Oxidation ponds

OXIDATION PONDS : 

OXIDATION PONDS

INTRODUCTION : 

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

STABILIZATION PONDS : 

STABILIZATION PONDS 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

INTRO.(contd) : 

INTRO.(contd) 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 : 

OXIDATION PONDS 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

Facultative ponds : 

Facultative 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.

DESIGN OF FACULTATIVE PONDS : 

DESIGN OF FACULTATIVE PONDS 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.

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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)

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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. The BOD surface loading rate at 20̊ C is 253kg BOD/ha/day.(BOD Surface loading rate is constant at different temperatures) 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

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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.

MATURATION (TERTIARY) PONDS : 

MATURATION (TERTIARY) PONDS 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.

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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

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FISH DAPHNIA CYCLOPS PHYTOPLANKTON ROTIFERS DIAPTOMUS

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FISH ROTIFERS DIAPTOMUS CYCLOPS DAPHNIA PHYTOPLANKTON Fig. The effect of over-stocking with fish on the food-web in a fish pond system

RIVER PURIFICATION LAKES : 

RIVER PURIFICATION LAKES 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.

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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.

HIGH-RATE AEROBIC STABILIZATION PONDS : 

HIGH-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.

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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

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THANKS FOR YOUR KIND COOPERATION