44 Organic waste recycling: technology and management
The clarified juice contains approximately 88% water. Two-thirds of this
water is evaporated in vacuum multiple-effect evaporators. Crystallization takes
place in a single-effect vacuum pan where the syrup is evaporated until saturated
with sugar. It is at this point that the sugar first appears as fine crystals, which
are then built up to the size required for the final product. The mixture of
crystals and syrup is then concentrated to a dense mass (massecuite) and the
contents of the pan are discharged into a mixer or crystallizer.
The massecuite from the mixer or crystallizer is drawn into revolving
machines called centrifugals. The sugar crystals are retained and the mother
liquor, molasses, passes through. The final molasses or blackstrap is a heavy
viscous material containing approximately one-third reducing sugars, and the
remainder ash, organic non-sugars, and water. It may be used as cattle feed or as
a raw material for distilleries, to produce ethanol.
Sugar industry wastewater effluents
The sugar industry uses large amounts of water, mainly for cane washing,
condensing of vapor, boiler feed water, and other miscellaneous uses. Sources of
wastewater and sludge generation during the manufacturing of sugar are given in
Figure 2.6. There are air emissions from the sugar manufacturing process which
include a variety of pollutants such as particulates, SOx, NOx, CO, and
hydrocarbons; these pollutants have to be removed to the required air emission
standards set by the appropriate regulatory agency.
Table 2.13 Wastewater characteristics of Crescent Sugar Mills, Pakistan (Qureshi 1977)
Characteristics Range Fifty percentile Ninety percentile
pH 6.7-10.0 8.15 9.11
Alkalinity 260-490 370 494
Total solids 4,520-10,790 6,500 10,300
Fixed solids 1,850-6,150 4,000 6,100
Volatile solids 1,825-4,600 2,700 4,500
COD 607-3,680 2,030 3,500
Nitrogen 15-50 33 -
Phosphorus 6.7-11.25 9.25 -
All values are in mg/L, except pH
The characteristics of wastewater from a sugar mill in Pakistan are given in
Table 2.13. Its pH varied from 6.7 to 10 and the COD range was 600 - 3,700 mg/L.
The wastewater contained rather high solid concentrations in which the ranges of
total solids and volatile solids were 4520 - 10790 and 1820 - 4600 mg/L,
respectively. Wastewater data from a sugar mill in U.S.A. are also given in Table