Septic tanks are formed from a wide variety of materials, of which concrete and plastic are the most common, although steel is available as a much less popular option.
Offering many advantages over both concrete and steel, plastic septic tanks feature high resistance to cracking, zero rust, light-weight, high corrosion resistance, water-tightness and low costs. Of the plastic materials, polyethylene (PE), a light, chemical-resistant thermoplastic, is the most commonly used plastic resin used for plastic septic tanks.
Polyethylene resins can be linear high-density (HDLPE), cross-linked high-density (XLPE), or linear low-density (LLDPE) resins. Following are polypropylene, polyvinyl chloride and polyvinylidene fluoride. In comparison to polyethylene, polypropylene (PP) is a light, durable thermoplastic that is denser, stiffer, and stronger than polyethylene and has a much higher melting point of 320ºF.
Polyvinyl chloride (PVC) is a durable, transparent thermoplastic that is resistant to non-essential oils but cannot be exposed to some solvents. Polyvinylidene fluoride (PVDF) is another durable thermoplastic that has high resistance to chemicals and abrasion.
Septic systems consist of two major parts: a drainfield and a septic tank. The drainfield functions to remove contaminants and impurities from the liquid that emerges from the septic tank, and can also be referred to as either a leach field or a leach drain. The septic tank itself allows wastewater to enter in the first chamber of the tank.
After solids have settled and scum starts to float they are anaerobically digested, thus reducing the volume of solids within the septic tank. Next, the liquid component flows through the dividing wall into the tank’s second chamber. In this chamber further settlement occurs, with the excess liquid then draining into the drain field. The majority of plastic septic tanks are formed using either a molding or welding process.
Plastic tank molding, including rotational, blow, and injection molding, involves using a model shape to form the tank. Plastic tank welding is a process in which two pieces of heat-softened plastic are joined through the application of pressure. Examples of this process are hot air and extrusion welding. Septic tanks differ in size and capacity, ranging from small tanks holding a few gallons to tanks exceeding 35 feet in length, with a storage capacity of 50,000 gallons or more.