Lime stabilization, or applying hydrated lime (calcium hydroxide) to septage as a treatment method is well documented. The United States Environmental Protection Agency (USEPA) has endorsed this methodology as effective, and it has been used in that country to treat septage for many years. The full recommendations on the use of lime stabilization from the US EPA can be viewed at the following website:
http://www.epa.gov/OWM/mtb/septage.pdf
In addition, the Department of Health in the Philippines (DOH) also recognizes that lime stabilization is a viable septage treatment option in their Operations Manual on the Rules and Regulations Governing Domestic Sludge and Septage (August 2008). While the technology is recognized, it had yet to be fully tested in the Philippines, which was the purpose 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 the treated septage 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.
Technology Description
The Process
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, and 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 was 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 was constructed, and the soils compacted to limit leaching. 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 (as were the conditions for this pilot study), simply wetting and compacting the clay is sufficient to keep the septage from leaching, where it might contaminate 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 at the landfill site have high levels of clay particles, so no liner was required.
3. Adding lime to the septage. 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, the lime is added and mixed. When adding the lime from bags, workers must take care to wear dust masks. The worker will open the bags and add the lime to the septage one shovel-full at a time, taking care to avoid breathing in the reactive powder.
4. Mixing can be manually performed 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 don’t 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 30 minutes, which is enough time to accomplish this task.
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 agricultural lands.5. Monitoring. Monitoring is done with a handheld pH meter and a watch. The goal is to achieve a pH of 12 for 30 minutes, or a pH of 11.5 for one hour, or a pH of 11 for 2 hours. The speed of the pH rise and the
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. To accelerate the drying process, a covered drying bed can be easily constructed to achieve adequate drying in between 15- 30 days.
Laboratory Analysis and Phase 1 Results
Treatment 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:
• 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 metal concentrations from commercial septage loads may be higher, it appears that this treatment method is acceptable for domestic septage sources.
Costs of the Program and Scaling Up
For the San Fernando study, the cost of the hydrated lime was P455 per 50 kg bag, which included shipping to the treatment site at the landfill. The soils were sufficiently clayey, so wetting and compacting the soils with the excavator was sufficient to prevent leaching. When figuring in two staff persons, monitoring, excavation and miscellaneous costs, the total cost for treatment was P1,000 for the 5 cubic meter truck load, or roughly P200 per cubic meter of septage treated.
For this study, as the effluent from the process was directed to the existing leachate pond for the landfill, no additional costs for effluent treatment and disposal were incurred. For a new facility where effluent treatment is required, costs for building a pond or constructed wetlands should be factored in. As for the biosolids that remained after treatment, these can be allowed to air dry in place and then hauled off for use on agricultural land, or used as daily cover in the landfill.
Applicability
Lime stabilization is just one of many potential methods of treating septage. It is ideally suited for small operations of less than 15 cubic meters per day and in a location where there is sufficient space to install mixing pits, drying beds and solids storage facilities.
