OXIDATION PONDS

INTRODUCTION

�There are many innovative biological technologies for the

treatment of wastewaters and sludges.

�Four such technologies that have now become widely

accepted worldwide are –

• Stabilization ponds

• Plants & wetlands

• Biological Nutrient Removal(BNR) systems

• Composting

INTRODUCTION

�A stabilization pond is any natural or man-made lentic

(enclosed) body of water.

�In this organic waste is oxidized by natural activity.

�These are simply large shallow lagoons enclosed by earthen

embankments.

�These are common in areas where land is readily available &

cheap & there is plenty of sunshine.

�Also used for initial treatment of strong industrial wastewaters

or for the tertiary treatment of domestic wastewaters

�They are used to treat a wide variety of wastewaters including

sewage.

�They can be operated as either anaerobic- or aerobic-based

systems that can be loosely classified into 3 groups-

�Anaerobic ponds or lagoons

�Oxidation ponds

�Aeration lagoons

�Oxidation ponds are aerobic systems

�The oxygen required by the heterotrophic bacteria is provided

not only by transfer from the atmosphere but also by

photosynthetic algae.

�There are 4 different types of stabilization ponds –

�Facultative ponds

�Maturation ponds

�River purification lakes

�High-rate aerobic stabilization ponds

�In facultative ponds the algae use the inorganic

compounds(N,P,CO2) released by aerobic & facultative bacteria

for growth using sunlight for energy.

�They release oxygen into solution that inturn is utilized by the

bacteria completing the symbiotic cycle

�Oxygen also occurs naturally by oxygen transfer ,which is

increased by turbulence.

�There are 2 distinct zones in facultative ponds:

�Upper aerobic zone - where bacterial (facultative) activity

occurs.

�Lower anaerobic zone – where the solid settle out of suspension

to form a sludge that is degraded anaerobic ally

�These ponds have a complex ecology with many predator-prey

association, with phyto- & zooplanktonic forms predominating.

�Ponds are constructed to a depth of between 1.2-1.8m to

ensure maximum penetration of sunlight, and appear dark green

in color due to dense algal development.

�The concentration of algae is measured as chlorophyll a & varies

between 500 & 1500µg/l.

�Organic loading is expressed as a surface loading rate either as

kg BOD/Ha/day or gm BOD/m²/day that varies significantly with

temperature.

�The maximum loading rates in facultative ponds are very low

compared to other biological systems.

�The final effluent is rich in algae that have a high BOD (i.e.

upto 70-90% of the total BOD).

�Overall facultative ponds achieve a BOD removal of about

50%,however when the algae is filtered out in the laboratory

this rises to >90%.

�The BOD contribution of the algae in the effluent can be

approximated as 1mg of BOD for every 35µg of chlorophyll a

present.

�The BOD surface loading is calculated using equation –

λs=10 LiQ/Af

Where

λs is the surface loading rate(kg/ha/day)

Li the BOD of the influent wastewater(g/m³)

Q the flow rate(m³/day)

Af the area of the pond(m²)

EXAMPLE-

Design a facultative pond for an influent wastewater BOD

of 550mg/l and a flow rate of 120m³/day where the

average temperature is 20̊ C.

(a)The BOD surface loading rate at 20̊ C is 253kg

BOD/ha/day.(BOD Surface loading rate is constant at

different temperatures)

(b) The surface area of the pond required is calculated as-

Af =10LiQ/λs=10*550*120/253=2608.7m²

(c) The Hydraulic Retention Time (HRT) of the facultative

pond фf assuming a depth Df of 1.5m is-

фf=Af*Df/Q= 2609*1.5/120=32.6/day

�Facultative ponds are generally used in series

after an anaerobic lagoon and both of these

systems are used primarily for the removal of the

organic matter.

�The effluent from an anaerobic facultative system

does not produce water that can be safely used for

irrigation, especially in warmer climates, due to a

high level of pathogen contamination.

�Removal of parasites (including helminthes) &

other pathogens (bacterial, viral & protozoan),is

significantly enhanced by using maturation ponds

in series after the facultative pond.

�This type of oxidation ponds is widely used throughout the world

as a tertiary treatment process for improving the effluent quality

from secondary biological processes.

�Effluent quality is improved by removing suspended solids,

reducing ammonia, nitrate & phosphate concentration, and by

reducing the number of pathogens.

�They are of the same depth as facultative ponds,1.0-1.5m

�The retention period is normally 10-15 days

�Shorter periods can be used for either suspended solids (4 days) or

phosphate removal (7-10 days).

�They are also used to improve the quality of lowland river water

prior to potable water treatment.

�Maturation & facultative ponds have been used for the

fish production for many centuries.

�In Western Europe, roach & carp are the prime species

farmed, with chub & perch to a lesser extent.

�The main operational problem is disease due to high

density of fish stocked.

�Fish ponds require careful management to ensure that

the rate of organic loading does not disturb the ecological

balance

FISH

DAPHNIA

CYCLOPS

PHYTOPLANKTON

ROTIFERS

DIAPTOMUS

FISH

ROTIFERS

DIAPTOMUS

CYCLOPS

DAPHNIA

PHYTOPLANKTON

Fig. The effect of over-stocking with fish on the food-web in a fish pond system

�Diffuse pollution is extremely difficult to control and often results

in serious degradation of water quality, especially in large rivers.

�Polluted river can be treated by the development of large

impounded lakes to remove & degrade this residual pollution.

�Such lakes are very similar to maturation ponds.

�E.g..- 5 such lakes have been constructed on the river

Rhur(Germany) with lake Baldeney, the largest with a retention

time of 60 hours at low flows. The lake provides organic treatment

capacity equivalent to a population of 100,000/day. Upto 170 tones

of phosphorus per annum,together with heavy metals, is removed

by sludge settlement in the lake.

�The solids are removed from the main body of water by

settlement in the lake & which are subsequently removed by

dredging.

�The effect on water quality is quite dramatic with the

suspended solids reduced by 56% & the BOD by 34% overall.

-RATE AEROBIC

STABILIZATION PONDS

�High rate aerobic stabilization ponds are not designed for optimum

purification of wastewater but for algal production.

�These are very shallow (20-50cm deep),with an HRT of 1-3 days,and

are mixed at night to prevent settlement and to maintain aerobic

conditions.

�Algae are harvested for biomass or single-cell protein.

�Green algae such as Chlorella & Scenedesmus, have a protein

content of 50% (dry weight) compared to 60-70% for the BGA

Spirulina.

�It is possible to control the cellular constituents of algae by altering

the growth conditions, for ex- by limiting the nitrogen availability the

protein content of the algae decreases while its lipid content

increases.

�Light availability is the most critical factor controlling algal growth

& so such ponds are restricted to those areas where there is plenty

of sunshine.

�The filamentous algae(BGA) Osciallatoria & Spirulina are

preferred as they are easier to harvest & slightly more digestible

than single-celled algae, such as Euglena & Scenedesmus

COOPERATION