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Waste Water Treatment (Primary Treatment (Tube and Plate Settlers…

- - - - Inlet Configuration
        
        Centre Feed
        
        Most common
        
        Wastewater is introduced to the center of the tank through a center riser into a circular feed well
        
        Diameter of feed well 10-15% tank diameter
        
        Depth of feed well 1-2.5m
        
        Velocity through the orifices of the well 0.075-0.15 m/s
        
        Peripheral Feed
      - Outlet Configuration
        
        Centre Feed
        
        Double Weir Troughs
        
        Cantilevered
        
        Suspended
        
        Submerged Orificies
        
        V-notch Weirs
        
        Clarified water overflows into an effluent trough
        
        Peripheral Feed
      - Diameter
        
        Controlled by overflow rate
        
        Range from 3-90m diameter (12-45m typical)
      - Depth
        
        Should be deep enough to accommodate sludge removal equipment, store settled solids (when desludging intermittently), avoid scour, and avoid carryover of solids in the effluent
        
        Range from 3-5m
      - Flow splitting
        
        Flow is split between multiple sedimentation units so that hydraulic load and the solids load are in proportion to the design limits of the tanks
        
        A flow splitter (weirs and notches) is used to split the flow equally
      - Sludge Removal
        
        Travelling bridge
        
        Scrapper plows the settled sludge progressively into a centrally located sludge hopper
        
        Straight multiblade scrapper
        
        Most common
      - Scum removal
        
        Rotating skimmer arm and wiper attached to the scraper mechanism
        
        Scum is pushed over the beach plate by the wipers, and then into the scum outlet piping
    - - Inlet Configuration
        
        Stilling Wall
        
        Prevents incoming water jetting into the tank which reduces retention time
        
        Channel or Flume
        
        Inlet Structures are designed to distribute water over the entire cross-section
        
        When rectangular tanks are constructed side-by-side, water is distributed by a single channel that runs perpendicular to the flow through the tank
      - Sludge Removal
        
        Chain and flight
        
        Combined scum and sludge removal
        
        Travelling Bridge
        
        Travels back and forth along the tank
      - Common in large-scale water treatment plants
      - Length 30-60m
      - Width 3-24m
      - Depth 4m typical
        
        Too shallow - no room for sludge collecters or settlement
        
        Too deep - inefficient
      - Floor slopes towards the sludge hopper
      - If numerous tanks are required, rectangular tanks with common walls are constructed
      - Outlet Configuration
        
        Water leaving the sedimentation basin should be collected uniformly across the width of the basin
        
        Consists of launders running parallel to the length of the tank
        
        V-notch weirs are attached to the launders
      - Scum removal
        
        Returning chain-and-flight scraper at the surface of the tank (for chain-and-flight sludge removal system
        
        A surface scraper and a scum pipe (for travelling bridge sludge removal system)
  - - - Makes primary effluent nutrient deficient
      - May help achieve phosphorus consent
      - Hamper secondary biological treatment which requires nutrients
  - - - 5-45% for suspended solids
      - 5-50% for BOD5
- - - - Aerobic Ponds
        
        Shallow ponds less than 1m depth - DO decreases with depth
        
        Light penetrates to the bottom maintaining algal photosynthesis during daytime and UV disinfection
        
        DO is maintained by algal photosynthesis
        
        Surface re-aeration by wind mixing
        
        Organic water degradation mainly through the action of aerobic bacteria
      - Anaerobic Ponds
        
        Deep ponds (2-5m) that receive high organic loadings - has a smaller footprint because don't need to worry about DO
        
        O2 consumption rate much higher than O2 production rates which creates anaerobic conditions
        
        Anaerobic bacteria break down the organic matter, releasing methane and carbon dioxide, which can collected as a fuel (biogas). Sludge is deposited on the bottom.
        
        Typical BOD reduction efficiency is of the order of 50-70%
      - Facultative Ponds
        
        1-2m deep
        
        BOD reduction by aerobic bacteria at the pond surface and by anaerobic bacteria at the bottom
        
        The soluble BOD is aerobically stabilised and suspended and colloidal BOD tend to settle and is decomposed by anaerobic bacteria
        
        BOD reduction 70-85%
      - Aerated Ponds
        
        Mechanical diffused aerators provide oxygen and allow for a greater proportion of organic material to remain suspended
        
        Suspended solids and organic colloidal materials (BOD) settles to the bottom to form a sludge layer that undergoes anaerobic decomposition
    - - Conventional Activated Sludge
        
        Aerator
        
        Long, rectangular basin
        
        Aerated for 6-8 hours
        
        Keeps microbes in suspension
        
        Settling Tank
        
        Liquid-solids separation, produces sludge
        
        Recycle System
        
        A portion of the sludge is wasted everyday to maintain the proper amount of microbes to efficiently oxidise the biodegradable organics
      - Step aeration activated Sludge
      - Contact Stabilisation
      - Complete Mix Activated Sludge
      - Extended Aeration
      - Oxidation Ditch
      - Nutrient Removal
      - Micro-organisms are mixed thoroughly with the organic compounds present in wastewater
      - As the micro-organisms grow and are mixed by the agitation of air, the individual organisms flocculate to form an active mass of microbes called activated sludge
      - Growth Kinetics
        
        Food to micro-organism ratio F/M describes the degree of starvation of the biomass
        
        F/M > 1 log (max) rate of growth of biomass, large sludge volume generated but smaller reactor volume required
        
        Endogenous growth: F/M <<1, amount of food availability limits the biomass growth rate , smaller sludge generation but larger reactor volume required
      - BOD removal is approximately 85%
      - 80-99% removal of bacteria (sunlight, temperature, aggressive micro-organisms, predation by ciliated protozoans, competition from other bacteria, adsorption to sludge solids)
      - 90-99% removal of viruses (mostly through solids settling, but also bacterial antiviral products and predation)
  - - - Trickling Filter
        
        Advantages
        
        Resistant to shock loads
        
        Low biosolids load
        
        Low power requirements
        
        Simple operation
        
        Disadvantages
        
        High suspended solids in effluent (sloughing of biofilm)
        
        Very little operational control
        
        Relatively low BOD removal (85%)
        
        Types
        
        Standard or low rate trickling filter
        
        Single stage rock media units
        
        Loading rate: 1-4m3 wastewater/m2 area filter-day
        
        Large area required
        
        High rate trickling filter
        
        Single-stage or two stage rock media units
        
        Loading rate: 10-40m3 wastewater/m2 area filter-day
        
        Re-circulation ratio 1:3
        
        Plastic media
        
        Loading rate: 25-50m3 wastewater/m2 area filter-day
        
        Deep bed 3-12m
        
        Re-circulation ratio 1:6
      - Rotating Biological Contactor
        
        Consists of closely spaced discs (lightweight, 3-3.5m diameter), mounted on a horizontal shaft
        
        The discs are rotated while about 1/2 of their surface area is immersed in wastewater
        
        Microbes in wastewater adhere to the rotating surfaces and form a biofilm
        
        Oxygen is supplied to the attached biofilm by the air while the air is out of the liquid
        
        As the biofilm passes through the wastewater they absorb organic compounds for oxidation
        
        Discs
        
        Provide media for the buildup of attached microbial growth
        
        Bring the biofilm into contact with the wastewater
        
        Aerate the wastewater and the suspended microbial growth in the reservoir
        
        Simple operation, short retention time due to large active surface, low power, minimal odour and low sludge production
        
        Performance Factors
        
        Number of stages
        
        Organic loading rate
        
        Hydraulic loading rate
        
        Recirculation rate
        
        Submergence
        
        Rotational speeds
        
        Oxygen levels
- - - - Ferric Chloride
      - Aluminium sulphate
      - Lime
        
        Lime use is reducing due to increase in sludge production, pH control requirements, and operation and maintenance problems with storage, handling and feeding of lime
    - - Well-established technology with readily available chemicals
      - Completely enclosed systems, self-operating and low maintenance
    - - Competing reactions, varying levels of alkalinity and
        other factors make calculation of proper chemical
        dosages impossible.
      - Overdosing can diminish the effectiveness of the
        treatment.
      - Requires working with corrosive chemicals, increasing operator safety concens
      - Addition of treatment chemicals, especially lime may increase the volume of waste sludge by up to 50%
      - Large amounts of chemicals may need to be transported to the (potentially remote) treatment location
      - Polymers can be expensive
  - - - Pore or grain size
      - Shape
      - Medium gradation
      - Specific gravity
      - Depth
      - Hardness
      - Solubility of materials used
    - - Turbidity buildup - controlled by adding chemicals or polymers
      - Mudball (agglomerations of biological floc, dirt and the filter media) formation - reduces effectiveness of filtration and backwashing
      - Buildup of emulsified grease - increases head loss
      - Development of cracks and contraction of filter bed
      - Loss of filtering medium during backwash
- - - - Bar Racks/ Coarse Screens
      - Fine Screens
      - Very Fine Screens
      - Micro-screens
      - Trash Racks
  - - - Aerated
        
        Wastewater flows through the chamber in a spiral pattern
        
        Air is introduced along one side of the chamber
        
        Lighter organic particles remain suspended and are carried out of the tank
        
        Heavier particles settle to the bottom
        
        Quantity of air is adjusted to provide the roll
        
        Increases path length - increases retention time
        
        Uses diffusers to promote spiral path
        
        Uses centrifugal forces
      - Vortex Flow
        
        Wastewater enters the chamber tangentially
        
        Grit settles by gravity into the bottom of the tank
        
        Effluent exits at the top of the tank
        
        Outlet is wider than the inlet causing a lower exit velocity than the influent velocity. This prevents the grit being drawn into the effluent flow
        
        Airlift pump removes the grit from the grit sump
        
        Tangential inlet promotes circular flow.
      - Horizontal Flow
        
        Velocity controlled channel
        
        Dimensions of the channel - shape
        
        Special influent distribution gates
        
        Special weir sections at the influent end
        
        Designed to allow sufficient retention time for particles to settle
  - - - Macerators
        
        Slow-speed grinders that use two sets of counter-rotating blade assemblies
      - Grinders
        
        Pulverise the solids by a high-speed rotating assembly
      - Comminutors
        
        Uses a stationary horizontal screen to intercept the solids in the flow and a rotating or oscillating cutting bar to shear the material
  - - - Flow to full of 3DWF
      - Store >3DWF - 6DWF for 2 hours in storm tanks
      - Spill when full
      - Upstream pumps automatically spill >6DWF