Wastewater Daily Discussion - Stabilizat

Expert 'Tip & Tricks' from out friends at all-about-wastewater-treatment.com

Several factors may significantly affect or aid, the hydraulic and biological behavior of waste stabilization ponds. Some, not all, can be taken care of during design.

**Wind**

Ponds should be designed to induce churning by wind. We know, this results in uniform distribution of BOD, DO, algae and microorganisms all through the depth of water. It also moves oxygen down. This is particularly important when there is nil or insufficient photosynthetic activity. On the flip side, strong winds may produce high waves and erosion to the embankment slopes.

**Temperature**

Temperature directly influences the physical, chemical and biological activities in a pond system. Rate of photosynthesis and cellular metabolism are directly proportional to the pond temperature. Ponds should be designed for most adverse temperature conditions. At lower temperature, dissolved oxygen present has a tendency to remain in pond longer. As the temperature rises, dissolved oxygen is likely to be liberated to atmosphere, especially under supersaturated conditions. The oxygen production by Algae through photosynthesis is also temperature dependent. All ponds perform well on a sunny, cloudless day at an air temperature above 20 deg C and mild wind conditions. At a temperature above 35 deg C, the rate of photosynthesis declines rapidly and at temperatures above 45 deg C, it altogether stops. High temperatures stimulate growth of bluegreen algae at the expense of more efficient green algae. At the same time, aerobic bacteria consume oxygen at higher rate creating conditions to form anaerobic patches in the pond. Sudden reduction in temperature slows down algae activity and oxygen production. Algae will move to lower layers, the green color will reduce and pond performance will drop.

**Rainfall**

Rainfall influences pond performance. Detention time reduces when it rains. Besides, heavy shower dilutes the contents of shallow ponds reducing the food available to biomass. Rainfall adds oxygen to a pond system by increasing turbulence.

**Solar radiation**

Solar radiation directly relates to photosynthesis by the algae.
However, the rate of increase of photosynthesis declines when radiation intensity exceeds certain limits. Oxygen production also reaches a constant level. Actually, light is the factor for oxygen production in low light intensity conditions. And temperature is the guiding factor in areas of high light intensities. Latitude of the location and mean sky clearance factors help in determining the light intensity throughout the year. So, these are important parameters in designing the pond system, particularly the facultative pond. Too much solar radiation has adverse effects on pond performance.

**Evaporation and seepage**

These causes excessive loss of water resulting in increase in solid concentration which upsets the ecological balance. An evaporation rate in excess of 5 mm depth per day (50 cubic m)/hectare/day water
loss) is excessive and needs special attention. Soil characteristics along with knowledge of ground water, hydrology are important when selecting the site. If ponds have to be built on permeable soils, they must be lined to minimize seepage.

Physical factors:

**Surface area**

The surface area is a function of organic loading (BOD ) applied 5 per day (especially in case of facultative ponds). In warmer climates, surface loading from 150 - 400 kg BOD has been successfully deployed, though exceeding 250 kg BOD may cause odor problem.

**Water depth**

Stabilization ponds operate at constant depth as designed. Depths, less than 0.9 m cause growth of aquatic plants, surface weeds and mosquitoes. Depths exceeding 2 m in facultative ponds may limit sunlight penetration. So, anaerobic condition at the bottom layer may be created. A design depth of 1.5 m in facultative ponds has shown good results.

**Short circulating**

Incorrect positioning of inlet and outlet and poorly shaped ponds may produce short-circulating (dead or stagnant zones) within the pond. They may also transport the incoming wastewater quickly to the outlets, thus affecting pond performance.

Chemical factors:

**pH value**

Anaerobic and facultative ponds work well under slightly alkaline condition. So, industrial wastewater with high pH values should be appropriately controlled at the source before entry to ponds.

Anaerobic ponds situated in warm climates are usually biased to an alkaline pH value. In facultative ponds, if the pond turns deep green, the pH value can be taken to be in the alkaline range. If the pond water is yellowish green or milky, it is acidic.

However, facultative ponds display a natural diurnal variation in pH value. In the mornings, the pH value is low, due to excess carbon dioxide while in the late afternoon, the pH value rises due to the consumption of carbon dioxide by algae.

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