which can cause oxygen depletion in water bodies
- High content of C, N and P which can cause eutrophication in lakes
- Heavy metals which have a toxic effect on aquatic life and humans.
Define BOD and what is it typically used for?
BOD is Biochemical oxygen demand.
This is the quantity of oxygen required to stabilize the carbonaceous material through biological processes.
Typically used to determine organic pollution strength.
Define COD and what is it typically used for?
COD is chemical oxygen demand.
This is the quantity of oxygen required to stabilize the carbonaceous material through chemical processes (e.g. oxidation).
What is usually involved in primary treatment, including what it achieves?
Process which removes solids, including grit settling and scum floatation.
Usually though grit screening , grit removal, primary sedimentation and/or DAF plant.
This lowers the organic load therefore there is smaller process volumes, lower requirements and less secondary sludge during the next phases.
-Protection of the wastewater transport devices, subsequent treatment units, and receiving bodies.
What is usually involved in secondary treatment, including what it achieves?
Process which is mainly for the removal of organic matter, colloidal and dissolved contaminants through a biological process.
It is used to accelerate the decomposition of material that would naturally occur.
Usually through activated sludge reactors , and aerated filters.
What is usually involved in tertiary treatment, including what it achieves?
physiochemical process to further reduce organics, turbidity, nitrogen, phosphorous, metals and pathogens. Specifically TSS and inorganic nutrients
Most processes involve treatment such as disinfection, filtration, activated carbon or reverse osmosis.
It also provides additional protection for wildfire after discharging into rivers or lakes.
Reduction of disease causing organisms is through ponds, explain how and what modifications can be made to improve the reduction?
Oxidation ponds are used for the reduction of disease causing organisms in wastewater.
Sludge is produced at the bottom and is slowly decomposed through anaerobic microorganisms. (producing CO2 and CH4 gas)
The dissolved organic matter will remain suspended at the top in a zone called the aerobic zone. This is where the process requires oxygen which is supplied due to photosynthesis undertaken by algae in the pond.
Improvements:
- Increase detention time (more time)
- Shallow depth (more light penetration and photosynthesis)
- Large surface area (increases pH and therefore has greater photosynthesis)
List the factors which must be considered in the design of trickling filters?
(3)
- Organic loading Rate
- Wetting rate
- Flushing Intensity
Often the packing material is also important.
What are the 6 advantages of a membrane bioreactor compared to a secondary clarification?
- Very high effluent quality
- ensures bacteria and virus free effluent
- Colloids, solids, turbidity, and BOD removal
- Shorter reactor HRT (Hydraulic Retention Time)
- Less sludge retention
- Less space required.
Explain Nitrification and Denitrification
Nitrification:
Ammonia to nitrites to nitrates
This process required oxygen (aerobic) and it uses alkalinity.
Denitrification:
Nitrate to nitrogen gas
This requires carbon and performed under anoxic conditions
Alkalinity is produced in this step.
What are ammonia oxidising bacteria? Name three things that affect its growth rate? How to test?
Ammonia oxidising bacteria are called nitrifying bacteria.
The bacteria transform ammonia into nitrites and then the nitrites into nitrates.
These bacteria are chemoautotrophs where carbon dioxide is the main source of carbon and the energy is obtained through oxidation of an inorganic substrate.
Three things:
- Requires oxygen to grow
- Alkalinity is required for ammonia oxidizing (pH should be around 7.5 for optimal growth.
- Temp (25 degC)
Can be measured by the NBOD where the BOD test is completed and then continued for 6-10 days.
What are the limitations of the BOD Test? (4)
- Low levels of BOD can be found in the case that the microorganisms responsible for the decomposition are not adapted to the waste
- Heavy metals and other toxic substances can kill or inhabit the microorganisms
- The test takes five days, not useful for operational control
- The inhibition of the organisms responsible for the oxidation of ammonia is necessary
What are the limitations of the COD Test? (5)
- Theoretical > measured
- No info on biodegradability
- Both the biodegradable and the inert fraction are oxidized.
- No information about consumption rate
- Can oxidize inorganic material which disrupts the results
List three types of screening methods:
Band screen:
- Large Surface area, 90% removal
- Capture up to 1mm
- good flow
- protects membranes
- 90deg flow inlet flow balls up
Bar Screens:
- 12-25 mm gaps (poor screen capture)
- suffers wear
- big obstructions not captured
- can be mechanically raked.
Horizontal rotating drum screens:
- 1mm to 0.5mm
- used for larger municipal flows
- need near continuous high pressure spray for cleaning of wedge wire drum and periodic hot water blasting for fat.
- good screen capture due to 90deg turn
- Head loss occurs and so pumping is required.
What are three grit chambers and their properties?
Spiral Flow/ Aerated grit chamber
- Air introduced on the side to give spiral flow pattern
- Grit is given velocity so falls to bottom
- consistent removal for a range of flows
- low organinc content in grit
- high power requirement
- aeration can drive off volatile substances
- needs ventilation
- can be inefficent
Detritor
- Settling basin with short retention time
- Large chamber with rakes at the bottom,
- constant velocity.
- low head loss
- maintained to get uniform flow,
- high organic content in the grit
- loss of grit disturbed by rake
Vortex grit removal plants:
- spiral type flow
- wider range of velocity
- small retention time
- high percentage of fine grit removed
- no submerged bearings or equipment
- low footprint
- head loss
- paddles can collect rags, - - - FOG accumulation
- high/deep structure
Why would you use a DAF plant?
Used if the particulate material has a low density, flotation may be more suitable then sedimentation
Works well with FOG
What is the attached growth process?
The metabolic conversion which takes place inside the biofilm.
- substrate transport occurs by diffusion processes, initially through the liquid film in the liquid/biofilm interface and later through the biofilm.
- The products of the oxidation and reduction reactions are transported in the opposite direction, to the exterior of the biofilm.
- The substrate donor and the electron acceptors must penetrate the biofilm for the biochemical reaction to take place.
The limitation is the mass transfer.
Name three types of reactors/tanks/filters that will aid the attached growth process?
Trickling Filter
A trickling filter consists of a tank filled with a packing medium made of material which has highly permeability. The wastewater percolates in the direction, as drops or by jets (trickles over the membrane), in a downwards direction which allows bacterial growth on the surface of the packing material. The wastewater passes over the biofilm, promoting contact between the microorganisms and the organic matter.
- Aerobic system
- can cover for odour
Moving Bed Biofilm Reactor
Media is free to move around the reactor (suspended), advantage is that you can generate different environmental conditions for the biomass. Can have control on what the biomass is doing. Can design for both aerobic or anoxic. Need to ensure that the density isn’t to high as don’t want them to sink, what them to be floating. Can get much better treatment from them, can get very low BOD.
Submerged Aerated filter
A tank is filled with porous material through which wastewater and air flow through. The porous medium is maintained under total immersion by the hydraulic flow, which has three phases:
- Solid ( support medium)
- Liquid (movement throughout)
- Gas (aeration throughout)
This system can be submerged aerated biofilters or just filters (use beds which do not retain any suspended biomass so you need a secondary settling tank).
