Equalization Basins

Generally speaking, flow equalization is not a treatment process or treatment method; it's a method to improve wastewater treatment processes, whether they are physicochemical processes or biological processes. The purpose of flow equalization is to balance out the process parameters, such as flow rate, organic loading, strength of wastewater streams, pH, and temperature over a 24-hour period. This practice is often applied either at the very beginning of the wastewater treatment plant—aiming at minimizing or controlling fluctuations in wastewater characteristics in order to provide optimum conditions for the subsequent treatment processes in the plant—or at the point right before discharging of effluent. For the former, the target is often focused on toxicity of the influent; the latter is squared at maintaining a predetermined discharging volume.

Flow equalization usually involves construction of large basins to collect and hold wastewater streams, from which the wastewater is pumped to treatment facilities at a constant rate. These basins usually are located after pretreatment facilities such as screens, comminutors, and grit chambers. In the case of industrial wastewater discharging into a municipal wastewater treatment plant, the location of the basins should be placed in the industrial site before discharging to smooth flow rate and characteristics of the industrial wastewater stream. Mixing is usually provided to ensure adequate equalization in basins and prevent depositing of solids on the bottom of the basins. Additionally, these basins also provide some treatment functions by oxidizing the reduced compounds in the waste-water and reducing BOD through air stripping. The mixing may be achieved by a number of ways: distribution of inlet flow or baffling, turbine mixing, diffused air aeration, and mechanical aeration.

The basic types of flow equalization systems may be categorized as follows:

• Alternating flow diversion (see Fig 3.2a). The system alternates filling and discharging one of two flow equalization basins for consecutive time periods. The advantages of this system are constant flow rate and constant pollutant level in the discharging effluent to the treatment facilities. The disadvantage of the system is the high cost of construction, because a large capacity of basins is needed to hold incoming waste-water streams one basin at a time.

• Intermittent flow diversion (see Fig 3.2b). The basin is used to divert significant variance in wastewater flow when needed; the diverted stream can be added back to the wastewater stream before it enters the treatment facilities. The diversion tends to run on short time periods.

• Completely mixed, combined flow (see Fig 3.2c). This system completely mixes several incoming wastewater streams at the front end of the treatment facilities in a mixing basin in order to level out the variances among the streams. This system works well if the flows are rela-

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Figure 3.2. (a) Alternating flow diversion equalization system; (b) intermittent flow diversion system; (c) completely mixed, combined flow system; (d) completely mixed, fixed flow system.

Figure 3.2. (a) Alternating flow diversion equalization system; (b) intermittent flow diversion system; (c) completely mixed, combined flow system; (d) completely mixed, fixed flow system.

tively compatible, because any large variance in parameters will create a shock load to the treatment facilities. • Completely mixed, fixed flow (see Fig. 3.2d). The difference between this system and the completely mixed, combined flow system is that there is a large holding basin that not only completely mixes the incoming wastewater streams but also equalizes the flow parameters in the basin before the effluent of the basin goes into the treatment facilities, thus providing a constant wastewater effluent with relatively constant flow parameters.

Design of the equalization facility should begin with the investigation of characteristics of the wastewater and its variability. A detailed analysis of the pollutants in the stream and flow data collection are a must in order to gain an appreciation of the effect of the nature of the wastewater on downstream wastewater treatment. Several flow parameters are the most important to be included in a detailed study of the nature and variability of the wastewater stream: mass flow rate, BOD5, TSS, TOC, etc. The data gathered from the study of the wastewater stream tends to be time-series, and a statistical analysis is needed to determine the effect of variability on the data of the parameters.

Equalization basins may be designed to achieve flow equalization, concentration of pollutants, or both. For flow equalization, a plot of the cumulative flow volume versus time over 24 hours is compared to the straight line of the average daily flow rate on the same diagram. The equalization volume required is the vertical distance from the point of tangency to the straight line representing the average flow rate multiplying by a safety factor (e.g., 110%). If the cumulative inflow rate curve goes beyond the line of the average daily flow rate, draw two straight lines that bound the cumulative inflow rate curve and are parallel to the average daily rate line. The equalization volume is the distance between the two straight lines that are tangent to the extremities of the curve.

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