Why are my Peaking Factor Method flows higher than I would expect?
How to only “peak” Sanitary Flows The Peaking Factor Method is a traditional (old) method of determining the probable peak flow in a sanitary sewer system – given the Average Daily Flow.
It was a practical solution when the primary calculating tool was the slide rule, but today, high speed computers eliminate the reason to use this method and allowing much more accurate approaches.
There are still some good reasons to use this method:

The municipality has enough experience with the Peaking Factor Method that they can adjust the Peaking Factor Curves as required for the conditions at hand and so are able to obtain quite accurate results. 

There is no metered data, making it hard to “calibrate” a collection system if more realistic methods and compute algorithms are used. 
The only caveat here is that if a municipality is able to meter its flow to assure themselves that their Peaking Factor Curves are resulting is dependable results, then they have the data to use more accurate methods and have no reason to be limited to the Peaking Factor Method.
Even though I personally have reservations about using the Peaking Factor Method, I will have to admit that it may have its place and so I not only implemented this method in the Hydra® Engine, I also expanded its capability. Sort of the “If you can’t beat them, then join them” approach.
As a result, HYDRA allows either method to be used via a single “switch” so it’s easy to use both methods and compare the results.
There is a more technical discussion later in this paper called Disadvantages of the Traditional Peaking Factor Method. If you are inclined, you might read that section, but for now, let us see how to use the Peaking Factor Method.
The Magic SwitchSET PEA
On/Off {San/InfSan}
The “SET PEA” is required as it defines the set switch to be used. The parameters that follow the “SET PEA” determine the
design criteria you desire.
The 2^{nd }parameter can be set to either “On” or “Off”, which tells the HydraEngine if it is to use the Peaking Factor Method or Hydrographic Method for sanitary flows.
The 3^{rd }parameter is optional. It tells the HydraEngine if the Peaking Factor Method is to be applied to only the sanitary
flow or to the combination of both sanitary and infiltration flows. If this parameter is not entered, the HydraEngine defaults to peaking the Sanitary + Infiltration flows (See NOTE below).
EXAMPLES:
SET PEA On
Use the Peaking Factor Method, peaking the Sanitary+Infiltration
SET PEA Off
Use the Hydrographic Method (Default for the HydraEngine)
SET PEA On InfSan
Use the Peaking Factor Method, peaking the Sanitary+Infiltration
SET PEA On San
Use the Peaking Factor Method, peaking the Sanitary only
NOTE: Intuition should immediately tells you that the Peaking Factor should only be based on the Sanitary flow, but intuition is not always correct. The problem is that before jumping too quickly, you need to first determine how the Peaking Factor Curve you decide to use was developed.
Historically, this was done by measuring the average daily flow at some point in the system and then determining the peak flow for that point.
This was typically done during a dry period, but probably there was still infiltration but not RDII – Rainfall Dependent Infiltration and Inflow. If this was the case, then to reverse this process and determine what the peak flow is for any point in the system you should use the flows that the Peaking Factor Curve was based on – the Sanitary+Infiltration flow. HYDRA gives you the tools to go either way, just be careful.