Influence of Selected Operating Parameters on Fungal Biomass Production in Corn-Ethanol Wastewater

Abstract

Wastewater from a corn wet-milling ethanol plant was treated with Rhizopus microsporus mold in a continuous biofilm reactor (attached growth system). Plastic composite support tubes, composed of 50% (w/w) polypropylene and 50% (w/w) agricultural products were used as support media. The effects of operating pH (3.5, 4.0, and 4.5) and hydraulic retention times (HRTs) (5.0, 3.75, 2.5, and 1.25 h) on fungal growth, chemical oxygen demand (COD) removal and unwanted bacterial growth were evaluated under nonaseptic conditions. COD removal and biomass production were highest at pH 4.0 with lowest bacterial competition. Maximum COD removal of up to 80% was achieved at a 5.0 h HRT with a biomass yield of 0.44 g volatile suspended solids per g COD removed. A higher biomass yield was achieved at a shorter HRT of 2.5 h due to increased substrate availability; however, the biofilm was more sensitive to changes in wastewater composition. A HRT of 3.5–4 h was considered optimal in achieving organic removal and fungal biomass production. Significant loss of fungal biomass due to washout occurred at a 1.5 h HRT. Undesirable bacterial populations as a fraction of total biomass decreased with reducing HRT, excluding the 1.25 h HRT. Reductions in COD removal and biomass production were observed with decreases in aeration rate (1.0–0.25 L/min or 0.8–0.2 vvm (air volume per reactor working volume per minute). The recovered fungal biomass was found to contain protein of up to 40% (dry mass basis), which could serve as a source of high-value animal feed.