EO 012.01 TP2 LIQUID WASTE MANAGEMENT
REFERENCES • Environmental Engineering 5th Edition, pgs 554 - 665
CESSPOOLS • Cesspools are covered, open-joint, or perforated walled pits that receive raw sewage • Not recommended where groundwater serves as a water supply • Should be located downgrade from water sources and 200 – 500 away • Bottom should be at least 4 feet above the highest groundwater level
SOIL PERCOLATION TEST • Suitability of soil can be determined by a study of soil characteristics and a soil percolation test • A measure of the rate at which clear water will seep out of a standard-size test hole that has been previously saturated • Movement of effluent through soil removes pathogenic microorganisms by various processes • Soils containing clay, silt and sand will remove most microorganisms found in sewage effluent • Course-textured granular soils less effective
SEPTIC TANKS • a watertight tank designed to slow down the movement of raw sewage and wastes passing through so that solids can separate or settle out and be broken down by liquefaction and anaerobic bacterial action • does not purify the sewage, eliminate odours, or destroy all solid matter • conditions the sewage so that it can be disposed of to a subsurface absorption system without prematurely clogging the system
SEPTIC TANKS • Tank sizes are based on estimated daily flows which are determined by the population the tank will be serving • Tanks should not have a liquid volume of less than 750 gallons • 75 gal/person and 150 gal/bedroom • Detention time for large septic tanks should not be less than 24-72 hrs
SEPTIC TANKS • Scum: Substances lighter than water (oil, grease, fats) float to the top, where they form a scum layer. This scum layer floats on top of the water surface in the tank. Aerobic bacteria work at digesting floating solids • Sludge: The "sinkable" solids (soil, grit, bones, unconsumed food particles) settle to the bottom of the tank and form a sludge layer. It forms a flat layer along the tank bottom. Underwater anaerobic bacteria consume organic materials in the sludge, giving off gases in the process and then, as they die off, become part of the sludge • Effluent: Effluent is the clarified wastewater left over after the scum has floated to the top and the sludge has settled to the bottom. It is the clarified liquid between scum and sludge. It flows through the septic tank outlet into the drainage field
SEPTIC TANKS • Septic tanks for private homes generally require cleaning every 3-5 years depending on occupancy, but should be inspected once per year • Routine inspection and cleaning will prevent solids being carried over and clogging the treatment or leaching systems • Septic tanks are generally cleaned by septic tank cleaning firms • Contents called “septage” should be emptied into a sanitary sewer or wastewater treatment plant
ABSORPTION FIELD SYSTEM • Designed to receive effluent from the septic tank and equally disperse it throughout the area of the field • Perforated piping should be laid in narrow trenches (18-24 inches) not more than 2 feet below the ground surface • Highest seasonal groundwater level should be at least 2 feet and preferably 4 feet below the bottom of the trenches. • Absorption fields should be protected from excessive water runoff, seeded with grass or shallow-rooted plants, and mapped out (if possible) prior to fill.
ALTERNATIVE WASTEWATER DISPOSAL • Where soils or sites are unsuitable for a conventional septic tank subsurface absorption system, alternative systems are available but usually more complex and costly. These include: • Modified absorption systems • Capillary seepage trench • Absorption-evapotranspiration system • Sand filter system • Aerobic treatment unit • Mound system • Raised bed or built-up systems
Mound system • A mound system is an alternative to the traditional rural septic systemleach field. The mound system is an engineered leaching field used in areas where septic systems are more prone to failure due to having extremely slow or extremely fast permeable soils, • soil with shallow cover over porous bedrock, and soils that have high seasonal water table
Capillary Seepage Trench • the capillary seepage trench (CST), is similar to conventional systems except that an impermeable liner is placed at the bottom of the trench. The liner prevents direct percolation and causes the wastewater to move upwardly and horizontally by capillary action before percolating downwards.
STABILIZATION PONDS • Waste stabilization ponds (oxidation ponds, sewage lagoons) may be a practical solution in areas where ample space is available, they can be located 1000 feet from habitation and with consideration given to prevailing winds • can be operated as high-rate aerobic ponds, aerobic-anaerobic ponds, aerated ponds, which are mechanically aerated, or anaerobic ponds (lagoons)