9.20 A 2 g sample of MLSS obtained from an aeration basin is placed in a 1,000 mL graduated
cylinder. After 30 min settling, the MLSS occupies 600 mL. Does the following sludge have
good, acceptable, or poor settling characteristics?
Solution:
600 mL mL
300
2 g MLSS g
SVI = =
Since the SVI is well above 200, the sludge sample has poor settling characteristics.
9.21 Figure 9.16 showed the modified Ludzak– Ettinger (MLE) process, which is used to
configure a biological reactor to remove nitrogen. Explain the role of the two compartments in
terms of: (a) whether they are oxygenated; (b) whether CBOD is removed in the compartment;
(c) whether ammonia is converted in the compartment; (d) whether nitrogen is removed from the
aqueous phase in the compartment; and (e) the primary electron donor(s) and electron
acceptor(s) in each compartment.
Solution:
See discussion in the chapter text (especially Section 9.9.1).
9.22 Investigate the specific mechanisms by which ammonia nitrogen, total nitrogen, and
phosphorus are treated or recovered at your local municipal wastewater treatment plant. Are the
processes chemical or biochemical (or a combination)? Discuss your answer.
Solution:
Students’ responses will be specific to student’s location.
9.23 Investigate the specific mechanisms that your local municipal wastewater treatment plant
uses for aeration. Is it surface aeration, fine- or coarse-bubble aeration, or natural aeration (via a
facultative lagoon or attached growth system)?
Solution:
Students’ responses will be specific to student’s location.
9.24 A wastewater treatment plant will receive a flow of 35,000 m3/d (~10 MGD) with a raw
wastewater CBOD5 of 250 mg/L. Primary treatment removes ~25% of the BOD. Calculate the
volume (m3) and approximate hydraulic retention time (hr) of the aeration basin required to run
the plant as a “high rate” facility (F/M = 2 kg BOD/kg MLSS-day). The aeration basin MLSS
concentration will be maintained at 2,000 mg MLSS/L.
Solution:
9.25 Table 9.28 provides suspended solids concentrations in several different wastestreams at a
Table 9.28. Suspended solids concentration for different process streams in Problem 9.25
Process stream
Suspended Solids Concentration (mg SS/L)
Plant influent
200
Primary sludge
5,000
Secondary sludge
15,000
Aeration basin Effluent
3,000
If the design hydraulic retention time of each of four aeration basins operated in parallel equals 6
hours and the total plant flow is 5 million gallons per day, what is the F/M ratio in units of lbs
BOD5/lbs MLVSS–day. (b) Suppose the plant engineer wishes to increase the concentration of
microorganisms in the biological reactor because she expects the substrate level to increase.
What would she command the operator to do to accomplish this goal?
Solution:
9.26 Determine the minimum surface area for a facultative lagoon treating municipal wastewater
to be sited in Tampa, Florida. Assume the conversion efficiency of the algae is 3.5% and that
24,000 kJ of sunlight are needed to produce one kg of algae biomass. The average daily
concentration of CBOD in the wastewater to be treated is 250 mg/L and the average flowrate is 4
MGD.
Solution:
9.27 Determine the minimum surface area for a facultative lagoon treating municipal wastewater
to be sited in your local community. Assume the conversion efficiency of the algae is 3% and
that 24,000 kJ of sunlight are needed to produce one kg of algae biomass. Use the solar
radiation characteristics of your local area and obtain an average daily concentration of CBOD in
the wastewater and average wastewater flow rate from your instructor that is for your area.
Solution:
Students answers may vary, though use the method is example 9.8 as in the solution to problem
9.26.
9.28 The community of San Antonio is located in the Caranavi Province, Bolivia. According to
the 2005 year survey, there are 420 habitants of this community. The population is estimated to
increase to 940 by the year 2035. The average peak flow is currently 1.2 L/sec and is expected to
increase to 2.14 L/sec by 2035 The organic load is estimated to be 45 gram BOD5/capita–day.
The community is considering a free surface wetland to treat their wastewater. (a) What is the
BOD5 loading generated in the year 2035 (kg/day)? (b) Use the BOD loading to estimate the
maximum surface area (ha) required for a free surface wetland that would serve the community
in 2035 and remove BOD and TSS to 30 mg/L. (c) Assuming you are now considering sizing a
facultative lagoon instead of a free surface wetland. Quickly estimate the required surface area
(m2) for a facultative lagoon to handle a peak flow in 2035, assuming a design water depth of 4
m and a hydraulic detention time of 20 days.
Solution:
9.29 (a) What size septic tank would you recommend for a 2 bedroom cottage assuming the
available tank sizes are: 750; 1,000; 1,200; and 1,500 gallons? Assume you wish to have three
days of residence time for the pollutants in the tank. (b) How would your problem change for a 4
bedroom house with a two day residence time?
Solution:
9.30 Table 9.1 showed that pit latrines are considered an improved technology for treating
wastewater. Determine the depth required for a pit latrine 1 m × 1 m in area that serves a
household of 7 people and has a design life of 10 yr. Assume the pit is dug above the water table
and the occupants use bulky or nonbiodegradable materials for anal cleansing (e.g., corncobs,
stones, newspaper); therefore, the solids accumulation rate is assumed to be 0.09
m3/person/year. Allow a 0.5 m space between the ground surface and the top of the solids at the
end of the design life which is the point when the pit is filled.
Solution:
9.31 (a) Estimate the volume of gas production produced from one metric ton of food waste and
one ton of wastewater solids. (b) Based on the results from part (a), assuming the mass of food
waste and wastewater solids generated in a community is the same, would you recommend a
municipality develop a program to collect and digest (with energy recovery) food waste or
wastewater solids? Explain your answer based on potential energy production but also an
implementation stand point. Assume the methane production potential of wastewater solids is
120 m3/ metric ton, food waste has 3 times the methane production potential per volume of
wastewater solids, and methane makes up 60% of the total gas produced from anaerobic
digestion.
Solution.
9.32 (a) If methane has an energy content of 39 MJ per m3 and digester gas is approximately
60% methane, what volume of total digester gas must an anaerobic digester produce annually to
provide potable water for a family of six for a year? (b) If the methane is provided from
anaerobic digestion of food waste, how many pounds of food waste would a family have to
generate per day to provide this energy to heat water (lbs/day)? The United Nations states that
the minimum potable water requirement is provide drinking water sanitation and hygiene is 20 L
per person per day. Assume the water has an initial temperature of 25oC and you must raise the
temperature to 100oC to produce potable water. The energy required to raise water up 1oC is
equal to 4,200 J/L-oC and there is 39 MJ of energy per m3 of methane. Assume that 1 metric ton
of food waste produces 600 m3 of total gas.
Solution:
9.33 If 1 kg of volatile solids (VS) produces 0.5 m3 of methane but only half of the VS added to
the digester will be broken down to gaseous compounds. If you want to produce 120 L of
methane per day, how many pigs will you need to maintain to contribute waste to the digester?
Assume a 60-kg pig produces 5 kg of manure per day with 10% being VS.
Solution:
9.34 Waste must be held in the digester for a period of time for digestion to occur but the length
of time depends on temperature. Using the data in Table 9.29, calculate the appropriate digester
capacity and dimensions (diameter and height) in m for each temperature listed assuming 20 L of
input per day. Fix the digester dimensions at 1:5 diameter to height.
Table 9.29 Data for Problem 9.34
Temperature (°C)
Retention Time (days)
minimum recommended
10
55
20
20
30
8
Solution: