cmutsvangwa: Wastewater Engineering, Dept. of Civil and Water Engineering, NUST 11/10/2006 11-1
FACULTATIVE PONDS
Facultative Ponds
The bacterial reaction include both aerobic and anaerobic decomposition and hence
the term facultative pond. It is a dual layer system operating aerobically near the
surface and anaerobically at the bottom (Fig. 3). Aerobic digestion of the
putrescible organic biodegradable materials from the incoming wastewater will
occur in the upper layer is both quicker and efficient and preferably should include
most of water depth in the pond. Waste organics in suspension are broken down by
bacteria releasing nitrogen and phosphorous nutrients, and carbon dioxide. Algae
use these inorganic compounds for growth, along with energy from sunlight,
releasing oxygen to solution. Dissolved oxygen is in turn taken by the bacterial,
thus closing the symbiotic cycle. Some of the biodegradable material comes from
the bottom anaerobic layer due to partial anaerobic digestion.
Fig. 3
The basic biological reactions of bacteria and algae in a facultative
pond
The oxygen is received from the atmosphere through the water interface of the
shallow pond and the photosythetically produced oxygen from the activity of the
masses of algae. Heavy blooms of algae result in the rise of pH of water which is
not conducive to aerobic microorganisms. And all available oxygen can be stripped
from the pond during the night by the respiring algae. Aeration can be increased by
making use of the prevailing wind. Wind breaks up stratification by creating
turbulence. Stratification is the development of distinct thermal layers in the water
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Chapter 11
WSP_Facultative ponds
cmutsvangwa: Wastewater Engineering, Dept. of Civil and Water Engineering, NUST 11/10/2006 11-2
with the less dense warm liquid being on the top and the denser cold liquid at the
bottom. Any stratification will limit the aerobic treatment due short circuiting as the
incoming wastewater is warmer thereby short-circuiting across the surface and
discharged partially treated wastewater without the benefit of the normal retention
period. This situation is aggravated by algae and therefore the major axis of the
pond should be in the direction of the prevailing wind. No trees and solid fences
should be allowed within 150 m of pond edge.
The settleable solids are deposited at the bottom to form a layer of sludge and if
temperature is greater than 15o-19oC anaerobic digestion will commence. The
decomposition yields inorganic nutrients and odorous compounds like hydrogen
sulphide and organic acids. The latter are generally oxidised in the aerobic surface
thus preventing their emission to the atmosphere. The partial anaerobic digestion is
due to the limited sludge thickness of about 0.25 m.
Facultative ponds are either designed for incoming anoxic effluent from the
anaerobic ponds or from the raw wastewater. Facultative ponds are built in series
because the BOD of a well-mixed facultative pond should not exceed 60 mg/l. If the
BOD of the pond is appreciably in excess of 60 mg/l then it will no longer be
facultative but anaerobic, and if there is a pond in series following a similar pond
producing a 60 mg/l BOD effluent then it will nearly certainly be fully aerobic.
The optimum depth is approximately 1.5 m so as to prevent the growth of rooted
plants but not so deep to prevent solar radiation penetrating through most of the
ponds depth. The shallower the pond the more extensive is the water-air interface
and greater is the direct absorption of the atmospheric oxygen.
Design approaches for the facultative ponds
There are several design approaches for the facultative ponds and some of them
are as follows:
‰
‰
‰
Hermann & Gloyna Equation
McGarry & Pescod
First Order Complete mix Equation
Hermann & Gloyna Equation.
The equation is based on experimental results from a series of ponds and related to
the variation in temperature by the Arrhenius type equation. Hermann & Gloyna
equation considered the optimum temperature for the operation of ponds to be 35oC
and required retention period at this temperature to achieve 90% BOD reduction to
be 7 days. The pond area can be calculated from: -
L
Q
t r (35) i 1.072 (35−T ) f ' f
200
D
t r(35) =Optimum retention time at 35oC
A=
Where:
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WSP_Facultative ponds
(5)
cmutsvangwa: Wastewater Engineering, Dept. of Civil and Water Engineering, NUST 11/10/2006 11-3
1.072
D
Q
T
Li
=Arrhenius coefficient
=Depth of pond
=Inflow
=Design temperature
=Influent BOD mg/l
f and f’ are factors to correct for any toxicity of the wastewater to algae present and
to correct for the oxygen demand by sulphur bacteria
McGarry & Pescod
MaGarry and Pescod (1970) suggested a simple relationship between pond loading
at the point of failure and the temperature:
λ f = 11.2(1.054)T
Where:
λf
Tf
(6)
f
=Maximum pond loading at the point of failure, kg BOD5/ha.d
=Temperature in degrees Fahrenheit
By using the maximum permissible loading before pond failure occurred is not a
good practice to design, and therefore the equation was later modified by Mara
(1997) to a liner relationship:
λ s = 20T − 120
(7)
in which T is in degrees Celsius. Recently Marks (1993) has suggested that for
Zimbabwean conditions the equation should be:
λ s = 20T − 90
(8)
The surface BOD loading is given as:
λs =
Where:
Li
Q
A
10 Li Q
, kg/ha.day
A
(9)
= influent BOD, mg/l
=Inflow, m3/day
=Area in m2
Therefore the area required becomes:
A=
10 Li Q
, m2
20T − 90
(10)
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Chapter 11
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cmutsvangwa: Wastewater Engineering, Dept. of Civil and Water Engineering, NUST 11/10/2006 11-4
A=
Li Q
, m2
2T − 9
(11)
The retention time in the pond as:
θf =
Where;
D
Qm
Af D
Qm
, days
(13)
=Pond depth, m
=Mean flow, m3/day
The mean flow is the mean of the influent and effluent flows. The latter being the
former less net evaporation and seepage thus the retention time becomes:
θf =
Af D
[
1
Qi + Qe
2
(13)
]
If the seepage is negligible, Qe is given by
Qe = Qi − 0.001 A fe
(14)
Where e is the net evaporation in mm/day. Equation becomes:
θf =
[2Q
i
2Af D
− 0.001A f e
]
(15)
A minimum value of 5 days should be adopted for temperatures below 20oC and 4
days for temperatures above 20oC. This is to minimise hydraulic short-circuiting
and to give algae sufficient time to multiply.
Example 2
Determine the sizes for the WSP system using the MacGary and Pescod approach.
Design data in example 1 is also applicable.
Without anaerobic ponds the area required is;
A=
430 × 40600
2 × 14 − 9
=918842 m2
With anaerobic ponds, the influent BOD is reduced by 48% and therefore the area
will be;
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cmutsvangwa: Wastewater Engineering, Dept. of Civil and Water Engineering, NUST 11/10/2006 11-5
A=
224 × 40600
2 × 14 − 9
θf =
[2Q
i
=478652m2
2Af D
− 0.001A f e
]
= θf =
2 × 478652 × 1.5
[2 × 40600 − 0.001× 47865 × 5]
=18.2 days
Say 19 days. (The net evaporation has been assumed to be 5 mm/day).
First Order Complete mix Equation
This is a rational approach based on the following assumptions:
‰ The pond is a completely mixed reactor
‰ There is a continuous inflow of raw wastewater and a continuos outflow of
treated effluent.
‰ There will be an instantaneous immediate perfect mixing of the influent with
the whole pond contents with no loss by evaporation or seepage.
The mean hydraulic retention time is given as:
θf =
V
AD
=
Q
Q
(16)
The design equation is given as follows:
A=
Where:
Q( Li − 60)
18D(1.07 )
T − 20
T
,
m2
(17)
= Mean temperature of the coldest month in oC.
References
1. Ellis K., (1995), Unpublished Lecture Notes in Wastewater Engineering,
Loughborough University, UK
2. IWPC, (1972), Manual for Small Works, UK
3. Mara D., (1976), Sewage Treatment in Hot Climates, John Wiley, UK
4. Mara D., (1997), Design of Waste Stabilisation Ponds in India, Lagoon
Technology, UK
5. Smith .M., (1995), Unpublished Lecture Notes in Wastewater Engineering,
Loughborough University, UK
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