Patent ID: 8151999

Claim:
A molded tank made of thermoplastic for receiving, treating or containing waste water when buried beneath soil, comprising an elongate integral tank body having opposing ends spaced apart along the tank body length and a wall which forms (a) a bottom, (b) a top having at least one opening for access to the interior of the tank, (c) opposing lengthwise-running sides; the tank body having a center section located between said openings, where said wall runs circumferentially around the length axis of the tank and said wall has a multiplicity of circumferentially-running peak corrugations and valley corrugations; wherein the tank wall is comprised of (a) an inner layer LI of solid thermoplastic for containing liquid within the tank; (b) a middle layer LM of closed cell foam thermoplastic, resistant to passage of both liquid and heat and structurally connecting the inner layer and an outer layer; and, (c) an outer solid thermoplastic layer LX, resistant to the force of external mechanical objects and passage of liquid; wherein, said middle layer LM is integrally fused to both the inner layer LI and outer layer LX; wherein, each of said three layers is free of substantial voids and extends continuously within the tank body; the tank formed by a molding process which integrally fuses said layers to each other during formation of each, to thereby provide structural strength to said wall.