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CVEN 5534, Fall 2010
Homework 2 Solutions
1. a. WWTPs keep record flow data more or less continuously so the average daily influent
flow (“InfAvgFlo”) is the average of the flow measurements throughout a given day. The
maximum daily flow is the highest daily average value over the period of record, and the
average daily flow is the average of all the “InfAvgFlo” measurements reported. The
maximum month average day (MMAD) is the average daily flow during the month with
the highest average daily flow rate. In general plant capacity is rated in influent flow, not
effluent. The reason for this is that plants must be designed to handle influent flow.
Effluent flow rates are lower than influent due to losses of water primarily with sludge
removal. Average daily flow values are used for almost all secondary process unit sizing.
Clarifiers are designed to handle peak flows. For the Longmont WWTP data set
Parameter MGD
Average daily flow 7.1
Maximum daily flow 9.7
MMAD (June) 7.7
Comments: The difference between the average and maximum daily flow are an
indication of a flow peaking factor with a duration of a day or longer and the MMAD is
an indicator of a seasonal peaking factor. The MMAD has been used for secondary
process unit sizing with an hourly peak flow also used in clarifier design. At Longmont,
the difference between the MMAD and the average daily flow is about 10%, indicating
consistent wastewater flows and probably little infiltration/inflow even suring the high
runoff period in June. The average maximum flow (~hourly peak) is 10 MGD, indicating
a short-duration flow peak of 1.43, which is very moderate.
b. Influent COD:TBOD ratio calculated from averages of the measurements of the two
parameters = 2.53, which is about 10 to 70% higher than reported typical values between
1.5 and 2.3. Explanations include either soluble or particulate organic matter in sewage
which is not rapidly biodegradable under the conditions of a 5-day BOD test. That could
be components like oil and grease from restaurants and cafeterias, industrial wastes like
solvents and degreasers (although hopefully they would not comprise a large fraction of
influent organics). There could be toxic organic compounds. Again one would hope not
in the concentration that would produce a high COD:BOD ratio. One of the students in
class, Mitch Clement, offered an excellent hypothesis: that the readily degradable matter
in the sewage was degraded in the collection system before it reached the plant, and
supported his explanation by evaluating the association of the COD:TBOD to flow. A
few people mentioned agricultural drainage and nutrients to account for higher-than
expected COD. First, the COD test does not measure nitrogen, phosphorus or salts, the
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commonest constituents of ag drainage. Second, what little agricultural drainage is
generated in rapidly urbanizing Longmont would probably not find its way into a sanitary
sewer.
c. Graph of total effluent ammonia (NH4-N + NH3-N) daily and 30-day average values are
shown below.
General formula for going from water quality standard to permit limits:
Where Cw and Qw are wastewater treatment plant ammonia-nitrogen species concentration and
plant normalized effluent flow rate, respectively; Cus and Qus are ammonia-nitrogen species
concentration and normalized stream flow rate upstream of the plant discharge, respectively; Cds
and Qds are ammonia-nitrogen species concentration and normalized flow rate downstream (after
discharge), respectively.
Upstream total ammonia-N (mg/l)1 0.53
Upstream unionized NH3-N (mg/l) 1 0.026
Acute Water quality standard total ammonia (Cds, mg/l) 8.4
Chronic Water Quality standard unionized ammonia (Cds, mg/l) 0.06
WWTP average total ammonia-nitrogen (mg/l) 1.13
WWTP maximum 30-day average total ammonia-nitrogen (mg/l) 1.56
Calculated WWTP avg unionized NH3-N (mg/l) 0.07
Calculated WWTP max. 30-day average unionized NH3-N (mg/l) 0.09
1 average from CDPHE stream water quality sampling, LWWTP permit rationale.
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12/18/2008 2/6/2009 3/28/2009 5/17/2009 7/6/2009 8/25/2009
Total Ammonia (mg/L as N)
LWWTP Effluent Total Ammonia
Total NH3-N
30-d avg total NH3-N
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Normalized flows:
Qw = 1; Qus = 0.54 (low flow condition), Qds = 1.54
To meet acute standard = 8.4 mg/L total ammonia nitrogen, 30-day average effluent ammonia
nitrogen must be less than:
This is no problem for the Longmont WWTP to meet the acute WQS for ammonia. This
highest ever reported daily value for total ammonia nitrogen is 7.74 mg/l, and more relevant to
the permit, the highest 30-day average value was 1.56 mg/l, significantly below where a WQS-
based effluent limit would be set.
The chronic standard is another story.
The chronic limit would be associated with a total effluent ammonia limit of 1.3 mg/l. The plant
average total ammonia nitrogen is 1.1 mg/l about 15% less than the WQBEL. More important
their 30-day average total ammonia nitrogen exceeded the WQBEL for 29 days in April-May
2009. Meeting a new WQBEL based on the chronic ammonia standard could be very
problematic for the plant.
d. For nitrate, the upstream average nitrate-nitrogen concentration is 3.2 mg/l, and the
calculated new WQBEL for NO3-N is:
The 30-day average for NO3-N exceeded 13.7 mg/l for 25 days in April 2009, not surprisingly
when ammonia was low. Also, the overall average effluent NO3-N for the entire data period was
13 mg/l – only 5% lower than the standard. Moreover, the 30-day average effluent nitrate
nitrogen exceeded the WQBEL of 13.7 mg/l on 25 days in April 2009. With current treatment,
the plant is in kind of a double bind. As they get better at meeting stringent ammonia WQBEL,
their nitrate levels will increase. If a 10 mg/l NO3-N WQS were adopted – for example if there
was a possibility for the St. Vrain getting a designated use as a drinking water supply, it would
be very difficult for the Longmont WWTP to meet the new WQBEL for nitrate.
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e. Effluent fecal coliform has two permit limits: 30-day average < 251 CFU/100 ml and 7-
day average < 502 CFU/100 ml. The plant would have no problem meeting either of
these standards, as well as new standards based on E. coli, by either conversion
estimation method.
WWTP Data Fecal
Coliform
E. coli
(0.63 conversion)
E. coli
(0.77 conversion)
Plant effluent fecal coliform (avg CFU/100
ml)
28 18 22
Plant effluent fecal coliform (max 30-d avg
CFU/100 ml)
60 38 46
Plant effluent fecal coliform (max 7-d avg
CFU/100 ml)
117 74 90
Estimated discharge E. coli limits for 30-day average based on conversion of 251 CFU/100 ml
fecal coliform limit would be 158 CFU/100 ml or 193 CFU/100 ml, depending on which
conversion factor was used. For the 7-day average effluent limit, 502 CFU/100 ml fecal
coliform, the new 7-day averages would be316 CFU/100 ml or 387 CFU/100 ml depending on
which conversion factor was used. In all cases: 30-day and 7-day average effluent limits,
regardless of conversion factor used to estimate E. coli, the Longmont WWTP would have
no problem meeting an E. coli based limit, just as it has no problem meeting the current
fecal coliform limit.
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2. Factors to consider (by no means an exhaustive list).
Technology-based effluent limits (TBELs):
Merits Problems
Typically national standards which are uniform
and easy to understand (and hard to contest)
Do not consider unique receiving water
conditions
Achievable with available technology which
could consider secondary and tertiary treatment
May induce complacency with current
technology
Historically have produced improved water
quality
May not protect very high quality waters with
anti-degradation goals
Consistency produces equal cost burdens on
public treatment works
May not be easy to lower standards once they
are set
Non-point source pollution not factored in –
WWTPs responsible only for their own
effluent
New designated uses such as water reuse are
hard to add into permit limits. They become
voluntary
Some improvements such as energy
conservation, nutrient recovery, are technology
based and could be encourage by TBELs
Water quality-based effluent limits (WQBELS)
Merits Problems
Based on scientific knowledge of physical,
chemical, biological, ecological factors in
particular stream
Require significant investment in research and
many factors and combination of factors may
be neglected. Example, alkalinity significantly
changes toxicity of heavy metals.
Incorporate designated use for most waters
(except antidegradation waters) which
recognizes human factors
Changes to designated use offers opportunity
to downgrade receiving water quality due to
economic or social pressure
WQBELs can change to incorporate new
science
Frequent changes (e.g., over 5- or 7-year
permit cycle can impose significant hardship
on WWTPs where major process changes take
place over decadal or longer cycles.
Could inspire new technology based on
emerging standards or contaminants
Adding new contaminants can be done on a
local level
Research costs linking contaminants to impacts
would be borne locally
Non-point source (NPS) pollution may be
factored in explicitly
Utilities bear the cost burden of NPS