Collection and Preliminary Treatment Flashcards
Specific Gravity of Grit, Sand and Gravel
2.6
Grit Removed from Wastewater Contains
Inorganic: Sand, Silt, Small Gravel, Shells
Organic: Rice, Corn, Coffee Grounds
Combined Sewers handle what kind of flow?
Domestic Sewage and Storm Water
- these are no longer made; storms can increase hydraulic loading to the point they are diverted into rivers and streams
What problems can inadequate grit removal from raw sewage cause?
- Wear on Pumps and Mechanical Equipment
- Affect downstream treatment process
effectiveness/efficiency :- build up in primary treatment/ aeration basins
- accumulate in digesters
Units that shred, cut or grind rags and large solids in raw wastewater and return shredded solids to the wastewater flow
- Comminutor
- Barminutor
- Muffin Monster
Most common flow measuring device installed where influent enters the WWTP
Parshall Flume
What occurs when the velocity in the grit channel is too slow?
Light organics settle with the grit. The organics then putrify. This can cause odors due to producing hydrogen sulfide.
Putrification is the decomposition of matter in the absence of oxygen
Pre-aeration of raw sewage can produce this benefit to later treatment.
Primary clarifier settling is significantly improved
Why pre-chlorination of raw sewage is not recommended or practiced
Pre-Chlorination of raw or untreated sewage can produce undesirable chlorinated organic compounds that are suspected carcinogens (cancer causing)
The minimum design velocity in a gravity sewer
Minimum design velocity is 2 feet per second (fps)
The optimum velocity of sewage flow in a grit channel.
Optimum velocity is 0.8 - 1.2 feet per second (fps)
Term for typical variation in domestic sewage flow to a treatment plant.
□ Diurnal Flow :
Sewage flow reflects the water use of a community
and typically has two “peaks” and two “valleys”
during each 24 hour period.
Common velocity control devices used at the end of grit channels.
□ Parshall flume
□ Proportional weir
Used to remove rags and large debris from wastewater.
□ Bar screen
□ Trash rack
Terms used to describe storm water and groundwater flow into a sanitary sewer during wet weather conditions
□ Inflow
□ Infiltration
Stormwater flow into a wastewater treatment plant can cause this
Hydraulic overload of the treatment plant
Sewer that flows as a result of the slope of the pipe.
Gravity sewer
Sewer discharge pipe from a sewage pumping station.
Force main
Methods used to determine the condition of a sanitary sewer.
□ TV camera inspection
□ Sewage flow monitoring at manholes
□ Visual inspection of manholes
Treatment plant problems that can occur if shredding equipment is not operating properly.
□ Plugging of pumps
□ Fouling of aeration equipment
□ Plugging of trickling Filter (distributor arm openings and
media)
□ Build-up in digesters
Typical operating mode for two identical pumps in a sewage pumping station.
The “lead” - “lag” status of the pumps should be switched automatically after each pumping cycle.
Component parts of a pre-cast manhole,
□ Base
□ Riser section(s)
□ Cone
Terminology used to describe sewer lines:
Lateral
Main
Trunk or Outfall
□ Lateral sewer receives no flow from other sewer lines
□ Main sewer receives sewage flow from laterals
□ Truck or outfall sewer carries the sewage flow from an entire area. The pipe that directly discharges into the treatment plant
Bottom of sewer pipe.
Sewer invert
Function of storm sewer catch basin
Settle out grit and sand from storm water flows
Sanitary sewer infiltration caused by this.
□ Faulty joints in laterals and mains
□ Leaky manholes
□ Broken sewer lines
□ Poorly constructed or damaged house laterals (usually most
important!)
Smoke testing a sanitary sewer can provide this information
□ Illegally connected house gutter drains to the sanitary sewer
□ Broken sewer lines
□ Faulty house lateral joints
A significant difference or discrepancy in this may indicate problems with pumping
station pumps or controls.
The run-time (hour meter reading) for each pump over a period of several days.
Wastewater confined for an extended period in a wet well, sewer main or force main can
result in this.
Sewage will become septic resulting in the formation of hydrogen sulfide gas.
These factors determine the carrying capacity of a gravity sewer.
Pipe diameter, pipe slope or grade, and pipe interior smoothness
A slope of 0.2% is this “rise over run.”
0.2 ft per 100 ft
These problems can occur if the flow velocity in a gravity sewer is too low.
□ Solids settle out of the sewage and can plug the sewer pipe
□ Sewage becomes septic, resulting in odors from HS gas
□ Grease and grit are not scoured, and can plug the sewer
These factors contribute to the amount of HS gas released from sewage in gravity sewers
□ Low pH
□ Anaerobic (septic) conditions
□ Warm sewage temperatures
□ Build-up of bacteria slime
□ Low, non-scouring, sewage flow velocity
Methods used to clean gravity sewers
□ Balling and pigging (not commonly used today)
□ High velocity jetting (most common method used today)
□ Flushing
Recommended frequency to rake a manually cleaned bar screen.
Clean often enough to provide free flow of sewage.
The volume of screenings typically removed from raw sewage per MG (million gallons).
0.5 - 5 cu. ft. per million gallons
Chemical precipitation is used to remove these from industrial wastewater.
Copper, zinc, and other toxic metals
These materials can interfere with the operation of a wastewater treatment plant.
□ Toxic metals such as copper, zinc, chromium, etc.
□ pH extremes (less than pH 5 or greater than pH 9)
□ Oil and grease
□ High BOD, COD or TSS
Acids most often used to neutralize wastewater with high pH.
□ Sulfuric acid HSO,
□ Hydrochloric acid HCl
Chemicals most often used to neutralize wastewater with an low pH
□ Hydrated lime Ca(OH)2
□ Soda ash CaCO,
□ Caustic soda NaOH
POTW
Publicly Owned Treatment Works
Communities with industries that discharge to the sanitary sewer must have this in place
Pretreatment Program that insures industrial dischargers do not adversely impact the operation of the POTW
This pollutant is oxidized to neutralize toxicity
Cyanide
Common industrial pretreatment processes.
□ Neutralization (acidic or alkaline wastes)
□ Oxidation (cyanide, hexavalent chromium, etc.)
□ Dissolved air floatation (suspended solids, fat, oil and grease)
□ Screening (particulate solids)
□ Chemical precipitation (dissolved metals such as copper, zinc, cadmium, etc.)
□ Aeration (BOD and COD)
Problems created by sewer lines with cracked pipes, offset joints or joint failure
- Root intrusion
- Groundwater infiltration (flow into the sewer pipe)
□ Sewer exfiltration (sewage flow out of the sewer pipe)
Sulfide corrosion of gravity sewer pipes.
- HS is produced by anaerobic bacteria growing in the slime layer below water level
- HS is released to air space above
- Sulfur oxidizing bacteria convert the sulfide to sulfuric acid which corrodes concrete
Methods of disposing of wastewater screenings.
- Disposal in a sanitary landfill (most common)
- Incineration
Flow equation used to estimate flow in gravity sewers
Manning equation. The cfs flow can be calculated when pipe diameter, depth of flow,
pipe slope, and pipe roughness are provided.
Installed in sewer manholes to measure flows.
Palmer-Bowlus flume
FOG
Fats, oils and grease
Problems associated with excess FOG in raw sewage.
□ Fowling and plugging of equipment, piping and instrumentation probes
* Increased load on sludge digestion and/or secondary treatment process
□ Interference with settling in primary or secondary clarifiers
* Excess levels are not effectively removed by typical treatment processes
Sources and characteristics of fats, oils and grease (FOG) in sewage.
- Petroleum based lubricating oil and grease (industry). Generally non-polar and non-
biodegradable - Animal fat and dairy fat (restaurants and food processing). Generally polar and
biodegradable - Plant based cooking oils (restaurants and food processing). Generally polar and
biodegradable
Floatable FOG in raw sewage is removed by this treatment process.
Primary clarifier scum remova
Reducing fats, oils and greases entering raw sewage depends primarily on this.
Installation and proper operation of grease interceptors and oil/water separators at
facilities
Typical manhole placement in gravity sewer systems.
□ 300 - 500 ft apart
□ Manholes also located at junctions and changes in direction, slope, pipe size
Peaking factor (ratio of peak to average flow) changes with this
Decreases as the size of the collection system increases
Lift station wet well size should optimize this
- Limit the pump cycles per hour
- Minimize solids settling and accumulation
Maximum infiltration allowed in a sanitary sewer system
500 gal/day * mile * inch (500 gal/day per mile of pipe, per inch of pipe diameter)
Sources of sewer inflow.
- Roof drains and foundation drains connected to the sanitary sewer
□ Holes in manhole covers - Interconnection between sanitary and storm sewers
Spacing of bars in bar screens (trash racks)
0.25 - 0.5 in (6.35 - 12.7 mm)
(3/8 to 2 inches pearson)
Grit removal processes used in preliminary treatment
- Grit channel (approx. 1 ft/sec velocity maintained in channel)
□ Aerated grit chamber (air diffusers produce spiral rolling motion that results in
approx. 1 ft/sec velocity) - Vortex settling chamber (sewage enters circular tank on a tangent and produces spiral
flow)
□ Detritus tank (small rectangular settling tank) - Hydrocyclone (pumped grit slurry or primary clarifier sludge is separated from water
and light organics by cyclonic centrifugal action)
CMOM
Checklist and guide developed by the EPA to evaluate a wastewater Collection system’s
Management, Operation and Maintenance activities
Primary goal of CMOM program
Better management and operation of collection systems to prevent sanitary sewer
overflows (SSO)
Typical concentration of FOG in domestic sewage.
30 - 50 mg/L
