Conventional and Modified On-Site Systems

Conventional on-site systems are the most common and the simplest on-site wastewater disposal systems. Installed properly, conventional systems are very reliable and require minimal maintenance.

Sometimes conventional systems must be modified to work properly in areas with thin soil layers, steep slopes, or high water tables. There are a number of standard modifications to the conventional system that can make on-site systems function well in these difficult conditions.


The amount of sewage that flows from a house depends on a number of factors, such as the number and age of the occupants, water-using habits, time spent at home, the size of the house, condition and age of the plumbing fixtures, and the income level of the occupants. Sewage flows from restaurants, businesses, and commercial buildings vary widely with the type of business and activity in the building. The amount of sewage that an on-site system must handle is important to the design, construction, and installation of an on-site system because the amount of sewage determines the size of the septic tank, pipelines, and the size of the treatment and disposal field. This section discusses how to estimate how much flow an on-site system can receive.


A design unit is used to determine the amount of sewage that will flow from a house, business or other facility.

The design unit method is used in the North Carolina Laws and Rules for Sewage Treatment and Disposal Systems.

For example, the dwelling unit for a house is a bedroom. The number of bedrooms determines bow many people may live in the house ( design unit). It is assumed that there will be two people living in the house for each bedroom and that each person will use 60 gallons of water per day, so each bedroom contributes 120 gallons per day to the sewage flow from a house. More bedrooms means that there are more people using water and that there is more flow to the on-site system. A 3-bedroom house would have 3 bedrooms at 120 gallons per day or a 360 gallon-per-day flow.

Under the rules, the minimum flow from a house, for design purposes, is 240 gallons per day, whether the house has one or two bedrooms. This minimum flow is used so that the on-site system will be adequately sized to account for water used by the basic fixtures expected to be present in all houses, regardless of size ( e.g. bathroom sink, toilet, and shower; kitchen sink and dishwasher; and a laundry machine).

In the same way, sewage flows from businesses can be determined by how many design units the business has. For example, the design unit for a restaurant is 40 gallons of sewage per day for each customer seat. So, a 100-seat restaurant would need an on-site system designed to handle 4,000 gallons of sewage per day ( 40 gallons per seat per day x 100 seats = 4,000 gallons per day). Motels generate 120 gallons per day for each room and day schools produce 10-15 gallons of sewage per day for each student.

The following points show how to use design µnits to determine sewage flow.

  • The minimum flow from a house is 240 gallons per day.

  • Each bedroom more than two bedrooms adds 120 gallons per day'.

  • If there are more than two people per bedroom, then the sewage flow is 60 gallons per day for each person living in the house.

  • For businesses and other buildings see Table 5. 1. 1 for the flow per design unit.

  • Design of sewage treatment and disposal systems for facilities not identified in this table shall be determined using available flow data, water-using fixtures, occupancy or operation patterns, and other measured data.

Daily Sewage Flow for Establishments

Daily Sewage Flow for Establishments


The actual flow of sewage from certain places may vary considerably from the flow rate listed in Table 5.1.1. For instance, if a business installs fixtures that use only small amounts of water, such as ultra-low flow toilets and ultra-low flow showerheads, then the daily sewage flows will be lower than listed in Table 5.1.1. This means that the treatment and disposal field for an on-site system can be made smaller under the rules. A smaller system requires less land area and is less expensive to construct.

  • Adjustments are not allowed for single-family houses or most other homes. Only places listed in Table 5.1.1 can request to adjust the daily flow of sewage so that they can install smaller treatment and disposal fields.

  • Establishments that request adjustments of flow may be required to install a flow equalization tank to smooth out the bursts of sewage that may flow at certain times of high usage. For example, a restaurant that serves only evening meals may have high peak flows for only a few hours a day and nothing during the rest of the day. Here, a grease trap and flow equalization tank that releases the sewage slowly to the septic tank and treatment and disposal field will be very helpful. The grease trap and flow equalization tank can keep grease and oil from being carried through the septic tank to the treatment and disposal field.

  • It is important to study the characteristics of the sewage to determine the strength of the wastewater. Restaurants and other food service facilities usually need to determine the amount of grease and oil and the strength of the waste before a smaller treatment and disposal field can be approved. Other establishments may need to test for other characteristics.

If an establishment wishes to apply for an adjustment to the daily flow of sewage, it must show that the daily flows are lower than the amount of sewage usually expected from a similar establishment. Two ways to show that the daily sewage flows are lower than Table 5.1.1 are given below.

The facility may keep records of the water used each month for 12 months in a row, and for the water used each day for 30 days of use. Frequently, records of water use can be obtained from water bills or from the public water utility company. To get a good estimate of the water used each day, the 30-day record should be taken when the water use is heavy. Next, the highest monthly reading is divided by the sum of the 30 days' water usage to get a peaking factor. The peaking factor shows how much difference there is between the normal daily flow during the peak flow month and taken to normal daily flow for the 30-day daily flow monitoring period. Then the highest three readings of the 30 days' records are averaged and multiplied by the peaking factor. The final result is an adjusted daily flow to use for design of an on-site system.

Keeping daily records of water usage is necessary because it is very difficult to determine a daily flow from yearly or monthly water usage. A daily flow calculated from a yearly or monthly usage will not show high peak flows. An example is given below.

  • Example. Suppose a 24-hour restaurant with 30 seats begins using disposable dinnerware, which greatly reduces flows from dishwashing, and a specially designed advanced pretreatment unit which assures waste­water characteristics do not exceed domestic strength. The restaurant manager requests a permit for an on-site system that is smaller than called for in Table 5.1.1. They agree to keep records of their water use for 12 months in a row and for a 30-day straight period to show that a smaller on­site system will be adequate. Of the 12 monthly records, the highest monthly water usage is 52,721 gallons. The 30-day record occurred during the month ofJune, which is a time when water usage is expected to be higher than normal. The sum of the 30 days' water usage records is 49,842 gallons and the three highest daily flows recorded are 1,468 gallons, 1,537 gallons, and 1,562 gallons.

    The peaking factor is the first number needed. It can be found by dividing the highest monthly water usage by the sum of the 30 days' water usage. In this example, the highest monthly water usage is 52,721 gallons and the sum of the 30 days' water usage is 49,842 gallons. Dividing 52,721 by 49,842 equals a peaking factor of 1.06. Next, the three highest daily flows from the 30 days' water records are averaged. In the example, the three highest daily flows are 1,468 gallons, 1,537 gallons, and 1,562 gallons. The average is the sum of the three daily flows divided by 3, which gives an average daily flow ofl,522 gallons per day. To find the adjusted daily flow, multiply the average daily flow by the peaking factor, or 1,522 gallons per day times 1.06 equals an adjusted daily flow of 1,613 gallons per day. A shorter version of the calculations is presented below.

From Table 5.1.1, the daily flow for a 24-hour restaurant with 30 seats is 2,250 gallons. As you can see, the adjusted daily flow, 1,613 gallons, is less than the flow listed in Table 5.1.1, which means that a much smaller treatment and disposal field can be used.

From Table 5.1.1, the daily flow for a 24-hour restaurant with 30 seats is 2,250 gallons. As you can see, the adjusted daily flow, 1,613 gallons, is less than the flow listed in Table 5.1.1, which means that a much smaller treatment and disposal field can be used.


In addition to the volume of sewage, it is important to know the characteristics of the wastewaters that are handled by on-site systems. The characteristics of the wastewater help determine the size and type of the on-site system. In some cases, additional septic tanks, filters, aerobic units, or large treatment and disposal fields may be required, especially where very strong wastewater must be treated. Table 5.1.2 lists a number of wastewater characteristics that can be important in designing on-site systems.

Sewage from most households has a common range of values for many of the characteristics listed in Table 5.1.2. These typical values are listed in Table 5.1.3, Typical Characteristics of Household Wastewater.

Sewage from restaurants, commercial buildings, and industries can vary widely from site to site. Because the strength of the wastewater can be very different, it is best to have the applicant test the wastewater for selected characteristics to help determine the type and size of the on-site system needed.


In recent years, much effort has been made to treat, recycle, or even reuse different types of wastewater from normal home use. The following definitions describe wastewater from homes.

  • Black water is the wastewater from toilets, urinals, and other devices that receive human or animal wastes. This sewage is very dangerous because it is polluted with high concentrations of bacteria and solids.

  • Gray water is wastewater from bathroom sinks, showers, baths, laundries, dishwashers, and kitchen sinks. This wastewater contains some pollutants and moderate concentrations of bacteria. The solids content can be high, especially in water from laundries, which contains lint. Gray water must be treated and disposed in an approved on-site system.

  • Some homes and facilities that must conserve water use systems that reuse gray water for flushing toilets. Thus, the gray water is used twice before it is discharged to the on-site system, saving water.

Wastewater Characteristics

Wastewater Characteristics

Typical Characteristics of Household Water

Typical Characteristics of Household Water