08.08.2015   BIOETHANOL

Liquid waste (stillage)

Whilst the slop from molasses is very dark in color, cassava liquid waste has a light yellowish color with a lower COD (40,000-60,000) and BOD (15,000-30,000) values. The characteristics of waste water from the ethanol factories using cassava and molasses as feedstock are shown in Table 9. In consideration of this, the waste water from cassava-based process is much easier to handle than the waste obtained from molasses. This implies less investment and operational costs. In China, cogeneration of biogas obtained from waste water treatment in ethanol factory operating with cassava is reported to be able to cover all electricity needs in ethanol production process and still have some excess to supply to the grid (Dai et al., 2006). The practice for using thin stillage in Thailand is also for biogas production.

Characteristic

Factory using cassava chips

Factory using molasses

1. Chemical Oxygen Demand (COD, mg/L)

40,000-60,000

100,000-150,000

2. Biological Oxygen Demand (BOD, mg/L)

15,000-30,000

40,000-70,000

3. Total Kjedahl Nitrogen (TKN, mg/L)

350-400

1,500-2,000

4. Total Solids (mg/L)

60,000-65,000

100,000-120,000

5. Total Suspended Solid (TSS, mg/L)

3,000-20,000

14,000-18,000

6. Total Volatile Solids (mg/L)

20,000-40,000

n. a.

7. Total Dissolved Solids (mg/L)

50,000

105,000-300,000

8. pH

3.5-4.3

4.1-4.6

Source: Sriroth et al., 2006; n. a. = not applicable

Table 9. Characteristics of stillage obtained from ethanol factories in Thailand.