Advanced Treatment Flashcards
Required for proper operation of “rapid sand” filters.
Filter aid polymer
“Rapid sand” gravity filters.
- Mixed media
anthracite
medium silica sand
fine garnet sand - Dual media
anthracite
silica sand - Deep bed mono-media
anthracite or medium silica sand
Media sorting during filter backwash is due to this characteristic.
As the filter expands and is fluidized during backwash, the specific gravity of the media allows the formation of distinct layers.
TOP anthracite 1.4 spec gravity
MIDDLE silica sand 2.6 spec gravity
BOTTOM garnet 4.0 spec gravity
Indications of adequate filter backwash.
*20- 25% expansion of media
*media become fluidized
Typical backwash rate for rapid sand filters
□ 15-20 gpm/sq ft filter surface or
□ 2-2.7 ft/min backwash rise rate
Continuous backwash occurs in these filters.
□ Moving bridge filter
□ Continuous backwash upflow “clarifier
Problems associated with inadequate filter backwash.
□ Mudballs
□ Debris on filter surface
□ Filter surface cracking
Filter status is determined by continuously monitoring these.
□ Effluent turbidity
□ Headloss
This initiates filter backwash.
□ Turbidity breakthrough or
□ Filter reaches “terminal” headloss (6-8 ft) or
□ Filter run time has been reached
Bacteria associated with the nitrification process.
- Nitrosomonas sp. (Converts ammonia to nitrite)
- Nitrobacter sp. (Converts nitrite to nitrate)
Characteristics of the nitrification process:
Overall reaction
Oxygen requirements
Aerator requirements
Optimum pH range ■ J
Alkalinity and pH ]relationship
Solids retention time required
Source of nitrifying bacteria
Characteristics of the nitrification process:
* NH, — NO, (NH,+202 -HNO,+H,0)
* Nitrification is an acrobic process: 4.5 lb 02 consumed per lb of ammonia nitrified
* 40- 60% additional aeration is needed to support nitrification
* 7.5-8.5
* Nitrification consumes alkalinity (natural buffer in water) and can drive the pH below 6.0
* SRT necessary for nitrification isdependant on the aeration basin temperature.
~5 days if the temp is 16 °C
~12 days if temp is 10 °C
* A population of nitrifying bacteria must be “grown” in the treatment plant (aerator MLSS, TF, RBC, etc)
Nitrite “lock”
- Nitrification stuck in the intermediate step allowing nitrite to accumulate
- Low pH is typically the cause of incomplete nitrification
- Nitrite in the effluent interferes with disinfection by reacting with and destroying chlorine residual
Characteristics of denitrification:
Overall reaction
Conditions required
Byproducts
- Nitrate or nitrite is converted to nitrogen gas
- Respiring (food eating) bacteria and anoxic (oxygen depleted) conditions. Adding readily available food like
sucrose or methanol can stimulate denitrification. - In addition to N2 gas, alkalinity is produced and the pH is raised
- Respiring (food eating) bacteria and anoxic (oxygen depleted) conditions. Adding readily available food like
Treatment processes that can accomplish nitrogen removal by BNR.
- Treatment processes that promote nitrification and denitrification of wastewater
- Activated sludge aeration basin partitioned to provide aerobic nitrification zone and anoxic denitrification zone
- Extended aeration oxidation ditch operated to produce alternating aerobic and anoxic zones
- Sequencing batch reactor (SBR) operated to provide time periods of aerobic and anoxic treatment
PAOs
phosphate accumulating organisms
Most treatment plants achieve phosphorus removal by this method.
Chemical precipitation using alum, ferric sulfate or lime
Summary of phosphorus removal by BNR
Exposing MLSS to a continuous cycle of anaerobic and aerobic conditions stimulates the growth of PAOs and the “luxury” uptake of 80 - 90% of the soluble phosphate in wastewater.
Process overviewof phosphorus removal by BNR
- Influent wastewater and RAS are fed to an anaerobic phase basin:
PAOs multiply and accumulate in MLSS over time
Under anaerobic conditions PAOs actually release phosphorus into the MLSS\ - MLSS then enters the aerobic phase (aeration basin):
PAOs take up, remove 80 - 90% of soluble phosphate in the MLSS - After settling in the clarifier, the settled sludge is returned to the anaerobic phase:
Settled sludge must not beallowed to become anoxic in the clarifier - phosphate is released back into the
wastewater - Wasting of the high-phosphate activated sludge provides actual removal of phosphorus
Why N and P are removed from wastewater.
- Nitrogen and phosphorus are plant nutrients - they stimulate algae blooms which choke waterways.
- Algae blooms contribute to “aging,” the long-term decline in lakes called eutrophication.
- Algae bloom die-off produces high oxygen demand and low D.O. in receiving streams.
- Ammonia is toxic to some fish and exerts oxygen demand on the receiving stream
How constructed wetlands accomplish BNR.
As wastewater flows through beds planted with bulrushes and cattails, Nitrogen and phosphorus are taken up by wetlands plants. The N and P nutrients become part of the plant material and must be periodically harvested from the beds.
Percent of filtered water needed to provide filter backwash
About 3% is normal
Odor control methods.
- Wet air scrubbing
Foul air is piped to one or more scrubbers where chemical sprays (NaOH and hypochlorite) strip out and
neutralize odor compounds - Biofiltration
Foul air flows upward through moist media (compost, mulch or peat).
Microorganisms grow on moist media and utilize odor compounds for food - Activated carbon adsorption
Foul air is piped through vessels filled with activated carbon. Odor molecules are adsorbed into highly
porous carbon surfaces.
Odor compounds removed by odor control methods
- Hydrogen sulfide
- Organic sulfides like mercaptan and dimethyl sulfide
- Ammonia
