Science of Water : Concepts and Applications

294 The Science of Water: Concepts and Applications
Based on experience and according to the Texas Water Utilities Association’s Manual of Water
Utility Operations, 8th edition, typical pretreatment processes include the following:
1. Removal of debris from water from rivers and reservoirs that would clog pumping
equipment.
2. Destratifi cation of reservoirs to prevent anaerobic decomposition that could result in reducing
iron and manganese from the soil to a state that would be soluble in water. This can
cause subsequent removal problems in the treatment plant. The production of hydrogen
sulfi de and other taste- and odor-producing compounds also results from stratifi cation.
3. Chemical treatment of reservoirs to control the growth of algae and other aquatic growths
that could result in taste and odor problems.
4. Presedimentation to remove excessively heavy silt loads prior to the treatment processes.
5. Aeration to remove dissolved odor-causing gases such as hydrogen sulfi de and other
dissolved gases or volatile constituents, and to aid in the oxidation of iron and manganese,
although manganese or high concentrations of iron are not removed in the detention
provided in conventional aeration units.
6. Chemical oxidation of iron and manganese, sulfi des, taste- and odor-producing compounds,
and organic precursors that may produce trihalomethanes upon the addition of
chlorine.
7. Adsorption for removal of tastes and odors.
√ Note: An important point to keep in mind is that in small systems, using groundwater as a
source, pretreatment may be the only treatment process used or required.
√ Note: Pretreatment generally involves aeration or the addition of chemicals to oxidize contaminants
that exist in the raw water. It may be incorporated as part of the total treatment process or
it may be located adjacent to the source before the water is sent to the treatment facility.
AERATION
Aeration is commonly used to treat water that contains trapped gases (such as hydrogen sulfi de)
that can impart an unpleasant taste and odor to the water. Just allowing the water to rest in a
vented tank will (sometimes) drive off much of the gas, but usually some form of forced aeration is
needed. Aeration works well (about 85% of the sulfi des may be removed) whenever the pH of the
water is