last lecture pt 2 Flashcards
(24 cards)
- For the transfer of gas molecules from the gas phase to the liquid phase, slightly soluble gases encounter the primary resistance from the liquid film
- Very soluble gases encounter the primary
resistance to transfer from the gaseous film
Aeration
a physical mass transport across gas film and liquid film
Two-film theory
Aeration (cont)
- Aim of the aerator - to increase O₂ transfer from liquid film to the bulk liquid at a rate sufficient to meet the O2 demands of metabolism
- A major energy consuming process
- Ka is the overall oxygen mass transfer coefficient. It is a function of the equipment, tank geometry and wastewater characteristics
- Function of O₂ in activated sludge is a two stage process
- Aeration provides the DO (electron acceptor) for aerobic metabolism
- DO of 0.5 - 2 mg/L is necessary for aerobic condition
- Aeration must balance the oxygen uptake by the microorganisms
Separation of the treated wastewater
from the solids
- Occurs after the _____
- A physical process of settling generally in a separate tank
- In some processes this removal of solids can
also occur in the same tank but separated in
time
biological or transformation
process
- A physical separation process to settle the
solids (microorganisms, particulate) from the
clarified effluent - Thicken sludge is returned to the reactor tank
- Design for 3 x ADWF or PWWF
- Class III and IV settlings; depth is relevant
- Weir overflow rate < 250 m³/m.d
Final Sedimentation Tank
Final Sedimentation Tank (cont)
- Involves 2 important functions
1. ____ - Hydraulic loading must not exceed the settling velocity of the slowest settling particle
- V5 = Q/A ie. 30 - 40 m³/m².d for activated sludge
- HRT ≈1.5 to 2h
- Thickening capacity is based on the ____ Theory
- A concept of maximum quantity of solids that can be handled by a settling tank at a given underflow removal rate without affecting performance. It involves the solids loading rate
- GL = Q(1 + R)X/(1000A) kg/m².d
- Measuring the settleability of sludge, SVI
Clarification; solids Flux theory
- A criterion for measuring the settleability of sludge
- It is related to the recycling of activated sludge
- SVI is defined as the settled volume of sludge (mL/L) in 30 minutes per unit MLSS (mg/L)
- SVI of 50 - 100 mL/g indicate good dense sludge
- SVI > 150 mL/g are light, poorly compacting
(bulking sludge)
Sludge Volume Index (SVI)
Factors affecting SVI
- ____ composition; relationship between
zoogloeal and filamentous growth are
dependent on industrial wastes, carbohydrates
etc - Degree of ___ mixing in reactor tank;
plug flow is less prone to bulking - ___ conditions and ___ systems result in low SVI
Sewage composition
longitudinal mixing
Anoxic conditions; nitrifying systems
- Represents the underflow of the final
clarifier to the reactor tank - An essential feature of the activated sludge
system to maintain the desired MLSS - its value varies from 20 to 150% of ADWF
Return Activated Sludge (RAS)
Types of Activated Sludge Systems
____
- Operates at F/M ratios of 0.2 to 0.5
- Design to remove BOD and may also nitrify
- Plug flow, limited longitudinal mixing, spiral flow along tank through diffusers
- Reactor tanks are long, narrow up to 150 m length; W:D = 1:1 to 2.2:1; D = 3 to 5 m; W = 6 to 12 m
- Limited resistance to shock and toxic loads
Conventional activated sludge
Types of Activated Sludge Systems
______
* Systems operate with low organic loadings (F/M); high and high HRT
* Process minimises sludge handling, consequently have no primary sedimentation tanks
* Increased endogenous respiration results in less sludge, but increase O₂ demand
* Exhibits completely mixed; hence more stable to fluctuations in flow and loading (organic); requires less stringent recycle
* Examples are continuous oxidation ditches eg. carousels
Continuous extended aeration process
- Biological oxidation and final clarification occur in the same tank: functions are only separated in time
- Primary treatment is not necessary
- Treated and clarified water is decanted intermittently but raw sewage is fed continuously
- Sludge is wasted during the aeration cycle to
maintain a constant MLSS of 3500 - 5000 mg/L
Intermittent Decanting Extended Aeration (IDEA)
- An example of the intermittent decanting extended aeration process serving 500 to 2000 ep
- 4 hours operating cycle for normal operation; 3 phases per cycle controlled by an automatic timing device
- Aeration = 2.5 h
- Settling = 1.0 h
- Effluent decanting = 0.5 h
Pasveer Oxidation Ditch
Major advantages of the IDEA process
- Cheaper than continuous activated sludge systems
- Easily modified to remove nutrients
- Easy operation and minimum attendance
Disadvantages of the IDEA process
* High operating energy requirements
* Sludges are often difficult to settle
* Not suitable for large flows
- Secondary treatment will remove up to 98% of microorganisms and 105 - 107/100 mL of coliform remains
- Chlorine remains the common disinfection agent
- A contact time of 20 - 30 minutes is required
- Much debate continues on the use of chlorine
- Other environmentally friendly methods are
preferred such as: UVL, ozone, membrane filtration, artificial wetlands
Disinfection
- The major components of nutrients in wastewater are nitrates and phosphates.
- They contribute to the eutrophication of receiving water.
- Total nitrogen may be about 35 mg/L and total phosphorus 8 mg/L after secondary treatment
- Raw sewage composition of C:TN:TP≈ 100:25:6
- Normal plant growth only need C:TN:TP of
100:15:1
Nutrient Removal
In the nitrogen cycle, organic and ammonium
nitrogen are converted first to nitrite and then to nitrate
Sources:
Organic nitrogen (40%)
Ammonium-nitrogen (NH+-N 60%)
Nitrite-nitrogen (NO2-N)
Nitrate-nitrogen (NO-N)
Nitrification
Nitrification (cont)
- ____ in wastewater is toxic to fish; it has a high O2 demand; it increases Cl₂ demand during disinfection
- Primary treatment removes < 20% influent nitrogen
- Secondary treatment removes about 30% cumulative
- Limit for ammonium-N in treated effluent < 2 mg/L
- Nitrification is a 2-stage process by different types of aerobic autotrophic bacteria
- Operating pH ____
- Nitrification reduces alkalinity (7.1 g of alkalinity as CaCO₂ is exhausted by 1 g NH4+-N) nitrified)
- Nitrification is adversely affected by F/M > 0.4 – 0.6
- Minimum DO 1.5 mg/L is required
Ammonia; 6.5-8
-
Conversion of nitrates (derived from nitrification) to
nitrogen gas - ____ is a type of respiration carried out by facultative heterotrophs; a process known as ___ as NO3 is the terminal electron acceptor
- Alkalinity is increase but by about half the amount removed by nitrification
Denitrification; anoxic
Denitrification (cont)
- DO inhibits denitrification
- A carbon source must be available (external or recycled endogenous carbon)
- Some BOD is removed but more slowly than aerobic respiration
- Denitrification can be induced in the anoxic part of fixed growth systems by making the filter bed deeper (2.5-3 m) but use of activated sludge is the normal process
- In conventional activated sludge, the anoxic zone within the reactor tank may be 30 – 40% of volume and precedes the aerobic zone
*In carousel systems, the establishment of sequential aerobic zones coupled with long HRT and high promote endogenous denitrification - Denitrification can be achieved in separate reactors using suitable organic source
Sources are from domestic wastewater, trade and agricultural wastes; usually present in 3 forms
- Orthophosphate (removed by chemical/or biological processes)
- Polyphosphate
- Organic phosphorus
- Polyphosphate and organic phosphorus are less easily removed until transformed to orthophosphate after secondary treatment
Phosphorus Removal
Phosphorus Removal (cont)
- About 10% of insoluble phosphorus can be removed by _____ settling
- ____ biological treatment removes a further 15-35% by assimilation during biomass growth, but a well designed BNR (biological nutrient removal) plant can remove up to 95% of P
- Almost all soluble phosphorus can be removed by ___ precipitation
primary settling
conventional biological treatment
chemical precipitation
Phosphorus Removal (cont)
- A more efficient process is ____
- One example is the modified University of Cape Town model (UCT) for biological nutrient removal
- Denitrifying plants can be modified by a fermentation zone at the head of the aeration tank
- Selective growth of bacteria (acinetobacter) absorbs the phosphorus
- Daily wasting of activated sludge removes the stored phosphorus
biological phosphorus removal
