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PHYSICAL CHARACTERISTICS The physical characteristics of wastewater include those items that can be detected using the physical senses. They are temperature, color, odor, and solids. Temperature The temperature of wastewater varies greatly, depending upon the type of operations being conducted at your installation. Wide variation in the wastewater temperature indicates heated or cooled discharges, often of substantial volume. They have any one of a number of sources. For example, decreased temperatures after a snowmelt or rainfall may indicate serious infiltration. Changes in wastewater temperatures affect the settling rates, dissolved oxygen levels, and biological action. The temperature of wastewater becomes extremely important in certain wastewater unit operations such as sedimentation tanks and recirculating filters. Color The color of wastewater containing dissolved oxygen (DO) is normally gray. Black-colored wastewater usually accompanied by foul odors, containing little or no DO, is said to be septic. Table 10-2 provides wastewater color information. Table 10-2.-Significance of Color in Wastewater
Odor Domestic sewage should have a musty odor. Bubbling gas and/or foul odor may indicate industrial wastes, anaerobic (septic) conditions, and operational problems. Refer to table 10-3 for typical wastewater odors, possible problems, and solutions. Solids Wastewater is normally 99.9 percent water and 0.1 percent solids. If a wastewater sample is evaporated, the solids remaining are called total solids. The amount of solids in the drinking water system has a significant effect on the total solids concentration in the raw sewage. Industrial and domestic discharges also add solids to the plant influent. There are many different ways to classify solids. The most common types are dissolved, suspended, settleable, floatable, colloidal, organic, and inorganic solids. Part of the total solids is dissolved in wastewater. Much like sugar dissolves in coffee, many solids dissolve in water. Dissolved solids pass through a fine mesh filter. Normal wastewater processes using settling or flotation are designed to remove solids but cannot remove dissolved solids. Biological treatment units such as trickling filters and activated sludge plants convert some of these dissolved solids into settleable solids that are then removed by sedimentation tanks. Those solids that are not dissolved in wastewater are called suspended solids. When suspended solids float, they are called floatable solids or scum. Those suspended solids that settle are called settleable solids, grit, or sludge. Very small suspended solids that neither float nor Table 10-3.-Odors in Wastewater Treatment Plant
Odor Location Problem Possible Solutions Earthy, Musty Primary and No problem None required Secondary Units (Normal) Hydrogen sulfide Influent Septic Aerate, chlorinate, (H2S), "Like oxonizate rotten eggs" " Primary Clarifier Septic Sludge Remove sludge " Activated Sludge Septic More air or less BOD, Aeration Tanks Conditions recirculation rate, Trickling Filters (Anaerobic) HTH, flood " Secondary " Remove sludge and/or Clarifier grease " Chlorine Contact " Remove sludge Tank " General Plant " Good housekeeping Chlorinelike Chlorine Contact Improper Adjust chlorine Tank chlorine dosage controls dosage Industrial Odors General Plant Inadequate Enforce sewer use pretreatment regulation settle are called colloidal particles. Colloidal particles are often removed in the biological treatment units. They may also be removed by chemical treatment followed by sedimentation. All the solids discussed above may be either organic or inorganic. Organic solids always contain carbon and hydrogen and when ignited to high temperatures (500C to 600C) burn to form carbon dioxide, water, and sometimes various other compounds. The burning or volatilization of organic solids has led to the term volatile solids. All solids that burn or evaporate at 500C to 600C are called volatile solids. These solids serve as a food source for bacteria and other living forms in a wastewater treatment plant. Most organic solids in municipal waste originate from living plants or animals. Those solids that do not burn or evaporate at 500C to 600C, but remain as a residue, are called fixed solids. Fixed solids are usually inorganic in nature and may be composed of grit, clay, salts, and metals. Most inorganic solids are from nonliving sources. Table 10-4 summarizes the types and amounts of the solids discussed in the preceding paragraphs. |
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