Slow Rate Systems

Slow-rate (SR) systems are typically nondischarging systems. The size of the project site will depend on the operating season, the application rate, and the crop. The number of months of possible wastewater application is presented in Figure 2.2. The design flow to the SR system can be calculated from Equation 2.10. The land area will be based on either the hydraulic capacity of the soil or the nitrogen loading rate. The area estimate given in Equation 2.10 includes an allowance for preapplication treatment in an aerated pond as well as a winter storage allowance. The expected effluent (percolate) quality is BOD < 2 mg/L, TSS < 1 mg/L, total N < 10 mg/L (or lower if required), and total P < 0.1 mg/L:

FIGURE 2.2 Approximate months per year that wastewater application is possible with slow rate land treatment systems.


Asr = Slow rate land treatment project area (ac; ha).

Qm = Average monthly design flow to the SR site (gal/mo; m3/mo).

q = Average monthly flow from pretreatment (gal/mo; m3/mo).

ts = Number of months storage is required.

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