Organic matter release characteristics of brown rice: A promising solid carbon source with unique dual-enzyme system for wastewater treatment

Solid carbon source (SCS) has attracted increasing research interests considering its merits of sustainable organic matter release capacity, safe transportation, easy management, and no need for frequent addition. In this study, the organic matter release capacities of five selected natural (milled rice and brown rice) and synthetic (PLA, PHA, PCL) SCSs were systematically investigated. The results showed that brown rice was the preferable SCS with high COD release potential, release rate and maximum accumulation of 309.2 mg-COD/g-SCS, 581.3 mg-COD/L·d and 6183.3 mg-COD/L, respectively. The cost for COD supply of brown rice stood at $1.0/kg-COD with considerable economic viability. The organic matter release process of brown rice could be well depicted by Hixson-Crowell model with a rate constant of −1.10. The addition of activated sludge could enhance the organic matter release of brown rice, evidenced by the increased release of VFAs with a proportion up to 97.1 % in the total organic matter. Moreover, the mass flow of carbon showed that the addition of activated sludge could improve the carbon utilization rate, and the peak value could achieve 45.4 % in 12 days. The unique dual-enzyme system, consisting of exogenous hydrolase from microorganisms in activated sludge and the endogenous amylase from brown rice, was supposed to be the main reason for the superior carbon release capacity of brown rice over other SCSs. This study was expected to offer an economic and effective SCS for the biological treatment of low-carbon wastewater.