Practitioner's Tool / Septage Treatment and Reuse
Biological Sludge Thickening
Generally the first step in the process of septage or sludge treatment is thickening. Wastewater from residential septic tanks and treatment plants (after aerobic treatment processes) usually contains solid concentrations of 1-4%. Thickening is used to reduce the volume of waste and can decrease capital and operating costs of remaining processing downstream because of reduced volume. Thickening can also provide additional benefits including blending, sludge equalization, and increased ease of storage. Grit removal, gas stripping and clarification are also more easily accomplished when sludge is thickened. Thickening is generally performed using one of these types of processes:
- Gravitational thickening is typically accomplished by applying raw sludge or septage to sand beds. The sand beds allow the water to separate both through gravity and by capillary forces, which draw water into the sand and leave solids behind.
- Flotation is usually utilized in lagoons or tanks. Dissolved oxygen is added through mechanical aeration devices, which produce tiny air bubbles that adhere to suspended matter in the water, bringing suspended matter to the surface where it is skimmed off and removed.
- Centrifugal concentration uses mechanical equipment to force the water from the solids by spinning it rapidly in perforated drums, similar to a clothes washing machine on the spin cycle. While energy intensive, this process is highly efficient and fast, allowing for dewatering high volumes of septage in a limited space.
Other processes for dewatering sludge or septage include vacuum filters, pressure filters, belt filter presses, solar inclined beds, screw presses, perched beds and solar dryers.
Stabilized septage and sludge is less odorous and has fewer pathogenic organisms, making stabilization of prime importance in the disposal of septage and sewage sludge. Basic techniques for stabilization include aerobic and anaerobic digestion, dual digestion, lime stabilization, chlorine stabilization, vertical tube reactors, heat treatment, and composting. When applying septage or sludge to land, it should be relatively odorless and pathogen free, so in addition to lowering pathogen levels, odor problems need to be reduced or eliminated for public acceptance. Stabilization processes are not required for all secondary treatment plant sludges but are always required for septage application.
Alkali (Lime) Stabilization
According to the United States Environmental Protection Agency's (USEPA's) Fact Sheet for Septage Treatment and Disposal, hydrated lime or other alkaline material can be added to liquid domestic septage to stabilize the waste when allowed to react for a minimum of 30 minutes to raise the pH to 12. Although septage characteristics and lime requirements vary depending on the waste, mixing is not difficult. Twenty to twenty-five pounds of lime are used for every 1,000 gallons of septage. The USEPA considers the most common stabilization approaches used prior to land application to be adding lime slurry to pumper trucks before septage is pumped (if the truck has a stainless steel tank), to pumper trucks while septage is being pumped, or to tanks or pits storing septage already discharged. The septage and lime may be mixed using a coarse bubble diffuser system located in the tank or truck, but it may be more beneficial to use a separate storage tank for lime and septage mixing because it allows for more uniform mixing, which results in a more complete reaction and easier sampling, monitoring, and control [Note: using a fire pump with a three-inch hose is a simple way to mix lime with septage in a pit].
Septage and sludge from wastewater treatment plants and contain significant quantities of pathogenic organisms including viruses, bacteria, parasites, and fungi. Human and animal wastes, biological laboratory wastes, industrial wastes, and food wastes all contribute potentially harmful organisms. These pathogens must be removed via disinfection, which deactivates or destroys the bacteria if the sludge is to be applied to land as a fertilizer or soil conditioner. The sludge stabilization process (discussed above) necessitates adding hydrated lime (calcium hydroxide) to wastewater at a rate of 25 kilograms per 4,000 liters for 30 minutes. This and other processes, including air drying, pasteurization, long-term storage, and high-energy radiation can serve to significantly reduce the number of pathogens in sludge.
Dewatering is the removal of liquid from septage or sludge during treatment. This is a critical process and should be maximized, as it will reduce sludge trucking costs, is normally required prior to composting, and is absolutely required when sludge is disposed of in landfills because it reduces the amount of leachate produced in landfill sites.
Dewatered sludge or septage should contain solids concentrations of from 20 - 40%, depending on the process used and whether or not the sludge is conditioned (as described below). Reliability of the dewatering process is a critical element in selecting a treatment method.
Conditioning is the process of adding heat or chemicals such as polymers and alum to wastewater to simplify the dewatering process because dry sludge is easier to dispose of.
Some conditioning processes also serve to disinfect the sludge, deodorize, physically alter the sludge, improve solids recovery, and reduce the solids content. Conditioning equipment like in the Philippines, the Dagat-dagatan septage facility operated by Maynilad Water Services, Inc., shown left, facilitates the entry and mixing of chemicals with the septage.
Thermal processes use heat to either remove water from the sludge, as in heat drying, or reduce the sludge volume by both evaporation and the destruction of organic matter, as in incineration or starved air combustion (pyrolysis). Temperatures in these processes range from 300-400°C for heat drying to 760-870°C for pyrolysis. The most significant problem with these processes are the energy needed to produce high temperatures, the high capital costs associated with the facilities, and the need for extensive air pollution equipment. NOTE: Thermal processes are expensive and may be restricted under the Philippines Clean Air Act. Check with local Department of Environment and Natural Resources officials for more information.
Composting, defined here as the aerobic decomposition of organic constituents in sludge to form a relatively stable humus-like material similar to soil, is a stabilization process as well as a process which serves as a last step in the resource recovery of sludge. Compost, which is musty in odor, brown in color, and relatively pathogen free, can supply some of the nutrients required for most soils and can help soil to retain moisture.
Composting techniques can be divided into three processes: windrow composting, aerated static pile composting, and mechanical composting. Several basic principles are involved in these processes. The sludge must be dewatered, and then a bulking agent comprised of wood chips, sawdust, cocopeat, rice hulls or even shredded municipal solid waste is added to bring the solids content of the mixture up to 40-50%. The mixture must be stable, porous, and capable of sustaining decomposition without added fuels.
Bioreactors can be used to enhance composting. This bioreactor pictured to the left is manufactured by the I. M. Bongar Corporation of Muntinlupa, Philippines and represents an attractive septage treatment alternative for local governments. Dewatered sludge is typically mixed with a bulking agent, such as rice hulls, coconut dust, or shredded municipal solid waste, and then fed to the bioreactors, which hasten the composting process and produce uniformly processed materials suitable for resale as soil amendment. Major benefits include reduced odor compared with compost piles and a greater volume of compost processed for the available space when compared with windrow composting.
Reuse Of Treated Septage
If septage is properly treated, in can be used for agricultural purposes, but should not be used on food crops intended to be eaten raw unless stringent controls are in place. The Philippine Department of Health Regulations has made the following regulations for agriculturally applied reused septage for food crops.
Requirements for Land Application
Treatment methods for sludge and septage described above have been developed to render biosoilds safe for land application in the case of food crops, as long as the following requirements are met. These are based on the USEPA guidelines for biosolids treatment processes that significantly reduce pathogens. One of these methods must be used if land application for food crops is to be practiced:
- Aerobic digestion between 40 days at 20°C and 60 days at 15°C
- Anaerobic digestion between 15 days at 35-55°C and 60 days at 20°C
- Air dry for at least three months, and two of the months must have average daily temperatures above freezing
- Compost with temperatures greater than 40°C for five days. The temperature must be greater than 55°C for four hours during the five days.
- Lime stabilization by adding sufficient hydrated lime to raise the pH greater than 12 for 30 minutes
Testing to verify appropriate pathogen reduction is required if agricultural reuse is intended. These methods have been recognized to reduce the number of helminthes eggs to levels that are determined to be acceptable by the World Health Organization (WHO) for land application purposes for food crops. While a standard for helminthes eggs in biosolids has not been codified into law by the Philippines Department of Health, there is guidance from the WHO on acceptable limits of these parasitic organisms as described below:
Nematodes, for a number of reasons, are the indicators of choice for testing for the presence of helminthes in biosolids destined for agricultural reuse. The WHO guideline of 1 nematode egg per liter of treated wastewater (or septage) used for vegetable irrigation (WHO, 1989), and an average manuring rate of 2–3 tons per hectare per year should be followed.
Testing for nematode eggs is a relatively simple procedure that should be used to check the treatment efficiency and acceptability of biosolids prior to land application. This should become an integral component of any biosolids program that reuses the treated product as a soils amendment for agricultural purposes.
If this sounds too complicated, keep it simple by reusing treated sludge and septage on landscaping or non-food crops such as bamboo, tree farms or reforestation projects for beneficial reuse of treated septage or sludge.
For more information, visit the website of the Philippines Department of Environment and Natural Resources or the USEPA.