sizes of industrial activities. Normally, 80 to 120 gallons per day per permanent resident and 30 to 50 gallons per day per transient and community labor personnel can be used as a rough volume estimate for flow.

PATTERNS OF FLOW

The amount of wastewater a treatment plant receives fluctuates from hour to hour. Changing seasons also affect the pattern flow. Peak flow of domestic wastes normally reaches a plant just after breakfast and for several hours in the early evening. Industrial wastes may reach the plant during the industry's period of operation. If the industry has two or three shifts, flow will be more constant.

The size and topography of the area served by a treatment plant also affects the flow pattern. Small plants may have large differences between peak and low flow periods. Larger plants normally have more uniform rates of flow. The period of lowest flow is usually between 2400 and 0500 hours. Unusual flow patterns help operating personnel identify and correct abnormal surges in flow in the wastewater system.

CHARACTERISTICS OF SEWAGE

Sewage is composed of many materials that are broken down into three general areas. These areas are the physical, chemical, and biological characteristics of wastewater. This section will aid you in identifying these various characteristics.

WASTEWATER COMPOSITION

The concentrations of most materials in wastewater are expressed in milligrams per liter (mg/I) and denote the strength of the wastewater. The higher the concentration, or mg/I, the higher the strength. Table 10-1 lists the most important materials that compose wastewater.

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