Derivation of design flows of wastewaters

The accurate determination of wastewater flows is the first fundamental step in the design of wastewater facilities. To ensure proper design, accurate and reliable data must be available. This entails proper selection of design period, accurate population projection, and the determination of the various flow rates. Again, the discussion that follows addresses the particulars of the community; however, parallel concerns should also be directed to the determination of industrial wastewaters and other types of wastewaters.

1.6.1 Design Flows

Different types of flow rates are used in design of wastewater facilities: average daily flow rate, maximum daily flow rate, peak hourly flow rate, minimum daily flow rate, minimum hourly flow rate, sustained high flow rate, and sustained low flow rate. Each of these flow rates has its own use in design. To ensure accurate values for these parameters, a statistically sufficient amount of data should be available for their derivation. Ideally, the amount of data should be infinite or, at least, exhaustive.

The average daily flow rate is the mean of all the daily flow rate values obtained from an exhaustive length of flow record. For example, if the total measured flow at a treatment plant for one year (365 days) is 9,000,000 cubic meters, the average

daily flow rate is 9,000,000/365 = 24,658 m per calendar day. If the duration of record is more than one year, an average daily flow rate value is established for each year. All these averages are considered together to arrive at a single average daily flow rate. The longer the record, the more accurate the value will be. The ideal value is one in which the length of record is exhaustive.

Note that the average daily flow rate for wastewater is not being given a probability of occurrence. As in water consumption, the average daily flow rate for wastewaters is the average daily flow rate of all daily flow rates available on record. This is the long-term average as was discussed in the case of water. Again, only one value exists for the particular parameter of average daily flow rate. Also, only one value exists for each of the parameters of maximum daily flow rate, maximum hourly flow rate, and so on, with no attached probability of occurrence, although, in concept, one may ask for a particular value corresponding to a certain probability. If a probability of occurrence is attached, this means that some of the wastewaters will not be treated. For example, if a wastewater treatment plant is designed to treat a maximum flow that is equaled or exceeded 10% of the time, this means that 10% of the time, some sewage will not be treated. Sewage, however, must be treated at all times.

As in water consumption, average daily flow rates are usually normalized against the contributing mid-year population. Thus, if the contributing mid-year population for the example given in the previous paragraph is 50,000, the average daily flow rate is also given as 24,658/50,000 = 0.49 m per capita per day.

Analogous to the average design flow rates used for water, average daily flow rate values for wastewater are used in developing ratios in design such as the ratio of maximum daily flow rate to the average daily flow rate, the peak hourly flow rate to the average daily flow rate, and so on. Thus, knowing the average daily flow rate and the ratio, the maximum daily flow rate or the peak hourly flow rate, and the like, can be determined.

The maximum daily flow rate is the largest total flow that accumulates over a day as obtained from an exhaustive length of flow record. For example, suppose the flow on March 10 of a certain year was 49,316 m accumulated over the day. If the whole flow record is examined and no one flow summed over the length of one calendar day exceeds this value, then 49,316 m /day is the maximum daily flow rate. On the other hand, the minimum daily flow rate is the smallest total flow that accumulates over a day as obtained from an exhaustive length of flow record. This is just the reverse of the maximum daily flow rate.

The peak hourly flow rate is the largest accumulation of flow in an hour during a particular day as obtained from an exhaustive length of flow record. For practical purposes, the peak hourly flow rate may be considered as the instantaneous peak flow rate. The minimum hourly flow rate is the smallest accumulation of flow in an hour during a particular day as obtained from an exhaustive length of flow record. This is just the reverse of the peak hourly flow rate. As with peak hourly flow rates, the minimum hourly flow rate may be considered as the instantaneous minimum flow rate.

The sustained peak flow rate is the flow rate that is sustained or exceeded for a specified number of consecutive time periods as obtained from an exhaustive length of flow record. Note the use of the word "or," signifying union of two events. For example, pick any record and take a set of seven-day consecutive flow rates such as 49,300, 48,600, 47,689, 46,000, 46,000, 44,000, and 44,000 all in m3 per day. The mean of these values (46,513 m /day) refers to the "sustained" which, in this case, the flow is being sustained in seven days. The mean of all sets of seven-day consecutive flow rates may be computed similarly. The result of the calculations will then be an array of sustained flow rates, which can be arranged in descending order. As in the previous methods of determining the maximum values using the probability distribution, the sustained peak flow rate is the sustained flow rate that has the probability of being equaled or exceeded by 0%.

The sustained minimum flow rate is the flow rate that is sustained or not exceeded for a specified number of consecutive time periods as obtained from an exhaustive length of flow record. Again, note the use of the word "or" for the union of two events. This is just the reverse of the sustained high flow rate; the array, instead of being arranged in descending order, is arranged in ascending order. The sustained minimum flow rate is then the sustained flow rate that is equaled or not exceeded by 0%.

In the previous discussion, the phrases average daily flow rates, maximum daily flow rates, maximum hourly flow rates, minimum daily flow rates, and so on are often simply shortened to average daily flow, maximum daily flow, maximum hourly flow, minimum daily flow and so on, respectively. Again, in the development of these flow rates, sufficient or exhaustive lengths of data should be available to produce statistically acceptable values.

1.7 deriving design flows of wastewaters from field survey

Design flows can be established in several ways: use values in the literature, use the treatment plant record, and conduct a field survey. The use of literature values is the least desirable because flow quantities are dependent on the habits of the people and the characteristics of the land area. Land characteristics affect sewage flow, because the nature of the land affects the depth of groundwater, which, in turn, determines the amount of infiltration into the sewer.

The habits of people also affect the quantity of sewage flow. The habits of the people in Asia are different from the habits of the people in America. Farming communities have different habits from city and urban communities. They produce different quantities of sewage as well as different constituents of sewage. The sewage in Ocean City, MD probably contains a lot of crab shells as compared with the sewage in Salt Lake City, UT. Thus, any literature value may be meaningless, because there may be no way to determine whether or not the value conforms to the land characteristics and habit of the people in the proposed design.

If the use of plant record is futile because of the aforementioned infiltration, a field survey may be conducted. Conducting a field survey is desirable but suffers from two defects: it is very expensive and, by necessity, is conducted over a very short time. Given these drawbacks, however, conducting a field survey is probably the most accurate of the three methods mentioned previously.

For an expansion to an existing system, the flow measurement can be done easily, because the system already exists. For brand-new systems, the metering can be done in a nearby town or village, where the characteristics of the system and the habits of the people are similar to those of the proposed. The requirement of similarity is very important. To have accurate predictions of sewage flows, it is obvious that the reference town should have characteristics similar to those of the proposed. To illustrate the various methods of determining design flows, Figures 1.6 and 1.7 were derived. These figures also show what a flow pattern would look like if a sewer system were monitored in a field survey.

In practice, before any monitoring is done, the as-built sewer system plans must first be obtained to determine where the monitoring apparatus should be set. Once this is established, the contributing population must also be determined. This population is used to normalize the measured flow rates on a per capita basis in order for the data to be transportable; and, most important, the nearby precipitation record should be consulted to determine the months for the dry and wet seasons. If the purpose of the field survey is to determine the dry-weather flows, then the monitoring should be done on the dry-weather months as evidenced from the records of the nearby precipitation station. If the purpose of the field survey is to determine infiltration, then the monitoring should be done on the wet-weather months, again, as evidenced from the records of the nearby precipitation station. In Maryland, the dry-weather months are normally June to August.

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