Practitioner's Tool / Lime Stabilization of Septage

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In mid-2008, engineers and staff from the San Fernando City health, planning, environment and engineering offices engaged in the first stage of a pilot demonstration project to determine the effectiveness of lime stabilization as a method of treating domestic septage in the Philippines.

The Department of Health in the Philippines (DOH) also recognizes that lime stabilization is a viable septage treatment option in their manual: Rules and Regulations Governing Domestic Septage and sludge (August 2008). While the technology is recognized, it has yet to be fully tested in the Philippines, which is the purpose of Phase 2 of this pilot study.

Phase 1 of the study involved treating one load (approximately 5 cubic meters of septage) using the lime stabilization method and performing laboratory tests to verify that it meets the requirements for septage reuse as provided for in the DOH manual. The tests show that the process was effective in complete destruction of pathogenic organisms and that all other listed parameters were within the specifications stated by DOH.

Phase 2 is now proposed to expand upon the success of Phase 1 and further document the environmental and cost effectiveness of the process. Phase 2 will involve treating 24 loads of septage (approximately 120 cubic meters of septage) in 8 three-load batches. Four of the three-load batches will be extensively monitored by sampling and laboratory analysis. This represents 50% of the total volume of septage that will be analyzed in this study, which is more than enough to show that the results of the Phase 2 study are statistically significant. This will be the basis of review by DOH for other lime stabilization applications throughout the Philippines.

The Phase 2 study will last 8 weeks. It will process one batch of three septage loads a week for 8 weeks.

National Government, Local Government, and Private Sector Partnership

In order to effectively carry out the program from which results may be applied to other lime stabilization project applications in the Philippines, a partnership between the Local Government of San Fernando and the National Agencies of the DOH and the Environment Management Bureau (EMB) of the Department of Environment and Natural Resources (DENR) is proposed. There is no funding request from any of these agencies as this is a project that is totally funded by the income collected through tipping fees, which are less than currently incurred to deliver the septage to Baguio for proper treatment. What will be required is a temporary Environmental Sanitation Clearance for the stationary facility from both DOH and DENR to conduct this Phase 2 investigation. Both DOH and EMB will be requested to review the plans for carrying out the Phase 2 investigation as detailed in this document. Both agencies will also be invited to monitor the operations of the Phase 2 investigation. Once completed, the results will be shared with both of the agencies. The temporary clearance will be required to enable the city to levy the tipping fee.

The other partners involved will be the private desludgers that will work cooperatively to bring loads of septage to the temporary facility and pay the tipping fee. All participating private desludgers will be required to properly complete septage manifest forms for each septage load as required by the DOH manual. This will also help verify the quantity and origin of the septage, which will be useful in interpreting the results of the study. Septage manifest forms will be available from the Community Environment and Natural Resources Office.

For more information on the agencies and entities mentioned in this section, visit our page, Regulatory Roles and Responsibilities in the Philippines.

Technology Description

To treat septage using the lime stabilization method, hydrated lime, otherwise known as calcium hydroxide, is added to the septage and mixed manually or with a simple mixing pump. The ensuing chemical reaction elevates the pH, which kills the pathogens and separates the water from the solids. The remaining byproducts are then suitable for reuse as a soil amendment for either agricultural or non-agricultural crops, or the solids may be dried and used as daily cover on the landfill.

The Process

Hydrated lime
  1. Choosing a site. The first step is to choose a suitable site for the treatment process. Sites should be selected that are far away from residential areas, at least 25 meters from any groundwater wells or floodplains of streams or rivers. The site should be relatively flat and zoned for commercial or industrial purposes. Additionally, the site should be accessible to septage hauling trucks. For this project, the San Fernando City sanitary landfill is the chosen site.
  2. The mixing pit. In order for the process to be effective, the septage must be mixed with the lime for a minimum of 30 minutes. To facilitate this, a mixing pit will be constructed. Mixing pits are small excavations that must be lined with plastic or concrete if soils are permeable. For impermeable soils with high concentrations of clay, simply compacting the clay is sufficient to keep the septage from contaminating the groundwater. The mixing pit in the San Fernando study was excavated to a depth of 1.5 meters and was 3 meters wide by 4 meters long. The soils were impermeable clay so no liner was required. The same pit will be used for the Phase 2 study.
  3. The mixing process. Septage is discharged from the truck to the mixing pit simply by opening up the valve on the back of the truck. As the septage dumps into the pit, workers monitor the loading and assist the driver as needed. When the pit is filled to the required level (for this study, this will be three truck loads), the lime is added and mixed.
  4. Mixing can be performed manually or with a mixing pump. For the San Fernando tests, a small 3 horsepower mixing pump was made available but not used. Instead, manual mixing was performed. To mix using the mixing pump, lower the suction hose into the pit, and spray the discharge from the pump back into the pit. The recirculation process helps keep the lime in suspension so that it properly contacts and thoroughly dissolves in the septage. This is similar to what happens when you add a teaspoon of sugar to a cup of coffee. If you do not stir it, the sugar will not dissolve effectively. The same is true with the lime in the septage. Without proper mixing, the pH will not elevate to the desired level. To manually mix, two or three operators with large wooden paddles can keep the lime in suspension for one hour. As an alternative, lime can be mixed directly into the truck, as shown in the image. Here lime slurry is mixed with the septage as it is being pumped from the septic tank. This process makes it simple to apply the stabilized septage directly from the truck to the land.
  5. Monitoring. Monitoring is carried out with a handheld pH meter and a watch. The goal is to achieve a pH of 12 for 30 minutes, a pH of 11.5 for one hour, or a pH of 11 for 2 hours. The speed of the pH rise and the ultimate level of the pH will depend upon the amount of lime used per cubic meter of septage, the quality of the hydrated lime, and the mixing. The San Fernando trials show that an application rate of 50 kilograms of lime per 5 cubic meters of septage is adequate to achieve the required level of treatment (pH 11 for 2 hours).
  6. After the treatment. The treatment process achieves two goals. First, it kills the pathogens in the septage. Second, it separates the water from the solids. After the septage has been treated with lime, the mixing pump is turned off and the solids allowed to separate from the liquid by settling. The pH will also begin to drop back toward neutral. After 24 hours, the clear liquid can be siphoned off and discharged to a leachate pond, used to irrigate agricultural land, or for landscaping purposes. The solids may be applied as a soil amendment or dried and used as daily cover for sanitary landfills. For the purposes of this study, a drying bed will be constructed (details provided below) to accomplish separation of the solids from the liquids.

Laboratory Analysis and Phase 1 Results

Laboratory analysis was performed for the Phase 1 San Fernando trial following Department of Health requirements. The results show total destruction of all pathogens including the following:

  • Fecal streptococci
  • Total coliform bacteria
  • Infective parasites
  • Salmonella bacteria

Additionally, metals analysis including zinc, lead, copper, chromium, nickel, mercury and cadmium all showed levels well below the legal limits. While metals from commercial septage loads may be higher, it appears that this treatment method is acceptable for domestic septage sources.

In order to statistically prove the effectiveness of the technology for consideration and use by other applicants in other locations, additional testing is proposed to include the following:

  1. Four of the 8 batches equating to 50% of the total number of septage batches will be tested.
  2. In addition to the above analysis, the supernatant (the clear liquid that separates from the solid content) will be analyzed for biochemical oxygen demand, total suspended solids, and nutrients to verify its suitability for discharge in compliance with DENR standards.
  3. Nutrients including total nitrogen, total phosphorus, and potassium will be tested separately for the supernatant and the solids. This will identify the potential of using the treated septage for agricultural purposes, such as fertilizer or soils amendment.
Phase 2 Procedures to be Followed

1. Mixing pit.Soils in the area chosen for the mixing pit contain enough clay that, when compacted by heavy equipment, it will be impermeable and adequate for the program. To prepare the pit, the city will use the track hoe on site at the landfill to compact the bottom of the pit using the bucket of the machine. First, the operator will wet the soil, then compact the pit with the bucket. This will be suitable to ensure that septage does not leak out of the bottom of the pit into the soils during the test. After removing the stabilized septage, the operator will re-compact the pit using the same procedure described above to prepare it for the next batch.

2. Mixing the lime and septage.The lime and septage will be mixed as it was during the Phase 1 test except that three truckloads of septage will be processed at the same time. All three trucks will discharge their septage prior to starting the stabilization process. Ideally, the filling of the pit will occur within one 24-hour period. Each truck will hold an estimated 5 cubic meters, so a total of 15 cubic meters of septage will enter the pit for treatment. Once the pit is filled with the three truck loads of septage, the operators will begin shoveling in the lime. 150 kilograms, or three sacks, of lime will be added to the septage in the pit. The lime will be added by shovel, one scoop at a time. Workers will use rubber gloves and dust masks. The operators will take care to shovel the lime evenly over the surface of the septage. Once all of the lime is added, the septage/lime mixture will be mixed. During Phase 1, operators demonstrated a simple method of mixing using long boards as paddles. This is an acceptable method as long as the operators can keep the lime in suspension by keeping the mixing action going. Recommended would be three workers with paddles who will mix the septage with the lime in it for one hour. Alternatively, the mixing pump can be used as described above. The mixing will last for one hour after which the mixture of septage and lime will be allowed to settle in the pit for 24 hours.

3. Monitoring for pH. During the mixing process, the operator will monitor the pH with a handheld pH meter every 15 minutes, then once an hour for 4 hours. Finally, after the 24-hour settling period, the operator will monitor pH one final time. All monitoring records will be logged in the log book. If after 30 minutes, the pH does not reach 11, an additional bag of lime (50 kilograms) will be added and mixed in with the septage.

4. Discharging the stabilized septage. Under full-scale production, the stabilized septage will be reclaimed and applied to agricultural fields to recapture the nutrient value in the septage. Studies have shown that the nitrogen, phosphorus and potassium plus the micronutrients in the stabilized septage make it an ideal fertilizer for landscape and certain agricultural crops. However, it is understood that until such time that the product is determined to be safe through a statistically significant laboratory-backed study performed here in the Philippines, there will be some resistance and acceptability by farmers. Until that time, the stabilized septage will need to be disposed of. The procedure for disposing of the septage will be as follows:

  • a) City staff will construct one septage drying bed that will be 20 square meters in area and have a depth of .5 meters. The bed will be filled with a 50/50 mixture of sand and gravel placed in a compacted earthen basin. The basin will drain to the leachate pond through a perforated under-drain pipe.
  • b) The stabilized septage will be loaded onto the drying beds either with the pump or with the bucket from the track hoe. The purpose of the drying beds is to quickly separate the water from the solids, which is readily accomplished with lime stabilized septage. The clear liquid will drain through the gravel and sand to the pond and the solids left behind will dry in the sun. It will not matter if the solids get rained upon as the water will pass through the solids into the gravel and sand and pass through to the pond. This is a simple and low-cost method of separating septage solids that has been tested in a documented pilot study in Thailand. The costs for the drying bed will be fully covered by the project (see budget below).
  • c) The stabilized septage from the 24 loads will be loaded onto the drying bed at a rate of 1 batch (15 cubic meters) once a week for 8 weeks. The amount of residual solids will only add a few centimeters to the solids depth on the top of the drying bed for each batch. Studies show this type of drying can be operated for up to two years with no clogging. At the end of the study, the drying bed may be expanded or decommissioned with the gravel and solids being placed safely in the landfill as daily cover.

5. Sampling and analysis: Four of the 8 batches of septage will be analyzed. Two of these batches will have the samples split and run by a separate laboratory to maintain quality control. Two sample streams will be tested: a) the stabilized septage as it is loaded onto the drying beds and b) the clear water as it exits the drying beds to the leachate pond. The samples taken will be as follows:

  • a) Stabilized septage:
    • 1. Fecal streptococci
    • 2. Total coliform bacteria
    • 3. Infective parasites
    • 4. Salmonella bacteria
    • 5. Zinc
    • 6. Lead
    • 7. Copper
    • 8. Chromium
    • 9. Nickel
    • 10. Mercury
    • 11. Cadmium
    • 12. Total nitrogen
    • 13. Total phosphorus
    • 14. Potassium
  • b) Drying bed effluent:
    • 1. Biochemical oxygen demands
    • 2. Total suspended solids
    • 3. Total nitrogen
    • 4. Total phosphorus
    • 5. Potassium

6. Worker health and safety. Septage is infectious material and must be handled with caution. Additionally, hydrated lime is extremely reactive and also must be handled with care and with proper personal protective equipment (gloves, goggles, dust mask). A written health and safety plan will be prepared and provided to the septage operators prior to initiation of the program. Weekly safety meetings will be hosted by the Community Environment and Natural Resources Office and all operators and septage handlers required to attend.


This project will sustain itself completely through the tipping fee set at 3,375 pesos per 5 cubic meter load. This is considerably less than the cost currently charged to haul the septage to the current disposal location.

It should be noted that if the Phase 2 project is successful and this program is in full operation, there will be a significant cost savings as the main expenses will be limited to the cost of the lime and the labor.