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WLS-014 How to Monitor an Onsite Wastewater Treatment System (OWTS)

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Version: 05/17/2023


To provide the process for monitoring an Onsite Wastewater Treatment System (OWTS) utilizing supplemental treatment as required by the Sonoma County Operational Permit Program. The property owner is required to monitor the OWTS utilizing supplemental treatment twice a year during the months of March and October, representing the wet and dry times of the year.

Routine monitoring will help ensure the longevity of the septic system. A Self-Monitoring Form, specific to the system, will be emailed directly to the property owner prior to March and October.


The Regional Water Quality Control Board allows the County to permit OWTS utilizing supplemental treatment, provided a program is in place to monitor the performance of these systems. To enter the monitoring program, the original OWTS applicant must have executed an Operational Permit Easement which is binding on current and future property owners. The purpose of the Operational Permit Easement is to allow County and State officials to enter the property for the purpose of monitoring the system.

General Instructions

The following instructions address the minimum items that the property owner should be observing during a monitoring inspection. The Self-Monitoring Form should be completed as the observations are being made. OWTS utilizing supplemental treatment vary by type and date of installation and may not include all the items addressed in these instructions. The form should be completed using the link emailed to the property owner. A copy of the completed form is available on the website.

Performance Wells


Performance wells (also called Monitoring wells) aid in evaluating the performance of an OWTS. They are usually three- or four-inch diameter perforated pipes with a lid or cap located near the ground surface.

  • The performance wells may be protected by a small round box. Do not mistake the boxes over the purge valves or adjusting valves for the performance wells.
  • The number of performance wells vary with each system. There should be at least one located upslope of the leach field, several in or around the leach field, and at least one or two below the area of the leach field.
  • The performance wells located 25 feet downslope of the leach field are used as a sampling location to evaluate the effectiveness of the sewage treatment. The upslope well is the background reading and is used to compare to the downslope wells.
  • Performance well locations can be found on the plot plan for the septic system, which can be viewed at Permit Sonoma, or with the consultant who designed the system.

Performance wells indicate whether the necessary separation to ground water is being maintained for proper sewage treatment or whether a biomat is forming within the soil which slows down vertical permeability. Within a mound or sand filter system, they indicate whether a biomat is forming at the gravel/sand interface or at the sand/soil interface. Within an at-grade system, they indicate whether a biomat is forming at the gravel/soil interface.


The number of performance wells vary for each non-standard septic system.

  1. Add as many performance wells as the system has by selecting the green plus sign on the right side of the blue box until you have a place to input a reading for each performance well. 
  2. A sketch of the layout of the system identifying the locations and numbers of the performance wells may be uploaded through the link provided on the electronic Self-Monitoring Form.
  3. In the appropriate location on the electronic Self-Monitoring Form, measure and record the distance in inches from ground level down to the top of any water in the performance well.
  4. If there is no water in the performance well, indicate “dry” for that well.
  5. It is very important to keep a copy of the system diagram and numbered performance well locations for future use.

Control/Alarm Box (with or without timers)


The control/alarm box contains the breakers and fuses serving the pump, control floats and alarms. It also contains the dose counter which is used to calculate the average amount of water in gallons per day that is discharged to the septic system. Each system is designed for a maximum number of gallons per day and this maximum should not be exceeded. Exceeding this maximum may cause the system to fail. The instructions for completing Sections C and D pertaining to these gallons per day are found below.

Instructions: Control/Alarm Box (with or without timers)

The dose counter indicates the accumulated number of doses to the system since installation. Some systems utilize a timer instead of a dose counter (see Alarm Boxes with Timers below). The questions regarding the Control/Alarm Box will be answered by selecting Yes, No, or N/A in the drop down menu.

  1. To check the dose counter, press, and release the lever labeled either “auto/off/manual” or “hand/auto” located on the side of the box.
    1. When the lever is released the dose counter should increase by one.
    2. If it does not increase by one, the dose counter is not working, and a qualified septic contractor should be contacted.
  2. Check the alarm. It can be audio, visual or both.
    1. If the system does not have an audio or visual alarm, select “N/A” in the appropriate drop-down menu on the monitoring form.
    2. Press the lever or button labeled “test” or “alarm test”. If the audible or light alarms are not working, contact a qualified septic contractor.

Instructions (without timer): Dose counter reading

  1. Enter the current dose counter reading and the date the reading is taken in the appropriate boxes on the Self-Monitoring Form.
  2. The previous dose counter reading and date should be pre-populated. Enter the previous dose counter reading from the previous Self-Monitoring Form and the date it was recorded.
  3. If this information is not available, contact Permit Sonoma OPR via email at to obtain the previous reading and date.
  4. All calculations will be shown on the completed form.

Instructions (without timer): Average gallons per day

  1. To obtain the average gallons per day pumped to the septic system, divide the number of doses by the number of days to obtain the number of doses per day. The gallons per dose for your septic system can be found located at the upper right portion of your Self-Monitoring Form. In addition, gallons per dose is printed on the Operational Permit for the OWTS with supplemental treatment and is also located on the plans for the system.
  2. Multiply the gallons per dose by the number of doses per day to obtain the gallons per day. Compare these gallons per day to the gallons per day the system is designed for. The designed gallons per day can be found at the upper right corner of the monitoring form, on the septic plans, or on the Operational Permit.

The system should be operating below the designed gallons per day. Operating above this design may cause the system to fail or shorten the life of the system. If the designed maximum flow is exceeded, check for plumbing leaks such as running toilets, review daily water use, and/or contact a septic contractor to assist in determining the cause for the high dose readings.

Instructions (with timer): Dose counter reading and average gallons per day

  1. The Orenco alarm box uses a timer and a scrolling LCD display. To obtain the dose numbers, use only the up/down arrows to find “D Pump Ct” (Discharge Pump Count) and “ETM” (Elapsed Time in Minutes).
    1. Write down the “D Pump Ct” number for the doses in the space for current dose count reading.
    2. Write down the “ETM” in the space for current minutes/hours reading, which indicates how many minutes or hours the system has discharged to the leach field. The doses are set to last for a specified number of minutes or hours. The number of minutes or hours per dose should not change over time unless changed with approval from Permit Sonoma Well and Septic staff.
  2. If the system has only an “ETM” or similar timer and no dose meter, follow these steps:
    1. Determine the number of gallons pumped to the system. If, for example, the prior timer reading was 800 and is now 1,000, then 200 minutes, not doses, have elapsed.
    2. Divide the number of minutes by the number of days since the previous reading (200 for example) and the answer is one minute per day. NOTE: Some timers display hours, be sure to check whether minutes or hours are displayed. See the example calculation below.

Example calculation: Number of minutes (200) divided by number of days (200) = 1 minute per day. Then multiply one minute per day by the designed gallons per minute*, for example 10 gallons per minute to obtain the answer of 10 gallons per day.

* The disposal rate in gallons per minute can be obtained from the consultant who designed the system, the contractor who installed the system, or from the approved septic system plans. The upper right portion of the Self-Monitoring Form includes the dose setting, not the time for the dose or the gallons per minute. It is highly recommended that the parameters of the system be reviewed with a service provider prior to scrolling through the LCD.
Instructions for drip systems (with flow meters) 
There may be one or two flow meters which can be found within the headworks, located within a Christy box. The flow meter on the return line reflects the gallons flushed through the system in order to clean out the lines.
To determine the amount of effluent being discharged to the leach field:

  1. Read the flow meter on the supply line, which is displayed in gallons.
  2. On the form where it asks you to “Choose your Flow Reading Type” select Flow Meter from the drop-down menu.
  3. If there is a second flow meter (located on the return line) subtract those gallons from the supply line’s flow meter reading. This total should be written in the space for “Today’s reading (Flow Meter)”.

Note: The LCD screen does not need to be read if the system includes a flow meter. However, comparing the actual gallons per day (flow meter readings) with the LCD display of minutes lapsed, may indicate that the gallons per minute are decreasing over time. This is a useful diagnostic tool to determine if the drip emitters are plugged, thereby reducing the flow per minute.

General Condition of the System

Please note any maintenance and/or repairs performed, and who performed them, since the last monitoring, i.e., tank pumped, alarm repaired, pump replaced, system purged & balanced.

Additional Routine Maintenance Recommendations

Diversion Valves: Diversion valves switch the effluent flow between two or more separate leach fields. Often there will be two functioning leach fields separated by a diversion valve. Switching periodically between leach fields allows the leach fields to dry out prolonging their longevity. Often the system higher in elevation or above ground is utilized during the winter months in order to achieve a separation to ground water.

Manual Diversion Valve: A manual valve will often be found within an access container made of ABS, PVC, or corrugated pipe with an access lid at the ground surface. The valve will be located after the sump tank or after the septic tank if there is not a sump tank.

Automatic Diversion Valve: An automatic diversion valve is often found within a Christy box and may consist of valve levers controlled electronically (there will be wires connected to the valves) or several pipes exiting from a chamber such as with the Hydrotek valve.

To determine if a Hydrotek valve is operational:

  1. Have one person activate the pump and send one dose of effluent and have a second person observe the effluent flowing through the clear piping exiting the valve during a dose.
  2. One person sends a second dose while the second person observes the effluent. The effluent should be traveling through a different pipe. If it is not, contact a qualified septic contractor for assistance.

Electronically controlled diversion valve: To determine if it is working, first ascertain when the valve is set to open. This information should be available from the approved septic plans or from the contractor or consultant who worked on the system. If the diversion valve is set to switch between fields after every dose:

  1. Observe which valves are open during a dose, and then observe if the valves rotate during the next dose. If they don’t rotate between doses, contact a qualified septic contractor.
  2. If the valves are not set to rotate after each dose, contact the contractor or consultant who worked on the system for assistance.  

Valve boxes: Locate the valve boxes, as shown on the approved septic plans. Check to see if they are filled with dirt or effluent and remove any excess dirt. Dirt will hinder the ability to adjust the valves when purging and adjusting is necessary. Effluent may indicate that a biomat is forming on the gravel bed and that the system needs purging and adjusting.

Vegetation: Follow proper vegetation practices. Refer to WLS-017, Landscape Guidelines for Nonstandard Septic Systems.

Soil cover: Fill in any holes in the leach field area with a permeable soil similar to the existing cover.

Interceptor drains: Interceptor drains are sub-surface drains used to catch sub-surface water in order to divert it around the leach field. Not all septic systems employ an interceptor drain. If one is used, the location of the interceptor drain should be on the approved septic plans. Locate the outfall and the cleanouts and clear any obstructions. The best time to check to see if the drain is functioning is during or right after a rainfall. Water should be running out of the outfall.

Expansion area: Determine if the system has an expansion area.

  1. For any new improvements such as, structures, driveways, drainage ways, horse corrals, etc. setbacks from the expansion areas must be met. For questions, contact Permit Sonoma Well and Septic staff prior to the start of work.

Pretreatment units: Determine if the system has a pretreatment unit and review the maintenance manual for service instructions.

  1. Make sure the unit is always turned on.
  2. When properly working no odors should be present when the unit is closed.
  3. If there are odors present when the unit is closed, contact a qualified septic contractor for assistance.

Drip Systems: For drip systems, refer to the maintenance manual if available.

  1. Check the approved septic plans for location of the drip system, including the headworks, located in a Christy box, and the air vent/vacuum breakers.
  2. Be sure that the headworks, and air vents are free of excess dirt.
  3. Check visible plumbing for any leaks.
  4. Check for wet areas within the system which may indicate a broken line.
  5. If there are wet areas or plumbing leaks, contact a qualified septic contractor.