DPH: Determining Design Sewage Flow

Determining Design Sewage Flow

The Public Health Code specifies design requirements for subsurface sewage disposal systems serving residential buildings which are different from those serving non-residential buildings. There are two practical reasons for this. Firstly, it is logical to relate the size of the sewage disposal system to architectural features of the building served, wherever possible, since the system is a permanent attachment to the building. This can conveniently be done by basing the size of the sewage disposal system of a residential building on the number of bedrooms it contains. Secondly, subsurface sewage disposal systems serving owner-occupied dwellings must be designed on a much more conservative basis than those serving other buildings since it is almost impossible to condemn such a dwelling because of a failing sewage disposal system which cannot be corrected. The economic and socialhardships presented by putting a family out of a home in which they have invested their life savings are such that regulatory officials usually must resort to less satisfactory abatement methods, such as holding tanks and reduced water use, which are objectionable to the residents and difficult to enforce. Non-residential buildings present a different situation, of course. A restaurant or other high water use facility may be converted to a retail store or low water use facility, without any undue economic hardship. Also, there is more latitude for the use of water reducing fixtures and water conservation. It probably also would be possible to condemn a non-residential building within the legal and political structure if abatement is impossible by any other means.


The size of the septic tank and leaching system serving a residential building is related to the number of bedrooms, without consideration of the number of occupants or the water consumption. The requirements in the Technical Standards may appear to be extremely conservative, considering that the size of the average family has been decreasing and now consists of less than three persons, and considering that studies have shown per capita water consumption to average approximately 50 gallons per day. On the other hand, it must be realized that sewage disposal requirements cannot be based on average figures, since if this were done, one-half of all the systems would be substandard and in danger of failing. A factor of safety of 1.5 is required to bring the confidence level to over 90 percent, for the reasons previously described. Therefore, in water usage terms, the design flow for each bedroom has been set at 150 gallons per day. This is based on two occupants. each averaging 50 gallons per day, with a 1.5 safety factor applied. The 150 GPD per bedroom usage factor would be utilized whenever performing hydraulic analysis calculations for residential buildings. The leaching system sizing tables in the Technical Standards utilized this flow rate to determine the effective leaching area per bedroom. No new residential building should be constructed except on this basis of design.

REVIEWING THE HOUSE PLANS: The design of sewage disposal systems in repair situations is relatively simple due to the fact that the number of bedrooms in an existing house can be provided by the licensed installer, the design engineer or the property owner during the application phase of the repair process. If there is a question, the sanitarian could request the property owner to allow access to the dwelling in order to confirm the basis of design. This process becomes much more complex with respect to proposed new home construction, particularly when permits are requested and approved prior to the final determination of what the house may look like. For that reason, it is essential that the basis of design be based on very detailed house plans and those plans be incorporated as part of the sewage disposal review process. In order to reduce the risk of any miscommunication, a copy of the house plans should be signed off by the health department and forwarded to the town building official prior to issuance of a building permit.

DEFINITION OF A BEDROOM: Within today’s custom homes it is not uncommon to see exercise rooms , sewing rooms, studies, offices, dens, family rooms and other similarly labeled non-bedroom spaces shown on residential house plans. However, these same rooms can and are used as bedrooms when a family grows or the house is sold to another family which has different needs. To make sure the home is served by a sewage disposal system which is sized properly, the system must be based on the potential number of bedrooms in the house.

There are certain standards by which a room can be deemed a potential bedroom. They provide:

  • A defined habitable space per Building Code requirements.   The exception to this statement would pertain to obvious future habitable space (such as the unfinished second floor in a "cape" style home) which has the appropriate structural shell but has not been "finished" to meet Building Code standards for habitable space.

  • Privacy to the occupants.

  • Full bathroom facilities (containing either a bathtub or shower) which are conveniently located to the bedroom served.

  • Entry from a common area, not through a room already deemed a bedroom.

Consideration should be given to the number of rooms in a new dwelling which may be used as bedrooms, even though the builder may not intend to use them as such. This is particularly true for homes built on speculation, since the builder has no control over who purchases the dwelling. Generally, all rooms on the second floor of a two story house, except for the bathroom and hallway, are considered bedrooms. Two bedrooms houses are allowed by the Public Health Code. However, such buildings would be expected to be relatively small in total floor area.

A significant number of homes are being constructed with habitable space above a two or three car garage. This space may be accessible from either the first or second floors or both. They are typically labeled as second floor playrooms or bonus rooms, may be quite large in area and have the potential to be a bedroom. Using the above criteria, this space should be deemed a bedroom when access is from the second floor and a full bathroom is readily available. The same designation would apply if access were provided from both the second and first floor. It would not be designated a bedroom if the only way to gain access to this area above the garage were perhaps from a first floor stairway when the first floor does not have a full bathroom facility, or access is from the garage.

Some latitude can be applied to the above when dealing with large homes, consisting of more than 5 bedrooms. It would not be unusual for this type of home to have a truly functional library, an exercise room, or a home office. However, before a bedroom designation can be made there should be some architectural feature which would typically exclude it from being used as a bedroom (such as, bookshelves around perimeter of library, sauna built into exercise room, etc.).

Rooms on the first floor of two story homes are generally easier to deal with. If rooms do not have access to full bathroom facilities on the first floor or are constructed with large archways, or, where entrance is through another room.

Basement areas can be utilized for bedrooms in certain circumstances. Walk-out basements with large windows, sliding glass or conventional doors could allow the area to be converted to a bedroom in the future. The key to this situation is the availability of plumbing fixtures on this level of the house. Plumbing plans should be examined at the time of initial construction to determine if plumbing will be "roughed in" which would provide access for future bathroom facilities. If a full bathroom (with a tub or shower) is shown on the plans then all rooms in the basement area shall be considered bedrooms when they meet the aforementioned "potential bedroom" standards.

It is also a phenomenon of the 90’s that large homes are being built for "small" families. The two person occupancy per bedroom used for design purposes is not realistic for many single family homes that exceed four (4) bedrooms (there are just not a lot of families which consist of 10 or more people). It is for that reason that a reduction in the sizing tables for leaching systems serving single family homes is being considered for homes which exceed four (4) bedrooms.



There may be a situation where a residence will be served by more than one subsurface sewage disposal system and the total sewage flow divided between the two systems, in accordance with the sanitary fixtures which they serve. This is not very desirable from the design standpoint since the characteristics of the wastes and the functioning of the sewage disposal systems may be altered. The Public Health Code requires that the subsurface sewage disposal system receiving the toilet wastes be large enough to meet the requirements for the entire house, and the other system to be at least one-half the size required for the full house.

This requirement is based on the following normal distribution of sewage flow from a residence, with a factor of safety.


Percent of Total Flow



Bath and Shower






In most split systems, the toilet and bath water goes to one system and the kitchen and laundry to the other, although occasionally only the laundry system is separated.

The volume of sewage flow from a residence will fluctuate considerably during the course of a day, and from day to day. However, the peak discharge rate is not a critical factor in the design of a residential sewage disposal system. Peak flows are unlikely to exceed 100 gallons per hour or 20 gallons a minute, and these should not interfere with the functioning of a properly designed septic tank.


Non-residential buildings are designed on the basis of the estimated 24 hour sewage flow. A list of estimated flows for certain non-residential buildings is included in the Technical Standards. These figures include a factor of safety. Non-residential buildings also may be designed on specific flow figures obtained for the particular type of facility to be constructed. However, the design engineer must include a factor of safety in this figure. For instance, water consumption figures may be available for a chain of fast food restaurants or supermarkets which would be acceptable as a design basis for similar facilities in Connecticut. In such a situation, an average flow figure for 3 to 5 such establishments maybe used with a factor of safety of 1.5 to 2.0. Lacking any specific information, the flow figures in the Technical Standards should be used.

Unlike residential buildings, the peak flow for certain non-residential buildings may be a critical design consideration. Buildings such as churches and athletic stadiums have extremely high one day flows, but relatively low weekly average flows. In such a situation, the septic tank is normally designed for the peak day flow, but the leaching system could be designed for an average flow over a few days to a week providing there is sufficient storage volume in the leaching system to hold the peak flows. Sewage would fill up the leaching system during the peak day and leach away into the soil before the next peak.

Leaching galleries or pits are usually used in order to provide sufficient storage of peak flows. Some facilities such as parks and recreational camp grounds have very high three day flow on certain week-ends, but lower flows during other times. The subsurface sewage disposal system should be designed for these peak flow periods.


Subsurface sewage disposal systems serving new buildings normally should not be based on a low design flow due to the use of sanitary fixtures which reduce the amount of water used. Such sanitary fixtures do not always prove to be acceptable to the users, and they may subsequently be replaced by conventional fixtures. This is difficult to prevent, particularly in residential buildings. However, there are situations where the use of low flow sanitary fixtures is desirable in order to abate an existing sewage overflow. The only reliable way to produce a significant volume reduction is by the use of special toilets or toilet appurtenances. Tank inserts may be used which reduce the volume of flushing water in the tank. Some toilets have adjustable flush controls which allow either a large volume or a limited volume flush. Other types have a specially designed bowl for a reduced flush volume. Connecticut has passed legislation which requires that all new toilets discharge a maximum 1.6 gallons per flush. In general, these types of low water flush toilets will reduce the volume of toilet wastes by 25 to 50 per cent and reduce the total sewage flow by 5 to 15 per cent produced from fixtures used in older homes. There are also available special toilets which provide only a minimum bowl rinse, or which use vacuum or compressed air assisted flushing water. In general, these toilets will use only about one gallon per flush and will reduce total sewage volume by 20 to 30 per cent. There are also non discharging toilets which would reduce the volume of sewage generated in a household by about 40 per cent. A more detailed discussion of the various types of low water use toilets may be found in Part II of the manual.

Pressure reducing attachments on shower heads and sink faucets also will tend to reduce water consumption. However, it is doubtful that it will produce much over 5 to 10 per cent reduction total sewage volume. The amount of water used for sanitary fixtures other than toilets is controlled mainly by the habits of the users, not by the sanitary fixture itself. When the desire is strong enough, it is possible to make extreme reductions in water consumption. This has occurred in some cases, such as where a holding tank is used which must be pumped periodically at a considerable expense. However, it is not advisable to rely on reducing sewage volume in this manner.

Content Last Modified on 12/19/2007 8:52:55 AM