Influence of humic substances on bioavailability of Cu and Zn during sewage sludge composting

► Both water-soluble Cu and Zn increased at the initial stage of composting. ► HA-Cu and HA-Zn were positively correlated with HA and H/F respectively. ► The distribution coefficients of HA-Cu and HA-Zn reached 27.50% and 3.33%. ► The HA-Cu/HA-Zn ratio increased from 1.29 to 2.73 over the composting period.