NOVELTY - A septic tank computing method comprises establishing a mathematic model of the septic tank using effective volume of septic tank, total number of septic tank, total quantity of accumulated degradable sludge, and clearing time of sludge; determining sum S of degradable sludge accumulated within T time; establishing a corresponding relationship among data according to the computation result of the mathematic model; and establishing data corresponding relationship table of the septic tank or using the mathematic model to manufacture the computing application software of the septic tank.    USE - Septic tank computing method.    ADVANTAGE - Retention time of the sludge in the septic tank is prolonged.    DETAILED DESCRIPTION - A septic tank computing method comprises:    (A) restraining a boundary condition of digestion, comprising affirming, presuming, and dating the relative environment condition corresponding to the target object;    (B) establishing dynamic relationship of a digestion tank under mass conservation principle: M=Min-Mout, where the formula represents that the matter in the digestion tank is the difference between the matter entering the digestion tank and the matter discharged from the digestion tank;    (C) restraining and analyzing relationship of the matters preserved in the septic tank; W=W1+W'+W'', where the formula represents that efficient volume of the septic tank is the sum of the sewage volume, the degradable sludge volume and the non-degradable sludge volume;    (D) establishing a mathematic model of the septic tank: W=W1+W'+W''=Naqt/24x 1000+(1-b)K2( alpha TaNK1/(1-d)1000+S/(1-c)1000), where W is the effective volume of the septic tank with unit of m3, W1 is the sewage part volume in the septic tank with unit of m3, W' is the non-degradable sludge volume in the septic tank with unit of m3, W'' is the degradable sludge volume in the septic tank with unit of m3, N is designed total number of the septic tank with unit of a person; q is sewage quota for per person per day with unit of L/person.day, t is retention time of the sewage in the septic tank with two units of 12 hours or 24 hours; a is the percentage of number of actually using the sanitary ware to the designed total number; alpha is the converge confluence coefficient equal to 0.7 L/person.day, or effluent-division coefficient equal to 0.4 0.7 L/person.day, b is the moisture content of sludge, T is the clearing time of the sludge with unit of day; d is the moisture of the concentrated non-degradable sludge, K1 is the non-degradation of the mixed sludge, K2 is the mixture interception rate flowing the digestion tank, S is the total quantity of the degradable sludge accumulated within T time, and c is the moisture content of the concentrated degradable sludge;    (E) determining the sum S of degradable sludge accumulated within T time: S=S1(1-K)+S1(S1-K)2+S1(1-K)3+...+ S1(1-K) to the power of n, converging the degradable sludge quantity S; in the formula, S1 is the total quantity of degradable sludge entering the septic tank in a fermentation period time section; K is the digestion shrinkage in the fermentation period;    (F) establishing a corresponding relationship among data according to the computation result of the mathematic model; and    (G) establishing data corresponding relationship table of the septic tank or using the mathematic model to manufacture the computing application software of the septic tank.