Biological wastewater engineering, Civil Engineering

a. We put a lot of time and effort into cultivating our activated sludge microbial community. So why do we waste sludge out of it?
b. A wastewater discharges to Puget sound, a marine water where nitrogen is the limiting nutrient for algal growth. Sketch what the wastewater plant activated sludge process probably looks like.
c. A wastewater plant has been designed to handle an average daily flow of 5MGD with a commensurate BOD concentration of 400mg/L. The plant PEAK flows, however, are 13MGD: at that flow rate, the BOD is diluted to 250mg/L. Upstream of the one AS basin are two primary clarifiers that split the influent flow equally between them; the clarifiers typically remove 25% of the BOD load.
i) What are the BOD loads to the plant at each influent flow rate?
ii) And to the Activated sludge (AS) basin?
iii) If the influent quality is to stay same at each of these loads, how will the plant adjust their wasting rate (do not need a number, just a description).
iv) If the MLSS concentration (X) is 4500mg/L, what is the maximum specific growth rate we could expect at the average BOD concentration?
v) The aeration basin is 1MG. Assuming Y is 0.6, b is 0.08 and the permit is written for a BOD effluent concentration of 15mg/L, what SRT should the plant maintain to keep X at 4,500mg/L under average daily conditions?
vi) Under peak flow conditions?
vii) What is the F:M ratio of each of the flow conditions if the MLSS concentration is 4,500mg/L?
viii) During peak flow event, one of the primary clarifiers is hydraulically overwhelmed and can only remove about 10% of the influent BOD. The other clarifier functions as normal. The peak flows also washed some of the bacteria out of the system before the plant had time to adjust their wasting rate, so X has dropped to 3,500mg/L. What will be the BOD concentration out of the AS basin under these circumstances? Assume Y and b are the same as in (v) above, and that the peak flow SRT calculated in (vi) is still the target SRT.
ix) The peak flow only last for four hours, but the grit chamber can barely keep up with a flowrate of 10MGD. The plant has made up for this deficit by providing equalization (EQ) storage to hold the excess flow until the influent flowrate drops. How big does the EQ basin have to be to prevent the grit chamber from overflowing?
x) The flows are from snow that piled up during a blizzard. The weather is now warm enough to melt all the snowmen and wash them into the combined sewers that feed the plant. In preparation for and during the blizzard, the city put sand on all the roadways, which is also now washing into the combined sewers.
xi) What influent parameter will the sand most affect?
xii) Where will the sand largely end up?
2. A plug flow treatment reactor has an influent flow with a concentration of 150mg/L of total toxic organics (TTO) and a flow rate of 380L/min. The reaction is first-order with a rate constant of
0.4hr-1 Determine the required and reactor volume to achieve an effluent concentration of 20mg/L.
b. A completely-mixed treatment reactor has an influent flow with a concentration of 150mg/L of total toxic organics (TTO) and a flow rate of 380L/min. The reaction is first-order with a rate constant of
0.4hr-1 Determine the required and reactor volume to achieve an effluent concentration of 20mg/L.
c. How many times larger than the plug-flow reactor must the completely mixed reactor be to achieve 80% removal?
3. Primary effluent is to be treated by two parallel trains of the complete mixed activated sludge process. Assume average flow conditions, and the primary sedimentation performance is described in question two above. Assume the following for the activated sludge process:
i) plant effluent BOD of 8mg/L
ii) biomass yield of 0.55kg biomass/kg BOD
iii) endogenous decay rate (kd)= 0.04day-1
iv) Solids retention time= 8days
v) MLVSS concentration in the aeration tank of 3000mg/L
vi) waste and recycle solids concentration of 12,000mg/L
a. Determine the aeration tank volume in cubic meters
b. Determine the return cycle (recycle) flow rate in cubic meters per day (and in MGD)
c. Determine the food to microorganism ratio (F/M) for the aeration tank in kg BOD per day per kg MLVSS
d. Determine the design hydraulic detention time in hours

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