20. Despite sections 18 and 19, an effluent having undergone biological treatment and having reached the acute lethality level may be combined with another effluent provided that(1) the average removal rate, measured in BOD5 reduction from biological treatment, is at least 90% for the month preceding toxicity control sampling;
(2) the toxicity of the effluent having undergone biological treatment is lower than the maximum lethality level; and
(3) the mill has reduced its annual water consumption by at least 50% since 1985, calculated in cubic metres per ton of production, except if the water consumption is lower than 40 m3/t or the mill was built after 31 December 1971.