How to improve raw water quality?

The primary cause of bad water quality concerns is stagnant, unmixed, oxygen deficient water. By making sure that there are adequate oxygen levels throughout the water column, we maintain good raw water quality. The ADS fine bubble aeration tubing provides optimal oxygen transfer and mixing.

Introduction

Anoxic (oxygen-deprived) bottom waters and sludge fill with contaminants such as organic chemicals and trace metals. Water quality problems are due to these contaminants.

The undesirable taste and odor often encountered with raw water is the result of dissolved organics, iron, and manganese, which also cause brownish water color (staining). Foul taste and odor in raw water sources are the result of hydrogen sulfide, which also highly increase chlorine demand.

The release of phosphorus from sediments is increased by the lack of dissolved oxygen. It is important to remember that algae growth is encouraged by phosphorus which constitutes a nutrient. Oxygen depletion during biodegradation of algae and weeds contribute to poor taste and odor. The "flat" taste of water can also result of little oxygen concentration.

Reservoirs often have thermal stratification problems. The reservoir surface as natural atmospheric aeration can only provide oxygenated conditions to a depth of 2 to 3 m. Thus, less dense and generally better water quality resides in the upper layer close to the surface. The bottom of the reservoir constitute a dense poor water quality zone where the water intakes are often located.

Free chlorine reacts with organic matter to form trihalomethanes (THM), suspected cancer-causing compound. They are measured in raw water sources as total organic carbon (TOC). The reduction of THM formation in chlorinated drinking water is the result of the decrease of TOC levels in raw water sources. The depletion of oxygen levels leads to the rise of TOC levels, especially when dissolved oxygen levels drop below 5 mg/L. The rise of TOC levels is also greatly encourage by algae growth.

Solution

An ADS aeration sytem supplies oxygenation at the bottom of the reservoir, which induces complete circulation and introduces oxygen at the sludge water interface. The small size of the bubbles produced avoid them to roil up the sludge. Prevention of formation of thermoclines and anaerobic zones is ensured by designing systems which completely mix reservoirs a number of times per day.

By increasing oxygen levels, we prevent bottom waters from becoming anoxic, reducing the release of iron, manganese and phosphorous from accumulated sediments. Tastes and odors that emanate from anaerobically decaying organic matter are oxidized and eliminated. It also provides a useful food which supports a mix of organisms which can act to out-compete undesirables, such as algae, for available nutrients.

Destratification reservoirs is cost effective with ADS systems. The elimination of the "good" and "bad" water layers convert the entire body of water to fully oxygenated conditions and better water quality. Water quality is uniform throughout the reservoir and bottom intakes does no longer constitute a problem. Destratification ensure reservoir temperatures equalization. Algae growth and evaporation are reduced due to mixing to the surface of cooler water.

The reduction of TOC levels and the prevention of THMs formation necessite to maintain dissolved oxygen levels above 5 mg/L, which is possible with an efficient ADS aeration and mixing system. The reduction THM formation potential can be obtained by forcing algae into a photorespiratory state resulting of stressful conditions (high light intensity and high dissolved oxygen).

Result

When a raw water reservoir is aerated, treatment plant chemical usage can be significantly reduced from levels normally required to treat the water. The reduction of the amount of organic reaching the treatment plant is the result of the organic removal occuring in the reservoir itself. Furthermore, treating water biologically with a minimum amount of chemicals is a cost effective and environmental friendly solution.