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Activated Sludge Module 1 ... Basic Concepts


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Which two laboratory analyses are required to calculate the F/M ratio?

? Influent CBOD5 and MLSS
? Effluent CBOD5 and TSS
? MLVSS and influent TOC
? Influent CBOD5 and MLVSS
F is food, or CBOD5; and M is mass or volatile microorganisms.
What type of phosphorus is soluble and not particulate?

? Poly-phosphate
? Organic-phosphate
? Ortho-phosphate
? Agoro-phosphate
This type of phosphorus usually requires some form of chemical treatment to achieve low levels
Which two flow streams typically combine together to make-up mixed liquor?

? Influent and secondary effluent
? Influent and primary effluent
? Primary effluent and waste activated sludge
? Primary effluent and return activat
In plants without primary treatment, mixed liquor suspended solids (MLSS) is a combination of influent wastewater and return activated sludge (RAS).
What typically happens to the activity rate of the microorganisms as the temperature of the wastewater decreases?

? It speeds up
? It slows down
? Temperature of the wastewater has nothing to do with activity rate of the bugs
Just like us, when the temperature gets colder we move slower!
Bacterial activity rate doubles with every 10 degree C increase in temperature.
What process control parameter is the result of aeration total solids inventory divided by total solids removed from the process?

? Gould sludge age (GSA)
? Solids retention time (SRT)
? Mean cell residence time (MCRT)
? Slu
SRT is usually based on total solids and MCRT is based on volatile solids.
What type of BOD is measured when the sample being analyzed is first filtered?

The "S" stands for soluble.
When the sample is filtered, the BOD value in the filtrate is typically lower than the BOD value of the unfiltered sample.
Which type of activated sludge process typically has an F/M ratio of about 0.2 to 0.5 and an SRT of about 5 to 15 days?

? High rate aeration
? Conventional aeration
? Endogenous respiration
? Pure oxygen
This is know as the declining growth phase ... about the middle of the growth curve.
Which statement best describes denitrification?

? NH3 + O2 = NO2
? NO2 + carbon = NH3
? NO3 + O2 = NO2
? NO3 + carbon = N2
Denitrification converts nitrates into free nitrogen gas that is displaced into the atmosphere.
This is an anoxic process that requires active bugs, low-to-no dissolved oxygen and a source of carbon (CBOD5).
What term is used to describe liquid that is removed from a digester, between an upper layer of scum and a lower blanket of sludge?

? Subnatant
? Effluent
? Filtrate
? Supernatant
Supernatant is typically removed from either an aerobic or anaerobic digester.
Liquid that is removed from the underside of a blanket of floating sludge is referred to a subnatant (like in a dissolved air flotation [DAF] process).
Which type of bacteria must have free dissolved oxygen to survive?

? Anaerobic
? Anoxic
? Facultative
? Aerobic
This oxygen must be supplied from an external source of energy ... like mechanical aerators or diffused aeration with blowers.
What is another way of saying mg/L?

? ppm
? 7.48 gals per ft3
? 8.34 lbs per million gallons
Both a and c
ppm and 8.34 lbs per million gallons are both the same as mg/L.
One gallon per million gallons is 1 ppm.
What condition can result in sludge on the surface of a secondary clarifier typically caused by a septic environment?

? Bulking sludge
? High rate aeration
? Rising sludge
? Over-wasting
Rising sludge originally settled properly and then gas caused it to rise to the surface.
This may indicate a lack of D.O. in the aeration system, an RAS rate that is too low, or plugged draft tubes or broken plows.
What is the correct incubation time and temperature for the BOD test?

? 5 days at 20°F
? 5 days at 20°C
? 20 days at 5°C
? 5 days at 68°C
This is the standard time and temperature for CBOD5 values that are used for F/M ratio calculations.
What bacterial activity measurement is expressed as mg/L/hr?

? Respiration rate (RR)
? Dissolved oxygen (D.O.)
? Settleometer
? Oxygen uptake rate (OUR)
The OUR identifies the amount of oxygen used per unit of time.
It is referred to as a quantitative test (the quantity of oxygen used).
However, OUR is not related to the quality of bugs in the system. That would be the RR test.
A high rate aeration system (not pure oxygen) typically has a low MLSS concentration and high F/M ratio ⬦ is this system considered to be over-loaded or under-loaded?

? Over-loaded
? Under-loaded
? F/M ratio has nothing to do wi
Loading refers to the amount of CBOD5 entering the aeration system ... and not the amount of solids in the system.
So, an overloaded system typically has high F/M ratio
Given the following data, what is the CBOD5 loading on the aeration system of this facility?

- Plant flow is 2.75 mgd
- Influent CBOD5 is 225 mg/L
- Primary clarifier CBOD5 removal is 30%

? 5,160 lbs/day
? 1,548 lb
(225 mg/L x 0.7) x 2.75 mgd x 8.34 lbs/gal
Note: 0.7, or 70% is the remaining CBOD5 that leaves the primary clarifier and enters the aeration system.
Which flow pattern typically combines the influent wastewater (or primary effluent) and RAS before it enters the beginning of a rectangular aeration tank. The combined flow then travels straight through the tank and leaves at the other end?

All flow (influent and RAS) enters the beginning of the aeration tank and "plugs" through the tank's length being displaced from the end of the tank
Which aeration system is typically operated in the endogenous respiration growth phase?

? Conventional activated sludge
? High rate aeration
? Extended aeration
? Log growth
This is on the far right side of the growth curve where older bugs use more oxygen to break down CBOD5.
The MLSS concentration in an extended aeration process can typically be very high.
What is it called when mixed liquor never settles properly and remains either suspended or rising in the secondary clarifier?

? Rising sludge
? Bulking sludge
? Sludge blanket
? Nitrification
Bulking is different from rising sludge because it never settles to start with.
However, rising sludge originally settled, and then for some reason (typically caused by septic gas), rose to the surface.
What is an intermediate oxidation state between ammonia nitrogen (NH3) and nitrate nitrogen (NO3)?

? CO2
? NO2
Nitrite is usually very unstable and "wants" to be something else ... typically either nitrate or free nitrogen gas.
Nitrite stabilization can result in toxic kill of microorganisms, or at the very least, a high demand for chlorine.
Which growth phase is predominant in extended aeration?

? Declining growth
? Endogenous respiration
? Log growth
? Filamentous
This growth phase is typical for activated sludge with high SRT and low F/M ratio
Which conditions best describe potential filamentous over-growth (bulking)?

? High D.O. and low food
? High D.O. and high F/M ratio
? Low D.O. and low F/M ratio
? Zero D.O. and low SRT
Because filamentous bacteria are strictly aerobic, they can out-compete heterotrophic bacteria for oxygen ... especially with low substrate (CBOD5) values
Which parameter best describes the balance of food to microorganisms?

? F/M
F is the food (or CBOD5) available to the carbon-eating bugs; and M is the mass or volatile microorganisms available to eat the food.
F/M ratio is basically the diet of the bugs!
What test result is necessary to convert OUR to RR?

? D.O.
The respiration rate (RR) is calculated by dividing the OUR, mg/l/hr by the mixed liquor volatile suspended solids, MLVSS,in gm/l.
The units for RR are mg of oxygen per gm of volatile bug per hour of time.
Which best describes sludge volume index?

? Numerical indicator of bug population
? Volume-to-weight ratio of mixed liquor
? Volume-to volume ratio of mixed liquor
? Removal efficiency of BOD
SVI is a numerical indicator of the settling rate of MLSS.
By definition, the SVI is the volume of sludge in ml occupied by one gram of mixed liquor.
Differently said, SVI is the volume-to-weight ratio of the mixed liquor.
Which growth phase does the stabilizer tank of the contact stabilization process typically operate in?

? Log growth to declining
? Declining growth to endogenous
? Endogenous respiration to log
? Filamentous
Depending on the MLSS concentration, F/M ratio and the SRT, the stabilizer tank bugs could be in the old declining growth to young endogenous respiration phase.
Which growth phase provides the best oxygen utilization efficiency . . . lowest pounds of oxygen per pound of CBOD5 converted?

? Endogenous respiration
? Declining growth
? Extended aeration
? Log growth
Typically, in this log growth phase, the bugs are in their most active state and the oxygen utilization efficiency is at its most optimum
Which condition may indicate too much food is still available in the MLSS leaving the aeration tank?

? Low OUR/RR in aeration inlet
? High OUR/RR in aeration inlet
? Low OUR/RR in aeration outlet
? High OUR/RR in aeration ou
The oxygen uptake rate (OUR) typically remains high when CBOD5 (food) is still available in the aeration tank outlet.

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