NOVELTY - The continuous method comprises discharging hydrolyzate (20) from a pulp washing system, storing the hydrolyzate in a first stage pretreatment tank for a first stage of pretreatment, determining a chemical oxygen demand (COD) level (45000-90000 ppm) of the discharged or stored hydrolyzate, adding a dilution liquid to the hydrolyzate as the hydrolyzate is transferred from the first stage of pretreatment to a second stage of pretreatment, and determining an amount of the dilution liquid to be added to the hydrolyzate or a ratio of the dilution liquid to the hydrolyzate.    USE - The continuous method is useful for treating hydrolyzate with biological organisms including methanogenic bacteria and acidogenic bacteria (claimed) in a biogas plant for production of methane gas, which is useful for boilers driving steam turbines and gas turbines that generate electricity to power a pulp mill.    ADVANTAGE - The continuous method ensures economical and energy-efficient treatment of hydrolyzate with biological organisms thus efficiently and easily producing the methane gas and maintaining the health of the microorganisms in the bioreactor tanks.    DETAILED DESCRIPTION - The continuous method comprises discharging hydrolyzate (20) from a pulp washing system, storing the hydrolyzate in a first stage pretreatment tank for a first stage of pretreatment, determining a chemical oxygen demand (COD) level (45000-90000 ppm) of the discharged or stored hydrolyzate, adding a dilution liquid to the hydrolyzate as the hydrolyzate is transferred from the first stage of pretreatment to a second stage of pretreatment, determining an amount of the dilution liquid to be added to the hydrolyzate or a ratio of the dilution liquid to the hydrolyzate based on the determined COD level, transferring the diluted hydrolyzate with the nutrients from the second stage of pretreatment to a reactor feed tank combined with a recycle stream from a bioreactor having the biological organisms, transferring the diluted hydrolyzate from the reactor feed tank to a bioreactor tank having the biological organisms, generating a combustible gas as the diluted hydrolyzate is consumed by the biological organisms in the bioreactor tank, adding nutrients to the diluted hydrolyzate as the hydrolyzate is transferred from the first stage of pretreatment to the second stage of pretreatment, and balancing growth rates of the biological organisms by pretreating the hydrolyzate in the first and second stages of pretreatment. The second stage of pretreatment comprises storing the diluted hydroylzate in a second stage pretreatment tank. A ratio of dilution liquid added to the hydrolyzate is 1.5-3 of dilution liquid to hydrolyzate. The pulp washing system discharges pulp produced from chips digested with water or other non-alkali cooking liquor. The digesting of the chips avoids use of alkali cooking liquor (28). The washed pulp is used to form a corrugating medium. The hydroylzate continuously flows from the pulp washing system to the reactor feed tank through the first stage of pretreatment and second stage pretreatment. The biological organisms are contained in an anaerobic environment in the biological reactor tank. The determined COD level is measured with a refractometer, and is the sole factor used to determine the amount or the ratio for adding the dilution liquid. An INDEPENDENT CLAIM is included for a method for pretreating hydrolyzate with biological organisms.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic block diagram of a pulping process having pulp washing stages and a biogas stage.    Chip feed system (12)    Digester (14)    Biogas plant (18)    Hydrolyzate (20)    Feed screw (24)    White cooking liquor. (28)