BTEMWT – Assignment 1B: Mikkeli WWTP
The Mikkeli Kenkävero WWTP is a mechanical, biological and chemical treatment of wastewater,
based on “activated sludge” in Finland. Later in this document you find a short process description
and key plant data for Mikkeli WWTP
In the following we shall try to calculate the size of the key process reactors in the plant.
For this we take some key values from the plant load and performance data:
Key load data
The mass of organic matter (BOD) in the inlet wastewater, average (mBOD,avg,i): 3,150 kg BOD/d
Nitrogen load in raw wastewater (mN,avg,i): 760 kg N/d
Phosphorous load in raw wastewater (mP,avg,i): 110 kg P/d
Suspended solids (or suspended matter) in raw wastewater (mSS,avg,i): 4200 kg SS/d
Flow, average (Qavg,i): (4,941,504 m3/year) /(365 d/year) = 13,538 m3/d
Assumptions:
 From flow variation data is it clear that not much rain water is taken to the plant (separate
sewers).
 The yearly average water temperature is Tavg = 16 oC.
 The minimum water temperature (two-week average) is Tmin = 8 oC.
 The removal efficiency of organic matter (BOD) in the primary settling tank is 30 %
 The ratio of sludge growth per kg food (BOD) supplied is 0.7 kg SS/kg BOD. This ratio is
used to estimate total daily sludge production (SP) at a given plant load (SS is suspended
solids, equivalent to filterable dry matter).
Questions
1) Using the definition of 1 PE = 60 g BOD/d, calculate how many PE the average plant load is
equal to.
2) With this PE number and the yearly average flow, calculate the daily flow per PE (m3/PE/d)
Does this seem like reasonable water consumption for a person in Mikkeli? Assuming that
storm water is discharged in separate sewers, could there be other sources of water in the
sewage pipes but water from the consumers (households)?
3) Use the detailed flow data to fine the maximum ratio between peak flow [m3/d] in 2012 and
the average flow. Assuming that the BOD load exhibit the same variation, what is then
maximum daily BOD load [kg BOD/d]?
4) Using this value, the average BOD removal rate, and the ratio for sludge growth per Kg
BOD, calculate the max biological sludge production in one day: SPmax,d [kg SS/d]. (“SS” is
suspended solids, a measure of filterable substances – i.e. mass of particles in the water)
5) The required aerobic sludge age for obtaining nitrification at 8oC is approx. 14 days.
Calculate the max production of sludge SP14 over 14 days as SP14 = 0.85·SPmax,d [kg
SS/d]·14d . The 0.85 ratio compensate for the fact that the max BOD load [kg BOD/d] will
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be smaller the more days we use to measure it. Using 365 days we are down to the average,
and hence considerably below the max (cf. question 3).
6) We now assume that the concentration of activated biological sludge in the aeration tank is
XA = 4 kg SS/m3. Calculate based on this the tank volume [m3] that ensures a sludge age
(SA) of 14 days. (Sludge age is the average retention time for sludge in the aeration tank, i.e.
Mass of sludge in the aeration tank divided by the daily sludge production).
7) Biological excess sludge contain approx. 1 – 1.5 % phosphorous by mass. Use a value of 1.2
% and calculate how much P [kg P/d] is removed from the water with the biological excess
sludge.
8) Assuming that the suspended solids (SS) in the raw wastewater contain 1 % P, and assuming
that the primary settling tanks remove 50 % of SS in the raw wastewater. How much P is
removed from the raw wastewater with the SS that settles in the primary clarifier? [kg P/d]
9) In order to achieve the performance for removal of P (see below plant description), how
much P remains to be removed after sedimentation of SS in the primary clarifier and
removal of excess biological sludge? [kg P/d]
10) Using a molar ratio for dosage of iron of 1.2 – i.e. 1.2 mole Fe per mole P chemically
removed. Calculate the required dose of iron to the water in mole/d. Using the molar weight
of iron (Fe) calculate the required iron dose in [kg Fe/d].
11) This iron precipitate as a complex of FePO4 and Fe(OH)3 components. The SS production is
in the order of 2.2 kg SS/kg Fe. Calculate the average chemical sludge production [kg SS/d].
Compare this to the biological sludge production.
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Background info on Mikkeli Kenkävero Wastewater Treatment Plant (WWTP)
Areal photo of Mikkeli WWTP and the town of Mikkeli, Finland.
Key process information
Short process description
The first step in the treatment process is a mechanical pretreatment. The wastewater is passed
through a screen to remove “large” objects that could otherwise damage the plant equipment and/or
be difficult to remove in the process. These objects are anything from bicycle handles to plastics,
cotton sticks etc. The screen is typically from 1.5 – 5 mm in bar spacing.
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Following the screen the water arrives in the aerated grit and grease chamber. Here sand is setteled
and taken out. A mild turbulence caused by aeration keeps organic particles in suspension, to
remove as little organic matter with the sand as possible. The sand is taken out sp that it does not
form permanent deposits in the subsequent reactors, and so that abrasion and wear in pumps are
minimized. Iron chloride is added to the water.
The water is passed into large “primary” settling tanks. The purpose of these is to remove organic
particles by sedimentation. The iron chloride added in the git and grease chamber will precipitate
and help take small particles and aggregate them to bigger particles that can be precipitated. Some
of the dissolved phosphorous in the water will also be precipitated and removed as iron-phosphatehydroxide solids. In a primary settling tank 40 – 80 % of the suspended solids are removed, and 25
– 40 % of the organic matter (BOD) in the wastewater is removed and taken out at the bottom of the
tank as “primary sludge”. This is rich in organic matter and must be treated, often in a biogas plant
in which methane is formed and used for heat or power production.
The water-phase is taken from the top of the primary settling tank, now being reduced for much of
it’s organic content. This water is mixed with a “soup” of microorganisms called “activated sludge”.
This sludge contain a culture of microorganisms - various bacteria and small animals – that are
adapted to feed on the type of organic matter in the wastewater. The mix of wastewater and sludge
in aerated for the degradation of organic matter and nitrification (aeration of reduced nitrogen such
as NH4+ to nitrate NO3-). Dissolved phosphorous in the wastewater (PO43-) is precipitated by
addition of iron (Fe3+) to a solid PO4Fe complex. Lime is added to control pH as the iron solution is
acidic, and the biological processes may decrease pH.
Now the mix of purified water and activated sludge is taken to the final (or secondary) clarifiers (or
settling tanks). Here sludge is settled to the bottom and pumped back into the aeration tanks, and the
purified wastewater is decanted from the top of the clarifiers.
Final disinfection or filtration of the purified water may take place to remove the last part of
particles and microorganisms from the water.
The activated sludge grows as result of the decomposition of the organic matter in the wastewater.
Every day some sludge must be taken out of the circulation. This part of the activated sludge is
called excess (or surplus) sludge. In Mikkeli this sludge is pumped to the inlet of the primary
settling tank and taken out with the primary sludge to the biogas plant, although the biogas potential
of this sludge is much less compared to the primary sludge. After processing (digestion) of the
sludge, the sludge is dewatered and taken to storage, before final disposal. The sludge can be taken
to incineration or can be applied on farmland as texture improvement and fertiliser, provided that
the content of heavy metals and other harmful substances is low.
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Incomming wastewater load:
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2012 performance data:
2012
Whole
Quarter 1 Quarter 2 Quarter 3 Quarter 4 year
treshold
/ quarter
*
BOD7 ATU mg/l
7
5
4
6
5
≤ 10
Efficiency % of purification
98
98
98
97
98
≥ 96
Total Phosphorus mg/l
0,3
0,19
0,3
0,45
0,31
≤ 0,5
Efficiency % of purification
97
98
96
95
96
≥ 96
0,31
(annual
average)
≤4
NH4 -N mg/l
0,03
0,97
0,08
0,18
≥ 90
Efficiency % of purification
100
98
100
100
99
(annual
average)
Solids mg/l
10
7
10
15
11
≤ 35
Efficiency % of purification
97
98
97
94
97
≥ 90
COD Cr mg/l
35
30
29
39
33
≤ 125
Efficiency % of purification
94
94
95
92
94
≥ 75
Flow to plant 2012:
Treated water
m3/d
min
average
max
m3/month
total
January
February
March
April
May
June
11014
9886
10626
11595
11772
8913
13375
11304
12024
17724
15631
11727
15976
12140
14623
22624
22460
13828
414636
327815
372735
531712
484568
351813
July
August
September
October
November
December
9573
9766
9809
13172
13838
9449
13078
11619
12495
15905
15422
11679
19779
15190
20402
23379
17693
13416
405414
360202
374846
493048
462665
362050
Total in 2012
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