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Small Flow Treatment Facilities

Small flow treatment facilities (SFTFs) are sewage facilities designed to treat 2,000 gallons of sewage per day or less. Pennsylvania has nearly 3,000 NPDES-permitted SFTFs, most of which serve individual residences. These facilities do not require the use of a certified wastewater operator. SFTFs must generally submit an “Annual Maintenance Report (AMR)” (PDF) (DOC) (3800-PM-BPNPSM0093e) to DEP by June 30th of each year.

Many of the questions concerning operation of SFTFs deal with disinfection using chlorine. The following sections address two common issues.


Chlorine Tablets

Chlorine tablets used for swimming pool disinfection are unique and should NOT be used for disinfection as part of a wastewater treatment system. Many residential wastewater treatment system operators mistakenly use the incorrect type of chlorine tablets, which may cause a hazardous situation.

Acid based products, such as common swimming pool tablets, disinfect slowly and produce non-degradable chlorine residual while lowering effluent pH. Even occasional use of these acid-based products in wastewater treatment can result in the discharge of inadequately treated wastewater and create potentially harmful or hazardous gases.

Pool tablets are designed to soak in water rather than just having water pass by them as is done for wastewater treatment. If improperly used in a wastewater treatment system, the pool tablets can emit hazardous gasses.

Only chlorine tablets containing calcium hypochlorite should be used for disinfection at SFTFs.

For more information on this subject see DEP's fact sheet, Single Residence or Small Flow Wastewater Treatment Facility Chlorine Tablet Disinfection Guidelines (PDF) (3800-FS-DEP4406).

Provided for your convenience is a small listing of online websites where you can buy chlorine tablets for your wastewater system:


Total Residual Chlorine (TRC) Monitoring

The PAG-04 General Permit requires that TRC concentrations be within the range of 0.3 to 0.5 milligrams per liter (mg/L). Levels below this range may not provide for adequate disinfection of the sewage effluent, and levels above this range have a greater likelihood of negatively impacting the receiving waters.

TRC testing must be done monthly and reported on the AMR. If an initial measurement provides a TRC concentration outside of this range, SFTF operators should make necessary adjustments and test again. Sometimes high levels are found because improper chlorine tablets are used, as discussed above.

Sewage facilities most often use the DPD colorimetric method for testing and reporting TRC.

However, SFTFs regulated by a PAG-04 General Permit or an individual NPDES permit may use either the DPD colorimetric method or individual chlorine test strips for monitoring TRC.

The chlorine test strip method uses an indicator chemical that develops a color when dipped into chlorinated water. The color of the test strip turns darker with higher chlorine residuals. Users compare this color to a color scale to read the chlorine residual in milligrams per liter (mg/L).

Generally, if there is no color change, there is no chlorine present in the water. Users must be able to discern between colors to effectively use color comparative test methods.

Users must ensure that the selected test strips are capable of reading in the range necessary to provide accurate results. Refer to your NPDES permit to determine your specific permit limit prior to purchasing chlorine test strips.

Here are some tips for preventing errors when using test strips:

  • Ensure you are using the correct test strip for your application. Most NPDES permits require testing of Total Residual Chlorine, NOT Free Residual Chlorine.
  • Dip the test strip in a 50 ml water sample and adjust dip time according to the temperature compensation chart.
  • Remove the test strips and shake once to remove excess water. Read results in 20 seconds.
  • Match the colors in a well-lit area. Do not match colors in strong, direct light.
  • Do not touch the reactive pad. This may contaminate the pad and affect results.
  • Store the strips at room temperature and keep them away from excessive humidity.
  • Use a different method if any oxidants, such as permanganate, bromine, or iodine are present.

There are many providers of chorine test strips. A small sampling of those available to test for TRC at levels necessary for SFTF reporting include Hach, Test Kit Central, Filter Water and National Safety Products.

There are also handheld meters available for testing of Total Residual Chlorine, including:HI711 Checker® HC (Handheld Colorimeter), Hach, YSI.

Many more meters are available when searching the internet for "Total Residual Chlorine colorimeter". Meters or color strips must be capable of measuring TRC concentrations below 0.3 mg/L.